Convenient Synthesis of Bis(alkoxy)rhenium(VII) Complexes

Full text of DOI:10.1021/ic951181m

1408
Inorg. Chem. 1996, 35, 1408-1409
since a similar approach would be impeded by the disposition
of the phenyl groups (III).
The bis(alkoxy)rhenium(VII) complexes react with triph-
enylphosphine in dry benzene at room temperature to yield
MTO, triphenylphosphine oxide, and olefin:
Convenient Synthesis of Bis(alkoxy)rhenium(VII)
Complexes
Zuolin Zhu, Ahmad M. Al-Ajlouni, and
James H. Espenson*
CH3Re(O)2(OCR2CR2O) + PPh3 f
CH3ReO3 + Ph3PdO + CR2CR2 (2)
Ames Laboratory and Department of Chemistry,
Iowa State University, Ames, Iowa 50011
ReceiVed September 8, 1995
Release of an alkene is strongly enhanced by the phosphine,
and it occurs essentially upon mixing. Without phosphine,
alkenes are released only slowly, if at all, from these Re(VII)
derivatives; if heated, the MTO is destroyed. In wet acetonitrile,
the reactions of styrene epoxide and 2,3-dimethyl-2-butene
epoxide with MTO also gave bis(alkoxy)rhenium(VII) com-
plexes in the presence and in the absence of H2O2. The yield
(∼10-15%) was much less than that obtained in dry benzene
or in chloroform. In the absence of hydrogen peroxide, alkenes
were released slowly over 3-5 days at room temperature. In
addition, perrhenate ions and methanol were formed as decom-
position products of MTO in peroxide-containing solutions.13
Because epoxides are also formed from the reaction of the
monoperoxorhenium complex A with alkenes, reaction 3 is
reversible.
The study of high-oxidation-state organorhenium compounds
has been a field of continuing activity, thanks to the success of
methylrhenium trioxide (CH3ReO3 or MTO) in catalytic pro-
cesses. This catalyst is effective in oxidations,1-3 olefin
metathesis,4 the olefination of aldehydes,5 and the preparation
of other compounds with three-membered rings.6 The syntheses
of some rhenium compounds derived from MTO have been
reported.5 Epoxide formation is a key reaction,7-9 and it bears
directly on these findings, as we now report.
Re(VII) complexes containing a chelated bis(diolate) ligand
can be synthesized by refluxing MTO with 2,3-dimethyl-2,3-
diol.10 Here we report a more convenient method for this
preparation. A different series of related compounds consists
of chelated bis(diolates) of the Cp*Re-oxo series, Cp*ReO-
(diolate).11,12
Results and Discussion
The reaction between MTO and an epoxide, eq 1, leads to
diolate complexes. Five epoxides were used in this study: 2,3-
The reaction of the diperoxo-Re(VII) complex, B ([H2O2]
) 0.5 M and [MTO] ) 0.02 M), with styrene was complete in
3-4 h. Under the same conditions, in the absence of H2O2,
the 1H-NMR spectrum showed that less than 50% of the styrene
oxide had reacted with MTO after 5 days. Because A and B
exhibit similar reactivities toward the epoxidation of olefins,9
these results indicate that the reverse rate constant in eq 3 is
much larger than the forward one and so the equilibrium
constant is much less than unity. In the presence of H2O2, A
and B are present, so any alkene formed reacts rapidly with A
or B and could not be observed. Oxygen was transferred from
styrene epoxide (0.1 M) to 2,3-dimethyl-2-butene (0.1 M) in
acetonitrile in the presence of MTO (0.02 M). The reaction
produced styrene and 2,3-dimethyl-2-butene epoxide in ∼5-
10% yield, eq 4. After 3 days, only ∼20% of styrene epoxide
dimethyl-2-butene epoxide, styrene epoxide, cis-cyclododecane
epoxide, cis-stilbene oxide, and trans-stilbene oxide. All except
the last react with methylrhenium trioxide to give the corre-
sponding bis(alkoxy)rhenium(VII) compounds (I) in nearly
quantitative yield.
We suggest that the first step is the approach of the oxygen
atom of the epoxide to the rhenium atom, at a site remote from
the Re-C bond (II). Unfavorable steric interactions may
and 2,3-dimethyl-2-butene were consumed. Other products,
1-phenyl-1,2-ethanediol, 2,3-dimethyl-2,3-butanediol, and the
bis(alkoxy)rhenium(VII) complexes, were also observed. In the
absence of MTO, no styrene or 2,3-dimethyl-2-butene epoxide
were observed, clearly substantiating the need for a catalyst.
2,3-Dimethyl-2-butene epoxide most probably results from
the reaction of 2,3-dimethyl-2-butene with A, formed from
account for the failure of trans-stilbene oxide to react with MTO,
(1) Zhu, Z.; Espenson, J. H. J. Org. Chem. 1995, 60, 1326.
(2) Vassell, K. A.; Espenson, J. H. Inorg. Chem. 1994, 33, 5491.
(3) Abu-Omar, M. M.; Espenson, J. H. J. Am. Chem. Soc. 1995, 117,
272.
(4) Herrmann, W. A.; Wagner, W.; Flessner, U. N.; Volkhardt, U.;
Komber, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1636.
(5) Herrmann, W. A.; Roesky, P. W.; Wang, M.; Scherer, W. Organo-
metallics 1994, 13, 4531.
(8) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1638.
(9) Al-Ajlouni, A.; Espenson, J. H. J. Am. Chem. Soc. 1995, 117, 9243.
(10) Herrmann, W. A.; Watzlowik, P.; Kiprof, P. Chem. Ber. 1991, 124,
1101.
(11) Herrmann, W. A.; Herdtweck, E.; Floel, M.; Kulpe, J.; Kusthardt, U.;
Okuda, J. Polyhedron 1987, 6, 1165-1182.
(12) Gable, K. P. Organometallics 1994, 13, 2486.
(13) Abu-Omar, M. M.; Hansen, P. J.; Espenson, J. H. Submitted for
publication.
(6) Zhu, Z.; Espenson, J. H. J. Org. Chem. 1995, 60, 7090.
(7) Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch, M. H. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1157.
0020-1669/96/1335-1408$12.00/0 © 1996 American Chemical Society

Notes
Inorganic Chemistry, Vol. 35, No. 5, 1996 1409
Scheme 1
Scheme 2
styrene epoxide with MTO as shown in eq 4. We have not
explored the unimolecular reactions that lead to alkene release.
On the other hand, the literature reports the slow unimolecular
release of alkene from rhenium(V) diolates.12,14
of epoxycyclohexane in tert-butyl alcohol-water solutions;19
with MTO alone, ring opening is catalyzed but the reaction is
slower.
The most substantial difference between Re(V) and Re(VII)
diolates is not so much the oxidation state of the metal but the
nature of the rhenium compound that remains were each diolate
to dissociate an epoxide. The Re(V) compound would produce
R-ReO2, and Re(VII), MTO or R-ReO3. The latter is a stable
substance, whereas the former, although known,5,15,16 is not
particularly stable. On the other hand, were each diolate to
dissociate an alkene, then the Re(V) compound would produce
R-ReO3, and Re(VII), the monoperoxide A, at least from a
stoichiometric argument. Such differences must surely influence
the transition state energies, favoring the Re(VII) compound in
practice.
There is, we believe, a close interrelation of three processes:
(a) alkene epoxidation with hydrogen peroxide catalyzed by
MTO that proceeds through the peroxorhenium compound CH3-
Re(O)2(O2), A; (b) diolate formation from an epoxide with MTO
as described herein; and (c) alkene release from the diolate,
which has been better characterized for Re(V).12,14,17,18 In other
words, eq 1 as well as eq 3 is reversible, since A is a an
intermediate common to both.
The interrelationship between these processes, in part con-
jectural, is diagramed in Scheme 1. This diagram depicts the
epoxidation occurring via the peroxide A, proceeding through
one or more intermediates to either the epoxide or the diolate.
In practice, epoxidation is very rapid, as is the conversion of
MTO to A by reaction with hydrogen peroxide. Although not
depicted, the diperoxo rhenium complex B would be expected
to react in a parallel fashion. Thus diolate formation is of
minimal importance until the supply of peroxide is exhausted
(assuming the alkene was taken in excess); alternatively, as in
the procedure described here, no peroxide was used, allowing
the transformation of the epoxide to the diolate. Hydrogen
peroxide accelerates the MTO-catalyzed ring opening reaction
Certain possibilities for the excision of an alkene from a
rhenium(V) diolate are shown in Scheme 2.14,17,18 In particular,
Hammett correlations have been used to rule out some pos-
sibilities, and CH2 migration is the avenue suggested. By the
same token, CH2 migration (to O, not Re; see IV, a species
with an extra O atom derived from A) may provide the avenue
for the reaction under discussion here.
Experimental Section
A solution of methylrhenium trioxide (250 mg, 1 mmol) in 15 mL
of dry methylene chloride was treated with the desired epoxide (1.2
mmol). After 1 day at room temperature, during which the solution
changed from colorless to yellow to deep red, the solvent was removed
under vacuum. The light yellow bis(alkoxy)rhenium(VII) compounds
were purified by vacuum sublimation. These procedures were carried
out under argon. 2,3-Dimethyl-2-butene epoxide was prepared as
described,20 and the other epoxides, available commercially, were
purified using standard methods.21
Spectroscopic data for the isolated products in CDCl3 are as follows.
(a) For R1 ) R2 ) R3 ) R4 ) Me: 1H-NMR δ 2.38 (3H), 1.34
(12H); 13C-NMR δ 96.45 (C-Me2), 42.70 (Me-Re), 25.75 (C-Me2);
1H-NMR in CD3CN δ 2.39 (3H), 1.36 (12 H).
(b) For R1 ) R2 ) R3 ) H, R4 ) Ph: 1H-NMR δ 2.44 (3H), 4.15
(1H), 4.52 (1H), 5.35 (1H), 7.05 (5H); 13C-NMR δ 136.54, 129.52,
1
128.85, 126.31, 87.18, 85.25, 42.83; H-NMR in CD3CN δ 2.45 (s,
3H), 4.78 (dd, 1H), 5.23 (t, 1H), 5.54 (dd, 1H), 7.32 (m, 3H), 7.47 (m,
2H).
(c) For the reaction between MTO and cyclododecane epoxide: 1H-
NMR δ 2.41 (3H), 4.24 (2H), 2.17 (4H), 1.82 (4H), 1.35 (8H), 1.01
(4H); 13C-NMR δ 94.54, 42.07, 31.22, 28.73, 28.73, 26.18, 24.75, 22.83.
(d) For the reaction between MTO and cis-stilbene: 1H-NMR δ 2.47
(3H), 4.31 (2H), 7.43 (10H); 13C-NMR δ 134.69, 127.52, 126.03,
125.88, 98.76, 42.72.
Acknowledgment. This research was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
Division of Chemical Sciences, under Contract W-7405-Eng-82.
(14) Gable, K. P.; Juliette, J. J. J. Am. Chem. Soc. 1995, 117, 955.
(15) Felixberger, J. K.; Kuchler, J. G.; Herdtweck, E.; Paciello, R. A.;
Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1988, 27, 946.
(16) Herrmann, W. A.; Wang, M. Angew. Chem., Int. Ed. Engl. 1991, 30,
1641.
(17) Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116, 833.
(18) Gable, K. P.; Juliette, J. J. J. Abstracts of Papers; 209th National
Meeting of the American Chemical Society, Anaheim, CA, 1995;
American Chemical Society: Washington, DC, 1995.
IC951181M
(19) Herrmann, W. A.; Fischer, R. W.; Rauch, M. U.; Scherer, W. J. Mol.
Catal. 1994, 86, 243.
(20) Camps, F.; Coll, J.; Messeguer, A.; Pericas, M. A. Tetrahedron Lett.
1981, 22, 3859.
(21) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemical,
3rd ed.; Butterworth-Heinemann: Oxford, U.K., 1988.