Push-pull and pull-push effects in isatylidenes

A series of C-8 mono- and disubstituted isatylidenes were prepared in order to examine the influence of the substituent on the polarization of the ethylenic carbons C-3=C-8. Donor–acceptor C-8 substitution resulted in an inversion of the polarization of the ethylenic carbons, as evidenced by the 13C NMR spectra, and because these compounds can exist in solution as equilibrium mixtures of the E- and Z-isomers. The dual donor–acceptor character of the heterocycle is controlled by the cross-conjugated C-3=C-8 double bond, in which the C-3 carbon atom has the possibility of conjugating with either the donor or the attracting portion of the heterocyclic system, consistent with contributions from isatylidene resonance forms which induce the C-8 carbon atom to be charged either positively or negatively, thus allowing an opposite sense of the push–pull effect.