Vic-Dibromides

The reductive dehalogenation of vic-dibromides to give the alkenes, using sodium sulphide [32] or sodium trithiocarbonate [33] is aided by the addition of quaternary ammonium salts. Anri-elimination normally occurs is good yield, but is susceptible to steric factors [34], Other functional groups are not reduced by the sulphide. 

Considering that diaryl tellurides debrominate vic-dibromides, forming diarylteUurium dibromides (see Section 4.1.12.1), which in tnm can be easily hydrolysed to the corresponding tellnroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Problem 6.32 (a) What does each of the following observations tell you about the mechanism of the addition of Brj to an alkene (i) In the presence of a Cl salt, in addition to the vic-dibromide, some vic-bromochloroalkane is isolated but no dichloride is obtained, (ii) With cis-2-butene only rac-2.3-dibromobutane is formed, (iii) With rra/u-2-butene only m o-2,3-dibromobutane is produced, (h) Give a mechanism compatible with these observations. ... 

Reduction of l92-dibromides. Chlorobis(cyclopentadienyl)titanium, generated in situ by zinc reduction of (C5H5)2TiCl2 in THF, effects rapid reductive denomination of vic-dibromides to alkenes by an anft-elimination.1 Examples ... 

Bis[4-methoxyphenyl] tellurium dibromide is an intermediate in the debromination of vic-dibromides to olefins in a water/benzene system containing potassium disulfite. The organic tellurium compound added as bis[4-methoxyphenyl] tellurium catalyzes this reaction1. 

Phenoxathiin 10,10-dichlorides and dibroinides9, phenoxaselenin dibroinide10, vic-dibromides and diiodides9, and iodobenzene dichloride10 were also used to convert phenoxatellurins to the corresponding 10,10-dihalides. 

The reagent is also useful for dehydrohalogcnation of. v -alkyl bromides and of vic-dibromides, for some Michael additions, and for aldol condensation. ... 

Bisdehydrobromination of vic-dibromides.u A route to N-acyl-l,2-dihydropyri-dines (3) involves bromination of an N-acyltetrahydropyridine followed by bisdehydrobromination of the vic-dibromide (2). Usual methods, including DABCO or DBU in DMF,... 

Debrominadon of ic-dibromides.1 vic-Dibromides are reduced to alkenes by treatment with Na2S or NaSH in benzene/H20 in the presence of catalytic amounts of meth-yltrioctylammonium chloride (Adogen 464). The debromination occurs by stereoselective aim-elimination. The phase-transfer catalyst is not required with water-soluble substrates. 

E2 debromination of vicinal vic-dibromides is most favoured when the two bromine atoms are periplanar and anti (Scheme 4.6). 

Aryl-substituted vic-dibromides undergo debromination to produce the corresponding E-alkenes when treated with indium metal in MeOH. Since debromination occurs by the usual trans-elimination, meso/erythro- and d,Z-/f/zreo-vic-dibromides would give trans- or cis-alkenes, respectively, as shown in Scheme 4.6. It is thus suggested that in this case the reaction occurs via a common relatively stable radical or anion intermediate, which directly collapses to -alkene. 

More recently organotellurides have been used for the stereoselective debromination of vic-dibromides. ... 

Indium is reported to be an efficient catalyst for the debromination processes of vic-dibromides. An illustration of the utility of this method is in the debromination of aryl-substituted vic-dibromides to produce the corresponding (E)-alkenes with indium metal in MeOH. In general, the reaction proceeds via a common, relatively stable radical intermediate, collapse of which leads directly to the (E)-alkenes, and methanol is found to be the best solvent for this reaction. ... 

The reductive elimination of v/c-bromochlorides and vic-dichlorides with metals such as zinc is much less stereoselective than the corresponding vic-dibromides. However, with Nal in DMF the reductions are highly stereoselective and proceed with a net syn elimination. These observations have been used to effect an alkene inversion as shown in Scheme 1 for the preparation of the sex pheromone (3) of the oriental fruit moth. 7 Treatment of v/c-dichlorides with sodium phenylselenolate also results in a syn elimination. ... 

Debromination. Aryl-substituted vic-dibromides (both meso and dl isomers) undergo debromination with indium in refluxing methanol to afford f j-alkenes. 

A reagent for dehydrohalogenations is 1,5-diazabicyclo(4.3.0)-5-nonene has been shown to be versatile and capable of being used under mild conditions. The reaction of vic-dibromides... 

Catalytic debromination of vic-dibromides under heterophase conditions using violo-gen, J Am. Chem. Soc., 1984, 106, 1124 R. Sato, Y. Kobayashi, Y. Tomiya,... 

Debromination. Ho and Wong have reported that some vic-dibromides are debrominated by this very hindered base (DMF, reflux) in surprisingly high yields. On the other hand, indene dibromide does not undergo this reaction. T.-L. Ho and C. M. Wong,Syn. Commun., 5, 87 (1975). 

Dehalogenation (1, 1089). Debromination of vic-dibromides can be carried out conveniently in a two-phase system (H2 O—toluene) using hexadecyltributyl-phosphonium bromide (5, 322-323) as phase-transfer catalyst. The combination of sodium iodide and sodium thiosulfate (4, 466) is used as the reagent sodium iodide is needed in catalytic amounts only, since it is regenerated by the thiosulfate, meso-Dibromides react faster than the d/-isomers and afford only trans-iilkcnes the d/-isomers give mixtures of cis- and frans-alkenes. ... 

Alkenes. vic-Dibromides undergo debromination with EtMgBr-(dppe)NiCl2 in THF at 0°. Alkynes are obtained from dibromoalkenes. 

The relative halophilicity of trivalent phosphorus compounds, n-Bu3P> Ph3P > (EtO)3P, has been demonstrated from their reactions with vic-dibromides (15). Their reactivities parallel the softness of the central phosphorus atoms. 

Debromination (Section 7.10) The elimination of two atoms of bromine from a vic-dibromide, or, more generally, the loss of bromine from a molecule. 

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