Highly stereoselective synthesis of conjugated polyenes via a homocoupling reaction of unsaturated silanes

A new method for the synthesis of conjugated polyenes containing up to eight double bonds with all-E configuration is reported. The procedure is based upon a homocoupling reaction of dienyl-, trienyl-, or tetraenylsilanes, promoted by PdCl2 in methanol and in the presence of LiCl and CuCl2. Configurational and conformational assignments were rigorously made on the basis of NMR spectra. The compounds obtained represent a novel interesting class of symmetrically substituted polyenes with potential optical and electrooptical properties. By changing the solvent, the homocoupling process can be switched to the halogenation of the silyl-substituted terminal double bond, thus leading to polyenyl halides.