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, o xazole, and imidazole), their corresponding benzo-fused analogs, and b is(2-azolyl)methanes (azolyl groups as above). In this way it is possi ble 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power of these heterocycles in terms of charge demands C-X, a quantity representing the fraction of pi negative charge withdrawn (delocalized) by the ring.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 in dicate that C-thiaz > C-oxaz > C-imidaz; furthermore, benzoazoles are more efficient than monocyclic systems in delocalizing the negative ch arge. The charge demand C-X of imidazole is the smallest among the het eroaromatics so far considered, being even smaller than that of the ph enyl ring. As a consequence, the negative charge in the anion of 2-ben zyl-N-methylimidazole is predominantly transferred from the carbanioni c carbon to the phenyl group rather than to the imidazolyl residue. Th e high double bond character of the bond linking the carbanionic and i pso phenyl ring carbons leads to room temperature C-13 shift anisochro ny 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 hete rocycle. A single set of resonances is presented by the bis(heteroaryl )methyl carbanions. pi-Charge/shift relationships allow for the accura te pi-charge mapping in these carbanionic systems, and the results poi nt to considerable delocalization of the electron pair(s) of the oxyge n and pyrrolic nitrogen atoms at position 1 in oxazole and imidazole t oward the pyridic nitrogen at position 3 of the rings (in both the neu trals and the carbanionic species). On the contrary, not only does the sulfur atom in thiazole derivatives not delocalize any negative charg e in the anions but it is barely involved in any pi-donation to the py ridic nitrogen atom at position 3 also in the neutrals.