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 Inter

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• 作者：W. von E. Doering ; Juris L. Ekmanis ; Kevin D. Belfield ; F.-G. Kl&auml ; rner ; and Bernd Krawczyk
• 刊名：Journal of the American Chemical Society
• 出版年：2001
• 出版时间：June 13, 2001
• 年：2001
• 卷：123
• 期：23
• 页码：5532 - 5541
• 全文大小：137K
• 年卷期：v.123,no.23(June 13, 2001)
• ISSN：1520-5126

文摘

A series of cyclobutanes substituted 1,2- by polyenes of increasing radical-stabilizing power hasbeen 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) andcycloreversion (fragmentation) to 1 when heated in the range 72.1-118.2 ges/entities/deg.gif">C: anti-2 ges/entities/rarr.gif"> syn-2, ges/gifchars/Delta.gif" BORDER=0 >H^{ges/entities/thermod.gif">} = 30.3kcal mol^-1, ges/gifchars/Delta.gif" BORDER=0 >S^{ges/entities/thermod.gif">} = 0.2 cal mol^-1 K^-1; anti-2 ges/entities/rarr.gif"> 1, ges/gifchars/Delta.gif" BORDER=0 >H^{ges/entities/thermod.gif">} = 32.8 kcal mol^-1, ges/gifchars/Delta.gif" BORDER=0 >S^{ges/entities/thermod.gif">} = +8.0 cal mol^-1 K^-1.Agreement with an enthalpy of activation predicted by assuming full allylic stabilization in a hypotheticaldiradical intermediate is good. An example of further activation by a radical-stabilizing group is manifestedby 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 43.6ges/entities/deg.gif">C. In both systems 2 and 4, volumes of activation for stereochemical interconversion and cycloreversionhave been determined and found to be essentially identical within experimental uncertainties, ges/gifchars/Delta.gif" BORDER=0 >V^{ges/entities/thermod.gif">} = +10.2± 1.0 and +12.6 ± 1.4 cm³ mol^-1, respectively (weighted means). These strongly positive values are consistentwith the rate-determining step being the first bond-breaking, while the near identity of the volumes of activationargues against the indispensable second bond-breaking being a determining factor in fragmentation. Theseresults 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 byenthalpy and entropy, as in the transition state model, but by entropy alone.