Conformational Control of Metallocene Backbone by Cyclopentadienyl Ring Substitution: A New Concept in Polyphosphane Ligands Evidenced by “Through-Space” Nuclear Spin−Spin Coupling. Application in Heteroaromatics Arylation by Direct C−H Activation

The present study deals with the conformational control of the metallocene backbone within ferrocenyl polyphosphane ligands and their performance in the highly topical palladium-catalyzed heteroaromatics arylation by direct C−H activation. New substituted cyclopentadienyl rings were synthesized, which allowed the assembling of original tri- and diphosphanes. The bulky cyclopentadienyl lithium salts diphenylphosphino-3-(triphenyl)methylcyclopentadienyllithium (4) and 1,2-bis(diphenylphosphino)-4-(triphenyl)methylcyclopentadienyllithium (5) were prepared in excellent yield. The assembling of these new hindered cyclopentadienyl salts (Cp) with other Cp fragments was performed in order to prepare ferrocenyl ligands with controlled conformation. A comparison of conformations of 1,1′,2-tris(diphenylphosphino)-3′,4-di-tert-butylferrocene (3) and 1,1′,2-tris(diphenylphosphino)-3′-(triphenyl)methyl-4-tert-butylferrocene (6) allowed us to determine, for the first time, the conditions of an efficient control of the orientation of the phosphino substituents on the ferrocene backbone in the absence of an ansa-bridge. The characterization of these metallo-ligands, by multinuclear NMR in solution and by X-ray diffraction in the solid state, focused on nonbonded J PP spin−spin couplings. These unusual couplings are especially useful for assessing the conformation of the ferrocene backbone in solution. The palladium complexes of the triphosphane ligand 6 and the diphosphane 1,1′-bis(diphenylphosphino)-3,3′-di(triphenyl)methylferrocene (7) were successfully used in the direct C−H activation of electron-rich heteroaromatics for coupling to demanding aryl bromides, whether electron rich and/or sterically congested. Products such as 2-butyl-5-(4-methoxyphenyl)furan (10), 2-butyl-5-o-tolylfuran (11), and thiophene analogues (12 and 13) were obtained in yields higher than 90%. The NMR examination of the reaction of [PdCl(η3-C3H5)]2 with ligands 6 and 7 allowed us to determine the coordination chemistry of the precatalysts. The allylic palladium complexes 8 and 9 confirmed the unusual conformation present in 6. As a consequence, the selective and “genuinely” tridentate bonding of a triphosphane to one palladium center is reported. This seldom observed coordination mode is a direct consequence of the successful ferrocene backbone conformation control.