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Grubbs catalyst

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1st Generation Grubbs Catalyst
Identifiers
Properties
C43H72Cl2P2Ru
Molar mass 822.97 g·mol−1
Appearance Purple solid
Melting point 153 °C (307 °F; 426 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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2nd Generation Grubbs Catalyst
Names
IUPAC name
[1,3-bis-(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro(phenylmethylene)(tricyclohexylphosphine)ruthenium
Identifiers
Properties
C46H65Cl2N2PRu
Molar mass 848.98 g·mol−1
Appearance Pinkish brown solid
Melting point 143.5-148.5 °C
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
2nd Generation Hoveyda-Grubbs Catalyst
Identifiers
Properties
C31H38Cl2N2ORu
Molar mass 626.63 g·mol−1
Appearance Green solid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Grubbs' Catalysts are a series of transition metal carbene complexes used as catalysts for olefin metathesis. They are named after Robert H. Grubbs, the chemist who first synthesized them. There are two generations of the catalyst, as shown on the right.[1][2] In contrast to other olefin metathesis catalysts, Grubbs' catalysts tolerate other functional groups in the alkene, are air-tolerant and are compatible with a wide range of solvents.[3][4] For these reasons, Grubbs' catalysts have become popular in synthetic organic chemistry.[5]

First generation catalyst

The first well-defined ruthenium catalyst for olefin metathesis was discovered in 1992.[6] It was prepared from RuCl2(PPh3)4 and diphenylcyclopropene.

First Grubbs-type catalyst
First Grubbs-type catalyst

This initial ruthenium catalyst was followed in 1995 by what is now known as the first generation Grubbs catalyst. It is easily synthesized from RuCl2(PPh3)3, phenyldiazomethane, and tricyclohexylphosphine in a one-pot synthesis.[7][8]

Preparation of the 1st generation Grubbs Catalyst
Preparation of the 1st generation Grubbs Catalyst

The first generation Grubbs catalyst, while largely replaced by the second generation catalyst in usage, was not only the first catalyst to be developed other than those developed by Richard R. Schrock (Schrock carbenes), but is also important as a precursor to all other Grubbs-type catalysts.

Second generation catalyst

The second generation catalyst has the same uses in organic synthesis as the first generation catalyst, but generally with higher activity. This catalyst is stable toward moisture and air, thus is easier to handle in the lab.

Shortly before the discovery of the 2nd generation Grubbs' catalyst, a very similar catalyst based on an unsaturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)imidazole) was reported independently by Nolan[9] and Grubbs[10] in March 1999, and by Fürstner[11] in June of the same year. Shortly thereafter, in August 1999, Grubbs reported the 2nd generation catalyst, based on a saturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)dihydroimidazole):[12]

Synthesis of the 2nd Generation Grubbs Catalyst
Synthesis of the 2nd Generation Grubbs Catalyst

In both the saturated and unsaturated cases a phosphine ligand is replaced with an N-heterocyclic carbene (NHC), which is characteristic of all 2nd generation type catalysts.[3]

Both the 1st and 2nd generation catalysts are commercially available, along with many derivatives of the 2nd generation catalyst.

"Hoveyda-Grubbs" Hoveyda-Blechert Catalyst

In the "Hoveyda-Grubbs" Catalyst the benzylidene ligand has a chelating isopropoxy group attached to the benzene ring. Its commonly used name "Hoveyda-Grubbs" is in fact a misnomer, since this catalyst was independently discovered in the research groups of Amir Hoveyda[13] and Siegfried Blechert[14]: a more correct name is thus 'Hoveyda-Blechert Catalyst', which should only be used in the literature. The chelating oxygen atom replaces a phosphine ligand, which for the 2nd generation catalyst gives a completely phosphine-free structure. The "Hoveyda-Grubbs" catalysts, while more expensive and slower to initiate than the Grubbs catalyst from which they are derived, are popular because of their improved stability.[3] Hoveyda-Grubbs catalysts are easily formed from the corresponding Grubbs catalyst by the addition of the chelating ligand and the use of a phosphine scavenger like copper(I) chloride:[13]

Preparation of Hoveyda-Grubbs Catalyst from the 2nd generation Grubbs Catalyst
Preparation of Hoveyda-Grubbs Catalyst from the 2nd generation Grubbs Catalyst

The 2nd generation Hoveyda-Grubbs catalysts can also be prepared from the 1st generation Hoveyda-Grubbs catalyst by the addition of the NHC:[14]

Preparation of Hoveyda-Grubbs Catalyst from the 1st generation version
Preparation of Hoveyda-Grubbs Catalyst from the 1st generation version

In one study a water soluble Grubbs catalyst is prepared by attaching a polyethylene glycol chain to the imidazolidine group.[15] This catalyst is used in the ring-closing metathesis reaction in water of a diene carrying an ammonium salt group making it water-soluble as well.

Ring closing metathesis reaction in water with water soluble
Ring closing metathesis reaction in water with water soluble

Fast-Initiating Catalysts

The initiation rate of the Grubbs' catalyst can be altered by replacing the phosphine ligand with more labile pyridine ligands. By using 3-bromopyridine the initiation rate is increased more than a million fold:[16]

Preparation of fast initiating catalysts
Preparation of fast initiating catalysts

The principle application of the fast-initiating catalysts is as initiators for ring opening metathesis polymerisation (ROMP). Because of their usefulness in ROMP these catalysts are sometimes referred to as the 3rd generation Grubbs' catalysts.[17] The high ratio of the rate of initiation to the rate of propagation makes these catalysts useful in living polymerization. Living polymerization can be used in the synthesis of block copolymers with interesting side chain functionality, taking full advantage of the high functional group tolerance of the Grubbs' catalyst.

Applications

Olefin metathesis is a reaction between two molecules containing double bonds. The groups bonded to the carbon atoms of the double bond are exchanged between molecules, to produce two new molecules containing double bonds with swapped groups. Whether a cis isomer or trans isomer is formed in this type of reaction is determined by the orientation the molecules assume when they coordinate to the catalyst, as well as the sterics of the substituents on the double bond of the newly forming molecule.

On October 5, 2005, Robert H. Grubbs, Richard R. Schrock and Yves Chauvin won the Nobel Prize in Chemistry in recognition of their contributions to the development of this widely used process.

References

  1. ^ Grubbs, Robert H. (2003). Handbook of Metathesis (1st ed.). German: Wiley-VCH. ISBN 3-527-30616-1.
  2. ^ Grubbs, R.H.; Trnka, T.M.: Ruthenium-Catalyzed Olefin Metathesis in "Ruthenium in Organic Synthesis" (S.-I. Murahashi, Ed.), Wiley-VCH, Germany, 2004.
  3. ^ a b c Vougioukalakis, G. C. (2010). "Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts". Chemical Reviews. 110 (3): 1746–1787. doi:10.1021/cr9002424. PMID 20000700. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ Trnka, T. M.; Grubbs, R. H. (2001). "The Development of L2X2Ru=CHR Olefin Metathesis Catalysts: An Organometallic Success Story". Accounts of Chemical Research. 34 (1): 18–29. doi:10.1021/ar000114f. PMID 11170353.
  5. ^ Cossy, Janine; Arseniyadis, Stellios; Meyer, Christophe (2010). Metathesis in Natural Product Synthesis: Strategies, Substrates and Catalysts (1st ed.). Germany: Wiley-VCH. ISBN 3-527-32440-2.
  6. ^ Nguyen, S.T.; Johnson, L.K.; Grubbs, R.H.; Ziller, J.W. (1992). "Ring-opening metathesis polymerization (ROMP) of norbornene by a Group VIII carbene complex in protic media". Journal of the American Chemical Society. 114 (10): 3974–3975. doi:10.1021/ja00036a053.
  7. ^ Schwab, P.; France, M.B.; Ziller, J.W.; Grubbs, R.H. "A Series of Well-Defined Metathesis Catalysts - Synthesis of [RuCl2(=CHR')(PR3)2] and Its Reactions". Angewandte Chemie-International Edition in English. 34 (18): 2039–2041. doi:10.1002/anie.199520391.
  8. ^ Schwab, P.; Grubbs, R. H.; Ziller, J. W. (1996). "Synthesis and Applications of RuCl2(=CHR')(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity". Journal of the American Chemical Society. 118 (1): 100–110. doi:10.1021/ja952676d.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  9. ^ Huang, J.-K.; Stevens, E.D.; Nolan, S.P.; Petersen, J.L. (1999). "Olefin Metathesis-Active Ruthenium Complexes Bearing a Nucleophilic Carbene Ligand". Journal of the American Chemical Society. 121 (12): 2674–2678. doi:10.1021/ja9831352.
  10. ^ Scholl, M.; Trnka, T.M.; Morgan, J.P.; Grubbs, R.H. (1999). "Increased Ring Closing Metathesis Activity of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Imidazolin-2-ylidene Ligands". Tetrahedron Letters. 40 (12): 2247–2250. doi:10.1016/S0040-4039(99)00217-8.
  11. ^ Ackermann, L.; Fürstner, A.; Weskamp, T.; Kohl, F.J.; Herrmann, W.A. (1999). "Ruthenium Carbene Complexes with Imidazolin-2-ylidene Ligands Allow the Formation of Tetrasubstituted Cycloalkenes by RCM". Tetrahedron Letters. 40 (26). doi:10.1016/S0040-4039(99)00919-3.
  12. ^ Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. (1999). "Synthesis and Activity of a New Generation of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with 1,3-Dimesityl-4,5-dihydroimidazol-2-ylidene Ligands". Organic Letters. 1 (6): 953–956. doi:10.1021/ol990909q.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ a b Garber, S.B.; Kingsbury, J.S.; Gray, B.L.; Hoveyda, A.H. (2000). "Efficient and Recyclable Monomeric and Dendritic Ru-Based Metathesis Catalysts". Journal of the American Chemical Society. 122 (34): 8168–8179. doi:10.1021/ja001179g.
  14. ^ a b Gessler, S.; Randl, S.; Blechert, S. (2000). "Synthesis and metathesis reactions of phosphine-free dihydroimidazole carbene ruthenium complex". Tetrahedron Letters. 41 (51): 9973–9976. doi:10.1016/S0040-4039(00)01808-6.
  15. ^ Soon Hyeok Hong and Robert H. Grubbs (2006). "Highly Active Water-Soluble Olefin Metathesis Catalyst". Journal of the American Chemical Society. 128 (11): 3508–3509. doi:10.1021/ja058451c. PMID 16536510.
  16. ^ Love, J.A.; Morgan, J.P.; Trnka, T.M.; Grubbs, R.H. (2002). "A Practical and Highly Active Ruthenium-Based Catalyst that Effects the Cross Metathesis of Acrylonitrile". Angewandte Chemie-International Edition in English. 41 (21): 4035–4037. doi:10.1002/1521-3773(20021104)41:21<4035::AID-ANIE4035>3.0.CO;2-I.
  17. ^ Leitgeb, Anita; Wappel, Julia; Slugovc, Christian (2010). "The ROMP toolbox upgraded". Polymer. 51 (14): 2927–2946. doi:10.1016/j.polymer.2010.05.002.
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