- Mechanism of metal-free hydrogen transfer between amine-boranes and aminoboranes
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The kinetics of the metal-free hydrogen transfer from amine-borane Me 2NH?BH3 to aminoborane iPr2N=BH 2, yielding iPr2NH?BH3 and cyclodiborazane [Me2N-BH2]2 via transient Me 2N=BH2, have been investigated in detail, with further information derived from isotopic labeling and DFT computations. The approach of the system toward equilibrium was monitored in both directions by 11B{1H} NMR spectroscopy in a range of solvents and at variable temperatures in THF. Simulation of the resulting temporal-concentration data according to a simple two-stage hydrogen transfer/dimerization process yielded the rate constants and thermodynamic parameters attending both equilibria. At ambient temperature, the bimolecular hydrogen transfer is slightly endergonic in the forward direction (ΔG1° (295) = 10 ± 7 kJ?mol-1; ΔG 1(295) = 91 ± 5 kJ?mol-1), with the overall equilibrium being driven forward by the subsequent exergonic dimerization of the aminoborane Me2N=BH2 (ΔG 2°(295) = -28 ± 14 kJ?mol-1). Systematic deuterium labeling of the NH and BH moieties in Me 2NH?BH3 and iPr2N=BH2 allowed the kinetic isotope effects (KIEs) attending the hydrogen transfer to be determined. A small inverse KIE at boron (kH/kD = 0.9 ± 0.2) and a large normal KIE at nitrogen (kH/kD = 6.7 ± 0.9) are consistent with either a pre-equilibrium involving a B-to-B hydrogen transfer or a concerted but asynchronous hydrogen transfer via a cyclic six-membered transition state in which the B-to-B hydrogen transfer is highly advanced. DFT calculations are fully consistent with a concerted but asynchronous process.
- Leitao, Erin M.,Stubbs, Naomi E.,Robertson, Alasdair P. M.,Helten, Holger,Cox, Robert J.,Lloyd-Jones, Guy C.,Manners, Ian
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Read Online
- METHODS OF PREPARATION OF AMINOBORANES AND APPLICATIONS FOR BORYLATION AND SUZUKI COUPLING
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The present invention relates to a process for the preparation of aminoboranes (RR′N—BH2), and aminoborane-amine complexes (RR′N—BH2:NRR′R″), wherein R, and R′ of the aminoborane can be the same or different alkyl groups, and R, R′, and R″ of the amine complex can be the same or different alkyl groups or hydrogen. The dialkylaminoboranes have been utilized for the preparation of aryl and alkenylboronate esters via dehaloborylation of the corresponding aryl or alkenyl halides, followed by treatment with alcohols or diols. The process and the products thereof are within the scope of this disclosure.
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- Polyaminoborane main chain scission using N-heterocyclic carbenes; Formation of donor-stabilised monomeric aminoboranes
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The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH 2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2NBCl2 was characterized by single crystal X-ray diffraction.
- Stubbs, Naomi E.,Jurca, Titel,Leitao, Erin M.,Woodall, Christopher H.,Manners, Ian
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supporting information
p. 9098 - 9100
(2013/09/24)
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- Metal-free dehydrogenation of amine-boranes by an N-heterocyclic carbene
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The dehydrogenation of primary and secondary amine-boranes (RNH 2·BH3 and R2NH·BH3; R = alkyl groups) was studied using the bulky N-heterocyclic carbene IPr (IPr = [(HCNDipp)C:]; Dipp = 2,6-iPr2C6H3) as a stoichiometric dehydrogenation agent. In the case of primary amine-boranes, carbene-bound adducts IPr·BH2-NH(R)-BH3 were obtained in place of the desired polymers [RNH-BH2]n. The secondary amine-borane iPr2NH·BH3 participated in dehydrogenation chemistry with IPr to afford the aminoborane [iPr2NBH2] and the dihydroaminal IPrH 2 as products. Attempts to induce H2 elimination from the arylamine-borane DippNH2·BH3 yielded a reaction mixture containing the known species IPr·BH2NHDipp, IPr·BH2NH(Dipp)-BH3, free DippNH2 and IPrH2. The new hindered aryl-amine borane adduct Ar*NH 2·BH3 [Ar* = 2,6-(Ph2CH) 2-4-MeC6H2] underwent a reaction with IPr to give IPr·BH3 and free Ar*NH2, consistent with the presence of a weaker N-B dative bond in Ar*NH2· BH3 relative to its less hindered amine-borane analogues. The Royal Society of Chemistry 2013.
- Sabourin, Kyle J.,Malcolm, Adam C.,McDonald, Robert,Ferguson, Michael J.,Rivard, Eric
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p. 4625 - 4632
(2013/05/08)
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- Synthesis and the thermal and catalytic dehydrogenation reactions of amine-thioboranes
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A series of trimethylamine-thioborane adducts, Me3N· BH2SR (R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C6F 5 [2e]) have been prepared and characterized. Attempts to access secondary and primary amine adducts of thioboranes via amine-exchange reactions involving these species proved unsuccessful, with the thiolate moiety shown to be vulnerable to displacement by free amine. However, treatment of the arylthioboranes, [BH2-SPh]3 (9) and C6F 5SBH2·SMe2 (10) with Me2NH and iPr2NH successfully yielded the adducts Me2NH· BH2SR (R = Ph [11a], C6F5 [12a]) and iPr 2NH·BH2SR (R = Ph [11b], C6F5 [12b]) in high yield. These adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me2N=BH 2, derived from the cyclic dimer [Me2N-BH 2]2 (13), and iPr2N=BH2 (14), respectively. Attempts to prepare the aliphatic thiolate substituted adducts R2NH·BH2SR′ (R = Me, iPr; R′ = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form aminothioboranes, R2N= BH(SR′). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and 12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr2N=BH(SPh) (22) and iPr2N=BH(SC6F5) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2 mol % loading of [Rh(μ-Cl)cod]2 in the case of the former species. Similar studies with adduct 11a evidenced a competitive elimination of H2 and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated analogue 12a was found to be resistant to dehydrogenation.
- Robertson, Alasdair P. M.,Haddow, Mairi F.,Manners, Ian
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p. 8254 - 8264
(2012/09/22)
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- Catalytic redistribution and polymerization of diborazanes: Unexpected observation of metal-free hydrogen transfer between aminoboranes and amine-boranes
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Ir-catalyzed (20 °C) or thermal (70 °C) dehydrocoupling of the linear diborazane MeNH2-BH2-NHMe-BH3 led to the formation of poly- or oligoaminoboranes [MeNH-BH2]x (x = 3 to >1000) via an initial redistribution process that forms MeNH 2?BH3 and also transient MeNH=BH2, which exists in the predominantly metal-bound and free forms, respectively. Studies of analogous chemistry led to the discovery of metal-free hydrogenation of the B=N bond in the "model" aminoborane iPr2N=BH2 to give iPr2NH?BH3 upon treatment with the diborazane Me3N-BH2-NHMe-BH3 or amine-boranes RR′NH?BH3 (R, R′ = H or Me).
- Robertson, Alasdair P. M.,Leitao, Erin M.,Manners, Ian
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supporting information; experimental part
p. 19322 - 19325
(2012/01/13)
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