Borane complexes stability

The mechanism involves the dissociation of the coordinated borane 15 to generate a monoborane intermediate 16. Coordination of the alkene would generate the alkene borane complex. A /3-borylalkylhydride with B-H stabilization is certainly an important resonance structure of 17. An intramolecular reaction would extrude the alkyl boronate ester product and coordination of HBcat would regenerate the monoborane intermediate. 

NEUTRAL BORANE COMPLEXES 24.2.1 General Considerations and Stability... 

Since the discovery of NH3-BF3 by Gay-Lussac and Thenard12 in 1809, many neutral borane complexes have been described, mainly with N, P, As, O and S donor molecules. Their stabilities vary considerably many are formed at low temperatures only, whereas in other cases... 

The stability of the coordinative bond in the borane complexes is determined by the electrophilicity of the borane, by the nucleophilicity of the ligand and by steric factors. The electrophilicity of the borane, in turn, depends on the electronegativity of the groups bonded to the boron and on the overlap between the unoccupied p orbital of the boron and the occupied p orbitals of the atoms directly bonded to it (p -p interaction). The predominance of the latter is reflected by the fact that the relative acidities of halogenoboranes increase in a sense opposite to that expected from electronegativity considerations and is supported by the very weak Lewis acidity of the alkoxy- and amino-boranes. The importance of steric factors is pointed out by Brown and co-workers.13-15... 

In a number of cases, e.g. for CO, PH3, PF3 and thioethers, the existence of complexes or their surprising stability was explained in terms of a special n bond.16 However, experimental data and theoretical considerations seem to contradict the assumption that such kinds of n bonds exist.25 Borane complexes in which donor atoms of appropriate nucleophilicity are part of the group attached to the boron, e.g. BCl2(NMe2), BH2(PMe2), BBr2SMe, constitute a special case and exist due to intermolecular coordination, in the form of dimers, trimers or polymers.17-19... 

Based on the data in Table 1, on additional semiquantitative data and upon qualitative observations2,9,11 the following order of relative stabilities could be established for borane complexes ... 

For borane complexes of intermediate strength the order of relative stabilities can also be estimated from equilibrium data as well, obtained in gas phase or in solution.11,23 From the temperature dependence of the equilibrium constant for the dissociation in the gas phase the A% values can be determined with great accuracy.22... 

In addition to these limited procedures a number of experimental methods (vibrational spectroscopy, dipole moment measurements, electron diffraction, NMR, etc.) have been employed to determine the relative stabilities of these complexes.11,23 Intense effort has been directed towards establishing some kind of correlation between NMR parameters and stability of the borane complexes. The chemical shifts alone rarely show good correlation. However, complexation shifts (the chemical shift difference between the free and complexed borane or ligand) and various spin-spin coupling constants correlate better with calorimetric data, especially for ligands or boranes belonging to structurally similar series (Table 2).10,24... 

The stability and equilibrium properties of neutral borane complexes are not well known. Instead, the recent chemistry of the borane complexes is predominantly characterized by preparative and structural investigations. This is even more obvious for the charged borane complexes. 

The regioseledivity of the last reaction in Scheme 5.13 is not only because of the greater acidity of the methylene group, but also because some secondary and tertiary amides (e.g. /3-arylamides, /3-vinylamides, or /3-(phenylthio)amides, or borane complexes of /3-phosphino propionamides [132, 133]) are deprotonated at the /3 position under kinetic control to yield chelate-stabilized carbanions [58, 134], Illustrative examples of such remarkable metalations are shown in Scheme 5.14. 

Many phosphine-borane complexes Y3P BZ3 have been characterized. They include compouuds where Y = alkoxy, aUcyl, amino, halide, and hydride groups, and Z = alkyl, halide, and hydride. The stabilities of these complexes vary widely depending on the Lewis acidity and basicity of the boron and phosphorus moieties, respectively. The relative stabilities of Lewis acid-base complexes with BH3 are R3P > R3N > R3AS > R3Sb, but with BF3 the order is R3N > R3P > R3AS > RsSb. The stabihties of the borou halide complexes of phosphines follow the same order as the amine complexes BI3 > BBrs > BCI3 > BF3. 

This 1 1 complex, a white crystalline solid of considerable stability, is prepared either by heterogeneous absorption of diborane by ethylenediamine in a high vacuum apparatus or by reaction of ethylenediamine with the tetrahydrofurane-borane complex ... 

Intensive studies also showed that many transition metal complexes are able to catalyze aromatic C-H borylation of various arenes (Scheme 7), e.g., Cp Ir(H)(Bpin)(PMe3) [64,65], Cp Rh(Ti4-C6Me6) [65,66], ( 75-Ind)Ir(COD) [67], (776-mesitylene)Ir(Bpin)3 [67], [IrX(COD)]2/bpy (X = Cl, OH, OMe, OPh) [68-70]. A very recent study by Marder and his coworkers showed that [Ir(OMe)(COD)]2 can also catalyze borylation of C-H bonds in N-containing heterocycles [71]. For the Ir-catalyzed borylation reactions, it is now believed that tris(boryl)iridium(III) complexes [67,69], 40c, [72] are likely the reactive intermediates and a mechanism involving an Ir(III)-Ir(V) catalytic cycle is operative [67,69]. A recent theoretical study [73] provided further support for this hypothesis. A mechanism, shown in Scheme 8, was proposed. Interestingly, there are no cr-borane complexes involved in the Ir-catalyzed reactions. The very electropositive boryl and hydride ligands may play important roles in stabilizing the iridium(V) intermediates. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 84 Borane, thiocyqnato-halogenohydro-, 88 Borane, trialkoxy-amine complexes, 88 Borane, trifluoro-complexes Lewis acids, 87 van der Waals complexes, 84 Borane complexes aminecarboxy-, 84 aminehalogeno-, 84 amines, 82,101 B-N bond polarity, 82 preparation, 83 reactions, 83 bonds B-N, 88 B-0,88 B-S,88 31 bonds, 82 carbon monoxide, 84 chiral boron, 84 dimethyl sulfide, 84 enthalpy of dissociation, 82 halogenohydro, 84 hydro, 83 monocyano, 84 neutral, 81-90 stability, 81 NMR, 83... 

The nature and concentration of boron-containing compounds affect substantially the film formation process, chemical and phase composition, structure and surface morphology of the resultant films. The ability of compounds to be a source of boron depends on either kinetic stability (i.e. the symmetry of anion) in the case of higher boranes or B-N bond strength for the borane complexes. 

Diphenylmethyl bromide allowed to react 5 hrs. at room temp, with triethylamine-borane complex in nitromethane -> diphenylmethane. Y ca. 100%. - The yields depend on the stability of the carbonium ions derived from the halides. Prim, halides can not be reduced by this method. F. e., also in liq. SO2, s. S. Matsumura and N. Tokura, Tetrah. Let. 1968, 4703 with diborane in nitromethane s. Tetrah. Let. 1969, 363. 

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