Vinyl copper(I) compounds: Part II: Stereospecific addition of homocuprates to 1‐alkynes in tetrahydrofuran

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Abstract
Homocuprates [RCuR”]MgX, 1 (R = alkyl; R” = R or BuC≡C) appeared to be useful reagents for the introduction of an R group into 1‐alkynes R′−C≡C−H 2 (R′ = Ph, H, alkyl) in tetrahydrofuran as a solvent. Depending on R and R′ terminal alkenes RR′CCH2 4a, non‐terminal alkenes E−R′CHCHR 4b or mixtures of 4a and 4b were isolated after protic work‐up. Except for the reaction of t‐Bu2CuMgCl with Ph−C≡C−H and t‐Bu−C≡C−H only cis additions were observed.In two cases, viz. for the reaction of Me2CuMgCl and t‐Bu2CuMgCl with Ph−C≡C−H, evidence was obtained for accompanying proton abstraction from the terminal alkyne 2 by the cuprate. This proton abstraction could be suppressed completely by carrying out the addition in the presence of LiBr. Moreover, a strong accelerating influence of LiBr on the rate of the R transfer from the cuprate 1 to the 1‐alkyne 2 was observed.