Boranes from diborane

Softer bases will also react with diborane and cause symmetric cleavage. A typical preparation for ammonia borane from diborane that seeks to avoid the salt structure would be to cleave the diborane symmetrically with a soft base, forming a BH3 Soft Base adduct. The soft base is then displaced by ammonia to yield the product, in the desired BH3NII3 form [53]. 

As is true for most reagents, there is a preference for approach of the borane from the less hindered face of the alkene. Because diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered alkenes. Table 4.4 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result from both the low regioselectivity and from addition to both faces of the double bond. Even 7,7-dimethylnorbornene shows only modest preference for endo addition with diborane. The selectivity is enhanced with the bulkier reagent 9-BBN. 

NaB2Hs + 4HC1 2B4H10 + 4NaCl + 8H2 Higher boranes can be produced from diborane by pyrolysis. 

The conversion product tetra(carbahexa)borane 167 (molecular structure shown in Fig. 8) can be obtained from diborane 165 (Scheme 26) via a 2,5-diborabicyclo-[2,l,l]-hexane 166 and its structure have been reported by Enders et al.104 Synthesis of carbaboron compounds has also been reviewed previously.105,106... 

Aromatic boranes are synthesized from diborane and arylmagnesium halides [i 5], arylmercury halides [iS5], or diarylmercury compounds [/S i]. Their oxidation furnishes phenols in variable yields. A large excess (up to 16 mol) of BH3 is necessary for high yields (equations 604 and 605). 

The background to this method is provided by the hydroboration of alkenes with diborane, B2H6 (Section 4.8.2). A borane from the very bulky chiral alkene a-pinene (35) is 36, which is of general form R2BH. This borane is monomeric, and is formed by cis addition of diborane to the double bond in each of two a-pinene molecules. In this chiral diborane, the bulk of a-pinene prevents formation of a trialkylborane of type R3B. However, addition of R2BH 36 (Scheme 8.3) to a smaller prochiral alkene occurs preferentially to one face, and is the basis of a synthesis of alcohols with a high degree of enantiomeric purity. 

Octahydrotriborate (1 — )ion, [B3H8], is an important intermediate in the synthesis of higher boranes, polyhedral borane anions, and transition-metal complexes. Salts containing this ion have usually been prepared1 from diborane, which is toxic, spontaneously flammable, and expensive. In addition, pressure facilities are often required for the synthesis. 

Amongst the earliest separations of hydrides of boron carried out in the 1950s and 1960s was the separation of the borane homologues from diborane to pentaborane. Low column temperature and an inert system were needed to minimize the on-column degradation, but some interconversion of species was seen due to catalysis by the substrate. Separation of boron hydrides have also been... 

The known fluorophosphine-borane adducts are listed in Table VIII. Trifluorophosphine-borane can be made directly from diborane and excess trifluorophosphine (255) or by displacement of carbon monoxide from BHa CO (255), which it closely resembles. 

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-... 

A number of compounds [R2PBH2] have now been prepared (4-1, HO), some by pyrolysis of borane adducts obtained from diborane and secondary phosphines, for example,... 

While the four-center transition structure for BH3 addition is a widely used model, other reaction pathways have also been considered. In a synthesis of optically active (-)-l-butanol-l-d, Streitwieser and co-workers used the optically active borane formed from diborane and (-l- )-a-pinene (R2BH) to carry out the hydroboration-oxidation of (Z)-l-butene-l-d. To explain the observed stereochemistry of the reduction, they proposed that hydroboration involves a n complex between R2BH and the alkene (Figure 9.41). Note the close resemblance of such a complex to the cyclic structures... 

As a final example of a process in which enantioselective synthesis is achieved using an optically active reagent, let us consider some of the results of hydroboration with optically active boranes. The boranes are normally prepared by reaction of diborane with one of several available terpene-derived alkenes. One such reagent is diisopinocampheylborane, which is prepared from diborane and a-pinene. ... 

A mixture of -propyldiboranes is obtained along with tri- -propyI-borane from the reaction of diborane with cyclopropane at about 95 C. Apparently the banana bonds of cyclopropane can provide sufficient electron density to form the expected pi-complex intermediate (124). 

Carbon Monoxide- C—B)Borane BH3CO The unstable compound carbon monoxide-(C—B)borane (12), BH3CO, prepared from diborane and carbon monoxide under elevated pressures (20S), has been the subject of intensive spectroscopic investigations. Electron diffraction measurements (204) indicate that the molecule has its B—C—O... 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Diborane occupies a special place because all the other boranes can be prepared from it (directly or indirectly) it is also one of the most studied and synthetically useful reagents in the whole of chemistry.B2H6 gas can most conveniently be prepared in small quantities by the reaction of I2 on NaBH4 in diglyme [(MeOCH2CH2)20], or by the reaction of a solid tetrahydroborate with an anhydrous acid ... 

On the other hand, the observed syn preference of 59a is consistent with a study of hydroboration of 59a with diborane by Schueler and Rhodes [127], who obtained a mixture of the monoalcohols (symanti = 74 26) upon oxidative work-up. A similar magnitude of. yyn-preference was found (syn anti = 73 27) in the hydroboration with a bulkier borane, 2,3-dimethyl-2-butylborane (thexyl borane) [127]. This lack of effect of the bulk of the reagent in the hydroboration of 59a is consistent with the idea that the n face of 59a is free from steric bias [127], and that the syn preference of 59a found in dihydroxylation and epoxidation is non-sterically determined [128]. 

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