C-13 CHEMICAL-SHIFTS OF ALKENE CARBONS IN ACYLIDENE-3,5-DIARYL-2,3-DIHYDRO-1,3,4-THIADAZOLES AND RELATED BENZOTHIAZOLES AND BENZOSELENAZOLES, 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 ylid ene-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-dihydrobenzose lenazoles of known geometry, indicating appreciable charge polarizatio n in these compounds as in other push-pull olefins. Substitution that promotes more extensive charge delocalization results in the C alpha s ignal shifting to the higher-frequency end of the chemical shift range . The observed shifts are compared with those calculated according to the Pretsch scheme.