Lone-pair cooperativity in the N-alkylation of some derivatives of 2,2'-biimidazole

Abstract

The relative rates of N-alkylation of 1,1'-dimethyl-2,2'-biimidazole (6), 5,6-dihydrodiimidazo- [1,2-a:2',1'-c]pyrazine (7) and 6,7-dihydro-5H-diimidazo[l,2-a:2',1'-c][l,4]diazepine (8) with a variety of alkylating agents are reported. Irrespective of the alkylating agent, the order of reactivity was (8) (where the biimidazole has an additional three-carbon bridge) > (7) (two-carbon bridge) > (6) (no bridge), with a maximum of (8)/(6) = 317 for reaction with chloro(methoxy)methane. Since reaction of (8) is most susceptible to steric hindrance, but where the nitrogen lone pairs are in close proximity, these results support the idea of lone-pair cooperativity in the alkylation reactions. Variation in the rate ratios with changes in alkylating agent are discussed.