Diheteroarylmethanes. 8. Mapping charge and electron-withdrawing power of the 1,2,4-triazol-5-yl substituent

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-p henyl-1,2,4-triazole (13) have been synthesized and the carbanions 11(-) an d 13(-) investigated in DMSO by multinuclear NMR spectroscopy. By applying previously proposed and widely used pi-charge/C-13 shift relationships on t he spectra of the anions, it is possible to rank the pi electron-withdrawin g power of the 1,2,4-triazol-5-yl group in terms of charge demand c(x), a q uantity that represents the fraction of pi negative charge delocalized by t he heterocyclic ring. Our results indicate that the charge demand ex of thi s heterocycle is considerably greater than that of other 1,3-azoles (2-imid azolyl, 2-oxazolyl, 2-benzoimidazolyl), being close to that of some mono an d diazinyl substituents. A single set of resonances is presented by both ca rbanions 11(-) and 13(-), thus showing that they exist either as a single g eometric isomer species or as a mixture of isomers in a rapid (on the NMR t ime scale) equilibrium. C-13 and N-16 shift/pi-charge relationships allow a ccurate 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 fir st of the bis(heteroaryl)methyl carbanions that we have studied, in which a ll of the negative charge originated by deprotonation of the carbon acid 11 is hosted on the nitrogen atoms without, any appreciable involvement of th e heteroaromatic carbon frame.