Reduction of CO2 with ammonia borane and selective formylation of amines in the presence of imidazolium halides†

Abstract

The formylation of N-methylaniline (1a) with CO₂ and ammonia borane, NH₃·BH₃ (AB) produced selectively N-methyl-N-phenylformamide (3a) in the presence of the halide imidazolium salts ([IL]X). In contrast, a mixture of two products, formylated 3a and methylated 2a (N,N-dimethylaniline), was obtained in the presence of non-halide imidazolium salts, containing BF₄⁻, PF₆⁻ or OS (octyl sulfate) anions. Reactions of other primary and secondary amines with AB and CO₂ carried out in the presence of halide salt, [dbim]Cl (dbim = 1,3-dibutyl imidazolium cation) selectively lead to the formation of formamides as the only products. The effect of ionic liquids on the selectivity of amine formylation was explained by their ability to stabilize different forms of boron formates. In the presence of halide salts, the reaction of AB with CO₂ results in the formation of BH(OCHO)₂ which is responsible for the formylation of amines to formamides. In contrast, the formation of a mixture of boron formates, BH_n(OCHO)_3−n (n = 1, 2), in the presence of non-halogenated imidazolium salts results in the production of a mixture of formylated and methylated amine derivatives.