Bifunctional catalyst

A catalyst with two contrasting functional components often a Lewis acid and Lewis base.

Overview

The broadest description of the most common definition of a bifunctional catalyst is a catalyst that contains Lewis acidic and Lewis basic component. This definition describes bifunctionality as a form of amphoterism. At times it more convenient to describe the catalyst as containing an electrophilic and nucleophilic, however, this description is arguably just a different phrasing of the first definition. Other times its more convent to be more specific with the description of the contrasting groups. For example bifunctional hydrogenation catalyst contain a positions which can accept and donate a proton and another which can accept and donate a hydride. Starting from the oxidized form of the catalysts the proton acceptor is a Lewis base and the hydride acceptor is a Lewis acid.

Many enzymes are believed to be bifunctional applying Lewis acids and Lewis bases to substrate as required for the given transformation.

Bifunctional catalysts have been used to facilitate a variety or reactions. As already noted common examples include hydrogenation and transfer hydrogenation catalyst. [Guan, H.; Iimura, M.; Magee, M.; Norton, J.; Zhu, G. "J. Am. Chem. Soc." 2005, "127", 7805.] These catalysts are especially good at delivering hydrogen to unsaturated polar bonds such as ketones. In this process the oxygen atom receives a proton and the partial positive carbon alpha to the oxygen receives a hydride. In direct hydrorgenation hydrogen is used as the reducing agent. In transfer hydrogenation the hydrogen is derived from a sacrificial alcohol such as isopropanol which delivers a hydrogen equivalent to the catalysts and in the process produces acetone. A number of these catalysts can preform the hydrogengenation enantioselectively producing chiral alcohols.

Bifunctional Hydrogenation Mechanisms

The Shvo catalyst has been studied exstensively and is believed to deliver hydrogen through a concerted mechanisms. Other catalysts do not deliver hydrogen through a concerted pathway but still retain the identity of the hydride. [Hashiguchi, S.;Fujii, A.; Takehera, J.; Ikariya, T.; Noyori, R. "J. Am. Chem. Soc." 1995, "117", 7562.] [Haack, K.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. "Angew. Chem. Int. Ed." 1997, "36", 285.] [ Yi, C.; He, Z. "Organometallics", 2001, "20", 3641.] [Samec, J.; Backvall, J.; Adersson, P.; Brandt, P. "Chem. Soc. Rev." 2006 "35" 237-248.] This pathway if sometimes referred to as the monohydride pathway. This can be contrasted with the dihydride pathway more common in hydrogenation catalysts which are not bifunctional. [Magee, M. Norton, J. "J. Am. Chem. Soc." 2001, 123, 1778] . [Bollock, M. "Chem. Eur. J." 2004 "10", 2366.] In theres systems the hydride and proton lose their respective identities and are isotopically scrambled in the hydrogenated product.

The concept of bifunctional catalyst has not been limited to a transition metal hydride acceptors. There is also examples of using boron as a hydride acceptor for the metal free cleavage of hydrogen and hydrorgonenation of easily reduced polar unsaturated bonds such as imines. [Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, and Douglas W. Stephan (17 November 2006) "Science" 314 (5802), 1124.]

Bifuntional hydrogenation catalysts are not limited to hydrogenation chemistry. A number of such systems have been previously studied as hydrogenation catalysts have been found to have application in the oxidation and possibly the production in of hydrogen. Other bifunctional catalysts have been developed for the express purpose of their elctrochemistry and associated interactions with hydrogen. [Curtis, C. J.; Miedaner, A.; Ciancanelli, R.; Ellis, W. W.; Noll, B. C.; Rakowski DuBois, M.; DuBois, D. L. "Inorg. Chem." 2003 "42(1)" 216-227.] [Appel, A. M.; DuBois, D. L.; Rokowski DuBois, M. "J. Am. Chem. Soc." 2005 "127(36)" 12717-12726.]

References