Radical carboxylations of iodoalkanes and saturated alcohols using carbon monoxide

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 amide s and the second method, remote carboxylation, converts saturated alcohols into delta -lactones. Both methods rely upon radical carbonylation chemistr y to introduce carbon monoxide, but the key steps are conceptually differen t. 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 delta -carbon is successfully coupled with the carbonylation-oxidation seq uence. Carboxylations of alkyl halides by transition metal catalyzed method s are often problematic because of the inherent weakness of alkyl-metal bon ds. Existing methods for carbonylative delta -lactone synthesis using trans ition metal catalysts are limited to unsaturated alcohols. Thus, these two radical carboxylation methods nicely complement existing transition metal c atalyzed carboxylations.