New Ruthenium(II) Indenylidene Complexes Bearing Unsymmetrical N-Heterocyclic Carbenes

Abstract

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...

SIMes/PCy3 mixed ligand‐coordinated alkyl group‐tagged ruthenium indenylidene complexes: Synthesis, characterization and metathesis activity

“…One of the N ‐mesityl groups is positioned towards the indenylidene moiety and shows π–π stacking interactions (3.427(2) Å, 16.2(4)°; and 3.954(2) Å, 16.5(4)° for the five‐ and six‐membered rings, respectively, in the case of 4f and 3.513(5) Å, 12.7(1)°; and 4.005(4) Å, 14.4(1)° for the five‐ and six‐membered rings, respectively, in the case of 4g ) between the two aromatic layers to stabilize the arrangement. This arrangement of the geometry is similar to that of most of ruthenium complexes of this type …”

In a Quest for Selectivity Paired with Activity: A Ruthenium Olefin Metathesis Catalyst Bearing an Unsymmetrical Phenanthrene‐Based N‐Heterocyclic Carbene

“…[47] In line with the trend of investigating uNHC as ligandsf or olefin metathesis, our group published as eries of works on Naralkyl-N'-aryl NHC-based complexes.T hese provedu seful as olefin-metathesis precatalysts, not only,i ns tandard conditions under an argon atmosphere, but also in commercial grade nondegassed solvents (Figure 1, L10-B). [48,49] Importantly,t hose complexes were successfully applied in thee thenolysis of ethyl oleate (Figure 1, L12-B,C) [50] and afforded low isomerization in self-metathesis of a-olefins ( Figure 1, L11-C). [51] They were also used in reactions with 1ppm pre-catalyst loading yielding 232 735 TON (Figure 1, L14-B).…”

Ruthenium Complexes Bearing Thiophene‐Based Unsymmetrical N‐Heterocyclic Carbene Ligands as Selective Catalysts for Olefin Metathesis in Toluene and Environmentally Friendly 2‐Methyltetrahydrofuran

“…The class of ruthenium indenylidene precatalysts with symmetrical NHC carbenes ( 2 a – c ) has been extensively studied . In contrast, ruthenium indenylidene complexes bearing unsymmetrical NHC carbene subunits are largely unexplored –,,,…”