Initiation and Termination Mode of Enyne Cross-Metathesis and Metallotropic [1,3]-Shift Controlled by Remote Substituents

The cross-metathesis of terminal alkyne and alkene using Ru-based Grubbs catalyst generally undergoes alpha-insertion. In this study, excellent control over alpha- and beta-insertion of ruthenium alkylidene into terminal alkynes has been achieved by using a substituent at a remote site from the reaction center. While the origin of this regioselectivity of insertion and metallotropic shift remains to be established, the trend indicates that the alpha-insertion with a concomitant metallotropic shift is favored with small substituents whereas the beta-insertion without a metallotropic shift is favored with bulky substituents such as tert-butyl and trialkyl silyl groups.