Thermal rearrangement of 2-acetoxy-2,6,6-trimethylbicyclo[3.1.0]hexane: Theoretical elucidation of the mechanism

Bicyclohexenes are believed to be the immediate precursors of aromatic compounds. As a part of the exploratory study of thermal aromatization reactions, 2,6,6-trimethylbicyclo[3.1.0]hexan-2-ol and its ester derivative 2-acetoxy-2,6,6-trimethylbicyclo[3.1.0]hexane were synthesized. Pyrolysis of 2-acetoxy-2,6,6-trimethylbicyclo[3.1.0]hexane at 350 °C gave 1,3,3-trimethyl-1,4-cyclohexadiene instead of the expected product, 2,6,6-trimethylbicyclo[3.1.0]hex-2-ene. Computational methods such as PM3, HF/6-31G∗, B3LYP/6-31G∗, UHF/6-31G∗, UB3LYP/6-31G∗, and UMP2/6-31G∗ 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 °C in which the expected product 2,6,6-trimethylbicyclo[3.1.0]hex-2-ene is the intermediate. The first step is the 1,2-elimination of the ester, leading to 2,6,6-trimethylbicyclo[3.1.0]hex-2-ene. The second step is the sigmatropic rearrangement of 2,6,6-trimethylbicyclo[3.1.0]hex-2-ene via concerted homodienyl 1,5-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 1,4-cyclohexadiene from such an intermediate through a biradical transition state involving 1,2-hydrogen migration does not seem to be plausible.