TORQUOSELECTIVITY IN THE ELECTROCYCLIC CONVERSION OF BENZOCYCLOBUTENES TO ORTHO-XYLYLENES

The conrotatory electrocyclic opening of benzocyclobutene to o-xylylene was studied by means of ab initio molecular orbital calculations. The theory developed earlier to predict the torquoselectivity of ring opening of 3-substituted cyclobutenes was found to be applicable. Experimentally, the ring opening of several 7-substituted benzocyclobutenes, such as the cyano, methoxycarbonyl, and formyl derivatives was examined. o-Xylylenes were obtained in which cyano or ester groups had rotated outwards, whereas the formyl group turned inwards. N,N-Dimethylbenzocyclobutene-7-carboxamide exhibited 75% inward torquoselectivity upon ring opening. A reversal of mode was seen with the 7-methyl derivatives of benzocyclobutene-7-carbonitrile and methyl benzocyclobutene-7-carboxylate in that the 7-methyl substituent manifested overwhelming outward motion.