A general method has been developed for the overall transformation of a-bromo ketones to a-alkyl or a-aryl ketones, with benzotriazole being used as a synthetic auxiliary. a-Benzotriazolyl ketones, when converted into their phenylhydrazones, reacted smoothly with alkyl and aryl Grignard reagents, which displaced benzotriazolate, to give the corresponding a-alkyl or a-aryl hydrazones. In some cases, these hydrolysed directly to the a-alkyl or a-aryl ketones. In each case, the product was treated with 2,4-dinitrophenylhydrazine to isolate the target ketones as the corresponding 2,4-dinitrophenylhydrazones.
ChemInform Abstract A novel and general method for replacing the bromine atom in α-bromo ketones of type (I) by nucleophiles is presented; the resulting ketones (11 examples given) are isolated as 2,4-dinitrophenylhydrazones such as (IX). The procedure employs the benzotriazole moiety as an auxiliary group (cf. (IV), (V)), whose elimination results in the formation of the likely intermediate (VIII). Control experiments indicate that this process (and thus the subsequent attack of the Grignard reagent) is greatly facilitated by an equilibration of the isomers (IV)/(V) via ionic dissociation.
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