Hybrid Phosphine/Amino-Acid Ligands Built on Phenyl and Ferrocenyl Platforms: Application in the Suzuki Coupling of o-Dibromobenzene with Fluorophenylboronic Acid.

We describe the synthesis and characterization of two classes of hybrid phosphino ligands functionalized with amino ester or amino acid groups. These compounds are built either on a rigid planar phenyl platform or on a functionalized - conformationally controlled - rotational ferrocene backbone. Modifications at the -PR2 phosphino groups (R=aryl and alkyl, with various steric bulk, Ph, Mes, i-Pr, Cy) and at the amino acid/amino ester functions are reported, showing a valuable high modularity. The coordination chemistry of these compounds regarding palladium and gold was investigated, in particular with respect to the coordination mode of the phosphino groups and the preferred interaction with metals for the amino ester and amino acid functions. For all the hybrid ligands, based either on ferrocenyl or phenyl platforms, the (P,N)-chelating effect dominates in solution for coordination to Pd(II), while linear P-Au(I) complexes without interaction with the amino groups are assumed. The investigation of the catalytic activity of these new ligands in the demanding palladium-catalyzed Suzuki-Miyaura coupling of o-dibromoarenes with fluorophenylboronic acid underlined the importance of the amino ester dicyclohexylphosphinoferrocene for avoiding the deleterious homocoupling and arene oligomerization side-reactions that were otherwise observed with the other phosphine ligands.

Palladium complexes of N-heterocyclic carbenes displaying an unsymmetrical N-alkylfluorenyl/N'-aryl substitution pattern and their behaviour in Suzuki-Miyaura cross coupling.

A series of new Pd-PEPPSI complexes containing imidazolylidene ligands with a mixed 9-alkyl-9-fluorenyl/aryl N,N'-substitution pattern have been synthesised. Single crystal X-ray diffraction studies were carried out for four complexes, which revealed that the N-heterocyclic carbene ligands display a semi-open, unsymmetrical space occupancy about the metal. Despite their particular unsymmetrical shape, the new complexes were found to perform as well in Suzuki-Miyaura cross coupling (dioxane, 80 °C) as previously reported, highly active analogues bearing two sterically protecting 9-alkylfluorenyl substituents. They were further found to be considerably more active than the standard Pd-PEPPSI complexes [PdCl2IMes(pyridine)] and [PdCl2IPr(pyridine)]. Surprisingly, unlike the latter, the unsymmetrical complexes of this study were practically inactive in isopropanol at 80 °C. Under these conditions the complexes appear to decompose with formation of non-stabilised nanoparticles.