"Olefin metathesis is a child of industry and, as with many catalytic processes, it was discovered by accident." As part of ongoing work in what would later become known as
Ziegler–Natta catalysis Karl Ziegler discovered the conversion of
ethylene into
1-butene instead of a saturated long-chain hydrocarbon (see
nickel effect). In 1960 a
Du Pont research group polymerized
norbornene to
polynorbornene using lithium aluminum tetraheptyl and
titanium tetrachloride (a patent by this company on this topic dates back to 1955), : a reaction then classified as a so-called
coordination polymerization. According to the then proposed
reaction mechanism a RTiX titanium intermediate first coordinates to the double bond in a
pi complex. The second step then is a
concerted SNi reaction breaking a CC bond and forming a new alkylidene-titanium bond; the process then repeats itself with a second monomer: : Only much later the polynorbornene was going to be produced through
ring opening metathesis polymerisation. The DuPont work was led by
Herbert S. Eleuterio.
Giulio Natta in 1964 also observed the formation of an unsaturated polymer when polymerizing
cyclopentene with tungsten and molybdenum halides. In a third development leading up to olefin metathesis, researchers at
Phillips Petroleum Company in 1964 described olefin
disproportionation with
catalysts
molybdenum hexacarbonyl,
tungsten hexacarbonyl, and
molybdenum oxide supported on alumina for example converting
propylene to an equal mixture of
ethylene and
2-butene for which they proposed a
reaction mechanism involving a
cyclobutane (they called it a quasicyclobutane) – metal complex: : 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
Nissim Calderon at the
Goodyear Tire and Rubber Company described a novel catalyst system for the metathesis of
2-pentene based on
tungsten hexachloride,
ethanol, and the
organoaluminum compound EtAlMe2. The researchers proposed a name for this reaction type: olefin metathesis. Formerly the reaction had been called "olefin disproportionation." : In this reaction 2-pentene forms a rapid (a matter of seconds)
chemical equilibrium with
2-butene and
3-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 of
methanol. The Goodyear group demonstrated that the reaction of regular 2-butene with its all-
deuterated isotopologue yielded C4H4D4 with deuterium evenly distributed. In this way they were able to differentiate between a
transalkylidenation mechanism and a
transalkylation mechanism (ruled out): : In 1971 Chauvin proposed a four-membered
metallacycle intermediate to explain the statistical distribution of products found in certain metathesis reactions. This mechanism is today considered the actual mechanism taking place in olefin metathesis. : Chauvin's experimental evidence was based on the reaction of
cyclopentene and
2-pentene with the
homogeneous catalyst tungsten(VI) oxytetrachloride and
tetrabutyltin: : 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
cyclobutadiene a few years earlier independently came up with a competing mechanism. It consisted of a tetramethylene intermediate with sp3
hybridized carbon atoms linked to a central metal atom with multiple
three-center two-electron bonds. : 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 Robert H. Grubbs got involved in metathesis in 1972 and also proposed a metallacycle intermediate but one with four carbon atoms in the ring. 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) In 1975 Katz also arrived at a metallacyclobutane intermediate consistent with the one proposed by Chauvin 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: : 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: : In this reaction the
ethylene product distribution (d_4,d_2,d_0) 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. In a model reaction isotopically labeled carbon atoms in
isobutene and
methylenecyclohexane switched places: : The Grubbs group then isolated the proposed metallacyclobutane intermediate in 1980 also with this reagent together with 3-methyl-1-butene: : They isolated a similar compound in the
total synthesis of
capnellene in 1986: : In that same year the Grubbs group proved that metathesis polymerization of norbornene by Tebbe's reagent is a
living polymerization system and a year later Grubbs and Schrock co-published an article describing living polymerization with a
tungsten carbene complex 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. This prompted
Robert 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 using
ruthenium trichloride,
osmium trichloride as well as tungsten alkylidenes. 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: : The corresponding
tricyclohexylphosphine complex (PCy3)2Cl2Ru=CHCH=CPh2 was also shown to be active. This work culminated in the now commercially available 1st generation
Grubbs catalyst.
Schrock catalysts Schrock entered the olefin metathesis field in 1979 as an extension of work on
tantalum alkylidenes. The initial result was disappointing as reaction of with
ethylene yielded only a metallacyclopentane, not metathesis products: : But by tweaking this structure to a (replacing
chloride by
t-butoxide and a
cyclopentadienyl by an
organophosphine, metathesis was established with cis-2-pentene. In another development, certain tungsten oxo complexes of the type were also found to be effective.
Schrock alkylidenes for olefin metathesis of the type {{chem2|Mo(NAr)(CHC(CH3)2R){OC(CH3)(CF3)2}2}} were commercialized starting in 1990. : The first asymmetric catalyst followed in 1993 : With a Schrock catalyst modified with a
BINOL ligand in a
norbornadiene ROMP leading to highly stereoregular cis,
isotactic polymer. ==See also==