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 supervised their synthesis. Several generations of the catalyst have also been developed. Grubbs catalysts tolerate many functional groups in the alkene substrates, are air-tolerant, and are compatible with a wide range of solvents. For these reasons, Grubbs catalysts have become popular in synthetic organic chemistry.
Grubbs, together with Richard R. Schrock and Yves Chauvin, won the 2005 Nobel Prize in Chemistry in recognition of their contributions to the development of olefin metathesis.
First-generation Grubbs catalyst
In the 1960s, ruthenium trichloride was found to catalyze olefin metathesis. Processes were commercialized based on these discoveries. Ill-defined but highly active homogeneous catalysts based on this remain in industrial use.
thumb|400px|none|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 synthesized from RuCl<sub>2</sub>(PPh<sub>3</sub>)<sub>3</sub>, phenyldiazomethane, and tricyclohexylphosphine in a one-pot synthesis.
thumb|400px|none|Preparation of the first-generation Grubbs catalyst
Second-generation Grubbs catalyst
The second-generation catalyst has the same uses in organic synthesis as the first generation catalyst, but generally with higher activity.
The second-generation catalyst based on a saturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)dihydroimidazole) was reported in 1999:
thumb|450px|none|Synthesis of the second–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 second-generation-type catalysts. In the following year, the second-generation of the catalyst was described in nearly simultaneous publications by two independent lab groups. By changing the steric and electronic properties of the chelate, the initiation rate of the catalyst can be modulated, such as in the Zhan catalysts.
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:
500px|none|Preparation of the first-generation Hoveyda–Grubbs catalyst from the first-generation Grubbs catalyst
The second-generation Hoveyda–Grubbs catalysts can also be prepared from the 1st generation Hoveyda–Grubbs catalyst by the addition of the NHC:
thumb|500px|none|Preparation of the second-generation Hoveyda–Grubbs catalyst from the second–generation Grubbs catalyst
In 2006, a water-soluble Grubbs catalyst was prepared by attaching a polyethylene glycol chain to the imidazolidine group. 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.
thumb|500px|none|Ring closing metathesis reaction in water
Third-generation Grubbs catalyst (fast-initiating catalysts)
The 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 times. Both pyridine and 3-bromopyridine are commonly used, with the bromo- version being 4.8 times more labile resulting in even faster rates. The catalyst is traditionally isolated as a two pyridine complex, however one pyridine is lost upon dissolving and reversibly inhibits the ruthenium center throughout any chemical reaction.
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The principal 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. The high ratio of the rate of initiation to the rate of propagation makes these catalysts useful in living polymerization, yielding polymers with low polydispersity.
Applications
Grubbs catalysts are of interest for olefin metathesis. They are mainly applied to fine chemical synthesis. Large-scale commercial applications of olefin metathesis almost always employ heterogeneous catalysts or ill-defined systems based on ruthenium trichloride.