Borane, addition

When E = P, R = 2,4,6-trimethylphenyl, and R = 2,4,6-tri-f-butylphenyl (Eq. 26), the same reduction can be accomplished using BH3-SMe2.59 The borane addition product is also obtained (Mes2GeH—PArBH2). 

Borane addition shifted the 29Si signal at higher frequencies. At this point the question of a hydride or fluoride spatial interaction with silicon can be answered by the 29Si 8 of 2-aminopyridine derivative (+6.48) compared with those of 4-aminopyridine (+6.70). In the latter, only electronic effects through the ring can be... 

A similar reaction performed with P(NMe2)3 and 2-aminopyridine gives trisaminophosphorus compound. Borane addition gives the corresponding phosphorus and pyridinic adducts (Figure 15). 

When there is only a small difference in the steric hindrance between both sides of a double bond, the borane addition results in lower selectivity. In certain cases, this difficulty is circumvented by the application of sterically demanding borane derivatives, e.g., dialkylboranes. Among these derivatives, 9-borabicyclo[3.3.1]nonane (9-BBN) is reactive, thermally stable and highly sensitive to subtle differences in steric enivronment20-21. 

Scheme 5.3. (a) Stereospecificity (within experimental error) of crotyl borane additions to aldehydes, R = Me, Et, i-Pr, Ph [26]. (b) Transition structures for stereospecific addition of crotyl boron compounds to aldehydes. 

CHEMICAL PROPERTIES thermally unstable water reactive hydrolyzes in water to hydrogen and boric acid reacts with ammonia to form diborane diammoniate reacts slowly with bromine to form boron bromides reacts with hydrocarbons or organoboron compounds to give alkyl- or aryl-boron compounds reacts with metal alkyls to form metal borohydrides reacts with strong electron pair donors to form borane addition compounds FP (-90°C, -130°F) LFL/UFL (0.9%, 98%) AT (40-50°C, 104-122°F) HF (35.6 kJ/mol gas at 25°C). 

The pyrrolidyl enamines of 3-oxo-steroids (176) are reduced by diborane to give the saturated 3a- (177) and 3j8-pyrrolidino-steroids (178) in good total yield. The mechanism of saturation of the olefinic bond is discussed in terms of norma borane addition to give amino-borane derivatives (Scheme 4) the BHj group at C-2 is probably displaced internally by a hydride ion. The 2)5-steroidal borane derivative (179 R = H) is stable in the absence of a 10/5-methyl group (oestrane series) and can be oxidized in the usual way to give the 3a-amino-2)5-hydroxy-derivative (180). ... 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

Even though borane addition to alkenes happens without a catalyst, the catalytic version is important because it has usefully different chemo-, regio and stereoselectivities (Section 9.5). Enantiomeric excesses as high as %% can be obtained with a Rh( R -binap) catalyst in the conversion of norbor-nene to exo-norborneol, and additions to allylic alcohols which give a 10 90 ratio of synianti product in the absence of a catalyst switch to a % 4 ratio with Wilkinson s catalyst." ... 

Organoboranes are of special importance because they are easily formed in borane addition to C=C bonds (hydroboration). The high electronegativity of B (2.0) means that the B—C bond is not very polar and BR3 species are usually water, although not air stable but sufficiently reactive to be useful. In contrast to most other reagents, a B—H bond adds in an anti-Markownikov manner to an alkene to... 

Even though borane addition to alkenes happens without a catalyst, the x t alytic version is important because it has usefiilly different chemo-, legio-, and... 

To better understand the nature of final hydroboration products before subsequent oxidation, we considered competing pathways of hydroboration reactions (Scheme 2). The end product of the first hydroboration step using an activated BH3 is a mixture of monosubstituted boranes with a single double bond. Further borane addition can take place in three different ways by intermolecular hydroboration via the attached BH2 substituent (pathway A in Scheme 2), by another activated BH3 molecule (pathway B in Scheme 2), or by intramolecular hydroboration via another monosubstituted borane (pathway C in Scheme 2). Covalent polymers can thus form via pathway C [17], following the intermolecular hydroboration reaction pathway. In pathway C, subsequent hydroboration of R2BH species... 

MESOGENIC MIXTURES CONTAINING aoso-BORANE ADDITIVES... 

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