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

• Published in: Tetrahedron letters Vol. 153
• Main Authors: Nowienski, Anita, Rosenberg, Murray G., Brinker, Udo H.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 19-12-2024
• Subjects: 1,4-Diradicals; Aldehyde synthesis; Arrhenius parameters; Bicyclo[2.2.0]hexanes; Radical stabilization energy; Swern oxidation; Thermolysis; 1,4-Diradicals; Radical stabilization energy; Bicyclo[2.2.0]hexanes; Arrhenius parameters; Aldehyde synthesis; Swern oxidation; Thermolysis
• ISSN: 0040-4039
• DOI: 10.1016/j.tetlet.2024.155369

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•First synthesis of (bicyclo[2.2.0]hex-1-yl)methanal.•Arrhenius parameters for thermal rearrangement to 2-methylenehex-5-enal.•Radical stabilization energy of CHO-group is –10.9 kcal/mol.
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 (Tavg = 343.30 K). Seven rate constants kT were used to plot log kT vs. 1/T, which provided the Arrhenius parameters for the rearrangement: activation energy (Ea) = 25.4 ± 1.0 kcal/mol and pre-exponential factor (A343) = 1.66 × 1012 s−1. The Ea is 10.6 kcal/mol below that of the unsubstituted archetype bicyclo[2.2.0]hexane. This sizable change in Ea reflects the radical stabilization energy of the CHO-group in 1-formylcyclohexane-1,4-diyl.