Carbonylative Approaches to α,β-Unsaturated Acyl Radicals and α-Ketenyl Radicals. Their Structure and Applications in Synthesis

Abstract

Abstract The carbonylation of vinyl radicals gives α,β-unsaturated acyl radicals. This transformation was successfully applied to tandem radical reactions, resulting in assembling three and four components. In these reactions, both halogen abstraction from vinyl halides and hetero atom radical additions to alkynes are used to generate the parent vinyl radicals. Ab initio calculations and density functional methods predict that α,β-unsaturated acyl radicals and the isomeric α-ketenyl radicals are not canonical forms, but are isomeric species that undergo interconversion. Calculations also indicate that α,β-unsaturated acyl radicals are more stable than α-ketenyl radicals, whereas α-ketenyl radicals containing a heteroatom, such as Si, Ge, and Sn, at α-position are more stable than the corresponding α,β-unsaturated acyl radicals. This represents a promising resource for developing new synthetic applications that involve the use of the α-ketenyl radicals. Indeed, following the prediction by calculation, we succeeded in trapping of a tin-attached α-ketenyl radical by imines and amines in an intramolecular fashion. We were also able to achieve the intermolecular trapping of α-ketenyl radicals, providing a new method for alkyne carbonylation by hybrid radical/ionic reactions.