Enantioselective Organocatalytic Cyclopropanations. The Identification of a New Class of Iminium Catalyst Based upon Directed Electrostatic Activation

A new method for enantioselective organocatalytic cyclopropanation is described. This study outlines the identification of a new class of iminium catalyst based on the concept of directed electrostatic activation (DEA). This novel organocatalytic mechanism exploits dual activation of ylide and enal substrates through a proposed electrostatic activation and stereodirected protocol. Formation of trisubstituted cyclopropanes with high levels of enantio- and diastereoinduction is accomplished for a variety of α,β-unsaturated aldehydes and sulfonium ylides. In addition, mechanistic studies have found that this cyclopropanation reaction exhibits enantioselectivity and reactivity profiles that are in accord with the proposed DEA step.