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

The hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) proceeds at 100 degrees C. For conversion at room temperature three different organocalcium catalysts have been investigated: the calcium hydride complex [DIPPnacnacCaH center dot(THF)](2) (1, DIPPnacnac CH{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)), Ca[2Me(2)N-alpha-Me3Si-benzyl)(2)center dot(THF)(2) (2) and DIPPnacnacCa(H-BBN)center dot(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 Ph(2)CHCH(2)Bcat but (Ph2CHCH2)(3)B is formed instead. Organocalcium compounds catalyze the decomposition of HBcat to B-2(cat)(3) and BH3 (or B2H6) and the latter is involved in hydroboration of DPE. The calcium-catalyzed decomposition of HBcat was investigated with B-11 NMR and the signals were assigned to the following species: B-2(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)center dot(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( H(2)Bpin)] 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. (C) 2011 Elsevier B. V. All rights reserved.