Olefin metathesis
Olefin metathesis | |
---|---|
Reaction type | Carbon-carbon bond forming reaction |
Identifiers | |
Organic Chemistry Portal | olefin-metathesis |
RSC ontology ID | RXNO:0000280 |
In
Catalysts
The reaction requires
Grubbs catalysts, on the other hand, are ruthenium(II) carbenoid complexes.
Applications
Olefin metathesis has several industrial applications. Almost all commercial applications employ
- The Phillips Triolefin and the Olefin conversion technology. This process interconverts propylene with ethylene and 2-butenes. Rhenium and molybdenum catalysts are used. Nowadays, only the reverse reaction, i.e., the conversion of ethylene and 2-butene to propylene is industrially practiced, however.[6]
- alpha-olefins) for conversion to detergents. The process recycles certain olefin fractions using metathesis.[7]
- silicaand MgO.
- 1,5-Hexadiene and 1,9-decadiene, useful crosslinking agents and synthetic intermediates, are produced commercially by ethenolysis of 1,5-cyclooctadiene and cyclooctene. The catalyst is derived from Re2O7 on alumina.
- Synthesis of pharmaceutical drugs,[8]
Homogeneous catalyst potential
Molecular catalysts have been explored for the preparation of a variety of potential applications.[9] the manufacturing of high-strength materials, the preparation of cancer-targeting nanoparticles,[10] and the conversion of renewable plant-based feedstocks into hair and skin care products.[11]
Types
Some important classes of olefin metathesis include:
- Cross metathesis (CM)
- Ring-opening metathesis (ROM)
- Ring-closing metathesis (RCM)
- Ring-opening metathesis polymerization(ROMP)
- Acyclic diene metathesis (ADMET)
- Ethenolysis
Mechanism
Hérisson and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis.[12] The direct [2+2] cycloaddition of two alkenes is formally symmetry forbidden and thus has a high activation energy. The Chauvin mechanism involves the [2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallacyclobutane intermediate. The metallacyclobutane produced can then cycloeliminate to give either the original species or a new alkene and alkylidene. Interaction with the d-orbitals on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures.
Olefin metathesis involves little change in enthalpy for unstrained alkenes. Product distributions are determined instead by
Cross metathesis and ring-closing metathesis are driven by the entropically favored evolution of
Cross-metathesis is synthetically equivalent to (and has replaced) a procedure of
Historical overview
"Olefin metathesis is a child of industry and, as with many catalytic processes, it was discovered by accident."[1] As part of ongoing work in what would later become known as
In 1960 a
a reaction then classified as a so-called
Only much later the polynorbornene was going to be produced through
In a third development leading up to olefin metathesis, researchers at
This particular mechanism is symmetry forbidden based on the Woodward–Hoffmann rules first formulated two years earlier. Cyclobutanes have also never been identified in metathesis reactions, which is another reason why it was quickly abandoned.
Then in 1967 researchers led by
In this reaction 2-pentene forms a rapid (a matter of seconds)
The Goodyear group demonstrated that the reaction of regular 2-butene with its all-
In 1971 Chauvin proposed a four-membered metallacycle intermediate to explain the statistical distribution of products found in certain metathesis reactions.[21] This mechanism is today considered the actual mechanism taking place in olefin metathesis.
Chauvin's experimental evidence was based on the reaction of
The three principal products C9, C10 and C11 are found in a 1:2:1 regardless of conversion. The same ratio is found with the higher oligomers. Chauvin also explained how the carbene forms in the first place: by alpha-hydride elimination from a carbon metal single bond. For example, propylene (C3) forms in a reaction of 2-butene (C4) with tungsten hexachloride and tetramethyltin (C1).
In the same year Pettit who synthesised
Experimental support offered by Pettit for this mechanism was based on an observed reaction inhibition by carbon monoxide in certain metathesis reactions of 4-nonene with a tungsten metal carbonyl[23]
Robert H. Grubbs got involved in metathesis in 1972 and also proposed a metallacycle intermediate but one with four carbon atoms in the ring.[24] The group he worked in reacted 1,4-dilithiobutane with tungsten hexachloride in an attempt to directly produce a cyclomethylenemetallacycle producing an intermediate, which yielded products identical with those produced by the intermediate in the olefin metathesis reaction. This mechanism is pairwise:
In 1973 Grubbs found further evidence for this mechanism by isolating one such metallacycle not with tungsten but with platinum by reaction of the dilithiobutane with cis-bis(triphenylphosphine)dichloroplatinum(II)[25]
In 1975 Katz also arrived at a metallacyclobutane intermediate consistent with the one proposed by Chauvin[26] He reacted a mixture of cyclooctene, 2-butene and 4-octene with a molybdenum catalyst and observed that the unsymmetrical C14 hydrocarbon reaction product is present right from the start at low conversion.
In any of the pairwise mechanisms with olefin pairing as rate-determining step this compound, a secondary reaction product of C12 with C6, would form well after formation of the two primary reaction products C12 and C16.
In 1974 Casey was the first to implement carbenes into the metathesis reaction mechanism:[27]
Grubbs in 1976 provided evidence against his own updated pairwise mechanism:
with a 5-membered cycle in another round of isotope labeling studies in favor of the 4-membered cycle Chauvin mechanism:[28][29]
In this reaction the ethylene product distribution at low conversion was found to be consistent with the carbene mechanism. On the other hand, Grubbs did not rule out the possibility of a tetramethylene intermediate.
The first practical metathesis system was introduced in 1978 by Tebbe based on the (what later became known as the)
The Grubbs group then isolated the proposed metallacyclobutane intermediate in 1980 also with this reagent together with 3-methyl-1-butene:[31]
They isolated a similar compound in the total synthesis of capnellene in 1986:[32]
In that same year the Grubbs group proved that metathesis polymerization of norbornene by Tebbe's reagent is a living polymerization system[33] and a year later Grubbs and Schrock co-published an article describing living polymerization with a tungsten carbene complex[34] While Schrock focussed his research on tungsten and molybdenum catalysts for olefin metathesis, Grubbs started the development of catalysts based on ruthenium, which proved to be less sensitive to oxygen and water and therefore more functional group tolerant.
Grubbs catalysts
In the 1960s and 1970s various groups reported the ring-opening polymerization of norbornene catalyzed by hydrated trichlorides of ruthenium and other late transition metals in polar, protic solvents.
The corresponding tricyclohexylphosphine complex (PCy3)2Cl2Ru=CHCH=CPh2 was also shown to be active.[40] This work culminated in the now commercially available 1st generation Grubbs catalyst.[41][42]
Schrock catalysts
Schrock entered the olefin metathesis field in 1979 as an extension of work on tantalum alkylidenes.[43] The initial result was disappointing as reaction of CpTa(=CH−t−Bu)Cl2 with ethylene yielded only a metallacyclopentane, not metathesis products:[44]
But by tweaking this structure to a PR3Ta(CHt−bu)(Ot−bu)2Cl (replacing
Schrock alkylidenes for olefin metathesis of the type Mo(NAr)(CHC(CH3)2R){OC(CH3)(CF3)2}2 were commercialized starting in 1990.[47][48]
The first asymmetric catalyst followed in 1993[49]
With a Schrock catalyst modified with a
See also
References
- ^ ISBN 978-0-471-23896-6.
- S2CID 98046245.
- Nobelprize.org. 5 October 2005.
- .
- ^ Ileana Dragutan; Valerian Dragutan; Petru Filip (2005). "Recent developments in design and synthesis of well-defined ruthenium metathesis catalysts – a highly successful opening for intricate organic synthesis". Arkivoc: 105–129. Archived from the original on 12 May 2006. Retrieved 6 October 2005.
- ^ S2CID 103664623.
- ISBN 3-527-28838-4.
- PMID 20163176.
- PMID 22731677.
- PMID 18452296.
- ^ "Dow Corning and Elevance Announce Partnership to Market Naturally Derived Ingredients in Personal Care Applications" (Press release). Elevance Renewable Sciences. 9 September 2008. Archived from the original on 9 December 2011. Retrieved 19 January 2012.
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- ^ A. W. Anderson and N. G. Merckling, U. S. U.S. patent 2,721,189 (18 October 1955)
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Further reading
- "Olefin Metathesis: Big-Deal Reaction". 80 (51). 2002: 29–33. )
- "Olefin Metathesis: The Early Days". 80 (51). 2002: 34–38. )
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- S2CID 35370749.
- Samojłowicz, C.; Grela, K. (2009). "Ruthenium-Based Olefin Metathesis Catalysts Bearing N-Heterocyclic Carbene Ligands". Chemical Reviews. 109 (8): 3708–3742. PMID 19534492.
- Vougioukalakis, G. C.; S2CID 4589661.
- Trnka, T. M.; S2CID 22145255.
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- Grela, K. (2010). Grela, K. (ed.). "Progress in metathesis chemistry (Editorial for Open Access Thematic Series)". Beilstein Journal of Organic Chemistry. 6: 1089–1090. PMID 21160917.