The DMSO-Catalyzed CO₂ Reduction with 9-BBN as a Source for the Selective N-Formylation, Room Temperature N-Methylation of Amines, N-Hydroxymethylation of Heterocycles, and C–N Couplings

The reduction of CO₂ (1 atm) with 9-BBN in DMSO in the presence of amines at 90 °C selectively gave <i>N</i>-formamides in good to excellent yields. This method works with a range of substrates, including heterocyclic secondary amines and primary and secondary aliphatic and aromatic amines. Remarkably, the same CO₂ (1 atm) reduction in the presence of a catalytic amount of DMSO and aromatic primary and secondary amines in toluene at room temperature afforded <i>N</i>-methylamine products isolated in 44-97% yields, representing the first room temperature method using hydroborane. Interestingly, the reduction of CO₂ (1 atm) in the presence of carbazole and indole derivatives gave the N-hydroxymethylated products in good to excellent yields, representing the hitherto unknown direct conversion of CO₂ under mild conditions. More interestingly, the addition of indole or <i>N</i>-methylindole to the reaction mixture containing the hydroxymethylated products at 60 °C resulted in the formation of new C-N-coupled products. The proposed mechanism, based on several control experiments, suggests that the transfer of a formyl group from the formoxyborane occurs, which then leads to the N-methylation after reduction with 9-BBN. Another possible route is the nucleophilic attack of formoxy- or methoxyborane on the aminoborane formed in the reaction mixture.