An [(NHC)(NHC<sub>EWG</sub>)RuCl<sub>2</sub>(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring‐Closing Metathesis

Vorfalt, Tim; Leuthäußer, Steffen; Plenio, Herbert

doi:10.1002/ange.200900935

Cited by 33 publications

(20 citation statements)

References 34 publications

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“…[11] Somewhat unexpectedly, the performance of complexes 5 ad in the RCM of diethyl dimethallylmalonate is comparable, despite significant differences in the electron donation of the respective NHC ewg ligands.…”

Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 89%

“…All of the newly synthesized complexes are characterized by superior performance relative to complex 1 and even the least efficient one of the new complexes gives better yields than complex 1 for this test reaction. [11] The best results in terms of final yield and rate of the RCM reaction are obtained for complex 5 g with an unsymmetrical NHC ewg ligand (R 1 = Me, R 2 = iPr). This complex is characterized by a combination of high activity and relatively fast initiation.…”

Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 97%

“…[ 11] Here we are only going to discuss the work of others published in 2009 and the substrate conversions not studied previously.…”

Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 99%

“…On testing such complexes in RCM reactions, it was discovered that [(NHC)-A C H T U N G T R E N N U N G (NHC ewg )RuCl 2 A C H T U N G T R E N N U N G (CHPh)] 1 renders catalysts with unprecedented activities for RCM (ring-closing metathesis) reactions leading to tetrasubstituted olefins. [11] The synthesis of such olefin-metathesis products still poses significant problems with Grubbs-type complexes and until recently 2.5-5 mol % of catalyst and elevated temperatures were required for such reactions. [12] More recently, some improvements were reported and attributed to a reduced bulk of the NHC ligand, [13] to the use of NHC ligands with restrictedconformational freedom, [14] and by applying low polarity, perfluorinated solvents.…”

Section: A C H T U N G T R E N N U N G (Nhc Ewg )Rucl 2 a C H T U N Gmentioning

confidence: 99%

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[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Sashuk

Peeck

Plenio

2010

Chemistry A European J

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101

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Imidazolium salts (NHC(ewg)HCl) with electronically variable substituents in the 4,5-position (H,H or Cl,Cl or H,NO(2) or CN,CN) and sterically variable substituents in the 1,3-position (Me,Me or Et,Et or iPr,iPr or Me,iPr) were synthesized and converted into the respective [AgI(NHC)(ewg)] complexes. The reactions of [(NHC)RuCl(2)(CHPh)(py)(2)] with the [AgI(NHC(ewg))] complexes provide the respective [(NHC)(NHC(ewg))RuCl(2)(CHPh)] complexes in excellent yields. The catalytic activity of such complexes in ring-closing metathesis (RCM) reactions leading to tetrasubstituted olefins was studied. To obtain quantitative substrate conversion, catalyst loadings of 0.2-0.5 mol % at 80 degrees C in toluene are sufficient. The complex with the best catalytic activity in such RCM reactions and the fastest initiation rate has an NHC(ewg) group with 1,3-Me,iPr and 4,5-Cl,Cl substituents and can be synthesized in 95 % isolated yield from the ruthenium precursor. To learn which one of the two NHC ligands acts as the leaving group in olefin metathesis reactions two complexes, [(FL-NHC)(NHC(ewg))RuCl(2)(CHPh)] and [(FL-NHC(ewg))(NHC)RuCl(2)(CHPh)], with a dansyl fluorophore (FL)-tagged electron-rich NHC ligand (FL-NHC) and an electron-deficient NHC ligand (FL-NHC(ewg)) were prepared. The fluorescence of the dansyl fluorophore is quenched as long as it is in close vicinity to ruthenium, but increases strongly upon dissociation of the respective fluorophore-tagged ligand. In this manner, it was shown for ring-opening metathesis ploymerization (ROMP) reactions at room temperature that the NHC(ewg) ligand normally acts as the leaving group, whereas the other NHC ligand remains ligated to ruthenium.

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Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 89%

Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 97%

“…[ 11] Here we are only going to discuss the work of others published in 2009 and the substrate conversions not studied previously.…”

Section: Rcm Reactions Utilizing [(Nhcmentioning

confidence: 99%

Section: A C H T U N G T R E N N U N G (Nhc Ewg )Rucl 2 a C H T U N Gmentioning

confidence: 99%

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[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Sashuk

Peeck

Plenio

2010

Chemistry A European J

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101

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“…Such as ingle-pot protocol is obviously advantageousi nt ermso fc ost due to the use of the same noble metal source to carry out twodifferent transformations efficiently.W hat is also remarkable in this case is that high enantioselectivities can be obtained from the mixture of Ru species present at the endo f the metathesis reactions.I ndeed, it is well-known that asymmetric hydrogenation is av ery sensitive transformation, andp ractitioners in this field usuallyt ake great care of using very pure metal precursors.T his may not be needed, as long as av ery active and enantioselective catalyst is formed from am ixture of precatalysts as seems to be the case in our protocol. Considering the latest development in Ru-based metathesis where the formation of tri-and tetra-substituted pro-chiral olefins is becoming more common, [13,14] we believe that our methodology could be applied to an increasing number of substrates and therefore be of great value for synthetic chemists.…”

Section: Pnmrmentioning

confidence: 99%

Assisted Tandem Catalysis: Metathesis Followed by Asymmetric Hydrogenation from a Single Ruthenium Source

et al. 2015

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DedicatedtoP rof.Stephen L. Buchwald on the occasion of his 60 th birthday.Supporting information for this article is availableonthe WWW under http://dx.doi.org/10.1002/adsc.201500359.Abstract: Here we report the first example of at andem metathesis-asymmetric hydrogenation protocolw here the prochiral olefin generated by metathesisi sh ydrogenated with high enantioselectivity by an in situ formed chiral ruthenium catalyst. We show that either the ruthenium metathesis catalysts or the ruthenium species formed during the metathesisr eaction can be converted into an efficient asymmetric hydrogenation catalyst upon addition of ac hiral ligand and an alcohol. Thep erformancei na symmetric hydrogenation appears to be very dependent on the solvent, the chiral ligand, and the prochiral substrate.Keywords: asymmetric catalysis;h ydrogenation; metathesis; ruthenium;tandem catalysisIn sequential tandemcatalysis, [1] two consecutive catalytic transformationsa re carried out within as ingle reactionv esselw here the product of the first catalytic cycle is the substrate of the second cycle.F ogg et al. [1b] in their review on the taxonomy of tandem reactions proposed the term "assisted tandem catalysis" for processes where the initial catalyst is converted into an ew species able to catalyze the second reaction upon the addition of ar eagento rachangei nt he reactionc onditions.I na ddition to the main advantages associated with every one-pot multi-step transformation (i.e., high process andw ork-up efficiency), such ap rocess allowsm ultiple use of the catalyst or, at least, of its precious metal component.Ru-catalyzed olefin metathesis [2] followed by hydrogenation is atypical example of assisted tandem catalysis andh as been the subject of numerous publications, [3] initiallyi nt he field of polymers [4] and later in the synthesiso fs mall molecules.[5] Although Ru is commonly used in asymmetric hydrogenation, [6] there are no reports about as equential tandem metathesisasymmetric hydrogenation. We anticipatedt hat this could be achieved via addition of ac hiral ligand after the metathesis step andp rior to introducing hydrogen. However, such an approach is risky considering the fact that different Ru species are presenta tt he end of the metathesis reaction. [7] Under hydrogen pressure,t hese may form different complexes which may or may not contain the chiral ligand, and which will have dissimilar efficiency in asymmetric hydrogenation. In this communication,w er eport our stepwise approachw here we first show that Grubbsm etathesis catalysts can be converted into enantioselective hydrogenation catalysts followed by several examples of sequential tandem metathesis-asymmetric hydrogenation.In initial experiments,w et ested whetherH oveydaGrubbs2 nd generation metathesis catalyst (HG-II)i n the presence of (S)-BINAP [2,2'-bis(diphenylphosphino)-1,1'-binaphthyl] as ac hiral ligandw as active in the enantioselective hydrogenation of 2-acetamidoacrylic acid methyl ester (1)( Scheme in Table 1). The choice of HG-II was based ...

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N ‐Heterocyclic Carbene–Ruthenium Complexes: A Prominent Breakthrough in Metathesis Reactions

Mavila

Lemcoff

2014

N‐Heterocyclic Carbenes

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An [(NHC)(NHC_EWG)RuCl₂(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring‐Closing Metathesis

Cited by 33 publications

References 34 publications

[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Assisted Tandem Catalysis: Metathesis Followed by Asymmetric Hydrogenation from a Single Ruthenium Source

N ‐Heterocyclic Carbene–Ruthenium Complexes: A Prominent Breakthrough in Metathesis Reactions

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An [(NHC)(NHCEWG)RuCl2(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring‐Closing Metathesis

Cited by 33 publications

References 34 publications

[(NHC)(NHCewg)RuCl2(CHPh)] Complexes with Modified NHCewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

[(NHC)(NHCewg)RuCl2(CHPh)] Complexes with Modified NHCewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

Assisted Tandem Catalysis: Metathesis Followed by Asymmetric Hydrogenation from a Single Ruthenium Source

N ‐Heterocyclic Carbene–Ruthenium Complexes: A Prominent Breakthrough in Metathesis Reactions

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An [(NHC)(NHC_EWG)RuCl₂(CHPh)] Complex for the Efficient Formation of Sterically Hindered Olefins by Ring‐Closing Metathesis

[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins

[(NHC)(NHC_ewg)RuCl₂(CHPh)] Complexes with Modified NHC_ewg Ligands for Efficient Ring‐Closing Metathesis Leading to Tetrasubstituted Olefins