Substituted Boranes

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Several oxygen- and sulfur-substituted boranes have been reported (125—130). 1,3,2-Benzodioxaborole [274-07-7] (catecholborane, CB) (15) is the one best studied. It is commercially available or can be prepared by the reaction of catechol with borane-THF (57,131), or by other procedures (132). The product is a Hquid existing as a monomer, remarkably stable to disproportionation. No... 

Boranes. Borane (BH3) and substituted boranes reduce aldehydes and ketones in a manner similar to their addition to C=C bonds (15-16). That is, the boron adds to the oxygen and the hydrogen to the carbon ... 

The complexes of borane (BH3) itself with amines, phosphines, dialkyl sulfides and carbon monoxide, the first representatives prepared only in die 1930s were thoroughly discussed in several books and reviews.2,9,25-36 Therefore, in this section, only a brief summary will be given outlining the most basic aspects of these compounds, whereas complexes of substituted boranes will be discussed in more detail. 

Several complex hydrides have been prepared, such as AgBH4 these were all thermally unstable.383 More stable complexes were obtained with substituted boranes when isolated as phosphine derivatives.386-388... 

Addition of borane to 2,3-dimethylbut-2-ene gives 2,3-dimethyl-2-butylborane (thexylborane) which is also a synthetically useful substituted borane. 

Pyrazaboles substituted on either the ring or the boron may be obtained by reaction between the appropriately substituted pyrazole and the appropriately substituted borane.11 For example the 2,6-dibromo-4,4,8,8-tetraethylpyrazabole can be made by the reaction between 4-bromopyrazole and pyrophoric triethylborane in xylene at reflux temperature. Another route to such compounds involves the dihydrobis(l-pyrazolyl)borate... 

As a result of the hydridic nature of the hydrogen attached to boron, amine-boranes are interesting and useful reducing agents and have been employed in the reduction of numerous organic carbonyl compounds. They have been utilized in studying the kinetics and mechanism of hydride reactions and are precursors for the synthesis of substituted boranes, borazines, boronium ions, higher boron hydrides, and carboranes. 

As mentioned previously, the addition of borane, or an alkyl-substituted borane, to a carbon-carbon double bond is very sensitive to steric hindrance. The preceding vinylborane does not add a second boron because of the steric bulk of the ethyl group and the boron group on the end of the double bond. If the boron is considered to be about the same size as a carbon, then this vinylborane corresponds to a trisubstituted alkene. 

Figure 4 Boryl complexes derived from N-substituted boranes aminoboryl complexes 4 and 5, diborane(4)yl complexes 6 and 7, rj -borazine complexes 8...

Stereoselectivity in the hydroboration of asymmetric substrates is described in Section D.2.5.2.1.1. The organoborane products shown there are usually oxidized to the corresponding alcohols. In a similar manner to the symmetric substrates above, cyclic, asymmetric and functionalized alkenes, e.g., a-pinene24 and cholesterol2S, are attacked by borane and more selectively by substituted boranes from the less hindered side16-20-21 26-29. 

Intermediate enolates derived from Michael-type processes can be isolated. For example, enantiomerically pure enolate (5 )-131 can be isolated, as is the case of the almost enantiomerically pure enolate (5 )-131, prepared by 1,4-addition of the phenyl-substituted borane 130 to enone 129, in the presence of a substoichiometric amount of the chiral rhodium mediator [Rh(OMe)(COD)]2-(5 )-BINAP (COD = 1,5-cyclooctadiene, equation 33). Protonation of (5 )-131 with methanol leads to cyclohexanone (5 )-132 in good yield with no loss of enantiomeric purity (equation 34). The protonation is presumably diastereoselective, taking place on the less hindered face of (5)-131, away from the neighbouring phenyl group, as can be inferred from the stereochemical outcome (133)... 

Most calculations " have considered the interaction of unsubstituted borane and ethylene. They suggest the likely formation of a loose three-center complex, which then reorganizes to a four-center transition state with little or no activation energy. The experimental evidence shows, however, that there may be significant activation energies in reactions of some hindered alkenes with substituted boranes. ... 

A few calculations have considered reactions of substituted boranes or ethylenes, 9 f-8 and reasonable qualitative accounts of observed selectivities can sometimes be produced. 

Boranes. Borane (BH3) and substituted boranes reduce aldehydes and ketones... 

IV. Nuclear Resonance (NMR) Studies of the Boranes, Substituted Boranes,... 

The reactions of a-TMS-substituted boranes with aldehydes proceed with eliminadon of the TMS group to give alkenylboranes as in equations (45) and (46). Such products can be hydrolyzed to alkenes or oxidized to aldehydes. The intermediate in the case of the dimesi lborane reaction was directly oxidized to aldehyde in 95% overall yield, a good formyl homologation process. ... 

In addition to BH3, alkyl- and alkoxy-substituted boranes can also undergo insertion into a B-H bond. The higher stability and commercial availability of these boranes (e.g., BBN, catecholborane) has made them attractive as hydroboration reagents. These substituents can also be utilized to effect stereocenters upon insertion. Since hydroboration proceeds with controlled cis stereochemistry, the use of chiral boron substituents... 

---