Dose Stevens Rearrangement Really Proceed via a [1,2] Radical Rearrangement?

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Stevens rearrangement is a typical reaction of ammonium ylides. The mechanism has been the subject of much study, and a [1,2] radical rearrangement pathway in the solvent cage has been considered. In the papers that reported high yields of Stevens products, most of the migrating groups were benzyl groups.In my previous studies of chemical behavior of N,N-dimethylbenzylammonium alkylides, produced by fluoride ion induced desilylation of N-benzyl-N,N-dimethyl-alpha-(trimethylsilyl) alkylammonium halides, I reported that Stevens products were formed from isotoluenes which were formed by a [2,3] sigmatropic rearrangement of the alkylides (two steps mechanism). There is no [1,2] shift from the alkylides to the Stevens products.In this research project, I investigated the chemical behavior of N-alkyl-N,N-dimethylammonium benzylides. When a migrating group (R) was an alkyl, no rearrangement occurred but moderate yields of Stevens products were obtained when R was CH_2CN.In the two steps mechanism, is chirality of the migrating group retained? Stevens product prepared from optically-active S-(-)-N,N-dimethy-N-(trimethylsilyl) methyl-1-phenethylammonium iodide retained the chirality during a [2,3] rearrangement followed by a [1,3] shift.These results revealed that Stevens rearrangement has two reaction mechanisms.