Vinylogous Electrochemical Carboxylation of Dienones.

Among the reconversion strategies of carbon dioxide, its electrochemical fixation as a C1 synthon onto organic scaffolds (electrochemical carboxylation) displays an enormous synthetic potential and is thus receiving increasing attention. Examples of electrochemical carboxylation are reported via the activation of C-X (X = halide or pseudo-halide), C-H, or C-C bonds, or of unsaturated systems comprising CC, CN, and CO bonds. In this work, we report the electrochemical carboxylation of dienones, achieving the synthesis of 6-oxo-carboxylic acid derivatives in useful yields up to 56%. We show that electrochemical reduction of dienones drives their umpolung reactivity as nucleophiles toward carbon dioxide, promoting a δ-selective electrochemical carboxylation. The electrochemical reactivity was expanded to polyconjugated carboxylic derivatives such as α,β,γ,δ-unsaturated esters, thioesters, and amides. This work provides to the best of our knowledge the first example of vinylogous electrochemical reactivity in extended conjugated carbonyls involving carbon dioxide as the partner reactant. The reactivity and regioselectivity are rationalized through a mechanistic investigation that integrates cyclic voltammetry analysis and DFT calculations: this supports the reactivity with the CO electrophile of nucleophilic doubly reduced species of the parent compound, preferentially occurring at the vinylogous position. The role of CO in this process is also discussed. Considering the large synthetic versatility of carboxylic acids, our new protocol may become a useful tool for accessing novel synthons in drug design and general scientific development. We believe that these results will provide a guide for future studies on CO fixation.