J. Am. Chem. Soc. 1997, 119, 3507-3512
3507
Aqueous Catalysis: Methylrhenium Trioxide (MTO) as a
Homogeneous Catalyst for the Diels-Alder Reaction
Zuolin Zhu and James H. Espenson*
Contribution from the Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
ReceiVed September 9, 1996X
Abstract: The title compound proves to be an effective and efficient catalyst for the Diels-Alder reaction when the
dienophile is an R,â-unsaturated ketone or aldehyde. It is especially effective in water. Equal amounts of any such
dienophile and any of six representative dienes (isoprene, 2-methyl-1,3-pentadiene, 2,3-dimethyl-1,3-butadiene,
cyclopentadiene, 1,2,3,4,5-pentamethylcyclopentadiene, and 1,3-cyclohexadiene) were used, along with 1% MTO.
The reactions gave usually >90% isolated yield of the cycloaddition product except for the larger dienophiles. Nearly
exclusively, there was formed one product isomer, the same one that usually predominates. The reactions were
often run in chloroform (mostly) and in other organic solvents. A select number were carried out in water, where
the reactions gave a greater product yield in a considerably shorter time. Water, itself, is known to enhance the rates
of Diels-Alder reactions, but MTO exerts an additional accelerating effect. Kinetics studies were carried out to
show that the rate is proportional to the catalyst concentration. The products do not inhibit the reaction. The desirability
of MTO as a Diels-Alder catalyst stems from a combination of favorable properties: the inertness to air/oxygen,
the tolerance for many substrates, the use of an aqueous medium, and the absence of product inhibition. The initial
step appears to be the (weak) coordination of the carbonyl oxygen to the electropositive rhenium center. Steric
crowding around rhenium inhibits reactions of the larger dienophiles.
Introduction
be used, although the former is a corrosive material that requires
an almost stoichiometric concentration.6 [TiCp2](CF3SO3)2 and
[ZrCp2(THF)](CF3SO3)2, stable for a period of time in air, are
insoluble in nonpolar solvents.7 [Re(salen)(NO)(H2O)]SbF6,
being insufficiently soluble in many solvents, requires special
solvent conditions.1 The combined catalyst 4Ph2SnS/AgClO4
must be used at -78 °C.8
Methylrhenium trioxide (CH3ReO3, abbreviated as MTO),
easily prepared from dirhenium heptoxide and tetramethyl tin,9
is soluble in all common organic solvents and in water and is
not sensitive to oxygen or acids. That MTO might be able to
function as a catalyst was suggested by its ability to coordinate
to R,â-unsaturated aldehydes, without giving the cyclic trimer
of the aldehyde; this was noteworthy in light of the trimerization
of other aldehydes in the presence of MTO.10 Thus we have
investigated DA reactions involving the activation of R,â-
unsaturated aldehydes and ketones. The data obtained show
the effectiveness of MTO in comparison with other catalysts in
organic solvents.
The Diels-Alder (DA) reaction provides an important
synthetic method for cyclization, but a catalyst is often required
for the reactions carried out in this study, since the reaction is
otherwise prohibitively slow.1 The noncatalyzed reaction in
water is much faster.2-4 Transition metal complexes are known
to give higher stereoselectivity than the spontaneous process.
Traditional catalysts are Lewis acids, such as BCl3, AlX3, TiX4,
and SnX4, but they are not without their limitations.1 These
catalysts are sensitive to water, and must often be used in nearly
stoichiometric quantity. To summarize matters, it appears that
the traditional Lewis acids bind to the oxygen atoms of the
dienophile and product; the turnover frequency decreases during
the reaction owing to product inhibition. Indeed, certain
catalysts, such as BF3, AlCl3, and TiCl4, can polymerize or
destroy the substrate.
The ideal DA catalyst should be soluble in common solvents,
capable of substantial rate accelerations, insensitive to oxygen
or water, free from product inhibition, and able to bind the
dienophile rapidly and reversibly.
The DA reaction in water without catalysts is now well
recognized.2,11 Recently, the catalysis of the DA reaction in
water by Co2+, Ni2+, Cu2+, and Zn2+ ions was reported; the
interaction between a specifically designed dienophile and
cyclopentadiene was investigated.12 This study established the
effectiveness of MTO as a catalyst for the DA reaction in water.
Several nontraditional catalysts meet some of these re-
quirements.5-8 One example is [CpFe(CO)P(OMe)3(THF)]BF4,
but it cannot tolerate oxygen and is not soluble in all of the
useful solvents.5 Catechol boron bromide or [FeCp2]PF6 can
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
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(5) Olson, A. S.; Seitz, W. J. Tetrahedron Lett. 1991, 32, 5299.
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Tetrahedron Lett. 1989, 30, 1357.
Results
Reactions in Chloroform. The reactions in chloroform were
carried out with 10 mmol each of the diene and the dienophile
in 20 mL of chloroform at room temperature. The MTO catalyst
(9) Becker, B. F.; Fritz, H. P. Chem. Ber. 1975, 108, 3292.
(10) Zhu, Z.; Espenson, J. H. Submitted for publication.
(11) Larsen, S. D.; Grieco, P. A. J. Am. Chem. Soc. 1985, 107, 1768.
(12) Otto, S.; Bertoncin, F.; Engberts, J. B. F. N. J. Am. Chem. Soc.
1996, 118, 7702.
(7) Hollis, T. K.; Robinson, N. P.; Bosnich, B. Organometallics 1992,
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(8) Mukaiyama, T.; Watanabe, K.; Shiina, I. Chem. Lett. 1995, 1.
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