Catalysts diboran

Boron tribromide [10294-33A], BBr, is used in the manufacture of diborane and in the production of ultra high purity boron (see Boron, ELEMENTAL BoRON COMPOUNDS). Anhydrous aluminum bromide [7727-15-3], AIBr., is used as an acid catalyst in organic syntheses where it is more reactive and more soluble in organic solvents than AlCl. Tballium bromide [7789AOA], TlBr, is claimed as a component in radiographic image conversion panels (39). 

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. 

Used industrially in the manufacture of diborane, ultra high purity boron, and semiconductors used as a catalyst in polymerizations, alkylations, and acylations. 

The rhodium-catalyzed borylation of methyl C-H bonds is compatible with several moieties containing oxygen, nitrogen, and fluorine.154 For example, the reaction of pinacol acetal of 2-hexanone with bis(pinacolato)diborane (B2pin2) in the presence of Cp Rh(774-C6Me6) catalyst gives the alkylboronate ester in 74% yield (Equation (111)). The rhodium-catalyzed C-H activation and borylation occur at the least hindered and least electron-rich methyl group. 

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

Enantioselective reduction of ketones.1 The ability of diborane in combination with the vic-amino alcohol (S)-2-amino-3-methyl-l,l-diphenyl-l-butanol (12, 31) to effect enantioselective reduction of alkyl aryl ketones involves formation of an intermediate chiral oxazaborolidine, which can be isolated and used as a catalyst for enantioselective borane reductions (equation I). 

Boron trichloride is used as a catalyst in polymerization reactions. Other applications include refining of alloys soldering flux and as a component in certain fire extinguishers. It also is used to prepare boron libers and other boron compounds including diborane, sodium borohydride and several adducts. 

Chlorine-hydrogen mixture can explode in the presence of sunlight, heat or a spark. Also, it can explode when mixed with acetylene or diborane at ordinary temperatures, and with ethylene, fluorine, and many hydrocarbons in the presence of heat, spark or catalysts. 

Figure D shows some olefin insertion reactions. Hydride additions to olefins have been known for a long while. Among these many examples, manganese hydrocarbonyl, and cobalt hydrocarbonyl, magnesium hydride, diborane, alkylalu-minum hydrides, germanium and tin hydrides all add quite readily to olefins. These last two cases are questionable because the mechanism is not clear. Some of these additions occur without a catalyst some are speeded up by ultraviolet light some are catalyzed by Group VIII metals. So it is not clear whether all these reactions are the same or whether there are several different mechanisms.

The following amine boranes are manufd by the Callery Chemical Co a)Dimethylamine-borane, (CH,)aNH BHS, wh solid b)Tri-methylamine-borane, (CHS)SN BH, wh solid e)Pyridine-borane, CBHSN BHb, col liquid These amine-boranes are relatively stable complexes and are of interest because they act as selective reducing agents, polymerization catalysts, anti-oxidants and stabilizing agents. They may also be used for the prepn of diborane and as petroleum additives. Further information may be obtained from Tech Bull C-200(Ref 2)... 

Meerwein, however, had not explored the nature of the polymer or pursued alternative preparation methods. Since then other catalysts such as diborane, aluminum... 

Deuterioammonia, 188, 190 Deuteriobor ation, 191 Deuteriobromic acid, 214 Deuteriodiborane, 191 Deuterio-Raney nickel, 215 Deuterium gas and a surface catalyst, 199 Deuterium and tritium gas, 179 3,17/8-Diacetoxyestra-3,5-diene, 486 3,20-Diacetoxypregna-3,5,20-triene, 411 (20S)-2/3,3j3-Diacetoxy-5a-pregn-7-en-6 one-20-carboxylic acid methylester, 301 Diborane, 89, 100... 

Both chemical and catalytic methods are available to convert chromanones to chromans, and flavones to flavans. Clemmensen s method is the most commonly used and usually gives good yields, but diborane is sometimes preferable. The Wolff-Kishner method appears to degrade the pyran ring and is rarely used on chromanones. The conditions of catalytic reduction have to be rather more drastic in order to produce chromans and flavans but much depends on the activity of the catalyst and on the solvent used. If the chromanol is first formed, more severe conditions are usually required to complete the reduction (77HC(31)207>. 

Rhodium-catalyzed 1,4-addition reactions of bis(pinacolato)diborane(4) and bis(neopentyl glycolato)diboron to a,(3-unsaturated ketones give the corresponding boron derivatives 222, 223, and 224-230 (Scheme 40).131 This is the first reported example in which a rhodium catalyst was used in addition reaction of diboron reagents to ot,(3-unsaturated electron deficient alkenes. 

The solvent-free, microwave-assisted coupling of thienyl boronic acids and esters with thienyl bromides, using aluminum oxide as the solid support, served to rapidly check the reaction trends on changing times, temperature, catalyst, and base and easily optimize the experimental conditions to obtain the desired product in fair amounts. This procedure offers a novel, general, and very rapid route to the preparation of soluble thiophene oligomers. Quaterthiophene 265 was obtained in 6 min by reaction of 2-bromo-2,2 -bithiophene with bis(pinacolato)diborane(4) in 65% yield, whereas dithiophene 266 was obtained with 70% yield. The synthesis of new chiral 2,2 -bithiophenes also was reported. The detailed... 

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