Isomerization aluminum bromide-hydrogen

Parameters needed for the conformational analysis of the 10 isomeric monomethyldecalins are in Table XVI. The bridgehead methyl isomers are included for completeness, although they would not occur in the products of hydrogenation. Good agreement is shown between the computed equilibrium and one determined experimentally by isomerization with aluminum bromide-hydrogen bromide-hydrocarbon complex by the method described by Schneider e< al. (77). 

Isomerization Aluminum bromide. Boron trifluoride-Hydrogen fluoride. Dimethyl sulfoxide. Formic acid. N-Lithioethylenediamine. Mercuric acetate. Perchloric acid. Potassium r-butoxide. Potassium hydride. Potassium hydroxide. Zinc dust. 

Aluminum bromide-hydrogen bromide. The effect of olefins upon the isomerization of n-butane catalyzed by aluminum bromide-hydrogen bromide is summarized in Table III. 

Effect of Olefins upon the Isomerization of n-Butane Catalyzed by Aluminum Bromide-Hydrogen Bromide... 

The isomerization of branched chain hexanes in the presence of aluminum bromide-hydrogen bromide has been studied (Bishop, Burk, and Lankelma, 44). The summary of the results obtained, given in Table XXVII, shows that both 2-methylpentane and 3-methylpentane can isomerize with but a trace of cracking. The effect of reaction time on the conversion of 3-methylpentane shows two points of interest the five-fold increase in cracking with the longer reaction time and the marked change in the relative concentration of the hexanes. It w as found that 2,2-dimethyl-... 

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). 

Brown, H. C., Jungk, H. The catalytic halides. XI. The isomerization of o- and p-xylenes and some related alkylbenzenes under the influence of hydrogen bromide and aluminum bromide the relative isomerization aptitudes of alkyl groups. J. Am. Chem. Soc. 1955, TT, 5579-5584. 

In support of this hypothesis it has been observed that when aluminum bromide is treated with oxygen a reaction occurs which liberates bromine,48 and further, the halogen atoms rather than the hydroxyl group are responsible for the catalytic activity of HOAlBr2. Thus when the compound DOAlBr2 is prepared, the hydrogen-deuterium exchange with n-butane is not at all proportional to isomerization.4 ... 

The most important catalysts found for this reaction are aluminum chloride and aluminum bromide. The early investigators reported that these aluminum halides are catalysts for the isomerization of n-butane to isobutane later investigators found, however, that the reaction proceeds only when traces of water or hydrogen halide are used in conjunction therewith. More recent work indicated that under certain controlled conditions, even aluminum halides-hydrogen halides do not catalyze the isomerization of butanes unless traces of olefins or their equivalent are present. 

The isomerization of butanes using a high vacuum technique for the purification and introduction of the reactants was investigated (Leighton and Heldman, 5). It was found that completely anhydrous aluminum bromide did not act as a catalyst for the isomerization at 85° the addition, however, of a small amount of hydrogen bromide to the reactants caused n-butane to isomerize. 

The complex resulting from the action of water on aluminum bromide, freed from any noncombined hydrogen bromide, was used as a catalyst for the isomerization of n-butane. It was found that by contacting n-butane at 25 for 20 hours with the catalysts, isomerization occurred when the molal ratio of water-aluminum bromide was 1, 2, or 3 (Table IX, B). When the ratio was 4, slight isomerization occurred at 25°, but appreciably more occurred at 80°. With a ratio of 6, no isomerization occurred. No appreciable evolution of hydrogen bromide was noticed in these experiments. The results obtained demonstrate that the isomerizing catalyst formed by the action of water on aluminum bromide is not equivalent to an aluminum bromide catalyst, inasmuch as the latter requires hydrogen bromide and traces of olefins to cause the isomerization of n-butane. 

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... 

Aluminum chloride. The reaction of aluminum chloride with water is, to some extent, similar to the one of aluminum bromide namely, the more water added, the less hydrogen chloride liberated. With a 1 molal ratio of water to aluminum chloride, 0.71 mole hydrogen chloride was formed when the water-aluminum chloride ratio was increased to 2, only 0.29 mole hydrogen chloride was generated per mole aluminum chloride reacted (Table XI). The catalysts produced from the actioii of water on aluminum chloride were less active than those obtained from aluminum bromide and therefore the isomerization experiments were carried out at 80-100°. Unlike aluminum chloride, catalysts produced by the action of water on aluminum chloride did not require the presence of olefins or hydrogen chloride to promote the isomerization of 7i-butane. 

In order to test the validity of the postulated mechanism of isomerization, deuterium bromide-aluminum bromide instead of hydrogen bromide-aluminum bromide was used as an isomerization catalyst (Pines and Wackher, 23). The reactions were carried out under controlled conditions using a high-vacuum technique. The experimental conditions are summarized in Table XIV. 

Effect of oxygen. Two hypotheses were proposed to explain the effect of oxygen upon the isomerization of n-butane in the presence of either aluminum chloride or aluminum bromide (Pines and Wackher, 8). Both hypotheses assume the formation of hydrogen halide and alkyl halide the latter acting as a chain starter for isomerization. 

It has been shown that the isomerization of n-pentane in the presence of aluminum chloride or aluminum bromide containing catalysts was accompanied by side reactions, characterized by the formation of butanes and tarry material. These side reactions can be inhibited by the use of hydrogen or organic additives. 

Concurrently, a series of papers began to appear on the isomerization and hydrogen-exchange reactions of hydrocarbons with acid catalysts. Over slightly hydrated aluminum bromide at room temperature propane containing carbon-13 at one end (C Hq-I-C )was found to isomerize toward a statistical mixture with... 

In addition, as discussed above, oxidation reactions and reactions which use CO2 as a reagent as well as a solvent are worth investigating. Examples of both are discussed below. Finally, electrophilic processes may be advantageously transferred to supercritical CO2, as demonstrated by the improved isomerization of C4-C12 paraffins catalyzed by aluminum bromide. 2,44) Below, we describe three catalytic reactions which appear promising by these criteria asymmetric catalytic hydrogenation of enamides, ruthenium-catalyzed two-phase oxidation of cyclohexene, and the catalytic copolymerization of CO2 with epoxides. 

The Freund reaction of 3,4-dimethylpyridine methobromide and benzylmagnesium bromide affords a mixture of two isomeric di-hydropyridines (46a, b). The partial reduction of the latter isomer to l,3,4-trimethyl-6-benzyl-3-piperideine (47) was performed by hydrogenation (Pd/BaS04), or by the action of sodium borohydride, or on treatment of the perchlorate with lithium aluminum hydride.39... 

---