Wv. Doering et Yh. Wang, CryptoCope rearrangement of 1,3-dicyano-5-phenyl-4,4-d(2)-hexa-2,5-diene. Chameleonic or centauric?, J AM CHEM S, 121(47), 1999, pp. 10967-10975
The "centauric" model for evaluation of the effect of radical-stabilizing p
erturbations on the Cope rearrangement conjectures independent action of su
bstituents that make conflicting electronic demands on the two halves of th
e transition region. The present test of this conjecture compares 1,3-dicya
no-[5-protio]-hexa-1,5-diene (1(H)) and 2-phenylhexa-1,5-diene, with 1,3-di
cyano-5-phenylhexa- 1,5-diene (1(Ph)). Thermochemical information required
for a proper comparison includes new data of the van't Hoff type on conjuga
tive interaction of cyano with the carbon-carbon double bond, reevaluation
of the radical-stabilizing potential of the cyano group on secondary and al
lyl radicals, comparison with the (reevaluated) stabilizing effect of cyano
in "nodal" positions of the Cope transition region, and determination of t
he enthalpy and entropy of activation of the cryptoCope rearrangement of ot
herwise Cope-incompetent, thermodynamically more stable hexa-2,5-dienes rel
ated by prototropy to the Cope-competent hexa-1,5-dienes above. The "chamel
eonic" model is concluded to be unsatisfactory, while the "centauric" is in
better, if not complete, accord with experiment.