Study of electron transfer processes involving lithium dialkylamides

Abstract

In this study the potential of a class of lithium dialkyamides to serve as mechanistic probes was investigated. Lithium N-butyl-4-pentenylamide 31 in THF rearranged smoothly to give a mixture (after protonation) which contained 1-butyl-2-methylpyrrolidine 33, formed in an unusual cyclization reaction. Better success was achieved with the use of N-lithio-N-butyl-5-methyl-1-hex-4-enamine 41 as a mechanistic probe. The aminyl radical formed by the thermolysis or the photolysis of the tetrazene 55 was observed to form the cyclized products 49 and 58 in ca. 30% yield. The reaction of 41 with oxiziridine 60 and the radical cation of thianthrene 64 gave evidence for an initial single electron transfer (SET) process. However, in the reduction of benzophenone by 41, no cyclic products were obtained. Treatment of N-butyl-N-(5-methyl-4-hexen-1-yl)benzamide with phenyllithium gave an intermediate which did lead to some aminyl radical formation as evidenced by the formation of the cyclic product 49. The latter results suggest that the aminyl radical, had it been formed in the reaction of 41 with benzophenone, would have had time to cyclize and thus, an initial SET process was not important in this reaction. The reduction of benzophenone by lithium dialkyamides is a typical example of a broad class of reactions for which electron transfer mechanisms have been inferred from the detection of oxidant derived radical anions. A scheme in which the ketyl is formed in secondary reactions after the initial concerted (beta)-hydride transfer is presented. Results indicate that the benzophenone ketyl observed in these reactions arises from the reaction of the benzophenone dilithium dianion with benzophenone. The dianion is formed slowly by deprotonation of the lithium salt of benzhydrol by excess lithium amide base. Further, the scheme which is proposed can be restated in a general form to account for the production of oxidant derived radical anions in other reactions of nucleophiles or bases with aryl ketones.