In our search for more-selective olefin metathesis catalysts, a series of Hoveyda–Grubbs-type second-generation complexes bearing unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized and tested in model reactions. It was found that the N-benzyl substituent in NHC has a positive influence on the selectivity of the newly obtained catalysts in the self-metathesis reaction of α-olefins. As expected, a typical relationship between activity and selectivity with respect to the N-aryl substituent used was observed. Dipp-containing complexes exhibited higher stability at elevated temperature, while Mes-bearing complexes typically gave better yields than their Dipp analogues.

New robust and air-stable ruthenium(II) indenylidene secondgeneration precatalysts with unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized. These complexes were studied using ring-closing metathesis (RCM) of diethyl diallylmalonate (5; DEDAM) as a model substrate. Two new complexes containing OMe and NMe 2 groups in the NHC ligand, namely [1-(2,4,6-trimethylphenyl)-3-(2-methoxybenzyl)-2-imidazolidinylidene]-dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (4a) and [1-(2,4,6-trimethylphenylshow increased activity in comparison to the parent [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (Umicore M2). Interestingly, the complex [1-(2,4,6-trimethylphenyl)-3-(2-thiomethylbenzyl)-2-imidazolidinylidene]dichloro(3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) (4c), bearing a SMe substituent, showed very low activity in the model RCM reaction with diethyl diallylmalonate (5). Application of the active complexes for metathesis reactions with a broad spectrum of olefins in commercial-grade solvents in air was examined.O lefin metathesis is a versatile tool in organic chemistry for the formation of carbon−carbon double bonds. 1 In this respect it was widely acknowledged by a Nobel Prize to R. R. Schrock, Y. Chauvin, and R. H. Grubbs. 2−4 Olefin metathesis still attracts a great deal of attention from organic chemists not only in academia 5 but also in industry. 6 The state of the art of olefin metathesis catalysts open up new vistas for more efficient methods in tedious natural product synthesis. 7 Nowadays, there are a great number of precatalysts known with various NHC ligands designed for olefin metathesis, and many of them are commercially available. 8 It is well-known that modifications on the NHC's moiety can induce profound changes in the reactivity pattern of the resulting precatalysts. 9 Moreover, the unsymmetrical NHC ligands allow diversification of the steric bulkiness in the vicinity of the ruthenium and therefore change the catalytic properties of the resulting complexes. 10−12 The range of the ruthenium metathesis precatalysts developed up to this point is broad, but there is still need for more complexes which are air-stable and remain robust in solvents of commercial-grade purity. Therefore, we turned our attention to ruthenium indenylidene precatalysts, since they are robust and efficient in the olefin metathesis reactions. 13 We expected that formal replacement of one of the traditional mesityl groups in the NHC ligand by a flexible benzyl group bearing a donor substituent might allow additional coordination of the latter to the metal.The synthesis of the new imidazolinium salts 3a,b was achieved by the condensation of N-(2,4,6-trimethylphenyl)-1,2-diaminoethane and the respective easily accessible aldehydes. The resulting diimines were reduced in situ to diamines 2a,b, which were further cyclized with triethyl orthoformate to the corresponding NHC salts. 14 Synt...

New robust and air-stable ruthenium(II) indenylidene type second-generation precatalysts with unsymmetrical N-heterocyclic carbene (NHC) ligands were synthesized. The reaction profiles of these complexes were studied in commercial-grade solvents in air with model substrates leading to the di-, tri-, and tetrasubstituted olefins. In addition, application of selected precatalysts for olefin metathesis reactions on a broad spectrum of substrates with different functional groups was examined. Studies of solvent effects for the selected precatalysts as well as thermal activation of the corresponding complexes showed significant differences in their activities. Observed relationships in connection with single-crystal X-ray analysis revealed the influence of the unsymmetrical NHC ligands on the initiation rate of precatalysts. Dissociation of the phosphine ligand, the rate-determining step for our most active precatalysts, is accelerated by the steric repulsion of the dangling benzyl arm and the tricyclohexylphosphine ligand. A precatalyst with a hemilabile benzyl arm evinced decreased activity, probably due to coordination of the heteroatom to the ruthenium core taking place after dissociation of the phosphine ligand.

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