Previously, Wei et al. have conducted a mechanistic study on the chosen aldehyde selective reaction (Figure .C). We performed a preliminary mechanistic study with a simplified COBI catalyst (ie Cat-A). The obtained results are applicable to all other reactions and our conclusions align with those of Wei et al. (Figure and) The complexation between the COBI catalyst and the aldehyde leads to INT1, where the formed B-O bond is syn to the N-H bond. The ΔG of the complexes with the opposite stereochemistry at the boron atom are at least 3.3 kcal mol -1 higher compared to INT1 across the various reaction systems in this study (SI Figure .6). The addition of the diazo on INT1 gives an adduct, INT2, which undergoes a nitrogen elimination. Sequentially, a 1,2 shift or oxygen insertion occurs to give the products, which can be a ketone, aldehyde or epoxide.