Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

Balof, Shawna L.; P’Pool, Steven J.; Berger, Nancy J.; Valente, E.; Shiller, Alan M.; Schanz, Hans‐Jörg

doi:10.1039/b809793c

Cited by 81 publications

(73 citation statements)

References 67 publications

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“…C NMR chemical shifts are referenced to SiMe 4 (d = 0 ppm) using the residual proton impurities of the deuterated solvents as internal standards. 31 P NMR spectra were referenced using H 3 PO 4 (d = 0 ppm) as an external standard. Spectra are reported as follows: chemical shift (ppm), multiplicity, coupling constant (Hz), and integration.…”

Section: Methodsmentioning

confidence: 99%

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

Costabile

Mariconda

Cavallo

et al. 2011

Chemistry A European J

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The synthesis of Ru-based precatalysts with N-heterocyclic carbene (NHC) ligands bearing syn- and anti-methyl groups on the NHC backbone and aryl N-substituents with differing steric bulk was carried out. The catalytic behavior of the monophospine Ru precatalysts (7a, 7b, 8a, and 8b) was compared to the corresponding family of phosphine-free catalysts (9a, 9b, 10a and 10b) in the ring-closing metathesis (RCM) of olefins. These catalysts showed high efficiency in RCM reactions and the syn-isomers 7a and 9a, in particular, proved to be among the most active catalysts in the formation of tetrasubstituted olefins through RCM. DFT studies on the entire RCM catalytic cycle of hindered olefins were performed to rationalize the different behaviors of catalysts with syn- and anti-methyl groups on the NHC backbone. Theoretical results not only disclosed how NHC symmetry influences the overall activity of the catalyst, but also gave relevant and more general indications on the crucial steps of the RCM of olefins.

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Section: Methodsmentioning

confidence: 99%

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

Costabile

Mariconda

Cavallo

et al. 2011

Chemistry A European J

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“…The Mulliken atomic charges for the Ru center in complex 11 were nearly identical to complex 10 which is not surprising with respect to the similar activities for catalysts 2 and 4 in ROMP and RCM reactions. [19] More interestingly, the charges became less positive with increasing protonation to complexes 11Ј and 11ЈЈ, however, these charges are still much more positive than for complex 9. This is somewhat surprising since the H 2 ITap ligand becomes positively charged and thus an increase in the charge at the metal center could have been expected.…”

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confidence: 95%

“…Such partial protonation of the basic phosphane ligand was observed before with by the addition of aqueous hydrochloric acid to complex 4. [19] The PCy 3 ligand is more Brønsted basic than the aniline nitrogen atoms by approx. four orders of magnitude.…”

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confidence: 99%

“…[12] We have recently reported catalysts 4 and 5 coordinated by the H 2 ITap [bis-1,3-(4Ј-aminophenyl-N,N,3Ј,5Ј-tetramethyl)-4,5-dihydroimidazol-2-ylidene] ligand. [19] The catalysts contain two pH-responsive NMe 2 groups at the phenyl rings which can be protonated with strong acid. Whereas the neutral catalysts exhibited similar or even slightly higher activities in ROMP and ring closing metathesis (RCM) reactions as than their very active, commercially available counterparts 2 and 3 in organic solvents, the catalysts failed to perform ROMP of cationic exo-7-oxanorbornene 6 derivative in acidic protic media (Scheme 1).…”

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confidence: 99%

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Ru‐Based Olefin Metathesis Catalysts Bearing pH‐Responsive N‐Heterocyclic Carbene (NHC) Ligands: Activity Control via Degree of Protonation

Balof

Lowe

et al. 2009

Eur J Inorg Chem

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= 1,3-bis-(2Ј,6Ј-dimethyl-4Ј-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene] were used for the ring-opening metathesis polymerization (ROMP) of exo-7-oxanorbornene derivative 7 in the presence of various amounts of acid. Upon gradual protonation of the NMe 2 groups of the H 2 Tap ligand, the metathesis activity of both catalysts were gradually reduced due Over the last decade, olefin metathesis has emerged as a powerful technique in organic [1] and polymer synthesis.[2] In particular Ru-based, single-site catalysts such as Grubbs' first-and second-generation catalysts 1 and 2, and Hoveyda-Grubbs catalyst 3 have gained scientific and commercial importance due to their high tolerance towards moisture and functional groups, [3][4][5] as these catalysts have significantly expanded the scope of metathesis substrates. In terms of overall activity and thermal stability, catalysts 2 and 3 bearing an N-heterocyclic carbene (NHC) ligand, most commonly the H 2 IMes [bis(2,4,6-trimethylphenyl)dihydroimidazol-2-ylidene] ligand, are superior. [4,5] This is due to their slower rates of initiation which is overcompensated by the fact that these catalysts promote extremely fast metathesis propagation. [6,7] On the flip side however, the high propagation rates accomplished with these catalysts makes them unsuitable for controlled ring-opening metathesis polymerization (ROMP) in contrast to catalyst 1. In fact, only small fractions of catalysts 2 and 3 initiate in a standard ROMP reaction. [7] Apart from reaction temperature and structural parameters, only a few controls have been recognized to influence the activity of Ru-based olefin metathesis catalysts. The addition of Brønsted [8][9][10][11][12][13][14][15] or Lewis [15][16][17] to electronic changes of the N-heterocyclic carbene (NHC) ligand donor capability. The investigation of the ROMP polymer 8, DFT calculations and measurements of the initiation kinetics prove that the reduced activity is solely due to reduced rates of propagation.

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“…[14,15] To date, just a few examples of ammonium-salt-tagged NHC-metal complexes are known in the literature; in most cases, an NHC-metal complex was quaternized with HCl. [16][17][18] As SIPr, SIMes, IPr, and IMes ligands are the most common ligands in NHC-based catalysis, Wang et al described ammonium-salt-tagged SIPr-Cu I complexes with different alkyl residues on the ammonium group, and different counterions. [19] Zwitterionic gold-NHC complexes bearing sulfonate and pyridinium groups were found to catalyze the cyclization of alkynoic acids.…”

Section: Introductionmentioning

confidence: 99%

Ammonium‐Salt‐Tagged IMesAuCl Complexes and Their Application in Gold‐Catalyzed Cycloisomerization Reactions in Water

Belger

Krause

2014

Eur J Org Chem

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Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

Cited by 81 publications

References 67 publications

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

The Pivotal Role of Symmetry in the Ruthenium‐Catalyzed Ring‐Closing Metathesis of Olefins

Ru‐Based Olefin Metathesis Catalysts Bearing pH‐Responsive N‐Heterocyclic Carbene (NHC) Ligands: Activity Control via Degree of Protonation

Ammonium‐Salt‐Tagged IMesAuCl Complexes and Their Application in Gold‐Catalyzed Cycloisomerization Reactions in Water

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