Elucidating the mechanism and regioselectivity of phosphine-catalyzed transformation of MBH carbonate

The detailed mechanism and the role of catalyst in the PPh3-catalyzed [3+6] annulation reaction of MBH carbonate and dicyanoheptafulvene have been systematically investigated using a DFT method. Based on the calculations, the transformation of MBH carbonate includes several steps: (1) nucleophilic addition of PPh3 to MBH carbonate, (2) cleavage of the C-O bond, (3) & alpha;-H elimination for the formation of allylic phosphorus ylide, (4) regioselective & alpha;-addition of allylic phosphorus ylide with dicyanoheptafulvene, (5) intramolecular ring-closure, and (6) regeneration of the PPh3 catalyst. The C-O bond cleavage is considered to be the rate-determining step and the reaction between allylic phosphorus ylide and dicyanoheptafulvene is the regioselectivity-determining step with the & alpha;-addition mode occurring prior to & gamma;-addition. The value of local nucleophilicity calculated using the Parr function proves that the & alpha;-carbon is more reactive than & gamma;-carbon thus leading to a lower energy barrier. NCI and AIM analyses also show that the transition states involved in & alpha;-addition possess stronger weak interactions, which are responsible for stabilizing the corresponding transition state. The ELF shows that PPh3 promotes the release of BocO(-) anions mainly by alleviating the electron density between the C & gamma;-O1 bonds.

Automated summary

The detailed mechanism and role of the catalyst in the PPh3-catalyzed [3+6] annulation reaction of MBH carbonate and dicyanoheptafulvene were systematically investigated using a DFT method. The C-O bond cleavage was found to be the rate-determining step, while the reaction between allylic phosphorus ylide and dicyanoheptafulvene determined the regioselectivity with α-addition occurring prior to γ-addition. PPh3 promoted the release of BocO(-) anions primarily by alleviating the electron density between the Cγ-O1 bonds.