Enantioselective Synthesis of Axially Chiral Styrene-Carboxylic Esters by Rhodium-Catalyzed Chelation-Controlled [2+2+2] Cycloaddition.

Axially chiral styrene-carboxylic esters were synthesized in high yields with excellent enantioselectivity by the cationic rhodium(I)/H -BINAP complex-catalyzed chelation-controlled [2+2+2] cycloaddition reactions of 1,6- and 1,7-diynes with 1,3-enyne-carboxylic esters. The diastereo- and enantioselective synthesis of C symmetric axially chiral cis and trans-stilbene-dicarboxylic esters was also achieved by the double [2+2+2] cycloaddition reactions of two molecules of the 1,6-diyne with 2,3-dialkynylmaleate and 2,3-dialkynylfumarate, respectively. In these reactions, the 1,3-enyne-carboxylic esters coordinating to rhodium with a five-membered chelate were more reactive than those coordinating to rhodium with a six-membered chelate, although both chelation modes realized excellent enantioselectivity. The enantioselection mechanism of the cationic rhodium(I)-catalyzed chelation-controlled [2+2+2] cycloaddition was elucidated by DFT calculations.