1-[(E)-2-Arylethenyl]-2,2-diphenylcyclopropanes: Kinetics and Mechanism of Rearrangement to Cyclopentenes

Kinetic measurements for the thermal rearrangement of 2,2-diphenyl-1-[(E)-styryl] cyclopropane (22a) to 3,4,4-triphenylcyclopent-1-ene (23a) in decalin furnished DHisom(sic) = 31.0 +/- 1.2 kcal mol(-1) and Delta S-isom((sic)) = -6.0 +/- 2.6 e.u. The lowering of Delta H-(sic) by 20 kcal mol(-1), compared with the rearrangement of the vinylcyclopropane parent, is ascribed to the stabilization of a transition structure (TS) with allylic diradical character. The racemization of (+)-(S)-22a proceeds with Delta H-rac((sic)) = 28.2 +/- 0.8 kcal mol(-1) and Delta S-rac((sic)) = 5 +/- 2 e.u., and is at 150 degrees 106 times faster than the rearrangement. Seven further 1-(2-arylethenyl)-2,2-diphenylcyclopropanes 22, (E)- and (Z)-isomers, were synthesized and characterized. The (E)-compounds showed only modest substituent influence in their k(rac) (at 119.4 degrees) and k(isom) (at 159.3 degrees) values. The lack of solvent dependence of rate opposes charge separation in the TS, but a linear relation of log k(rac) with log p.r.f., i.e., partial rate factors of radical phenylations of ArH, agrees with a diradical TS. The ring-opening of the preponderant s-trans-conformation of 22 gives rise to the 1-exo-phenylallyl radical 26 that bears the diphenylethyl radical in 3-exo-position, and is responsible for racemization. The 1-exo-3-endo-substituted allylic diradical 27 arises from the minor s-gauche-conformation of 22 and is capable of closing the three- or the five-membered ring, 22 or 23, respectively. The discussion centers on the question whether the allylic diradical is an intermediate or merely a TS. Quantum-chemical calculations by Houk et al. (1997) for the parent vinylcyclopropane reveal the lack of an intermediate. Can the conjugation of the allylic diradical with three Ph groups carve the well of an intermediate?