Having demonstrated the scope of this process, consideration to the mechanism of this cascade was given. Initial association of tBu-BIMP 7 to the oxindole carbonyl of substrate I by hydrogen-bonding is assumed to direct deprotonation of the allylic ether-BIMP complex II to give III, with subsequent concerted [2,3]sigmatropic rearrangement giving IV with high enantioselectivity. Subsequent protonation and catalyst release gives isolable V. A subsequent base (tBu-BIMP or DBU) catalysed anion-accelerated [1,3]-rearrangement that proceeds with retention of configuration at the carbinol centre generates [1,2]-Wittig product VI (Fig. ). Building on this framework, DFT analyses were used to probe three mechanistic possibilities for the unconventional [1,3]-rearrangement from either the neutral alcohol or anionic alkoxide; concerted [1,3]sigmatropic rearrangement, homolytic fragmentation followed by recombination, or anionic fragmentation and recombination (Fig. ) . In these initial calculations the N-trityl group was truncated to an N-methyl and the ammonium counterion was omitted to reduce the complexity of the calculations; DFT calculations in both diastereoisomeric series were considered and showed similar energetic trends (see SI for further information), although for simplicity only those for the major diastereoisomer are illustrated. Extensive modelling was unable to locate a cyclic transition state without constraining the bond-forming and bond-breaking distances, thus only an approximation to the cyclic process could be obtained via a constrained cyclic transition state. Although examples of [1,3]-sigmatropic rearrangements exist in the literature, most exhibit high activation barriers or occur with smaller, more flexible, or more sterically accessible systems than I . Consideration of the reaction barriers/intermediate energy differences (see SI for further information) for the homolytic and ionic pathways from either the neutral alcohol or alkoxide revealed that those proceeding via the alkoxide 66 are significantly favoured, consistent with an anion-accelerated fragmentation process . Furthermore, comparison of bond lengths of the breaking C-C bond within the neutral (1.6 Å) and deprotonated (1.7 Å) species indicate that significant bond lengthening is observed upon deprotonation, consistent with weakening of this bond as proposed by Evans . Overall, both the constrained cyclic transition state (ΔG ‡ = 56.9 kcal•mol -1 ) and the homolytic diradical fragmentation/recombination pathway (ΔG ‡ = 63.2 kcal•mol -1 ) were significantly disfavoured compared to an ionic fragmentation/recombination pathway (ΔG ‡ = 2.0 kcal•mol -1 ). Fig shows a free energy surface for the ionic fragmentation/recombination pathway from deprotonated intermediate 66 to product 67 . This stepwise dissociative process proceeds through bond cleavage via TS A that generates a delocalised allylic anion and isatin, that recombines in an effectively barrierless process to generate 67. The calculated low barrier to C-C cleavage was postulated to be a consequence of the computational omission of the assumed ammonium counterion ([tBu-BIMP-H] + or [DBU-H] + ) that would be expected to raise this barrier . Indeed, when this ionic fragmentation pathway was explored with [tBu-BIMP-H] + included as the ammonium counterion the barrier to C-C bond cleavage via ionic fragmentation was raised to 7.8 kcal•mol -1 (see Fig. ). This barrier is consistent with that calculated in a stepwise dissociation-recombination process in an anion-accelerated amino-Cope rearrangement (8.6 kcal•mol -1 ) by Houk and Njardarson . The observed retention of configuration with high enantiospecificity within the products is consistent with the bond-breaking process to generate the allylic anion and isatin, followed by recombination upon the re-face of the isatin, occurring at a faster rate than conformational change and bond rotation to allow si-face addition of the isatin that leads to reduced enantioselectivity. Further evidence for an anionic pathway can be taken from the known reversibility of crotyl and allylic Grignard additions to aldehydes and ketones . To experimentally probe the validity of this proposed anionic fragmentation pathway, taking isolated racemic [2,3]-rearrangement products that differed in the electronic effect of C(1')-aryl substituents with tBu-BIMP 7 showed that inclusion of electron withdrawing substituents led to enhanced reaction rates 50 . Hammett analysis (Fig. ) revealed a r value of +0.76 when plotted against the substituent constant σ-, consistent with the build-up of negative charge within the rate limiting transition state of the reaction and the proposed anionic fragmentation. Given the importance of understanding fundamental stereochemical chemical processes the enantiospecificity observed in this pathway will have broader implications for a plethora of other synthetic transformations.