Organocuprate addition to α,β-unsaturated compounds: synthetic and mechanistic aspects

Abstract

A mechanistic model for t h e addition of lithium dimethylcuprate to enoates and enones is proposed on t h e basis of chemical and NMR-spectroscopic studies. The first s t e p of t h e reaction is t h e reversible formation of a copper($ alkene n-complex by coordination of t h e C -C group to one of t h e copper a t o m s in t h e dimeric cluster s t r u c t u r e (Me Culi&. The following steps lead to t h e irreversible formation of t h e new carbon-carbon bond. With chiral substrates t w o diastereomeric n-complexes a r e postulated, t h e stereoselectivity being determined by t h e relative r a t e s for their irreversible decay. In t h e reaction between methyl S-ortho-dimethylaminoethylcinnamate, 10, and lithium dimethyl-and lithium diphenylcuprate t h e diastereomeric product ratios a r e ca 5: 1. 2

Mechanisms of Copper‐mediated Addition and Substitution Reactions

“…XANES investigation of a complex formed between a trans-cinnamate ester and Me 2 CuLiÁLiI in THF indicated elongation of the CbC double bond and an increase in the coordination number of the copper atom. NMR studies on the organic component in the complexes indicated loosening of the olefinic bond [72,74]. Very recently, Krause has determined the kinetic activation energies (E a ¼ 17-18 kcal mol À1 ) of some conjugate addition reactions for the first time [75].…”

On the Role of PIII Ligands in the Conjugate Addition of Diorganozinc Derivatives to Enones

“…[8] However, in spite of the importance of this promising catalytic process, very few mechanistic studies have been undertaken to elucidate its mechanism. In the related area of the conjugate addition of Gilman cuprates [9] early kinetic studies by Krauss and Smith [10] and subsequent NMR studies by Ullenius and Smith [11] clearly pointed to a mechanism, where an intermediate formed in equilibrium with the starting material goes through an irreversible step to give the conjugate adduct. Further support came from mechanistic studies by Krause, who identified such an intermediate, followed by elegant investigations on kinetic isotope effects by Singleton.…”

Nucleophilic Addition–Electrophilic Coupling with a Carbanion Intermediate

“…190,191 Simple alkylcopper(I) reagents are less reactive than their corresponding dialkylcopper(I)metals; this comparitive lack of reactivity in 1,4-addition reactions results in relatively rare use in vicinal difunctionalizations of a,~-unsaturatedcarbonyl substrates. Equation (44) illustrates that vinylcopper initiates dialkylation of a cyclopentenone,192 but most organocoppers in fact are inert. In part, this inertness may be explained to the lack of solubility of the copper reagents in common solvents for conjugate additionenolate trapping reactions, such as THF or diethyl ether.…”