Probing electron-transfer pathways in reductions by lithium dialkylamides
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1983
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Abstract
An intriguing and unexpected reduction product was obtained from the reaction of lithium dialkylamide bases with (E)-2-tert-butyl-3-phenyloxaziridine (la) in tetrahydrofuran at 0(DEGREES). This reduction product, N-tert-butylbenzaldimine (2a), was the major product of the reaction, while the expected isomerization product, N-tert-butylbenzamide (3), accounted for the remainder of the product material. The yields of 2a and 3 from the above reaction were sensitive to the nature of the lithium dialkylamide base employed in the reaction, while they were somewhat less sensitive to the concentration of the lithium dialkylamide base. Lithium 2,2,6,6-tetramethylpiperidide gave the highest yield of 2a (89%), while lithium diisopropylamide (LDA) gave the widest range and lowest yield of 2a (58-73%) with increasing amide base concentration (0.05-0.20 M). The reduction reaction of la with lithium amide bases apparently involves initial single electron transfer (SET) from the amide base to la with subsequent formation of an intermediate that eventually produces 2a. The isomerization reaction of la occurs through simultaneous deprotonation and ring opening of la to give the anion of amide 3. Lithium N-n-butylcyclopropylamide (6) and lithium N-n-propylcyclobutylamide (7) were prepared and evaluated as probes for SET pathways involving lithium dialkylamides. Amide 6 was thermally unstable at or below 0(DEGREES) while amide 7 was thermally stable at 0(DEGREES). Amide 7 behaved as a normal lithium amide base in deprotonations of aliphatic ketones and esters, while evidence for SET from 7 appeared in reactions of 7 with oxaziridine 1a, benzophenone, and other oxidants, e.g., tetrakis{iodo(tri-n-butylphosphine)silver(I)}.
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Chemistry