Diborane preparation

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

The first method is commonly used for preparing diborane. 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Silane, pure or doped, is used to prepare semiconducting siUcon by thermal decomposition at >600° C. Gaseous dopants such as germane, arsine, or diborane maybe added to the silane at very low concentrations in the epitaxial growing of semiconducting siUcon for the electronics industry. Higher silanes, eg, Si H and Si Hg, are known but are less stable than SiH. These are analogues of lower saturated hydrocarbons. 

BCl, BBr, and BI undergo exchange reactions to yield mixed boron hahdes. Exchange reactions also occur with trialkyl, triaryl, trialkoxy, or triaryloxy boranes and with diborane. Anhydrous metal bromides and iodides can be prepared by the exchange reaction of the metal chloride or oxide and BBr or BI (21)-... 

Dlbor ine(6). This compound is manufactured by Gallery Chemical Co. ia Gallery, Peimsylvania. Laboratory-scale preparations are given ia equations 4—64 5 6, of which the last may be the most convenient method. Diborane is the most important starting material for all the other boron hydrides. 

G in the presence of a catalytic amount of a Lewis base such as dimethylether, (GH2)20. In addition to the gas-phase pyrolysis of diborane, can be prepared by a solution-phase process developed at Union Garbide Gorp. Decaborane is a key intermediate in the preparation of many carboranes and metaHa derivatives. As of this writing, this important compound is not manufactured on a large scale in the western world and is in short supply. Prices for decaborane in 1991 were up to 10,000/kg. 

Aminoboranes can be prepared from diborane to protect a tertiary amine during oxidation they are cleaved by refluxing in ethanol or methanolic sodium carbonate. ... 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

Hydroboration affords an efficient preparation of the 5a-A -system (141, for example) from A" -3-ketones. Reaction with diborane followed by decomposition of the organoboron intermediate with refluxing acetic anhydride gives good yields of olefins. Ketones must be protected, and alcohols are transformed to acetates. A -7-Ketones yield 5oc-A -olefins (for example, 138). 

Diborane reduction of an ortho ester that is prepared from a triol with CH3C(OEt)3, PPTS. ... 

Diborane occupies a special place because all the other boranes can be prepared from it (directly or indirectly) it is also one of the most studied and synthetically useful reagents in the whole of chemistry.B2H6 gas can most conveniently be prepared in small quantities by the reaction of I2 on NaBH4 in diglyme [(MeOCH2CH2)20], or by the reaction of a solid tetrahydroborate with an anhydrous acid ... 

In a 200-ml three-necked flask fitted with a dropping funnel (drying tube) is placed a solution of 13.4 g (0.12 mole) of 1-octene in 35 ml of THF. The flask is flushed with nitrogen and 3.7 ml of a 0.5 M solution of diborane (0.012 mole of hydride) in THF is added to carry out the hydroboration. (See Chapter 4, Section I regarding preparation of diborane in THF.) After 1 hour, 1.8 ml (0.1 mole) of water is added, followed by 4.4 g (0.06 mole) of methyl vinyl ketone, and the mixture is stirred for 1 hour at room temperature. The solvent is removed, and the residue is dissolved in ether, dried, and distilled. 2-Dodecanone has bp 119710 mm, 24571 atm. (The product contains 15 % of 5-methyl-2-undecane.) The reaction sequence can be applied successfully to a variety of olefins including cyclopentene, cyclohexene, and norbornene. 

A dry 5(X)-mI flask equipped with a thermometer, pressure-equalizing dropping funnel, and magnetic stirrer is flushed with nitrogen and then maintained under a static pressure of the gas. The flask is charged with 50 ml of tetrahydrofuran and 13.3 ml (0.15 mole) of cyclopentene, and then is cooled in an ice bath. Conversion to tricyclo-pentylborane is achieved by dropwise addition of 25 ml of a 1 M solution of diborane (0.15 mole of hydride see Chapter 4, Section 1 for preparation) in tetrahydrofuran. The solution is stirred for 1 hour at 25° and again cooled in an ice bath, and 25 ml of dry t-butyl alcohol is added, followed by 5.5 ml (0.05 mole) of ethyl bromoacetate. Potassium t-butoxide in /-butyl alcohol (50 ml of a 1 M solution) is added over a period of 10 minutes. There is an immediate precipitation of potassium bromide. The reaction mixture is filtered from the potassium bromide and distilled. Ethyl cyclopentylacetate, bp 101730 mm, 1.4398, is obtained in about 75% yield. Similarly, the reaction can be applied to a variety of olefins including 2-butene, cyclohexene, and norbornene. 

A mixture of d- and l- hexoses also results from the hydroboration of these 5-enes. Hydroboration results in anti-Markownikoff, cw-hydration of the double bond and the amount of each hexose formed varies according to the nature of the substituent groups. For example, hydroboration (23) of methyl 6-deoxy-a-D-ryZo-hex-5-enopyranose (3) affords methyl a-D-glucopyranoside and methyl / -L-idopyranoside in the ratio of 1 2.5 respectively whereas hydroboration of the fris-trimethylsilyl ether of 3 afforded them in the ratio 1 0.6 respectively. The hydroboration method can be used to achieve specific labelling of hexoses with tritium methyl-/ -L-idopyranoside[5-H3] and methyl a-D-glucopyranoside [5-H3] were thus prepared (23). Similarly, hydroboration of the D-Zt/ro-hex-5-eno derivative (14) with diborane-H3 followed by removal of the isopropyli-dene group, afforded methyl a-D-mannopyranoside [5-H3] and methyl / -L-gulopyranoside [5-H3] in the ratio of 1 2 respectively (23). 

War research forced us to explore new synthetic routes and we discovered the alkali metal hydride route to diborane. This solved the synthetic problem. At the same time we discovered sodium borohydride and developed simple synthetic methods for its preparation and manufacture. 

Use of lumps of the solid aluminate, rather than its ethereal solution, and of peroxide-containing etherate [1], rather than the peroxide-free material specified [2], caused an explosion during the attempted preparation of diborane. 

Safety considerations are paramount in any boron hydride synthesis. The energy yield from the oxidations of boron hydrides is too high for any cavalier treatment of boron hydrides. Exclusion of air is the critical consideration in diborane reactions. Decaborane(14) is less reactive, generally, in a kinetic sense, but the thermodynamic potential is comparable. In addition, all volatile boron hydrides are toxic. The procedures described in the latter two preparations are within our experience non-hazardous. These procedures should be followed in every detail improvisation is not recommended. 

Borazine originally was obtained by the reaction of ammonia with diborane.1 Mixtures of lithium or sodium tetrahydro-borate with ammonium chloride also have been pyrolyzed to yield this product.2 More recently, the reduction of B,B, B"-trichloroborazine with alkali metal hydroborates has proved to be a convenient laboratory-scale method for the preparation of this compound.3-7 The procedure described herein is a variation of the last method as reported by Dahl and Schaeffer.7 This method is effective for the synthesis of iV-substituted alkyl- and arylborazines, i.e., compounds of the formula (HBNR)3 where R is CH3, C2H6, C6H , C6H5, p-C6H4CH3, or p-C6H4OCH3.5... 

Borazine can also be prepared directly by the reaction of diborane with ammonia. 

Each boron atom is surrounded by four hydrogen atoms in an arrangement that is approximately tetrahedral. Diborane can also be prepared by the reaction of BF3 with NaBH4. 

Several less general methods for preparing organotin hydrides are also used, though often with less satisfactory yields. However, reduction of N, A-diethylaminostannanes by dire -butylaluminum hydride or diborane occurs with high yields224 ... 

Like iminoboranes, imine-adducts of boron compounds have only recently been prepared and investigated. The adduct obtained from the reaction of diphenylketimine and diborane readily loses hydrogen, even at 20°, to form 1,3,5-diphenylmethylborazine 41) ... 

To prepare the diborane in tetrahydrofuran, add 0.3 M NaBH4 (or LiBH4) and 0.4 M BF3 in total 200 ml tetrahydrofuran and keep dry in refrigerator, or generate the diborane in the reaction flask as follows To a well-stirred suspension of 3.4 g NaBH4 in 150 ml tetrahydrofuran and 0.3M of the styrene or propenylbenzene, add over one hour at room temperature, 15.1 ml BF3 in ether in 20 ml tetrahydrofuran (keep temperature at room temperature) let stand one hour at room temperature and decompose the excess hydride with water then add the NaOH and chloramine (or hydroxyl-amino-O-sulfonic acid) and proceed as above to get the amine. 

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