Hydrogen bromide, elimination bromine

The fact that the rate law of hydrogen bromide elimination is first order with respect to the base may be interpreted by an E2 mechanism. The antiperiplanar position of the hydrogen and the bromine atoms in Ih also makes this mechanism very likely. Earlier the same mechanism was proposed for the elimination reaction of some tertiary a-halo ketones (ref. 19). Other mechanism, such as ElcB or El, seems to be very improbable considering the lack of any deuteration at C-2 or the lack of any rearrangement and the fact that the generation of a-keto cations requires acidic conditions (ref. 20). 

For the hydrogen bromide elimination an E2 mechanism was considered, as tetra-O-acetyl-a-D-mannopyranosyl bromide and tri-O-acetyl-a-o-rhamno-pyranosyl bromide, which contain the bromine atom and 2-acetoxy group in the trans positions, could not be dehydrobrominated. 

Where functionalization is required adjacent to the site of direct bromination, it is sometimes possible to move the halide through hydrogen bromide elimination... 

Phenylpropiolic acid. This is an example of an aromatic acetylenic acid, and is made by adding bromine to the ethylenio linkage in ethyl cinnamate, and treating the resulting dibromide with alcohobc potassium hydroxide which eliminates two molecules of hydrogen bromide ... 

Robertson et al.261 measured rates of bromination of some aromatic hydrocarbons in acetic acid containing sodium acetate (to eliminate protonation of the aromatic by liberated hydrogen bromide) and lithium bromide (to reduce the rate to a measurable velocity ) at 25 °C, the second-order rate coefficients for 3-nitro-N,N-dimethylaniline and anisole being 14.2 and 0.016 respectively the former compound was thus stated to be about 1012 times as reactive as benzene (though no measurement of the latter rate coefficient, inferred to be 1.33 xlO-11, could be found in the literature) and this large rate spread gives one further indication of the unreactive nature of the electrophile. Other rates relative to benzene were ... 

The ethylene addition product is saturated, that of benzene, on the other hand, is more unsaturated than is benzene itself, since the symmetrical cancelling of residual valency (Thiele) is upset and the aromatic character is destroyed. In order to re-establish this character it is only necessary for hydrogen bromide to be eliminated, which takes place with liberation of energy. The elimination occurs with extraordinary speed, even before the other double bonds, which have become reactive, have time to take up bromine. 

Although apparently undergoing electrophilic substitution, the reaction of pyran-2-one with bromine involves addition to the double bond and subsequent elimination of hydrogen bromide (Scheme 31) (69JOC2239). In general electrophilic substitution of pyran-2-ones is a difficult process and forcing conditions are usually required. 

The indirect dehydrogenation to pyranones has received rather more attention. The allylic bromination of 5,6-dihydropyran-2-one by NBS, followed by the elimination of hydrogen bromide, is described in detail and offers the attractions of mild conditions and easy isolation of the product (770S(56)49). A similar approach was used to synthesize the steroidal pyran-2-ones from the fully saturated lactone (354), with a combination of dehydrogenation and dehydrobromination to achieve oxidation (64MI22400). 

It was envisaged that the enones were produced following abstraction of H-1 (a process facilitated by the ability of sulfur atoms to stabilize radicals on bonded carbon centers), radical bromination, elimination of hydrogen bromide to give substituted glycals, allylic bromination at C-3, and loss of acetyl bromide. In the formation of compound 6, hydrogen abstraction from C-5 was deemed to compete with that from C-1, and to lead to substitution at the former site with the formation of a relatively stable product. 

By addition of bromine at low temperature methyl 2-siloxycyclopropanecarboxylates are regioselectively cleaved to give a-bromo-y-oxoesters 126 quantitatively. These can be isolated in singular cases, but usually they are directly transformed to oe,P-unsaturated y-oxoesters 127 by subsequent elimination of hydrogen bromide with triethylamine 80). Thereby -alkenes are obtained in good overall yields, which are interesting acceptor building blocks for further synthetic operations. 

Also, allyl dithiocarbamates (243) react with bromine to form the dithiolanylium salts (244) which on thermolysis eliminate hydrogen bromide to form the 1,3-dithiolylium bromides (245) <80AHC(27)151>. 

Since the excess trimethylsilyl bromide was difficult to remove, an alternative sequence was investigated (Scheme 10). After bromination of the silyl enol ether, the reaction mixture was poured into water to hydrolyze both the trimethylsilyl bromide and the anhydride. On heating this bromoacid as before, an unexpected compound was formed. This can be rationalized as follows The reaction proceeds from the enol form, and the mechanism is formally 1,5 elimination of hydrogen bromide with concomitant loss of carbon dioxide. The second decarboxylation is analogous to the one seen earlier, and would be expected of the a,8-unsaturated ketone. 

The a-bromine atom is stable during lactone formation from a,y-dibromo-butyryl bromide the yield of a-bromo lactone is 94%. Under similar conditions the /3-bromine atom of /3,y-dibromohexanoic acid is eliminated as hydrogen bromide to give the lactone of 4-hydroxy-2-hexenoic acid. Cyclization of alkali salts of 15-bromopentadecanoic acid has been studied using various solvents and concentrations. Best yields of the O)-lactone are obtained from the potassium salt in methyl ethyl ketone. ... 

Alternatively, the ketone may be converted to the enol acetate and this may be brominated (Scheme 3.85). Elimination of hydrogen bromide from a-bromo ketones leads to a,P-unsaturated ketones. 

Bis(trifluoromethyl)-5,6,7,8-tetrahydroquinazoline is brominated by A -bromosuccinimide at both benzylic positions. The resulting dibromo derivative 3 undergoes elimination of hydrogen bromide in the presence of a tertiary amine to afford 2,4-bis(trifluoromethyl)quinazoline... 

Triethylarsine dibromide, (C3H5)3AsBrj.— Triethylarsine sulphide is treated with concentrated hydrobromic acid, or triethylarsine with bromine in alcoholic solution. It crystalli.ses from chloroform in deliquescent needles, easily soluble in water and alcohol, insoluble in ether. Chlorine or nitric acid eliminates the bromine, and with concentrated sulphuric acid decomposition takes place with evolution of hydrogen bromide. 

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