Mechanism of the Diels—Alder Reactions of Butadiene

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Abstract
Kinetic data on the pyrolysis of 1,5‐cycloöctadiene to give butadiene and 4‐vinyl cyclohexene by parallel paths are analyzed and shown to be quantitatively consistent with a common biradical precursor, the octadien−2,6,‐diyl−1,8 biradical. It is further shown that these data are in excellent agreement with independent studies of the reverse reactions; pyrolysis of 4‐vinyl cyclohexene to butadiene and the dimerization of butadiene. A fourth study of the pyrolysis, in solution of 1,2‐divinyl cyclobutane to give butadiene, 1,5‐cycloöctadiene, 4‐vinyl cyclohexene, and cis—trans isomerization is shown to fit the same scheme with only minor modifications in two of the rate constants. The analysis lends support to the 12.6‐kcal assignment of the allyl stabilization energy. It also resolves an apparent anomaly in the rate parameters for the reverse Diels—Alder pyrolyses of cyclohexene, 3‐methyl cyclohexene, and 4‐vinyl cyclohexene. Arrhenius parameters for all the elementary rate constants are assigned and shown to be reasonable compared to similar processes. Important rotation‐controlled rates in the scheme are examined and conclusions drawn about the contributions of different rotomeric forms of the biradical.