The dichotomy between competitive [1,2]-and [2,3]-rearrangements of onium ylides has been of longstanding interest to the synthetic community. Stevens reported the first [1,2]-rearrangement of a benzylic ammonium ylide in 1928, with the mechanism of this rearrangement thoroughly investigated and of widespread interest as concerted [1,2]-rearrangements are forbidden by the Woodward-Hoffman rules. Wittig 2 and Hauser 3 demonstrated that [1,2]-rearrangement proceeds with retention of configuration at the migrating carbon. The most widely-accepted mechanistic explanation comes from detailed experiments by Ollis, which indicate homolytic C-N bond cleavage of the ammonium ylide generates a geminate radical pair that undergoes preferential "intramolecular" radical-radical recombination (Figure ). Experiments using an enantiopure benzylic ammonium salt 1 showed up to 99% enantiospecificity in viscous solvents, leading to the suggestion that radical-radical recombination takes place within the solvent cage at a faster rate (estimated at >10 11 s -1 ) 7 than molecular rotation or cage escape/diffusion of either of the radical partners (Figure ). The [1,2]-rearrangement of benzylic ammonium ylides has been most-widely studied as the competing [2,3]-rearrangement process (Sommelet-Hauser) is significantly disfavoured due to the requirement for transient dearomatisation. In contrast, allylic ammonium ylides typically give product mixtures that are ascribed to competitive [1,2]and [2,3]-rearrangement, with the [1,2]-product usually being a minor reaction pathway. A range of factors influence the ratio of [1,2]-and [2,3]-products; strong bases and aprotic solvents, higher reaction temperatures, and the use of sterically-congested and unstabilised ylides typically lead to a greater proportion of [1,2]-rearrangement. The current state-of-the-art understanding of these processes has been provided by Singleton and co-workers, with [1,2]-and [2,3]-rearrangement product mixtures shown to arise from a common transition state in which homolytic C-N bond cleavage is promoted by a loose [2,3]-transition state, leading to 20% of the [1,2]-product at 90 ˚C (Figure ). Current asymmetric approaches to [1,2]-rearrangements of ammonium ylides are limited, and generally rely upon either stoichiometric chiral auxiliaries or stereospecific rearrangements through N-to-C chirality transfer. Catalytic enantioselective approaches are rare, with the difficulty in controlling radical-radical recombinations leading to this being a recognised challenge in enantioselective synthesis. The current state-of-the-art is represented by the work of Arnold and coworkers who recently established a biocatalytic [1,2]-Stevens type ring expansion of aziridines. Single examples of a phosphite-catalysed enantioselective [1,2]-rearrangement of an N-benzylic isoquinoline derivative (94:6 er), and a copper-catalysed enantioselective [1,2]-rearrangement of an allylic sulfonium ylide (69:31 er) have been demonstrated. During the finalisation of this work, enantioselective variants involving ring-expansion of conformationally restricted azetidine-derived allylic ammonium ylides that proceed through a stepwise intramolecular ion-pair pathway have been reported. In previous work, we developed the isothiourea-catalysed enantioselective [2,3]-rearrangement of allylic ammonium ylides (Figure ). Experimental and computational studies led to the proposal of a highly stereodefined transition state TS-I, in which stereocontrol is governed by: (i) the (Z)- configuration of the enolate-like ylide; (ii) a 1,5-O•••S chalcogen interaction that locks the conformation of the acylated catalyst; (iii) the phenyl substituent of the catalyst promoting facial-selectivity for rearrangement; and (iv) a π•••isothiouronium interaction between the cinnamyl Ph substituent and the acylated catalyst. Building upon Singleton's proposition of common [1,2]-and [2,3]-transition states, we rationalised that access to pre-organised [2,3]-transition state TS-I could be exploited to develop a catalytic enantioselective [1,2]-rearrangement through incorporating steric hindrance at the terminal allylic substituent while exploiting solvent polarity and increasing reaction temperature (Figure , grey box). Herein is described a catalytic protocol using a commercially available isothiourea catalyst, tetramisole hydrochloride, to provide unnatural α-amino acid derivatives in excellent enantioselectivity.