Ammonium radical pair intermediates

Mechanistically the rearrangement is formulated to proceed via an intermediate radical-pair or ion-pair. In either case the initial step is the formation of a nitrogen-ylide 2 by deprotonation of the ammonium species with a strong base. The abstraction of a proton from the a-carbon is facilitated by an electron-withdrawing group Z—e.g. an ester, keto or phenyl group ... 

Radical mechanisms are also known to be involved in 1,2-rearrangement reactions, such as the R alkyl group migration leading from ammonium ylids 16 to tertiary amines 19 (Scheme 3). Whilst the possible ion pair intermediate 17 is destabilized by the electron-withdrawing carbonyl group, the radical pair 18 is favored by the captodative stabilization of one of the radicals. 

The yield and ratio of the two Stevens rearrangement products are dependent on both the base and the solvent however, the ortho-rearrangement product was only detected when butyllithium was used in hexane. Variation of the halide ion had only a small effect on the ratio of rearrangement products. Several mechanisms were considered 93) as routes to the formation of the rearrangement products, 1) ion-pairs, 2) cationic rearrangement, 3) carbenoid, 4) dimetallated intermediates, 5) free-radical, 6) predissociation of the ammonium salt and, 7) sigmatropic shift. 

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