Organocopper formation

The more reactive lithium dialkenylcuprates (R2CuLi) cannot be directly acylated even at low temperature, because further addition to the product takes place. These reagents can be modified in situ so that the actual species undergoing reaction is an organozinc (requires palladium catalysis) however, other methods for generating organozincs are available that do not involve prior organocopper formation (Section 1.13.4.4). Stoichiometric vinylcopper and other copper-mediated acylations were not directly applicable to the case shown in equation (58) (however, see Section 1.13.2.1). 

Lithium (71.2 mg, 10.3 mmol) and naphthalene (1.592 g, 12.42 mmol) in freshly distilled THF (10 ml) were stirred under argon until the Li was consumed ( 2 h). A solution of CulPBus (3.666 g, 9.333 mmol) and PBus (2.89 g, 14.3 mmol) in THF (5 ml) was added via cannula to the dark-green lithium naphthalenide solution at 0°C and the resultant reddish-black active copper solution was stirred for 20min. 1-Bromooctane (0.9032 g, 4.677 mmol) and GC internal standard -decane (0.1725 g, 1.212 mmol) in THF (5 ml) were added rapidly via cannula to the active copper solution at -78°C. The organocopper formation was typically complete within 20min at -78°C. 2-Cyclohexen-l-one (0.1875g,... 

Organocopper compounds used for carbon-carbon bond formation are called Gilman reagents in honor of Henry Gilman who first stud led them Gilman s career in teaching and research at Iowa State spanned more than half a century (1919-1975)... 

The mechanism of the reaction involves initial formation of a tri-organocopper intermediate, followed by coupling and loss of RCu. The coupling is not a typical polar nucleophilic substitution reaction of the sort considered in the next chapter. 

The mechanism of conjugate addition reactions probably involves an initial complex between the cuprate and enone.51 The key intermediate for formation of the new carbon-carbon bond is an adduct formed between the enone and the organocopper reagent. The adduct is formulated as a Cu(III) species, which then undergoes reductive elimination. The lithium ion also plays a key role, presumably by Lewis acid coordination at the carbonyl oxygen.52 Solvent molecules also affect the reactivity of the complex.53 The mechanism can be outlined as occurring in three steps. 

The stoichiometric reaction of organolithium or Grignard reagents with copper halides allows direct formation of organocopper species(103). 

The intrinsic instability of organocopper] 11) compounds is most probably associated with the redox properties of copper. Decomposition of organocopper] 11) compounds can occur by two different routes (i) formation of an organocopper]I) compound and an organic radical R" that can undergo further reactions, which formally represents a one-electron reduction process, and (ii) direct formation of R-R and Cu]0), which is formally a two-electron reduction process (reductive elimination cf Eqns. 1 and 2 in Scheme 1.3). 

Finally, some organocopper compounds undergo charge disproportionation under the influence of ligands that bind strongly to copper. Treatment of mesityl-copper with l,2-bis-(diphenylphosphino)ethane (DPPE), for example, results in the formation of bis(mesityl)copper anions and a copper cation to which four phosphorus atoms of two DPPE molecules are coordinated [75]. 

An illustrative example is the formation of the symmetric biaryl from the reaction between CuC6H4NMe2-2 and IC6H4NMe2-2, which has been studied in detail in the authors laboratory [95]. When this reaction is carried out in benzene as a solvent, the reaction stops when one third of the original organocopper compound has been consumed (Eqn. 1 in Scheme 1.20). 

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