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Thermal rearrangements of cis-2-alklcyclopropanecarboxylates and methyl 2-and 3-pentenoates Chiu, Norman Wing Kwai

Abstract

Seven alkyl substituted methyl cyclopropanecarboxylates having an alkyl group on C-2 cis to the ester group have been found to rearrange thermally to their corresponding if, [formula omitted] esters at temperatures ranging from 240° to 300° . The corresponding trans isomers are unaffected under identical reaction conditions. It is proposed that the rearrangement takes place by an intramolecular 1,5-hydrogen transfer via a six-membered cyclic transition state. The kinetics for these reactions have been studied and show first-order plots which are linear to 70% reaction. The heat of activation [formula omitted] was found to be between 25 to 38 kcal/ mole and the entropy of activation [formula omitted] was found to be between - 8 and -37 eu from measurements made over a 20° range. This and other information fully support a cyclic hydrogen transfer mechanism. Four methyl cis-2-pentenoates have been found to undergo both geometric and positional thermal isomerizations to yield equilibrium mixtures of 2- and 3-pentenoates at temperatures ranging from 240° to 300°. The 3-pentenoates produced from the corresponding trans-2-pentenoates were found to be formed via the cis - 2-pentenoate3. Studies using β-isopropylacrylate and methacrylate showed that the rate of positional isomerization was much faster than that of geometric isomerization. For these systems it was therefore possible to study the kinetics of the positional isomerism in the absence of competing geometric isomerization, The kinetic data provided first-order plots. The heat of activation [formula omitted] was found to be between 18 and 38 kcal/mole and the entropy of activation [formula omitted] was found to be between -8 to -50 eu. An intramolecular 1,5-hydrogen transfer mechanism involving a six-membered cyclic transition state similar to that for the cyclopropane series is indicated and is suggested to be the general mechanism for conversion of [formula omitted] esters to [formula omitted] esters. The series of 2-alkylcyclopropanes have been prepared by the decomposition of the corresponding pyrazoline3. Some of these pyrazolines have been newly synthesized for this work and include [formula omitted] esters required for this research. Several, however, have also been synthesized by the modified Wittig reaction.

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