Radical carboxylations of iodoalkanes and saturated alcohols using carbon monoxide

Abstract

This review covers two radical carboxylation methods using carbon monoxide, both of which were developed by our group. The first method, atom transfer carbonylation, converts alkyl iodides into carboxylic acid esters or amides and the second method, remote carboxylation, converts saturated alcohols into δ-lactones. Both methods rely upon radical carbonylation chemistry to introduce carbon monoxide, but the key steps are conceptually different. The first method utilizes an atom transfer reaction from an alkyl iodide to an acyl radical leading to an acyl iodide and the latter employs a one-electron oxidation reaction to convert an acyl radical into an acyl cation. The iodine atom transfer carbonylation process is reversible and therefore highly inefficient unless it is performed in concert with an ionic system to shift the equilibrium in the direction of an acyl iodide. In the latter process, a 1,5-translocation scheme to shift the radical from oxygen to the δ-carbon is successfully coupled with the carbonylation–oxidation sequence. Carboxylations of alkyl halides by transition metal catalyzed methods are often problematic because of the inherent weakness of alkyl–metal bonds. Existing methods for carbonylative δ-lactone synthesis using transition metal catalysts are limited to unsaturated alcohols. Thus, these two radical carboxylation methods nicely complement existing transition metal catalyzed carboxylations.