Pentaborane, alkylation

Alkyl derivatives of the nido-carborane, 2-CB5H9, are found as side products from the thermal (200°C) reaction between pentaborane (9) and acetylenes (see 5.3.2.7.3). These alkyl derivatives of 2-CB5H9 are formed from the internal 1,1 addition of two B—H bonds of the polyborane to the acetylene. 

Until recently, the standard preparative route to 2,3-R2C2B4H6 compounds (R = H or alkyl) was the gas-phase reaction of alkyne with pentaborane(9), bsh9, at elevated temperature.6,7 The scale of this reaction is limited by considerations of safety and practicality which center on (1) the utilization of large-... 

OTHER COMMENTS used as a chemical intermediate in the preparation of higher bo-ranes, alkyl boranes, ethyl pentaborane, and various hydrides useful as jet and rocket fuels, catalysts, corrosion inhibitor fluxing agents, and oxygen scavengers. 

A ntimber of penta-alkyl-l,5-dicarba-c/oso-pentaboranes(5) (9 R = Me, R = Me, Et, or Pr) have been produced from reactions between the dialkyl(l-alkynyl)boranes RjBC R and the tetra-alkyldiboranes (R2BH)2. Tris(diethyl-boryl)methane, (Et2B)3CH, has also been reported, and this gives, on treatment with triethylaluminium, both 2,3,4-triethyl-l,5-dicarba-c/oso-pentaborane(5) (10) and the adamantane derivative (11). ... 

Alkylation of the apical boron atom in pentaborane(9) is effected by olefins or alkyl halides in the presence of a Lewis acid catalyst such as aluminum chloride (249-254). (Fig. 3). Alternatively, trialkylborates (255),... 

There are several different methods [Eqs. (106)-(108)] by which 2-alkyl-ated pentaboranes can be prepared. The reaction of pentaborane at elevated... 

Pyrolysis of ethylene-diborane and trimethylborane-diborane mixtures (271, 272) yields 2-alkylpentaboranes, as well as mixtures of polyalkylated pentaboranes and alkylated decaboranes. The B NMR data for the alkylated pentaboranes obtained from the ethylene-diborane reaction (271) should be reinterpreted in favor of a downfield chemical shift effect of the alkyl group. 

The alkylpentaboranes are sensitive to air oxidation and to hydrolysis, the derivatives with smaller alkyl groups resembling pentaborane(9) more closely in this respect (249,267). Known alkylpentaboranes are summarized in Table III. 

The nature of this reaction appears consistent with the previously reported isotopic exchange between diborane and pentaborane(ll) (27J). Other methods include the direct interaction of ethylene with pentaborane(ll) at 25° C (274), and a reaction at elevated temperatures (100° C) between tetra-borane and a methyldiborane mixture (242). From H NMR and infrared evidence (242, 274) the monoalkylated product is most probably 2-alkyl-pentaborane(ll). The presence of two methyl resonances in the H NMR of 2-methylpentaborane( 11) is attributed to a mixture of the possible exo and endo forms (Fig. 4) (242). 

Although the structure of the dialkyl derivative of pentaborane(ll) has not been unambiguously determined, it is believed that the two alkyl groups are attached to different boron atoms. This assignment is partially supported... 

Products which can be predicted on the basis of the above equilibria [Eqs. (109) and (110)] and from known pentaborane(l 1) chemistry have been isolated from decomposition studies on the alkylated pentaboranes(ll). One such decomposition gave evidence for formation of a trialkylated pentaborane(ll) 242). 

In contrast to the monoalkylation of pentaborane(9), Friedel-Crafts alkylation of decaborane(14) with alkyl halides or olefins yields a mixture of derivatives (250, 275-282). 

Pyrolysis of an ethylene-diborane mixture has yielded a complex product which contains, in addition to alkylated pentaboranes (Section II, C), a mixture of mono- (probably 5-substituted), di-, tri, and tetraethyldeca-boranes (271). 

Increasing the number of alkyl groups about the alkyne triple bond enhances the reactivity toward pentaborane, and results in increased yields of the corresponding 2,3-dicarbahexaborane(8) 315). 

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