Diheteroarylmethanes .5. E-Z isomerism of carbanions substituted by 1,3-azoles: C-13 and N-15 pi-charge shift relationships as source for mapping charge and ranking the electron-withdrawing power of heterocycles

Previously proposed pi-charge/shift relationships have been applied to C-13 and N-15 shifts Of the carbanions of a-benzylazoles (thiazole, oxazole, and imidazole), their corresponding benzo-fused analogs, and bis(2-azolyl)methanes (azolyl groups as above). In this way it is possible to rank the pi electron-withdrawing power of these heterocycles in terms of charge demands C-X, a quantity representing the fraction of pi negative charge withdrawn (delocalized) by the ring. The results indicate that C-thiaz > C-oxaz > C-imidaz; furthermore, benzoazoles are more efficient than monocyclic systems in delocalizing the negative charge. The charge demand C-X of imidazole is the smallest among the heteroaromatics so far considered, being even smaller than that of the phenyl ring. As a consequence, the negative charge in the anion of 2-benzyl-N-methylimidazole is predominantly transferred from the carbanionic carbon to the phenyl group rather than to the imidazolyl residue. The high double bond character of the bond linking the carbanionic and ipso phenyl ring carbons leads to room temperature C-13 shift anisochrony of the meta and meta' and ortho and ortho' positions of the phenyl ring. In all of the other cases, hindered rotation is observed at room temperature between the carbanionic carbon and position 2 of the heterocycle. A single set of resonances is presented by the bis(heteroaryl)methyl carbanions. pi-Charge/shift relationships allow for the accurate pi-charge mapping in these carbanionic systems, and the results point to considerable delocalization of the electron pair(s) of the oxygen and pyrrolic nitrogen atoms at position 1 in oxazole and imidazole toward the pyridic nitrogen at position 3 of the rings (in both the neutrals and the carbanionic species). On the contrary, not only does the sulfur atom in thiazole derivatives not delocalize any negative charge in the anions but it is barely involved in any pi-donation to the pyridic nitrogen atom at position 3 also in the neutrals.