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 eth ylenic carbons C-3=C-8. Donor-acceptor C-ti substitution resulted in an inv ersion of the polarization of the ethylenic carbons, as evidenced by the C- 13 NMR spectra, and because these compounds can exist in solution as equili brium mixtures of the E- and Z-isomers. The dual donor-acceptor character o f 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 eith er the donor or the attracting portion of the heterocyclic system, consiste nt with contributions from isatylidene resonance forms which induce the C-8 carbon atom to be charged either positively or negatively, thus allowing a n opposite sense of the push-pull effect. Copyright (C) 2000 John Wiley & S ons, Ltd.