Halide ion promoted

Alkyl-substituted acetylenes can react with HCI by either the Ad S or the Ad 2 mechanism. The Ad S mechanism leads to anti addition. The preference for one or the other mechanism depends on the reactant structure and the reaction conditions. Added halide ion promotes the Ad S mechanism and increases the overall rate of reaction. " Reaction of 4-octyne with TFA in CH2CI2 containing O.l-l.OMBr leads mainly to Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the reaction as compared to reaction with TFA alone, indicating the involvement of the Br in the rate-determining protonation step. " ... 

By the same token, in base-promoted E2 dehydrohalogenations, the rate of elimination of the halogen as a halide ion is expected to be I > Br > Cl F This element effect has indeed been documented in many instances, a few examples of which are listed in Table I... 

For many metals and alloys the determination of /p is complex, and its magnitude is governed by many factors such as surface finish, rate of formation, alloying constituents, and the presence of those anions, such as halides, that promote localised breakdown. In many instances the attack on passive films by halide ions shows a temperature and concentration dependence similar to the effect of hydrogen ions, i.e. the rate of film dissolution increases with concentration in accordance with a Freundlich adsorption relationship... 

Hydride-promoted reactions are also well known, such as the acrylic and vinylacrylic syntheses (examples 7-10, Table VII). Some less-known compounds, which form in the presence of halide ions added to tetracar-bonylnickel, have been described by Foa and Cassar (example 11, Table VII). Reaction of allene to form methacrylates, and of propargyl chloride to give itaconic acid (via butadienoic acid), have been reported (examples 13 and 14, Table VII). 1,5-Hexadiene has been shown to be a very good substrate to obtain cyclic ketones in the presence of hydrogen chloride and tetracarbonylnickel (example 15, Table VII). The latter has also been used to form esters from olefins (example 16, Table VII). In the presence of an organic acid branched esters form regioselectivity (193). 

The reactivity of iron(II) cations [FeX]+, where X = H, Me, C3H5, NH2, OH, F, Cl, Br, and I, have been examined by reactions with acetone (177). The C-C bond activation was the major process for the iron halide cations. The [FeF]+ ion promoted C-H bond activation as well as C-C bond activation and C-H bond activation was also promoted by the other [FeX]+ ions. The relative reactivities of the [FeX]+ ions toward acetone have been correlated with the thermochemical and electronic properties of the substituents X. 

These final model experiments indicate that thioglycosides may be used in the block synthesis of 1.2- cis- linked glycosides with DMTST as promoter. With unreactive acc.eptors it may be advisable to refrain from transforming the reaction into a halide-assisted one by adding halide ion to the mixture. When the acceptor reactivity is sufficiently high, however, the presence of halide ion may significantly improve the stereospecificity and the yield of the desired product. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Advantageous use of homochiral cyclohexadiene-cis-l,2-diol, available by means of biocatalytic oxidation of chlorobenzene with toluene dioxygenase, has enabled the synthesis of all four enantiomerically pure C18-sphingosines (Nugent, 1998), which are known inhibitors of protein kinase C and important in cellular response mediation for tumor promoters and growth factors. The four requisite diastere-omers of azido alcohol precursors were accessed by regioselective opening of epoxides with either azide or halide ions. 

Ammonia present in the sample also contributes to COD. However, in presence of excess free Cl-, it is converted into HH4C1 and is not oxidized. Long chain aliphatic compounds are often difficult to oxidize. A catalyst, Ag2S04, is therefore added to promote such oxidation. Halide ions present in the sample may, however, react with Ag2S04 forming silver halide. Such halide interference may be partially overcome by adding HgS04. 

Because of the possibility of charge-transfer interactions between polar head and halide ion, ion specific interactions can be expected to be particularly marked for alkylpyridinium halides. From the CMCs counterion dependence3, as well as from counterion dissociation studies, binding is found to follow the sequence I- > Br > CP. The size of hexadecylpyridinium micelles is very sensitive to the anion of added salt, aggregation being promoted according to the sequence227 F [Pg.52]

The base-promoted halogenation takes place by a nucleophilic attack of an enolate ion on the electrophilic halogen molecule. The products are the halogenated ketone and a halide ion. 

Finally, nucleophilic halide ions can enhance the reactivity by promoting the removal of generated acyl moieties from the metal centers ... 

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