Synthesis and conformational study of 1,1′-ethano-9-9′-bifluorenyl. Anti and gauche conformers of a 9,9′-bifluorenyl derivative and chair and twist conformers of a dibenzo-1,5-cyclooctadiene derivative

Abstract

A Hofmann-type elimination of the sulphonium salt ( 18) under basic conditions led to the formation of a novel o-xylylene derivative ( 11) which dimerized regioselectively to give the bifluorenyl/dibenzo-1,5-cyclooctadiene derivative ( 3). The reaction is shown to be kinetically controlled and non-concerted. The two rigid conformers ( 3a) and ( 3b) were characterised by their 1H NMR spectra and they represent respectively the first observed examples of an anti bifluorenyl derivative and a twist dibenzo-1,5-cyclooctadiene derivative. A semiempirical molecular orbital PM3 calculation supported the observed results. Dynamic 1H NMR studies indicated an interconversion process ( 3a) ⇄ ( 3b) at higher temperatures involving a conformational barrier estimated at 65.2 kJ mol −1.