Thermal reactions of anti- and syn-dispiro[5.0.5.2]tetradeca-1,8-dienes: Stereomutation and fragmentation to 3-methylenecyclohexenes. Entropy-dictated product ratios from diradical intermediates?

A series of cyclobutanes substituted 1,2- by polyenes of increasing radical-stabilizing power has been investigated to test the proposition that stabilization energies obtained independently from apposite, cis,-trans geometric isomerizations can be successfully transferred to another system, in this paper, cyclobutanes. The first member of the series, 3-methylenecyclohexene (1), is photodimerized to anti- and syn-dispiro[5.0.5.2]-tetradeca-1,8-dienes (anti-2 and syn-2), which undergo stereomutation (stereochemical interconversion) and cycloreversion (fragmentation) to 1 when heated in the range 72.1-118.2 °C: anti-2 → syn-2,ΔH‡ = 30.3 kcal mol^-1, ΔS‡ = 0.2 cal mol^-1 K^-1; anti-2 → 1, ΔH‡ = 32.8 kcal mol^-1, ΔS‡ = +8.0 cal mol^-1 K^-1. Agreement with an enthalpy of activation predicted by assuming full allylic stabilization in a hypothetical diradical intermediate is good. An example of further activation by a radical-stabilizing group is manifested by the ∼20 000-fold acceleration in rate shown by the system 1-phenyl-3-methylenecyclohexene (3) and anti- and syn-2,9-diphenyldispiro[5.0.5.2]tetradeca-1,8-dienes (anti-4 and syn-4), measured, however, only at 43.6 °C. In both systems 2 and 4, volumes of activation for stereochemical interconversion and cycloreversion have been determined and found to be essentially identical within experimental uncertainties, ΔV‡= +10.2 ± 1.0 and +12.6 ± 1.4 cm³ mol^-1, respectively (weighted means). These strongly positive values are consistent with the rate-determining step being the first bond-breaking, while the near identity of the volumes of activation argues against the indispensable second bond-breaking being a determining factor in fragmentation. These results are consistent with the theoretically based construct of Charles Doubleday for the paradigm, cyclobutane, in which the ratio between two channels of exit from a "generalized common biradical" is not controlled by enthalpy and entropy, as in the transition state model, but by entropy alone.