Borane , compounds with dimeric

Boron Hydrides The boron hydrides, or boranes, are volatile, molecular compounds with formulas B Hm. The simplest is diborane (B2H6), the dimer of the unstable BH3. Diborane can be prepared by the reaction of sodium borohydride (NaBH4) and iodine in an appropriate organic solvent ... 

When alkenes are treated with borane in ether solvents, BH3 adds across the double bond. Borane cannot be prepared as a stable pure compound (it dimerizes to diborane, B2Hg), but it is commercially available in the form of... 

It is known that hydroboration reaction of cyano groups gives iminoborane species which dimerize to form B-N four-membered ring (cyclodiborazane) in the case of appropriate borane used [16]. When this reaction is used for the bifunctional monomers, formation of polymeric materials consisting of cyclodiborazane units can be expected. Concerning the preparation of poly(cycIodiborazane)s, we have recently reported two new methods, i.e., allylboration polymerization of dicyano compounds with triallylborane [17], and polycondensation between bis(silylimine)s and chlorodialkylboranes [18]. 

Diborane [19287-45-7] the first hydroborating agent studied, reacts sluggishly with olefins in the gas phase (14,15). In the presence of weak Lewis bases, eg, ethers and sulfides, it undergoes rapid reaction at room temperature or even below 0°C (16—18). The catalytic effect of these compounds on the hydroboration reaction is attributed to the formation of monomeric borane complexes from the borane dimer, eg, borane-tetrahydrofuran [14044-65-6] (1) or borane—dimethyl sulfide [13292-87-0] (2) (19—21). Stronger complexes formed by amines react with olefins at elevated temperatures (22—24). 

Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65]. 

The boron-11 n.m.r. spectrum of the compound [(CH3)3CCH=N-B(C4H9)2]n exhibits a strong signal at -7,4 ppm and a weak one at -38,8 ppm. At 120 °C the signal intensities reverse thus indicating that the dimer reverts to the monomer with increasing temperature 6>. The chemical shifts of imine-borane derivatives (c.f. Eq. (5)) are observed near +1,0 ppm 7>. 

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. 

Boron has only three electrons in the valence shell, and therefore its compounds are electron deficient and there is a vacant p-orbital on the boron atom. Borane (BH3) exists as a mixture of BaHe/BHs, as dimerization partially alleviates the electron deficiency of the boron. This equilibrium is fast, and most reactions occur with BH3. The addition of borane to a double bond is a concerted process going through a four-centered transition state. The formation of the C-B bond precedes the formation of the C-H bond so that the boron and the carbon atoms are partially charged in the four-centered transition state. 

Telluroaldehydes have been generated and trapped for the first time by the reaction of benzylidenetriphenylphosphorane with "activated" tellurium (a method analogous to that previously used to prepare selenoaldehydes) (Scheme 22).71 A wide range of reactive ylides have been converted into the adducts (118) by reaction with borane.72 On heating, (118) rearrange to triphenylphosphine-monoalkylborane adducts (119) which undergo the expected hydroboration reactions with alkenes. A new route to phosphaalkenes (121) is available from the reaction of phosphinomethylenetriphenylphosphoranes (120) with Lewis acids.73 In the case of (120, R2=NPr 2) the compounds (121) can be isolated and in one case an X-ray structure was obtained. However, similar reactions of (120, r2=Bu ) lead to the dimers (122). 

A complication in these reactions is the similar properties for trans dimer 6 and the cis dimer 20. The only noticeable difference in the NMR spectrum is the shift of one H resonance by 0.01 ppm, and a shift in the pyrazine l3C resonances by about 0.1 ppm, while the optical rotations are identical ([a]D = +82°). However, the compounds have very different solubilities - trituration of the mixture with ethanol yields the trans dimer upon filtration, while the crude cis dimer is obtained by evaporation of the filtrate. This separation procedure enabled purification of the mixture obtained from reaction of 2ot,3a-diaminocholestane (obtained by borane reduction of 22) and 2,3-diketocholestane (obtained by oxidation of 3-cholestanone with potassium t-butoxide and oxygen) (Figure 11). 

Reaction of compound 56 with Li20 yields the anhydro dimer 57, which is the N,N-dimethyl derivative of anhydro dimer 53, and this provides another route to the anhydro dimers. In analogy with the reaction to form the 1-azonia-2-boratanaphthalene 56, l-oxonia-2-boratanaphthalenes 58a—fare formed by the reaction of phenylethyne with arylchloro(phenyloxy)boranes (Scheme 23). 

Zwitterionic borane adducts were reacted with trityl cation salts to provide a series of primary borenium cation derivatives (Scheme 12). For example, the A-heterocyclic carbene (NHC) borane (45) was prepared from the corresponding NHC (44). With the addition of the trityl salt, the dicationic dimer (46) was isolated. Product (46) was characterized by X-ray crystallography and trapped by nucleophiles. Its observation by NMR resonance (5 + 10.6) is in accordance with the GIAO calculated value (5 + 0.8). Similar borenium dications were generated from amine nucleophiles, such as compound (47). The authors considered the question why some borenium ion salts... 

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