Journal of Scientific & Industrial Research Vol 65, December 2006, pp 957-965 Metathesis catalysts: Historical perspective, recent developments and practical applications Okram Mukherjee Singh* Department of Chemistry, Manipur University, Canchipur 795 003 Received 29 May 2006; accepted 02 August 2006 Metathesis, a recently developed synthetic chemical methodology, has widespread applications in industries in large- scale production of organic compounds, pharmaceutical products and polymeric materials. Keywords: Catalysis, Drugs, Mechanism, Olefin -metathesis, Polymer Introduction [Mo(CO) 6] on alumina}, led to ethylene and 2- Nobel Prize in Chemistry for 2005 has been butenes. In 1967, 2-butene and 3-hexene were awarded 1 to Frenchman Yves Chauvin and Americans synthesized from 2-pentene by employing a catalytic Richard Schrock and Robert Grubbs for their system formed from tungsten hexachloride, ethanol contributions to metathesis. Chauvin 2 proposed a new and ethyl-aluminium dichloride and used the term mechanism of metathesis, whereas Schrock 3 and olefin metathesis for the first time 8. Calderon 9 applied Grubbs 4 developed new catalysts for metathesis. the metathesis to ring-opening polymerization of Metathesis is a kind of organic synthesis where cyclo-olefins to polyalkenamers. Bradshaw et al 10 carbon compounds from very simple and small suggested a reaction mechanism through a four- molecular weight to very large, complex polymers; centered cyclobutanic intermediate. First order kinetic macromolecules and even natural products are studies on propene metathesis in the heterogeneous synthesized. In metathesis, double bonds are broken phase were reported by Begley & Wilson 11 and on 2- and made between carbon atoms in ways that cause pentene metathesis in the homogeneous phase by atom groups to change places. This happens with the Hughes 12 . Although the results obtained by Begley & assistance of special catalyst molecules. Wilson 11 indicate a first order kinetics for the reaction Metathesis of olefins is the reaction in which the of propene, contradicting the mechanism through a molecules of these hydrocarbons are formally cyclobutanic intermediate, further investigations fragmented at their double bonds and new olefin carried out in homogeneous catalysis 13-14 seemed to molecules result by recombination of fragments indicate a second order kinetics. The real kinetics of originating from different molecules (Scheme 1). the process was remaining uncertain until the Metathesis reactions (Schemes 2-6) are generally discovery of Chauvin mechanism 2, which was reversible and, with the right catalyst system, awarded Nobel Prize in Chemistry for 2005. equilibrium can be attained in seconds, even with 15 4 In 1970, Chauvin & Herisson suggested a chain substrate/catalyst ratios of 10 . mechanism for metathesis and ring opening polymerization in the homogeneous phase, through History of Catalytic Metathesis Non-catalytic carbonic intermediates. In 1973, Chauvin published 16 metathesis of olefins has been known since 1931, 5 that WCl + MeLi mixture catalyzes the formation of when Schneider & Frolich obtained ethene and 2- 6 propene by reaction of 2-butene, which was proposed butene by pyrolyzing propene at 852°C. Under first 6,7 to proceed via methylation of tungsten, followed by catalyzed metathesis reactions , propene heated with -elimination in the tungsten-carbon bond of W–CH molybdenum {in the form of the metal, oxide or 3 to form W CH 2 (H), then metathesis. Carbonic chain _____________ mechanism suggested by Chauvin was supported later 17-30 *E-mail: [email protected] by the new experimental results . 958 J SCI IND RES VOL 65 DECEMBER 2006 Scheme 1 Scheme 2 Scheme 3 Scheme 4 Scheme 5 Scheme 6 SINGH: METATHESIS CATALYSTS: DEVELOPMENTS & APPLICATIONS 959 Developing Catalyst Molecules With catalytic metathesis, shorter synthetic routes to Catalyst systems for the olefin metathesis generally the products give higher yields. Metathesis is contain a transition metal compound, but often require significant to green chemistry because: 1) Reactions the presence of a second compound (co catalyst), and are more efficient (fewer steps, fewer resources, less sometimes a third (promoter). System most waste); 2) Reactions are easier to use (stable under commonly used is based on the chlorides, oxides, or ambient conditions); and 3) Environmentally benign other easily accessible compounds of Mo, Ru, W, Re, (non-hazardous solvents, less hazardous waste). For Os, Ir, Ti, V, Cr, Co, Nb, Rh, or Ta compounds. example, Grubbs used a green approach, from RuCl 3 Typical co catalysts are EtAlCl 2, R 3AlCl 2, R 3Al and in water to the sophisticated chiral version of the R4Sn(R=Ph, Me, Et, Bu), while promoters often second-generation ruthenium benzylidene catalyst, to contain oxygen as O 2, EtOH or PhOH. It has now provide the organic and polymer communities with been realized that metathesis could assume great most environment friendly metathesis catalysts, importance for organic synthesis if reliable and because of their stability to air and even to some effective catalysts could be found. Molybdenum and extent to aqueous media and their high compatibility tungsten soon appeared to be the most suitable metals. with most organic functionalities. A few examples of Some catalysts were produced with those metals, but the synthetic applications of metathesis in there was still uncertainty as to what groups would pharmaceuticals are as follows: bind to the metal to give stable yet active alkylidene Metathesis was used in the synthesis of a complexes. 7-membered azepin compound called balanol, which A breakthrough came in 1990 when Schrock is used as anticancer agents and in controlling 31-33 et al reported construction of a group very active, inflammation, cardiovascular disorders, central well-defined Mo catalyst (Scheme 7). With this nervous system dysfunction, and even HIV infection. discovery, chemists began to realize that olefin Fuerstner 38 & Thiel 39 claimed that by using ring- metathesis could be used for general purposes in closing metathesis, total synthesis of this compound organic synthesis. Metathesis gained increasing was shortened by 4-5 steps from the earlier methods attention among researchers active in synthetic (Scheme 9). chemistry. Yet another breakthrough in the Another anticancer drug epothilone 40 and its development of metathesis catalysts came in 1992 34-36 derivatives were also prepared by using the ring when Grubbs et al discovered a catalyst with the closing metathesis (Scheme 10). Nicolaou et al 41 metal ruthenium. It was stable in air and demonstrated reviewed the efficiency of ring closing metathesis in higher selectivity but lower reactivity than Mo the total synthesis of macrocyclic lactones and several catalysts. New catalyst also had the ability to initiate natural products. metathesis in the presence of alcohols, water and Similar type of ring closing metathesis was applied carboxyl acids (Scheme 8). to produce Civetone 42 (cyclo-9-heptadecenone) used Grubbs’ catalysts have become the first well- in the perfume industry from ethyl oleate. Another defined catalysts for general metathesis applications class of natural compounds, easily obtainable through in ordinary laboratories. Catalyst 2 in Scheme 8 is metathesis are insect pheromones, which are useful as generally named Grubbs’ catalyst and has become a environment friendly pest-control agents. Pederson standard with which all new catalysts are compared. et al 43 reported a typical metathesis reaction to yield The general applicability of Grubbs’ catalyst has the pheromone, 11-tetradecenyl acetate from a long given rise to future prospects of the possibilities of chain olefinic ester and 3-hexene catalyzed by Grubbs organic synthesis. Grubbs bases his catalyst design on catalyst B (Scheme 11). detailed mechanical studies. Materia Medica, an international drug company has Practical Applications of Metathesis produced three pheromones: i) ( E)-5-decenyl acetate, Pharmaceutical Industries a pheromone of peach twig borer ( Anarsia lineatella ); Metathesis is an important tool for new ii) a mixture of ( E)- and ( Z)-11-tetradecenyl acetate, pharmaceuticals used in treating major diseases such as of omnivorous leafroller ( Platynota stultana ); and (iii) bacterial infections, hepatitis C, cancer, Alzheimer’s different mixture of (E)- and (Z)-11-tetradecenyl disease, Down’s syndrome, osteoporosis, arthritis, acetate, of Sparganothis fruit worm, a pest of inflammation, fibrosis, HIV/AIDS, migraine, etc 37 . cranberries and blueberries. 960 J SCI IND RES VOL 65 DECEMBER 2006 Scheme 7 Scheme 8 Scheme 9 SINGH: METATHESIS CATALYSTS: DEVELOPMENTS & APPLICATIONS 961 Scheme 10 Scheme 11 962 J SCI IND RES VOL 65 DECEMBER 2006 Hydrocarbon Industries disproportionated by metathesis to detergent range First metathesis process applied on an industrial internal olefins (C 11 -C14 ) in the disproportionation scale, developed by Phillips Petroleum Co., USA, and reactor, and the product is fractionated in distillation 44 known as The Triolefin Process , produces high- columns, into fractions of higher olefins. purity ethene and but-2-ene from propene. This Shell Chemicals began large-scale production of process operated for 6 years from 1996 by detergent alcohols and esters, which are made of long Shawanigan Chemicals near Montreal in Canada chain alkenes that are prepared from ethane using (capacity, 50,000 tons/year). It is based on the metathesis procedure 45 . Shell