Beyond Butadiene: Activation Parameters for Thermal Stereomutations and Cycloreversion of Cyclobutanes from a Photochemical Cyclodimerization of the Rigid, All-Trans Triene 4a-Methyl-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 incorporated into a model for thermal cyclodimerization (and its reverse, cycloreversion) of polyene based on the ethylene-cyclobutane paradigm. This model accommodates the butadiene-divinylcyclobutane equilibrium satisfactorily. When the model is applied to polyenes of higher order, enthalpies of activation in both directions are predicted to be strikingly lowered as the order of the polyene increases. Photochemical dimerization at −75 °C of optically pure triene (R)-3 of the title, produces a single dimer (of three allowed), to which a cyclobutane structure in the anti and, tentatively, endo,endo configuration, (R,R)-4_a,n,n, is assigned. The remarkable stereospecificity is ascribed to an intermediate excimer, in which maximum overlap and minimal steric repulsion are hypothetically achieved in either of the two incipiently anti arrangements. Irradiation of racemic (R,S)-3 at −75 °C produces two of six possible isomers, now racemic 4_a,n,n and racemic 4_a,n,x. Each of the photochemically favored cyclobutanes begins to equilibrate above −20 °C (very rapidly at 20 °C and above) with the other two stereoisomers in its set. From rates measured between °22.7 and 0.0 °C, Eyring parameters, ΔH^‡ = 21.2 kcal/mol and ΔS^‡ = 2.7 eu, are obtained. In the temperature range of 0.0 to 22.0 °C, the mixture of dimers, continuously maintained at equilibrium, undergoes thermal, [2 + 2] cycloreversion to the triene (R)-3 with Eyring parameters, ΔH^‡ = 24.2 kcal/mol and ΔS^‡ = 5.8 eu. Although syn-anti rearrangement competes with cycloreversion in divinylcyclobutanes, the gap in enthalpy of activation between the two paths is significantly larger in these dibutadienylcyclobutanes.