Thermal rearrangement of α,β- to β,γ-unsaturated esters. Evidence for a 1,5-hydrogen transfer mechanism

• Published in: Canadian journal of chemistry 1968-07, Vol.46 (13), p.2225-2232
• Main Authors: McGreer, Donald E, Chiu, Norman W. K
• Format: Article
• Language: English
• ISSN: 0008-4042; 1480-3291
• DOI: 10.1139/v68-362

Methyl cis-4-methyl-2-pentenoate ( 1 ) has been found to rearrange at 252 °C by a simple first-order and presumably unimolecular mechanism to methyl 4-methyl-3-pentenoate (3) with a rate constant k of 4.28 × 10 −5  s −1 . The heat of activation ΔH†, was determined to be 37.5 ± 2 kcal/mole and the entropy of activation, ΔS†, was determined to be −8.2 ± 4 e.u. These activation parameters and the steric requirements of the reaction support a mechanism involving a cyclic 1,5-transfer of hydrogen. Studies on the isomerization of methyl cis-2-pentenoate, methyl cis-2,4-dimethyl-2-pentenoate, and methyl cis-2-methyl-2-pentenoate to the corresponding 3-pentenoates suggest that the 1,5-hydrogen transfer mechanism could well be general for the equilibration of α,β- and β,γ-unsaturated esters.An isopropyl group cis to a methyl or a carbomethoxy group on a double bond is shown by nuclear magnetic resonance to prefer a conformation with the methyls of the isopropyl group held away from the methyl or carbomethoxy group. A cis hydrogen does not restrict the conformation of the isopropyl group.