In the presence of base, diphenylphosphine oxide (4a) and VBX 1a can react through an a-attack to give intermediate G, which could undergo double 1,2-shifts to reach the terminal alkene 5a via carbene H (pathway IV). Alternatively, the regioselectivity can be rationalized through a phospha-Michael reaction, which is a versatile and powerful tool for phosphorus-carbon bond formation. NMR experiments in THF-d8 were carried out to observe any reaction intermediates. In the absence of VBX 1a, no reaction was observed between DBU and 4a. When 1a was added, product 5a started forming immediately. Likewise, no adduct could be observed upon mixing 1a and 4a in the absence of DBU.