Based on the previous observations on the electron-withdrawing group (EWG)-directed allylic functionalization and the notion on the capacity of B(MIDA) to accept adjacent electron density, a possible reaction mechanism was proposed (Scheme 5a). The reversible olefin coordination/nucleophilic substitution/selenide oxidation forms two regioisomeric intermediates A and B. Both are prone to undergo dehydrodeselenenylation or nucleophilic displacement of the selenium moiety. However, the activation effect from B(MIDA) by stabilizing the developing negative charge in the transition state TS-C would facilitate a regioselective syn-dehydrodeselenenylation, thereby shifting the equilibrium towards the alkenyl MIDA boronate formation. Interestingly, previous evidence from Tunge has suggested that the dehydrodeselenenylative elimination is the rate-limiting step. However, kinetic isotope effect studies in our case by using deuterated allyl MIDA boronate revealed a small KIE value of 1.2 (Scheme 5b). This result indicates that the C-H cleavage is no longer involved in the rate-limiting step, in accordance with an activation effect of B(MIDA) (by lowering the energy of TS-C) in this reaction.