Grubbs' catalyst

Grubbs' Catalyst is a transition metal carbene complex named after the chemist by whom it was first synthesized, Robert H. Grubbs. 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 and are compatible with a wide range of solvents.^[3] For these reasons, Grubbs' Catalysts are extraordinarily versatile.

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First generation catalyst

The 1^st Generation Catalyst is often used in organic synthesis to achieve olefin cross-metathesis (see below), ring-opening metathesis polymerization (ROMP), and ring-closing metathesis. It is easily synthesized from RuCl₂(PPh₃),^[4] phenyldiazomethane, and tricyclohexylphosphine in a one-pot synthesis.^[5] Grubbs' Catalyst is a relatively stable compound in air, which makes handling very easy. The IUPAC name of the 1^st Generation Catalyst is benzylidene-bis(tricyclohexylphosphine)dichlororuthenium.

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. Other catalysts are effective for this reaction, notably those developed by Richard R. Schrock (Schrock carbene).

Second generation catalyst

The Second Generation Catalyst has the same uses in organic synthesis as the First Generation Catalyst, but has a higher activity. This catalyst is also air stable and is easily synthesized from the combination of the 1^st Generation Catalyst and alkoxy-protected 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene.^[4] This ligand is an N-heterocyclic carbene (NHC) and now ruthenium is coordinated to two carbene groups. The IUPAC name of the Second Generation Catalyst is benzylidene[1,3- bis(2,4,6-trimethylphenyl)-2- imidazolidinylidene]dichloro(tricyclohexylphosphine)ruthenium. Both generations of the catalyst are commercially available.

Hoveyda-Grubbs Catalyst

In the first generation Hoveyda-Grubbs Catalyst, one of the phosphine ligands is replaced by an isopropyloxy group attached to the benzene ring. Its second generation has the other phosphine ligand replaced by NHC.

In one study a water soluble Grubbs catalyst is prepared by attaching a polyethylene glycol chain to the imidazoline group. The imidazolinium salt is deprotonated by potassium hexamethyldisilazide (KHMDS) in situ to give the N-heterocyclic carbene, which displaces one phosphine ligand to give the modified ruthenium complex:^[6]

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.

Applications

An interesting application of Grubbs' Catalyst is in the aerospace industry. A spaceship's hull is a necessarily very strong material, but over time small microcracks in the structure can form. A new material, with potential application in the construction of spaceship hulls, contains Grubbs' Catalyst, as well as capsules of dicyclopentadiene, which can undergo ring opening metathesis polymerisation. When a crack in the hull forms, the capsules are ruptured and come into contact with Grubbs' Catalyst, which polymerizes dicyclopentadiene and seals the crack.^[7]

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, R.H. Handbook of Metathesis; Wiley-VCH, Germany, 2003.
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. ^ 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.
4. ^ ^{a b} 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.
5. ^ 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.
6. ^ 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.
7. ^ S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, S. Viswanathan (2001). "Autonomic healing of polymer composites". Nature 409: 794-797. doi:10.1038/35057232.