Elimination of Halide

In the arylations of enamines with very reactive aryl halides (352,370) such as 2,4-dinitrochlorobenzene, the closely related mechanistic pathway of addition of the enamine to the aromatic system, followed by elimination of halide ion, can be assumed. The use of n-nitroarylhalides furnishes compounds which can be converted to indolic products by reductive cycliza-tion. Less reactive aryl halides, such as p-nitrochlorobenzene, lead only to N-arylation or oxidation products of the enamines under more vigorous conditions. 

Halobenzenes undergo nucleophilic aromatic substitution through either of two mechanisms. If the halobenzene has a strongly electron-withdrawing substituent in the ortho or para position, substitution occurs by addition of a nucleophile to the ring, followed by elimination of halide from the intermediate anion. If the halobenzene is not activated by an electron-withdrawing substituent, substitution can occur by elimination of HX to give a benzyne, followed by addition of a nucleophile. 

This reaction is a principal method of forming IIIB-transition-metal cr bonds. The formation of thermodynamically favored alkali-metal halides or related salts and acids HX enhances the easy formation of those bonds. A second possible interaction between anionic metal bases and group-IIIB halides is a simple acid-base relationship without elimination of halide anions. However examples of this are rare, and they have been described often for group-IIIB compounds without halogen ligands ( 6.5.3.2). 

Burkholderia cepacia strain 2CBS is able to degrade ort/jo-halogenated benzoates by dioxygenation to catechol with the elimination of halide and decarboxylation. The enzyme contains a ferredoxin-and a Rieske-type [2Fe-2S] center. These could be distinguished on the basis of their EPR spectra, and the results were compared with those for other [2Fe-2S] clusters (Riedel et al. 1995). 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

The majority of reactions involving loss of halogen are the addition-elimination (AE) type, in which the nucleophile adds at the carbon carrying the halogen, followed by elimination of halide. This occurs most readily when the halogen is a or y to ring nitrogen, e.g. Scheme 132. 

One-electron reduction of organyl halides often results in the spontaneous elimination of halide and the formation of organyl radicals according to Scheme 1-9 ... 

The kRrlka ratios for the elimination-rearrangement process at 120° are 20,46 and 41 for R = H, Me and MeO, respectively. Bond-breaking is therefore important, and slow elimination of halide ion from vinyl carbanions, which are formed in pre-equilibrium, seems plausible (Jones and Damico, 1963). 

This sequence explains Price s observations adequately and seems to be required in this particular case. The oxidative elimination of halide ion from salts of phenols does not always follow this course, however. In the peroxide-initiated condensation of the sodium salt of 2,6-dichloro-4-bromophenol (Reaction 23) molecular weight continues to increase with reaction time after the maximum polymer yield is obtained (Figure 5) (8). Furthermore, Hamilton and Blanchard (15) have shown that the dimer of 2,6-dimethyl-4-bromophenol (VIII, n = 2) is polymerized rapidly by the same initiators which are effective with the monomer. Obviously, polymer growth does not occur solely by addition of monomer units in either Reaction 22 or 23 some process leading to polymer—polymer coupling must also be possible. Hamilton and Blanchard explained the formation of polymer from dimer by redistribution between polymeric radicals to form monomer radicals, which then coupled with polymer, as in Reaction 11. Redistribution has indeed been shown to occur under... 

The mechanism of the above elimination reaction is similar to the E2 mechanism of dehydrohalogenation. The reaction proceeds by nucleophilic attack of a base on silicon, simultaneous elimination of halide ion and formation of double bond (Scheme 4.4). 

A useful, although somewhat limited, method of preparing vinyl azides involves the addition of azide ion to vinyl halides with subsequent elimination of halide ion. This reaction is only possible when the halogen is located to a group which can effectively stabilize the intermediate carbanion. A number of alkyl and aryl substituted )3-chlorovinyl ketones (23) have been found to react in this way ". In many cases, however, the resulting /3-azidovinyl ketones (26) cannot... 

Direct elimination of halide from the ring may occur by the hydrolytic, reductive, or decarboxylative displacement of halogen the products then undergo oxidation followed by disruption of the aromatic ring. 

The formation of anhydro sugars must involve attack by the internal hydroxyl group, instead of generation of a carbonium ion, as the rate at which they are formed is very much greater than the rate of elimination of halide either in fully protected halo sugars or in those in which anhydride formation is not sterically possible. 

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