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Computational and Experimental Evidence of Two Competing Thermal Electrocyclization Pathways for Vinylheptafulvene
Author(s) -
Frandsen Benjamin N.,
Skov Anders B.,
Cacciarini Martina,
Brøndsted Nielsen Mogens,
Kjaergaard Henrik G.
Publication year - 2019
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201800437
Subject(s) - conrotatory and disrotatory , ring (chemistry) , polarity (international relations) , solvent , chemistry , molecular switch , computational chemistry , molecule , organic chemistry , biochemistry , cell
The thermal electrocyclic ring‐closure reaction of vinylheptafulvene (VHF) to form dihydroazulene (DHA) is elucidated herein by using DFT and 1 H NMR spectroscopy. Two different transition states were found computationally; one corresponds to a disrotatory pathway, which is allowed according to the Woodward–Hoffmann selection rules, whereas the other corresponds to a conrotatory pathway. The conrotatory pathway is found to be zwitterionic in the transition state, whereas 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 at the C1 position. To support these findings, we chemically ring‐opened diastereomerically pure 1‐(benzothiazol‐2‐yl)‐DHA to give the VHF form, then subsequently thermally reconverted the VHF to DHA in a range of solvents with various polarities. We found that, depending on solvent polarity, different ratios of anti ‐ and syn ‐diastereoisomers of DHA were formed in a systematic manner, which supports the existence of two distinct thermal ring‐closure pathways for VHF.