Initial explorations of divergent N-heterocycle formation. Studies were initiated by selecting the donor-acceptor vinyl diazoester 2a to determine if Rh-catalyzed addition of methyleneaziridine 39 1a to 2a would lead to: 1) ring expansion via opening of the aziridinium ylide by C1 of INT1 to form the azetidine 3a or 2) opening at C3 of INT1' to furnish the dehydropiperidine 4a (Scheme 2A). Somewhat to our surprise, 3a and 4a were formed in 1:1 ratio and 75% NMR yield using Rh2(OAc)4 as the catalyst (Scheme 2A, entry 2). Increasing the size of the bridging carboxylates on the dinuclear Rh complex (entries 3-5) improved the selectivity for 4a over 3a to 2:1 using Rh2(esp)2. However, the increased steric bulk in the Davies catalyst (entry 6, Rh2(R-BTPCP)4, Rh2tetrakis[(R)-1-(4-bromophenyl)-2,2-diphenylcyclopropane carboxylate) gave no products at rt; heating the reaction gave only dimerization of 2a. A brief solvent screen for carbene transfer catalyzed by Rh2(esp)2 was carried out (entries 7-9), with CH2Cl2, toluene, 1,2-dichloroethane (DCE) and ethyl acetate are proving to be viable solvents. However, there was no significant change in the ratio of 4a:3a, suggesting that solvent polarity does not play a key role. Increasing the size of the Me group on 1a to a C5H11 group (1b) in reaction with 2a also gave ~1:2 mixtures of 3ba and 4ba. Increasing the bulk on the ester group of 2a to a benzoate in 2b provided the same selectivity for the dehydropiperidine 4ab over azetidine 3ab 1.8:1, indicating sterics on the ester do not influence the reaction outcome. A solid-state X-ray crystal structure of 4ab was obtained to verify its structure, as NMR spectroscopy can often be misleading in terms of assigning product identities. To better understand the details of this transformation and explore how we might control the selectivity of the ring expansion, Density Functional Theory (DFT) calculations were performed according to our previous calculations on related systems, derives from the nucleophilic addition of the aziridine 1a to the corresponding Rh2-carbenoid intermediate. Our results indicate that the barriers associated with the aziridine ring-opening and concomitant four/six-membered ring formation (via TS1/TS1') are rather low (< 5 kcal/mol), with the barrier leading to 3aa slightly favored (G ≠ = 1.5 kcal/mol). Despite this, the formation of 4aa is strongly thermodynamically favored; when coupled with the negligible energy difference between the key ylide intermediates INT1 and INT1' (G = 0.3 kcal/mol) and a room-temperature feasible rotational barrier between them (G ≠ = 10 kcal/mol), results in an equal possibility of formation of both bicyclic products.