Abstract
Abstract In order to evaluate the electron transfer ability of organocopper reagents, the reactions of appropriate Michael acceptors with methyl- and butylcopper reagents were investigated. The reaction of trimethyl ethylene-1,1,2-tricarboxylate (1a) with methylcopper reagents gave a mixture of the conjugate adduct 2a and reduction product 3a in good to high yields. The ratio of 2a and 3a varied with the copper species and decreased in the following order: MeCu(CN)Li > MeCu > Me2Cu(CN)Li2 >> Me2CuLi > Me3CuLi2. It was confirmed by 13C NMR studies and oxidation experiments of the intermediate copper species that the reduction product 3a was produced via an electron transfer process and that the conjugate adduct 2a was afforded through a nucleophilic attack of MeCuILn. Consequently, the electron transfer ability of methylcopper reagents is in the following order: Me3CuLi2 > Me2CuLi >> Me2Cu(CN)Li2 > MeCu > MeCu(CN)Li. By similar experiments using 3-cyclopropyl-2-methoxycarbonyl-3-phenyl-2-propenoate (9) as a Michael acceptor, we concluded that the electron transfer ability of butylcopper reagents is in the order of Bu2CuLi > BuCu(CN)Li ≈ Bu2Cu(CN)Li2 > BuCu, and that its ability of butylcopper reagents is higher than that of the corresponding methylcopper reagents.
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