Organocopper compounds coupling

Organocopper compounds couple readily with allyl halides under mild conditions. In some instances the reactions are reported to be vigorous. Coupling of ethynylcopper compounds with allyl halides can be effected at the ambient temperature in the presence of halide or cyanide ligands 221). Isolation of some rearranged coupling products in certain reactions 44, 66) may indicate the intermediacy of allylcopper(III) species. The reactions are summarized in Table VII. 

During the last decade, a substantial number of novel (sometimes even stereoselective) strategies for the preparation of allenic prostaglandins have been devised. The approach used by Patterson involves a three-component coupling via a 1,4-addi-tion of the organocopper compound 121 to the enone 120, followed by alkylation of the enolate formed with the bromide 122 (Scheme 18.40) [121]. However, due to the notoriously low reactivity in the alkylation of the mixed copper-lithium enolate formed during the Michael addition [122], the desired product 123 was obtained with only 28% chemical yield (the alkylation was not even stereoselective, giving 123 as a 1 1 mixture of diastereomers). 

As a last example of an uncatalyzed C,C coupling of a neutral organocopper compound Figure 16.9 depicts the alkynylation of a copper acetylide with a bromoalkyne which is easily accessible via bromination of a terminal alkyne ... 

Organocopper compounds of stoichiometric ratio RCu do not always react with alkyl bromides (29, 44, 66, 147, 181). Where possible, a reaction may be effected at higher temperatures (128), in a more strongly coordinating solvent (31, 297), or by means of the cuprate species (297). An unsubstantiated coupling reaction between phenylcopper and butyl bromide in an aromatic solvent was mentioned (24), although it is doubtful that an organocopper species was employed (285). 

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. 

However, good yields of ketones can be obtained for an equivalent quantity of acid halide per group attached to copper. Suitable conditions exist for the coupling of dialkylcuprates with acid chlorides which contain other functional groups (233), such as carbonyl, carboxy, and cyano. The order of reactivity of these groups towards organocopper compounds is CHO > CO > COgR > CN. Benzoyl fluoride reacts very slowly with lithium dimethylcuprate. 

Since then, experiments have been reported which indicate that (1) organocopper compounds will couple with aryl halides (2) arylcopper compounds can be oxidatively and thermally dimerized (3) arylcopper compounds are intermediates in the Ullmann reaction (4) organocopper compounds are intermediates in copper-catalyzed decarboxylations and (5) copper-promoted coupling reactions are not restricted to aromatic halides. The copper(I) oxide-promoted coupling reactions, however, have still to yield firm evidence of a copper intermediate. 

Other examples of coupling between alkynyl halides and organocopper compounds can be found in Reference 559. 

In this chapter, the substitution reactions of organometallic reagents with organic halides and related electrophiles are reviewed. - The major portion of the chapter is devoted to a discussion of organocopper compounds, which first transformed the alkylation of nonstabilized carbanions into a reaction of general synthetic utility. More recently, transition metals other than copper have also found widespread application in such coupling reactions, and developments in this area are outlined in Section 1.5.3. 

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