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2-Phenylbenzimidazoline (PBI) as a mild, selective, and convenient reducing agent was efficiently generated in situ from o-phenylenediamine and benzaldehyde in alcohols. A generally applicable method for the selective reduction of carbon–carbon double bonds of a variety of electron-deficient olefins with an alcoholic solution of PBI is described. The reduction of α,β-unsaturated ketones to the corresponding saturated ketones could also be accomplished (but, less effectively) with PBI with the aid of a Lewis-acid catalyst. 1-Methyl-2-(o-nitrophenyl)benzimidazoline prepared and isolated by the reaction of o-nitrobenzaldehyde with N-methyl-o-phenylenediamine reduced benzylidenemalononitrile to give benzylmalononitrile and 1-methyl-2-(o-nitrophenyl)benzimidazole in high yields. This shows the validity of PBI to be the actual reducing species in the present reduction system. From a mechanistic study, the present reductions could be interpreted in terms of a mechanism involving a synchronous transport of a pair of hydrogens or a sequential transfer of a hydride and a proton from PBI to the olefins.

The reaction of 2-lithiobenzothiazole with a variety of electrophiles such as aldehydes, ketones, carboxylic esters, lactones, nitriles, and amides afforded the expected addition and substitution products. Trimethylsilyl chloride readily reacted with the benzothiazolyl anion to give 2-trimethylsilylbenzothiazole, while common alkyl halides including primary iodide and benzyl halides, as well as epoxides, did not react with the anion. This characteristic nucleophilicity of the anion was also demonstrated by its reaction with phenacyl halides and 5-chloro-2-pentanone leading to the formation of benzothiazolyl-substituted small-ring ethers. In order to demonstrate the value of 2-lithiobenzothiazole as a masked formyl anion, 2-(α-hydroxyalkyl)benzothiazoles were transformed into α-hydroxy carbonyl compounds in three reaction steps without masking the α-hydroxy groups. Quaternization of various 2-(α-hydroxyalkyl)benzothiazoles with methyl iodide in DMF afforded the corresponding 2-(α-hydroxyalkyl)-3-methylbenzothiazolium iodides in high yields and the subsequent alkylation or reduction of the C=N+ link was readily accomplished with organolithium and Grignard reagents or sodium borohydride leading to the corresponding 2-(α-hydroxyalkyl)-3-methylbenzothiazolines with or without another 2-substituents. Hydrolysis of former compounds performed in aqueous acetonitrile, buffered to pH 7 and containing AgNO3, gave the desired α-hydroxy ketones.

Reduction of various a,P-unsaturated ketones ( 3 a -g ) and (4a-4) in methanol by the benzothiazoline (1) in the presence of aluminium chloride gives, in all cases, the corresponding saturated ketones ( 5 a -g ) and (6a-d) without any of the unsaturated or saturated alcohol. Reduction of a,P-unsaturated esters (7a,b) similarly gives the saturated esters (9a,b), while reaction of cinnamaldehyde (8) with compound (1) does not occur at all. Among the Lewis acids examined, aluminium chloride gives the best results. Reduction of 2'-azachalcone (21) with 2phenyl [2-2H] benzothiazoline reveals that, in the reduction product, the deuterium atom is located

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