The reaction requiresmetal catalysts. Most commercially important processes employheterogeneous catalysts. The heterogeneous catalysts are often prepared by in-situ activation of a metal halide (MClx) usingorganoaluminium ororganotin compounds, e.g. combining MClx–EtAlCl2. A typicalcatalyst support isalumina. Commercial catalysts are often based on molybdenum and ruthenium. Well-definedorganometallic compounds have mainly been investigated for small-scale reactions or in academic research. The homogeneous catalysts are often classified as Schrock catalysts andGrubbs catalysts. Schrock catalysts feature molybdenum(VI)- and tungsten(VI)-based centers supported byalkoxide andimido ligands.[4]
Commercially available schrock catalysts
Grubbs catalysts, on the other hand, are ruthenium(II) carbenoid complexes.[5] Many variations of Grubbs catalysts are known. Some have been modified with achelating isopropoxybenzylideneligand to form the relatedHoveyda–Grubbs catalyst.
Olefin metathesis has several industrial applications. Almost all commercial applications employheterogeneous catalysts using catalysts developed well before the Nobel-Prize winning work on homogeneous complexes.[6] Representative processes include:[1]
The Phillips Triolefin and theOlefin 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]
Molecular catalysts have been explored for the preparation of a variety of potential applications,[8] such as the synthesis ofpharmaceutical drugs,[9] the manufacturing of high-strength materials, the preparation of cancer-targetingnanoparticles,[10] and the conversion of renewable plant-based feedstocks into hair and skin care products.[11]
Hérisson and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis.[12] The direct [2+2] cycloaddition of two alkenes is formallysymmetry forbidden and thus has a highactivation 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 thed-orbitals on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures.
Olefin metathesis mechanism
Olefin metathesis involves little change in enthalpy for unstrained alkenes. Product distributions are determined instead byle Chatelier's Principle, i.e.entropy.
Classification of Olefin metathesis reactions
Cross metathesis and ring-closing metathesis are driven by the entropically favored evolution ofethylene orpropylene, which can be removed from the system because they are gases. Because of this CM and RCM reactions often usealpha-olefins. The reverse reaction of CM of two alpha-olefins,ethenolysis, can be favored but requires high pressures of ethylene to increase ethylene concentration in solution. The reverse reaction of RCM, ring-opening metathesis, can likewise be favored by a large excess of an alpha-olefin, oftenstyrene. Ring-opening metathesis usually involves a strained alkene (often anorbornene) and the release of ring strain drives the reaction. Ring-closing metathesis, conversely, usually involves the formation of a five- or six-membered ring, which is enthalpically favorable; although these reactions tend to also evolve ethylene, as previously discussed. RCM has been used to close larger macrocycles, in which case the reaction may be kinetically controlled by running the reaction at high dilutions.[13] The same substrates that undergo RCM can undergo acyclic diene metathesis, with ADMET favored at high concentrations. TheThorpe–Ingold effect may also be exploited to improve both reaction rates and product selectivity.
Cross-metathesis is synthetically equivalent to (and has replaced) a procedure ofozonolysis of an alkene to two ketone fragments followed by the reaction of one of them with aWittig reagent.
"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 asZiegler–Natta catalysis Karl Ziegler discovered the conversion ofethylene into1-butene instead of a saturated long-chain hydrocarbon (seenickel effect).[14]
a reaction then classified as a so-calledcoordination polymerization. According to the then proposedreaction mechanism a RTiX titanium intermediate first coordinates to the double bond in api complex. The second step then is aconcertedSNi reaction breaking a CC bond and forming a new alkylidene-titanium bond; the process then repeats itself with a second monomer:
This particular mechanism is symmetry forbidden based on theWoodward–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.
In this reaction 2-pentene forms a rapid (a matter of seconds)chemical equilibrium with2-butene and3-hexene. No double bond migrations are observed; the reaction can be started with the butene and hexene as well and the reaction can be stopped by addition ofmethanol.
The Goodyear group demonstrated that the reaction of regular 2-butene with its all-deuteratedisotopologue yielded C4H4D4 with deuterium evenly distributed.[20] In this way they were able to differentiate between atransalkylidenation mechanism and atransalkylation mechanism (ruled out):
In 1971 Chauvin proposed a four-memberedmetallacycle 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.
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) withtungsten hexachloride and tetramethyltin (C1).
In the same year Pettit who synthesisedcyclobutadiene a few years earlier independently came up with a competing mechanism.[22] It consisted of a tetramethylene intermediate with sp3hybridized carbon atoms linked to a central metal atom with multiplethree-center two-electron bonds.
Experimental support offered by Pettit for this mechanism was based on an observed reaction inhibition bycarbon monoxide in certain metathesis reactions of 4-nonene with a tungstenmetal 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 withplatinum by reaction of the dilithiobutane withcis-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 ofcyclooctene, 2-butene and 4-octene with amolybdenum 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 asrate-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 theethylene 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)Tebbe reagent.[30] In a model reaction isotopically labeled carbon atoms inisobutene andmethylenecyclohexane switched places:
The Grubbs group then isolated the proposed metallacyclobutane intermediate in 1980 also with this reagent together with 3-methyl-1-butene:[31]
In that same year the Grubbs group proved that metathesis polymerization of norbornene by Tebbe's reagent is aliving polymerization system[33] and a year later Grubbs and Schrock co-published an article describing living polymerization with atungsten 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 morefunctional group tolerant.
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.[35][36][37] This promptedRobert H. Grubbs and coworkers to search for well-defined, functional group tolerant catalysts based on ruthenium. The Grubbs group successfully polymerized the 7-oxo norbornene derivative usingruthenium trichloride,osmium trichloride as well as tungsten alkylidenes.[38] They identified a Ru(II) carbene as an effective metal center and in 1992 published the first well-defined, ruthenium-based olefin metathesis catalyst, (PPh3)2Cl2Ru=CHCH=CPh2:[39]
The correspondingtricyclohexylphosphine complex (PCy3)2Cl2Ru=CHCH=CPh2 was also shown to be active.[40] This work culminated in the now commercially available 1st generationGrubbs catalyst.[41][42]
Schrock entered the olefin metathesis field in 1979 as an extension of work ontantalum alkylidenes.[43] The initial result was disappointing as reaction ofCpTa(=CH−t−Bu)Cl2 withethylene yielded only a metallacyclopentane, not metathesis products:[44]
But by tweaking this structure to aPR3Ta(CHt−bu)(Ot−bu)2Cl (replacingchloride byt-butoxide and acyclopentadienyl by anorganophosphine, metathesis was established with cis-2-pentene.[45] In another development, certain tungsten oxo complexes of the typeW(O)(CHt−Bu)(Cl)2(PEt)3 were also found to be effective.[46]
Schrock alkylidenes for olefin metathesis of the typeMo(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 aBINOL ligand in anorbornadieneROMP leading to highly stereoregular cis,isotactic polymer.
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