Abstract
For over a century, ketenes have intrigued organic chemists with their unusual physical properties and unique spectrum of chemical reactivity; it is not an exaggeration to say that they represent one of the keystone reactive intermediates of physical organic chemistry. Their synthetic potential, originating in part from their high reactivity, has proven to be powerful as well, from the Staudinger synthesis of β-lactams to the photochemical Wolff rearrangement. It is perhaps no surprise that the past several decades have also shown that ketenes are excellent precursors for catalytic asymmetric reactions, creating chiral centers mainly through addition across their CC bonds. Due to their electrophilic nature, ketenes readily react with nucleophiles to form zwitterionic enolates that form the basis for much asymmetric chemistry. Through catalytic, asymmetric methodology, researchers have successfully utilized ketenes in a range of reaction manifolds, including [2+2] and [4+2] cycloadditions, reductive couplings, nucleophilic SN2 substitutions, as well as both electrophilic and nucleophilic additions. In this review, these reaction pathways are discussed in terms of such diverse reactions as ketene alcoholysis and aminolysis, α-halogenation, α-amination, α-hydroxylation, and the formation of β-, γ-, and δ-lactones and β- and γ-lactams.