1883-36-9Relevant academic research and scientific papers
Calcium-mediated hydroboration of alkenes: "trojan horse" or "true" catalysis?
Harder, Sjoerd,Spielmann, Jan
, p. 7 - 14 (2012)
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-iPr 2C6H3N)}2), Ca[2-Me 2N-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 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 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(H 2Bpin)]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.
