8-PURINYL VERSUS 2-BENZIMIDAZOLYL CARBANIONS - CHARGE DEMANDS OF THE HETEROCYCLES AND LIGAND PROPERTIES OF THE BIS(HETEROARYL)METHANES

Two new purinyl derivatives, 8-benzyl-7-methylpurine (8) and bis(7-met hylpurin-8-yl)methane (9), have been synthesized and their correspondi ng carbanions investigated in DMSO. The application of our previously proposed pi-charge/shift relationships to the C-13 and N-15 shifts Of the carbanions has made it possible to map rr-charges and obtain the v alues of the charge demand of the heterocyclic rings, a resonance inde x of the pi-electron-withdrawing power of the substituent. The charge demand of the 7-methylpurin-8-yl substituent is found to be the highes t among the previously investigated azinyl and azolyl substituents and is comparable with that of the strongest classical electron-withdrawi ng functions. The replacement of the fused benzene ring in the benzimi dazolyl substituent by a pyrimidine ring containing the nitrogen atoms in appropriate positions causes a considerable increase in the electr on-withdrawing capacity of the heterocycle. Spectroscopic and reactivi ty data confirm the strong electron-withdrawing nature of the purinyl ring. The C-13 NMR spectrum shows the existence of the carbanions of b oth purinyl derivatives as a mixture of geometric isomers, a consequen ce of the high double-bond character along the bond linking the carban ionic carbon to C(8) of the purinyl ring. Bis(7-methylpurin-8-yl)metha ne can be easily converted to its stable NH tautomer by means of basic catalysis. It behaves as an ''active methylene'' compound giving high -yield condensations with electrophiles. Finally, unlike the correspon ding benzimidazol-2-yl derivative, bis(7-methylpurin-8-yl)methane reac ts with metal acetates to give neutral methanates [ML2] (M = Zn, Cu, C o; LH = bis(7-methylpurin-8-yl)methane), where the ligand is present a s an anionic system. On the basis of this and previous data, it is con cluded that charge demand plays a strategic role in obtaining stable c helates of this sort.