A ligand-assisted proximity effect allows for H2-driven copper hy-dride chemistry under mild conditions

A bifunctional copper(I)/N-heterocyclic carbene complex bearing a highly basic 2-iminopyridine subunit effects a variety of copper(I) hydride-based reductive transformations at low H2 pressure. The bifunctional catalyst allows for the first time to employ only catalytic amounts of alkoxide additive and is the copper(I) catalyst with the highest reactivity towards H2 re-ported so far. We can demonstrate that efficient alkyne semihydrogenations, conjugate reductions as well as 1,2-reductions of carbonyl compounds can be carried out at very low H2 pressure. The new protocol circumvents the need for previously required high-pressure equipment. At the same time, use of the new iminopyridine-based ligand allows for the tolerance of an unprecedented variety of functional groups in the realm of copper(I)-catalyzed hydrogenations. A possible working model featuring an iminopyridine-mediated proximity effect that coordinates the key alkoxide close to the reactive cop-per(I) center is proposed to account for the observed significant rise in reactivity. Mechanistic studies directed at support-ing this current working hypothesis are presented.