- Catalytic Hydroboration of Esters by Versatile Thorium and Uranium Amide Complexes
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The challenging hydroboration of esters is achieved using simple uranium and thorium amides, U[N(SiMe3)2]3 and [(Me3Si)2N]2An[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)] (An = Th or U) acting as precatalysts in the reaction with pinacolborane (HBpin). All three complexes showed impressive catalytic activities, reaching excellent yields. A large scope of esters was investigated including aliphatic, aromatic, and heterocyclic esters that were transformed cleanly to the corresponding hydroborated alcohols, which readily hydrolyzed to the free alcohols. Moreover, the actinide catalysts demonstrated unexpected high functional tolerance toward nitro, halide, cyano, and heteroaromatic functional groups. The reaction exhibited excellent selectivity toward the ester when additional double and triple unsaturated C-C bonds were present. Lactones and poly caprolactone have been successfully cleaved to the monomeric units, showing a great promise toward polymer degradation and recycling. Detailed kinetic studies are provided in order to determine the rate dependence on the concentration of catalyst, HBpin, and ester. A plausible mechanism is proposed based on stoichiometric reactions, DFT calculations, thermodynamic measurements, and deuterium-labeling studies.
- Makarov, Konstantin,Kaushansky, Alexander,Eisen, Moris S.
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p. 273 - 284
(2022/01/03)
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- A Versatile NHC-Parent Silyliumylidene Cation for Catalytic Chemo- And Regioselective Hydroboration
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This study describes the first use of a silicon(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]I (1, IMe =:C{N(Me)C(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h-1 reaction time: 0.17 h, yield: 98%, TOF = 58.7 h-1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC(-O)H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.
- Leong, Bi-Xiang,Lee, Jiawen,Li, Yan,Yang, Ming-Chung,Siu, Chi-Kit,Su, Ming-Der,So, Cheuk-Wai
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supporting information
p. 17629 - 17636
(2019/11/11)
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- Hydroboration of Carbon Dioxide Using Ambiphilic Phosphine-Borane Catalysts: On the Role of the Formaldehyde Adduct
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Ambiphilic phosphine-borane derivatives 1-B(OR)2-2-PR′2-C6H4 (R′ = Ph (1), iPr (2); (OR)2 = (OMe)2 (1a, 2a); catechol (1b, 2b) pinacol (1c, 2c), -OCH2C-(CH3)2CH2O- (1d)) were tested as catalysts for the hydroboration of CO2 using HBcat or BH3·SMe2 to generate methoxyboranes. It was shown that the most active species were the catechol derivatives 1b and 2b. In the presence of HBcat, without CO2, ambiphilic species 1a, 1c, and 1d were shown to transform to 1b, whereas 2a and 2c were shown to transform to 2b. The formaldehyde adducts 1b·CH2O and 2b·CH2O are postulated to be the active catalysts in the reduction of CO2 rather than being simple resting states. Isotope labeling experiments and density functional theory (DFT) studies show that once the formaldehyde adduct is generated, the CH2O moiety remains on the ambiphilic system through catalysis. Species 2b·CH2O was shown to exhibit turnover frequencies for the CO2 reduction using BH3·SMe2 up to 228 h-1 at ambient temperature and up to 873 h-1 at 70 °C, mirroring the catalytic activity of 1b. (Figure Presented)
- Declercq, Richard,Bouhadir, Ghenwa,Bourissou, Didier,Légaré, Marc-André,Courtemanche, Marc-André,Nahi, Karine Syrine,Bouchard, Nicolas,Fontaine, Frédéric-Georges,Maron, Laurent
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p. 2513 - 2520
(2015/04/22)
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- Role of catechol in the radical reduction of B-alkylcatecholboranes in presence of methanol
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Mechanistic investigations on the previously reported reduction of B-alkylcatecholboranes in the presence of methanol led to the disclosure of a new mechanism involving catechol as a reducing agent. More than just revising the mechanism of this reaction, we disclose here the surprising role of catechol, a chain breaking antioxidant, which becomes a source of hydrogen atoms in an efficient radical chain process.
- Povie, Guillaume,Villa, Giorgio,Ford, Leigh,Pozzi, Davide,Schiesser, Carl H.,Renaud, Philippe
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scheme or table
p. 803 - 805
(2010/06/12)
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