Borane adducts amine-boranes

Despite the fact that many boron hydride compounds possess unique chemical and physical properties, very few of these compounds have yet undergone significant commercial exploitation. This is largely owing to the extremely high cost of most boron hydride materials, which has discouraged development of all but the most exotic appHcations. Nevertheless, considerable commercial potential is foreseen for boron hydride materials if and when economical and rehable sources become available. Only the simplest of boron hydride compounds, most notably sodium tetrahydroborate, NajBHJ, diborane(6), B2H, and some of the borane adducts, eg, amine boranes, are now produced in significant commercial quantities. 

Amine—borane adducts have the general formula R3N BX where R = H, alkyl, etc, and X = alkyl, H, halogen, etc. These compounds, characterized by a coordinate covalent bond between boron and nitrogen, form a class of reducing agents having a broad spectmm of reduction potentials (5). 

Compounds of the type LBH2X can be prepared by the reaction of the appropriate amine borane and hydrogen haHdes or halogens. The synthesis of the trimethyl amine iodoborane [25741-81-5] adduct (eq. 8) yields a precursor for the preparation of the trimethyl amine isocyanoborane [60045-36-5] adduct as shown in equation 9 (15). 

Amine-borane adducts have the general formula R3NBX3 where R = alkyl, H, etc., and... 

The nature of the bonding in amine-boranes and related adducts has been the subject of considerable theoretical discussion and has also been the source of some confusion. Conventional representations of the donor-... 

Recently, Manners and co-workers have found another type of reaction, the inorganic process corresponding to Rh-catalysed dehydrocoupling of borane adducts, which is homogeneous or heterogeneous catalysed depending on the substrate, in particular the nature of the Lewis base, phosphine or amine respectively [15]. 

In aprotic solvents that can act as electron pair donors such as ethers, tertiary amines, and sulfides, borane forms Lewis acid-base adducts. 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

Dissociation Enthalpies De [kcal/mol] of Selected Borane- and Alane-Amine Adducts... 

The B-N distances in as-described borane-amine adducts were found to be shorter in the solid state (156.4 161.6pm) than in the gas phase (167.2 163.7pm). Moreover, it was found that the B-N distance of such adducts in solution is strongly affected by the polarity of the solvent the more polar the solvent the shorter the B-N bond. Both findings indicate the strong influence of a dipolar field, either within a crystal or in polar solvents, on dative B-N bond distances. 

This cited reaction illustrates that the C=N double bond of iminoboranes is quite stable. Indeed, the C=N bond in these compounds tends to increase its bond order, forming corresponding nitriles, rather than to undergo further 1,2-additions leading to aminoboranes. This suggestion is confirmed by several reported transformations of iminoboranes to nitrile-borane adducts (Eq. (20)) M). Addition across the C=N double bond of iminoboranes is virtually unknown. This event is also true for related imines (e.g., dichloromethylenealkyl-amines) which yield imine-trihaloborane adducts with trihaloboranes rather than to undergo a 1,2-addition (c.f. Sect. VII). 

Borane, BH3, is an avid electron-pair acceptor, having only six valence electrons on boron. Pure borane exists as a dimer in which two hydrogens bridge the borons. In aprotic solvents that can act as electron donors such as ethers, tertiary amines, and sulfides, borane forms Lewis acid-base adducts. 

The formation of individual cycloborazanes can be achieved, in some cases, by transition metal-catalysed dehydrocoupling reactions. By using this strategy, secondary amine-boranes are converted to the four-membered ring (H2BNMe2)2 under mild conditions, whereas primary amine-boranes produce borazines. The cyclic pentamer is obtained as the exclusive product from ammonia-borane adduct H3B NH3 employing an iridium(fll) catalyst [see eqn (2.13) in Section 2.3)]. 

In reactions between Lewis acids and bases such as amines and boranes or boron halides, bulky substituents on one or both species can affect the stability of the acid-base adduct. Perhaps the most straightforward type of effect is simple steric hindrance between substituents on the nitrogen atom and similar large substituents on the boron atom. Figure 9.3 is a diagrammatic sketch of the adduct between molecules of tripropylamine and triethylborane. This phenomenon is known as front or F-... 

The reduction of amides with borane leads to the formation of borane-amine adducts, which can be resistant towards acylating agents or hydrolysis. Such borane complexes can be cleaved either by treatment with a secondary amine (e.g. piperidine, 60 °C [180]), or oxidatively, by treatment with iodine (Entry 3, Table 10.11 [181,182]). 

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