Transmetalation to copper

Addition of CuCl to a solution of a 2,3,4,5-tetraethylzirconacyclopentadiene in THF at room temperature leads to the precipitation of a yellow solid (34a) (Eq. 2.26) [6]. 

Although not completely characterized, this yellow solid was assumed to be a dicopper diene derivative. In order to dissolve the yellow precipitate, DMPU (dimethylpropylene urea) was required. The dicopper diene species 34a is unstable in solution and gradually decomposes to undefined compounds. Therefore, all reactions involving transmetalation to copper are performed in situ in a one-pot or one-step process. 

As described above, when CuCl is regenerated in the reaction, the process can be catalytic in copper. In other cases, a stoichiometric amount (2 equiv.) of CuCl is used. Although CuCN shows similar reactivity, CuBr and Cul are not so effective as compared to CuCl. Allylation benzene, naphthalene, and anthracene formation, as well as acylation are representative examples, which are described below. 

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Addition reactions depend on the metal used for the transmetalation of the zirconacyclo-pentadiene. After transmetalation to copper, an addition reaction occurs to the carbon-carbon double bond or to a carbon—carbon triple bond bearing electron-withdrawing groups (Michael addition reactions). On the other hand, transmetalation to Ni allows the use of carbon—carbon triple bonds bearing electron-donating groups. 

Subsequent developments indicated that transmetallation to copper was also possible in both catalytic and stoichiometric senses, with the use of the THF soluble copper salt CuCN.2LiCl being especially convenient.7 The formation of these functionalized copper reagents has allowed a substantial increase in the range of products which may be prepared from organozinc halides.8... 

On the other hand, transmetalation to copper and further reactions with electrophiles occur preferentially at the phenyl-bearing carbon of unsymmet-rically substituted zirconacyclopentadiene derivative 35 (Scheme 17). The presence of lithium salts should be avoided to prevent the double allylation reaction of the zirconacyclopentadiene derivative [46]. 

A rhodium(III)/copper(II)-mediated process was reported to provide tetra-substituted enol esters in a trans-selective fashion. Overall, the reaction consists of a heteroaryl acyloxylation of alkynes. The process was initiated by a rhodium(III)-catalyzed C-2-selective activation of electron-rich het-eroarenes, such as benzo[I>]furan, and furan. Upon addition across an alkyne, a transmetalation to copper(II) enabled reductive C—O bond formation (14AGE14575). 

Transmetallation of the organic group from zirconium to another metal opens up possibilities. The palladium-catalysed coupling reactions can be found in Section 2.4. Addition of dimethyl cuprate results In transmetallation to copper. The resulting cuprate then displays typical cuprate reactivity, such as addition to enones. More economically, small amounts of copper can catalytically activate the zirconium complex towards this kind of chemistry, although the precise mechanism is unclear. Additions to enones can also be achieved directly using nickel catalysis (Scheme 5.64). Transmetallation to zinc has also been demonstrated. ... 

The preparation of functionalized uracils and purines is of high interest due to the biological properties of these important classes of heterocycles [97]. Starting from various protected 5-iodouracils such as 168, the addition of iPrMgBr (—40 C, 45 min) leads to the formation of the corresponding magnesium compound 169 that can be trapped by various aldehydes, ketones and acid chlorides, leading for instance, after transmetallation to copper and reaction with benzoyl chloride to ketone 170 in 73% yield (Scheme 4.35) [98]. 

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