0 Search • Table of Contents Cover Story • C&EN Classifieds OLEFIN December 23, 2002 • Today's Headlines METATHESIS: BIG- • Cover Story Volume 80, Number 51 CENEAR 80 51 pp. 34-38 DEAL REACTION • Editor's Page ISSN 0009-2347 A boon to organic • Business synthetic chemists, • Government & Policy olefin metathesis als • Science & Technology proves useful for ma OLEFIN METATHESIS: THE EARLY industrial processes • ACS News DAYS through metal-carbe • Calendars catalysts • Books Recognizing the role of metal carbenes was key in • Career & Employment realizing the promise of olefin metathesis OLEFIN • Special Reports METATHESIS: THE • Nanotechnology A. MAUREEN ROUHI, C&EN WASHINGTON EARLY DAYS • What's That Stuff? Recognizing the role of metal carbenes w key in realizing the Back Issues Robert H. Grubbs of California Institute of Technology and Richard R. Schrock of Massachusetts Institute of Technology promise of olefin immediately come to mind at the mention of olefin metathesis 2002 Go! metathesis. However, the advent of olefin metathesis featured Related Stories Safety Letters many industrial researchers, who, while working at major Chemcyclopedia U.S. petrochemical companies, discovered the reaction. It also Rings Made Easy involved several academic and French Petroleum Institute [C&EN, Sept. 23, ACS Members can sign up to chemists, who helped establish the role of metal carbenes in 2002] receive C&EN e-mail the reaction. newsletter. New Initiator Is Olefin metathesis was Fast And Efficient first observed in the [C&EN, Aug. 28, 2000] 1950s by industrial chemists. In 1956, Herbert S. Eleuterio, at Metathesis Magic DuPont's petrochemicals [C&EN, Jan. 8, 2001] department, in Join ACS Wilmington, Del., Generic Tide Is obtained a propylene- Rising ethylene copolymer from [C&EN, Sept. 23, a propylene feed passed 2002] over a molybdenum-on- aluminum catalyst. Separating Olefins Analysis showed that the With A Redox output gas was a mixture Switch of propylene, ethylene, [C&EN, Jan. 8, and 1-butene. And when 2001] he tried the experiment with cyclopentene, "the How Methanol polymer I got looked like Turns Into Olefins somebody took a pair of [C&EN, Nov. 6, scissors, opened up 2000] cyclopentene, and neatly sewed it up again," he E-mail this tells C&EN. article to a friend Chemists at other petrochemical companies were getting Print this artic similar baffling results. Edwin F. Peters and Bernard L. Evering recorded in U.S. Patent 2,963,447, assigned in 1960 E-mail the 0 0 to Standard Oil Co. of Indiana, that propylene combined with editor molybdenum oxide on alumina treated with triisobutyl aluminum yields ethylene and butenes. In 1964, Robert L. Banks and Grant C. Bailey, of Phillips Petroleum, Bartlesville, Okla., reported the disproportionation of propylene to ethylene and butenes using molybdenum hexacarbonyl supported on alumina [Ind. Eng. Chem. Prod. Res. Dev., 3, 170 (1964)]. Like magic, the chemistry was spewing products that could not be explained by the reactions of olefins known at the time. In 1967, Nissim Calderon and others at Goodyear Tire & Rubber, Akron, Ohio, figured out what was going on. The unexpected products are due to cleavage and reformation of the olefins' double bonds. One carbon of the double bond of one olefin, along with everything attached to it, exchanges place with one carbon of the double bond of the other olefin, along with everything attached to it. The Goodyear researchers named the reaction "olefin metathesis" [Tetrahedron Lett., No. 34, 3327 (1967).] The Goodyear team performed experiments with butene and deuterated 2-butene in the presence of a homogeneous catalyst [J. Am. Chem. Soc., 90, 4133 (1968)]. At about the same time, Johannes C. Mol and others at the University of Amsterdam, in the Netherlands, independently reached the same conclusion with propylene and carbon-14-labeled propylene in the presence of a heterogeneous catalyst [Chem. Commun., 1968, 633]. THE RACE TO EXPLAIN the reaction was on. According to a once-popular hypothesis, which was favored by Calderon and is sometimes referred to as the conventional mechanism, a cyclobutane intermediate complexed to the metal is formed [J. Am. Chem. Soc., 90, 4133 (1968)]. However, olefin metathesis forms no cyclobutanes, and putting cyclobutanes into olefin metathesis systems does not produce alkenes. Thus in 1971, Roland Pettit, then a chemistry professor at the University of Texas, Austin, proposed a tetramethylene complex, in which four methylene units are bonded to a central metal atom [J. Am. Chem. Soc., 93, 7087 (1971)]. In the late 1960s, Grubbs was a postdoc at Stanford University, where his interests in organometallic chemistry, catalysis, and reaction mechanisms converged. He first heard of olefin metathesis at a seminar there. "The transformation is unique, and we had no idea how it happens," he tells C&EN. In an attempt to explain olefin metathesis, Grubbs--who by then had moved to Michigan State University, East Lansing-- proposed that the redistribution of groups around the double bonds was due to a rearranging metallacyclopentane intermediate [J. Am. Chem. Soc., 94, 2538 (1972)]. Later, he suggested that one mode of rearrangement could lead to formation of a cyclobutane complexed to a metal carbene [Inorg. Chem., 12, 2166 (1973)]. These proposals were incorrect. Had people been aware of 0 0 work by French chemists published in the early 1970s, some of these ideas might not have come up. In 1971, two chemists at the French Petroleum Institute, Yves Chauvin and his student Jean-Louis Hérisson, suggested that olefin metathesis is initiated by a metal carbene. The metal carbene, they proposed, reacts with an olefin to form a metallacyclobutane intermediate that breaks apart to form a new olefin and a new metal carbene, which propagates the reaction [Makromol. Chem., 141, 161 (1971); because of a typographical error in the journal's running head, this paper is sometimes erroneously cited with a 1970 publication date]. This paper, everyone agrees, was the first to envision correctly a key role for metal carbenes in olefin metathesis and the events that lead to exchange of groups around carbon- carbon double bonds. Chauvin's work gave the field "a chance to move away from its state of alchemy, although it took several years before the mechanism was experimentally supported and widely accepted," says K. C. Nicolaou, a chemistry professor at Scripps Research Institute and the University of California, San Diego. Chauvin, now retired, says three papers published in 1964 led him to the hypothesis. The first was from Ernst Otto Fischer at the University of Munich, Germany, about a new type of metal-carbon bond exemplified in the metal carbene (methylmethoxycarbene)pentacarbonyl tungsten--(CO)5W=C (CH3)(OCH3) [Angew. Chem. Int. Ed., 3, 580 (1964)]. The second paper was from Giulio Natta at the Industrial Chemistry Research Institute at Milan Polytechnic, Italy, describing the ring-opening polymerization of cyclopentene with triethylaluminum and hexachlorotungsten [Angew. Chem. Int. Ed., 3, 723 (1964)]. And the third was from Phillip Petroleum's Banks and Bailey on the disproportionation of propylene. Like magic, the chemistry was spewing products that could not be explained by the 0 0 reactions of olefins known at the time. "APPARENTLY, these papers had nothing in common," Chauvin tells C&EN. "But for me, they were a revelation." The papers of Natta, Banks, and Bailey indicated that cyclopentene polymerization and propylene disproportionation are the same reaction, he explains. Therefore, they must involve the same type of intermediate species. "With the paper of Fischer, I felt that these species could be metal carbenes," Chauvin continues. With Hérisson, Chauvin studied the coreactions of acyclic and cyclic olefins. They found that the main products of cyclopentene and 2-pentene were C9, C10, and C11 dienes in a ratio of 1:2:1. That three products are formed is key, because the conventional mechanism--the most popular hypothesis in the late 1960s--predicts only the C10 product. The results show complete exchange between the olefin starting materials, which is possible with a metal carbene intermediate but not with a cyclobutane intermediate. The reaction of cyclooctene and 1-pentene, however, gave almost entirely the C13 product predicted by the conventional mechanism. The French authors stated that their scheme for olefin metathesis--involving a reaction between metal carbenes and olefins--could not explain the result. For several years after the Chauvin paper was published, consensus regarding the involvement of metal carbenes in olefin metathesis did not emerge. For example, Mol concluded in 1975 that the available data supporting the Chauvin hypothesis did not exclude other mechanisms. And in 1977, Calderon wrote, "The present state of knowledge does not permit a clear-cut selection of a single scheme [among four proposed, including Chauvin's] over the rest." However, suggestions that metal carbenes are key were coming from various labs. For example, in 1972, Michael F. Lappert and coworkers at the University of Sussex, Brighton, England, showed that various rhodium complexes catalyze the metathesis of tetraaminoethylenes, which are highly electron rich olefins, and that rhodium carbenes are formed during the reaction [Chem. Commun., 1972, 927]. But the relevance of this reaction to ordinary alkenes was "an open question" at the time [Chem. Soc. Rev., 2, 99 (1973)]. Meanwhile, two years after proposing a role for metal carbenes, Chauvin showed that a small amount of propylene is formed from 2-butene in the presence of tungsten hexachloride and methyllithium or tetramethyltin [C. R. Acad. Sci. Paris, 276, 169 (1973)]. The product, Chauvin tells C&EN, could be explained by replacement of a ligand on tungsten with a methyl group and elimination of a methyl hydrogen to form a W=CH2 species, which reacts with 2- butene. 0 0 Perhaps the most significant clue came from the University of Wisconsin, Madison, in 1974.