Stereoselective Cyanation of Vinyl Halides Catalyzed by Tetracyanocobaltate(I)

Article abstract of DOI:10.1021/ja00370a019

Tetracyanocobaltate(I), ^3-, which is formed in an aqueous alkaline solution under a hydrogen atmosphere, catalyzes the cyanation of vinyl halides to form 2-alkenenitriles.The reaction is stereoselective, forming nitriles with retention of configuration, except for (Z)-2-bromobut-2-ene, which forms a mixture of nearly equimolar isomeric nitriles.Reactivity is dependent on the CN:Co ratio and is highest when the ratio is slightly lower than 5:1.Presence of excess cyanide ion inhibits the reaction, but a dropwise addition of the KCN solution to maintain CN:Co^3-, were detected as intermediates by 1H and 13C NMR spectroscopy, indicating that the reaction proceeds stepwise.In the first step, the ? complex is formed by the oxidative addition of a vinyl halide to ^3- via a radical nonchain process; in this step stereoselectivity is determined.In the second step, which is rate determining, a 2-alkenenitrile is formed by the reductive coupling of the vinyl and cyano ligands, regenerating ^3-.Clear NMR evidence has been obtained for the formation of ^3-, where the olefin is (E)- or (Z)-cinnamonitrile.A high degree of electron transfer from ^3- to olefin was indicated by the large upfield shifts of the olefinic carbon atom resonances by coordination.