Borane adducts

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). 

The synthesis of cZos o-borane dianions B H (1-7) relies principally on thermolysis reactions of boranes in the presence of either BH4 or amino-borane adducts. " The yields... 

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

The so-prepared compounds are very sensitive toward air and moisture, in particular in solution. H- and C-NMR spectra prove the formation of 1 1 adducts of the type dmap—M(R2)E(Tms)2- The proton resonances of the dmap molecule are shifted to higher field, as was observed for similar borane adducts... 

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]. 

Casanova jr., J., H. R. Kiefer, D. Kuwada, and A. Boulton 1,3-Diaza-2,4-Diboretidines. Isocyanide-Borane Adducts. Tetrahedron Letters 1965, 703. 

Figure 18 Syntheses of poly(phenylphosphinoborane) [PhPH-BH2] (25) and poly(isobutylphosphinoborane) p -BuPH-BH2]K (26) from their respective phosphane-borane adducts. (Adapted from ref. 45.)...

Figure 20 The primary phosphine-borane adduct RPH2-BH3 (29) with the highly electron-withdrawing aromatic group R = p-CF3C6H4. (Adapted from ref. 49.)...

It is shock-sensitive and highly flammable, like the bis(borane) adduct. 

Equilibria involving both forms 6.24,31,35) as Well as equlibria between the dimeric form and the nitrile-borane adducts, (III)24 31), are known to exist. 

Transformations of dimeric iminoboranes into the corresponding nitrile-borane adducts, due to temperature changes during sublimation or distillation, have been observed for the reaction products of pentafluorobenzonitrile and tribromoborane or organodibromoboranes 24) ... 

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). 

The thioesterimide-haloboranes, upon thermal dehydrobromation, yield C-S substituted iminoboranes, whereas treatment with trie thy lamine leads to the rather unstable thioesterimides and triethylamine-borane adducts. 

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