BEYOND BUTADIENE - ACTIVATION PARAMETERS FOR THERMAL STEREOMUTATIONS AND CYCLOREVERSION OF CYCLOBUTANES FROM A PHOTOCHEMICAL CYCLODIMERIZATION OF THE RIGID, ALL-TRANS TRIENE THYL-2,3,4,4A,5,6-HEXAHYDRO-2-METHYLENENAPHTHALENE

Enthalpies of stabilization of polyenyl radicals, recently determined by thermal cis-trans isomerization about double bonds, are incorporate d into a model for thermal cyclodimerization (and its reverse, cyclore version) of polyene based on the ethylene-cyclobutane paradigm. This m odel accommodates the butadiene-divinylcyclobutane equilibrium satisfa ctorily. When the model is applied to polyenes of higher order, enthal pies of activation in both directions are predicted to be strikingly l owered as the order of the polyene increases. Photochemical dimerizati on at -75-degrees-C of optically pure triene (R)-3 of the title, produ ces a single dimer (of three allowed), to which a cyclobutane structur e in the anti and, tentatively, endo,endo configuration, (R,R)-4a,n,n, is assigned. The remarkable stereospecificity is ascribed to an inter mediate excimer, in which maximum overlap and minimal steric repulsion are hypothetically achieved in either of the two incipiently anti arr angements. Irradiation of racemic (R,S)-3 at -75-degrees-C produces tw o of six possible isomers, now racemic 4a,n,n and racemic 4a,n,x. Each of the photochemically favored cyclobutanes begins to equilibrate abo ve -20-degrees-C (very rapidly at 20-degrees-C and above) with the oth er two stereoisomers in its set. From rates measured between -22.7 and 0.0-degrees-C, Eyring parameters, DELTAH(double dagger) = 21.2 kcal/m ol and DELTAS(double dagger) = 2.7 eu, are obtained. In the temperatur e range of 0.0 to 22.0-degrees-C, the mixture of dimers, continuously maintained at equilibrium, undergoes thermal, [2 + 2] cycloreversion t o the triene (R)-3 with Eyring parameters, DELTAH(double dagger) = 24. 2 kcal/mol and DELTAS(double dagger) = 5.8 eu. Although syn-anti rearr angement competes with cycloreversion in divinylcyclobutanes, the gap in enthalpy of activation between the two paths is significantly large r in these dibutadienylcyclobutanes.