Scheme 3. Synthesis of symmetrical geminal diborons from alkynes or alkenes. ^aReaction condition A: terminal alkyne (1.7 mmol), CuCl (5.0 mol%), KOtBu (0.1 equiv.), xantphos (6.0 mol%), HBpin (2.4 equiv.), toluene, 23 °C, 15 h. ^bReaction condition B: vinyl pinacol boronic ester (2.0 mmol), CuCl (5.0 mol%), KOtBu (0.1 equiv.), xantphos (6.0 mol%), HBPin (1.2 equiv.), toluene, 23 °C, 15 h.