Tin-catalyzed reductive coupling of amines with CO2 and H2

Abstract

Reductive coupling of amines with CO₂ and H₂ can be catalyzed by transition metals. However, functional group (FG) tolerance is improved when using auxiliary main group hydrides (instead of H₂), which makes them more suitable for the synthesis of functionalized molecules. Replacing auxiliary main group hydrides with frustrated Lewis pairs (FLPs) can, in theory, generate comparably selective hydrides in situ from H₂ activation. Herein, we report the selective N-formylation of amines via CO₂ hydrogenation catalyzed by a series of R₃SnX (R = alkyl, X = Cl, OTf, NTf₂, ClO₄) Lewis acids, which form an FLP with the amine substrate and/or 2,4,6-collidine. FLP dihydrogen activation leads to the in situ formation of an R₃Sn–H species with excellent selectivity toward CO₂ hydrogenation over other reducible FGs. Consequently, amines containing alkenes, amides, esters, and carboxylic acids can be N-formylated with CO₂ and H₂. Increasing the steric bulk of the alkyl substituents prevents the redistribution of the R₃SnX Lewis acid to R₄Sn and R₂SnX₂, which is the primary cause of catalyst decomposition. Cy₃SnOTf reached turnovers of >300 and amine conversions of up to 100%. In addition to synthesis, isolation, and testing of key intermediates for their reactivity, we identified an off-cycle intermediate by in operando ¹H NMR spectroscopy and therefore propose a mechanistic cycle. By avoiding the use of any transition metals or auxiliary main group hydrides, our procedure opens a pathway for developing transition-metal-free CO₂ hydrogenation methods utilising hydrogen gas.