Palladium(0)‐Catalyzed Anti‐Selective Addition‐Cyclizations of Alkynyl Electrophiles

Palladium(0)/monophosphine complexes catalyze anti‐selective alkylative, arylative, and alkynylative cyclizations of alkynyl electrophiles with organometallic reagents. The remarkable anti‐selectivity results from novel oxidative addition, that is, the nucleophilic attack of electron‐rich palladium(0) on the electrophile across the alkyne followed by transmetalation and reductive elimination (“anti‐Wacker”‐type cyclization). With regard to 5‐alkynals, triphenylphosphine (PPh3)‐ligated palladium(0) catalyzes the cyclization of terminal alkynes and conjugated alkenyl‐ or alkynyl‐substituted ones to afford 2‐cyclohexen‐1‐ol and 2‐alkylidene‐cyclopentanol derivatives, respectively. For 6‐alkyl‐ or 6‐aryl‐5‐alkynals, the cyclization does not proceed with the palladium/PPh3 catalyst; however, it does proceed with palladium/tricyclohexylphosphine (PCy3), to yield the former products predominantly. Remarkably, the latter catalyst completely switches the regioselectivity in the cyclization of the conjugated diyne‐aldehydes. Notably, palladium/PPh3‐catalyzed cyclizations also proceed with other organometallics or even without them. Palladium(0)/monophosphine complexes catalyze anti‐selective arylative cyclizations of alkynyl electrophiles with organometallic reagents. Remarkably, the regioselectivity in the cyclization of conjugated diyne‐aldehydes can be controlled by a choice of the phosphine ligand, with triphenylphosphine and tricyclohexylphosphine leading to the exclusive formation of 2‐alkylidene‐cyclopentanol and 2‐cyclohexen‐1‐ol derivatives, respectively.