Enantioselective Synthesis of Alkyne-Substituted Quaternary Carbon Stereogenic Centers through NHC−Cu-Catalyzed Allylic Substitution Reactions with (i-Bu)2(Alkynyl)aluminum Reagents

A catalytic enantioselective method for the formation of alkyne-substituted all-carbon quaternary stereogenic centers is reported. Additions of alkynylaluminums to alkyl-, aryl-, carboxylic ester-, or silyl-substituted allylic phosphates are promoted by 1.0−5.0 mol % loadings of NHC−Cu complexes derived from air-stable and commercially available CuCl2·2H2O. The requisite Al-based reagents are prepared through treatment of the corresponding aryl-, heteroaryl-, alkyl-, or alkenyl-substituted terminal alkynes with diisobutylaluminum hydride in the presence of 5.0 mol % Et3N at ambient temperature. The desired 1,4-enynes are obtained in up to 98% yield and >99:1 enantiomeric ratio. Selected Au-catalyzed cyclizations involving the alkyne unit of the enantiomerically enriched products are presented as a demonstration of the method’s utility in chemical synthesis.