Rhodium-Catalyzed Dimerization of Terminal Alkynes Assisted by MeI

Dimerization of terminal arylalkynes at ambient temperature catalyzed by Rh(CO)(PPh3)2Cl (2) in the presence of MeI leads to formation of enyne with high conversion and high regio- and stereoselectivity. A rhodium intermediate captured from oxidative addition of MeI was used for dimerization of alkyne with selectivity controlled by the use of solvents. In aprotic solvent (such as acetone, CH2Cl2, or THF) dimerization of terminal alkynes HC⋮C(p-C6H4X) (1, X = H, a; NO2, b; C(O)H, c; Me, d; CN, e; NMe2, f; CF3, g; F, h; Br, i; I, j) leads to the (E)-1,4-disubstituted enynes 6 (a−k) in high selectivity. However, when MeOH is used as a solvent, the dimerization of 1-arylalkynes containing an electron-withdrawing group affords selectively the (Z)-1,4-disubstituted enyne 8. Requirement of the presence of MeI for this conversion indicates that the process presumably involves initially a six-coordinated rhodium methylacetylide intermediate. Oxidative addition of ICH2CN to 2 yielded the catalytically inactive six-coordinated complex Rh(CO)(PPh3)2(C⋮CPh)(I)(CH2CN) (5a). The analogous complex 5b with a p-nitro group on the phenyl acetylide ligand is characterized by X-ray diffraction analysis.