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

Abstract

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) α-H elimination for the formation of allylic phosphorus ylide, (4) regioselective α-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 α-addition mode occurring prior to γ-addition. The value of local nucleophilicity calculated using the Parr function proves that the α-carbon is more reactive than γ-carbon thus leading to a lower energy barrier. NCI and AIM analyses also show that the transition states involved in α-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 Cγ–O1 bonds.