Trivalent Metal Lewis Acids Activate CO 2 in Transfer Hydrogenations

Abstract

Using γ-terpinene as a bio-derived H2 surrogate, trivalent metal MX3 (M = Al, Ga, In, Yb, X = Cl, OTf ) Lewis acids (LAs) catalyze CO2 hydrogenation to formate, yielding p-cymene as the by-product. The resulting formate produces up to 91% N-formamides in tandem hydrogenation-coupling reactions and up to 95% heterocycles, including azoles, via further in situ transfer formylation to ortho-substituted anilines and cyclization at 130 °C and 4 bar. But In(OTf )3 and a Lewis base fail to abstract a hydride from γ-terpinene. Unlike other LAs and transfer hydrogenation catalysts that induce hydride abstraction from 1,4-cyclohexadiene(s) over B(C6F5)3, alkali earth or noble metals, MX3 LAs activate CO2, so CO2 can directly accept a hydride from γ-terpinene during formate synthesis, as shown by density functional theory calculations. This triple role of MX3 LAs in promoting (1) CO2 activation, (2) tandem coupling reactions, and (3) transfer formylation at low pressure paves the way for sustainable CO2 hydrogenation processes, leveraging bio-derived H2 surrogates to develop efficient carbon capture and utilization systems and to synthesize valuable compounds from renewable feedstocks.