Carbon-13 chemical shifts of alkene carbons in 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadazoles and related benzothiazoles and -selenazoles, and their relationship to other push-pull alkenes

Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78-109 ppm (C alpha) and 154-164 ppm (C beta) for a series of 11 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, 3 2-acylidene-3-alkyl-2,3-dihydrobenzothiazoles, and 2 2-acylidene-3-alkyl-2,3-dihydrobenzoselenazoles of known geometry, indicating appreciable charge polarization in these compounds as in other push-pull olefins. Substitution that promotes more extensive charge delocalization results in the C alpha signal shifting to the higher-frequency end of the chemical shift range. The observed shifts are compared with those calculated according to the Pretsch scheme.