Reactions between lithium dimethylcuprate and alkenyl bromides, chlorides, and fluorides: synthetic, mechanistic, and stereochemical aspects

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Abstract
The reactions of cis- and trans-1-halogeno-2-phenylsulphonylethylenes (I)–(VI) and 1-halogeno-2-phenylethylenes (VII)–(X) with lithium dimethylcuprate in ether have been investigated. In most cases cross-coupling products are formed in good yield but reductive dehalogenation, which occurs with retention of configuration, may become an important process depending upon the nature of the substrate. At variance with the styrene system, in the sulphonyl compounds the stereochemical course of the substitution is influenced by the leaving group and by the temperature, a high degree of stereospecificity being observed only for bromides and chlorides at low temperature. Low and high leaving group effects have been observed respectively for the sulphonyl and styrene systems thus suggesting the operation of different mechanisms. These and related results are discussed in the light of the following pathways: (i) electron transfer; (ii) formation of a vinyl carbanion; (iii) concerted substitution of the halogen; (iv) addition–elimination. Mechanism (iv) appears a likely candidate for the reactions of the sulphonyl derivatives whereas mechanisms (ii) and/or (iii) may be followed by the halogenostyrene system.

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