Synthesis and structural assignment of 2,4 '-disubstituted benzylidenefluorenes and 4 '-substituted benzylidene-1-azafluorenes

In the course of a study aimed at the investigation of edge-to-face arene-arene interactions, substituted benzylidenefluorenes 8-16 and benzylidene-1-azafluorenes 20-24 were synthesized as mixtures of E-Z-isomers. The full structural assignment of these compounds, performed by 2D NMR experiments, is described together with a study of their E-Z isomerization under equilibrating conditions. For compounds 8-16 no correlation was observed between the electron-donating or electron-withdrawing nature of the substituent groups and the E/Z equilibrium ratios. In the case of 20-24 the predominance of the E-isomers seems to depend exclusively on the N-lone pair-benzylidene ring electronic repulsion rather than on the nature of the pala substituent on the aromatic ring. (C) 1998 John Wiley & Sons, Ltd.