Copper dehalogenations

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

As a consequence of facile homolytic cleavages, sulfonyl halides (I > Br > Cl F unsuitable) are able to add to unsaturated C—C systems. To prevent (or reduce) competing polymerizations, the additions of sulfonyl chlorides have been recommended to be carried out in the presence of copper(I/II) salts (Asscher-Vofsi reaction ). Comprehensive surveys have been published on the resulting j8-halogeno sulfones (or their vinyloguous compounds) as well as on their dehalogenation products (vinyl sulfones, 1-sulfonyl-l, 3-dienes, etc.). Table 5 reviews a series of sulfonyl halide additions and facile hydrogen halide eliminations. 

Reaction of the l-thia-2,5-diborole 63 with potassium leads to liquid 64a [85], and dehalogenation of (Z)-2,3-bis(dichloroboryl)hexene with copper vapor or Na/K alloy results in the formation of crystalline 64b [86], both in low yields (Scheme 3.2-34). 

In addition to the procedures summarized in Table 6, modifications of the zinc dehalogenation have appeared. For example, copper-activated zinc in the presence of ammonium chloride with methanol as solvent apparently gives better yields and, in some cases, is the method of choice.90,91 124 An example of this procedure is the reductive dechlorination of 8,8-dichloro- l-phenylbicyclo[4.2.0]octan-7-one, with zinc in acetic acid, to give l-phenylbicyclo[4.2.0]octan-7-one (7) in 30% yield. The use of zinc and ammonium chloride in methanol gave 7 in 70% yield.124 However, no general trend can be seen. Another variation is to replace ammonium chloride with hydroxylamine hydrochloride.116... 

Certain metals can be used for gas-phase defluorinations to give fluoroaromatic compounds. Thus, dehalogenations occur when the compounds in a gas phase are passed at high temperature over fillings or gauze made of copper,124 125 iron,126 131 iron oxides,132 or platinum,133... 

Fluorine is simultaneously eliminated with other halogens if it is the only halogen vicinal to iodine, bromine or chlorine. The elimination tendency decreases in the series IF > BrF > C1F. The group CF2CHFX is relatively stable towards elimination and requires stronger reaction conditions. The dehalogenation of saturated fluorohalo compounds with zinc dust or zinc/ copper and zinc/silver couples is one of the most useful methods for the preparation of fluoroalkenes (see Table 8). Zinc/zinc(II) chloride has also been used instead of zinc alone. 

The dehalogenation of naphthyridines with hydrogen over palladium on calcium carbonate in a weakly basic alcoholic solution gives excellent yields (90-95%) of reduced compounds.38,45,134,137,138 This method for removal of halogens has been extensively used and generally surpasses the classic hydrazine-copper sulfate reduction method. 

Bromo-,344 495 5-bromo-,318 and several polyfluorobenzo[6]thio-phenes100,110 have been prepared by decarboxylation of the corresponding 2-carboxylic acid with copper and quinoline. Good yields of fluoro compounds are obtained by this method, but those of the bromo compounds are rather low, probably owing to some simultaneous dehalogenation. 

When Y = F in [Cu2(XYL—F)]2+ (19), neither hydroxylation nor F migration occurs and intact XYL—F ligand is recovered. The lack of reactivity may be ascribed to the presence of both a deactivated ring and a very strong C—F bond. We note that when Y = Cl, reasonable yields of copper complexes with oxidatively dehalogenated ligand are obtained [168,169] the reaction also requires an external reductant. 

Vinylic halides also undergo high yielding alkoxy dehalogenation using copper(I) salts as catalysts in a mixed methanol/A-methylpyrrolidine solvent system245. The reaction proceeds with retention of the double-bond geometry (equation 29) and multi-halo vinyl halides may be used. 

A tri-dimensional electrode reactor geometry was studied by He et al. (2004a, b) to overcome the problem of low conductivity media (>1 S m-1). The reduction of model carbon tetrachloride was performed on a porous copper foam with good conversion rates and almost total dehalogenation of the substrate. 

Trans selectivity in chrysanthemates was obtained by using a bulky copper catalyst (36) together with a sterically4iindered dia/oester (1-methyl diazoacetate), whereas cu-pyrethraies can be obtained by dehalogenation of the cti-cyclopropane resulting from cydopropanation of 2Hmethyl-5,5,5-trichlorO 2-pentene 50. ... 

With PCI3, Cu atoms yielded smaller amounts of product. Copper also proved useful in dehalogenation reactions with alkyl halides ... 

Halogenation and Dehalogenation Reactions. Usually, reactions of this type take place readily without utilizing catalysts. However, when selectivity of the desired product is low or it is necessary to run the reaetion at a lower temperature, the use of a catalyst is desirable. Supported copper and silver halides can be used for the halogenation of hydrocarbons. Hydrochlorination reactions can be carried out with mercury copper or zinc halides. 

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