Metathesis reactions are among the most important processes in organic synthesis. The decisive breakthrough in making these reactions practical for industrial purposes, which range from the synthesis of polymers to pharmaceuticals, came with the discovery of the reaction mechanism by Yves Chauvin and the targeted development of transition-metal-based metathesis catalysts by Richard Schrock and Robert Grubbs. The winners of the Chemistry Nobel Prize in 2005 present first-hand accounts of these developments.

Substantially better initiators and therefore considerably more reactive than complexes of type A, the novel metathesis catalysts B are easily prepared by the reaction of [RuCl₂(PPh₃)₃] with the appropriate diazoalkanes and subsequent phosphane exchange. While the PPh₃ derivatives are excellent catalysts for living ROMP of norbornene, the complex with R = Cy, R′ = Ph is also a very efficient catalyst for the metathesis of acyclic alkenes.

Unsaturated halocarbene complexes can be synthesized in the first example of olefin metathesis with a directly fluorinated substrate. The ruthenium catalyst 1 reacts with 1,1-difluoroethylene to yield the corresponding methylidene [Ru]=CH₂ (2) and difluorocarbene [Ru]=CF₂ complexes (3). H₂IMes=1,3-dimesityl-4,5-dihydroimidazol-2-ylidene; Cy=cyclohexyl.

Reactive reagents can be prepared by means of olefin cross-metathesis. A wide variety of functionalized allyl boronates were synthesized and were found to react cleanly with aldehydes to afford homoallylic alcohols, without prior purification (see scheme). Olefins that bear allylic ethers, halides, protected aldehydes, and sterically encumbering groups are viable substrates for this reaction.

Stiff PEGs: Aqueous solutions of pyrene end-capped poly(ethylene glycol)s (PEGs) increase their viscosity in the presence of octafluoronaphthalene possibly as a result of aggregation by face-to-face stacking (shown schematically). The exploitation of the arene–perfluoroarene supramolecular synthon in solution is reported.

Stereoselective and chemoselective olefin cross metathesis can be viewed as a highly selective and efficient set of reactions that provide the same products as would selective CH activation and allylic oxidation (see scheme for an example). More active catalyst systems will provide an efficient process to functionalized products from readily available olefins. Cy=cyclohexyl.

A variety of highly A,B-alternating copolymers are formed from diacrylates and cycloalkenes in the presence of [Cl₂(PCy₃)(H₂IMes)RuCHPh] (1). The high conversion and the high selectivity come from the thermodynamically favored formation of a CC double bond between these monomers.

The highest initiation rate of any reported ruthenium-based catalyst was found for the new olefin-metathesis catalyst [(H₂IMes)(3-Br-py)₂(Cl)₂RuCHPh] (1), which was synthesized in one step from commercially available reagents. Complex 1 is highly efficient for the cross metathesis of acrylonitrile, which is generally a poor substrate for metathesis reactions (e.g., see scheme). Mes=2,4,6-trimethylphenyl.

Highly active with ultrafast initiation, the catalyst 1 promotes the controlled, living, ring-opening polymerization of various norbornene and 7-oxonorbornene derivatives (see scheme). The high k_i/k_p ratio associated with the polymerization reactions results in better molecular-weight control and narrower polydispersity indexes (PDIs, k_i=rate constant for initiation, k_p=rate constant for polymerization).

Trick or treat? Ruthenium alkylidene catalyzed ring-closing metathesis of crown ether like diene substrates around a dumbbell-shaped secondary ammonium ion affords [2]rotaxanes. The reversible nature of this process has been demonstrated through a “magic ring” synthesis, wherein the preformed olefinic macrocycle and dumbbell-shaped component equilibrate to form the hydrogen-bond-stabilized [2]rotaxane in the presence of a metathesis catalyst (see scheme).

Less is more: It is much less efficient to synthesize both components of a multivalent recognition site separately than it is to use one multivalent component to act as a template for the catalytically orchestrated construction of the other component, as demonstrated by the formation of the mechanically interlocked, triply threaded molecular bundle shown. The situation is reminiscent of nature.

Metathesis takes sides: The scope of asymmetric metathesis has been expanded with the use of chiral ruthenium catalysts for asymmetric ring-opening cross-metathesis and for the first example of an asymmetric cross-metathesis (see scheme, TIPS=triisopropylsilyl). Information about the mechanism of asymmetric ring-opening cross-metathesis should allow the development of more selective catalysts.

A small price to pay: The second block of a diblock copolymer is “sacrificed” in order to leave behind a monofunctionalized metathesis polymer with a hydroxy end group. By incorporation of a dioxepine unit into the copolymer, a breaking point is created between the block to be end-functionalized and the block to be sacrificed.

ROMPing around in water: Two well-defined, small-molecule olefin-metathesis catalysts (1 and 2) are introduced. While they are insufficiently stable to mediate most cross-metathesis reactions in water, these catalysts competently mediate ring-opening metathesis polymerization (ROMP) and ring-closing metathesis reactions in an aqueous environment.

Having a breakdown: Decomposition of the olefin metathesis catalyst [(biph)(PCy₃)Cl₂RuC(H)Ph] (biph= N,N′-diphenylbenzimidazol-2-ylidene, Cy=cyclohexyl) results in benzylidene insertion into an ortho CH bond of an N-phenyl group of the biph ligand. The ruthenium center further inserts into another ortho CH bond of the other N-phenyl ring to give a new RuC bond as a part of a five-membered metallacycle (see scheme).