Calcium-mediated hydroboration of alkenes: “Trojan horse” or “true” catalysis?

The hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) proceeds at 100 °C. For conversion at room temperature three different organocalcium catalysts have been investigated: the calcium hydride complex [DIPPnacnacCaH·(THF)]2 (1, DIPPnacnac = CH{(CMe)(2,6-iPr2C6H3N)}2), Ca[2-Me2N-α-Me3Si-benzyl)2·(THF)2 (2) and DIPPnacnacCa(H-BBN)·(THF) (3, BBN = 9-borabicyclo[3.3.1.]nonane). Although up to 96% conversion of DPE is found, the product of the reaction is not the expected Ph2CHCH2Bcat but (Ph2CHCH2)3B is formed instead. Organocalcium compounds catalyze the decomposition of HBcat to B2(cat)3 and BH3 (or B2H6) and the latter is involved in hydroboration of DPE. The calcium-catalyzed decomposition of HBcat was investigated with 11B NMR and the signals were assigned to the following species: B2(cat)3, B(cat)2−, HBcat, BH3(THF), BH4− and B2H7−. A tentative mechanism for the formation of these species was proposed. The intermediate DIPPnacnacCa(BH4)·(THF)2 (5) was independently prepared by reaction of 1 and BH3(Me2S) and was structurally characterized by X-ray diffraction. Stoichiometric reaction of 1 with pinacolborane (HBpin) gave a trimeric complex [DIPPnacnacCa(H2Bpin)]3 (6) which was structurally characterized by X-ray diffraction. This complex does not react with DPE, also not at elevated temperatures. The possible equilibrium between 6 and 1/HBpin is therefore fully at the side of 6. As 6 is unstable in the presence of HBpin, no further catalytic conversions have been investigated.