The key C-C bond formation between radical I and 2a could take place in several ways. Based on previous mechanistic studies with VBX and EBX , the radical coupling could proceed via -addition, -addition or a concerted radical coupling pathway to deliver product 3. The high stereoselectivity observed in the reaction, as well as formation of 3o-D as the only isomer indicates a concerted process. Density functional theory (DFT) studies of the three pathways using DHP 1a and VBX 2a showed that the lowest energy profile was found for the concerted radical pathway, proceeding via the depicted transition state TS1 (12.7 kcal/mol). Scheme 5. Mechanistic studies.