Transition‐Metal‐Catalyzed Cyclization of Alkynals via Oxametallacycle Intermediates
The transition-metal-catalyzed cyclization of alkynals viaoxametallacycle intermediates is a useful method for thestereoselective synthesis of cyclic allylic alcohol derivatives. The reductive cyclization reactions are catalyzed by titanium and nickel complexes using organosilanes, organoboranes, and organozincs as reducing agents. The alkylative, arylative, and alkenylative cyclization reactions are catalyzed by nickel complexes using organozincs and alkenylzirconiums. On the other hand, the recently developed rhodium-catalyzed reductive cyclization reactions of alkynals allow using dihydrogen as a reducing agent without the use of organometalloid and organometallic reagents. Furthermore, very recently, the rhodium-catalyzed acylative cyclization reactions of alkynals were accomplished by using aldehydes and acyl phosphonates as acylating reagents. Importantly, the use of the rhodium catalysts realized the enantioselective cyclization reactions of alkynals with excellent levels of enantioselection.