Boron-hydrogen

Elemental boron and the borates are not considered to be toxic, and they do not require special care in handling. However, some of the more exotic boron hydrogen compounds are definitely toxic and do require care. 

This was one of the great shocks of my life and was the way we discovered that sodium borohydride possesses a remarkable stability (for a simple boron-hydrogen compound) in water. 

For reference, terminal boron-hydrogen bonds normally exhibit J 1 1b h in the range 100-190 Hz, whereas when a hydrogen bridge is involved JUb-h is usually less... 

The most widely used method for the synthesis of iminoboranes involves the 1,2-addition of boron-element bonds such as boron-hydrogen, boron-halogen, boron-carbon, or boron-sulfur bonds across the C=N bond of nitriles thereby producing variously substituted iminoboranes (Eq. (2)). The formation of iminoboranes as well as the stability of the products depends on the substituent on the nitrile group, the nature of the boron-element bond to be cleaved during the 1,2-addition across the C N bond, and to a lesser extent on the non-reacting boron substituents 26T... 

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

Cmelins Handhuch der Anorganischen Chemie, 8th Edn., Boron Compounds, Part 18, Boron Hydrogen Compounds, 1978 Springer Publ. Heidelberg. 

It was suggested by Sidgwick76 that electron pairs are used for the boron-boron bond and four of the boron-hydrogen bonds and that one-electron bonds are formed between boron atoms and the two remaining hydrogen atoms. Structures based upon this suggestion were discussed in the previous editions of this book. 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate126 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H , satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

Rather than continue to progress from less complex to more complex boron-hydrogen compounds, it will be more convenient to jump to a complex but highly symmetric borohydride ion. lB,H 2f. It may be synthesized bv the pyrolvsis of the BjHJ ion ... 

According to the electronegativity chart (Figure 10-11), the boron-hydrogen... 

The hydroboration of an olefin involves a cis addition of a boron-hydrogen bond to an alkene linkage, and for unsymmetric olefins occurs in a counter-Markownikoff fashion. 1-Alkenes and simple 1,2-disubstituted olefins undergo rapid conversion to the corresponding trialkylborane, whereas addition of diborane to tri- and tetrasubstituted olefins may be conveniently terminated at the respective di- and monoalkylborane stage. 1-Alkenes yield trialkylboranes in which there is a preponderant (approximately 94%) addition of the boron atom to the terminal carbon.2,3... 

Although the term hydroboration is most commonly employed2-3 16 to denote the addition of a boron-hydrogen linkage to carbon-carbon multiple bonds, it has also been used for the two-step oxidative process to distinguish it from the process of reduction involving H-B addition and protonolysis. 18... 

The exchange equilibrium of Eq. (75) is also catalyzed by compounds containing boron-hydrogen bonds, such as (CH3)4B2H2 (140,141). 

Study of the kinetics of the reaction of the dimer sym-tetrasiamyldiborane with olefins has established that the reaction is second order, first order in each component [11], The hydroboration reaction involves cis addition of the boron-hydrogen linkage to the double bond and the hydroboration reaction is very powerfully catalyzed by ether solvents. Consequently, it was proposed that the hydroboration reaction involved a transition state in which the ether solvent serves to solvate and stabilize the leaving disiamylborane group (Fig. 1) [1]. 

Hydroboration is the term given to the addition of a boron-hydrogen bond to either the carbon-carbon double bond of an alkene (Equation B 1.1) or the carbon-carbon triple bond of an alkyne (Equation B1.2). The first examples of hydroboration were reported in 1956 from the laboratories of H.C. Brown and since then the reaction has found many important applications in organic chemistry. Indeed, in 1979 Brown was awarded the Nobel chemistry prize for his contributions to hydroboration and related areas of reactivity. 

The boron atom in BH3 is sf hybridized with a vacant p orbital perpendicular to the plane of the three boron-hydrogen bonds. Thus borane and its derivatives are electrophilic (Lewis acidic) and combine readily with electron-rich species. For example, borane interacts with one of the lone pairs on the oxygen atom of tetrahydrofuran as shown below. 

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