Tuneable reduction of CO2 – organocatalyzed selective formylation and methylation of amines

Abstract

The N-formylation and N-methylation of amines with carbon dioxide (CO₂) are important types of transformations which give access to a wide range of key intermediates and compounds. Efficient catalytic methods for both reactions are rare because of the challenge of controlling selectivity. Herein, we report the environmentally benign organocatalyzed N-formylation and N-methylation of primary and secondary amines using CO₂ as the C1 source in the presence of hydrosilanes as the reductant. Readily available methyltriphenylphosphonium methylcarbonate has been proven to be an efficient catalyst for the N-methylation of amines and CO₂ in the presence of polymethylhydrosiloxane (PMHS) under mild conditions. In contrast, the selective N-formylation is achieved in the presence of trimethoxysilane as the reductant. In both transformations, a wide range of substrates were selectively converted. 15 primary and secondary amines were N-methylated and N-formylated and the corresponding products were obtained in yields of up to 98% and 94%, respectively. Moreover, benzimidazoles are accessible from diamines and CO₂ in a one-pot reaction in yields of up to 83%. Mechanistic investigations revealed different reaction pathways for N-methylation and N-formylation depending on the reductant as well as the activation of CO₂ by the catalyst.