The conformational diastereomers of 7-substituted-(8Z)-8-chloro- 6,7-dihydro-9-(1,2-dihydronaphthalen-4-yl)-5H-benzo(7)annulene derivatives, the question of atropisomerism versus ring inversion

Conformational diastereomers of 7-substituted-(8Z)-8-chloro-6,7-dihydro-9-(1,2-dihydronaphthalen-4-yl)-5H-benzo[7]annulene 1 are observed at room temperature in solution. Two conformational processes are possible in 1, i.e. atropisomerism around the sp 2 -sp 2 pivot bond and ring inversion of cycloheptadiene moiety which together provide four minima structures. The 3 J calculation by Haasnoot equation on optimized structures is accordance with the 7-substituents in pseudo equatorial (exo) position in both forms. The barrier to conformational process is determined by dynamic 1 H-NMR spectroscopy to be ∆G ≠ (365K)=74.5±0.5 kJ/mol. Solvent dependent populations of the two forms are studied in DMSO-d6 and CDCl3, the population ratio is not sensitive to solvent polarity.