Our investigation of the COBI-catalysed reactions suggests that reaction dynamics play a significant role in controlling selectivity. In the ketone and epoxide selective reactions, the stereochemistry is controlled kinetically by the initial addition between the diazo and the catalyst-aldehyde complex (TS1). However, the process beyond TS1 is controlled by reaction dynamics, which influence the chemoselectivity of the outcome. In the aldehyde selective reaction, both the stereoselectivity and the chemoselectivity are dynamically controlled.