There are a wide variety of variants on this reaction as is discussed below. This metallacycle then breaks up in the opposite fashion to afford a new alkylidene and new olefin. If this process is repeated enough, eventually an equilibrium mixture of olefins will be obtained. Such cycloaddition reactions between two alkenes to give cyclobutanes is symmetry forbidden and occurs only photochemically. However, the presence of d-orbitals on the metal alkylidene fragment breaks this symmetry and the reaction is quite facile.
News Catalysts There have been roughly four distinct generations of olefin metathesis catalysts: "Black Box" heterogeneous catalysts consisting of a high valent transition metal halide, oxide or oxo-halide with an alkylating co-catalyst such as an alkyl zinc or alkyl aluminum. Some of these catalyst systems are placed on an alumina or silica support. While these catalysts are exceedingly active, they have an exceedingly low tolerance for functional groups because of their Lewis acidic nature.
Likewise, less than one percent of the material is an active catalyst, and nothing is known about the nature of the actual catalytic species in these systems. One commercial application still using these catalysts is the ROMP of dicyclopentadiene to produce tough plastics for use in golf carts, snow mobile hoods etc. Titanocene-based catalysts. These Ti-based catalysts are not nearly as active or tolerant of carbonyl functionalities as the later catalysts, but Grubbs has shown that these Ti complexes undergo stoichiometric Wittig-like reactions with ketones, aldehydes and other carbonyls to form the corresponding methylene derivatives.
The mechanism of this reaction is identical to that of the olefin metathesis reaction except that the final step is not reversible. The reactivity of these catalysts can be tuned very easily by changing the nature of the alkoxide ligands.
These catalysts have a high tolerance for functionality, although they are air and water-sensitive. The success of these catalysts stems from their coordinative and electronic unsaturation making them electrophilic and their bulky ligands prevents bimolecular decomposition.
RCM is the focus of this article Eq. In general, molybdenum catalysts display high activity but are unstable toward air or water; ruthenium catalysts are less active but exhibit good selectivity and functional-group compatibility.
RCM has been employed extensively in organic synthesis to establish both saturated and unsaturated rings; the reaction can be used to form carbocycles or heterocycles. In a cycloreversion step, a small olefin is expelled and new metal carbene intermediate 8 forms, which still contains a tethered alkene.
Intramolecular cycloaddition yields new metallacyclobutane 9, which undergoes cycloreversion to expel the metal carbene catalyst and generate the product cyclic alkene. In RCM reactions, reactants are typically designed so that the desired cyclic alkene is accompanied by a small gaseous olefin such as ethylene or propene, the loss of which drives the reaction forward.
Highly dilute conditions discourage intermolecular metathesis and thereby also promote RCM. Second-generation Grubbs catalysts 3 - 6 employ trans N-heterocyclic carbene ligands to accelerate the phosphine dissociation step. Schrock-type complexes with a stereogenic center at molybdenum are more often used as catalysts than ruthenium complexes with chiral ligands.
For example, chiral molybdenium complex 10 catalyzes the desymmetrization of vinyl ethers to form dihydropyrans with moderate to good enantioselectivity Eq. For example, Schrock-type complex 11 catalyzes the cyclization of an allylborane, which undergoes oxidation to yield a chiral diol with very high stereoselectivity and moderate yield Eq.
Molybdenum catalyst 1 was developed before the Grubbs-type catalysts and is highly active, but sensitivity of this catalyst to air and water limits its applicability. Second-generation Grubbs catalysts 4 - 6 include a strongly donating N-heterocyclic carbene ligand trans to the phosphine ligand, accelerating phosphine dissociation and increasing their activity relative to 2 and 3.
Ruthenium-based complexes have two general limitations. The first is their tendency to form stable Fischer carbenes in the presence of electron-rich olefins such as enol ethers Eq. The second is their susceptibility to coordination by Lewis bases, which limits their compatibility with functional groups such as amines and phosphines however, protection strategies can circumvent this limiation; see below.
The rate of olefin metathesis is strongly affected by the substitution pattern of the alkene s , with more substituted alkenes reacting more slowly. Steric hindrance near the reacting alkenes may have an effect similar to alkene substitution.
Synthesis of Carbocycles Molybdenum catalyst 1 and second-generation ruthenium complexes 4 and 5 are most effective for the synthesis of substituted cyclic alkenes. Terminal alkenes are usually the preferred substrates because of their relatively high reactivity. Although the use of air- and water-sensitive catalyst 1 is undesirable from a practical standpoint, it may be necessary in reactions that establish tetrasubstituted double bonds Eq.
Typical catalysts are ruthenium complexes 2 and 4. Conformational constraints are necessary in the substrate to promote cyclization, but geminal disubstitution between the reactive alkenes is often enough to promote cyclization. Substrates with greater rigidity can give rise to more structurally complex cyclooctenes Eq. For example, RCM was applied in a total synthesis of — -terpestacin to establish a fifteen-membered ring Eq.
The functionalized cyclic products thus prepared can then be employed in cross-coupling reactions Eq. Heterocycles containing a carbon-carbon double bond can also be prepared via RCM. Although nitrogen- and oxygen-containing rings are the most common products, heterocycles containing phosphorus, silicon, boron, sulfur, and other elements have also been prepared.Second-generation Grubbs catalysts 4 - 6 include a strongly donating N-heterocyclic carbene ligand trans to the phosphine ligand, accelerating phosphine dissociation and increasing their activity relative to 2 and 3. Olefin Metathesis General Information The olefin metathesis reaction the subject of Nobel Prize in Chemistry can be thought of as a reaction in which all the carbon-carbon double bonds in an olefin alkene are cut and then rearranged in a statistical fashion: If one of the product alkenes is volatile such as ethylene or easily removed, then the reaction shown above can be driven completely to the right. Limitations[ edit ] Many metathesis reactions with ruthenium catalysts are hampered by unwanted isomerization of the newly formed double bond, and it is believed that ruthenium hydrides that form as a side reaction are responsible.
Although one prochiral center is present the product is racemic. Although nitrogen- and oxygen-containing rings are the most common products, heterocycles containing phosphorus, silicon, boron, sulfur, and other elements have also been prepared.