Deciphering the Origin of Enantioselectivity on the Cis-Cyclopropanation of Styrene with Enantiopure Di-chloro,Di-gold(I)-SEGPHOS Carbenoids Generated from Propargylic Esters.
The stereoselective synthesis of cis-disubstituted cyclopropanes by the Au(I)/PPh3-catalyzed cycloaddition of propargylic esters and styrene has been studied using density functional theory calculations. The computed mechanistic scheme involves the rate-limiting 1,2-rearrangement of the propargylic ester with the π-coordinated gold complex, followed by the (2 + 1)-cheletropic reaction of styrene with the alkenyl-Au(I) carbene intermediate to afford the cis-disubstituted cyclopropane derivative in a high cis/trans diastereomeric ratio. With a ( R)-di-chloro,di-gold-DTBM-SEGPHOS complex as the catalyst, computations are consistent with a rate-determining (2 + 1)-cheletropic reaction, in which facial discrimination is proposed to result from a combination of subtle steric and electronic effects in the SiRe facial approach transition structure, which favor the formation of the cis-cyclopropane diastereomer of 1 R,2 S absolute configuration, as experimentally observed.