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Immobilized on Merrifield polystyrene, catalyst 1 shows high activity and stability in different types of olefin metathesis. The products can be isolated in high purity by simple filtration, while the immobilized catalyst 1 can be recycled and used up to four times in the ring‐closing metathesis of 2. Mes=methane sulfonate, Ts=toluene sulfonate, Cy=cyclohexyl.

Dedicated to Professor Ekkehard Winterfeldt on the occasion of his 75th birthdaySince the development of Grubbs catalysts of the first ([RuCl 2 (PCy 3 ) 2 (=CHÀC 6 H 5 )] (1; Cy= cyclohexyl) and second (2) generation, many modified ruthenium complexes for olefin metathesis have been reported, [1] for which strikingly different influences of the ligands on the efficiency of CÀC 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; IMesH 2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) [2] are especially wellsuited for cross metathesis with electron-deficient olefins such as a,b-unsaturated nitriles.[3] The stability and the initiation rate of the precatalyst could be tuned by steric and electronic effects of the substituents. [4] Additional intramolecular interactions of functional groups or atoms with the metal center have been reported for some ligands. [3c, 5, 6] Grubbs and co-workers recently found an accelerating effect in olefin metathesis through fluorineruthenium 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.[7] 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.[8] 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 C 2 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. PtO 2 and H 2 under ambient pressure led to the formation of a 3:1 mixture of the meso and racemic forms. Simple chromatography on silica gel Scheme 1. Synthesis of ruthe...

The exchange of the PCy 3 ligand in complex 2 by an o-isopropyl-phenylether ligand leads to the extremely stable and highly selective initiator 3 for cross metathesis reactions. For the first time, Ru-catalysed cross coupling with acrylonitrile can be performed in good yields.Ru-complexes with sterically demanding N-heterocyclic carbene (NHC) ligands are presently of growing interest ( Figure). 1 Compound 2 exhibits a substantially higher lifetime and activity than the classical Grubbs catalyst 1, due to the bulkiness and the increased basicity of the NHC ligand compared to PCy 3 . Complex 2 has been employed for the synthesis of tri-and tetra-substituted double bonds in ring-closing metathesis (RCM) and cross metathesis (CM) reactions. 2 The ligand exchange also leads to a high tolerance towards functional groups. Thus, 2 in contrast to 1 can be used for a variety of selective CM reactions with a,b-unsaturated esters, ketones and aldehydes. Acrylonitriles, however, do not react. 3 Figure Ru-catalysts for olefin metathesisWe suppose that the activity in metathesis reactions strongly depends on the electronic properties of the Rucarbene complex. Therefore, we have studied olefin metathesis reactions of complex 3. As expected, 3 is extremely stable, does not decompose when exposed to water and/ or air, and can be purified by chromatography on silica gel without any special precautions. We found that the replacement of the second phosphine ligand leads to different reactivities and selectivities. Complex 3 turned out to be perfectly suitable for selective CM reactions with a,bunsaturated nitriles.The preparation of complex 3 was published recently. 4 We have developed two further syntheses. 5 SchemeThe known complex 4 6 was treated with imidazolidine 5 to afford the new complex 6. The second PPh 3 ligand was replaced by a conceptionally new ligand exchange by RCM with phenylether 7 affording 3 in 40% yield. The overall yield of these two steps is moderate (36%), but this pathway avoids the use of the rather expensive and sensitive PCy 3 and the phenyldiazomethane usually employed for the synthesis of 1 (and thus of 2). 7 An alternative synthesis starts with complex 8, 8 described by Hoveyda, in a PCy 3 -NHC exchange reaction. 9In systematic studies we have found that 3, like 2, is an excellent catalyst for selective cross coupling reactions between terminal olefins and a,b-unsaturated carbonyl compounds. 9 In addition, with this catalyst -for the first time in Ru-carbene complex chemistry -highly selective CM with acrylonitrile can be accomplished. 10 Hitherto, such reactions were only known to be catalysed by Mo-complexes of the Schrock type. 11 However, these complexes are very air-and moisture-sensitive and show a restricted tolerance of several heteroatom functionalities. Acids, reactive carbonyl groups and alcohols cause problems. 12 We now provide a methodology for CM reactions of acrylonitrile with a variety of substrates that could not be functionalised so far. The products 13 such as those shown in the Ta...

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