Hydrogen-Boron Bonds

Borane exists primarily as a colorless gas called diborane. Diborane is a dimer—a molecule formed by joining two identical molecules. Because boron is surrounded by only six electrons, it has a strong tendency to acquire an additional electron pair. Two boron atoms, therefore, share the two electrons in a hydrogen-boron bond in unusual half-bonds. The hydrogen-boron bonds in diborane are shown... 

The stability of the bond between the cation and the hydroborate M-[BH4] and the stability of the hydrogen-boron bond M[B-H4] is different. This has an influence on the possible path of the thermal decomposition reaction. 

Only boron in the third periodic group, and then, only as the element, occurs in the solid state as a polymer. With the boranes (borohydrides), main chain bonds are only partly boron-boron bonds they also possess boron-hydrogen-boron bond character. 

Hydroboration involves addition of a hydrogen-boron bond to an aikene. From... 

By considering the reactions depicted in Schemes 10.3 and 10.4, you can see how the hydroboration-oxidation of an alkene gives an alcohol that is the product of overall anti-Markovnikov addition of the elements of H-OH to the carbon-carbon double bond. Remember, however, that the key step determining the regio-chemistry of the reaction is the Markovnikov addition of the hydrogen-boron bond across the Tr-bond. 

These results lend themselves to predicting the most likely product of hydroboration reactions, as the molecule with the lowest energy transition state should yield the kinetic product. These results are also in correlation with the anti-Markovnikov regioselectivity of the reaction the electropositive boron within a hydrogen-boron bond of the borane electrostatically favors the most negatively charged sp carbon within the molecule. Intriguingly, 1,3-cyclohex-adiene presents an important exception to the anti-Markovnikov rule, whereas transition state theory continues to predict the selectivity of the reaction. 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

Steric effects may be an even more important factor m controlling the regioselec tivity of addition Boron with its attached substituents is much larger than a hydrogen atom and becomes bonded to the less crowded carbon of the double bond whereas hydrogen becomes bonded to the more crowded carbon... 

Step 2 Anion of hydrogen peroxide acts as a nucleophile attacking boron and forming an oxygen-boron bond... 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

The most commonly accepted model for the hydrogen-acceptor pairs locates H at the BC site (see Fig. 4). This model was originally proposed for the H—B complex on the basis of satisfied bonds to explain the increased resistivity (Pankove et al., 1983), SIMS profiles (Johnson, 1985), and a hydrogen local-mode frequency consistent with a perturbed hydrogen-silicon bond (Pankove et al., 1985 Johnson, 1985 Du et al., 1985). The acceptor deactivation by atomic hydrogen was subsequently observed for Al, Ga, and In acceptors in silicon (Pankove et al., 1984). Hydrogen local-mode vibrations were identified as well for the H—Al and H—Ga complexes (Stavola et al., 1987). The boron vibrational frequency for the H—B pair was first identified by Stutzmann (1987) and Herrero and Stutzmann (1988a). 

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... 

Many of the boron hydrides have been shown to function as Brdnsted acids (See 34 for references). The bridging hydrogens are acidic. Pentaborane(9) is a monoprotic acid in the presence of a variety of bases (35,36,37). An example of such a reaction (38) is given in Scheme I, Reaction (4). By removing the bridging proton to give BsHs , a boron-boron bond is formed which is susceptible to insertion of electrophillic agents. 

Trimethylamine replaces PH3 quantitatively in BH3 PH3 to give BH3 N(CH3)3. Liquid or gaseous ammonia also replaces phosphine in the compound BH3 PH3 to the extent of 52-58% or up to 75%, respectively. The hydrogen atoms bonded to boron are partially, or completely, replaced by chlorine on treatment with hydrogen chloride, depending on the reaction conditions. The first stage of the reaction with hydrogen chloride is the formation of the colourless, viscous liquid BHjCl PH3 (see below). 

In systems derived from PH3 BH3, changes in the coupling constant, /pH, are found which are similar to those seen in compounds derived from PH3, i.e. compounds with phosphorus in coordination number 3. The observation that the alteration does not correspond to that seen for derivatives of PHi suggests that the very weak donor-acceptor bond in PH3 BH3 does not affect the hybridisation of phosphorus, as compared with that in free PH3, as much as might at first be expected. A small increase in the electron withdrawing action by halogen substitution of the hydrogen atoms bonded to boron causes an increase in the s character of the P-H bonds. 

The structural problem presented by these substances is not a simple one the fundamental difficulty is that there are not enough valence electrons in the molecules to bind the atoms together with electron-pair bonds. In BJT6, for example, there are twelve valence electrons all twelve would be needed to hold the six hydrogen atoms to boron by covalent bonds, leaving none for the boron-boron bond. 

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. 

The alkyl-boron bond is readily cleaved by hydrogen peroxide (these reactions are normally carried out in presence of alkali) according to... 

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