REARRANGEMENT AND CYCLOREVERSION OF DIELS-ALDER ADDUCTS OF CYCLIC 1,3-DIENES TO NORBORNADIENES - A NOVEL HOME-COPE REARRANGEMENT - EVIDENCEFOR COMPETITIVE PERICYCLIC AND DIRADICAL PROCESSES

On thermolysis of endo,endo-5, endo,endo-17, and endo,exo-17 (the endo ,endo and endo,exo Diels-Alder adducts of cyclopentadiene 7 or dimethy lfulvene 15 to norbornadiene 8 or 7-isopropylidenenorbornadiene 20), a novel type of homo-Cope rearrangement leading to 6, 21, and 23a, resp ectively, competes with the retro-Diels-Alder reactions. According to a force-field analysis of the kinetic parameters, the competitive reac tions (rearrangement and retro-Diels-Alder reaction) of endo,endo-5 oc cur in a pericyclic fashion whereas in the isopropylidene-substituted systems stepwise processes compete with the corresponding pericyclic r eactions. Indirect experimental evidence for this assumption comes fro m a stereochemical analysis of the retro-Diels-Alder reaction in the c is-5,6-dideuterionorbornene derivatives exo-36-d(2), endo-36-d(2) and exo-39-d(2), endo-39-d(2) occurring stereospecifically in the case of exo-36-d(2), endo-36-d(2) and non-stereospecifically in the case of ex o-39-d(2), endo-39-d(2). The change in mechanism is rationalized by a different (allyl vs, pentadienyl) stabilization of the potential dirad ical intermediates in the stepwise reactions.