Grubbs Catalysts for Olefin Metathesis

How olefin metathesis works: the Chauvin metallacyclobutane cycle

Metathesis literally means “changing places.” Two alkenes each break their carbon–carbon double bond and reconnect the pieces, so CH₂=CHR + CH₂=CHR′ can become RHC=CHR′ plus ethylene. The mechanism, proposed by Yves Chauvin in 1971, runs through a metal carbene (alkylidene) rather than pairing free alkenes.

The catalytic cycle proceeds as follows:

• The resting Ru=CHR species loses a phosphine (or opens its chelate) to give a reactive 14-electron alkylidene.
• An incoming alkene coordinates and undergoes [2+2] cycloaddition with the Ru=C bond, forming a strained four-membered ruthenacyclobutane.
• The metallacyclobutane fragments in the productive direction — a retro-[2+2] — releasing a new alkene and regenerating a new Ru alkylidene bearing a different carbon fragment.
• That new alkylidene reacts with the next olefin, and the cycle repeats.

Because every step is reversible, metathesis is fundamentally an equilibrium. Selectivity comes from driving that equilibrium: venting volatile ethylene, using ring strain, or precipitating product all pull the reaction toward the target.

The three flavors: Grubbs I, Grubbs II, and Hoveyda-Grubbs

Grubbs I (1995): (PCy₃)₂Cl₂Ru=CHPh. Two bulky, electron-rich tricyclohexylphosphines and two chlorides flank a benzylidene. It tolerates alcohols, water, and air — a dramatic advance over the moisture-sensitive Schrock molybdenum alkylidenes — but struggles with hindered or electron-poor alkenes.

Grubbs II (1999): one PCy₃ is swapped for a strongly σ-donating N-heterocyclic carbene, typically 1,3-bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene (H₂IMes). The NHC makes the ruthenium more electron-rich and the resulting 14-electron intermediate more reactive, raising activity toward di- and trisubstituted and electron-poor olefins by roughly 100-fold while keeping functional-group tolerance.

Hoveyda-Grubbs II (2000): the phosphine is replaced entirely by a chelating ortho-isopropoxybenzylidene. This oxygen tether snaps back onto the ruthenium after each turnover, producing an exceptionally robust, recyclable catalyst prized for electron-poor substrates such as acrylates and for immobilization on solid supports.

Conditions and reagents

Grubbs metathesis is usually run in dichloromethane, toluene, or benzene under an inert atmosphere, though the catalysts survive brief air exposure. Reactions proceed at room temperature to reflux (40–110 °C); the more slowly-initiating Grubbs II and Hoveyda-Grubbs often need 40–60 °C to reach useful rates. Catalyst loadings run from about 0.05 mol% for easy ring-closing to 5–10 mol% for difficult cross or tetrasubstituted cases.

• Dilution matters: ring-closing metathesis is run dilute (often 0.01–0.02 M) to favor intramolecular reaction over oligomerization.
• Ethylene removal: for terminal-olefin RCM, sparging or reduced pressure removes the ethylene byproduct and drags the equilibrium toward the ring.
• Quenching: residual ruthenium is scavenged with ethyl vinyl ether (which caps the alkylidene as an unreactive Fischer carbene), tris(hydroxymethyl)phosphine, activated carbon, or silica-bound thiols — important for pharma, where Ru limits are strict.

Additives such as CuI (to sequester released phosphine) or benzoquinone (to suppress alkene isomerization side reactions) are common troubleshooting tools.

Scope, selectivity, and limitations

Grubbs catalysts drive four main reaction classes: ring-closing metathesis (RCM) to build 5- to large rings, cross metathesis (CM) to stitch two different alkenes, ring-opening metathesis polymerization (ROMP) of strained cyclics like norbornene and cyclooctene, and acyclic diene metathesis (ADMET) polymerization.

Their signature strength is functional-group tolerance: the oxophilic Schrock catalysts choke on alcohols, aldehydes, and acids, whereas ruthenium ignores them and attacks the alkene. That chemoselectivity is why metathesis works on richly functionalized late-stage intermediates.

• Substrate ranking: terminal, unhindered olefins react fastest; 1,1-disubstituted and electron-poor alkenes need Grubbs II; tetrasubstituted alkenes remain very difficult.
• Failure modes: basic amines can bind and poison the metal (protect as ammonium salts or amides); nitriles and some sulfur groups slow catalysis; allylic strain and conformational bias can stall ring closures.
• Side reactions: Ru–H species formed by decomposition cause alkene isomerization, walking the double bond and giving ring-contracted or homologated products.

Stereochemistry: the persistent E/Z challenge

Classic Grubbs catalysts typically deliver the thermodynamically favored (E)-alkene in cross metathesis, because the metallacyclobutane can equilibrate before productive cleavage. Achieving high (Z)-selectivity was a long-standing gap, since many natural products and pheromones require the cis geometry.

A major advance came in 2011 when Grubbs and coworkers introduced Z-selective ruthenium catalysts bearing a chelating adamantyl-substituted NHC in which the carbene carbon is bonded directly to ruthenium. This constrained geometry forces the substituents onto the same face of the metallacyclobutane, favoring the cis alkene with selectivities exceeding 90%. Schrock's molybdenum and tungsten monoaryloxide pyrrolide (MAP) systems offer a complementary route to Z-alkenes. For ring-closing, the geometry of small and medium rings is largely dictated by the ring size rather than by the catalyst.

Applications and history

Olefin metathesis grew out of 1950s–60s industrial observations (the Phillips triolefin process, Goodyear polymer work) that alkenes disproportionate over ill-defined metal oxides. Chauvin's 1971 metallacarbene mechanism explained the puzzle; Richard Schrock built the first well-defined, highly active molybdenum alkylidenes in the 1980s–90s; and Robert Grubbs delivered the robust, functional-group-tolerant ruthenium catalysts that put metathesis into every synthetic lab. The three shared the 2005 Nobel Prize in Chemistry.

Impact spans the field:

• Total synthesis: RCM builds the macrocyclic rings of epothilones, and metathesis is a workhorse for complex molecules once thought too functional for organometallic catalysis.
• Pharmaceuticals: the hepatitis C protease inhibitors simeprevir and grazoprevir use ring-closing metathesis on manufacturing scale to form their macrocycles.
• Materials and polymers: ROMP of dicyclopentadiene gives tough thermosets; Grubbs-type ROMP underpins self-healing composites and precision block copolymers.
• Renewables and industry: cross metathesis of plant-oil olefins produces specialty chemicals and detergent feedstocks.

First- vs second-generation Grubbs and the Hoveyda-Grubbs variant

Feature	Grubbs I	Grubbs II	Hoveyda-Grubbs II
Ancillary ligand	Two PCy3	One PCy3 + NHC (H2IMes)	NHC + chelating isopropoxy-styrene
Activity / scope	Terminal, unhindered olefins	Hindered, electron-poor, di/trisubstituted	Recyclable, electron-poor olefins
Functional-group tolerance	Good	Excellent	Excellent
Thermal stability	Moderate	High	Very high (recovers by chelation)
Initiation rate	Fast	Slower initiation, fast propagation	Slow initiation, robust