The adduct [MeO → Bpin–Bpin]⁻ efficiently mediates the β-boration of α,β-unsaturated imines formed in situ. The use of chiral phosphines as additives, and in particular the chiral phosphine (S)-MeBoPhoz, enables the catalytic asymmetric reaction to proceed with higher enantioselectivity than the analogue copper(I) mediated reaction.