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

Abstract

Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78-109 ppm (Cu) and 154-164 ppm (CP) 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-dihydrobenzoselenaz0les 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 a 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.Key words: carbon-13 NMR, chemical shifts, push-pull olefins, 1.3.4-thiadiazoles, benzothiazoles, benzoselenazoles.RCsumC : On a observe les dtplacements chimiques du I3c des carbones des alcknes dans des rCgions allant de 78 B 109 ppm (Ca) et 156 i 164 (CP), d'une sCrie de 11 2-acylidkne-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, de 3 2-acylidkne-3-alkyl-2, 3-dihydrobenzothiazoles, et de 2 2-acylid~ne-3-alkyl-2,3-dihydrobenzostlnazoles de gComCtries connues; ces rCsultats indiquent qu'il existe une polarisation importante de la charge dans ces composCs, comme dans les autres olefines (( push-pull D.La substitution qui provoque la plus grande dklocalisation conduit B un dkplacement du signal C a vers les plus grandes frCquences de la plage des dkplacements chimiques. On a compart les dkplacements chimiques observts avec ceux calculCs B l'aide du schCma de Pretsch.