Electronic effects in asymmetric hydroboration

To determine whether electronic effects are operative in asymmetric hydroboration, a series of para-substituted 2-aryl-1-propenes were prepared and reacted with four asymmetric borane reagents. A significant correlation between the electronic nature of the para-substituent and the degree of asymmetric induction was observed only for a chloroborane–ether complex, not for any of several simple alkylboranes. A quantitative analysis of the relative reactivities is also given.