With an improved understanding of how the identity of the carbene precursor impacts the mechanism of the reaction to favor stereodivergent formation of either the azetidine or the dehydropiperidine product, the scope was explored (Scheme 5). First, a series of isomerically pure (E vs. Z) methyleneaziridines 1a-h were reacted with diazoesters 2d-n, where R 3 could be an alkyl, aryl or -CO2Me group. Based on computations, the lack of the conjugated CO2Me group directly attached to the C=C alkene of the carbenoid precursor kinetically favors the formation of the azetidine. Treatment of the gem-dimethylsubstited methyleneaziridine 1c with carbene precursors 2d-f showed that increasing the group on the β carbon from Me (2d) to Et (2e) to Ph (2f) increased the selectivity for formation of the azetidine from 2.1:1 to >15:1, which can be ascribed in part to an increased rotational barrier between the initial ylides and the increased energy difference between them. The inset in the left-hand column of Scheme 5 shows computations on the system Standard conditions: slow addition of the vinyldiazo (3.5-10 equiv) over 2 h to a mixture of 3 mol% Rh 2 (esp) 2 and aziridine (1.0 equiv) in CH 2 Cl 2 at rt. The yield of the minor product is given in parenthesis after the product ratio. The first letter of the product refers to the aziridine 1 employed, while the second letter is the carbene precursor 2. a The ethyl ester of the diazocarbene precursor was used. Substitution on the aryl ring was explored in reactions of 1c with carbene precursors 2f-j.