A more intriguing question was whether we might be able to achieve catalyst control over the size of the ring formation, independent of the identity of the carbene precursor. Reaction of 1c with 2c (Scheme 6A) with a series of dinuclear Rh complexes supported by bridging carboxylate ligands and no axial ligand showed the best selectivity for 4cc over 3cc using Rh2(esp)2 at 3:1 (Fig 5A, entry 2), but the others all gave an ~ 1:1 mixture of products (entries 1, 3, 4). We were curious if more drastic changes to the ligand might furnish valuable insights to drive future reaction and catalyst design. We tried two dinuclear Rh catalysts developed by the Darko group, (Rh2(OAc)3(p-OMe-ArTCB) and Rh2(OAc)3(p-NO2-ArTCB). These catalysts replace one of the -OAc ligands of Rh2(OAc)4 with a bridging N/S ligand, as shown in Scheme 6A; however, the presence of the N/S bridging ligand with sulfur position as an axial ligand gave no change in the product distribution (entries 6-7). We were pleased to find that a Rh2(espn)2Cl complex (entry 5), where the typical bridging carboxylate ligand has been replaced with a bridging amide ligand, reversed the product distribution in favor of the azetidine 3cc over the dehydropiperidine 4cc by a ratio of 3:1. This catalyst control was also observed using 1b (Scheme 6B), where Rh2(esp)2 gave a 3.2:1 ratio of piperidine 4bc over azetidine 3bc; the selectivity was reversed using Rh2(espn)2Cl to 7. of 3bc. We hypothesized that the amide groups of Rh2(espn)2Cl, which are bound to only one Rh atom of the dinuclear Rh complex (Scheme 6C), engage in hydrogen-bonding with the aziridine substrate. Specifically, we propose that the amides of the catalysts H-bond to both the carbonyl group of the carbamate and the carbonyl of ester from the carbene precursor. Attempts to methylate the amide nitrogen of Rh2(espn)2Cl to determine if disrupting hydrogen-bonding changed the reaction outcome were unsuccessful. Thus, we turned to computations (Scheme 6C, bottom) for support. Among the possible metal bound ylides involving aziridine 1a, the most stable species was computed to be B; this is the intermediate that ultimately leads to the experimentally observed formation of the four-member ring upon release of the transition metal fragment. Computations showed that this species is indeed stabilized by several NHꞏꞏꞏO=C hydrogen bonds. Note that the RhꞏꞏꞏC distance is rather long (ca. 2.7 Å), which indicates that the Rh2 fragment is only weakly bound to the ylide. We also observed catalyst control using a representative example of a dirhodium (II) metallopeptide class of catalysts developed by the Miller group (entry 8). To our delight, we not only saw selectivity for 3cc, but the first example of enantioselective induction into both 4cc (9% ee) and 3cc (8% ee). Ongoing studies in our group are focused on the elucidation of the details of the enantiodetermining transition state, as well as the development of improved enantioselective catalysts.