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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. 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. For these reasons, Grubbs' Catalysts are extraordinarily versatile.
Additional recommended knowledge
First generation catalyst
The 1st 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), phenyldiazomethane, and tricyclohexylphosphine in a one-pot synthesis. Grubbs' Catalyst is a relatively stable compound in air, which makes handling very easy. The IUPAC name of the 1st 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 1st Generation Catalyst and alkoxy-protected 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene. 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.
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:
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.
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Grubbs'_catalyst". A list of authors is available in Wikipedia.|