Desaktivierung von Ruthenium‐Metathesekatalysatoren durch intramolekulare Carben‐Aren‐Verknüpfung

Vehlow, Kati; Gessler, Simon; Blechert, Siegfried

doi:10.1002/ange.200702560

Cited by 36 publications

(17 citation statements)

References 32 publications

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“…Initiators 2, 3, and 4 have been prepared as described earlier. [30,54] The synthetic protocol used there was also applied to the synthesis of initiator 1 (Scheme 1, PCy 3 = tricyclohexylphosphine). This compound was prepared by means of a Pd-catalyzed N-arylation reaction of N-benzyl-N-(2-aminoethyl)amine with bromomesitylene.…”

Section: Resultsmentioning

confidence: 99%

“…(2), [54] [RuCl 2 (1-mesityl-3-methylimidazolin-2- (3) is described elsewhere. [30] N-Benzyl-N'-(2,4,6-trimethylphenyl)ethane-1,2-diamine: 3 ] (dba = dibenzylideneacetone; 1.0 mol %), (AE )-BINAP (BINAP = 2,2'bis-(diphenylphosphino)-1,1'-binaphthyl; 3.0 mol %), and toluene (25 mL) were mixed together.…”

Section: Molecular Weights Were Determined By Calibration Vs Polya C mentioning

confidence: 99%

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Alternating Ring‐Opening Metathesis Copolymerization by Grubbs‐Type Initiators with Unsymmetrical N‐Heterocyclic Carbenes

Lichtenheldt

Wang

Vehlow

et al. 2009

Chemistry A European J

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A series of Ru(IV)-alkylidenes based on unsymmetrical imidazolin-2-ylidenes, that is, [RuCl(2){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHPh)(pyridin)] (R = CH(2)Ph (5), Ph (6), ethyl (7), methyl (8)), have been synthesized. These and the parent initiators [RuCl(2)(PCy(3)){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHC(6)H(5))] (R = CH(2)C(6)H(5) (1), C(6)H(5) (2), ethyl (3)) were used for the alternating copolymerization of norborn-2-ene (NBE) with cis-cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE-alt-COE)(n) and poly(NBE-alt-CPE)(n) containing up to 97 and 91% alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1-3 and 5-8 were determined by using both a zero- and first-order Markov model. Finally, kinetic investigations using initiators 1-3, 6, and 7 were carried out. These revealed that in contrast to the 2nd-generation Grubbs-type initiators 1-3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE-alt-COE)(n) yielded a fully saturated, hydrocarbon-based polymer. Its backbone can formally be derived by 1-olefin polymerization of CPE (1,3-insertion) followed by five ethylene units and thus serves as an excellent model compound for 1-olefin polymerization-derived copolymers.

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Section: Resultsmentioning

confidence: 99%

Section: Molecular Weights Were Determined By Calibration Vs Polya C mentioning

confidence: 99%

Alternating Ring‐Opening Metathesis Copolymerization by Grubbs‐Type Initiators with Unsymmetrical N‐Heterocyclic Carbenes

Lichtenheldt

Wang

Vehlow

et al. 2009

Chemistry A European J

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“…This was seen as evidence for p-p interactions [46] between an aromatic ring at the NHC ligand and the Ru-benzylidene unit. In the meantime, it was shown that even in Grubbs II-type complexes with mixed aryl, alkyl-NHC [40,[47][48][49][50] or related ligands [49] the aryl groups are located above the benzylidene unit and the alkyl group above the empty coordination site. For bulky alkyl groups steric arguments account for the observed orientation of the N-substituents.…”

Section: Introductionmentioning

confidence: 98%

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

Leuthäußer

Schmidts

Thiele

et al. 2008

Chemistry A European J

139

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The electron-donating properties of eighteen N-heterocyclic carbenes (N,N'-bis(2,6-dimethylphenyl)imidazol)-2-ylidene and the respective dihydro ligands) with 4,4'-R substituted aryl rings (4,4'-R = NEt2, OMe, Me, H, SMe, F, Cl, Br, I) in the respective Grubbs II complexes were studied using electrochemical techniques. The nature of the 4-R substituent has a strong influence on the RuII/III redox potentials ranging between DeltaE1/2= +0.196 and +0.532 V. Three unsymmetrical Grubbs II complexes with 4-R not equal to 4-R' were also synthesized. Dynamic NMR spectroscopy revealed the restricted rotation around the (NHC)C-Ru bond (DeltaG = 89 kJ mol(-1) at 333 K) resulting in two atropisomers, respectively, with an isomer ratio close to unity. Each of the isomers, that is the two orientations of the 4-R/4-R' substituted mesityl ring with respect to the R=CHPh unit, gives rise to different redox potentials (4-R = NEt2, 4-R' = Br: DeltaE1/2= +0.232 and +0.451 V). In the oxidized Grubbs II complex (4-R = NEt2, H) and in the cathodic isomer the electron rich aryl ring is located above the Ru=CHPh unit. This orientational effect provides clear evidence for strong pi-pi through-space interactions in the RuIII complexes, assuming that the alternative through-bond transfer of electron density is equally efficient in both isomers.

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“…The metal coordination environment is no longer a square-pyramid, but rather a distorted octahedron. [9] As a result of twist there is a strong steric interaction between the ligand and chlorine atom Cl1. The Ru À Cl bond lengths of 2.350(3) are significantly shortened and the Cl1-Ru1-Cl2 bond angle of 158.57(6)8 is increased.…”

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confidence: 99%

A Novel Ligand for the Enantioselective Ruthenium‐Catalyzed Olefin Metathesis

et al. 2011

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Recently the interest for chiral olefin metathesis catalysts with respect to the synthesis of enantioenriched molecules, as well as enhanced product selectivities, has increased significantly.[1] For these kind of transformations, a range of molybdenum-catalysts containing one stereogen metal center have been very successful.[2] However, in comparison to these catalysts, ruthenium metathesis catalysts offer improved handling and stability. [3] A challenge with such catalysts is an efficient transfer of chirality from the N-heterocyclic carbene (NHC) to the metal center without substitution of the chlorine ligands, which are important for the reactivity.[4] Recently, we introduced chiral ruthenium metathesis catalysts of type 1, [5] which, compared to the catalysts of Grubbs et al. (e.g. 2), have a different orientation of the stabilizing N-aryl groups (Scheme 1) which arises from the monosubstitution in the NHC-backbone. [6] The C3 substituent in the NHC twists the framework and hinders rotation of the N-aryl substituent, while at the same time the absence of the C4 substitution allows a planar orientation for the mesitylene moiety. The resulting highly active catalyst was used for asymmetric ring-opening crossmetathesis providing excellent results.[5]Herein, we report a new type of chiral NHC-ligand in which rotation around the chirality transferring N-aryl bond is no longer possible, and the corresponding ruthenium metathesis catalysts. We decided to use a 2-substituted tetrahydroquinoline as the source of chirality, which is easy accessible via quinaldic acid 4. After esterification of 4 and hydration to rac-5, a kinetic enzymatic resolution leads to the desired key structure ent-5 in an overall yield of 41 % (! 99 % ee) (Scheme 2). [7] Protection of amine ent-5 (route A), ester saponification and subsequent amide coupling and deprotection, afforded amide 6. A further reduction with BH 3 ·SMe 2 led to the formation of diamine 7 in good yields. During this reaction a partial racemization and fluctuating ee-values were detected Scheme 1. Chiral ruthenium metathesis (pre)catalysts.

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Desaktivierung von Ruthenium‐Metathesekatalysatoren durch intramolekulare Carben‐Aren‐Verknüpfung

Cited by 36 publications

References 32 publications

Alternating Ring‐Opening Metathesis Copolymerization by Grubbs‐Type Initiators with Unsymmetrical N‐Heterocyclic Carbenes

Alternating Ring‐Opening Metathesis Copolymerization by Grubbs‐Type Initiators with Unsymmetrical N‐Heterocyclic Carbenes

π‐Face Donor Properties of N‐Heterocyclic Carbenes in Grubbs II Complexes

A Novel Ligand for the Enantioselective Ruthenium‐Catalyzed Olefin Metathesis

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