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Introduction to the Virtual Issue on Olefin MetathesisFundamentals and Frontiers ́ In this work, de Brito Sá, Rodriguez-Santiago, Sodupe, and Solans-Monfort propose a potential solution to the continuing challenge of cyclopropanation.7 The Tebbe reagent [Cp2Ti(μ2-Cl)(μ2-CH2)AlMe2], a landmark in the development of olefin metathesis and methylidene transfer reactions, is the prototypical “masked methylidene”. Thompson, Nakamaru-Ogiso, Chen, Pink, and Mindiola report the crystal structure of this seminal complex, which eluded crystallization for more than 40 years.8 Terminal methylidene (and methylidyne) complexes are typically difficult to isolate, because their steric accessibility leaves them susceptible to bimolecular reactions. Such complexes of the rare-earth metals have proved particularly elusive, although examples of methylene-bridged complexes have begun to appear. In the selected instances, Zhou, Li, Maron, Leng, and Chen describe the first example of a μ2-methylene rare earth complex,9 while Levine, Tilley, and Andersen present evidence that thermolysis of a bis(neopentyl)scandium precursor affords a terminal Sc CH2 intermediate, which rapidly dimerizes to yield a doubly methylene-bridged complex.10 Hill, Ward, and Xiong describe a convenient entry point into silylcarbyne complexes of tungsten, and, building on Templeton’s classic desilylation strategy,11 a stable, structurally characterized tungsten methylidyne complex.12 Ring-opening metathesis polymerization (ROMP), for decades pivotal to advances in metathesis, has seen a number of recent innovations. Hou and Nomura describe access to ultrahigh-molecular-weight polynorbornene via the first living vanadium arylimido initiators.13 Veige and co-workers overturn the usual orthogonality of alkene and alkyne metathesis with a tungsten alkylidyne complex supported by a trianionic pincer framework. Cycloaddition with ethylene generates a tethered alkylidene, which enables ring-expanding metathesis polymerization of norbornene to yield highly cis, syndiotactic cyclic polymers.14 June, Kang, Sohn, and Choi deploy the Grubbs Zselective catalyst to construct conjugated polyenes with a high proportion of six-membered backbone rings, via selective βaddition to 1,6-heptadiynes.15 The low oxophilicity of the ruthenium metathesis catalysts is key to their breadth of applications. Jana and Grela present a striking exhibition of this tolerance in the metathetical elaboration of endoperoxides, albeit tolerance stops short of hydroperoxides and benzoyl peroxide, which rapidly decompose the catalyst.16 More generally, the “reversed polarization” of the alkylidene, relative to earlier transition-metal catalysts, renders the [Ru]CH2 moiety susceptible to nucleophilic attack. In a survey of seven Grubbs-class catalysts, McClennan, Rufh, Lummiss, and Fogg demonstrate that sterically accessible donorsamines, ethers, and even waterdramatically accelerate abstraction of methylidene by liberated PCy3, accounting for the detrimental impact of stabilizing phosphine ligands on
T
his virtual issue is assembled around a Tutorial Review by Richard Schrock and Christophe Copéret, outlining the pathways by which high-oxidation-state alkylidene complexes are known to form and decompose.1 Beginning with Schrock’s pioneering synthetic methods based on α-abstraction from dialkyl precursors, the Tutorial proceeds through classic transformations and rearrangements to current frontiers. A highlight is the analysis of new evidence from heterogeneous systems, which suggests that olefins can bind to reduced metal surfaces to regenerate alkylidene sites. Such behavior need not be limited to surface species, as indicated by scattered reports of olefin-to-alkylidene rearrangements in low-valent molecular complexes.2 Why is this important? Because one of the great challenges in olefin metathesis, which unhappily continues to distinguish its catalytic chemistry from (e.g.) Pd-catalyzed cross-coupling, is facile, irreversible catalyst decompositionmore specifically, loss of the MCHR bond that enables metathesis. Alkylidene loss is a major problem, simply because the ligand is conventionally difficult to (re)install. Beyond its value in presenting historical advances, therefore, this work stands out in offering a perspective on alkylidene regeneration. The accompanying papers were chosen to showcase the diversity of advances in the chemistry of complexes containing a metal−carbon double bond. These range from new insights relevant to olefin metathesis, to the synthesis of new exemplars of the MC bond, and frontiers in the applications of metathesis and related chemistry. Organometallics and its sister journals, The Journal of the American Chemical Society, ACS Catalysis, and Organic Letters, have been home to many of the most significant findings reported on these topics: the present Virtual Issue celebrates these strengths by drawing together recent papers from all four journals. Computational methods have played a major role in advancing fundamental understanding. In a monumental analysis of experimental systems published over three decades, Solans-Monfort, Copéret, and Eisenstein dissect the structural features of unsubstituted d0 metallacyclobutanes (MCBs), the factors that underlie geometric preference and stability, and the consequences for olefin metathesis.3 Consistent with the core finding that a trigonal-bipyramidal (tbp), 14-electron structure enables metathesis via these complexes, the Schrock group describes the unusual stability of 16-electron, unsubstituted MCBs in which access to the tbp geometry is inhibited.4 A second computational report tackles the enduring question of the nature of the MC bond. While no consensus exists on the oxidation state of the metal in the ruthenium catalysts,5 Occhipinti and Jensen present DFT evidence to support assignment as Ru(IV).6 They propose that covalency should be regarded as the key defining feature of Schrock carbenes: accordingly, they classify group 3−7 alkylidenes as nucleophilic Schrock carbenes, and ruthenium alkylidenes as electrophilic covalent carbenes. A third computational study examines the tantalizing, long-sought goal of iron-catalyzed olefin metathesis. © 2017 American Chemical Society
Published: May 22, 2017 1881
DOI: 10.1021/acs.organomet.7b00325 Organometallics 2017, 36, 1881−1883
Organometallics
Editor's Page
metathesis productivity.17 The Hoveyda metathesis catalyst offers a phosphine-free platform, in which the stabilizing PCy3 group is replaced by a chelating styrenyl ether ligand. This catalyst was long regarded as slow to initiate, largely on the basis of the high molecular weights observed in ROMP chemistry. Labeling studies demonstrated that in fact it readily enters the catalytic cycle, but that facile recapture of the active methylidene species by free styrenyl ether regenerates the catalyst (and indeed inhibits metathesis from early stages of reaction).18 The recapture processfor which Hoveyda coined the memorable phrase the “boomerang mechanism”is examined in a new kinetics study by Griffiths, Keister, and Diver, which demonstrates that styrenyl ether is nearly 2 orders of magnitude more effective than 1-hexene as a substrate.19 The mobility of the N-heterocyclic carbene (NHC) ancillary ligand is widely viewed as key to both the stereoselectivity and decomposition of the Ru catalysts. Perfetto, Costabile, Longo, and Grisi describe routes to complexes with frozen NHC ligand conformations via backbone-functionalized ligands,20 while the Houk−Grubbs team embraces the unintended cyclometalation of an NHC substituent to develop phosphine-free catalysts with improved Z selectivity.21 The puzzling switch from E to Z selectivity in the metathesis of certain small olefins (including enol ethers and cyano-, halo-, or sulfido alkenes) by dichlororuthenium catalysts is explored in a study by Torker, Koh, Khan, and Hoveyda.22 Stereochemical control is connected to turnover-limiting MCB formation and cleavage, and the importance of stereoelectronic (rather than purely steric) effects is underlined. Going beyond the NHC paradigm, Pazio, Wozńiak, Grela, and Trzaskowski undertake a computational study of acyclic diaminocarbene ligands, which offer potentially heightened σ donicity, depending on their conformation, and could afford access to catalysts showing improved activity.23 While NHCs are the mainstay of highly reactive ruthenium catalysts, the Buchmeiser team has appropriated these ligands for use in the notoriously hard and demanding environment of the group 6 catalysts, with remarkable success.24 In their first report on these hybrid NHC−tungsten oxoalkylidene catalysts, turnover numbers of up to 10000 were achieved, opening up new possibilities for stereoelectronic tuning. Directly functionalized olefins remain a frontier in metathesis, as Schrock and Copéret note in concluding their Tutorial. Particularly challenging is cross-metathesis (CM) of halogenated olefins. Recent breakthroughs have been achieved in the Z-selective CM of cyclooctene with (Z)-1,2-dichloroethylene, in which the Schrock−Hoveyda team suggest that MoCHCl complexes are the active species.25 In the Ru systems, Takahira and Morizawa successfully drive the coupling reaction by pairing fluoroalkenes with electron-rich olefins,26 while forays into d10 chemistryand, possibly, non-Chauvin metathesisby the Baker group report cycloaddition of tetrafluoroethylene with Ni difluorocarbenes.27 A final series of papers is set within the classical domain of commodity-chemicals metathesis, with some defining 21stcentury twists. The Phillips Triolefin Process in the 1960s ran on silica-supported tungsten oxide catalysts. Metathesis is of course an equilibrium, and WOx/SiO2 remains the dominant catalyst for the reverse process (olefins conversion technology, OCT) used today to produce high-value, polymerization-grade propylene via ethylene−butene metathesis. Indeed, metathesis is a key technology for production of “on-purpose propylene” to address the shale gas-propelled deficit in supply.
Unsurprisingly, the search for insight into the nature of the active catalyst species remains intense. In an operando study of WOx/SiO2 activation utilizing transient temperature-programmed surface reaction (TPSR) spectroscopy, Lwin and Wachs make the point that “equilibrium metathesis” can look very different from either side of the reaction. Notably, propylene and 2-butene are found to generate highly active sites, while ethylene does not.28 The influence of dispersion and pretreatment is examined by Howell, Li, and Bell, who report that catalytic activity increases with W surface concentration up to the point where WO3 nanoparticles form: these authors propose a model to account for the low proportion of active tungstate species.29 The challenges involved in direct study of these surface metal species, detailed in a lucid review by Lwin and Wachs,30 underscore the motivation behind approaches focused on organometallic models. In addition to studies such as those highlighted above, much understanding has developed from “surface organometallic chemistry”, including work by Basset, Copéret, and others described in the Tutorial. A complementary study from the groups of Maron, Delevoye, and Taoufik correlates metathesis productivity31 with bipodal anchoring of tungsten to the silica surface (in contrast with the monopodal species proposed for the isoelectronic MoO3/ SiO2 system),32 behavior that the authors relate to increased catalyst stability. And finally, a joint paper from the Olivier-Bourbigou and Mauduit groups takes us, perhaps, back to the future. The Shell Higher Olefins Process (SHOP) has been for many years the largest-volume operation in olefin metathesis, used to produce detergent-range α-olefins from petrochemicals. The team reports the successful production of Fischer−Tropsch feeds from renewable resources, using a functional-group-tolerant ruthenium catalyst.33 The “Organometallics Tutorials” are designed to give the next generation of scientists direct access to the viewpoints of leading scientists on important topics in organometallic chemistry. Informed, sometimes opinionated, always thoughtprovoking, these perspectives combine deep expertise with a pedagogical objective. The genesis of the present collection of articles is the master class offered by Schrock and Copéret on the construction, function, loss, and regeneration of the metal− carbon double bond. While explicitly focused on highoxidation-state compounds, many of the lessons have much broader relevance, as the accompanying papers illustrate. These selected articles represent a few among many marvelous examples that set out continuing revelations in metathesis, and its implications for fundamental science, ongoing discoveries in catalysis, and opportunities for the global chemical community. We hope that this vista, from fundamentals to frontier applications, inspires a new generation to further expand the excitement and potential of the field.
Deryn E. Fogg*
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Department of Chemistry and Biomolecular Sciences, and Centre for Catalysis Research & Innovation, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.E.F.:
[email protected]. ORCID
Deryn E. Fogg: 0000-0002-4528-1139 1882
DOI: 10.1021/acs.organomet.7b00325 Organometallics 2017, 36, 1881−1883
Organometallics
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R. M.; Delevoye, L.; Taoufik, M. Organometallics 2016, 35, 2188− 2196. (32) Amakawa, K.; Wrabetz, S.; Kroehnert, J.; Tzolova-Mueller, G.; Schloegl, R.; Trunschke, A. J. Am. Chem. Soc. 2012, 134, 11462− 11473. (33) Rouen, M.; Queval, P.; Borre, E.; Falivene, L.; Poater, A.; Berthod, M.; Hugues, F.; Cavallo, L.; Basle, O.; Olivier-Bourbigou, H.; Mauduit, M. ACS Catal. 2016, 6, 7970−7976.
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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REFERENCES
(1) Schrock, R. R.; Copéret, C. Organometallics 2017, DOI: 10.1021/ acs.organomet.6b00825. (2) For a perceptive analysis, accompanied by examples from earlytransition-metal chemistry, see: Hirsekorn, K. F.; Veige, A. S.; Marshak, M. P.; Koldobskaya, Y.; Wolczanski, P. T.; Cundari, T. R.; Lobkovsky, E. B. J. Am. Chem. Soc. 2005, 127, 4809−4830. (3) Solans-Monfort, X.; Copéret, C.; Eisenstein, O. Organometallics 2015, 34, 1668−1680. (4) Sues, P. E.; John, J. M.; Schrock, R. R.; Müller, P. Organometallics 2016, 35, 758−761. (5) A representative statement: “Almost certainly it is best to view these species as Ru(II) complexes.” Schrock, R. R. J. Chem. Soc., Dalton Trans. 2001, 2541−2550. (6) Occhipinti, G.; Jensen, V. R. Organometallics 2011, 30, 3522− 3529. (7) de Brito Sá, E.; Rodríguez-Santiago, L.; Sodupe, M.; SolansMonfort, X. Organometallics 2016, 35, 3914−3923. (8) Thompson, R.; Nakamaru-Ogiso, E.; Chen, C.-H.; Pink, M.; Mindiola, D. J. Organometallics 2014, 33, 429−432. (9) Zhou, J.; Li, T.; Maron, L.; Leng, X.; Chen, Y. Organometallics 2015, 34, 470−476. (10) Levine, D. S.; Tilley, T. D.; Andersen, R. A. Organometallics 2017, 36, 80−88. (11) Jamison, G. M.; Bruce, A. E.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1991, 113, 5057−5059. (12) Hill, A. F.; Ward, J. S.; Xiong, Y. Organometallics 2015, 34, 5057−5064. (13) Hou, X.; Nomura, K. J. Am. Chem. Soc. 2015, 137, 4662−4665. (14) Nadif, S. S.; Kubo, T.; Gonsales, S. A.; VenkatRamani, S.; Ghiviriga, I.; Sumerlin, B. S.; Veige, A. S. J. Am. Chem. Soc. 2016, 138, 6408−6411. (15) Jung, K.; Kang, E.-H.; Sohn, J.-H.; Choi, T.-L. J. Am. Chem. Soc. 2016, 138, 11227−11233. (16) Jana, A.; Grela, K. Org. Lett. 2017, 19, 520−523. (17) McClennan, W. L.; Rufh, S.; Lummiss, J. A. M.; Fogg, D. E. J. Am. Chem. Soc. 2016, 138, 14668−14677. (18) Bates, J. M.; Lummiss, J. A. M.; Bailey, G. A.; Fogg, D. E. ACS Catal. 2014, 4, 2387−2394. (19) Griffiths, J. R.; Keister, J. B.; Diver, S. T. J. Am. Chem. Soc. 2016, 138, 5380−5391. (20) Perfetto, A.; Costabile, C.; Longo, P.; Grisi, F. Organometallics 2014, 33, 2747−2759. (21) Herbert, M. B.; Suslick, B. A.; Liu, P.; Zou, L.; Dornan, P. K.; Houk, K. N.; Grubbs, R. H. Organometallics 2015, 34, 2858−2869. (22) Torker, S.; Koh, M. J.; Khan, R. K. M.; Hoveyda, A. H. Organometallics 2016, 35, 543−562. (23) Pazio, A.; Wozńiak, K.; Grela, K.; Trzaskowski, B. Organometallics 2015, 34, 563−570. (24) Schowner, R.; Frey, W.; Buchmeiser, M. R. J. Am. Chem. Soc. 2015, 137, 6188−6191. (25) Lam, J. K.; Zhu, C.; Bukhryakov, K. V.; Müller, P.; Hoveyda, A.; Schrock, R. R. J. Am. Chem. Soc. 2016, 138, 15774−15783. (26) Takahira, Y.; Morizawa, Y. J. Am. Chem. Soc. 2015, 137, 7031− 7034. (27) Harrison, D. J.; Daniels, A. L.; Korobkov, I.; Baker, R. T. Organometallics 2015, 34, 5683−5686. (28) Lwin, S.; Wachs, I. E. ACS Catal. 2017, 7, 573−580. (29) Howell, J. G.; Li, Y.-P.; Bell, A. T. ACS Catal. 2016, 6, 7728− 7738. (30) Lwin, S.; Wachs, I. E. ACS Catal. 2014, 4, 2505−2520. (31) Grekov, D.; Bouhoute, Y.; Szeto, K. C.; Merle, N.; De Mallmann, A.; Lefebvre, F.; Lucas, C.; Del Rosal, I.; Maron, L.; Gauvin, 1883
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