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Synthesis and Reactivity of Homogeneous and Heterogeneous Ruthenium-Based Metathesis Catalysts Containing Electron-Withdrawing Ligands

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Abstract

The synthesis and heterogenization of new Grubbs–Hoveyda type metathesis catalysts by chlorine exchange is described. Substitution of one or two chlorine ligands with trifluoroacetate and trifluoromethanesulfonate was accomplished by reaction of [RuCl2([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (IMesH2 = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) with the silver salts CF3COOAg and CF3SO3Ag, respectively. The resulting compounds, [Ru(CF3SO3)2([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (1), [RuCl(CF3SO3)([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (2), and [Ru(CF3CO2)2([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (3) were found to be highly active catalysts for ring-closing metathesis (RCM) at elevated temperature (45 °C), exceeding known ruthenium-based catalysts in catalytic activity. Turn-over numbers (TONs) up to 1800 were achieved in RCM. Excellent yields were also achieved in enyne metathesis and ring-opening cross metathesis using norborn-5-ene and 7-oxanorborn-5-ene-derivatives. Even more important, 3 was found to be highly active in RCM at room temperature (20 °C), allowing TONs up to 1400. Heterogeneous catalysts were synthesized by immobilizing [RuCl2([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] on a perfluoroglutaric acid derivatized polystyrene-divinylbenzene (PS-DVB) support (silver form). The resulting supported catalyst [RuCl(polymer-CH2-O- CO-CF2-CF2-CF2-COO)([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (5) showed significantly reduced activities in RCM (TONs = 380) compared with the heterogeneous analogue of 3. The immobilized catalyst, [Ru(polymer-CH2-O-CO-CF2-CF2-CF2-COO)(CF3CO2)([DOUBLE BOND]CH-o-iPr-O-C6H4)(IMesH2)] (4) was obtained by substitution of both Cl ligands of the parent Grubbs–Hoveyda catalyst by addition of CF3COOAg to 5. Compound 4 can be prepared in high loadings (160 mg catalyst g−1 PS-DVB) and possesses excellent activity in RCM with TONs up to 1100 in stirred-batch RCM experiments. Leaching of ruthenium into the reaction mixture was unprecedentedly low, resulting in a ruthenium content <70 ppb (ng g−1) in the final RCM-derived products.

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