U.S. patent application number 09/177393 was filed with the patent office on 2001-05-24 for epoxidation of olefins.
Invention is credited to SHARPLESS, K. BARRY, YUDIN, ANDREI K..
Application Number | 20010001798 09/177393 |
Document ID | / |
Family ID | 26742663 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001798 |
Kind Code |
A1 |
SHARPLESS, K. BARRY ; et
al. |
May 24, 2001 |
EPOXIDATION OF OLEFINS
Abstract
An process for epoxidizing diversely functionalized olefins by
oxorhenium catalysis employs conditions which control water
concentration. By controlling water concentration, one can maximize
monoperoxo complex formation and increase turnover which
subsequently reduces diol side products obtained from epoxide ring
opening and increases the yield of the desired epoxide product. The
optimal range of water concentrations is 0.50-80.0 mol %. Using
less than 0.5 mol % water does not result in practical turnovers
and 1.0 equivalent of water (or more) is detrimental to the
lifetime of the active catalytic species formed. More particularly,
there are four aspects to controlling water concentration: 1)
anhydrous oxidants using trialkylsilyl peroxides and an in situ
source of BTSP eliminating the need for its isolation; 2) water
removal agents including molecular sieves (Aldrich, 3 .ANG., 4
.ANG.) and common inorganic dehydrating agents; 3) rhenium
catalysts; and 4) a boiling reactor process in the context of
oxorhenium catalyzed epoxidation.
Inventors: |
SHARPLESS, K. BARRY; (LA
JOLLA, CA) ; YUDIN, ANDREI K.; (SAN DIEGO,
CA) |
Correspondence
Address: |
THE SCRIPPS RESEARCH INSTITUTE
10550 NORTH TORREY PINES ROAD
MAIL DROP TPC 8
LA JOLLA
CA
92037
|
Family ID: |
26742663 |
Appl. No.: |
09/177393 |
Filed: |
October 23, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60062760 |
Oct 23, 1997 |
|
|
|
Current U.S.
Class: |
546/255 |
Current CPC
Class: |
C07D 301/14
20130101 |
Class at
Publication: |
546/255 |
International
Class: |
C07D 213/04 |
Goverment Interests
[0002] This invention was made with government support under Grant
No. GM 28384 awarded by the National Institutes of Health and Grant
No. CHE 9531152 awarded by the National Science Foundation. The
U.S. government has certain rights in the invention.
Claims
What is claimed is
1. An improved process for epoxidizing an olefin by
rhenium-catalysis, the process being of a type wherein a reaction
mixture is formed by combining the olefin with a ligand, an
oxidant, an organic solvent, a protic solvent and a catalytic
organo rhenium oxide under conditions suitable for epoxide
formation to occur, wherein the improvement comprises: said oxidant
is a silicon based anhydrous oxidant and said oxidant is combined
with the reaction mixture by a controlled slow addition.
2. The process as described in claim 1 wherein the silicon based
anhydrous oxidant is a trialkylsilyl peroxide represented by the
formulas (R).sub.3SiOOSi (R).sub.3, and (--(R).sub.2SiOO--) wherein
R is selected from the group consisting of C.sub.1-C.sub.6 alkyl
and tert-C.sub.1-C.sub.6 alkyl.
3. The process as described in claim 2 wherein the silicon based
anhydrous oxidant is bis(trimethylsilyl)peroxide.
4. The process as described in claim 1 wherein the slow addition of
the silicon based anhydrous oxidant transfers a peroxo group from
the oxidant to the rhenium oxide with the assistance of the protic
solvent thereby controlling excess water concentration and
maximizing monoperoxocomplex formation.
5. The process as described in claim 2 employing the further
addition to the reaction mixture of a water removal agent.
6. The process as described in claim 3 wherein the water removal
agent selected from the group consisting of Molecular sieves,
MgSO.sub.4, Na.sub.2SO.sub.4, NaSO.sub.4, CaCl.sub.2,
K.sub.2CO.sub.3, CaO, P.sub.2O.sub.5.
7. The process as described in claim 1 wherein the rhenium catalyst
is selected from the group consisting of (R)ReO.sub.3,
Re.sub.2O.sub.7, ReO.sub.3, ReO.sub.3(OH) , HReO.sub.4,
NH.sub.4ReO.sub.4, Re (metal), ReO.sub.2, and Me.sub.3SiOReO.sub.3
wherein R is selected from the group consisting of C.sub.1-C.sub.6
alkyl and tert-C.sub.1-C.sub.6 alkyl.
8. The process as described in claim 1 with the following
additional step: removing product water formed during the reaction
process by a boiling reactor for maintaining an aqueous
concentration in the reaction mixture low enough for retaining
activity of the oxorhenium catalyst.
9. The process as described in claim 1 wherein the olefin is a
mono-substituted olefin.
10. The process as described in claim 1 wherein the olefin is a
di-substituted olefin.
11. The process as described in claim 1 wherein the olefin is a
tri-substituted olefin.
12. The process as described in claim 1 wherein the olefin is a
tetra-substituted olefin.
13. The process as described in claim 1 wherein the ligand is
selected from the group consisting of pyridine, pyridine
derivatives containing electron withdrawing or electron donating
groups (nitro, esters, ketones, halogens, nitriles, sulphonic acid
esters), chiral pyridines (like cotinine), imines, oxazolines,
2-methylpyridine (2-picoline), 2-ethylpyridine, 2-propylpyridine,
2-phenylpyridine, 2-benzylpyridine, 2-fluoropyridine,
2-chloropyridine, 2-bromopyridine, 2-cyanopyridine,
2-hydroxypyridine, 2-pyridylcarbinol, 2-pyridineethanol,
2-pyridinepropanol, pyridine-2-carboxylic acid (picolinic acid) and
corresponding esters, 3-methylpyridine (3-picoline),
3-ethylpyridine, 3-butylpyridine, 3-phenylpyridine,
3-benzylpyridine, 3-fluoropyridine, 3-chloropyridine,
3-bromopyridine, 3-cyanopyridine, 3-pyridylcarbinol,
3-hydroxypyridine, 3-pyridinepropanol, pyridine-3-carboxylic acid
(nicotinic acid) and corresponding esters, 4-methylpyridine
(4-picoline), 4-fluoropyridine, 4-chloropyridine, 4-bromopyridine,
4-cyanopyridine, 4-ethylpyridine, 4-isopropylpyridine,
4-t-butylpyridine, 4-(1-butylpentyl)pyridine, 4-phenylpyridine,
4-benzylpyridine, 4-(4-chlorobenzyl)pyridine, 4-hydroxypyridine,
4-methoxypyridine, 4-nitropyridine, pyridine-4-carboxylic acid and
corresponding esters, 2,3-dimethylpyridine (2,3-lutidine),
2,4-dimethylpyridine (2,4-lutidine), 2,5-dimethylpyridine
(2,5-lutidine), 2,6-dimethylpyridine (2,6-lutidine),
3,4-dimethylpyridine (3,4-lutidine), 3,5-dimethylpyridine
(3,5-lutidine), 2,6-difluoropyridine, pentafluoropyridine,
pentachloropyridine, 2,6-dichloropyridine, 3,5-dichloropyridine,
2,3,5-trichloropyridine, 3,4-dicyanopyridine, 5-chloro-3-pyridinol,
2,3-pyridinecarboxylic acid and corresponding esters,
2,4-pyridinecarboxylic acid and corresponding esters,
2,5-pyridinecarboxylic acid and corresponding esters,
2,6-pyridinecarboxylic acid and corresponding esters,
2,6-diphenylpyridine, 2,6-di-p-tolylpyridine,
3,4-pyridinecarboxylic acid and corresponding esters,
2-pyridineethansulfonic acid, 4-pyridineethanesulfonic acid,
2,3-cyclopentenopyridine, 2,3-cyclohexenopyridine,
2,3-cycloheptenopyridine, 2,4,6-collidine, pyrazine,
2,3-pyrazinedicarbonitrile, pyrazinecarbonitrile,
2,6-dichloropyrazine, pyrazinecarboxylic acid and corresponding
esters, 2,3-dimethylpyrazine, 2,5-dimethylpyrazine,
2,6-dimethylpyrazine, 2,3-di-2-pyridylpyrazine, pyridazine,
3-methylpyridazine, 4-methylpyridazine, pyrimidine,
4-methylpyrimidine, 4,6-dimethylpyrimidine, 4-phenylpyrimidine,
2,4-dichloropyrimidine, 4,6-dichloropyrimidine,
2,4,5-trihydroxypyrimidine, 4-(trifluoromethyl)-2-pyrimidinol,
1,3,5-triazine, (-)-cotinine
(1-methyl-5-(3-pyridyl)-2-pyrrolidinone) pyridine-2,6-dicarboxylic
acid and corresponding esters, quinoline, isoquinoline,
2,2'-pyridyl, 2,2'-dipyridyl, 6-chloro-2,2'-bipyridine,
2,4'-dipyridyl, 4,4'-dipyridyl, 2,2':6',2"-terpyridine,
1,7-phenanthroline, 1,10-phenanthroline, 4,7-phenanthroline,
phenazine, 3,6-di-2-pyridyl-1,2,4,5-tetrazine,
2,2'-bipyridine-4,4'-carboxylic ester, 1,2-bis(4-pyridyl)ethane,
4,4'-trimethylenepyridine, quinoxaline, 2,3-dimethylquinoxaline,
1-nitropyrazole, 2,5-diphenyloxazole, 2,4,5-trimethyloxazole,
2,4,4-trimethyl-2-oxazoline, 3,5-dimethylisoxazole,
2,6-bis[(4S)-isopropyl-2-oxazolin-2-yl]pyridine,
1,2-dimethylimidazole, 1-butylimidazole, 2,3,3-trimethylindolenine,
and caffeine.
14. The method as described in claim 1 wherein the solvent is
selected from the group consisting of nitromethane
(CH.sub.3NO.sub.2), nitroethane (EtNO.sub.2), methylene chloride
(CH.sub.2Cl.sub.2), chloroform (CHCl.sub.3), carbon tetrachloride
(CCl.sub.4), 1,2-dichloroethane (CH.sub.2ClCH.sub.2Cl),
pentachloroethane (CCl.sub.3CHCl.sub.2), chlorinated aromatic
compounds: chlorobenzene, dichlorobenzene and other chlorinated
solvents, fluorinated solvents and chlorofluoro hydrocarbons,
acetonitrile (CH.sub.3CN), acetone, benzene, toluene, xylenes,
methanol, ethanol, propanol, isopropanol, butanol, t-butanol,
ethyleneglycol, diethylether, THF (tetrahydrofuran) and
supercritical CO.sub.2.
15. The process as described in claim 1 wherein the ligand is
present in 0.25 to 1.0 mol % overall concentration.
16. The process as described in claim 1 wherein the ligand is
present in 0.25 to 12.0 mol % overall concentration.
17. The process as described in claim 1 wherein the overall water
concentration is within the range of 0.5-80.0 mol %.
Description
RELATED APPLICATIONS
[0001] The present application claims priority from Provisional
Application Ser. No. 60/062,760, filed Oct. 23, 1997.
TECHNICAL FIELD
[0003] The invention is directed to an improved method for the
oxorhenium catalyzed epoxidization of diversely functionalized
olefins wherein the improvement comprises conditions which control
water concentration.
BACKGROUND
[0004] In recent years, the chemistry pertaining to the selective
oxidation of olefins was dominated by OSO.sub.4 and
O.sub.3Os.dbd.N--X species, the essential reactants in the
catalytic asymmetric dihydroxylation (AD) and aminohydroxylation
(AA) processes, respectively (Kolb et al. Chem. Rev. 1994, 94,
2483; Schlingloff et al. in Asymmetric Oxidation Reactions: A
Practical Approach, Katsuki, T., Ed.: Oxford University Press, in
press). Our continuing search for new transition metal-catalyzed
heteroatom transfer reactions has centered around osmium's
neighbors in the Periodic Table. Among the corresponding high
valent oxo derivatives, methylrhenium trioxide (CH.sub.3ReO.sub.3
or MTO) has been known for a long time (Beattie et al. Inorg. Chem.
1979, 18, 2318). It was only recently, however, that Herrmann and
others developed MTO into a well defined catalyst for a variety of
processes including olefin epoxidation with aqueous hydrogen
peroxide (H.sub.2O.sub.2) . For applications of MTO in organic
synthesis, see: Hoechst A G (Herrmann et al. ) DE 3.902.357 (1989);
Herrmann et al. Angew. Chem., Int. Ed. Engl. 1991, 30, 1638;
Herrmann et al. J. Mol. Catal. 1994, 86, 243; Herrmann et al.
Organomet. Chem. 1995, 500, 149; Al-Ajlouni et al. Am. Chem. Soc.
1995, 117, 9243; Pestovsky et al. J. Chem. Soc., Dalton Trans. 2
1995, 133; Adam et al. Angew. Chem. Int. Ed. 1996, 35, 533; Boelow
et al. Tetrahedron Lett. 1996, 37, 2717; Al-Ajlouni et al. J. Org.
Chem. 1996, 61, 3969; Herrmann et al. J. Mol. Cat. 1997, 118, 33;
Herrmann et al. Acc. Chem. Res. 1997, 30, 169; Espenson et al. Adv.
Chem. Ser. 1997, 253, 99; ARCO Chemical Technology (Crocco et al.,
H. S.) U.S. Pat. No. 5,166,372 (1992).
[0005] Regarding olefin oxidation, there is a fundamental
difference between OsO.sub.4 and CH.sub.3ReO.sub.3, for in contrast
to OsO.sub.4, MTO does not react directly with olefins (This is
true regarding olefin epoxidation. However, MTO is known to exhibit
metathesis activity (Herrmann et al. Acc. Chem. Res. 1997, 30,
169). Rather, the MTO-catalyzed epoxidation is believed to proceed
through the initial activation of H.sub.2O.sub.2 by the
electrophilic Re(VII) center resulting in the formation of
equilibrating mixture of mono- and bisperoxorhenium complexes that
transfer oxygen atoms to the corresponding olefins. Notably, the
OsO.sub.4/H.sub.2O.sub.2 system has little synthetic value for
olefin epoxidation. Even though epoxides are the primary products
in this system, significant amounts of diols and overoxidation
products are formed (Milas et al. J. Am. Chem. Soc. 1936, 58,
1302).
[0006] The major limitation of Herrmann's original
MTO/H.sub.2O.sub.2 epoxidation system is the acidity of the
reaction medium. The water molecule coordinated to the Re(VII)
center of the bisperoxo complex is highly acidic and sensitive
epoxides do not survive (The water molecule coordinated to the
rhenium center of the bisperoxo complex of MTO is highly acidic:
Herrmann et al. Angew. Chem. Int. Ed. Engl. 1993, 103, 1991).
Recent efforts in our laboratory led to a highly efficient olefin
epoxidation with 30% aqueous H.sub.2O.sub.2 where the catalytic
activity of MTO was uncoupled from acidity for the first time
(Rudolph et al. J. Am. Chem. Soc. 1997, 119, 6189; Coperet et al.
Chem. Commun. 1997, 16, 1565). The crucial features of this new
process are the requirement for a pyridine ligand and the solvent
switch from tert-butyl alcohol to methylene chloride which
additionally enhances the effectiveness of the pyridine-modified
rhenium catalyst (FIG. 1A).
[0007] We have previously disclosed on further improvements in this
epoxidation catalysis, specifically on the use of 3-cyanopyridine
as a ligand of choice for the epoxidation of terminal and
transdisubstituted olefins (Coperet et al. Chem. Commun. 1997, 16,
1565).
[0008] What is needed is an efficient and improved method for
oxorhenium epoxidization of diversely functionalized olefins
wherein the improvement increases turnover and which subsequently
reduces diol side products obtained from epoxide ring opening and
increases the yield of the desired epoxide product.
SUMMARY
[0009] One aspect of the invention is directed to an improved
process for epoxidizing olefins by rhenium-catalysis. More
particularly, the rhenium-catalyzed epoxidation is of a type
wherein a reaction mixture is formed by combining the olefin with a
ligand, a solution of oxidant, an organic solvent, a protic solvent
and a catalytic organo rhenium oxide under conditions suitable for
epoxide formation to occur. The improvement is directed to the use
of a a silicon based anhydrous oxidant and to its controlled slow
addition to the reaction mixture. In a preferred embodiment, the
silicon based anhydrous oxidant is a trialkylsilyl peroxide
represented by the formulas (R).sub.3SiOOSi(R).sub.3, and
(--(R).sub.2SiOO--).sub.n. In the above formulas, R is selected
from the group consisting of C.sub.1-C.sub.6 alkyl and
tert-C.sub.1-C.sub.6 alkyl. A preferred silicon based anhydrous
oxidant is bis(trimethylsilyl) peroxide. During the slow addition
of the silicon based anhydrous oxidant, a peroxo group is
transferred from the oxidant to the rhenium oxide with the
assistance of the protic solvent, thereby controlling excess water
concentration and maximizing monoperoxocomplex formation.
[0010] A further aspect of the invention is directed to the
addition of a water removal agent to the reaction mixture.
Preferred water removal agents include a group consisting of
Molecular sieves (Aldrich, 3 .ANG., 4 .ANG.), MgSO.sub.4,
Na.sub.2SO.sub.4, NaSO.sub.4, CaCl.sub.2, K.sub.2CO.sub.3, CaO,
P.sub.2O.sub.5. Preferred rhenium catalysts include (R)ReO.sub.3,
Re.sub.2O.sub.7, ReO.sub.3, ReO.sub.3(OH), HReO.sub.4,
NH.sub.4ReO.sub.4, Re (metal), ReO.sub.2, and Me.sub.3SiOReO.sub.3.
In the above formulas, R may be selected from the group consisting
of C.sub.1-C.sub.6 alkyl and tert-C.sub.1-C.sub.6 alkyl.
[0011] A further aspect of the invention is directed to the removal
of product water formed during the reaction process. Product water
is removed from the reaction mixture by use of a boiling reactor
process for maintaining an aqueous concentration in the reaction
mixture low enough for retaining activity of the oxorhenium
catalyst.
[0012] In a preferred mode, the olefin is a mono-substituted
olefin, a di-substituted olefin, a tri-substituted olefin, or a
tetra-substituted olefin. Preferred ligands include the following:
pyridine, pyridine derivatives containing electron withdrawing or
electron donating groups (nitro, esters, ketones, halogens,
nitriles, sulphonic acid esters), chiral pyridines (like cotinine),
imines, oxazolines, 2-methylpyridine (2-picoline), 2-ethylpyridine,
2-propylpyridine, 2-phenylpyridine, 2-benzylpyridine,
2-fluoropyridine, 2-chloropyridine, 2-bromopyridine,
2-cyanopyridine, 2-hydroxypyridine, 2-pyridylcarbinol,
2-pyridineethanol, 2-pyridinepropanol, pyridine-2-carboxylic acid
(picolinic acid) and corresponding esters, 3-methylpyridine
(3-picoline), 3-ethylpyridine, 3-butylpyridine, 3-phenylpyridine,
3-benzylpyridine, 3-fluoropyridine, 3-chloropyridine,
3-bromopyridine, 3-cyanopyridine, 3-pyridylcarbinol,
3-hydroxypyridine, 3-pyridinepropanol, pyridine-3-carboxylic acid
(nicotinic acid) and corresponding esters, 4-methylpyridine
(4-picoline), 4-fluoropyridine, 4-chloropyridine, 4-bromopyridine,
4-cyanopyridine, 4-ethylpyridine, 4-isopropylpyridine,
4-t-butylpyridine, 4-(1-butylpentyl)pyridine, 4-phenylpyridine,
4-benzylpyridine, 4-(4-chlorobenzyl)pyridine, 4-hydroxypyridine,
4-methoxypyridine, 4-nitropyridine, pyridine-4-carboxylic acid and
corresponding esters, 2,3-dimethylpyridine (2,3-lutidine),
2,4-dimethylpyridine (2,4-lutidine), 2,5-dimethylpyridine
(2,5-lutidine), 2,6-dimethylpyridine (2,6-lutidine),
3,4-dimethylpyridine (3,4-lutidine), 3,5-dimethylpyridine
(3,5-lutidine), 2,6-difluoropyridine, pentafluoropyridine,
pentachloropyridine, 2,6-dichloropyridine, 3,5-dichloropyridine,
2,3,5-trichloropyridine, 3,4-dicyanopyridine, 5-chloro-3-pyridinol,
2,3-pyridinecarboxylic acid and corresponding esters,
2,4-pyridinecarboxylic acid and corresponding esters,
2,5-pyridinecarboxylic acid and corresponding esters,
2,6-pyridinecarboxylic acid and corresponding esters,
2,6-diphenylpyridine, 2,6-di-p-tolylpyridine,
3,4-pyridinecarboxylic acid and corresponding esters,
2-pyridineethansulfonic acid, 4-pyridineethanesulfonic acid,
2,3-cyclopentenopyridine, 2,3-cyclohexenopyridine,
2,3-cycloheptenopyridine, 2,4,6-collidine, pyrazine,
2,3-pyrazinedicarbonitrile, pyrazinecarbonitrile,
2,6-dichloropyrazine, pyrazinecarboxylic acid and corresponding
esters, 2,3-dimethylpyrazine, 2,5-dimethylpyrazine,
2,6-dimethylpyrazine, 2,3-di-2-pyridylpyrazine, pyridazine,
3-methylpyridazine, 4-methylpyridazine, pyrimidine,
4-methylpyrimidine, 4,6-dimethylpyrimidine, 4-phenylpyrimidine,
2,4-dichloropyrimidine, 4,6-dichloropyrimidine,
2,4,5-trihydroxypyrimidine, 4-(trifluoromethyl)-2-pyrimidinol,
1,3,5-triazine, (-)-cotinine
(1-methyl-5-(3-pyridyl)-2-pyrrolidinone) pyridine-2,6-dicarboxylic
acid and corresponding esters, quinoline, isoquinoline,
2,2'-pyridyl, 2,2'-dipyridyl, 6-chloro-2,2'-bipyridine,
2,4'-dipyridyl, 4,4'-dipyridyl, 2,2':6',2"-terpyridine,
1,7-phenanthroline, 1,10-phenanthroline, 4,7-phenanthroline,
phenazine, 3,6-di-2-pyridyl-1,2,4,5-tetrazine,
2,2'-bipyridine-4,4'-carboxylic ester, 1,2-bis(4-pyridyl)ethane,
4,4'-trimethylenepyridine, quinoxaline, 2,3-dimethylquinoxaline,
1-nitropyrazole, 2,5-diphenyloxazole, 2,4,5-trimethyloxazole,
2,4,4-trimethyl-2-oxazoline, 3,5-dimethylisoxazole,
2,6-bis[(4S)-isopropyl-2-oxazolin-2-yl]pyridine,
1,2-dimethyimidazole, 1-butylimidazole, 2,3,3-trimethylindolenine,
and caffeine. Preferred solvents include nitromethane
(CH.sub.3NO.sub.2), nitroethane (EtNO.sub.2), methylene chloride
(CH.sub.2Cl.sub.2), chloroform (CHCl.sub.3), carbon tetrachloride
(CCl.sub.4), 1,2-dichloroethane (CH.sub.2ClCH.sub.2Cl),
pentachloroethane (CCl.sub.3CHCl.sub.2),chlorinat- ed aromatic
compounds: chlorobenzene, dichlorobenzene and other chlorinated
solvents, fluorinated solvents and chlorofluoro hydrocarbons,
acetonitrile (CH.sub.3CN), acetone, benzene, toluene, xylenes,
methanol, ethanol, propanol, isopropanol, butanol, t-butanol,
ethyleneglycol, diethylether, THF (tetrahydrofuran) and
supercritical CO.sub.2. In a preferred mode, the ligand is present
in 0.25 to 1.0 mol % overall concentration; alternatively, the
ligand may be present in 0.25 to 12.0 mol % overall concentration.
A preferred overall water concentration is within the range of
0.5-80.0 mol %.
DESCRIPTION OF FIGURES
[0013] FIG. 1 illustrates (a) shows the preferred mode of the MTO
epoxidation employing a pyridine ligand and methylene chloride as
the solvent; (b) shows the improvement viz.: replacement of aqueous
H.sub.2O.sub.2 with bis(trimethylsilyl)peroxide (BTSP) as an oxygen
atom source; (c) shows that MTO shows little to no reactivity
toward BTSP in CDCl.sub.3.
[0014] FIG. 2 shows a table of olefins (indicated entries) wherein
epoxidation of olefins with bis(trimethylsilyl)peroxide(btsp) was
catalyzed by high valent oxorhenium derivatives with the following
conditions: (a) 10 mmol scale; (b) 1.5 eq BTSP per double bond was
used; (c) A: CH.sub.3ReO.sub.3 (0.5 mol %), pyridine (1 mol %); B:
CH.sub.3ReO.sub.3 (0.25 mol %), pyridine (0.5 mol %); C:
Re.sub.2O.sub.7 (0.5 mol %), pyridine (1 mol %); D: Re.sub.2O.sub.7
(0.5 mol %); E: HReO.sub.4 (0.5 mol %); F: ReO.sub.3 (0.5 mol %);
G: Re.sub.2O.sub.7 (1 mol %); (d) syn-diepoxide was obtained
(>99:1); (e) anti-diepoxide was obtained (>99:1).
[0015] FIG. 3 shows basis for the observations that a trace of
water in the reaction mixture hydrolyzes BTSP to hydrogen peroxide
which subsequently adds to the rhenium center producing the peroxo
complex while releasing a water molecule which closes the oxidant
regeneration loop.
[0016] FIG. 4 illustrates A) (in the case of BTSP) formation of
peroxo complexes which must be accompanied by the silylation of the
rhenium-bound oxo ligand wherein this process is apparently much
slower than its protic counterpart; B) the need for a proton source
is accommodated where a regenerable XH species helps in ferrying
the peroxo group from silicon to rhenium.
[0017] FIG. 5 defines the scope of the oxorhenium process with
representative substrates including fairly unreactive olefins
and/or progenitors of sensitive epoxides with the following
conditions a) 10 mmol scale; (b) 1.5 eq BTSP per double bond was
used; (c) A: Re.sub.2O.sub.7 (0.5 mol %), H.sub.2O (5 mol %); B:
ReO.sub.3 (0.5 mol %), H.sub.2O (5 mol %); C: ReO.sub.3 (0.5 mol
%), H.sub.2O (1 mol %); D: Re.sub.2O.sub.7 (0.5 mol %), pyridine (1
mol %), H.sub.2O (5 mol %); E: 2py/HReO.sub.4 (0.5 mol %), H.sub.2O
(5 mol %); F: MTO (0.5 mol %), H.sub.2O (5 mol %).
[0018] FIG. 6 defines the scope of the oxorhenium process with
representative substrates including fairly unreactive olefins
and/or progenitors of sensitive epoxides with conditions as
described in FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The invention is directed to an improved method for
oxorhenium catalyzed epoxidization of diversely functionalized
olefins wherein the improvement comprises conditions which control
water concentration. By controlling water concentration, one can
maximize monoperoxo complex formation and increase turnover which
subsequently reduces diol side products obtained from epoxide ring
opening and increases the yield of the desired epoxide product. The
optimal range of water concentrations is 0.50-80.0 mol %. Using
less than 0.5 mol % water does not result in practical turnovers
and 1.0 equivalent of water (or more) is detrimental to the
lifetime of the active catalytic species formed. More particularly,
there are four aspects to controlling water concentration:
[0020] 1. Anhydrous oxidants
[0021] Trialkylsilyl peroxides R.sub.3SiOOSiR.sub.3; note that
drying agents or continuous water removal technologies are employed
for BTSP (R.sub.3SiOOSiR.sub.3) epoxidation); related trialkylsilyl
hydroperoxides R.sub.3SiOOH (see attached material for the
preparation) and polymeric derivatives (--R.sub.2SiOO--).sub.n
(R=organic functionality, see attached material for the
preparation);
[0022] bis(tert-butyl)peroxide (Aldrich) and benzoyl peroxide
(Aldrich) in combination with hexamethyldisilazane (Aldrich) (in
situ source of BTSP eliminating the need for its isolation),
tert-butyl hydroperoxide (Aldrich);
[0023] 2. Water removal agents
[0024] Molecular sieves (Aldrich, 3 .ANG., 4 .ANG.);
[0025] common inorganic dehydrating agents including MgSO.sub.4,
Na.sub.2SO.sub.4 CaSO.sub.4, CaCl.sub.2, K.sub.2CO.sub.3, CaO,
P.sub.2O.sub.5. (all Aldrich).
[0026] 3. Rhenium catalysts
[0027] RReO.sub.3 (for example, CH.sub.3ReO.sub.3),
Re.sub.2O.sub.7, ReO.sub.3, HReO.sub.4, NH.sub.4ReO.sub.4, Re
(metal), ReO.sub.2 (All Aldrich), Me.sub.3SiOReO.sub.3 (Strem)
[0028] 4. Boiling reactor process.
[0029] The boiling reactor process, in the context of oxorhenium
catalyzed epoxidation, comprises a method which continuously
removes water in the vapor stream by using appropriate solvent,
temperature, and pressure conditions, keeping the concentration of
the liquid phase low enough to maintain the activity of the
oxorhenium catalyst.
EXAMPLE 1
Bis(trimethylsilyl) Peroxide Extends the Range of Oxorhenium
Catalysts for Olefin Epoxidation
[0030] We disclose hereiin further improvements in the epoxidation
catalysis, the most significant being replacement of the
organometallic rhenium species (e.g. MTO) by cheaper and more
stable inorganic rhenium oxides (e.g. Re.sub.2O.sub.7,
ReO.sub.3(OH) and ReO.sub.3).
[0031] Among the known organometallic oxorhenium (VII) species
(R--ReO.sub.3) capable of catalyzing olefin epoxidation, MTO
appears to be the most stable toward oxidative and/or hydrolytic
removal of the alkyl group (vide infra; For a comprehensive study
on the base-induced decomposition of MTO, see: Abu-Omar et al. J.
Am. Chem. Soc. 1996, 118, 4966. In the presence of pyridine and
H.sub.2O.sub.2, MTO is slowly oxidized, producing pyridinium
perrhenate and CH.sub.3OH: Yudin, A. K.; Sharpless, K. B.
unpublished results).
[0032] Hence, catalyst modification by variation of the
R-substituent on the rhenium center was not rewarding despite
extensive efforts in the Herrmann laboratory. In addition,
R--ReO.sub.3 compounds, including MTO, are quite expensive (For
applications of MTO in organic synthesis, see: (a) Hoechst A G
(Herrmann, W. A.; Marz, D. W.; Kuchler, J. G.; Weichselbaumer, G.;
Fischer, R. W.) DE 3.902.357 (1989); (b) Herrmann et al. Angew.
Chem., Int. Ed. Engl. 1991, 30, 1638. (c) Herrmann et al. J. Mol.
Catal. 1994, 86, 243; (d) Herrmann et al. J. Organomet. Chem. 1995,
500, 149; (e) Al-Ajlouni et al. J. Am. Chem. Soc. 1995, 117, 9243;
(f) Pestovsky et al. J. Chem. Soc., Dalton Trans. 2 1995, 133; (g)
Adam et al. Angew. Chem. Int. Ed. 1996, 35, 533; (h) Boelow et al.
Tetrahedron Lett. 1996, 37, 2717; (i) Al-Ajlouni et al. J. Org.
Chem. 1996, 61, 3969; (j) Herrmann et al. J. Mol. Cat. 1997, 118,
33; (k) Espenson et al. Adv. Chem. Ser. 1997, 253, 99; (l) ARCO
Chemical Technology (Crocco, G. L.; Shum, W. P.; Zajacek, J. G.;
Kesling Jr., H. S.) U.S. Pat. No. 5,166,372 (1992). Herrmann et al.
Acc. Chem. Res. 1997, 30, 169. The most practical preparation of
MTO to date involves the reaction of tetramethyltin,
Re.sub.2O.sub.7, and hexafluoroglutaric anhydride in acetonitrile:
Herrmann et al. Inorg. Chem. 1992, 31, 4431.
[0033] These factors provided the incentive to seek water-free
epoxidation conditions which would hopefully extend the lifetime of
the MTO catalyst. This goal and much more was accomplished by
simply replacing aqueous H.sub.2O.sub.2 with
bis(trimethylsilyl)peroxide (BTSP) as an oxygen atom source (FIG.
1B) ((a) Cookson et al. J. Organomet. Chem. 1975, 99, C31; (b)
Taddei, M.; Ricci, A. Synthesis 1986, 633; (c) for a convenient,
large-scale preparation of BTSP from bis(trimethylsilyl)urea and
urea/H.sub.2O.sub.2 complex in dichloromethane, see: Jackson, W. P.
Synlett 1990, 536. The product obtained according to this method is
virtually free of hexamethyldisiloxane, a common by-product in
cognate BTSP preparations (see Supporting Material for details of a
one mole preparation); (d) Babin et al. Synth. Commun. 1992, 22,
2849.
[0034] Thermal stabilities of silylated organic peroxides have been
studied: Vesnovskii, B. P.; Thomadze, A. V.; Suchevskaya, N. P.;
Aleksandrov, Yu. A. Zh. Prikl. Khim. 1982, 55, 1005. We would like
to stress, however, that despite its great thermal stability, BTSP
is subject to facile hydrolysis in the presence of water and acids
which could result in formation of hazardous 100% H.sub.2O.sub.2.
However, pure BTSP has an active oxygen content of only 9% (cf.
tert-butyl hydroperoxide -17.8%; di-tert-butyl peroxide -10.9%;
hydrogen peroxide -47%). For applications of BTSP in organic
synthesis, see: (a) Brandes, D.; Blaschette, A. J. Organomet. Chem.
1973, 49, C6; (b) Brandes, D.; Blaschette, A. ibid. 1974, 73, 217;
(c) Tamao, K.; Kumada, M.; Takahashi, T. ibid. 1975, 94, 367; (d)
Salomon, M. F.; Salomon, R. G. J. Am. Chem. Soc. 1979, 101, 4290;
(e) Adam, W.; Rodriguez, A. J. Org. Chem. 1979, 44, 4969; (f)
Suzuki, M.; Takada, H.; Noyori, R. ibid. 1982, 47, 902; (g) Weber,
W. P. "Silicon Reagents in Organic Synthesis," Springer-Verlag: New
York, 1983; (h) Kanemoto, S.; Oshima, K.; Matsubara, S.; Takai, K.;
Nozaki, H. Tetrahedron Lett. 1983, 24, 2185; (i) Matsubara, S.;
Takai, K.; Nozaki, H. ibid. 1983, 24, 3741; (j) Matsubara, S.;
Takai, K.; Nozaki, H. Bull. Chem. Soc. Jpn. 1983, 56, 2029; (k) see
ref. 6b; (l) Hayakawa, Y.; Uchiyama, M.; Noyori, R. Tetrahedron
Lett. 1986, 2 7, 4195; (m) Curci, R.; Mello, R.; Troisi, L.
Tetrahedron 1986, 42, 877; (n) Kanemoto, S.; Matsubara, S.; Takai,
K.; Oshima, K.; Utimoto, K.; Nozaki, H. Bull. Chem. Soc. Jpn. 1988,
61, 3607; (o) Davis, F. A.; Lal, S. G.; Wei, J. Tetrahedron Lett.,
1988, 29, 4269; (p) Olah, G. A.; Ernst, T. D. J. Org. Chem. 1989,
54, 1204; (q) Camporeale, M.; Fiorani, T.; Troisi, L.; Adam, W.;
Curci, R.; Edwards, J. O. ibid. 1990, 55, 93; (r) Shibata, K.;
Itoh, Y.; Tokitoh, N.; Okazaki, R.; Inamoto, N. Bull. Chem. Soc.
Jpn. 1991, 64, 3749; (s) Chemla, F.; Julia, M.; Uguen, D. Bull.
Soc. Chim. Fr. 1993, 130, 547; (t) Irie, R.; Hosoya, N.; Katsuki,
T. Synlett 1994, 255; (u) Prouilhac-Cros, S.; Babin, P.; Bennetau,
B.; Dunogues, J. Bull. Soc. Chim. Fr. 1995, 132, 513; (v) Adam, W.;
Korb, M. N. Tetrahedron, 1996, 52, 5487; (w) Adam, W.; Golsch, D.;
Sundermeyer, J.; Wahl, G. Chem. Ber. 1996, 129, 1177; (x) Barton,
D. H. R.; Chabot, B. M. Tetrahedron 1997, 53, 487; (y) Barton, D.
H. R.; Chabot, B. M. ibid. 1997, 53, 511.
[0035] In addition to MTO, readily available inorganic rhenium
oxides (e.g. Re.sub.2O.sub.7, ReO.sub.3(OH) and ReO.sub.3) were
found to exhibit high catalytic activity in the present system.
FIG. 2 defines the scope of this new process with representative
substrates including fairly unreactive olefins and/or progenitors
of sensitive epoxides (The original MTO-based procedure Rudolph et
al. J. Am. Chem. Soc. 1997, 119, 6189 remains superior for the
preparation of highly acid-sensitive indene oxide). With the
present protocol, terminal olefins and dienes, problematic in the
original procedure, can be efficiently converted into the
corresponding epoxides.
[0036] We discovered that when water (1 eq with respect to the
olefin) is intentionally added at the beginning of the
MTO-catalyzed epoxidation of cis-4-octene, BTSP is hydrolyzed
within 10 minutes (as determined by GC), (Disappearance of BTSP in
the course of the reaction can be conveniently monitored by gas
chromatography) and poor conversions are observed presumably due to
the sensitivity of the generated epoxidizing species to excess
water (vide infra). In addition, significant amount of the diol
originating from the epoxide ring-opening is formed. On the other
hand, MTO-catalyzed epoxidations conducted in the presence of 4
.ANG. molecular sieves are extremely slow. Control experiments
demonstrate that MTO is not absorbed or inactivated by molecular
sieves under these conditions, ruling out catalyst removal as the
origin of lost activity. Similarly, very sluggish epoxidation is
observed when Re.sub.2O.sub.7 is used as a catalyst for the
epoxidation of 1-decene in anhydrous dichloromethane (ca. 7%
conversion after 2.5 hours). The reaction is dramatically
accelerated upon addition of 5 mol % water. On the basis of these
observations one could propose that a trace of water in the
reaction mixture hydrolyzes BTSP to hydrogen peroxide which
subsequently adds to the rhenium center producing the peroxo
complex while releasing a water molecule which closes the oxidant
regeneration loop (FIG. 3).
[0037] In contrast to epoxidation with aqueous H.sub.2O.sub.2 where
equilibrating bis- and monoperoxo complexes are produced
instantaneously upon exposure of the catalyst to the oxidant, the
reaction between equimolar amounts of MTO and BTSP has a
considerable induction period under anhydrous conditions. We
attribute this phenomenon to the necessity of "acidity build-up"
(the water molecule coordinated to the rhenium center of the
bisperoxo complex of MTO is highly acidic: Herrmann, W. A.;
Fischer, R. W.; Marz, D. W. Angew. Chem. 1993, 103, 1991).
Adventitious moisture can trigger an autocatalytic decomposition of
an acid-sensitive BTSP into H.sub.2O.sub.2 and
hexamethyldisiloxane. Alternatively, partial hydrolysis of BTSP
could afford Me.sub.3SiOOH which could act as an oxidant in the
present system. For the use of silyl hydroperoxides in epoxidation,
see: Dannley, R. L.; Jalics, G. J. Org. Chem. 1965, 30, 2417;
Rebek, J.; McCready, R. Tetrahedron Lett. 1979, 4337.
[0038] The intrinsic "slow addition" of hydrogen peroxide to the
oxorhenium precursor is managed by the "proton dependent" cycle
shown in FIG. 3 which accomplishes transfer of the peroxo group
from silicon to rhenium. In contrast, it is very difficult to
exercize such control in the H.sub.2O.sub.2 (aqueous or anhydrous)
MTO-catalyzed epoxidation processes; for example, slow addition of
H.sub.2O.sub.2 does not help in achieving higher conversions due to
faster catalyst decomposition at lower H.sub.2O.sub.2
concentrations (Ironically, MTO is stabilized at higher
H.sub.2O.sub.2 concentrations: Herrmann, W. A.; Fischer, R. W.;
Scherer, W.; Rauch, M. U. Angew. Chem., Int. Ed. Engl. 1993, 32,
1157).
[0039] Worthy of note, the aforementioned hydrolysis of BTSP to
H.sub.2O.sub.2 is the simplest of many scenarios which could
explain the requirement for H.sub.2O. In a more general way, the
need for a protic solvent is accommodated in FIG. 4b.
[0040] In accord with the previous observations, additives such as
pyridines serve to prevent sensitive epoxide ring opening by
buffering the highly acidic rhenium species. Notably, compared to
the original system, the amount of ligand necessary to achieve the
desired protection is now decreased from 12 to 0.5-1 mol % in both
MTO and Re.sub.2O.sub.7-catalyzed epoxidations (The use of 12 mol %
of pyridine completely arrested the reaction, presumably due to
base-mediated decomposition of MTO). In some instances MTO loadings
can be lowered to 0.25 mol % without affecting conversions--a
manifestation of prolonged catalyst lifetime under the present
conditions.
[0041] The use of Re.sub.2O.sub.7, ReO.sub.3(OH) and ReO.sub.3 as
catalyst precursors is a particularly important feature of the
present protocol. Catalytic activities of these inorganic rhenium
species for epoxidation with H.sub.2O.sub.2 were known to be very
poor. For the epoxidation of C.sub.2-.sub.20 olefins with
stoichiometric Re.sub.2O.sub.7 in the presence of pyridine, see:
Union Oil Co. of California (Fenton, D. M.) U.S. Pat. No.
3,316,279; (c) for early applications of Re.sub.2O.sub.7 in
olefin/H.sub.2O.sub.2 oxidation catalysis see: duPont de Nemours
and Co. (Parshall, G. W.) U.S. Pat. Nos. 3,657,292 and 3,646,130
(1972); (d) Warwel and co-workers found that Re.sub.2O.sub.7 is a
more effective epoxidation catalyst if the right solvent is chosen.
Their system employs 60% aqueous H.sub.2O.sub.2 in 1,4-dioxane at
90.degree. C. and 1,2-diols are isolated in good yields, the
initially formed epoxides being unstable in this system: Warwel,
S.; Rusch gen Klaas, M.; Sojka, M. Chem. Commun. 1991, 1578; (e)
Herrmann, W. A.; Correia, J. D. G.; Kuhn, F. E.; Artus, G. R. J.
Chemistry--A European Journal 1996, 2, 168.
[0042] Generally, the high acidity of these systems does not allow
epoxides to be isolated except in special cases such as from
ciscyclooctene (which affords an epoxide which is particularly
resistant to acid-catalyzed ring opening). In the present system,
rhenium oxides are comparable and in some cases superior to MTO
especially for the epoxidation of terminal olefins and dienes. The
cost of the process can be significantly reduced by using these
less expensive oxorhenium catalysts in combination with BTSP, now
more available through improved preparations. MTO and
Re.sub.2O.sub.7 were purchased from Strem Chemicals, Inc. ReO.sub.3
and HOReO.sub.3 were purchased from Aldrich Chemical and Co. For
industrial sources of rhenium, see: Peacock, R. D. "The Chemistry
of Technetium and Rhenium," Elsevier: Amsterdam, 1966.
[0043] This example illustrates improved conditions which were
developed under which simple inorganic oxorhenium species act as
efficient olefin epoxidation catalysts. It appears that the
hydrolytic stability of MTO has been the sole reason for its better
catalytic activity in the epoxidation so far. In conjunction with
BTSP, which can be easily and economically prepared on a large
scale, more readily accessible inorganic oxorhenium derivatives can
now be applied; the parent MTO-catalyzed processes benefit from
decreased catalyst loadings. In addition, the use of BTSP avoids
hazards associated with highly concentrated H.sub.2O.sub.2
solutions. Thus, these rhenium catalyzed epoxidations continue to
become more attractive for practical applications. In a more
general sense, perhaps our finding can be utilized in other
processes where lability of silicon-bound heteroatoms can effect
often desired controlled release of reagents. Lastly, elimination
of water from the reaction might lead to more pronounced influence
of the external ligand on the reactivity in the present homogeneous
epoxidation system. Perhaps, based on our study that emphasizes the
importance of limited water content, new hydrogen peroxide-based
epoxidations that incorporate continuous water removal throughout
the process, thereby extending catalyst lifetime, will emerge.
EXAMPLE 2
Olefin Epoxidation with Bis(trimethylsilyl) Peroxide Catalyzed by
Inorganic Oxorhenium Derivatives
[0044] This example illustrates the systematic investigation of the
oxorhenium catalyst precursors and the effects of various additives
on the efficiency of olefin epoxidation with BTSP. Discovery of the
beneficial effect of pyridine in the MTO-catalyzed epoxidation
prompted our detailed study of this phenomenon with the goal of
further improving the system. From the very beginning, salient
features of the pyridine-modified protocol seemed counterintuitive.
For example, base-mediated decomposition pathways of MTO in aqueous
H.sub.2O.sub.2 have been established (Herrmann et al. used N-bases
in order to suppress epoxide ring opening (Herrmann et al. Angew.
Chem. Int. Ed. Engl. 1993, 103, 1991) albeit at the expense of
detrimental effect on catalytic activity. For the most recent study
of the MTO/Lewis base catalysts in olefin epoxidation, see:
Herrmann et al. J. Organomet. Chem. 1997, 549, 319; For a
comprehensive study on the base-induced decomposition of MTO, see
Abu-Omar et al. J. Am. Chem. Soc. 1996, 118, 4966; in the presence
of pyridine and H.sub.2O.sub.2, MTO is slowly oxidized, producing
pyridinium perrhenate and CH.sub.3OH: Yudin--unpublished resultsThe
hydroperoxide (HOO--) species induces the decomposition of MTO into
methanol and catalytically inactive perrhenate (ReO.sub.4--).
Pyridine can be expected to facilitate this detrimental process by
increasing the pH of the medium. Indeed, pyridinium perrhenate is
formed during MTO-catalyzed epoxidations mediated by pyridine, but
this does not adversely affect the epoxidations of most olefins,
since full conversion is reached well before significant levels of
catalyst decomposition are reached. Another important role
attributed to pyridine in these systems is that of a buffer for the
Lewis acidic Re(VII) species present, thereby enabling even
sensitive epoxides to survive.
[0045] Despite the overall efficiency of the original
pyridine-modified system, lower conversions were observed for less
reactive substrates such as terminal olefins, due to the premature
destruction of the catalyst. Although 3-cyanopyridine provided a
remedy for this class of olefins, a more general way of extending
catalyst lifetime became a challenge.
[0046] Among the known organometallic oxorhenium (VII) species
(R--ReO.sub.3) capable of catalyzing olefin epoxidation, MTO is
most stable with respect to oxidative and/or hydrolytic removal of
the alkyl group (vide infra) . Hence, catalyst modification by
variation of the R-substituent on the rhenium center was not
rewarding despite extensive efforts in the Herrmann laboratory
(Herrmann et al. Inorg. Chem. 1992, 31, 4431; for the most recent,
and best, procedure, see: Herrmann et al. Angew. Chem. Int. Ed.
Engl. 1997, 36, 2652). Elimination of water from the reaction would
be an alternate path toward increased turnovers because catalyst
decomposition should be largely suppressed. At the same time, the
presence of pyridine should prevent the epoxide ring opening and,
perhaps allow one to observe ligand effects on the selectivity
features of the oxygen atom transfer event. A water-free
environment should also eliminate possible complications from phase
transfer effects.
[0047] Any process that involves H.sub.2O.sub.2 as the oxygen atom
source produces at least one equivalent of water as by-product,
which will defeat an anhydrous system unless an efficient water
removal can be incorporated in the process design. A possible
solution to the problem could be an oxidant that acts as an
"anhydrous" analog of H.sub.2O.sub.2. Readily accessible
bis(trimethylsilyl) peroxide (BTSP) has been previously used in
this capacity. Cookson et al. J. Organomet. Chem. 1975, 99, C31;
Taddei et al. Synthesis 1986, 633; for a convenient, large-scale
(0.5 mol) preparation of BTSP from bis(trimethylsilyl)urea and
urea/H.sub.2O.sub.2 complex in dichloromethane, see: Jackson, W. P.
Synlett 1990, 536. The product obtained according to this method is
virtually free of hexamethyldisiloxane, a common, albeit harmless,
by-product in cognate BTSP preparations; Babin et al. Synth.
Commun. 1992, 22, 2849. WARNING: Thermal stabilities of silylated
organic peroxides have been studied: Vesnovskii, B. P.; Thomadze,
A. V.; Suchevskaya, N. P.; Aleksandrov, Yu. A. Zh. Prikl. Khim.
1982, 55, 1005. Pure BTSP has an active oxygen content of only 9%
(cf. tert-butyl hydroperoxide -17.8%; di-tert-butyl peroxide
-10.9%; hydrogen peroxide -47%). We would like to stress, however,
that despite its great thermal stability, BTSP is subject to facile
hydrolysis in the presence of water and acids which results in
formation of hazardous 100% H.sub.2O.sub.2. Additionally,
professors Henri Kagan and Dieter Seebach recently brought to our
attention two reports from their respective laboratories that
document explosions upon contact between BTSP and metal needles:
Riant et al. J. Org. Chem. 1997, 62, 6733; Neumann et al. Chem.
Ber. 1978, 111, 2785. Thus, only plastic or glass pipettes should
be used to transfer BTSP. For applications of BTSP in organic
synthesis, see: Brandes et al. J. Organomet. Chem. 1973, 49, C6;
Brandes et al. ibid. 1974, 73, 217; Tamao et al. ibid. 1975, 94,
367; Salomon et al. J. Am. Chem. Soc. 1979, 101, 4290; Adam et al.
J. Org. Chem. 1979, 44, 4969; Suzuki et al. ibid. 1982, 47, 902;
eber, W. P. "Silicon Reagents in Organic Synthesis,"
Springwer-Verlag: New York, 1983; Kanemoto et al. Tetrahedron Lett.
1983, 24, 2185; Matsubara et al. ibid. 1983, 24, 3741; Matsubara et
al. Bull. Chem. Soc. Jpn. 1983, 56, 2029; Kayakawa et al.
Tetrahedron Lett. 1986, 27, 4195; Curci et al. Tetrahedron 1986,
42, 877; Kanemoto et al. Bull. Chem. Soc. Jpn. 1988, 61, 3607;
Davis et al. Tetrahedron Lett. 1988, 29, 4269; Olah et al. J. Org.
Chem. 1989, 54, 1204; Camporeale et al. ibid. 1990, 55, 93; Shibata
et al. Bull. Chem. Soc. Jpn. 1991, 64, 3749. (s) Chemla et al.
Bull. Soc. Chim. Fr. 1993, 130, 547; Irie et al. Synlett 1994, 255;
Prouilhac-Cros et al. Bull. Soc. Chim. Fr. 1995, 132, 513; Adam et
al. Tetrahedron 1996, 52, 5487; Adam et al. Chem. Ber. 1996, 129,
1177; Barton et al. Tetrahedron 1997, 53, 487; Barton et al. ibid.
1997, 53, 511.
[0048] To our surprise, however, MTO showed little to no reactivity
toward BTSP in CDCl.sub.3 (FIG. 1c) under stoichiometric
conditions, (We felt that the substantial Lewis acidity of
MeReO.sub.3 would enable it to react with BTSP. Curci et al. has
demonstrated that silylated peroxides are 50 to 100 times more
effective than the corresponding hydrido analogs in electrophilic
oxygen atom transfer to sulfides. Simple rate laws that do not
require acid catalysis were deduced for these processes) in marked
contrast to the reaction between MTO and aqueous H.sub.2O.sub.2
that instantaneously generates a mixture of mono- and
bisperoxorhenium complexes. As previously reported by Curci et al.,
BTSP (ARCO Chemical Technology (Crocco et al., H. S.) U.S. Pat. No.
5,166,372 (1992)) is at least 50 times more reactive than
H.sub.2O.sub.2 in stoichiometric oxidation of sulfides to
sulfoxides which may account for the observed lack of nucleophilic
reactivity of BTSP toward Re(VII) in the present system.
[0049] In the case of BTSP the expected formation of peroxo
complexes must be accompanied by the silylation of the
rhenium-bound oxo ligand (FIG. 4a, R=SiMe.sub.3) . This process is
apparently much slower than its protic counterpart (FIG. 4a, R=H).
Formation of the peroxo complexes in the MTO/BTSP system does
occur, but only upon addition of an equivalent amount of water.
Hydrolytic generation of H.sub.2O.sub.2 from BTSP accounts for this
result and we subsequently established that a catalytic amount of
MTO is sufficient to generate H.sub.2O.sub.2 in the BTSP/H.sub.2O
system (BTSP is hydrolyzed to H.sub.2O.sub.2 within 2 hours in the
presence of 1 eq H.sub.2O and 0.5 mol % MTO). Other proton sources
(e.g. CH.sub.3OH) are equally effective in promoting
hydrolysis.
[0050] In accord with the aforementioned observations, no olefin
epoxidation takes place in the MTO/BTSP system under water-free
conditions. A trace of a protic species (e.g. water) is essential
to enable rapid turnover of the catalytic cycle (An induction
period can be attributed to the necessity of "acidity build-up".
Adventitious moisture can trigger an autocatalytic decomposition of
acid-sensitive BTSP into H.sub.2O.sub.2 and hexamethyldisiloxane.
Alternatively, partial hydrolysis of BTSP could afford
Me.sub.3SiOOH which could act as an oxidant in the present system.
For the use of silyl hydroperoxides in epoxidation, see: (a)
Dannley, R. L.; Jalics, G. J. Org. Chem. 1965, 30, 2417. (b) Rebek,
J.; McCready, R. Tetrahedron Lett. 1979, 20, 4337). The scenario
shown in FIG. 3 involves the hydrolytic generation of free
H.sub.2O.sub.2 from BTSP. Thus, intrinsic "slow addition" of
hydrogen peroxide to the oxorhenium species is managed by the
"proton dependent" cycle A (FIG. 3) which accomplishes transfer of
the peroxo group from silicon to rhenium (cycle B) . This
represents a novel "controlled release" mode for the H.sub.2O.sub.2
addition in rhenium-catalyzed epoxidation processes. In contrast,
it is impossible to exercise such control in H.sub.2O.sub.2
(aqueous or anhydrous) protocols (Ironically, MTO is stabilized at
higher H.sub.2O.sub.2 concentrations: Herrmann et al. Angew. Chem.
Int. Ed. Engl. 1993, 32, 1157). Slow addition of H.sub.2O.sub.2
does not help in achieving higher conversions due to faster MTO
decomposition at lower H.sub.2O.sub.2 concentrations.
[0051] The most significant outcome of the limited water content in
the present system is high catalytic epoxidation activity of
inorganic high valent oxorhenium species. Thus, simple inorganic
derivatives (e.g. Re.sub.2O.sub.7, ReO.sub.3(OH) and ReO.sub.3)
efficiently catalyze epoxidations with BTSP despite the well known
hydrolytic instability of inorganic rhenium peroxides. Peroxo
perrhenic acid (H.sub.4Re.sub.2O.sub.13) was isolated in the
Re.sub.2O.sub.7/H.sub.2O.su- b.2 system as a highly hydrolytically
labile, explosive compound: Herrmann et al. Chemistry--A European
Journal 1996, 2, 168.
[0052] Although we have established that MTO catalyzes the
generation of H.sub.2O.sub.2 from BTSP, the requirement for protic
species in epoxidation can be explained by other closely related
scenarios. Thus, in a more general way, the need for a proton
source is accommodated in FIG. 4B where a regenerable XH species
helps in ferrying the peroxo group from silicon to rhenium.
[0053] Having established the requirement for a proton source, we
examined how the amount of such additive affects the reaction. For
instance, reactivity was found to be a function of water
concentration, and beyond a certain range (5-10 mol %) catalysis
became inefficient, either due to the limited number of turnovers
(cycle A) or due to the sensitivity of the catalytically active
species to water. In fact, we noted that when excess water (1 eq
relative to the olefin) is added at the beginning of the
MTO-catalyzed epoxidation of cis-4-octene, BTSP is hydrolyzed
within 10 minutes (as determined by GC), and poor conversions are
observed. In addition, a significant amount of the diol, resulting
from the hydrolytic ring opening of the epoxide, is formed. At the
other extreme, efforts to remove all traces of water by running the
process in the presence of 4 .ANG. molecular sieves almost stop the
epoxidation catalysis. Control experiments demonstrate that MTO is
not absorbed or inactivated by the molecular sieves under these
conditions, ruling out catalyst removal as the origin of lost
activity. Similarly, very sluggish reaction takes place when
Re2O.sub.7 is used as a catalyst for the epoxidation of 1-decene
under anhydrous conditions (ca. 7% conversion after 2.5 hours) .
The reaction is dramatically accelerated upon addition of 5 mol %
water and reaches 70% completion within the next hour. It appears
that the optimal water concentration range is 5-10 mol % depending
on the substrate (vide supra).
[0054] The Role of Pyridine Derivatives and Other Additives. In
accord with previous observations, additives such as pyridines
serve to prevent sensitive epoxide ring opening by buffering the
highly acidic rhenium species. Moreover, under present homogeneous
conditions, the modulation of reactivity at the rhenium center is
uncoupled from the phase transfer effects. Participation of water
as a potential ligand is also minimized (Variable temperature NMR
experiments indicate lability of the rhenium-bound pyridine
ligands. Thus, .sup.1H NMR spectra of MTO in presence of 2
equivalents of pyridine are identical at -65.degree. C. and at
25.degree. C. and show one kind of pyridine species). Compared to
the original system, the amount of the pyridine ligand that is
necessary for the desired epoxide protection, is now decreased from
12 to 0.5-1 mol %. In the case of aqueous H.sub.2O.sub.2/MTO
system, pyridine induces the formation of catalytically inactive
perrhenic acid which results in lower turnovers for the less
reactive substrates. In principle, inorganic oxorhenium catalysts
should easily tolerate pH increase since no labile R-Re bond are
present in the catalytically active species. At the same time,
catalytic turnover in the present system crucially depends on
efficient hydrolysis of BTSP and excess pyridine (10-12 mol %)
dramatically slows down the reaction for all substrates. The
optimal amount of the pyridine additive sufficient to maintain the
epoxide protection is 0.5-1 mol % with minimal detrimental effect
on the rate. We have also found that progenitors of highly
sensitive epoxides do not tolerate even relatively low water
concentrations. The preferred proton sources in such instances
should contain conjugate bases of low nucleophilicity which do not
participate in epoxide ring opening. In this regard, pyridinium
trifluoroacetate was found to be particularly effective as a
supplier of protons.
[0055] Catalyst Precursors. Comparable reactivity is observed among
all inorganic oxorhenium derivatives (see FIG. 5). However,
ReO.sub.3 is preferred (at least for laboratory scale epoxidations)
due to its non-hygroscopic nature relative to Re.sub.2O.sub.7.
Unlike the rest of the precatalysts, ReO.sub.3 does not dissolve in
the reaction medium at the start. As the reaction proceeds, Re(VI)
is rapidly oxidized to Re(VII), so that the solution acquires the
bright yellow color characteristic of peroxorhenium(VII)
species.
[0056] One of the most significant implications of the present
controlled H.sub.2O.sub.2 release version of epoxidation is overall
increase in the lifetime of oxorhenium catalysts. Indeed,
hydrolytic removal of the methyl group from the rhenium center of
the catalyst is no longer the origin of lost activity. Thus, the
catalytically active species are preserved throughout the process
and can be reused after evaporative removal of the solvent and the
epoxide.
[0057] Substrate Scope. FIG. 5 defines the scope of this new
process with representative substrates including fairly unreactive
olefins and/or progenitors of sensitive epoxides. The optimal
substrate concentration is in the 0.5-2M range with dichloromethane
as solvent (Such species had long been known to exhibit weak
activity as epoxidation catalysts: (a) for the epoxidation of
C.sub.2-.sub.20 olefins with stoichiometric Re.sub.2O.sub.7 in the
presence of pyridine, see: Union Oil Co. of California (Fenton, D.
M.) U.S. Pat. No. 3,316,279; for early applications of
Re.sub.2O.sub.7 in olefin/H.sub.2O.sub.2 oxidation catalysis see:
duPont de Nemours and Co. (Parshall, G. W.) U.S. Pat. Nos.
3,657,292 and 3,646,130 (1972); Warwel and co-workers found that
Re.sub.2O.sub.7 is a more effective epoxidation catalyst if the
right solvent is chosen. Their system employs 60% aqueous
H.sub.2O.sub.2 in 1,4-dioxane at 90.degree. C. and 1,2-diols are
isolated in good yields, the initially formed epoxides being
unstable in this system: Warwel, S.; Rusch gen Klaas, M.; Sojka, M.
Chem. Commun. 1991, 1578). No special precautions to exclude
moisture during the reaction need to be taken. We stress, however,
that because the catalytic turnover depends on the presence of
protic additives and is inhibited at high water concentrations, it
is good practice to account for any water beyond the deliberately
added amount. With this in mind we recommend anhydrous
dichloromethane. Upon completion of the reaction, the work-up
procedure simply involves destruction of the traces of
H.sub.2O.sub.2 with manganese dioxide and evaporation of
dichloromethane and hexamethyldisiloxane (Me.sub.3SiOSiMe.sub.3,
b.p. 100.degree. C.)
[0058] With the present system, terminal olefins, problematic in
the original procedure, are efficiently converted into the
corresponding epoxides within short reaction times. Disubstituted
olefins (both cis and trans) present little problem as do the tri-
and tetrasubstituted olefins. However, in the latter two cases
water has to be replaced by pyridinium trifluoroacetate as an
external proton source (vide supra). Thereby, a conjugate base of
lower nucleophilicity is involved which reduces the undesired
epoxide ring opening. Notably, the original H.sub.2O.sub.2/pyridine
system remains superior for the preparation of certain extremely
acid-sensitive epoxides such as styrene oxide and indene oxide.
Easily oxidizable substrates such as phenyl ethers are also beyond
the scope of this new protocol due to the competitive oxidation of
the aromatic ring.
[0059] Conditions were found under which simple inorganic
oxorhenium compounds act, for the first time, as efficient olefin
epoxidation catalysts (MTO and Re.sub.2O.sub.7 were purchased from
Strem Chemicals, Inc. ReO.sub.3 and HOReO.sub.3 were purchased from
Aldrich Chemical Co. For industrial sources of rhenium, see:
Peacock, R. D. The Chemistry of Technetium and Rhenium; Elsevier:
Amsterdam, 1966). The crucial factor enabling these inorganic
oxorhenium species to exhibit high activity for epoxidation
catalysis is thought to be the nearly anhydrous conditions which
are achieved and maintained by using BTSP as the oxygen atom
source, along with only a trace of a protic agent (e.g. H.sub.2O)
to catalyze slow transfer of the peroxide moiety from silicon to
rhenium. If, as seems probable, BTSP becomes available on a large
scale, this new "anhydrous" rhenium-catalyzed process could become
one of the most reliable and convenient methods available for
epoxidation of olefins at either laboratory or fine chemical
production scales. In addition to the fact that the inexpensive,
inorganic rhenium catalyst precursors are as effective as MTO, an
especially attractive feature of these "anhydrous" systems is the
simple work-up which entails only rotary evaporation of the
dichloromethane and hexamethyldisiloxane from the reaction
mixture.
[0060] While a preferred form of the invention has been shown in
the drawings and described, since variations in the preferred form
will be apparent to those skilled in the art, the invention should
not be construed as limited to the specific form shown and
described, but instead is as set forth in the following claims.
EXPERIMENTAL PROTOCALS
[0061] General .sup.1H and .sup.13C nmr spectra were recorded
either on a Bruker AM-250, a Bruker AMX-400 or a Bruker AMX-500
spectrometer. Residual protic solvent CHCl.sub.3
(.delta..sub.H=7.26 ppm, .delta..sub.C=77.0), d.sub.4-methanol
(.delta..sub.H=3.30 ppm, .delta..sub.C=49.0) and D.sub.2O
(.delta..sub.H=4.80 ppm, .delta..sub.C (of CH.sub.3CN)=1.7 ppm) or
TMS (.delta..sub.H=0.00 ppm) were used as internal reference.
Coupling constants were measured in Hertz (Hz). HRMS were recorded
using FAB method in a m-nitrobenzylalcohol (NBA) matrix doped with
NaI or CsI. Infra-red spectra were recorded on a Perkin-Elmer FTIR
1620 spectrometer. Enantiomeric excess was determined by HPLC using
a Daicel Chemical Industries CHIRALPAK AD column. Optical rotations
were measured with an Optical Activity AA-1000 polarimeter. Melting
points were taken on a Thomas Hoover capillary melting point
apparatus and are uncorrected. Column chromatography was performed
on Merck Kieselgel 60 (230-400 mesh). Analytical thin layer
chromatography was performed using pre-coated glass-backed plates
(Merck Kieselgel F.sub.254) and visualized by cerium
molybdophosphate or ninhydrin. Diethyl ether, tetrahydrofuran (THF)
and toluene (PhCH.sub.3) were distilled from sodium-benzophenone
ketyl, dichloromethane (DCM) and acetonitrile from calcium hydride.
Other solvents and reagents were purified by standard procedures if
necessary.
[0062] General procedure for controlled water epoxidation using
rhenium oxides as the catalyst and trialkylsilyl peroxides as the
anhydrous oxidant as exemplified for mono, di, tri and tetra
substituted olefins shown in FIG. 2, entries 1-8, FIG. 5, entries
1-11, and FIG. 6, entires 12-19; all listed olefins are purchase
from Aldrich; since variations in the preferred form will be
apparent to those skilled in the art, the invention should not be
construed as limited to the specific compounds shown and described,
but instead is as set forth in the following claims). In a 25 mL
scintillation vial equipped with a magnetic stirrer, the olefin
(1.41 g, 10 mmol) was placed followed by addition of 4 mL
dichloromethane (the solvent is not limited to dichloromethane, but
rather alernative solvents used in similar concentrations include:
the solvent is selected from the group consisting of nitromethane
(CH.sub.3NO.sub.2), nitroethane (EtNO.sub.2), methylene chloride
(CH.sub.2Cl.sub.2), chloroform (CHCl.sub.3), carbon tetrachloride
(CCl.sub.4), 1,2-dichloroethane (CH.sub.2ClCH.sub.2Cl),
pentachloroethane (CCl.sub.3CHCl.sub.2) ,chlorinated aromatic
compounds: chlorobenzene, dichlorobenzene and other chlorinated
solvents, fluorinated solvents and chlorofluoro hydrocarbons,
acetonitrile (CH.sub.3CN), acetone, benzene, toluene, xylenes,
methanol, ethanol, propanol, isopropanol, butanol, t-butanol,
ethyleneglycol, diethylether, THF (tetrahydrofuran) and
supercritical CO.sub.2). To this solution was added BTSP (2.8 g, 15
mmol; alternative related trialkylsilyl hydroperoxides used include
R.sub.3SiOOH and polymeric derivatives (--R.sub.2SiOO--) (wherein
in each case R=C.sub.1-C.sub.6alkyl); bis(tert-butyl)peroxide
(Aldrich) and benzoyl peroxide (Aldrich) in combination with
hexamethyldisilazane (Aldrich) (in situ source of BTSP eliminating
the need for its isolation)). The vial was immersed into ice/water
bath. After 5 minutes Re.sub.2O.sub.7 (24 mg, 0.05 mmol;
alternative rhenium oxide catalysts used in the same concentration
include: RReO.sub.3 (wherein R=C.sub.1-C.sub.6alkyl for example,
CH.sub.3ReO.sub.3), Re.sub.2O.sub.7, ReO.sub.3, HReO.sub.4,
NH.sub.4ReO.sub.4, Re (metal), ReO.sub.2 (All Aldrich),
Me.sub.3SiOReO.sub.3) was added followed by 10 mL water. The
reaction turned bright yellow and was allowed to warm up to room
temperature and stirred for 14 h. Disappearance of BTSP in the
course of the reaction was monitored by gas chromatography. Upon
completion, water (3 drops) was added followed by manganese dioxide
(ca. 5 mg) in order to decompose the remaining H.sub.2O.sub.2. The
destruction of H.sub.2O.sub.2 was evident by the disappearance of
yellow color. The mixture was then dried over Na.sub.2SO.sub.4.
Concentration afforded the epoxide (1.48 g, 94% yield) of a
colorless oil. Analytically pure sample is obtained by
distillation. 4. The remaining 4% consisted of dichloromethane and
traces (ca. 2%) of haxamethyldisiloxane. 5. When the stock solution
of Re.sub.2O.sub.7 in THF was used, stock solution of
Re.sub.2O.sub.7 in THF (0.042 M) can be stored for weeks at room
temperature with no signs of decomposition or lost activity in
epoxidation. 6. if trans-Stilbene is used as the olefin, it is not
completely dissolved at the beginning of the reaction. However, as
the epoxidation proceeds, the mixture becomes homogeneous. Optional
ligands which can be used to prevent sensitive epoxide ring opening
include pyridine at concentrations from 0.25 mol % to 12 mol % and
pyridine derivatives (same concentrations as pyridine if used in
lieu of pyridine) containing electron withdrawing or electron
donating groups (nitro, esters, ketones, halogens, nitrites,
sulphonic acid esters), chiral pyridines (like cotinine), imines,
oxazolines, 2-methylpyridine (2-picoline), 2-ethylpyridine,
2-propylpyridine, 2-phenylpyridine, 2-benzylpyridine,
2-fluoropyridine, 2-chloropyridine, 2-bromopyridine,
2-cyanopyridine, 2-hydroxypyridine, 2-pyridylcarbinol,
2-pyridineethanol, 2-pyridinepropanol, pyridine-2-carboxylic acid
(picolinic acid) and corresponding esters, 3-methylpyridine
(3-picoline), 3-ethylpyridine, 3-butylpyridine, 3-phenylpyridine,
3-benzylpyridine, 3-fluoropyridine, 3-chloropyridine,
3-bromopyridine, 3-cyanopyridine, 3-pyridylcarbinol,
3-hydroxypyridine, 3-pyridinepropanol, pyridine-3-carboxylic acid
(nicotinic acid) and corresponding esters, 4-methylpyridine
(4-picoline), 4-fluoropyridine, 4-chloropyridine, 4-bromopyridine,
4-cyanopyridine, 4-ethylpyridine, 4-isopropylpyridine,
4-t-butylpyridine, 4-(1-butylpentyl)pyridine, 4-phenylpyridine,
4-benzylpyridine, 4-(4-chlorobenzyl)pyridine, 4-hydroxypyridine,
4-methoxypyridine, 4-nitropyridine, pyridine-4-carboxylic acid and
corresponding esters, 2,3-dimethylpyridine (2,3-lutidine),
2,4-dimethylpyridine (2,4-lutidine), 2,5-dimethylpyridine
(2,5-lutidine), 2,6-dimethylpyridine (2,6-lutidine),
3,4-dimethylpyridine (3,4-lutidine), 3,5-dimethylpyridine
(3,5-lutidine), 2,6-difluoropyridine, pentafluoropyridine,
pentachloropyridine, 2,6-dichloropyridine, 3,5-dichloropyridine,
2,3,5-trichloropyridine, 3,4-dicyanopyridine, 5-chloro-3-pyridinol,
2,3-pyridinecarboxylic acid and corresponding esters,
2,4-pyridinecarboxylic acid and corresponding esters,
2,5-pyridinecarboxylic acid and corresponding esters,
2,6-pyridinecarboxylic acid and corresponding esters,
2,6-diphenylpyridine, 2,6-di-p-tolylpyridine,
3,4-pyridinecarboxylic acid and corresponding esters,
2-pyridineethansulfonic acid, 4-pyridineethanesulfonic acid,
2,3-cyclopentenopyridine, 2,3-cyclohexenopyridine,
2,3-cycloheptenopyridine, 2,4,6-collidine, pyrazine,
2,3-pyrazinedicarbonitrile, pyrazinecarbonitrile,
2,6-dichloropyrazine, pyrazinecarboxylic acid and corresponding
esters, 2,3-dimethylpyrazine, 2,5-dimethylpyrazine,
2,6-dimethylpyrazine, 2,3-di-2-pyridylpyrazine, pyridazine,
3-methylpyridazine, 4-methylpyridazine, pyrimidine,
4-methylpyrimidine, 4,6-dimethylpyrimidine, 4-phenylpyrimidine,
2,4-dichloropyrimidine, 4,6-dichloropyrimidine,
2,4,5-trihydroxypyrimidine, 4-(trifluoromethyl)-2-pyrimidinol,
1,3,5-triazine, (-)-cotinine
(1-methyl-5-(3-pyridyl)-2-pyrrolidinone) pyridine-2,6-dicarboxylic
acid and corresponding esters, quinoline, isoquinoline,
2,2'-pyridyl, 2,2'-dipyridyl, 6-chloro-2,2'-bipyridine,
2,4'-dipyridyl, 4,4'-dipyridyl, 2,2':6',2"-terpyridine,
1,7-phenanthroline, 1,10-phenanthroline, 4,7-phenanthroline,
phenazine, 3,6-di-2-pyridyl-1,2,4,5-tetrazine,
2,2'-bipyridine-4,4'-carboxylic ester, 1,2-bis(4-pyridyl)ethane,
4,4'-trimethylenepyridine, quinoxaline, 2,3-dimethylquinoxaline,
1-nitropyrazole, 2,5-diphenyloxazole, 2,4,5-trimethyloxazole,
2,4,4-trimethyl-2-oxazoline, 3,5-dimethylisoxazole,
2,6-bis[(4S)isopropyl-2-oxazolin-2-yl]pyridine,
1,2-dimethylimidazole, 1-butylimidazole, 2,3,3-trimethylindolenine,
caffeine, and polymer-supported ligands.
[0063] Standard Procedure for Preparation of Bis(trimethylsilyl)
Peroxide (BTSP) on a 1 mol scale.
[0064] Finely powdered urea/H.sub.2O.sub.2 complex (94 g, 1 mol)
was suspended in anhydrous dichloromethane (500 mL) in a 1 L
oven-dried round-bottom flask equipped with a magnetic stirrer. To
this mixture was added bis(trimethylsilyl)urea (204 g, 1 mol). A
reflux condenser was attached and the flask was immersed into an
oil bath maintained at 65-70.degree. C. overnight. The liquid phase
was distilled off (10 mm Hg) into an oven-dried round bottom 1 L
flask maintained at -78.degree. C. leaving slightly yellow urea
residue. Due to partial occlusion of the product in this residue,
it was further subjected to high vacuum bulb-to-bulb distillation.
Finally, combined fractions were redistilled (10 mm Hg) using a
short-path distillation head giving BTSP (133 g, 96% pure, 75%
yield) that was used for epoxidations without further
purification.
[0065] Standard Procedure for Preparation of R.sub.3SiOOH.
[0066] Finely powdered urea/H.sub.2O.sub.2 complex (1 eq) was
suspended in anhydrous dichloromethane in a 1 L oven-dried
round-bottom flask equipped with a magnetic stirrer. To this
mixture was added trialkylsilyl chloride R.sub.3SiCl (1 eq;
R=C.sub.1-C.sub.6alkyl; commerically available reagents from
companies such as Aldrich). A reflux condenser was attached and the
flask was immersed into an oil bath maintained at 65-70.degree. C.
overnight. Extraction with dichloromethane was followed by drying
over anhydrous sodium sulfate. Evaporation of dichloromethane
yielded R.sub.3SiOOH as an analytically pure material.
[0067] Standard Procedure for Preparation of Polymeric Derivatives
(--R.sub.2SiOO--).sub.n (R=organic functionality; eg.
C.sub.1-C.sub.6alkyl) Finely powdered urea/H.sub.2O.sub.2 complex
(1 eq) was suspended in anhydrous dichloromethane in a 1 L
oven-dried round-bottom flask equipped with a magnetic stirrer. To
this mixture was added dimethyldichlorosilane (1 eq). A reflux
condenser was attached and the flask was immersed into an oil bath
maintained at 65-70.degree. C. overnight. Extraction with
dichloromethane was followed by drying over anhydrous sodium
sulfate. Evaporation of dichloromethane yielded
(--R.sub.2SiOO--).sub.n as a white solid.
[0068] Standard procedure for epoxidation on a 10 millimol scale in
dichloromethane exemplified for 1-decene. In a 25 mL scintillation
vial equipped with a magnetic stirrer, 1-decene (1.41 g, 10 mmol)
was placed followed by addition of 4 mL dichloromethane. To this
solution was added BTSP (2.8 g, 15 mmol). The vial was immersed
into ice/water bath. After 5 minutes Re.sub.2O.sub.7 (24 mg, 0.05
mmol).sup.5 was added followed by 10 .mu.L of water. The reaction
turned bright yellow and was allowed to warm up to room temperature
and stirred for 14 h. Upon completion, water (3 drops) was added
followed by manganese dioxide (ca. 5 mg) in order to decompose the
remaining H.sub.2O.sub.2. The destruction of H.sub.2O.sub.2 was
evident by the disappearance of yellow color. The mixture was then
dried over Na.sub.2SO.sub.4. Concentration afforded 1-decene oxide
(1.48 g, 94% yield) of a colorless oil.
[0069] Standard procedure for epoxidation on a 10 millimol scale in
THF exemplified for trans-stilbene.
[0070] In a 25 mL scintillation vial equipped with a magnetic
stirrer, trans-stilbene (1.80 g, 10 mmol) was placed followed by
addition of 2.8 mL THF..sup.6 To this solution was added BTSP (2.8
g, 15 mmol). The vial was immersed into ice/water bath. After 5
minutes Re.sub.2O.sub.7 (24 mg, 0.05 mmol) was added dropwise as a
solution in 1.2 mL THF..sup.7 The reaction turned yellow and was
allowed to warm up to room temperature and stirred for 10 h. Upon
completion, water (3 drops) was added followed by manganese dioxide
(ca. 5 mg) in order to decompose the remaining H.sub.2O.sub.2. The
destruction of H.sub.2O.sub.2 was evident by the disappearance of
yellow color. The mixture was then dried over Na.sub.2SO.sub.4.
Concentration afforded trans-stilbene oxide (1.88 g, 96% yield) of
as an off-yellow solid.
[0071] The Boiling reactor process: This process comprises a method
which continuously removes water in the vapor stream by using
appropriate solvent, temperature, and pressure conditions, keeping
the concentration of the liquid phase low enough to maintain the
activity of the oxorhenium catalyst. The method is as follows:
[0072] A 500 mL autoclave-type reaction vessel equipped with the
liquid inlet (at the bottom) and gas outlet (on top) was charged
with the rhenium precursor (such as Re.sub.2O.sub.7), olefin, and
pyridine. The reactor was closed and heated as hydrogen peroxide
dissolved in organic solvent such as tert-butanol was slowly added
through a liquid inlet. As the reaction, maintained at the solvent
reflux temperature, proceeded, water was being collected at the
vapor outlet in the form of an azeotrope with organic solvent. To
keep the concentration of the catalytic species constant with
respect to the catalyst, olefin, pyridine, and epoxide, the solvent
that was collected at the outlet in the form of azeotrope with
water was passed through the drying agent and reintroduced at the
bottom. This mode of operation allowed to continuously remove water
in the vapor stream by using appropriate solvent, temperature, and
pressure keeping the concentration of the liquid phase low enough
to maintain the activity of the oxorhenium catalyst. At the end of
the reaction, the volatiles (mainly the product epoxide) were
removed and the rhenium catalyst was recycled in another run that
revealed no loss of catalytic activity.
* * * * *