Ligand Effects in Gold- and Platinum-Catalyzed Cyclization of Enynes:  Chiral Gold Complexes for Enantioselective Alkoxycyclization

Phosphine and bidentate N−N ligands inhibit the Alder-ene-type cycloisomerization of enynes catalyzed by Pt(II) and favor the alkoxycyclization process. The enantioselective Pt(II)-catalyzed alkoxycyclization has been studied in the presence of chiral mono- and bidentate phosphines, as well as chiral bidentate N−N ligands. Modest levels of enantioselection (up to 50% ee) have been obtained with Tol-BINAP as ligand. The alkoxycyclizations with a catalyst formed from [Au(L)Cl]/AgX proceed more readily, and up to 94% ee's have been obtained using [(AuCl)2(Tol-BINAP)] (47) as the precatalyst. The X-ray crystal structures of Au(I) complexes 47 and chloro-(R)-2-(tert-butylsulfenyl)-1-(diphenylphosphino)ferrocene gold(I) (39) show the AuCl fragments monocoordinated with the P centers of the chiral phosphine ligands.