Multicomponent Catalysis for Reductive Bond Formations

New concepts are outlined for reductive C–C- C–H-bond formations catalyzed by early transition metals in low oxidation states based on the use of chlorosilanes, silicon hydrides, or related oxophilic additives as mediators of the catalytic cycles. Amongst other examples, these new protocols account for the first instances of intramolecular carbonyl coupling reactions catalyzed by titanium, Nozaki–Hiyama–Kishi reactions catalyzed by chromium, pinacol couplings catalyzed by titanium, vanadium, or samarium, and reactions of enals or epoxyalkenes catalyzed by "Cp2Ti". A similar approach also permits catalysis of Barton–McCombie reductions by Bu3SnH.