The complexes [Cu(NHC)(NCMe)]BF₄ (NHC=N-heterocyclic ligand), with bis(catecholato)diboron (B₂(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]/[B₂(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, PhCH₂CH₂(Bcat). DFT calculations suggest that no oxidative addition processes take place at copper, but that intermediates containing coordinated -bonds are involved in the catalytic cycle.