A valuable, inexpensive Cu-I/N-heterocyclic carbene catalyst for the selective diboration of styrene

The complexes [Cu(NHC)-(NCMe)]BF4 (NHC = N-heterocyclic ligand), with bis(catecholato)diboron (B-2(cat)(2)) as the boron source, efficiently catalyze the diboration of styrene with very high degrees of conversion. With the appropriate NHC ligand, the reaction proceeds quantitatively toward the diborated derivative PhCH(Bcat)-CH2(Bcat). The [styrene]/[B-2(cat)(2)] ratio also has a strong effect on the selectivity: the use of an excess of styrene allows modification of the selectivity toward the formation solely of the monoborated derivative, PhCH2-CH2(Bcat). DFT calculations suggest that no oxidative addition processes take place at copper, but that intermediates containing coordinated sigma-bonds are involved in the catalytic cycle.