Atom-efficient
processes that occur via addition, redistribution,
or removal of hydrogen underlie many large-volume industrial processes
and pervade all segments of chemical industry. Although carbonyl addition
is one of the oldest and most broadly utilized methods for C–C
bond formation, the delivery of nonstabilized carbanions to carbonyl
compounds has relied on premetalated reagents or metallic/organometallic
reductants, which pose issues of safety and challenges vis-à-vis
large-volume implementation. Catalytic carbonyl reductive couplings
promoted via hydrogenation, transfer hydrogenation, and hydrogen autotransfer
allow abundant unsaturated hydrocarbons to serve as substitutes to
organometallic reagents, enabling C–C bond formation in the
absence of stoichiometric metals. This Perspective (a) highlights
past milestones in catalytic hydrogenation, hydrogen transfer, and
hydrogen autotransfer; (b) summarizes current methods for catalytic
enantioselective carbonyl reductive couplings; and (c) describes future
opportunities based on the patterns of reactivity that animate transformations
of this type.