are not observed in this system under typical reaction conditions (See SI Figure for details). As a consequence, the observation of crossover products is taken as a measure of the intermolecularity of the [1,2]-and [2,3]-rearrangement processes. In the [1,2]-rearrangement case this is assumed to be representative of competition between "in-cage" radical-radical recombination (considered as "pseudointramolecular") or diffusion of the radicals followed by recombination (considered as "intermolecular"). A radical trap experiment using 1 equivalent of TEMPO did not fully inhibit the reaction, but gave the [1,2]-rearrangement product in a lower product yield (30%) with drastically reduced product intermolecularity (0.9%) and a substantial increase in enantiopurity (97:3 er). TEMPO adduct 5 was isolated in 8% yield, consistent with TEMPO trapping an allylic radical intermediate (Figure ). The use of a viscous and polar solvent, ethylene carbonate, resulted in reduced intermolecularity (3.9%) with the [1,2]-rearrangement product again obtained with enhanced enantiopurity (94:6 er). Rearrangement of the corresponding 13 C-labelled diphenyl-substituted salts 2-[ 13 C1]-2e and 2'-[ 13 C1]-2e gave [1,2]-rearrangement product 3e in 77:23 er with significant levels of intermolecularity (32%). Performed in the presence of TEMPO, this rearrangement proceeded with remarkably reduced intermolecularity (0.4%) and increased product enantioselectivity (86:14 er) as well as giving 11% of the TEMPO-adduct 6. To further probe the consistent trend in higher product enantioselectivity with reduced intermolecularity, the [1,2]-rearrangement products arising from the C-labelled diphenyl-substituted salts 2-[ 13 C1]-2e and 2'-[ 13 C1]-2e (both in the absence and presence of TEMPO) were further analysed through preparative separation of the product enantiomers (from samples of 71:29 er and 84:16 er respectively). In both instances, higher intermolecularity was observed in the (S)-enantiomer (50% intermolecularity without TEMPO, 2% with TEMPO) than the (R)enantiomer (29% intermolecularity without TEMPO, 0.3% with TEMPO). Using this data, the enantioselectivitity of the inter-and pseudo-intramolecular processes can be calculated. In both the absence and presence of TEMPO, the pseudo-intramolecular process is highly enantioselective (calculated as 78:22 and 84:16 er, respectively), whilst the intermolecular process leads to significantly reduced product enantioselectivity giving close to racemic product (59:41 and 44:56 er, respectively). These experiments are consistent with "in-cage" radical-radical recombination being essential for high enantioselectivity, with the addition of TEMPO increasing product enantioselectivity by efficiently trapping the diffused radicals to minimise the intermolecular reaction that proceeds with reduced enantioselectivity. A cyclopropyl-substituted allyl substrate 2w was next prepared as a control to probe the rate of recombination within the radical cage (Figure ). Despite an assumed rate of ~10 8 s -1 for the ring opening of an intermediate cyclopropyl methyl radical, only the [1,2]-product 3w was observed, consistent with the rate of radical recombination (previously estimated as >10 11 s -1 ) 7 outcompeting ringopening of the cyclopropyl fragment (Figure ). The possibility of in situ identification of diffused radical intermediates was next sought. Given the significant 32% intermolecularity measured for the [1,2]-rearrangement of diphenyl substituted salt 2e, this substrate was selected for EPR studies (Figure ). Initial attempts to directly detect radical intermediates proved unsuccessful, largely hampered by strong microwave absorption of the reaction solution. To overcome this, an alternative approach was undertaken through monitoring the consumption of TEMPO by EPR in two distinct rearrangement reactions. Firstly, the rearrangement of diphenyl-substituted salt 2e (R = Ph), which proceeds with DFT was used to further investigate the mechanism and periselectivity of this reaction using tetramisole and allylic ammonium salt substrate 2a, with UM06/6-31G*/PCM(acetonitrile) at 50 °C with Grimme's D3 empirical dispersion corrections as implemented in Gaussian16. The proposed catalytic cycle is shown in Figure . Notably, the inclusion and location of the bromide counterion that participates in multiple non-covalent stabilising C-H/N-CH•••Br -interactions within all structures on the catalytic cycle was critical for attaining relative transition state energies that reproduce the experimentally observed preferential formation of the [1,2]-rearrangement product. In addition, exploration of the rearrangement for all transition structures involving both facial approaches of the reactive allylic group for [1,2]-and [2,3]-processes was established, with the lowest energy structures used to construct the catalytic cycle (Figure ). There are four factors that govern the selectivities observed in this reaction (Figure As expected, Re-Concerted-[2,3]-TS, which involves bond forming and breaking at both the primary and the tertiary carbons of the allylic group, experiences the greatest distortion, further explaining why the [1,2]-process is favored over both the concerted and the stepwise [2,3]processes. Computational analysis of a model system in which the allylic group bears just a single phenyl substituent predicts a switch in selectivity to now favour the concerted [2,3]process, in keeping with experimental results.