Olefin metathesis or
transalkylidenation is an
organic reactionOrganic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...
that entails redistribution of alkylene fragments by the scission of carbon - carbon
double bondsA chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
in olefins (alkenes). Its advantages include the creation of fewer sideproducts and
hazardous wasteA hazardous waste is waste that poses substantial or potential threats to public health or the environment. According to the U.S. environmental laws hazardous wastes fall into two major categories: characteristic wastes and listed wastes.Characteristic hazardous wastes are materials that are known...
s.
Yves ChauvinYves Chauvin is a French chemist and Nobel Prize laureate. He is honorary research director at the Institut français du pétrole and a member of the French Academy of Science. Chauvin received his degree from the Lyon School of Chemistry, Physics and Electronics in 1954.He was awarded the 2005...
,
Robert H. GrubbsRobert Howard Grubbs is an American chemist and Nobel laureate.As he noted in his official Nobel Prize autobiography, "In some places, my birthplace is listed as Calvert City and in others Possum Trot [NB: both in Marshall County]...
, and
Richard R. SchrockRichard Royce Schrock is an American chemist and Nobel laureate recognized for his contributions to the metathesis reaction used in organic chemistry.-Biography:...
shared the 2005
Nobel Prize in ChemistryThe Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...
for "the development of the metathesis method in
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
".
The reaction is
catalyzedCatalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by metals such as
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
,
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
,
rheniumRhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-white, heavy, third-row transition metal in group 7 of the periodic table. With an average concentration of 1 part per billion , rhenium is one of the rarest elements in the Earth's crust. The free element has...
,
rutheniumRuthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
and
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
. The reaction consists of an
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
double bondA double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
cleavageBond cleavage, or scission, is the splitting of chemical bonds.If the two electrons in a cleaved covalent bond are divided between the products, the process is known as homolytic fission and free redicals are generated by homolytic cleavage the process is known as homolytic fission or homolysis...
, followed by a statistical redistribution of alkylidene fragments. The general scope is outlined by the following scheme:
Applications
Olefin metathesis was first commercialized in
petroleumPetroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
reformation for the synthesis of higher olefins from the products (α-olefins) from the
Shell higher olefin processThe Shell higher olefin process is a chemical process for the production of linear alpha olefins via ethylene oligomerization and olefin metathesis invented and exploited by Royal Dutch Shell. The olefin products are converted to fatty aldehydes and then to fatty alcohols, which are precursors...
(SHOP) under high pressure and high temperatures. Traditional catalysts are derived from a reaction of the metal halides with alkylation agents for example WCl
6-EtOH-EtAlCl
2. A metathesis reaction is a chain reaction that begins when a metallocarbene and an olefin react to form a
metallacyclobutane. This intermediate then reacts further, decomposing into a new olefin (the product) and a new metallocarbene, which can then be recycled through the reaction pathway.
The
Grubbs' catalystGrubbs' Catalyst is a transition metal carbene complex named after Robert H. Grubbs, the chemist who first synthesized it. 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...
is a ruthenium carbenoid, whereas molybdenum(VI) and tungsten(VI)-based catalysts are known as
Schrock alkylideneA transition metal carbene complex is a organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and...
s. Related complexes can also catalyze
alkyne metathesisAlkyne metathesis is an organic reaction involving the redistribution of alkyne chemical bonds. This reaction is closely related to olefin metathesis. Metal-catalyzed alkyne metathesis was first described in 1968 by Bailey, et al. The Bailey system utilized a mixture of tungsten and silicon oxides...
and related polymerizations.
Reaction mechanism
Hérisson and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis. The direct [2+2] cycloaddition of two alkenes is formally
symmetry forbiddenThe Woodward–Hoffmann rules devised by Robert Burns Woodward and Roald Hoffmann are a set of rules in organic chemistry predicting the stereochemistry of pericyclic reactions based on orbital symmetry. These include electrocyclic reactions, cycloadditions , sigmatropic reactions, and group transfer...
and thus has a high
activation energyIn chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
. The Chauvin mechanism involves the [2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallocyclobutane intermediate. The metallocyclobutane produced can then cyclorevert to give either the original species or a new alkene and alkylidene. Interaction with the
d-orbitalsIn atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures.
Metathesis chemistry
Some important classes of
metathesisSalt metathesis is a molecular process involving the exchange of bonds between the two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations. This is represented by the general reaction scheme:These chemical species can either be ionic...
chemistry:
- Alkane metathesis
Alkane metathesis is a chemical reaction in which alkanes are rearranged to give longer or shorter alkane products. It is similar to olefin metathesis, except that olefin metathesis cleaves and recreates a carbon-carbon double bond, but alkane methathesis operates on a carbon-carbon single...
- Alkene metathesis
- Alkyne metathesis
Alkyne metathesis is an organic reaction involving the redistribution of alkyne chemical bonds. This reaction is closely related to olefin metathesis. Metal-catalyzed alkyne metathesis was first described in 1968 by Bailey, et al. The Bailey system utilized a mixture of tungsten and silicon oxides...
(AM)
- Cross-metathesis (CM)
- Ring opening metathesis (ROM)
- Ring-closing metathesis
Ring-closing metathesis or RCM is a variation on olefin metathesis that allows the closing of previously hard to make rings...
(RCM)
- Enyne metathesis
An Enyne metathesis is an organic reaction taking place between an alkyne and an alkene with a metal carbene catalyst forming a butadiene. This reaction is a variation of olefin metathesis.The general scheme is given by scheme 1:...
(EM)
- Ring opening metathesis polymerisation
Ring-opening metathesis polymerization is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins and a wide variety of catalysts have been discovered...
(ROMP)
- Acyclic diene metathesis
Acyclic diene metathesis or ADMET is a special type of olefin metathesis used to polymerize certain terminal dienes to polyenes:The new double bonds formed can be in cis- or trans-configuation....
(ADMET)
Like most chemical reactions, the metathesis pathway is usually driven by a thermodynamic imperative; that is, the final products are determined by the energetics of the possible products, with a distribution of products proportional to the exponential of their respective energy values.
Alkene metathesis is often generally driven by the evolution of
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, and alkyne metathesis is driven by the evolution of
acetyleneAcetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
, which are both gases; hence, metathesis reactions, if they are not ring-opening or ring-closing, usually use monosubstituted and 2,2-disubstituted alkenes and terminal alkynes. These are both dominated by the entropy gained by the net release of gas.
EnyneAn enyne is a functional group in organic chemistry consisting of a conjugated alkyne and alkene group....
metathesis cannot evolve a simple gas, and for that reason is usually disfavored unless there are accompanying ring-opening or ring-closing advantages. Ring opening metathesis usually involves a strained alkene (often a
norborneneNorbornene or norbornylene or norcamphene is a bridged cyclic hydrocarbon. It is a white solid with a pungent sour odor. The molecule consists of a cyclohexene ring bridged with a methylene group in the para position...
) and the release of ring strain drives the reaction.
Ring-closing metathesisRing-closing metathesis or RCM is a variation on olefin metathesis that allows the closing of previously hard to make rings...
, conversely, usually involves the formation of a five- or six-membered ring which is highly energetically favorable; although these reactions tend to also evolve
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
. RCM has been used to close larger macrocycles, in which case the reaction may be kinetically controlled by running the reaction at extreme dilutions. The
Thorpe-Ingold effectThe Thorpe–Ingold effect or gem-dimethyl effect, or angle compression is an effect observed in organic chemistry where increasing the size of two substituents on a tetrahedral center leads to enhanced reactions between parts of the other two substituents...
may be exploited to improve both reaction rates and selectivity.
Alkene metathesis is synthetically equivalent to (and has replaced) a procedure of
ozonolysisOzonolysis is the cleavage of an alkene or alkyne with ozone to form organic compounds in which the multiple carbon–carbon bond has been replaced by a double bond to oxygen...
of an alkene to two ketone fragments followed by the reaction of one of them with a Wittig reagent.
Scope
One study reported a ring-opening cross-olefin metathesis based on a Hoveyda-Grubbs Catalyst:
The metathesis reaction of
1-hexene1-Hexene is an organic compound with the formula CH2CHC4H9. It is an alkene that is classified in industry as higher olefin and an alpha-olefin, the latter term meaning that the double bond is located at the alpha position, endowing the compound with higher reactivity and thus useful chemical...
with the WCl
4(OAr)
2 catalyst yields 5-decene plus many byproducts from secondary metathesis reactions.
Historical overview
Known chemistry prior to the advent of olefin metathesis was introduced by
Karl ZieglerKarl Waldemar Ziegler was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compounds [which]...led to new polymerization reactions and ... paved the way for new and highly...
in the 1950s who as part of ongoing work in what would later become known as Ziegler-Natta catalysis studied
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
polymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
which on addition of certain metals resulted in
1-butene1-Butene is an organic chemical compound, linear alpha-olefin , and one of the isomers of butene. The formula is .-Stability:1-Butene is stable in itself but polymerizes exothermically. It is highly flammable and readily forms explosive mixtures with air...
instead of a saturated long-chain hydrocarbon (see nickel effect) .
In 1960 a Du Pont research group polymerized
norborneneNorbornene or norbornylene or norcamphene is a bridged cyclic hydrocarbon. It is a white solid with a pungent sour odor. The molecule consists of a cyclohexene ring bridged with a methylene group in the para position...
to polynorbornene using
lithium aluminum tetraheptyl and
titanium tetrachlorideTitanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...
(a patent by this company on this topic dates back to 1955 ),
a reaction then classified as a so-called
coordination polymerizationCoordination polymerization is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center...
. According to the then proposed
reaction mechanismIn chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
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 polymerisationRing-opening metathesis polymerization is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins and a wide variety of catalysts have been discovered...
.
Giulio NattaGiulio Natta was an Italian chemist and Nobel laureate. He won a Nobel Prize in Chemistry in 1963 with Karl Ziegler for work on high polymers.-Early years:...
in 1964 also observed the formation of an unsaturated polymer when polymerizing
cyclopenteneCyclopentene is a chemical compound with the formula 58. It is a colorless liquid with a petrol-like odor. It is one of the cycloalkenes.Cyclopentene is produced industrially in large amounts...
with tungsten and molybdenum halides .
In a third development leading up to olefin metathesis researchers at Phillips Petroleum Company in 1964 described olefin
disproportionationDisproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
with catalysts
molybdenum hexacarbonylMolybdenum hexacarbonyl is the chemical compound with the formula Mo6. This colorless solid, like its chromium and tungsten analogues, is noteworthy as a volatile, air-stable derivative of a metal in its zero oxidation state.-Structure and properties:Mo6 adopts an octahedral geometry consisting...
,
tungsten hexacarbonylTungsten hexacarbonyl is the chemical compound with the formula W6. This complex gave rise to the first example of a dihydrogen complex....
, and molybdenum oxide supported on alumina for example converting
propylenePropene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...
to an equal mixture of
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
and
2-butene2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-butene , shown at the right, and trans-2-butene , not shown.It is a petrochemical, produced by the catalytic cracking of crude oil...
for which they proposed a
reaction mechanismIn chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
involving a
cyclobutaneCyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
(they called it a quasicyclobutane) - metal complex:
This particular mechanism is symmetry forbidden based on the
Woodward-Hoffmann rulesThe Woodward–Hoffmann rules devised by Robert Burns Woodward and Roald Hoffmann are a set of rules in organic chemistry predicting the stereochemistry of pericyclic reactions based on orbital symmetry. These include electrocyclic reactions, cycloadditions , sigmatropic reactions, and group transfer...
first formulated two years earlier. Cyclobutanes have also never been identified in metathesis reactions another reason why it was quickly abandoned.
Then in 1967 researchers at the
Goodyear Tire and Rubber CompanyThe Goodyear Tire & Rubber Company was founded in 1898 by Frank Seiberling. Goodyear manufactures tires for automobiles, commercial trucks, light trucks, SUVs, race cars, airplanes, farm equipment and heavy earth-mover machinery....
described a novel catalyst system for the metathesis of 2-pentene based on
tungsten hexachlorideTungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...
,
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
the organoaluminum compound EtAlMe
2 and also 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 equilibriumIn a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
with
2-butene2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-butene , shown at the right, and trans-2-butene , not shown.It is a petrochemical, produced by the catalytic cracking of crude oil...
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
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
.
The Goodyear group demonstrated that the reaction of regular 2-butene with its all-deuterated
isotopologueIsotopologues are molecules that differ only in their isotopic composition. Simply, the isotopologue of a chemical species has at least one atom with a different number of neutrons than the parent....
yielded C
4H
4D
4 with deuterium evenly distributed . In this way they were able to differentiate between a
transalkylidenation mechanism and a
transalkylationTransalkylation is a chemical reaction of transfer of an alkyl group from one organic compound to another. Zeolite catalysts are often used. The reaction is widely used in the petrochemical industry to manufacture p-xylene, styrene, and other aromatic compounds....
mechanism (ruled out):
In 1971 Chauvin proposed a 4-membered
metallocycleIn organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center. Metallacycles appear frequently as reactive intermediates in catalysis, e.g. olefin metathesis and alkyne trimerization. In organic synthesis, directed...
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.
The active catalyst, a metallocarbene., was discovered by in 1964 by E. O. Fischer. Chauvins experimental evidence was based on the reaction of
cyclopenteneCyclopentene is a chemical compound with the formula 58. It is a colorless liquid with a petrol-like odor. It is one of the cycloalkenes.Cyclopentene is produced industrially in large amounts...
and 2-pentene with the homogeneous catalyst
tungsten(VI) oxytetrachlorideTungsten oxytetrachloride,also known as Tungsten tetrachloride oxide, is an inorganic compound with the formula WOCl4. This diamagnetic solid is used to prepare other complexes of tungsten. The orange-coloured compound is soluble in nonpolar solvents but it reacts with alcohols and water and...
and
tetrabutyltinTetrabutyltin is a stable organotin compound and combustible, colourless liquid at room temperature. It is incompatible with strong oxidizing agents. It has the molecular formula C16H36Sn. Sometimes the abbreviation SnBu4 and TTBT are used.Tetrabutyltin is the starting material of the tributyltin...
:
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
propylenePropene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...
(C3) forms in a reaction of 2-butene (C4) with
tungsten hexachlorideTungsten hexachloride is the chemical compound of tungsten and chlorine with the formula WCl6. This dark violet blue species exists as a volatile solid under standard conditions. It is an important starting reagent in the preparation of tungsten compounds. WCl6 is a rare example of a...
and tetramethyltin (C1).
In the same year Pettit who synthesised
cyclobutadieneCyclobutadiene is the smallest [n]-annulene , an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula 44 and a rectangular structure verified by infrared studies. This is in contrast to the square geometry predicted by simple Hückel...
a few years earlier independently came up with a competing mechanism . It consisted of a tetramethylene intermediate with sp
3 hybridized carbon atoms linked to a central metal atom with multiple
three-center two-electron bondA three-center two-electron bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a...
s.
Experimental support offered by Pettit for this mechanism was based on an observed reaction inhibition by
carbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
in certain metathesis reactions of 4-nonene with a tungsten
metal carbonylMetal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. These complexes may be homoleptic, that is containing only CO ligands, such as nickel carbonyl , but more commonly metal carbonyls contain a mix of ligands, such as Re3Cl...
Robert H. GrubbsRobert Howard Grubbs is an American chemist and Nobel laureate.As he noted in his official Nobel Prize autobiography, "In some places, my birthplace is listed as Calvert City and in others Possum Trot [NB: both in Marshall County]...
got involved in metathesis in 1972 and also proposed a metallacycle intermediate but one with 4 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
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
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
cycloocteneCyclooctene is a cycloalkene with an eight-membered ring. It is notable because it is the smallest cycloalkene that can exist as either the cis- or trans-isomer with the cis-isomer more common...
, 2-butene and 4-octene with a
molybdenumMolybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
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 stepThe rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
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
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
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
methylenecyclohexaneMethylenecyclohexane is a very useful compound in organic syntheses. It can be produced by a Wittig reaction or a Tebbe reaction from cyclohexanone, or as a side product of the dehydration of 2-methylcyclohexanol into 1-methylcyclohexene....
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 synthesisIn organic chemistry, a total synthesis is, in principle, the complete chemical synthesis of complex organic molecules from simpler pieces, usually without the aid of biological processes. In practice, these simpler pieces are commercially available in bulk and semi-bulk quantities, and are often...
of
capnelleneCapnellene is a naturally occurring hydrocarbon derived from Capnella imbricata, a species of soft coral found in Indonesia. Since the 1970s, capnellene has been targeted for synthesis by numerous investigators due to its stereochemistry, functionality, and the interesting geometry of the carbon...
in 1986:
In that same year the Grubbs group proved that metathesis polymerization of norbornene by Tebbe's reagent is a
living polymerizationIn polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
system and a year later Grubbs and Schrock co-published an article describing living polymerization with a
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
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 groupIn organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
tolerant.
Grubbs catalysts
In earlier work, Michelotti and Keaveney demonstrated that the ring-opening polymerization of norbornene was catalyzed by hydrated trichlorides of ruthenium, osmium, and iridium in alcoholic solvents, Francis W. Michelotti and William P. Keaveney, J. Polymer Science: Part A, Vol. 3, PP 895-905 (1965), Coordinated Polymerization of the Bicyclo-(2.2.1)-heptene-2 Ring System (Norbornene) in Polar Media. In 1965, Rinehart and Smith demonstrated that norbornene can be polymerized in emulsion using an iridium or ruthenium halide catalyst, Robert E. Rinehart and Homer P. Smith, Polymer Letters Vol. 3, PP.1049-1052 (1965). In 1974, Lido Porri et al. reported on the Ring-Opening Polymerization of Cycloolefins with Catalysts Derived from Ruthenium and Iridium, Die Makromolekulare Chemie, 175, 3097-3115 (1974). They conclude that the reaction of norbornene and aliphatic olefins proceed by a metathesis mechanism. This theme was picked up years later by the Grubbs group, who 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 such as (PPh
3)
2Cl
2Ru=CHCH=CPh
2:
The corresponding
tricyclohexylphosphineTricyclohexylphosphine is the tertiary phosphine with the formula P3. Commonly used as a ligand in organometallic chemistry, it is often abbreviated to PCy3, where Cy stands for cyclohexyl...
complex (PCy
3)
2Cl
2Ru=CHCH=CPh
2 This work culminated in the now commercially available Grubbs catalyst.
Schrock catalysts
Schrock entered the olefin metathesis field in 1979 as an extension of work on
tantalumTantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
alkylidenes.. The initial result was disappointing as reaction of CpTa(=CH-t-Bu)Cl
2 with
ethyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
yielded only a metallacyclopentane, not metathesis products:
But by tweaking this structure to a PR
3Ta(CHt-bu)(Ot-bu)
2Cl (replacing
chlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
by t-butoxide and a
cyclopentadienylIn organic chemistry, cyclopentadienyl is a cyclic group of atoms with the formula C5H5. Cyclopentadienyl are closely related to cyclopentadiene. Cyclopentadienyl have five carbon atoms bonded together in a pentagonal planar ring, all five of which are bonded to individual hydrogen atoms...
by an organophosphine, metathesis was established with cis-2-pentene. In another development, certain tungsten oxo complexes of the type W(O)(CHt-Bu)(Cl)
2(PEt)
3 were also found to be effective.
Schrock alkylidenes for olefin metathesis of the type Mo(NAr)(CHC(CH
3)
2R){OC(CH
3)(CF
3)
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
norbornadieneNorbornadiene is an organic compound. This bicyclic hydrocarbon is the most stable diolefin derived from the norbornane and norbornene. Norbornadiene is primarily of interest as a ligand in homogeneous catalysis, but it has been heavily studied due to its high reactivity and distinctive...
ROMPRing-opening metathesis polymerization is a type of olefin metathesis chain-growth polymerization that produces industrially important products. The driving force of the reaction is relief of ring strain in cyclic olefins and a wide variety of catalysts have been discovered...
leading to highly stereoregular cis, isotactic polymer.
Further reading