Computational and experimental evidence of two competing thermal electrocyclization pathways for vinylheptafulvene

The thermal electrocyclic ring-closure reaction of vinylheptafulvene (VHF) to form dihydroazulene (DHA) is elucidated in this work using Density Functional Theory and 1H-NMR spectroscopy. Two different transition states were found computationally, where one corresponds to a disrotatory pathway which is allowed according to the Woodward-Hoffmann selection rules, while the other corresponds to a conrotatory pathway. The conrotatory pathway is found to be zwitterionic in the transition state, while the disrotatory transition state varies in zwitterionic character depending on solvent and substituents in the molecular framework. The conrotatory and disrotatory transition states are found to have similar energy and their relative stability varies with solvent polarity and functionalization on the C1 position. To support these findings, we chemically ring-opened diastereomerically pure 1-(benzothiazol-2-yl)-DHA to the VHF form and subsequently thermally reconverted the VHF to DHA in a range of solvents of various polarity. We found that depending on solvent polarity, different ratios of antiand syn-diastereoisomers of the DHA were formed in a systematic manner which support the existence of two distinct thermal ring-closure pathways for VHF.