The Reaction Chemistry of Diborane

Diborane-ammonia Systems. Consider the formal analogy between the chloroammines of platinum and the ammonia adducts of diborane. It is useful to consider the hydrogen atoms attached to boron in the ammonia addition compounds of diborane as negative hydridic ligands, comparable to the chloride ions found in coordination chemistry. Although this concept may have been considered heresy some years ago, it was made easier to accept when Chatt (2) and his colleagues demonstrated the reaction ... 

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 ... 

In a study of the synthesis and chemistry of naphthalene-1,8-diboranes, dihydrodibenzoborepin 51 was prepared by reaction of boron tribromide with l,2-bis(2-trimethylsilanylphenyl)ethane 50, as shown in Equation (10) <2004JCD1254>. 

A fairly extensive metathetical chemistry of diboron compounds has been developed which provides the means for synthesis of specific derivatives from more commonly available starting materials such as tetrachloro-diborane(4) and the tetrakis(dialkylamino)diboron derivatives. Thus, reactions involving hydrolysis, aminolysis, alcoholysis, transamination, etc., are available for interconversions of diboron compounds by pathways analogous to those known in monoboron chemistry. Examples of such reactions include the following ... 

An outstanding feature of borane chemistry is the large number of reactions which result in the conversion of one or more boranes into others. These reactions make it possible to develop the whole of borane chemistry from one simple starting material, diborane. This point is emphasized in Chart 24.2, which shows only a small part of the very complex chemistry of boranes. Pyrolysis of B2H6 yields a number of the lower boranes including, for example, the thermally unstable BsHi 1, but this compound is preferably prepared by utilizing the equilibrium... 

Even when trimethylborane is taken in great excess for reaction with diborane, only four hydrogen atoms of the hydride are replaced by methyl groups. Tetramethyldiborane is fairly easily removed by fractional condensation from the reaction mixture. Since it behaves chemically as a source of (CH3)2BH fragments, it has been used to develop a considerable chemistry of the (CH3)2B-group, for example,... 

The chemistry of brazilin (21) and hematoxylin (22) has been reviewed by Robinson (62, 63) and Donnelly (20). In the first successful approach to a synthesis of brazilin the trimethyl ether (79) of an-hydrobrazilin (deoxybrazilone) (78) was prepared by treating 7-methoxy-3(3,4-dimethoxybenzyl)chroman-4-one (77) with phosphoric anhydride (16). The hydroxyl group at C-6a was introduced by a series of transformations (63). Later Kirkiacharian (41) found that hydration of the double bond can be achieved more directly by sequential treatment with diborane and alkaline hydrogen peroxide. The brazilin and hematoxylin derivatives obtained in this fashion were identical with products of earlier experiments. The course of the hydroboration reaction established the cis-fusion of rings B and C. 

In 1956 Herbert C. Brown (b. 1912) discovered that in ethereal solution diborane (B2H5) dissociates into borane (BH3), which can add to an alkene. The organoborane formed can be converted to an alcohol by treatment with hydrogen peroxide, and the overall result of this hydroboration reaction is the anti-Markovnikoff addition to the double bond. Brown was also responsible for the introduction into organic chemistry of the reducing agents sodium borohydride (sodium tetrahydridoborate(lll)) and lithium aluminium hydride (lithium tetrahydridoaluminate(lll)). 

Diborane is widely used in organic chemistry (hydrobnration) to convert alkcnes into alcohols. Consult an organic book and show reactions for this conversion. What is the advantage of this method ... 

Diborane(6) is an important reagent in synthetic organic chemistry, and reaction 12.8 is one convenient laboratory preparation. The structure of diglyme, used as solvent in reaction 12.8, is shown in diagram 12.2. 

Diborane finds such wide use in steroid chemistry that a new and very simple procedure for its preparation is welcomed. Tetra-alkylammonium borohydrides, which are readily extracted from an aqueous solution of Na BH and R4N -HSOT into CH2CI2, are used in the dried CH2CI2 solution addition of an alkyl halide (e.g. Mel) generates diborane in situ, and allows all the usual reactions of reduction or hydroboronation to be carried out conveniently and in high yields. 

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