Asymmetric Counteranion‐Directed Transition‐Metal Catalysis: Enantioselective Epoxidation of Alkenes with Manganese(III) Salen Phosphate Complexes

Abstract

Inspired by pioneering contributions on chiral Bronsted acid mediated reactions and our own studies in aminocatalysis, we are currently exploring asymmetric counteranion-directed catalysis (ACDC) as a general strategy for asymmetric synthesis. According to this concept, catalytic reactions that proceed via cationic intermediates can be performed highly enantioselectively by the incorporation of a chiral counteranion into the catalyst. After our initial proof of concept with organocatalytic transfer hydrogenations and epoxidations, ACDC has recently been extended to transition-metal catalysis with Toste s gold-catalyzed allene cyclizations, and our palladium-catalyzed Tsuji–Trost-type a-allylation of aldehydes. The further exploration of the potential of ACDC in transition-metal catalysis, especially applied to redox reactions, is of great interest and significance. Here we report a highly enantioselective epoxidation of olefins that is catalyzed by a chiral ion pair consisting of an achiral Mn–salen cation and a chiral phosphate counteranion. Stimulated by an important contribution from Kochi et al. , Jacobsen and Katsuki have significantly advanced the catalytic asymmetric epoxidation of unfunctionalized alkenes by introducing chiral Mn–salen catalysts. These complexes display a broad substrate scope although certain olefin classes still fail to be converted with high enantioselectivity. Interestingly, cationic Mn–salen complexes are C2-symmetrical and inherently chiral—even when the salen ligand itself is achiral. In case of the Jacobsen–Kastuki epoxidation, the chiral backbone of the salen ligand fixes the complex in one of the two enantiomorphic confirmations. The neutral donor ligands typically added increase reactivity and enantioselectivity by displacing the apically coordinated anion of the Mn complex. Chiral neutral donor ligands such as sparteine and chiral N-oxides have been used in combination with an achiral Mn–salen complex and shown to shift the equilibrium of enantiomeric conformational isomers of the cationic metal complex to one side. Reasonably good enantioselectivities have been achieved in the corresponding epoxidations. We hypothesized that a chiral counteranion should also be able to induce a preference for one of the two enantiomorphic conformations. Specifically, chiral binol-derived phosphate anions are ideally suited for our purposes, because in addition to possibly inducing one enantiomorphic conformation of the cationic complex, these ions may also amplify the chiral microenvironment around the metal center with suitable substituents at the 3,3’-positions (Figure 1). Overall, this may lead to a new type of chiral Mn–salen catalyst with unique properties.