Ruthenium-Diphosphine-Catalyzed Allylation of Phenols: A gem-Dialkyl-Type Effect Induces High Selectivity toward O-Allylation

Abstract

The catalytic performance of a series of novel cationic ruthenium(II) complexes with cyclopentadienyl and bidentate phosphine ligands was explored to establish a catalyst structure−performance relationship and gain mechanistic insight in the selective O-allylation of a phenol with allyl alcohol. It appears that catalysts containing bidentate phosphine ligands having geminal dialkyl substituents at the central atom of a C3-bridging group of the phosphine ligand are highly selective for O-allylation; apparently the presence of the substituents efficiently blocks the competitive and thermodynamically more favorable pathway to C-allylation. It appears that the electronic and structural properties of the Ru(II) precursor complexes in the solid state do not differ significantly from those of complexes containing unsubstituted analogous ligands, while the resulting catalysts show a vastly different catalytic performance. The complex [RuCp(dppp)](OTs), with the unsubstituted ligand, after six hours yields 70% conversion of phenol with a selectivity for O-allylation of only 27%, whereas the complex [RuCp(dppdmp)](OTs), with the dimethyl-substituted ligand, after six hours gives 60% conversion of phenol with 82% selectivity for O-allylation. The results suggest that the geminal dialkyl substitution at the central carbon of the C3 bridge of the ligand primarily leads to an increased kinetic stability of the bidentate chelate under reaction conditions, such as in the proposed intermediate [Ru(IV)(Cp)(diphosphine)(allyl)]2+ complexes. This implies that the high kinetic stability of the diphosphine chelate bound to Ru blocks the pathway to the thermodynamically favored C-allylation product. The results provide an interesting example in which the application of the geminal dialkyl substitution in the bridge of a bidentate ligand serves as a diagnostic tool to probe the nature of the selectivity-determining step in a catalytic pathway in homogeneous catalysis.