Copper compounds transmetalation

Addition of an alkene to a compound containing a metal-H bond usually results in insertion, and it does in this case, too, to give the stabler 1° alkylmetal. Addition of CuBr to this complex might result in transmetallation, to give a C2-Cu bond. Addition of the copper compound to the unsaturated imide gives conjugate addition, perhaps by coordination of the C3=C4 tz bond and insertion into the C2-Cu bond. Workup gives the observed product. 

A related preparation of diethyl 3-oxoalkyIphosphonates uses a zinc-mediated approach from dietliyl 2-bromoethylphosphonate. Thus, treatment of diethyl 2-bromoethylphosphonate with zinc dust in THF at 30°C affords the corresponding alkylzinc bromide in 90% yield. The addition of the soluble CuCN2LiCl at 0°C transmetallates the intermediate zinc compound to the copper compound. This copper-zinc reagent reacts with acyl chlorides in THF at 0°C to provide the corresponding diethyl 3-oxoalkyIphosphonates in high yields (84-96%, Scheme 7.57). ... 

The possibility of transmetalation reactions must be taken into account when active metals are used as reductants. For example, Coltrain and Jackels l have described the synthesis of zinc complexes of macrocyclic ligands, L, by the reduction of the corresponding copper compounds by metallic zinc, which is accompanied by transmetalation, Eq. 9.6 ... 

Similarly to the alkyl derivatives, the most common route for arylcopper compounds is the reaction of a copper halide and aryllithium compounds (Equation (4)). Organocuprates with aryl groups are obtained by using an appropriate excess of the lithium reagent. Magnesium aryls have also been employed in transmetallation reactions with Cu(l) salts to yield both arylcopper compounds and arylcuprates (Equations (5) and (6)). 

Generally, organocopper compounds can be prepared by transmetallation between copper salts and organometallic reagents such as RLi, RMgX, and RZnX.53,53a,53b Copper alkynides can be obtained by reaction of terminal alkynes... 

As mentioned above, for more than 20 years after the first preparation of zirconacyclopen-tadiene, no systematic carbon—carbon bond-forming reactions were investigated. The major reason was the low nucleophilicity of the zirconacyclopentadienes. Indeed, such was the reputation of zirconacyclopentadienes in the 1980s that they were referred to as dead-end compounds . This statement clearly emphasizes the assumption that zirconacyclopentadienes were completely inert with regard to the creation of carbon—carbon bonds. In this context, transmetalation of zirconacyclopentadienes to copper and subsequent carbon—carbon bond formation represents a milestone in zirconacyclopentadiene chemistry. 

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. 

The aforementioned dienyl dicopper derivatives show the characteristic reactivity of orga-nocopper compounds. However, one limitation to the use of copper is that an electron-withdrawing group is usually required for reaction with alkynes. In order to develop an insertion protocol for alkynes bearing electron-donating groups, transmetalation of zirco-nacyclopentadienes to nickel was investigated. 

This method can be extended to the preparation of alkenylcopper compounds. Thus, treatment of the iodoalkenyl azide 10 with nBuLi at —100 °C (Scheme 2.3), followed by transmetalation with Znl2 in THF and then by a second transmetalation with CuCN-2LiCl, produces the copper species 11. This then effects a cis-selective carbocupration of ethyl propiolate to furnish the ( , ) diene 12 in 81% yield. 

Transmetalation of thioethers to organocopper compounds can also be performed in some special cases. Thus, treatment of the ester 119 with Me2CuLiLiCN provides the copper reagent 120, which can be treated successfully with several electrophiles such as allyl bromide or acid chlorides to afford the expected products such as 121 (Scheme 2.54) [115, 116]. 

Scheme 2.62. Michael addition of an alkenylzirconium compound", by successive transmetalation into zinc and copper intermediates.

INT2, Scheme 10.7) undergoes further reaction (Li/Cu transmetalation) and generates a new organocuprate compound. (Note however that this difference could become more subtle since the product of conjugate addition (PD) might behave more like an a-cuprio(I) ketone complexed with a lithium cation [52] than a lithium enolate complexed with copper(I)). In neither reaction was any evidence of radical intermediates (i.e., SET) found by theoretical calculations [79]. 

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