Synthesis of Aromatic Aldehydes by C─H Formylation of Aromatics with Silyl Formates Prepared from CO2 and Hydrosilanes

Despite the recent remarkable progress in CO2 fixation reactions, the methods for the synthesis of aldehydes from CO2 are quite limited partly because of the lability of the resulting formyl group and difficulty in the controlled deoxygenative CO2 conversions leading to C─H and C─C bond formation. Here, we have developed the direct C─H formylation of electron-rich aromatics using silyl formates, prepared from CO2 and hydrosilanes, in the presence of BCl3 or BBr3. This is the first report on the direct C─H formylation of aromatics with silyl formates. Useful compounds including a biologically active compound and octaethylporphyrin were synthesized by fixing one to four CO2 molecules in a stepwise manner. DFT calculations have been done to elucidate the reaction mechanism including a dual role of BBr3 in the activation of silyl formate, HCO2SiMe2Ph, and electrophilic aromatic substitution.