Ito et al.

, p. 1043,1044 (1961)

Brown,Heseltine

, p. 1197,1201 (1967)

-

Hough et al.

, p. 864 (1955)

-

Molecular Quadrupole Moments, Magnetic Anisotropies, and Charge Distributions of Borazine, B-Trichloroborazine, N-Trimethylborazine, and B-Trichloro-N-trimethylborazine. Comparison with Benzene and Its Derivatives.

Dennis, Gary R.,Ritchie, Geoffrey L. D.

, p. 8403 - 8409 (1993)

Measurements of the dilute-solution molar Kerr constants, field-gradient birefringence constants, and Cotton-Mouton constants of borazine and three substituted borazines as solutes in cyclohexane or carbon tetrachloride at 25 deg C are reported.The observations yield the first direct experimental values of the effective polarizability anisotropies, electric quadrupole moments, and magnetic anisotropies, which are important descriptors of the molecular charge distributions.A comparison of the results for borazine with literature data for benzene shows that all three properties have the same signs in the two species but that all are considerably smaller in magnitude in borazine than in benzene.An analysis of the magnetizabilities indicates that the extent of electron delocalization in borazine is only about one-third of that in benzene, a conclusion which is consistent with a range of other evidence.

Meller,A.

, p. 1670 - 1679 (1968)

Rhodium-catalyzed formation of boron-nitrogen bonds: A mild route to cyclic aminoboranes and borazines

Jaska,Temple,Lough,Manners

, p. 962 - 963 (2001)

Secondary amine-borane adducts R2NH·BH3, which are stable to H2 elimination below 100 °C, undergo efficient catalytic dehydrocoupling at 25-45 °C in the presence of RhI or RhIII complexes to quantitatively form cyclic aminoboranes [NR2-BH2]2 (1: R = Me or 2: cyclo-C4H8); under similarly mild conditions, the analogous adducts NH3·BH3 and MeNH2·BH3 yield borazines [RN-BH]3 (3: R-H or 4: R = Me) in yields limited by intermolecular coupling reactions.

Bissot,Parry

, p. 3481 (1955)

Transition metal-catalyzed formation of boron-nitrogen bonds: Catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

Jaska, Cory A.,Temple, Karen,Lough, Alan J.,Manners, Ian

, p. 9424 - 9434 (2003)

A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me2NH·BH3 thermally eliminates hydrogen at 130 °C in the condensed phase to afford [Me2N-BH2]2 (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me2N=BH2 as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me2NH·BH3 was detected in the range 150-450 °C. The dehydrocoupling of Me2NH·BH3 was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(μ-Cl)]2 in solution at 25 °C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me2NH·BH3 was also demonstrated. This new catalytic method was extended to other secondary adducts RR′NH·BH3 which afforded the dimeric species [(1,4-C4H8)N-BH2]2 (2) and [PhCH2(Me)N-BH2]2 (3) or the monomeric aminoborane iPr2N=BH2 (4) under mild conditions. A new synthetic approach to the linear compounds R2NH-BH2-NR2-BH3 (5: R = Me; 6: R = 1,4-C4H8) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R2NH·BH3. The catalytic dehydrocoupling of NH3·BH3, MeNH2· BH3, and PhNH2·BH3 at 45 °C to give the borazine derivatives [RN-BH]3 (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(μ-Cl)]2 catalyzed dehydrocoupling of Me2NH·BH3 together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.

Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine-boranes using a [Rh(Xantphos)]+ catalyst

Johnson, Heather C.,Leitao, Erin M.,Whittell, George R.,Manners, Ian,Lloyd-Jones, Guy C.,Weller, Andrew S.

, p. 9078 - 9093 (2014)

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)]+ fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (Mn = 22 700 g mol-1, PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH 2) or elimination of amino-borane (with H3B· NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H 3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (Mn = 2 800 g mol-1, PDI = 1.8) or THF solvent (Mn = 52 200 g mol-1, PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the real monomer amino-borane H2B=NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.

Phillips,C.S.G. et al.

, p. 1202 - 1207 (1963)

Mechanisms of the thermal and catalytic redistributions, oligomerizations, and polymerizations of linear diborazanes

Robertson, Alasdair P. M.,Leitao, Erin M.,Jurca, Titel,Haddow, Mairi F.,Helten, Holger,Lloyd-Jones, Guy C.,Manners, Ian

supporting information, p. 12670 - 12683 (2013/09/23)

Linear diborazanes R3N-BH2-NR2-BH 3 (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me3N-BH2-NMe2-BH3 (1), Me3N-BH2-NHMe-BH3 (2), and MeNH 2-BH2-NHMe-BH3 (3). The mechanisms of the thermal and catalytic redistributions of 1 were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me3N·BH3 (8) and [Me2N-BH2]2 (9a)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H2B(μ-H)(μ-NMe 2)BH2] (5) and NMe3 as intermediates, with the former operating as a catalyst and accelerating the redistribution of 1. Intermediate 5 was then transformed to amine-borane 8 and the cyclic diborazane 9a by two different mechanisms. In contrast, under catalytic conditions (0.3-2 mol % IrH2POCOP (POCOP = κ3-1,3-(OPtBu 2)2C6H3)), 8 was found to inhibit the redistribution of 1 by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of 8 and Me2Ni - BH2 (9b), which spontaneously dimerizes to give 9a, with the absence of 5 and BH3 as intermediates. The mechanisms elucidated for 1 are also likely to be applicable to other diborazanes, for example, 2 and 3, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHi - BH2 (10) to form oligoaminoborane [MeNH-BH 2]x (11) or polyaminoborane [MeNH-BH2] n (16) following the initial redistribution reaction.