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?

• Published in: Journal of the American Chemical Society 2001-06, Vol.123 (23), p.5532-5541
• Main Authors: Doering, W. von E, Ekmanis, Juris L, Belfield, Kevin D, Klärner, F.-G, Krawczyk, Bernd
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja004128s

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.