Thermal rearrangement of 2-acetoxy-266-trimethylbicyclo[3.1.0]hexane: Theoretical elucidation of the mechanism

Abstract

Bicyclohexenes are believed to be the immediate precursors of aromatic compounds. As a part of the exploratory study of thermal aromatization reactions 266-trimethylbicyclo[3.1.0]hexan-2-ol and its ester derivative 2-acetoxy-266-trimethylbicyclo[3.1.0]hexane were synthesized. Pyrolysis of 2-acetoxy-266-trimethylbicyclo[3.1.0]hexane at 350 degrees C gave 133-trimethyl-14-cyclohexadiene instead of the expected product 266-trimethylbicyclo[3.1.0]hex-2-ene. Computational methods such as PM3 31G]* were employed in order to elucidate the mechanism of this reaction. The Gibbs free energy of activation and the reaction energy were calculated for the proposed polar and biradical mechanisms. The results showed that a two-step mechanism is plausible at 350 degrees C in which the expected product 266-trimetliylbicyclo[3.1.0]hex2-ene is the intermediate. The first step is the 12-elimination of the ester leading to 266-trimethylbicyclo [3. 1. 0]hex-2-ene. The second step is the sigmatropic rearrangement of 266-trimethylbicyclo[3.1.0]hex-2-ene via concerted homodienyl 15-hydrogen shift which is also the rate-determining step. UB3LYP/6-31G* calculations reveal that the cyclopropyl moiety of bicyclo[3.1.0]hex-2-ene can undergo homolytic bond cleavage to give an allylically stabilized biradical intermediate. However the formation of 14-cyclohexadiene from such an intermediate through a biradical transition state involving 12-hydrogen migration does not seem to be plausible. (c) 2007 Elsevier B.V. All rights reserved.