Boranes dimerization

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). 

Kinetic studies on simple olefins [ 6] have shown that the reaction with a dialkylborane in an ether solvent is of first order with respect to olefin and first order with respect to the borane dimer. It therefore seems probable that the transition complex should be represented as in Fig. 24, with a molecule of ether involved in coordination with the second B-H unit. It is significant that hydroboration occurs only in a solvent able to. perform this role. 

The bis-(3,5-dimethylpyrazolyl)borane dimer, [(CsN2H7)2BH]2 contains a ring of the type (49), in a flattened-chair conformation (dihedral angle of 12.7 between the NBN plane and the central plane).Steric effects of the bulky terminal pyrazolyl units force this structure to be adopted, by contrast to the boat conformation of pyrazolyldideuteriogallane. 

FIGURE 10.1 Two borane dimers used in chain transfer reactions (9-BBN = 9-borabicyclo[3.3.1]nonane H-B(Mes)2 = dimesitylborane). 

C2oH3oB2N8f Bis-(3,5 dimethylpyrazolyl)borane dimer, 40B, 369 C2oH36BaN8Sft, Tetra-B-isothiocyanatotetra-N-t-butylborazocine, 31B, 296... 

Pyridyl)hydrazine (Aldrich), 4-acetylpyridine (Acros), N,N,N -trimethylethylenediamine (Aldrich), methylrhenium trioxide (Aldrich), InQj (Aldrich), Cu(N0j)2-3H20 (Merck), Ni(N03)2-6Il20 (Merck), Yb(OTf)3(Fluka), Sc(OTf)3 (Fluka), 2-(aminomethyl)pyridine (Acros), benzylideneacetone (Aldrich), and chalcone (Aldrich) were of the highest purity available. Borane dimethyl sulfide (2M solution in THE) was obtained from Aldrich. Methyl vinyl ketone was distilled prior to use. Cyclopentadiene was prepared from its dimer immediately before use. (R)-l-acetyl-5-isopropoxy-3-pyrrolin-2-one (4.15) has been kindly provided by Prof H. Hiemstra (University of Amsterdam). 

Mono- and dialkylboranes usually exist as bridged dimers, yy -dialkyldiboranes and yy -tetraalkyldiboranes. Only very hindered alkylboranes, eg, bis(2,3-dimethyl-2-butyl) borane (39), are monomeric. However, for convenience of presentation monomers are shown in the equations. 

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

Under the phase-transfer catalysis conditions, 2-bromo-8-methylquino-line (67) was coupled with 2-pyridylboronic ester 68 to furnish 2-(2-pyridyl)-8-methylquinoline (69) in 56% yield (91JOC6787). At this point, it is opportune to mention that the simple 2-pyridylborane, in contrast to 3- and (4-pyridyl)boranes, is considered an unsuitable Suzuki coupling partner because it forms an unusually stable cyclic dimer resembling a dihydroanthracene. In this case, the obstacle was circumvented by using 2-pyridylboronic ester in place of 2-pyridylborane (Scheme 9). 

Free borane (2) exists as gaseous dimer—the diborane BaHg. In addition Lewis acid/Lewis base-complexes, as for example formed in an ethereal solvent, e.g. 4, are commercially available ... 

Fig. 12. Possible monomeric (I and II) and dimeric (III) structures for (acyloxy)boranes in gas phase, solution and solid-state...

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]. 

A series of (phoshinoyloxy)- [71], (phosphorylamino)- [72], (sulfonyloxy)-[73-78] and (seleninoyloxy)boranes [79] are also known, however, only monomeric or dimeric molecules with similar structures and conformations to I-III (Fig. 12) have been reported so far. 

However, in this structure, boron does not have an octet. It has an empty p orbital, (very similar to a carbocation, except there is no positive charge here). Therefore, borane is very reactive. In fact, it reacts with itself to give dimeric structures, called diborane ... 

Borane, BH3, having only six valence electrons on boron, is an avid electron pair acceptor. Pure borane exists as a dimer in which two hydrogens bridge the borons. 

Potential Non-Cvcllc Precursors of Preceramic Polymers. Boranes such as bis(trimethylsilyl(aminotrimethylsilylaminochloroboranes can be viewed as monomers for preceramic polymer and, ultimately, boron nitride production. Intermolecular dehydrohalogenation of this borane would be thus expected to yield either the dimer or the polymeric system. 

Equilibria involving both forms 6.24,31,35) as Well as equlibria between the dimeric form and the nitrile-borane adducts, (III)24 31), are known to exist. 

Although dimeric (t-butylmethyleneamino)dibutylborane can be prepared by the reaction of tri-t-butylborane with pivalonitrile at temperature above 150 °C by elimination of butene. preparation at lower temperatures involving triethylamine-borane according to Eq. (11), illustrates that the formation of this iminoborane proceeds via di-n-butylborane as an intermediate 6 ... 

Fluorocyanide was found to produce similar monomers 26). The iminoborane dimers obtained from chlorocyanide or bromocyanide with dimethylbromo-borane transform to monomers upon vacuum distillation. 

Transformations of dimeric iminoboranes into the corresponding nitrile-borane adducts, due to temperature changes during sublimation or distillation, have been observed for the reaction products of pentafluorobenzonitrile and tribromoborane or organodibromoboranes 24) ... 

Tris(diphenylmethyleneamino)borane was obtained according to Eq. (30). Exchange reactions of monomeric and dimeric iminochloroboranes with various fluorides could not be effected. However, this lack of reaction is to be expected, since it is well known from borazine chemistry that substitutions which would require a structural change in order to obtain a stable product will not occur 20>. (Diphenylketimino)trimethylsilane (rather than diphenylketimine lithium) has been utilized successfully to prepare (diphenylmethyleneamino) dihaloboranes. This type of reaction is in accordance with earlier observa-... 

Dimeric iminoboranes, on the other hand, are formed from fluoroacetonitrile and tris(organothio)boranes ... 

Dimeric(fluoromethylmethylthiomethyleneamino)bis(thiomethyl)borane monomerizes in CC14 solution 32>. 

Monomeric and dimeric species obtained from the reaction of alkylthio-cyanates with tris(phenylthio)borane have not been isolated in a pure state 32). The reaction of (organothio)dichloroboranes with nitriles as illustrated by... 

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