Synthesis of bulky arylphosphanes by rhodium-catalyzed formal [2+2+2] cycloaddition reaction and their use as ligands

Treatment of 1-alkynylphosphane sulfides with 1,6- or 1,7-diynes in the presence of a cationic rhodium catalyst results in a formal [2+2+2] cycloaddition reaction to afford the corresponding aromatic phosphane sulfides. The aromatic rings formed in the cycloaddition naturally bear one or two substituents at the ortho positions to the phosphorus atom, which creates a sterically hindered environment around the phosphorus atom. The following desulfidation of the products is facile under radical conditions or with the aid of tris(dimethylamino)phosphane, providing the corresponding bulky phosphanes. Dicyclohexyl(2,6-diphenylaryl)phosphane, which is available through this sequence, proves to serve as an efficient ligand in palladium-catalyzed cross-coupling amination reactions.