Bromination of acetylenes

Bromine, adsorbed on graphite, has been used for the stereoselective bromination of acetylenes to produce the ( )-a,/ dibromoalkenes. Isomerization of the E isomer to the Z isomer, which is normally catalyzed by bromine (equation 8), did not occur in the presence of graphite118. Table 1 lists the results of this study. 

TABLE 1. Stereoselective bromination of acetylenes in presence of graphite... 

By Addition to Unsaturated Substrates. Bromo- and iodo-functionalization of alkenes using a combination of the halogen and a mercury(II) salt has enabled the preparation of a series of 1,2-disubstituted products in which the second substituent, derived from the mercury salt, may be another halide or a series of other groups including esters, nitrate, thiocyanide, or a sulphone. Stereoselective bromination of acetylenes to furnish... 

Acetylene is condensed with carbonyl compounds to give a wide variety of products, some of which are the substrates for the preparation of families of derivatives. The most commercially significant reaction is the condensation of acetylene with formaldehyde. The reaction does not proceed well with base catalysis which works well with other carbonyl compounds and it was discovered by Reppe (33) that acetylene under pressure (304 kPa (3 atm), or above) reacts smoothly with formaldehyde at 100°C in the presence of a copper acetyUde complex catalyst. The reaction can be controlled to give either propargyl alcohol or butynediol (see Acetylene-DERIVED chemicals). 2-Butyne-l,4-diol, its hydroxyethyl ethers, and propargyl alcohol are used as corrosion inhibitors. 2,3-Dibromo-2-butene-l,4-diol is used as a flame retardant in polyurethane and other polymer systems (see Bromine compounds Elame retardants). 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Replacement, of acetylenic hydrogen atom by bromine, 46, 86 of acyl chlorine atom by fluoride atom using hydrogen fluoride, 46, 3... 

Some diamines carrying very bulky substituents like cardo groups can give colorless polyimides. For example, the bis-9,9-(4-aminophenyl)fluorene (FDA) or brominated and acetylenic FDA derivatives react with 6FDA giving copolymer films62 with low birefringence (low difference between in-plane and out-of-plane refraction index) (Fig. 5.8). A new cardo diamine l,l-bis[4-(4-aminophenoxy)phenyl]cyclododecane (Fig. 5.8) reacts with different aromatic dianhydrides with formation of colorless polyimides.63... 

Scheme 24 Unusually high reactivity of the triple bond of acetylene dicarboxylate in the bromination...

Recent studies by Pincock and Yates (32, 33) have demonstrated the intermediacy of vinyl cations in the electrophilic bromination of arylmethyl-acetylenes in acetic acid. The rates of addition of Brj to a number of substituted phenylmethylacetylenes in acetic acid follow the general equation... 

Action of bromine on acetylene, action of nitric acid on aromatic hydrocarbons. The accidents involving both reactions are very specific.They are the result of application of an unsuitable temperature. If it is too low, the reaction is too slow and causes an accumulation of reagent that is not converted and whose concentration increases, causing an acceleration of the reaction, i.e. a rise in temperature. The temperature then becomes too high and causes a more or less violent speeding-up of the reaction. 

Two examples have been given for the temperature/concentration action in the earlier reference to thermal effects (actions of bromine on acetylene and nitric acid on aromatic hydrocarbons). 

But catalytic reduction of the same phenyl propionic acid gives cis cinnamic acid. Therefore by adding hydrogen under various conditions, one can obtain a desired isomer. The conversion of acetylene into olefinic compounds has been carried out not only for the sake of obtaining the adduct, but Michael studied the various addition reactions for the sake of obtaining a desired product cis or trans. For example, he found that the addition of bromine to acetylene-dicarboxylic acid leads predominantly to the formation of trans isomer. 

Dibromoethane is a halogenated aliphatic hydrocarbon produced when gaseous ethylene comes in contact with bromine. The mixing of ethylene and bromine is accomplished in a variety of ways. One of the more common manufacturing processes involves a liquid-phase bromination of ethylene at 35°-85°C. After the bromination of ethylene, the mixture is neutralized to free acid and then purified by distillation. Other methods of 1,2-dibromoethane formation include the hydrobromination of acetylene and a reaction of 1,2-dibromoethane with water (Fishbein 1980 HSDB 1989). 

Replacement, of acetylenic hydrogen atom by bromine, 45, 86 of acyl chlorine atom by fluorine atom using hydrogen fluoride, 46, 3 of amino hydrogen atoms by thio-carbonyl group, 46, 19 of aromatic bromine atom by potassium /-butoxide, 46, 89 of a-chloro atom in ester using azide ion, 46, 47... 

Biedermann and Jacobson, who first prepared thieno[2,3-6]-thiophene (1) in 1886, characterized it as a 2,3,4,5-tetrabromo derivative with m.p. 172°. Later Capelle reported the isolation of a dibromo derivative of thienothiophene 1 with m.p. 122.5°, which was shown by Challenger and Harrison to be 2,3,5-tribromothieno[2,3-6]thiophene (m.p. 123°-124°). Capelle also obtained a tetrabromide, m.p. 223°, by bromination of the product of reaction of acetylene with sulfur. The tetrabromide seems to be identical with that prepared from the product of reaction of methane, acetylene, and hydrogen sulfide, m.p. 229°-230°, and is evidently 2,3,5,6-tetrabromothieno[3,2-6j-thiophene. ... 

The bromination with alkali hypobromite in aqueous solution gives good results with (hetero)arylacetylenes, enynes (RCH=CHOCH) and diynes (RC=CC=CH) all acetylenes that are more acidic than those acetylenes in the aliphadc or cycloaliphatic series with an isolated triple bond. For the conjugated systems the hypobromite method is superior to the reaction of metallated acetylenes with bromine. Various acetylenic alcohols are also brominated smoothly, which can be explained in part by their better solubility in water. Since in the case of primary and secondary ethynyl alcohols, oxidation of the alcohol can occur, the use of an excess of hypobromite should be avoided. The best procedure is drop wise additon of a small shot measure of hypobromite ro a mixture of alcohol and water. If the bromoalkynes to be prepared are not too volatile, small amounts of THF or dioxane may be added to effect a better solubility of the alkyne in the aqueous phase. Addition of a co-solvent may also be desired when the starting compound is a solid (e.g. ethynylcyclohexanol). 

Electrophilic additions of the halogens to alkynes have not been much studied. In acetic acid a given olefin reacts with bromine 103 to 105 times more rapidly than the corresponding acetylene. However, the relative rates are enormously solvent-dependent and decrease with solvent polarity. Thus bromina-tion of styrene is 2590 times faster than bromination of phenylacetylene in acetic acid, but only 0.67 times as fast in HzO. Solvation of the transition state must be... 

A simple, high yielding (65-90%) procedure has been described for the preparation of medium ring and macrocyclic acetylenic lactones which is mechanistically related to the well known Eschenmoser fragmentation reaction, and the following example is representative. Bromination of the tosylhydrazone of 1 was carried out with NBS at -10°C in a water/t-butanol/acetone mixture. The reaction mixture was then treated with aqueous NaHSC>3 solution and the resulting mixture heated at 50-60°C for one hour, which gave the acetylenic lactone 2. 

DMSO with Nal catalysis (Sonnet and Heath, 1980), by bromination of a terminal alkene followed by double elimination of HBr (Hoye el al., 1999), or by isomerization of an internal alkyne with the acetylene zipper reaction (Abrams, 1984). 

Let us now consider the synthesis of isoxazole 4.28, a drug for the treatment of bronchial asthma. The most direct preparation of isoxazolyl ketone 4.24 is the cycloaddition of unstable bromonitrile oxide 4.22 (prepared in situ by dehydrobromination of 4.21) with acetylenic ketone 4.23. Observe the regioselectivity of this reaction. Both electron-donating and electron-withdrawing groups on the acetylenic components in such cycloadditions tend to occur at the C5 position in the final isoxazole and not at C4. Bromination of ketone 4.24 affords bromoketone 4.25 which is 4.23 n... 

Forbes, Heidt and Sickman, J. Am. Chem. Soc., 57,1935 (1935). Acetylene dichloride, bromination of... 

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