Deactivation of Ruthenium Olefin Metathesis Catalysts through Intramolecular Carbene–Arene Bond Formation

Abstract

for olefin metathesis have been reported, for which strikingly different influences of the ligands on the efficiency of C C bond formation have been observed. For example, notable effects have been found in the class of alkoxy benzylidene ligands first introduced by Hoveyda and coworkers. Phosphine-free complexes such as the secondgeneration Hoveyda–Grubbs catalyst (4 ; IMesH2= 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) are especially wellsuited for cross metathesis with electron-deficient olefins such as a,b-unsaturated nitriles. The stability and the initiation rate of the precatalyst could be tuned by steric and electronic effects of the substituents. Additional intramolecular interactions of functional groups or atoms with the metal center have been reported for some ligands. 6] Grubbs and co-workers recently found an accelerating effect in olefin metathesis through fluorine– ruthenium interaction. Besides knowledge of such ligand and substituent effects, an understanding of catalyst decomposition and transformation to catalytically inactive ruthenium species is of major importance for the rational design and improvement of metathesis catalysts. Fundamental studies have been presented by Grubbs and co-workers and by Dinger and Mol. Herein, we report a novel deactivation reaction pathway transforming active ruthenium catalysts with alkoxy benzylidene ligands into catalytically inactive carbene complexes. During our study of diastereoselective ring rearrangement metathesis reactions (dRRM), we found that the neutral ligand that remains on the metal center has a pronounced effect on conversion and diastereoselectivity. While only moderate diastereoselectivities could be seen for the firstgeneration catalysts (1, 3), the sterically more demanding and more active second-generation catalysts (2, 4) led to significantly higher selectivities. Thus, we focused on the development of a bulky ruthenium carbene complex to increase the diastereoselective interaction between the olefine moiety and the catalytically active ruthenium species. To do this, we connected the N-aryl substituent with the N-heterocyclic carbene (NHC) through a C2 unit. Unlike in 2 or 4, the aromatic moiety in this new ruthenium complex (5, Scheme 1) should exert a much stronger steric influence on the ruthenium alkylidene moiety through torsion of approximately 458 and hindered rotation. The synthesis of the NHC ligand started from commercially available 2,2’-biquinoline (6). The first step was hydration to octahydrobiquinoline; the result of the hydration was strongly dependent on the catalyst system. PtO2 and H2 under ambient pressure led to the formation of a 3:1 mixture of the meso and racemic forms. Simple chromatography on silica gel