VOLUME OF CONCERT AND HEAVY-ATOM EFFECTS IN DIELS-ALDER REACTION-MECHANISMS

The role of heavy atom effects (HAE) in Diels-Alder (DA) reactions is proposed in support of a stepwise-diradical mechanism. The presence of heavy atoms in diene or dienophile causes cycloadditions that are nor mally stereospecific in the dienophile to become nonstereospecific, be cause the initially-formed singlet diradical undergoes ISC to the trip let which can rotate but cannot ring close until a second ISC back to singlet. A concerted mechanism cannot accommodate these HAE. The dimer ization of chloroprene, which had been analyzed via a dual mechanism b ased on activation-volume differences among the products, is now seen as unified in mechanism. Copyright (C) 1996 Elsevier Science Ltd