Development and Application of N-Trifluoromethyl NHC Ligands for Transition-Metal Catalysis

Abstract

Starting from N-trifluoromethyl benzimidazole, a series of unsymmetrical N-trifluoromethyl benzimidazolium salts have been prepared and fully characterized as precursors for N-heterocyclic carbene (NHC) ligands. IR analysis of [Ir(CO)2(NHC)Cl] complexes revealed that the trifluoromethyl substituent on nitrogen significantly decreases the σ-donating ability of the carbene carbon. On the other hand, the π-acceptor property of these novel ligands is enhanced. Examination of the 77Se NMR chemical shifts of [Se(NHC)] adducts and the redox potentials of [Rh(NHC)(COD)Cl] complexes further supports this assumption. In addition, the efficiency of these new N-trifluoromethyl NHC ligands was investigated in π-acidic Au(I)-catalyzed hydroxy alkoxylation of cyclohexene. The Au(I) complexes bearing N-trifluoromethyl NHC ligands thereby compete with [Au(PPh3)Cl] in terms of catalytic activity. We further report the synthesis of ruthenium metathesis catalysts containing unsymmetrical N-trifluoromethyl NHC ligands. These complexes have been fully characterized, and a Ru–F interaction has been identified in the solid state by X-ray crystallographic analysis for three catalysts with Ru–F distances between 2.629(2) and 2.652(2) Å. The influence of the N-trifluoromethyl NHC ligands on the initiation rates and activation parameters was studied. The activity of these catalysts was evaluated in benchmark olefin metathesis reactions and compared to the standard second-generation Grubbs catalyst. Remarkably, N-trifluoromethyl catalysts display an unusually high selectivity for the formation of terminal olefins (up to 90%) in the ethenolysis of ethyl oleate. Much improved selectivity is demonstrated for alternating copolymerization of cyclooctene and norbornene as well. Moreover, a library of 29 homologous Ru-based olefin metathesis catalysts has been tested for ethenolysis of cyclic olefins towards the goal of selectively forming α,ω-diene using cis-cyclooctene as a prototypical substrate. Dissymmetry at the NHC ligand was identified as a key parameter controlling the selectivity. The best-performing catalyst bearing an N-CF3 group significantly outperformed the benchmark second-generation Grubbs catalyst in ethenolysis of cis-cyclooctane. Application of this optimal catalyst to the ethenolysis of various norbornenes allows the efficient synthesis of valuable diene intermediates in good yields. The observed ligand effect trends could be rationalized through univariate and multivariate parameter analysis involving steric and electronic descriptors of the NHC ligand in the form of the buried volume and the 77Se NMR chemical shift, in particular the σyy component of the shielding tensor of [Se(NHC)] derivatives, respectively. Natural chemical shift analysis of this chemical shielding tensor shows that σyy probes the π-acceptor property of the NHC ligand, the key electronic parameter that drives the relative rate of degenerate metathesis and selectivity in ethenolysis with catalysts bearing dissymmetric NHC ligands.