| Popis: |
Three new triazolyl derivatives, bis(1H-1-phenyl-1,2,4-triazol-5-yl)methane (11), 1H-1-phenyl-5-(beta-styryl)-1,2,4-triazole (12), and 1H-5-benzyl-1-phenyl-1,2,4-triazole (13) have been synthesized and the carbanions 11(-) and 13(-) investigated in DMSO by multinuclear NMR spectroscopy. By applying previously proposed and widely used pi-charge/C-13 shift relationships on the spectra of the anions, it is possible to rank the pi electron-withdrawing power of the 1,2,4-triazol-5-yl group in terms of charge demand c(x), a quantity that represents the fraction of pi negative charge delocalized by the heterocyclic ring. Our results indicate that the charge demand ex of this heterocycle is considerably greater than that of other 1,3-azoles (2-imidazolyl, 2-oxazolyl, 2-benzoimidazolyl), being close to that of some mono and diazinyl substituents. A single set of resonances is presented by both carbanions 11(-) and 13(-), thus showing that they exist either as a single geometric isomer species or as a mixture of isomers in a rapid (on the NMR time scale) equilibrium. C-13 and N-16 shift/pi-charge relationships allow accurate pi-charge mapping of carbanionic systems. Our results clearly show that, in the case of benzyl carbanion 13-, all of the three nitrogen atoms are almost equally involved in delocalizing the negative charge. Also, the N-phenyl group contributes to charge delocalization. Anion 11(-) is the first of the bis(heteroaryl)methyl carbanions that we have studied, in which all of the negative charge originated by deprotonation of the carbon acid 11 is hosted on the nitrogen atoms without, any appreciable involvement of the heteroaromatic carbon frame |
