Carbonylative approaches to α,β-unsaturated acyl radicals and α-ketenyl radicals.: Their structure and applications in synthesis

The carbonylation of vinyl radicals gives alpha,beta-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 alpha,beta-unsaturated acyl radicals and the isomeric alpha-ketenyl radicals are not canonical forms, but are isomeric species that undergo interconversion. Calculations also indicate that alpha,beta-unsaturated acyl radicals are more stable than alpha-ketenyl radicals, whereas alpha-ketenyl radicals containing a heteroatom, such as Si, Ge, and Sn, at alpha-position are more stable than the corresponding alpha,beta-unsaturated acyl radicals. This represents a promising resource for developing new synthetic applications that involve the use of the alpha-ketenyl radicals. Indeed, following the prediction by calculation, we succeeded in trapping of a tin-attached alpha-ketenyl radical by imines and amines in an intramolecular fashion. We were also able to achieve the intermolecular trapping of alpha-ketenyl radicals, providing a new method for alkyne carbonylation by hybrid radical/ionic reactions.