Transmetallation copper reagent

Stannane 6a underwent facile transmetalation in tetrahydrofuran at — 78 °C on treatment with butyllithium to afford 6b. Addition of the lithium reagent 6b to a solution of 1.1 equivalents of copper(I) bromide-dimethyl sulfide in 1 1 diisopropyl sulfide/tetrahydrofuran at — 78 °C gave the copper reagent 6c, which reacted with methyl vinyl ketone at —78 "C in the presence of boron trifluoride-diethyl ether65, producing 7 in 55% yield65. 

Another recent disclosure examined silicon-to-copper transmetallation as a mild means of synthesizing alkenyl-copper reagents from stable precursors. The method requires activation of the silyl group by an allylic alcohol. Again, the silanes in this work are produced by circuitous means but should be accessible by ruthenium-catalyzed hydrosilylation. Treatment of the silyl alcohol with a stoichiometric amount of copper(l) /rz -butoxide results in the C-to-O migration of the silyl group to produce a vinylcuprate shown to be competent for subsequent allylation to produce 1,4-diene products (Scheme 17). 

Better results can be obtained by generating the boronate species with the aid of sodium methoxide. In this case, satisfactory transmetalation occurs on treatment with Cul. Thus, the functionalized copper reagent 55 can be alkynylated with 1-bromo-l-hexyne at —40 °C, furnishing the enyne 56 in 75% yield (Scheme 2.15) [32]. 

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

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]. 

Low chemical yields and a-selectivity on the allyl copper reagent are observed with the phenylthio analogues. These observations suggest that the pyridine nitrogen facilitates transmetalation and or cuprate reactivity and plays a role in the regio-selectivity of the reaction. 

A preliminary investigation directed to determine the influence of the substituents in both the telluride and the starting cuprate, employing cyclohexenone as substrate, showed that phenylvinyl tellurides are not appropriate as the source of the vinyl copper reagent, since both the vinyl and phenyl groups are transmetallated and add to the enone. This result could be anticipated since both the formed carbanions are sp hybridized. Otherwise, butyl vinyl tellurides undergo exclusive Te/vinyl transmetallation followed by the desired vinyl transfer. 

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... 

Copper halide transmetallation of a fluorovinylzinc reagent and refunctionalization of the copper reagent preparation of (Z)-1,1,2,5,5,5-hexafluoro-4-iodo-3-trifluoro methylpenta-1,3-diene15-16... 

The hydroboration of allylic silanes proceeds with high diastereoselectivity as demonstrated by Fleming and Lawrence.87 It is difficult to use the newly formed carbon-boron bond for making new carbon-carbon bonds due to its moderate reactivity. However, the B/Zn exchange converts the unreactive carbon-boron bond to a reactive carbon-zinc bond, as in compound 24. A further transmetallation with the THF soluble salt CuCN-2LiCl provides copper reagents, which can be allylated, alkynylated, or acylated (Scheme 6). 

Whereas uncatalyzed substitution reactions of organozinc compounds are limited to very reactive electrophiles, metal-transmetallated organozinc compounds are able to perform substitution reactions on various electrophiles. In the case of conjugated electrophiles, these zinc copper reagents can follow a Sn2 or Sn2 mechanism. 

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