Tetra borane

Fig. 2.1-20. Valence bond structure of the calculated ground state for n/ do-tetra-borane(8) and a suggested mechanism for the enantiomerization of B4H3R4- (R = H or alkyl).

The question of the molecular structure of the adducts of tetraborane(8) has now been resolved 144>. The boron-11 nmr spectra of the carbon monoxide and trifluorophosphine adducts have been analyzed 140> in terms of the 2112 topological geometry 141> proposed by Dupont and Schaeffer 142>. The dimethylaminodifluorophosphine adduct of tetra-borane(8) 143> which was prepared by the following reaction is stable at room temperature. 

Two new methods for the preparation of methyl derivatives of tetra-borane(lO) have been developed. The first method, which is useful on a synthetic scale, yields the new compound 2,2(CH3)2B4H8 in 49 per cent yield from the reaction of Nat Hs with (CH Bd 119>. This compound is thermally unstable decomposing at a moderate rate at room temperature even in the gas phase at low pressure. The structure is unambiguously deduced from the boron-11 nmr spectrum which is closely related to the spectrum of the parent compound, B4H10. 

Subsequently a number of other boranes have been discovered, so that today there are no less than 14 that are well-characterized chemically, and for most the structures are well-defined. A convenient nomenclature for the boranes specifies the number of hydrogen atoms with an arabic number in parentheses. For example B5H9 pentaborane(9), and B5Hn pentabor-ane(ll). The hydrogen number may be omitted where only one compound with the number of boron atoms in question is known, viz., B4H10 tetra-borane. A list of the better characterized boranes is given in Table 8-1. 

Diborane is not unique among the boron hydrides in its ability to form a carbonyl. High pressure reaction of either pentaborane-f / or tetra-borane-10 with carbon monoxide forms a substance which behaves like borane-carbonyl in its manner of decomposition. The formula of this polyborane-carbonyl has recently been established as OC B Hg (mp, — 114.5° bp, 59.6°) 41a). It reacts with trimethylamine without release of carbon monoxide. 

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). 

The milder metal hydnde reagents are also used in stereoselective reductions Inclusion complexes of amine-borane reagent with cyclodexnins reduce ketones to opucally active alcohols, sometimes in modest enantiomeric excess [59] (equation 48). Diisobutylaluminum hydride modified by zmc bromide-MMA. A -tetra-methylethylenediamme (TMEDA) reduces a,a-difluoro-[i-hydroxy ketones to give predominantly erythro-2,2-difluoro-l,3-diols [60] (equation 49). The three isomers are formed on reduction with aluminum isopropoxide... 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Ketones containing triple bonds in the a,)3-positions are reduced to the corresponding unsaturated alcohols with sodium cyanoborohydride or tetra-butylammonium cyanoborohydride in 64-89% yields [780]. Thus 4-phenyl-3-butyn-2-one gave 4-phenyl-3-butyn-2-ol [780]. If the same ketone was converted to its p-toluenesulfonylhydrazone and this was reduced with bis benzyloxy)borane, 1-phenyl-1,2-butadiene was obtained in 21% yield [786]. 

High yields of amines have also been obtained by reduction of amides with an excess of magnesium aluminum hydride (yield 100%) [577], with lithium trimethoxyaluminohydride at 25° (yield 83%) [94] with sodium bis(2-methoxy-ethoxy)aluminum hydride at 80° (yield 84.5%) [544], with alane in tetra-hydrofuran at 0-25° (isolated yields 46-93%) [994, 1117], with sodium boro-hydride and triethoxyoxonium fluoroborates at room temperature (yields 81-94%) [1121], with sodium borohydride in the presence of acetic or trifluoroacetic acid on refluxing (yields 20-92.5%) [1118], with borane in tetrahydrofuran on refluxing (isolated yields 79-84%) [1119], with borane-dimethyl sulflde complex (5 mol) in tetrahydrofuran on refluxing (isolated yields 37-89%) [1064], and by electrolysis in dilute sulfuric acid at 5° using a lead cathode (yields 63-76%) [1120]. 

The conversion product tetra(carbahexa)borane 167 (molecular structure shown in Fig. 8) can be obtained from diborane 165 (Scheme 26) via a 2,5-diborabicyclo-[2,l,l]-hexane 166 and its structure have been reported by Enders et al.104 Synthesis of carbaboron compounds has also been reviewed previously.105,106... 

An analogous description of each of the higher boranes can be produced by selecting not the traditional pyramids favored by ACS, but rather those solid geometry models which heuristically have a concept of "regularity" and, when that is not attainable, of "semi-regularity". For example, exactly as the tetra-boron molecule minimized Coulomb repulsion... 

Calcium bis[tetrahydroborate(l-)] may be prepared from sodium tetra-hydroborate( 1 -) and calcium dichloride by cation exchange11 in a suitable solvent such as dimethylformamide,12 an amine13,14 or an alcohol.13,14 A particularly good preparation of very pure Ca[BH4]2 involves the reaction of calcium dihydride with the triethylamine-borane adduct.15,16 This method may also be used for the preparation of other tetrahydroborates of alkali and alkaline earth metals. The triethylamine-borane adduct17 may be synthesized in a variety of ways, e.g., from triethylamine, sodium tetrahydro-borate(l —), and trichloroborane 18 from a trialkoxyborane, aluminum metal, and hydrogen in the presence of triethylamine 19 or by hydrogenation under pressure of a mixture of triethylborane and triethylamine.20 The triethylamine-borane adduct is a colorless liquid (mp — 2°). It is stable to air and moisture at room temperature and it is easily purified by vacuum distillation (bp 95-96°/12 torr). 

BORANE, COMPD. WITH N2H4 see HGU025 BORANE with DIMETHYLAMINE (1 1) see DOR200 BORANE, TRIFLUORO-, DIHYDRATE see BMG800 BORATES, TETRA, SODIUM SALT, anhydrous (OSHA) see DXG035... 

Alternatively, one might explore close-packed structures of boranes. Consider, for example, the difference between deltahedral and tetra-capped octahedral BioHio structures (Fig. 4). The former is stable as the dianion (11 cluster pairs) while, based on Extended Htickel calculations (60), the latter is slightly more stable for the +6 cation (7 cluster pairs). The capping principle predicts 7 cluster pairs for capped octahedra (61). A structurally characterized transition metal analogue of the tetra-... 

At the other extreme, the most stable derivatives of the B2X4 type appear to be the tetrakis(dialkylamino) compounds. Thus, tetrakis(dimethylamino)-diborane(4) is stable at its boiling point of 206° C 17). The compound decomposes at 300° C to form bis(dimethylamino)borane and involatile residues containing B—C bonds. It is reported that tetra(A -methylanilino)-borane(4) is pyrolyzed at 300° C by a different mechanism to yield N-methylaniline 18). 

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