Experimental and Theoretical Studies of Substituent Effects on an Orbital Symmetry Forbidden Electrocyclization

The disrotatory electrocyclizations of several transient bridged o-xylylenes to form benzocyclobutenes has been investigated experimentally. Electron-withdrawing groups at the o-xylyene termini have large effects on the activation energy for this orbital symmetry forbidden reaction, while electron-donating groups have smaller effects. Ab initio quantum mechanical calculations on models of the transition structures of disrotatory electrocyclizations of butadienes reproduce these trends and have been used to develop a qualitative hypothesis to explain the unusual substituent effects found in the experimental study. © 1989, American Chemical Society. All rights reserved.