Boron hydrides adducts

Ab initio molecular orbital theory at the HF/6-31G level has been used to investigate the structure of Lewis acid/base adducts of boron hydrides with argon and a variety of substrates that may be encountered in the mechanism for the oxidation of diborane. By use of fourth-order Moller-Plesset theory, i.e., MP4SDTQ, correlation effects are calculated at the HF/6-31G geometries. From HF/6-31G calculations, the following distances (in A) and angles for Ar-boron hydride adducts were found [22] ... 

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. 

Later, a zwitterionic monophosphino-[l,2,4]diazaphospholide 42 was obtained from the reaction of 3,5-bis(triphenylphosphonio)-[l,2,4]diazaphospholide chloride (38) with complex boron hydrides [43], In the initial step, selective extrusion of one PPh3 group takes place to yield the boron adduct of type 41, which on further reaction with triethylamine liberates zwitterionic diazaphospholide 42 (Scheme 12). 

A qualitatively similar dependence of ADh on boron orbital hybridization is noted in boron hydrides (104). -1bh = 81 cps for BHr where boron is tetrahedral while ABH = 136 cps for borazole where boron presumably is sp2. A ijh for BH3 adducts with ethers, amines, and phosphines range between 90 and 103 cps and for these compounds boron quadrupole coupling constants have been interpreted (22) in terms of a boron hybridization intermediate between sp2 and sp2. However, the simple dependence of... 

The adduct decomposed in about an hour at room temperature to a mixture of known boron hydrides. Reaction of the adduct with BF3 at —16° yielded a somewhat different mixture of boron hydrides. It is of interest that hydrogen gas was not formed in either case. 

Divalent cations of boron have been synthesized by partial displacement of bromide from boron tribromide adducts by using substituted pyridines.1 This reaction may lead to complete substitution of bromine with sufficiently basic amines or if the reaction conditions are not properly controlled. The present synthesis avoids this side reaction by blocking the fourth coordination position on boron with hydride which is not readily displaced by amines.2 The starting material in this case is an adduct of dibromoborane, (CH3)8NBHBr2, which is readily synthesized and is described in Sec. 20B. 

Discovery of the diammoniate of diborane (B2He, 2NH3) and bor-azene (B3N3H6), better known by its older name of borazole, by Stock (131) indicated that the boron hydrides could be used to prepare interesting boron-nitrogen compounds. This approach to boron chemistry was rapidly extended when the principles discussed in the preceding section were clearly recognized. Thus the compound (CH3)3N BH3, the first borane adduct to be characterized (39), pointed the way to the synthesis of a number of similar substances directly from diborane and amines for exam-... 

The compound borane-carbonyl occupies a special position in any discussion of molecules derived from boron hydrides with boron-carbon bonds. Carbon monoxide reacts with diborane (20 atm/room temperature) to give an adduct of the borane group which does not rearrange or lose hydrogen (39) The adduct OC BH3 does, however, dissociate into carbon monoxide and diborane at ordinary temperatures. No carbonyls of other Group III acceptor molecules have been prepared so far 133), and it has been suggested 27, 58) that borane-carbonyl owes its existence to the ability of the hydrogen atoms of borane to transfer electrons to a vacant p,-orbital in carbon monoxide. 

The inability of such photolyzate solutions to hydroborate 1-octyne rules out the presence of neutral boron hydrides. However, the detection of HD, upon the work-up with DOAc, and the formation of undeuterated toluene, upon treatment of the photolyzate with benzyl chloride and deuterolytic work-up, clearly support the presence of borohydrides, such as 45 (6). Finally, evidence supporting the generation of sodium diphenylborate(I) (46) or a similar product was obtained by conducting the photolysis in the presence of diphenylacetylene. Since monomeric 46 is formally isoelectronic with a carbene, adducts like 48... 

The ligands R can be exclusively halide, amide, hydride, or smart leaving groups like trimethylsilyl (TMS) or trimethylstannyl, or combinations of them. Some, but never all, of the sites R may be occupied by saturated or unsaturated organic substituents. In the case of boron hydrides, the Lewis base (D) adducts are preferred because they are much easier to handle than the pure boranes. 

Reaction of the alkyne adducts with aluminum and boron hydride reagents such as lithium aluminum hydride, sodium boro-hydride, or lithium triethylborohydride, however, leads to the formation of unexpected thiirane derivatives (eq 3). These can be desulfurized to give divinyl sulfides. 

Boron hydrides form a considerable number of Lewis base adducts, i.e., compounds in which the Lewis base has become bonded to the boron hydride with the boron framework intact. Such compounds have been observed for B5H9, B6H10, BsHi2, and B10H14. The reaction of B10H14 with a Lewis base to produce an intermediate necessary for the preparation of or /zo-carborane has been discussed above. The structure of this adduct is given in Fig. 14. 

FIGURE 14 The general structure of B10H12L2 compounds, the Lewis base adducts of B10H14. L represents a Lewis base such as CH3CN or (CH3)2S. [From Shore, S. G. (1975). In Boron Hydride Chemistry (E. L. Muetterties, ed.), p.137. Academic Press, New York, Figure 3.40.]... 

One of the differences between nonclassical and classical structures (as defined above) was the existence of vacancies on boron atoms (i.e., a trivalent boron with an empty or quasi-empty p-orbital). This property makes the clas.sical structure a better Lewis acid than the nonclassical structure. Thus, when a Lewis base coordinates to a boron hydride or carborane, often the classical structure is obtained. Table 6 indicates some of the boron hydrides and carboranes for which an adduct is known. 

The absence of an EPR signal in solution or in the solid state is indicative of a singlet ground state for the diradical ( BuBP Pr2)2- An indication of the radical character of this derivative is provided by a variety of facile oxidative addition reactions (Scheme 9.12). " For example, the treatment of ( BuBP Pr2)2 with diphenyl diselenide (or elemental selenium) produces a bicyclic compound in which a selenium atom bridges the two boron atoms. Trimethyl tin hydride reacts rapidly with ( BuBP Pr2)2 to give the trans adduct. Finally, ( BuBP Pr2)2 is slowly oxidised by deuterated chloroform to produce a R,R -dichloro adduct as a mixture of cis and trans isomers. 

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