Asymmetric Bifunctional Catalysis Using Heterobimetallic and Multimetallic Systems in Enantioselective Conjugate Additions

Abstract

The conjugate addition of a nucleophile to an α,β-unsaturated carbonyl compound (commonly referred to as Michael addition) is a classic carbon-carbon bond-forming reaction in organic synthesis [1]. It is also an important transformation for the formation of carbon-heteroatom bonds. As these reactions often result in the creation of a stereogenic center, the importance of controlling the stereoselectivity of such conjugate addition reactions has sparked intense and impressive developments by chemists in both academia and industry. The use of stoichiometric amounts of chiral reagents, ligands, and auxiliaries has led to the development of several asymmetric Michael reactions [2]. However, the need to develop more effective and atom-economic methods has led to the utilization of asymmetric catalysis in enantioselective conjugate additions (ECAs) [3]. In Nature, natural catalysts -enzymes -mainly use general base and acid catalysis to achieve carbon-carbon bond-forming reactions, with almost exclusive stereocontrol [4]. This is accomplished in the active site of the enzyme by the perfect arrangement and dual activation of the electrophilic acceptor and nucleophilic donor substrate. Sometimes, a metallic cofactor in cooperation with the neighboring amino acids of the active site is necessary to achieve C−C bond-formation. In this context, type II aldolase enzymes employ a Zn ion (which is a Lewis acid) to activate dihydroxyacetone phosphate, in cooperation with an amino acid residue (which acts as a Brønsted base) to generate, regioselectively, an enolate (Figure 4.1a) [4]. At the same time, the electrophilic aldehyde acceptor is a Brønsted acid activated by an amino acid residue, such that stereoselective C−C bond-formation will occur with excellent enantioselectivity. In Nature, millions of years of evolution were required order to achieve this perfectly orchestrated transformation in an aqueous medium. Consequently, in taking their inspiration from Nature, research chemists have developed asymmetric catalysts that may reach impressive levels of enantioselectivity in carbon-carbon bond-forming reactions [5]. One way to achieve such enantioselectivity is to employ a strategy similar to that utilized by type II aldolase enzymes, which includes asymmetric bifunctional catalysis and metal activation (Figure 4.1b) Catalytic Asymmetric Conjugate Reactions. Edited by Armando Córdova

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