No abstract

No abstract

No abstract

No abstract

Treatment of 2-methylbenzimidazole (la), 2-methyl-5-chlorobenzimidazole (Ib), 2-benzylbenzimidazole (IC), and l-(2-benzimidazolyl)-I-phenylpropane (3g) with 2 mol equiv of n-butyllithium in THF-hexane at 0" resulted in abstraction of the heterocyclic NH proton as well as an 01 hydrogen of the 2-alkyl substituent. Reactions of the resulting dilithio derivatives with alkyl halides, aldehydes, and ketones took place selectively at the sidechain carbanion center to produce 2-alkylbenzimidazoles and 2-(2-hydroxylalkyl)benzimidazoles, respectively. Attempted twofold deprotonation of 2-propylbenzimidazole (3a) with n-butyllithium afforded only the monolithio salt (7) even in the presence of TMEDA or HMPA. The weakly acidic character of the methyl protons of 2-methylbenzimidazole (la) has been demonstrated on a number of occasions.2 For example, la reacts with aromatic aldehydes in the presence of both acidic and basic catalysts to form 2-st~~ylbenzimidazoles.~~ Although the base-catalyzed reactions presumably involve an intermediate possessing carbanion character at the 2-methyl position,2a active hydrogen condensations at this site have previously been limited to those in ivhich unfavorable ionization of a side-chain proton is compensated for by a later, irreversible step such as dehydration of an intermediate aldol product. I t occurred to us that treatment of la with a suitable strong base should effect rapid ionization of the NH proton and that the benzimidazole nucleus might provide sufficient delocalization of negative charge to then H L la,R, = Rj = H 2a, R, = R2 = H b,Rl=H;Rj=C1 C. R, = CGH-]; RL = H b, R, = H; R2 = C1 C, R, = CGH;; R2 = K 3 r