Synthesis of (bicyclo[2.2.0]hex-1-yl)methanal and Arrhenius parameters for thermal rearrangement: Radical stabilizing effect of aldehyde substituents on highly strained CC bonds

Abstract

Bicyclo[2.2.0]hexane rearranges to hexa-1,5-diene via the cyclohexane-1,4-diyl diradical. The 1-formyl substituted derivative was sought to evaluate the effect of a CHO-group on the rate of rearrangement. (Bicyclo[2.2.0]hex-1-yl)methanal was prepared for the first time in a multistep synthesis starting from hexachlorocyclopentadiene. The pivotal last step was achieved by Swern oxidation of (bicyclo[2.2.0]hex-1-yl)methanol at T = –60 °C. The carbon skeleton of the alcohol precursor remains intact under those conditions. Thermolysis of the aldehyde to 2-methylenehex-5-enal follows a first-order rate law between T = 322.85–361.51 K (T_avg = 343.30 K). Seven rate constants k_T were used to plot log k_T vs. 1/T, which provided the Arrhenius parameters for the rearrangement: activation energy (E_a) = 25.4 ± 1.0 kcal/mol and pre-exponential factor (A₃₄₃) = 1.66 × 10¹² s⁻¹. The E_a is 10.6 kcal/mol below that of the unsubstituted archetype bicyclo[2.2.0]hexane. This sizable change in E_a reflects the radical stabilization energy of the CHO-group in 1-formylcyclohexane-1,4-diyl.