388 CHIMIA 2015, 69, No. 7/8 SCS Laureates and Awards & Fall Meeting 2015 doi:10.2533/chimia.2015.388 Chimia 69 (2015) 388–392 © Schweizerische Chemische Gesellschaft Metathesis by and Catalysts

Richard R. Schrock*

Paracelsus Prize 2014

Dedicated to Yves Chauvin, October 10, 1930 – January 27, 2015

Abstract: Carbon–carbon double bonds are an integral part of the chemical industry and are widely found in natural products, from the small and simple (ethylene) to the large and complex. The ability to manipulate carbon– carbon double bonds to make other carbon–carbon double bonds in a catalytic and stereospecific fashion has revolutionized the way organic molecules and polymers are made today. This article outlines the development of modern molybdenum and tungsten alkylidene catalysts that can be designed at a molecular level to achieve a given result. Carbon–carbon triple bonds also can be manipulated in a similar manner with the appropriate alkylidyne catalyst. Although the ‘ metathesis’ and ‘ metathesis’ reactions are now fifty to sixty years old, many problems remain that will require an even more detailed understanding of these most intricate, superficially simple reactions. Keywords: · · Metathesis · Molybdenum · Organic Synthesis · Tungsten catalysis

de Nemours and Company. He moved to correct one appeared first in a publication M.I.T. in 1975 where he became full pro- by Hérrison and Chauvin in 1971,[5] name- fessor in 1980 and in 1989 the Frederick ly the reversible reaction between a metal G. Keyes Professor of Chemistry. In 2005 complex that contains a metal–carbon he received the Nobel Prize in chemistry double bond (M=CHR) and a C=C bond to with R. H. Grubbs and Y. Chauvin. He yield an intermediate that contains a metal-

is a member of the National Academy of lacyclobutane (MC3) ring. A related reac- Sciences and is a Foreign Member of the tion that involves alkynes was discovered Royal Society of London. He has published to be catalyzed by a heterogeneous catalyst more than 570 research papers, and super- in 1968[6] and a homogeneous catalyst in Richard R. Schrock was born in vised over 180 PhD students and postdocs. 1972.[7] This ‘alkyne disproportionation’ Indiana, but spent his high school years reaction was proposed to consist of a re- in San Diego, California, and obtained versible reaction between a M≡CR bond his BA degree in 1967 from the University 1. Introduction (R≠H) and a C≡C bond to give all pos- of California at Riverside. He attended sible alkynes via a metallacyclobutadiene graduate school at Harvard University, In the early 1970s organometallic, intermediate.[8] Neither process resulted in from which he received his PhD degree organic, and polymer chemists were at- any positional isomerization of the C=C or in inorganic chemistry in 1971 (awarded tracted to a new type of catalytic reaction C≡C bonds. Although Fischer-type ‘low 1972) as a student of J. A. Osborn. He called Olefin Disproportionation by Banks oxidatation state’ carbene[9] complexes spent one year as an NSF postdoctoral fel- and Bailey, who published results in the were known at the time, and the synthesis low at Cambridge University working in open literature in 1964;[1] the reaction also of carbyne complexes soon followed,[10] the group of Lord Jack Lewis. In 1972 he was reported by Natta in 1964[2] and had they did not promote what came to be accepted a position in the group of George appeared in patents filed at duPont[3] and known as alkene metathesis[11] and alkyne Parshall at the Central Research and Standard Oil[4] several years earlier. The metathesis, respectively, in the rapid man- Development Department of E. I. duPont basic process, which was shown to be cata- ner often observed for what are now known lyzed by various molybdenum or tungsten, as ‘classical’ alkene and alkyne metathesis and later rhenium, compounds of unknown catalysts. However, experiments first pub- type, resulted in an ‘exchange’ of alkyli- lished in 1976 showed that polymerization dene (usually CHR, where R = H or alkyl) of cyclic olefins, metathesis of linear ole- units in alkenes with one another, e.g. pro- fins, enyne metathesis, and polymerization pylene could be equilibrated with ethyl- of could be initiated by certain *Correspondence: Prof. R. R. Schrock Department of Chemistry ene and cis and trans-2-butenes (Eqn. (1), Fischer-type carbene complexes, although Massachusetts Institute of Technology (MIT) Scheme 1) or norbornene could be polym- new alkylidenes of the same type as the 6-331 erized through a ‘ring-opening’ of its dou- initiator were not observed.[12] Classical 77 Massachusetts Ave. Cambridge, MA 02139, USA ble bonds (Eqn. (2), Scheme 1). Although metathesis catalysts often are formed from E-mail: [email protected] several mechanisms were proposed, the metal oxides on silica or alumina[13] or in SCS Laureates and Awards & Fall Meeting 2015 CHIMIA 2015, 69, No. 7/8 389

Scheme 1. pears to be closest to the transition state 2 CH3CH=CH2 CH23CH=CH2 CH2=CH2CH+ 2CH=CH3CH=CHCH2 + CH3CH=CHCH3 3 (1) Two (1)examples of for losing an olefin is a trigonal bipyramid re- in which the aryloxide is in one apical actions position and the imido group in the other (2) (2) (Fig. 2). A large aryloxide (e.g. a 2,6-di- substituted terphenoxide) was found to limit the that can be formed to those in which any substituents on the ring must point away from the large ary- solution from a variety of metal complexes. yields catalytically active alkyne metath- loxide ring.[26] Therefore, it is now possible An alkylating agent is often required, but esis catalysts for both Mo and W. to prepare Z olefins in a large variety from simply exposing the supported metal oxide Most Mo-based or W-based alkene two olefins or in a ring-opening metathesis to alkenes or alkynes at high temperatures metathesis[16,21] or alkyne metathesis[16,22] reaction (see below). It is now also possible will generate the catalyst through some un- catalysts today are four-coordinate. Four- to run a C=C metathesis coupling ‘in re- known mechanism or mechanisms. coordination allows an intermediate five- verse’, i.e. to consume a Z olefin selectively coordinate metallacyclobutane or metal- in a mixture of E and Z olefins through re- lacyclobutadiene complex to form readily, action with ethylene (‘ethenolysis’), leav- 2. Discovery of High Oxidation and reversibly. Both metallacyclobutane ing behind a pure E olefin, a process that is State Multiple Metal–Carbon Bonds and metallacyclobutadiene complexes leading to the formation of chemicals from have been isolated and characterized crys- seed oils, many of which are Z-oleic acid The first isolated and identified ‘high tallographically and shown to be viable derivatives, on an industrial scale.[27] In oxidation state’ metal complexes that catalysts. or aryloxide fact, the problem of selectively forming a Z contain M=CHR or M≡CR bonds are provide a ‘flexible’ coordination sphere olefin without subsequent isomerization to [14] E through metathesis was solved through Ta(CH-t-Bu)(CH2-t-Bu)3 and [Li(N,N'- both in terms of steric protection and elec- [23] a relatively simple principle, one that has dimethylpiperazine)][Ta(C-t-Bu)(CH2-t- tronic tuning through π and σ pathways. [15] been extended to Ru-based metathesis cat- Bu)3]. In these compounds M=CHR and At least one alkoxide or aryloxide M≡CR bonds are formed through what is is present in the vast majority of alkene or alysts;[28] formation of E olefins selectively essentially deprotonation of an α carbon alkyne metathesis catalysts today. (through kinetic control) is a problem that atom in a neopentyl ligand (to give a neo- In this article I will concentrate on some remains to be solved. pentylidene ligand) or subsequently de- recent developments in alkene metathesis protonation of a neopentylidene ligand (to catalysts that contain Mo or W, a few of give a neopentylidyne ligand), respective- which are shown in Fig. 1.[24] Other articles 4. Organic Synthesis ly. The ‘α abstraction’ method of are available that discuss recent advances preparing metal–carbon double and triple in olefin metathesis by Ru catalysts.[25] The formation of various types of di- bonds led to the syntheses of Mo and W substituted or trisubstituted alkenes stereo- alkylidene complexes that will metathesize selectively through olefin metathesis re- olefins[16] and Mo and W alkylidyne com- 3. Z and E Selectivity actions has been, and will continue to be, plexes that will metathesize acetylenes.[17] of tremendous benefit for synthesizing Rhenium alkylidene complexes that will One of the reasons why olefin metath- natural products, many of which contain metathesize olefins eventually also were esis has achieved what it has is that highly C=C bonds; olefin metathesis followed prepared through α hydrogen abstrac- reactive alkylidene complexes can be syn- by hydrogenation is also often a relatively tion reactions in high oxidation state Re neopentyl complexes.[18] Neopentyl and

neophyl (MCH2CMe2Ph) complexes are i-Pr i-Pr especially suited for forming alkylidene N N N O N N W O N and alkylidyne complexes that are rela- Mo W Mo tively stable toward bimolecular coupling O O O O PMe2Ph O of CHR or CR ligands, and are, or can be Br turned into (through ligand substitution) F3C Br TBSO CF catalysts for alkene or alkyne metathesis 1 2 3 3 4 reactions. The size and electronic nature of the ligands dictate the details of the me- Fig.1. Examples of some recent Mo and W olefin metathesis catalysts. tathesis reaction. Alkylidene and alkyli- dyne complexes are reformed during the thesized and manipulated in terms of struc- tures and reactivities in huge variety; this alkene or alkyne metathesis reaction, as R has been demonstrated through detection synthetic/mechanistic approach has been 4 of many alkylidene and alkylidyne analogs critical to catalyst development, and still is N R1 during and after a metathesis reaction. It critical for further catalyst development to- R3 R2 is interesting to note that today molyb- day. Most importantly, catalysts can be de- N M denum alkylidyne complexes for alkyne signed that yield products through kinetic R R O R R metathesis are prepared through oxidation control. An example is the formation of Z of Fischer-type alkylidyne complexes, olefins, which in many cases is the higher energy isomer of an acyclic olefin. This e.g. Mo(CPh)(CO)4Br to give Mo(CPh) R [19] R (1,2-dimethoxyethane)Br3, a close rela- discovery was greatly assisted through tive of W(C-t-Bu)(dme)Cl3, that was first the synthesis of mono aryloxide pyrrolide prepared in 1981;[20] replacement of the complexes such as 1, 3, and 4 (Fig. 1). The Fig. 2. A TBP metallacyclobutane derived from halides with alkoxide or aryloxide ligands metallacyclobutane intermediate that ap- a MAP complex. 390 CHIMIA 2015, 69, No. 7/8 SCS Laureates and Awards & Fall Meeting 2015

efficient method to form C–C bonds in Me OMe [29] OH rings. Several examples of natural prod- O S H O O Me ucts synthesized in the last few years that Me Me Me O MeO O involve one or more olefin metathesis steps HO Me N OH N O [30] Me Me O N Me HN are Epothilone C, (±)-Tetrapetalone O N [31] [30] Me Me A-Me Aglycon, Nakadomarin A, and Me O O O [32] O OH O OMe (+)-Neopeltolide (Fig. 3). Which of the N many different types of Mo- and W-based olefin metathesis catalysts best achieves each of the metathesis reactions is deter- Epothilone C (±)-Tetrapetalone A-Me Aglycon Nakadomarin A (+)-Neopeltolide mined through screening procedures, and Fig. 3. Several natural product synthesized using at least one olefin metathesis step. trends are beginning to emerge. Newer cat- alysts, e.g. Lewis acid activated[33] or high oxidation state complexes that contain an cyclopentadiene[41] yields isotactic or syn- nature of the ligands in the four coordi- NHC ligand,[34] have not yet been exam- diotactic polymers that are nate alkylidene imido (or oxo complex); ined to any significant degree as catalysts crystalline, high melting, relatively stable rates of the order of 100 s–1 allow these for various types of metathesis reactions. to oxygen, and therefore of commercial interconversions to be observed in variable It is becoming increasingly clear that small value. Tungsten oxo alkylidene complexes, temperature proton NMR studies.[45] The changes in a catalyst can have significant alkylidene obtained through rotation by especially when activated with B(C6F5)3, consequences in terms of yields, turnover, recently have been found to polymerize 90° can be stabilized through formation of and selectivities, and that access to a large norbornenes and norbornadienes that are a π bond employing a d orbital that lies in variety of catalysts therefore is required for difficult to polymerize stereoselectively, the N-M-C plane (Fig. 4). Usually the syn continued advances toward solutions to the or in some cases, to polymerize at all at­ isomer is the more stable with Keq values problems that remain. 22 °C with traditional Mo or W imido al- as high as 2000 or more, in part because The vast majority of metathesis reac- kylidene initiators.[42] The tacticities of of the agostic CH interaction. However, tions involve M=CHR intermediates in several cis,isotactic or cis,syndiotactic the anti isomer, which can be generated at which R is carbon-based or H. Some al- polymers have been proven through post –78 °C through photolysis, has been shown kylidenes in which R is a heteroatom have polymerization modification.[43] to be exceptionally reactive in several cas- now been isolated and appear to react with es. When the syn isomer is relatively un- olefins in a metathesis fashion.[35] Much reactive and the anti isomer is especially remains to be done in terms of developing 6. Syn and anti Isomers and reactive, a metathesis reaction can proceed catalysts for metathesis reactions that yield Alternating AB Copolymers via the anti isomer without any anti isomer directly functionalized olefins, especially being observed. those that can be employed subsequently A feature of high oxidation state In the process of exploring the synthe- in catalytic reactions that are not metath- M=CHR complexes of the type shown in sis of copolymers it was found recently[46] esis-based.[36] Fig. 1 is that they form two isomers, a syn that several alternating AB metathesis co- alkylidene isomer in which the R group polymers[47] could be prepared stereoselec- points toward the oxo or imido ligand and tively and with as high as 95% AB alterna- 5. Stereospecific Polymerization an anti alkylidene isomer in which the R tion from a Mo imido alkylidene initiator group points away from the oxo or imido (e.g. Scheme 3). All evidence suggests that Now that metathesis catalysts can be the mechanism consists of the reaction of ligand (Fig. 4); the CHα bond of the syn synthesized in large variety, it has been isomer is engaged in an ‘agostic’ interac- B with anti-MA to give syn-MB and one of possible to determine what catalyst is tion[44] with the metal. These syn and anti the trans C=C linkages, followed by the re- needed to polymerize cyclic olefins to give isomers can interconvert in the absence of action of A with syn-MB to give anti-MA a polymer with a single structure. Catalysts an olefin through rotation about the M=C and the other trans C=C linkage (Scheme for ring-opening metathesis polymeriza- bond by 180° at rates that can differ by as 3). The syn-MB isomer is in equilibrium tion (ROMP) have now been perfected that much seven orders of magnitude, e.g. from with the anti-MB isomer during polymer- produce cis,isotactic or cis,syndiotactic ~10–5 s–1 to ~102 s–1, depending upon the ization, and the anti-MB isomer is the one polymers with increasing reliability and variety.[37] In 1993 initiators that contain a biphenolate ligand (e.g. 2 in Fig. 1) were BiphenolateBiphenolate MAP MAP

found to direct an olefin to one side of

[ [ [ [ (3) (3) the initial M=C bond and all subsequent[ [ [ [ M=C bonds to give a cis,isotactic polymer (e.g. Eqn. (3) in Scheme 2 for norbornene cis,isotacticcis,isotactic cis,syndiotacticcis,syndiotactic itself).[38] Enantiomorphic site control of polymer formation is complemented by Scheme 2. Formation of stereoregular polymers from norbornene the more recent development of MAP ini- tiators (e.g. 1, 3, or 4) that promote forma- Fig. 4. The equilibrium tion of cis C=C bonds (vide supra) and in Ar Ar between syn and anti the process, formation of cis,syndiotactic Ar .. isomers. N 90° 90° N polymers as a consequence of the chirality rotation N rotation at the metal switching with each insertion M Hα M R R1O R1O of monomer into the M=C bond (stereo- R O M R O 1 R R 1 Hα genic metal control).[39] Hydrogenation anti H syn of pure cis,isotactic or cis,syndiotactic J ∼140 Hz J ∼125 Hz polymers made from norbornene[40] or di- CHα CHα Figure 4. SCS Laureates and Awards & Fall Meeting 2015 CHIMIA 2015, 69, No. 7/8 391

that is largely observed. Preliminary mod- active centers has never been proven, al- the number of different designs would ap-

eling studies suggest that kB is at least sev- though high oxidation state oxo alkylidene pear to be more limited than designs for eral hundred times larger than kA (Scheme complexes are the most likely suspects. homogeneous catalysts. 3). These findings suggest that syn and Now that tungsten oxo complexes have anti isomers continue to be unappreciated been prepared, it is time to determine if we features in many metathesis reactions cata- can prepare catalysts with a single structure 8. Comments lyzed by high oxidation state catalysts, i.e. on silica in which all sites are catalytically an unobservable alkylidene isomer that is active at low temperatures, and whose re- The olefin metathesis reaction is now present in low concentration may be the activities can be controlled to the degree approximately 60 years old, but a full un- one that determines the outcome of a me- that is possible now by close analogs in so- derstanding of its subtleties and how to tathesis reaction. The C=C bonds that are lution.[49] The first steps toward these goals develop catalysts to achieve a given result formed in each of the reactions shown in have now been taken.[50] For example,[50b] are only beginning to be understood.[51]

Scheme 1 are trans as a consequence of the the reaction of W(O)(CH-t-Bu)(dAdPO)2 The role of dispersive forces in sterically monomers being relatively ‘large’ (B) or (dAdPO = O-2,6-Adamantyl2C6H3) with crowded molecules of the type described ‘small’ (A) and therefore only trans metal- partially dehydroxylated silica (SiO2-700) here is a potential further complication lacycles being viable. yields a well-defined silica-­supported that remains to be considered.[52] Although

alkylidene complex, (SisurfO)W(O) progress in the last several years has been (CHCMe2Ph)(dAdPO), in high yield that significant, many problems remain to be 7. Catalysts Supported on Silica has been fully characterized through solid- solved in order to take full advantage of state NMR methods (Eqn. (4), Scheme 4). what can be achieved. Metathesis chem-

Olefin metathesis processes have been (SisurfO)W(O)(CHCMe2Ph)(dAdPO) is a istry seems likely to continue to grow, practiced on a commercial scale for de- highly active and relatively stable catalyst evolve, and become even more useful as cades,[48] often with a ‘classical’ heteroge- for the metathesis of internal alkenes with these remaining problems are solved. neous catalyst consisting of a metal oxide catalyst loadings as low as 50 ppm. It is al- supported on silica or alumina operating at so active for the homocoupling of terminal Received: May 28, 2015 several hundred degrees centigrade.[13] The alkenes, if ethylene is constantly removed largest metathesis process today is the con- in order to avoid formation of a relatively [1] R. L. Banks, G. C. Bailey, Ind. Eng. Chem. version of ethylene into 1-butene through unreactive square-pyramidal metallacy- Prod. Res. Dev. 1964, 3, 170. [2] G. Natta, G. Dall’asta, G. Mazzanti, Angew. dimerization, isomerization of 1-butene to clobutane complex. Supported catalysts of Chem. Int. Ed. Eng. 1964, 3, 723. 2-butenes, then metathesis between ethyl- this type contain only one ligand (dAdPO [3] a) H. S. Eleuterio, J. Mol. Catal. 1991, 65, 55; ene and 2-butenes to give propylene. This in Eqn. (4)) that can be varied and the na- b) H. S. Eleuterio, U. S. Patent 3074918, 1963. olefin conversion technology (OCT) uses a ture of the surface that surrounds the point [4] E. F. Peters, B. L. Evering, U. S. Patent catalyst made from tungsten oxide on sil- of attachment (OSi ) is likely to vary from 2963447, 1960. surf [5] J. L. Hérrison, Y. Chauvin, Macromol. Chem. ica. The precise nature of the catalytically one attached metal to another. Therefore, 1971, 141, 161. [6] F. Pennella, R. L. Banks, G. C. Bailey, Chem. Comm. 1968, 1548. Scheme 3. The mech- [7] a) A. Mortreux, M. Blanchard, Bull. Soc. anism of formation R MeO2C CO2Me Chim. France 1972, 4, 1641; b) A. Mortreux, of an alternating AB M. Blanchard, J. Chem. Soc., Chem. Commun. N copolymer. 1974, 786. [8] T. J. Katz, J. McGinnis, J. Am. Chem. Soc. P Mo (CH2)5 1975, 97, 1592. RF6O [9] E. O. Fischer, A. Maasböl, Angew. Chem., Int. syn-MB Ed. Engl. 1964, 3, 580. RF6O [10] E. O. Fischer, G. Kreis, C. G. Kreiter, J. Müller, G. Huttner, H. Lorenz, Angew. Chem., Int. Ed. Engl. 1973, 12, 564. [11] a) N. Calderon, H. Y. Chen, K. W. Scott, Tet. Lett. 1967, 34, 3327; b) K. J. Ivin, J. Mol, ‘Olefin kB kA + Metathesis and Metathesis Polymerization’, + CO2Me Academic Press: San Diego, 1997. [12] a) J. McGinnis, T. J. Katz, S. Hurwitz, J. Am. B A Chem. Soc. 1976, 98, 605; b) T. J. Katz, Angew. CO Me 2 Chem. Int. Ed. 2005, 44, 2; c) T. J. Katz in R ‘Handbook of Metathesis’, Vol. 1, Ed. R. H. N Grubbs, Wiley-VCH, Weinheim, 2003, p. 47. P [13] S. Lwin, I. E. Wachs, ACS Catal. 2014, 4, 2505. Mo [14] R. R. Schrock, J. Am. Chem. Soc. 1974, 96, (CH2)5 RF6O 6796. [15] L. J. Guggenberger, R. R. Schrock, J. Am. RF6O anti-MA MeO2C CO2Me Chem. Soc. 1975, 97, 2935. [16] R. R. Schrock in ‘Handbook of Metathesis’, Ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003, p. 8. Scheme 4. Synthesis [17] R. R. Schrock in ‘Handbook of Metathesis’, Ed. t-Bu t-Bu t-Bu t-Bu of a silica-supported R. H. Grubbs, Wiley-VCH, Weinheim, 2003, p. SiO2-700 SiO2-700 tungsten oxo alkyli- 173. O OH H O HO H dene catalyst. [18] a) R. Toreki, R. R. Schrock, J. Am. Chem. Soc. W W W W (4) (4) 1990, 112, 2448; b) R. Toreki, G. A. Vaughan, dAdPOH dAdPOH R. R. Schrock, W. M. Davis, J. Am. Chem. Soc. dAdPO dAdPOOPdAd OPdAd dAdPO dAdPOO O 1993, 115, 127. [19] a) A. Mayr, G. A. McDermott, A. M. Dorries, Sisurf Sisurf 2,6-Ad C2,6-AdH OH =C dAdPOH OH = dAdPO Organometallics 1985, 4, 608; b) B. Haberlag, 2 6 3 2 6 3 M. Freytag, C. G. Daniliuc, P. G. Jones, M. 392 CHIMIA 2015, 69, No. 7/8 SCS Laureates and Awards & Fall Meeting 2015

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