Pushing Electrons—Which Carbene Ligand for Which Application?
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Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany
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The isolation and scrutiny of metal carbene complexes is one of the core disciplines of organometallic chemistry and hence lies at the heart of Organometallics. Research has focused for many years on the development of carbon atom transfer reactivity after Fischer’s discovery of heteroatom-stabilized nucleophilic carbene complexes. (1) These efforts culminated in the development of Tebbe’s reagent (2) for methylene transfer as well as the application of carbene catalysts for cyclopropanation, (3) C–H insertion, (4) and the Nobel Prize winning olefin metathesis reaction. (5)
The field started to pick up even more speed with the isolation of the first stable acyclic free carbene, reported by Bertrand in the 1980s. (6) Synthesis of Arduengo’s crystalline N-heterocyclic carbene (NHC) in 1991, also known as “bottleable carbene”, eventually demonstrated that persistent carbenes are not merely laboratory curiosities. (7) Following these seminal discoveries, heterocyclic carbene ligands were introduced as ancillary ligands for catalysis, and the number of reports on NHC metal complexes skyrocketed. The success story of NHC ligands was, and still is, strongly connected with the names of Herrmann and Nolan. (8) Subsequently, further applications emerged in the fields of light/energy conversion and drug design. (9) Computational chemistry proved essential for the development of new carbenes and offered tools to decipher their electronic structures, causing carbene stability and reactivity trends. (11) After the early days of transition metal–NHC chemistry, it was again Bertrand’s group who was pushing the field, and within a few years, they isolated a remarkable series of stable acyclic and cyclic free carbenes. (10) These “unconventional” carbenes and derivatives showed a huge diversity of electronic properties, which was unprecedented for conventional NHCs. Currently, this new class of carbenes finds increasing application in organometallic coordination chemistry. Outstanding examples include the stabilization of radicals and low-valent complexes by cyclic alkyl amino carbene (CAAC) ligands or the isolation of metal complexes with nucleophilic carbene ligands derived from carbodiphosphoranes and carbodicarbenes. In fact, “classic” Schrock-type carbenes also are experiencing a remarkable renaissance. For instance, heterogeneous Schrock carbenes were shown to catalyze the metathesis of alkanes, (12) and smart ligand design (13) allowed for the development of stereoselective olefin metathesis catalysts.
Historically, traditional NHC ligands were believed to be pure σ-donors. (14) Accordingly, differences to Fischer- and especially Schrock-type carbenes were overemphasized, and they were classified as a new type of ligand. However, spotting the true differences between carbene ligands is very difficult, complex, and proved to be a challenging task. (15) Can diarylcarbenes be considered Schrock carbenes? Is a mesoionic carbene really a carbene or rather a heteroaryl-substituent? Is a particular carbene ligand a surprisingly strong π-acceptor (or is this behavior not surprising at all)? Is it a carbene, carbenoid, or carbocation? Is it a Schrock carbene, alkylidene, or nucleophilic carbene complex? How important are electrostatic considerations? What is the oxidation state of the coordinating metal and where are the electrons? Do electrophilic Schrock carbenes exist? A multitude of questions are waiting to be answered by a comprehensive tutorial (DOI: 10.1021/acs.organomet.7b00720).
Dominik Munz is an excellent choice for the assembly of such a tutorial review, because his academic education allowed him to obtain insights from several varying perspectives on carbene chemistry. During his graduate studies at TU Dresden, he studied the application of late transition metal NHC complexes for catalysis and the molecular modeling of the electronic properties of NHC complexes. In the group of Thomas Strassner in Dresden, he studied applications of NHC and mesoionic carbene complexes as light-emitting or -harvesting molecules. After a 1 year stint with Brent Gunnoe at the University of Virginia in the field of C–H bond activation, he continued with carbene research as a postdoctoral fellow in Guy Bertrand’s lab at UC San Diego. There, he worked on the synthesis of novel carbenes with small singlet–triplet gaps, functionalized CAAC ligands, and stable carbene radicals, as well as p-block carbene analogs. Currently, his research interests include the synthesis of carbene compounds of the electropositive metals as well as the application of ancillary carbene ligands for the stabilization of reactive transition metal intermediates.
I believe that this tutorial gives a concise and comprehensible introduction on the electronic structure of carbene ligands and helps to unveil how electronic effects translate into chemical behavior. The conceptual analysis of a variety of carbene ligands and their coordination chemistry can serve as an expedient guide to understand design principles and choice of specific carbene ligand for a particular application. Thus, I hope that chemists will be inspired to hop into the adventurous playground of carbene ligands, “push electrons”, and to discover the vast worlds beyond conventional NHCs and classic Fischer or Schrock carbene ligands.
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- Karsten Meyer, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany,
http://orcid.org/0000-0002-7844-2998,
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This article references 15 other publications.
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Not quite: Cyclic (alkyl)(amino)carbenes (CAACs) are strong σ-donors and have steric environments that differ dramatically from those of bulky, electron-rich phosphines and cyclic diaminocarbenes. These readily available carbenes are desirable ligands for catalysis, as illustrated by the palladium-mediated α-arylation of ketones and aldehydes with non-activated aryl chlorides at ambient temp.(c) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2006, 312, 722– 724 DOI: 10.1126/science.1126675Google ScholarThere is no corresponding record for this reference.(d) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. 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However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the pos. polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.(e) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206– 3209 DOI: 10.1002/anie.200705620Google Scholar10ehttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXlvVCrsrY%253D&md5=6fa04f9719bd47c20d7996f900fd6645Synthesis of an extremely bent acyclic allene (A "carbodicarbene"): a strong donor ligandDyker, C. Adam; Lavallo, Vincent; Donnadieu, Bruno; Bertrand, GuyAngewandte Chemie, International Edition (2008), 47 (17), 3206-3209CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Pushed to the limit: Pushing C=Cπ bonds to the breaking point by using a push-push substitution pattern forces allenes to bend (see structure; C light blue, N dark blue). An acyclic allene featuring a C=C=C bond angle of 134.8° has been isolated in which the typically sp-hybridized central carbon atom approaches a configuration that has two lone pairs of electrons, and acts as a very strong η1-donor ligand for transition metals.(f) Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5411– 5414 DOI: 10.1002/anie.200801176Google ScholarThere is no corresponding record for this reference.(g) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556– 559 DOI: 10.1126/science.1178206Google ScholarThere is no corresponding record for this reference.(h) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. 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- 15Mindiola, D. J.; Scott, J. Nat. Chem. 2011, 3, 15– 17 DOI: 10.1038/nchem.940Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3M%252Fks1eksw%253D%253D&md5=e460075cc63199f8e7a7470046a504beCarbenes and alkylidenes: Spot the differenceMindiola Daniel J; Scott JenniferNature chemistry (2011), 3 (1), 15-7 ISSN:.There is no expanded citation for this reference.
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- 8(a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290– 1309 DOI: 10.1002/1521-3773(20020415)41:8<1290::AID-ANIE1290>3.0.CO;2-YGoogle Scholar8ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38Xjt1ertbY%253D&md5=1e485792878215285d7a531ef505d020N-heterocyclic carbenes. Part 31. N-heterocyclic carbenes: A new concept in organometallic catalysisHerrmann, Wolfgang A.Angewandte Chemie, International Edition (2002), 41 (8), 1290-1309CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)A review. N-Heterocyclic carbenes have become universal ligands in organometallic and inorg. coordination chem. They not only bind to any transition metal, be it in low or high oxidn. states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chem., N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of org. syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphines will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chem. has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic labs. and in industry, a revolutionary turning point in organometallic catalysis is emerging.(b) Rovis, T.; Nolan, S. Synlett 2013, 24, 1188– 1189 DOI: 10.1055/s-0033-1339192Google ScholarThere is no corresponding record for this reference.
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Not quite: Cyclic (alkyl)(amino)carbenes (CAACs) are strong σ-donors and have steric environments that differ dramatically from those of bulky, electron-rich phosphines and cyclic diaminocarbenes. These readily available carbenes are desirable ligands for catalysis, as illustrated by the palladium-mediated α-arylation of ketones and aldehydes with non-activated aryl chlorides at ambient temp.(c) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2006, 312, 722– 724 DOI: 10.1126/science.1126675Google ScholarThere is no corresponding record for this reference.(d) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2007, 316, 439– 441 DOI: 10.1126/science.1141474Google Scholar10dhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXktlSkurw%253D&md5=c502ad5b8511938cb91fd13d49493491Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon centerFrey, Guido D.; Lavallo, Vincent; Donnadieu, Bruno; Schoeller, Wolfgang W.; Bertrand, GuyScience (Washington, DC, United States) (2007), 316 (5823), 439-441CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)In possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers and thus could potentially mimic their chem. behavior. Although singlet di(amino)carbenes are inert toward dihydrogen, it is shown that more nucleophilic and electrophilic (alkyl)(amino)carbenes can activate H2 under mild conditions, a reaction that has long been known for transition metals. However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the pos. polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.(e) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206– 3209 DOI: 10.1002/anie.200705620Google Scholar10ehttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXlvVCrsrY%253D&md5=6fa04f9719bd47c20d7996f900fd6645Synthesis of an extremely bent acyclic allene (A "carbodicarbene"): a strong donor ligandDyker, C. Adam; Lavallo, Vincent; Donnadieu, Bruno; Bertrand, GuyAngewandte Chemie, International Edition (2008), 47 (17), 3206-3209CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)Pushed to the limit: Pushing C=Cπ bonds to the breaking point by using a push-push substitution pattern forces allenes to bend (see structure; C light blue, N dark blue). An acyclic allene featuring a C=C=C bond angle of 134.8° has been isolated in which the typically sp-hybridized central carbon atom approaches a configuration that has two lone pairs of electrons, and acts as a very strong η1-donor ligand for transition metals.(f) Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5411– 5414 DOI: 10.1002/anie.200801176Google ScholarThere is no corresponding record for this reference.(g) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556– 559 DOI: 10.1126/science.1178206Google ScholarThere is no corresponding record for this reference.(h) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. 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Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.
- 11(a) Frenking, G.; Solà, M.; Vyboishchikov, S. F. J. Organomet. Chem. 2005, 690, 6178– 6204 DOI: 10.1016/j.jorganchem.2005.08.054Google Scholar11ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXht1CmtbrP&md5=67c6e3f0a8a95ea2ddeaa55961c456bdTheoretical studies of organometallic compounds. 53. Chemical bonding in transition metal carbene complexesFrenking, Gernot; Sola, Miquel; Vyboishchikov, Sergei F.Journal of Organometallic Chemistry (2005), 690 (24-25), 6178-6204CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)A review. The authors summarize recent theor. studies of their groups in which modern quantum chem. methods were used to gain insight into the nature of metal-ligand interactions in Fischer- and Schrock-type carbene complexes. With the help of charge- and energy-partitioning techniques it is possible to build a bridge between heuristic bonding models and the phys. mechanism which leads to a chem. bond. Questions about the bonding situation which in the past were often controversially discussed because of vaguely defined concepts may be addressed in terms of well defined quantum chem. expressions. The results of the partitioning analyses show that Fischer and Schrock carbenes exhibit different bonding situations, which are clearly revealed by the calcd. data. The contributions of the electrostatic and the orbital interaction are estd. and the strength of the σ donor and π acceptor bonding in Fischer complexes are discussed. The authors also discuss the bonding situation in complexes with N,N-heterocyclic carbene ligands.(b) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L. Coord. Chem. Rev. 2009, 253, 687– 703 DOI: 10.1016/j.ccr.2008.06.006Google ScholarThere is no corresponding record for this reference.
- 12(a) Schrock, R. R.; Copéret, C. Organometallics 2017, 36, 1884– 1892 DOI: 10.1021/acs.organomet.6b00825Google ScholarThere is no corresponding record for this reference.(b) Basset, J.-M.; Copéret, C.; Soulivong, D.; Taoufik, M.; Cazat, J. T. Acc. Chem. Res. 2010, 43, 323– 334 DOI: 10.1021/ar900203aGoogle ScholarThere is no corresponding record for this reference.(c) Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. Chem. Res. 2012, 45, 947– 958 DOI: 10.1021/ar3000713Google ScholarThere is no corresponding record for this reference.
- 13(a) Koh, M. J.; Nguyen, T. T.; Lam, J. K.; Torker, S.; Hyvl, J.; Schrock, R. R.; Hoveyda, A. H. Nature 2017, 542, 80– 85 DOI: 10.1038/nature21043Google Scholar13ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsFGlt74%253D&md5=440042192b1ae0babbd6cb2f986f76d3Molybdenum chloride catalysts for Z-selective olefin metathesis reactionsKoh, Ming Joo; Nguyen, Thach T.; Lam, Jonathan K.; Torker, Sebastian; Hyvl, Jakub; Schrock, Richard R.; Hoveyda, Amir H.Nature (London, United Kingdom) (2017), 542 (7639), 80-85CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)The development of catalyst-controlled stereoselective olefin metathesis processes has been a pivotal recent advance in chem. The incorporation of appropriate ligands within complexes based on molybdenum, tungsten and ruthenium has led to reactivity and selectivity levels that were previously inaccessible. Here we show that molybdenum monoaryloxide chloride complexes furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the com. available, inexpensive and typically inert Z-1,1,1,4,4,4-hexafluoro-2-butene. Furthermore, otherwise inefficient and non-stereoselective transformations with Z-1,2-dichloroethene and 1,2-dibromoethene can be effected with substantially improved efficiency and Z selectivity. The use of such molybdenum monoaryloxide chloride complexes enables the synthesis of representative biol. active mols. and trifluoromethyl analogs of medicinally relevant compds. The origins of the activity and selectivity levels obsd., which contradict previously proposed principles, are elucidated with the aid of d. functional theory calcns.(b) Nguyen, T. T.; Koh, M. J.; Shen, X.; Romiti, F.; Schrock, R. R.; Hoveyda, A. H. Science 2016, 352, 569– 575 DOI: 10.1126/science.aaf4622Google Scholar13bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xms1KisLw%253D&md5=f623cfa88cf87b371ec03684bdb8dec3Kinetically controlled E-selective catalytic olefin metathesisNguyen, Thach T.; Koh, Ming Joo; Shen, Xiao; Romiti, Filippo; Schrock, Richard R.; Hoveyda, Amir H.Science (Washington, DC, United States) (2016), 352 (6285), 569-575CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)In the presence of 1-5 mol% of molybdenum phenolate pyrrolide carbene complexes, alkenes (particularly aryl alkenes and hindered aliph. alkenes) underwent diastereoselective cross-metathesis reactions with (E)-1,2-dichloroethene and (E)-1-chloro-2-fluoroethene at ambient temp. to provide the thermodynamically disfavored (E)-alkenyl chlorides and fluorides in high yields and diastereoselectivities. The molybdenum carbene complexes may be delivered in the form of air- and moisture-stable paraffin pellets, allowing reactions to be performed without using a glovebox. Functionalized drugs and natural product analogs were prepd. by the method; the method was used to prep. a fragment of the natural product kimbeamide A and an intermediate in the synthesis of pitinoic acid B.
- 14Hu, X.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics 2004, 23, 755– 764 DOI: 10.1021/om0341855Google ScholarThere is no corresponding record for this reference.
- 15Mindiola, D. J.; Scott, J. Nat. Chem. 2011, 3, 15– 17 DOI: 10.1038/nchem.940Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC3M%252Fks1eksw%253D%253D&md5=e460075cc63199f8e7a7470046a504beCarbenes and alkylidenes: Spot the differenceMindiola Daniel J; Scott JenniferNature chemistry (2011), 3 (1), 15-7 ISSN:.There is no expanded citation for this reference.