Following a preliminary assessment of reaction variables (see ESI for full details), the desired conjugate addition product 4 was obtained in 61% NMR yield and ca. 4:1 d.r. using chloroacetic acid as a catalyst (entry 1). Based on this, we explored the use of chiral phosphoric acid catalysts 5 and 6 to induce enantioselective bond formation (entries 2-4); however, despite similar d.r. to entry 1, levels of asymmetric induction were poor. Enantioselectivity could not be improved despite broad screening (see ESI). Despite an enantioselective process appearing unsuccessful, promising diastereoselectivity was observed using simple carboxylic acid catalysts (e.g., entry 1). We therefore sought to develop a diastereoselective approach to the rare disubstituted aziridine products. Performing the reaction in aromatic solvents, such as PhMe (entry 5) gave improved yields; however, at the expense of diastereocontrol. In contrast, excellent diastereoselectivity was obtained using polar protic solvents such as i-PrOH (entry 6), but at lower yield. Mixtures of i-PrOH and PhMe gave results in between the extremes of the pure solvents (entries 7 and 8). No further improvements to reaction conditions could be made. The significant impact of these solvents compelled further investigation (Scheme 3). process. Experiment and theory are in agreement when the barriers for the back reaction are considered. The reversions to 8 by the two stereoisomers have barriers of 25.3 and 26.9 kcal/mol; these are lower than the rate-limiting barrier leading to 8 and thus reversion of 4 and equilibration of the two isomers is to be expected. The initial emergence of a modest excess of anti-4 (see ESI for details) is consistent with this isomer having the higher barrier for reversion. The calculations support a thermodynamically controlled preference, and suggest any energy difference between the syn and anti isomers is below the limit of what can be accurately computed, supportive of the nonstereoselective formation observed. Turning to the stereoselective degradation of 4, it was assumed that this depends on trace Lewis or Brønsted acid that can complex the quinoline nitrogen of 4, hence the reaction proceeding in the presence of both i-PrOH and silica. Further computational studies used a concentration of 1 × 10 -18 M of protonated i-PrOH, 4, and settings for solvation by i-PrOH to model these conditions. Protonation of 4 promotes fragmentation with computed barriers of 26.6 kcal/mol for fragmentation of syn-4 and 28.1 kcal/mol for anti-4. This is consistent with the direction and degree of stereoselectivity observed and with catalysis by trace acid. The origin of the stereoselectivity can be understood by considering the effect of protonating the quinoline nitrogen. This creates a cationic aromatic surface (Figure ) that makes a strong interaction with the electron rich aniline ring, renedering the system pseudo-cyclic. A further stabilizing influence is a throughspace interaction between the electron-rich belt around the Cl and the nearby NH + . In the syn-11 structure (shown looking down the fragmenting bond in Figure ), it is clear that the aniline-π cation interaction and Cl-NH interaction can be comfortably retained while permitting the methyl group to be anti-periplanar to the quinoline. Inversion to give anti-11 might require loss of the Cl-NH interaction or the methyl clashing with the quinoline. Instead, the preferred structure retains these features but in doing so requires the pseudo-cyclic structure held together by the aniline-π cation interaction to be boat-like. Thus, syn-4 is more readily protonated and fragments more rapidly than anti-4. With no further optimization of the conjugate addition step possible, we proceeded with the high d.r. method and combined with a base-mediated SNi to enable access to the desired 1,2disubstituted aziridine targets (Scheme 4).