Dehydrohalogenation catalytic

In early work, vinyl chloride had been heated with stoichiometric amounts of alkaU alkoxides in excess alcohol as solvent, giving vinyl ethers as products (210). Supposedly this involved a Williamson ether synthesis, where alkaU alkoxide and organic haUde gave an ether and alkaU haUde. However, it was observed that small amounts of acetylene were formed by dehydrohalogenation of vinyl chloride, and that this acetylene was consumed as the reaction proceeded. Hence acetylene was substituted for vinyl chloride and only catalytic amounts of alkaU were used. Vinylation proceeded readily with high yields (211). 

Both the above potentiating modifications were next included in a single molecule. Catalytic reduction of 173 affords the corresponding 5a-derivative. This is then taken on to the 2,4-dibromo compound (see the transformation of 12 to 21 for discussion) dehydrohalogenation gives the 1,4 diene, 9a-fluoropred nisolone acetate (176), ) a potent antiinflammatory agent. 

Formal isomerization of the double bond of testosterone to the 1-position and methylation at the 2-position provides yet another anabolic/androgenic agent. Mannich condensation of the fully saturated androstane derivative 93 with formaldehyde and di-methylamine gives aminoketone 94. A/B-trans steroids normally enolize preferentially toward the 2-position, explaining the regiospecificity of this reaction. Catalytic reduction at elevated temperature affords the 2a-methyl isomer 95. It is not at all unlikely that the reaction proceeds via the 2-methylene intermediate. The observed stereochemistry is no doubt attributable to the fact that the product represents the more stable equatorial isomer. The initial product would be expected to be the p-isomer but this would experience a severe 1,3-diaxial non-bonded interaction and epimerize via the enol. Bromination of the ketone proceeds largely at the tertiary carbon adjacent to the carbonyl (96). Dehydrohalogenation... 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

This procedure has been used to prepare a variety of substituted a-bromohydrocinnamic acids 2 p-acetyl-a-bromohydro-cinnamic acid was prepared for the first time by this method. The method illustrates a typical application of the Meerwein reaction for the arylation of unsaturated substrates.3 In this reaction a catalytic amount of a copper(I) salt is used to reduce an aryl diazonium salt forming an aryl radical and a copper(II) halide. Addition of the aryl radical to an unsaturated substrate forms an alkyl radical that is reoxidized by the copper(II) halide present forming an alkyl halide and regenerating the copper(I) salt catalyst. In this preparation, the product, an a-bromo acid, is formed in an acidic reaction mixture and dehydro-halogenation does not occur. However, dehydrohalogenation... 

The catalytic activity of polyethylene glycol (PEG) phosphonium salts has been evaluated, in phase-transfer dehydrohalogenation reactions, as slightly better than that of the corresponding PEG ammonium compounds886 (reaction 271). By comparison... 

The rate of catalytic dehydrohalogenation is influenced by the structure of the reactants, but the extent of this effect varies from one catalyst to another with change of mechanism, i.e. with the timing of the fission of the Ca—X and Cp—H bonds. This is best seen from the published data on the deuterium kinetic isotope effect in Table 8. Their significance for the elucidation of the mechanism will be dealt with in Sect. 2.4.4 and here we can simply state that the value of the isotope effect depends on the nature of the catalyst. However, with a different reactant and within a series of related catalysts, kH/kD values independent of the catalyst were obtained (Table 9) [183],... 

Besides the advantage of their high reactivity toward alkylation reactions, primary and secondary alkyl halides show little tendency for dehydrohalogenation in Nafion-H-catalyzed gas-phase reactions.235 Although a minor amount of olefin is reported to be formed, no polymer formation was observed on the catalyst. As a result, the catalytic activity of Nafion-H stays constant over prolonged on-stream periods. 

Rossen et al. [71a] has reported a synthesis of piperazine 113, a key intermediate in the synthesis of the HIV protease inhibitor Crixivan . The reaction between a preformed imine 110, f-butyl isocyanide, and formic acid afforded the Ugi product 111, which was dehydrohalogenated with triethylamine and cyclized with KO Bu to the tetrahydropyrazine 112. Catalytic hydrogenation in the presence of Rh-BINAP (97% ee) and deformylation with aqueous hydrazine gave the target piperazine 113 (Scheme 2.40). 

A catalytic amount of PC13 may be used in place of phosphorus in the reaction. (.h) Dehydrohalogenation of 2-bromobutanoic acid gives 2-butenoic acid. 

Further reactions [160], in which radical and organometallic chemistry have been successfully combined, are the regioselective dehydrohalogenation by Oshima [161] and the catalytic reductive ring opening of epoxides by Gansauer [162]. 

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