Pentaborane reactions

Pentaborane, Tech. Bull. LF202, Alton (II.), Olin Corp. Energy Div., 1960 Reaction of pentaborane with oxygen is often violently explosive. 

Phosphacarba- f6>-pentaborane [2-/Bu-l,2-PCB3Fl5] 25 was synthesized by gas-phase reaction of phosphaalkyne with tetraborane(lO) (see Figure 11). In the 31P NMR spectrum a signal at —501 ppm was detected for the apical phosphorus atom and confirmed by quantum mechanical calculations.120... 

Most of these reactions are reversible. Thus, heating tetraborane(lO) in the presence of a large excess of hydrogen reforms diborane. Pentaborane(ll) is thermally... 

While the //-dichloroboryl-pentaborane(9) obtained from KB5H8 and BC13 is stable only up to 0 °C [54], the 1-dichloroboryl isomer is a comparatively stable liquid. It is obtained from B5 H9 and BC13 in the presence of A1C13 as a catalyst in a Friedel-Crafts type reaction [55] ... 

This type of reaction has also been used to prepare 1-alkylpentaboranes [Eq. (45)]. Similarly, liquid l-chloropentaborane(9) is obtained from pentaborane(9) and chlo-... 

Fig. 2.1-24. Reactions of pentaborane(9) with ammonia, trimethylphosphine and diborane.

Carbaboranes with an arachno-CB4 skeleton, that is derivatives of 1-carba-arachno-pentaborane(lO), were identified as side products (Scheme 3.2-11) 18 in the reaction of dichloroethylborane with elemental Li [29], and 19 was obtained from pro-pyne and tetraborane(lO) [30], The reaction of dimethyl(dipropyn-l-yl)silane with a large excess of tetraethyldiborane(6) [ hydride bath which contains Et2BH2BEt2, other ethyldiboranes(6) and triethylborane] [31, 32] resulted in the formation of 20 in good yield [33]. 

Scheme 3.2-20. Formation of 1,5-dicarba-c/oso- derivative 39c. Its methylation leads to 39b, pentaborane derivatives 39 (R = CH2CMe3) via and reaction with RC2U to 39d,e (b X = Me c hydroboration of a borylalkyne with HBCI2 to X — Cl, d R — CMe3 e R — Ph). give 40a, which on heating yields the chloro...

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

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. 

Pentaazadienes, structure, 32 171-172 Pentaborane(9) reaction of with alkynes, 26 74 Pentacarbonyltungstenhydrogen sulfide anion, reaction with hexafluoroacetone, 30 297 Penta-coordinate complexes, 4 175-178 Pentacyanides... 

Bromine (80 mmole 12.8 g) is introduced by syringe into a 1-L reaction bulb (Fig. 1) containing a Teflon-coated magnetic stirring bar. The bulb is then placed on the vacuum line, cooled to -196°, and evacuated. Pentaborane(9) (78 mmole, 7.8 mL liquid volume at 0°) is distilled from a calibrated buret on the vacuum line and condensed into the reaction vessel, which is then allowed to warm to room temperature while the contents are vigorously stirred. A -196° bath is kept close at hand in the event the reaction proceeds too vigorously. After several hours, only a slight color remains. 

There are two synthetically useful routes to salts of the tetradecahydrononaborate(l — )ion, [B9H,4]. The first, which involves the reaction of NaH and pentaborane(9), B5H9, in tetrahydrofuran (THF) in a 1 2 molar ratio,1 suffers from two drawbacks ... 

Dipentoborylmeihane, BBH8CH2B5H8 mw 138.35 obtd in a yield of less than 1% by reaction of pentaborane with dich loro methane in the presence of AICI3. None of the desired product was isolated... 

Garboranes. The term carborane is widely used in American literature as a contraction of the IUPAC approved nomenclature carbaborane. The first carboranes, isomers of C2B3H5, C2B4H6, and C2B5H7, were prepared in the mid-1950s at Olin Mathiesen. These carboranes were obtained as a mixture in low yield from the reaction of pentaborane(9) with acetylene in a silent electric discharge. The discovery of the icosahedral... 

A sacrificial carrier polymer approach was reported by Sneddon in 1988.64 Here, an organic polymer with a pentaborane side group was prepared by addition polymerization as shown in reaction (18). Treatment of this polymer with ammonia converted the borane side group to a fused borazine (naphthalene-type) side unit. Subsequent pyrolysis in ammonia resulted in loss of the organic polymer and hydrogen and the formation of boron nitride. 

Although it was reported that pentaborane(9) did not react with sodium hydride in ethyl ether 170>, more recently, Onak, Dunks, Searcy, and Spielman 14> found that sodium hydride and lithium hydride do react slowly at room temperature in ethyl ether. More basic ethers such as diglyme (bis (2-methoxyethyl) ether) or tetrahydrofuran increase the rate of reaction up to 80 per cent of the hydrogen expected from the following reaction was obtained. 

The boron-11 nmr spectrum of the product obtained by Onak et al. bore no resemblence to that of pentaborane(9) and reaction of the product with anhydrous acid did not yield pentaborane(9). 

The synthesis and characterization of the [BsHs]- ion 15,16,17) js one of the most important developments in recent years in the chemistry of pentaborane(9) and its derivatives. Several examples of a new class of pentaborane(9) derivatives have been prepared in which substituents occupy bridging sites 172-175) and significant yields of hexa-borane(lO) 16>17>, hexaborane(12) 17>, or decaborane(14) 16>17> have been obtained from the reaction of B5H8 with B2H6 by careful control of the reaction conditions. 

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