Diboranes disproportionation

It should be noted that as of now there is no direct physical evidence (e. g., ESR spectrum) for the existence of the B2 H5 radical and much of the mechanistic discussion of the photochemistry of diborane(6) is speculative. If Eqs. (1) and (2) represent the correct mechanisms the rate of the quantum yields for H2 to B4H10 production would be unity. Although this ratio tends towards unity at lower pressures, this simple mechanism did not explain the ratio of two approached at-higher pressures. In order to explain this, an additional primary process was proposed, i. e., a disproportionation of two B2H5 radicals ... 

The density of liquid l-methyldiborane(6) at -126° is 0.546 g/mL, or 13.1 mmole/mL.s Its vapor pressure is 55 torr at -78.5° the compound is thermally unstable at this temperature and slowly disproportionates to diborane(6) and l,l-dimethyldiborane(6).1,6 Although the infrared spectrum can be used to identify this compound, similarities to the spectra of the disproportionation products present difficulties in assaying purity.7 The nB NMR spectrum is useful for identifying and assaying 1-CH3B2H5. At -110°, the "B NMR... 

The reaction has been found to be heterogeneous and of apparent second order. 1,3,2-Dioxaborolane, (CH20)2BH, disproportionates also quite easily at room temperature into diborane and (CH20)2BOCH2CH2OB-(OCH2)2, probably by the same mechanism (245). 

Dialkyldiborane is obtained by the reversible disproportionation of the monoalkylated diborane (283). 

Tetra(dimethylamino)diborane(4) was treated with o-phenylene isothio-cyanato-boronate, to give an interesting reaction which involve not only disproportionation but also cleavage of the B-B bond giving compounds 51 and 52. In addition a third product, 2-(l,3,2-benzodioxaborolo)-l,3,2-benzodioxaborole 12, was isolated and compared with those previously reported elsewhere58 (Scheme 10). 

The product boroxine (B3H3O3) disproportionates when condensed at 77 K and subsequently warmed to RT. Diborane and boric oxide are formed ... 

The B—C bond may be reduced in the presence of Hj at high P and T to afford boranes, e.g., alkylboranes and arylboranes are converted to organodiboranes and, in the presence of amines, to amine boranes , and also in processes in which solid borane polymer is formed from liquid organoboranes and Hj at 200°C. Partial hydrogenation of trialkylboranes to dialkylboranes is effected by treatment of the borane with Hj gas at 30.4 MPa and 120-200°C. Amine complexes of haloboranes are reduced by Hj gas to R3NBH3 species and ultimately to diborane . Amine complexes of triorganoboranes are aJso converted to amineborane by reaction with Hj gas. Finally, tetrahalodiboranes(4) are converted to the dihaloboranes by reaction with Hj below 0°C. The HjBX species disproportionate to yield ultimately BjH and BX3. 

Cage closure from nido or arachno to closo commonly occurs on heating, though disproportionation may occur simultaneously [Eq. (3.4)]. Reaction with diborane can be used to effect cage expansion [Eq. (3.5)] ... 

Lower dialkylboranes are reactive and hydroborate alkenes rapidly, even without a solvent below 0°C. The reactivity depends on the alkyl group, decreasing primary > secondary > cyclo-alkyl. Regioselectivity approaches that of RjBH with bulky substituents. Unfortunately, mixtures of products are formed because the reagents disproportionate and are in equilibrium with R3B and alkylated diboranes ... 

Trialkyl alanes may be used, like boron hydride derivatives (e.g., tetraalkyl diboranes), as catalysts for the alkyl exchange between two different trialkyl boranes (133). The A1R3 compounds are particularly suitable for bringing about the disproportionation of mixed dialkylalkenyl boranes (e.g., diethylalkenyl boranes), which result from the partial hydro-boronation of dienes with tetraethyldiboranes under the mildest possible... 

Dialkylboranes can disproportionate, although hindered alkylboranes are relatively stable, and this limits their utility in reactions with alkenes. One solution to this problem was the discovery that boranes containing heteroatoms stabilize the borane and are easy to prepare. Diborane reacts with alcohols such as methanol to... 

SiHij will give diborane in the presence of methyl radicals (271) or if heated in a sealed vessel at 150 to 155 C (86), There is good evidence that the first product is dichloroborane (HBCI2), which subsequently disproportionates to diborane. 

When the reduction of phenylboron dichloride is carried out with lithium aluminum hydride at the reflux temperature of dioxane (100 C) only the disproportionated products, triphenylborane and diborane, are found (7J). In contrast, a similar reduction of diethylphenylboronate in diethyl ether at room temperature gives the phenylborane (dimer) which is isolated as the pyridine adduct (74). Also, l-butoxy-3-methylboracyclopentane (VI) is reduced without difficulty by lithium aluminum hydride in ether solution to the corresponding tetrasubstituted diborane (VII) (75). 

The absence of diphenylborane (as either dimer or 1,1-diphenyldiborane) in the product mixture is attributed to a disproportionation, which must be more rapid than with the corresponding alkyl compounds, to give the 1,2-diaryldiborane and triphenylborane. Undoubtedly, the latter compound reacts further with diborane under these conditions to form more of the observed product (Section II, A, 1, a). 

Diphenyldiborane appears to be moderately stable toward disproportionation at room temperature (66, 74, 83), although at 100° C in dioxane a reaction which should have yielded this phenyl derivative resulted instead in the formation of triphenylborane and diborane (73). All attempts to obtain... 

Unsymmetrical trialkylboranes are isolated by hydroboration of alkenes with tetraalkyldiboranes (60-60b). In contrast, phenyldialkylboranes, prepared from 1,2-diphenyldiborane, undergo a rapid redistribution at room temperature into triphenylboron and a trialkylboron (126). 1,2-Diaryl-diboranes add to 1,3-dienes in toluene with the formation of 1-arylboracyclo-pentanes (XIII), which appear to be more stable toward disproportionation than the open chain analogs (126). 

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