Promoters hydrogen bromide

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

Dehydrochlorination of bis(tnfluoromethylthio)acetyl chloride with calcium oxide gives bis(trifluoromethylthio)ketene [5] (equation 6) Elimination of hydrogen chloride or hydrogen bromide by means of tetrabutylammonium or potassium fluoride from vinylic chlorides or bromides leads to acetylenes or allenes [6 (equation 7) Addition of dicyclohexyl-18-crown-6 ether raises the yields of potassium fluoride-promoted elimination of hydrogen bromide from (Z)-P-bromo-p-ni-trostyrene in acetonitrile from 0 to 53-71 % In dimethyl formamide, yields increase from 28-35% to 58-68%... 

There is considerable interest in amino derivatives of chroman and hydrogen bromide has been found to promote a stereoselective hydrogenation of an a-hydroxyoxime enabling the 5,5-aminochromanol 53 to be obtained in over 30% yield from chroman-4-one on a multi-kilo scale (Scheme 31) <00TL8021>. 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

The appearance of turbidity indicates saturation of alkyl halide. In this way both sodium thiosulfate and 2-bromopropane are nearly in a one-phase system, thus shortening significantly the heating period. Furthermore, the competitive hydrogen bromide elimination and the ensuing acid-promoted decomposition of thiosulfate into sulfur and sulfur dioxide are minimized, the checkers added 300 ml. of water over a period of 90 minutes. 

The intramolecular nature of most carbocationic isomerization was proved by means of labeling experiments. [l-13C]-Propane was isomerized in the presence of aluminum bromide promoted by hydrogen bromide to form [2-13C]-propane. None of the propane product contained more than one l3C atom per molecule.64 Similarly, very little label scrambling was observed in the isomerization of labeled hexanes over SbF5-intercalated graphite.65 Thus simple consecutive 1,2-methyl shifts can account for the isomerization of l3C-labeled methylpentanes (Scheme 4.3). 

Since the bond dissociation energy of hydrogen iodide is less than that of hydrogen bromide [DH-i, 70.5 kcal. DH-Br, 86.5 kcal. (3)], Reactions 6, 7, and 8 should occur more readily with hydrogen iodide. This would account for the fact that the promoting effect of iodine compounds on ignition is less than that of bromine compounds. 

The action of acids on siloles results in the cleavage of the two endocyclic Si—C bonds. Boiling concentrated hydrochloric acid, hydrogen bromide or glacial acetic acid promote the reaction, and the substituted butadienes, in which the geometry of the parent metallole is retained, are produced in high yield34,155 (equation 55). 

An entirely different approach to pyranoid 4-enes depends on eliminations from 5-bromo compounds obtained from pyranoid hexuronic acid derivatives by photobromination. Treated with zinc-acetic acid, the bromide 249 gives the glycal -like 250 (62%), while the 4-acetoxy compound 251 is formed when DBU is used to promote elimination. Similarly, base treatment of penta-0-acetyl-5-bromo-/J-D-glucose with DBU causes the analogous loss of hydrogen bromide and formation of the 4-acetoxy-4-ene, but use of zinc-acetic acid affords mainly the 5-exo-methylene alkene by the alternative available elimination process.237... 

The electron-releasing phosphine promotes oxidative addition of the bromo derivative to Pd(0) and, because of its bulkiness, readily generates free coordination sites by dissociation. Ethylene coordination and insertion then occur, followed by reductive elimination, triethylamine acting as a base to neutralize hydrogen bromide. As in most cases of transition metal-catalyzed reactions the fine details of the mechanism are still under investigation. Thus recent studies by Amatore s group suggest that the palladium(O) species formed by reduction of palladium acetate is an anionic acetato complex. 

Similarly, it will react with an alkane and will evolve a molecule of hydrogen bromide. The formation of bromine radicals is promoted by light ... 

The theoretical analysis presented in the previous section was applied to two model SCF isomerization systems. In one scheme, the catalytic isomerization of n-hexane is carried out using supercritical CO2 as solvent medium. CO2 also acts as activator for the Lewis acid catalyst promoted with hydrogen bromide or hydrogen chloride. Reported advantages over conventional processing include better mass transfer characterisitlcs, better selectivity towards isomerization products, and easier separation of reactants, product Isomers and catalyst (15). 

Phosphine-containing crown ethers, combined with [Rh(l,5-cycloocta-diene)Cl]2, promote hydrogenation of alkali-metal cinnamates [194]. It has been also demonstrated that ( 7r-C3H5PdCl)2, in the presence of such crown ethers, catalyzes the reaction of allyl bromide with powdered Nal or KI in benzene [194]. 

If we take the addition of hydrogen bromide to propene as an example, this change in regiochemistry may be achieved by the use of a peroxide, e.g. (RO)2, which promotes the formation... 

Another determining factor is the concentration of hydrogen bromide formed in the bromination step. The results just cited show that at a methanol furane ratio of 21 1 and a temperature of—10° the yield of tetramethoxybutene was 53% the first experiment cited was conducted at the same temperature (—10°) but with a methanol fiirane ratio of 12 1 and the yield of tetramethoxybutene rose to 83%. Hydrogen bromide thus promotes conversion of the dimethoxydihydrofurane (10) into the tetramethoxybutene (11). The high yield of (10) in the Bumess procedure is attributable largely to the lower reaction temperature. 

Base-promoted a-elimination of hydrogen bromide from dibromofluoromethane in the presence of an alkene affords 1-bromo-l-fluorocyclopropanes (see Houben-Weyl, Vol. E19b, pp 1497-1499). 

The effect of water upon the isomerization of n-butane that contains aluminum bromide also has been studied. These experiments are not comparable with those described above, as the hydrogen bromide formed was not removed from the reaction zone. It was found that aluminum bromide promoted by water, in spite of the free hydrogen bromide present, is a less active catalyst (Table X) than the corresponding catalyst described... 

In the absence of a promoter, aluminum bromide had no appreciable effect upon n-heptane at temperatures up to the boiling point of n-heptane. Using hydrogen bromide as a promoter, reaction occurred readily at a temperature as low as —20°. At room temperature and higher, cracking was the principal reaction. The conversion to lower boiling products was... 

Copper(I) salt promoted reaction of styryl bromides with dibenzoyl disulfide in a hot aprotic polar solvent produces 2,4-diarylthiophenes (421) in moderate yields (Scheme 84) <92CL1947>. The reaction may involve addition of PhCOS radical to the double bond of the substrate (activated by copper(I) salt), further addition of the resulting radical (420) to the substrate, and intramolecular radical substitution with extrusion of benzoyl radical followed by elimination of hydrogen bromide. 

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