A Pd-induced cascade B-C-vinyl coupling that produces multiple B-C-vinyl bonds starting from a single B-I bond has been demonstrated. The process is most probably stimulated by the geometrical disposition of the B-H bonds confronting the B-Pd sites, along with the hydride character of the B-H units. Two and one B-C-vinyl couplings on the metallacarborane substrate have been generally obtained, but formation up to six B-C-vinyl bonds has been observed. A theoretical reaction mechanism involving an unprecedented B-H activation is proposed to interpret the multisubstitution process.