Halogen-copper exchange

The reaction of lithium dimethylcuprate with perhalobenzenes (146) may be classified as a copper-halogen exchange reaction, but the metal... 

A reaction similar to the overall copper-halogen exchange observed on addition of perhaloarenes to lithium dimethylcuprate is also given by pentafluorobenzene, pentachlorobenzene, 2,3,5,6-tetrachloropyridine (146), and tetrafluorobenzenes (144). A small excess of methyllithium was again required, otherwise the reactions were very slow. Formation of methylcopper during the addition was not observed with the poly-fluoroarenes, but a mechanism was proposed similar to that for the perhaloarenes (146). Equation (22) represents the overall reaction for pentafluorobenzene. In most cases, methane was evolved nearly quantitatively. 

Perfluoro-fert-butylcopper was obtained by a copper-halogen exchange reaction 34) (see Section II,B). 

Furthermore, no reduction products were isolated from a similar reaction between tra i-j3-bromostyrene and lithium diphenylcuprate, although considerable copper-halogen exchange probably occurred. 

Diorganocuprates couple readily with vinylic halides and have been employed in the synthesis of some natural products, e.g., fulvoplumieren and Cecropia juvenile hormone (26, 27, 62, 63). Copper-halogen exchange can occur simultaneously 174, 309). Successful coupling reactions of organocopper compounds with vinylic halides are listed in Table VIII. 

Dibromonorcarane also reacts with dialkylcuprates 68, 69). Products from both coupling and a possible copper-halogen exchange were isolated, but apparently no norcarane or 7-bromo-7-alkylnorcarane. 

Copper-halogen exchange is major reaction for R acid and esters undergo Cu-Hal exchange only. 

Copper iodide acts as an efficient reagent for the nucleophilic displacement of 1-haloalkynes. It transforms 1-bromoalkynes (72) into 1-iodoalkynes (73) which, on further treatment with copper(II) bis(arenesulfinate), are converted into the corresponding alkynyl aryl sulfones (74). An electron transfer between 1-haloalkynes and copper(I) salts is believed to take place for the copper-assisted halogen-exchange reaction at the acetylenic carbon atom. 

This observation opens up the way for another transformation the copper catalyzed halogen exchange reaction on aromatic systems. When not only a catalytic, but a stoichiometric amount of iodide was added to 3-bromoquinoline in the presence of the copper catalyst system (7.83.), it was converted smoothly to 3-iodoquinoline, which was isolated from the reaction mixture in 95% yield.105... 

Aryl halides are excellent substrates for metal-halogen exchange by highly activated Rieke copper, affording aryl coppers at or below room temperature in fair to excellent yields. Functional groups such as nitriles, ketones and fluoride substituents are tolerated,... 

Klapars, A., Buchwald, S. L. Copper-Catalyzed Halogen Exchange in Aryl Halides An Aromatic Einkelstein Reaction. J. Am. Chem. Soc. 2002, 124, 14844-14845. 

In comparison, block copolymers prepared using a copper catalyst and the halogen exchange technique had predictable molecular weights and narrower molecular weight distributions (cf. Fig. 17) [91]. There was no evidence of slow initiation and the outer pMMA blocks were more uniform. When Moineau et al. used this system, the mechanical properties were greatly improved relative to the copolymers prepared with the Ni catalyst [179]. It has also been reported that the proper choice of solvent also improves block copolymerization [180]. 

---