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Catalytic Transfer Functionalization through Shuttle Catalysis Benjamin N. Bhawal, and Bill Morandi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02333 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016
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Catalytic Transfer Functionalization through Shuttle Catalysis Benjamin N. Bhawal and Bill Morandi* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany KEYWORDS catalysis, reversible, shuttle catalysis, alkene, isodesmic reactions
ABSTRACT: In this perspective, we describe an emerging type of catalysis that enables the catalytic reversible transfer of chemical entities beyond the well-established transfer hydrogenation reactions. Shuttle catalysis facilitates the transfer of small molecules (e.g. CO, HCN) or reactive intermediates between two substrates in an isodesmic process. In many cases, these often safer processes provide unprecedented synthetic flexibility and complement other catalytic bond-forming and bond-breaking reactions.
Introduction Catalytic reversible reactions have had an auspicious impact on organic synthesis. For example, the versatility of the alkene metathesis reaction has enabled the development of numerous, novel synthetic strategies, opening new avenues in retrosynthesis and leading to the improved preparation of bioactive compounds and materials.1 Another type of catalytic reversible reaction, transfer hydrogenation, has had a marked influence on organic synthesis and offers a simple, gas-free approach to shuttle a hydrogen molecule between two substrates (Scheme 1a).2 Using this strategy, both the hydrogenation and dehydrogenation of a substrate can be performed under similar reaction conditions, and the power of this reaction is best highlighted by the asymmetric transfer hydrogenation protocol developed by Noyori.3 The ability of this process to reversibly transfer a hydrogen molecule between a metal catalyst and an alcohol to transiently generate an activated functional group (e.g. a ketone) has also been used to alter in situ the reactivity profile of a given molecule in borrowing hydrogen reactions4 that have led to powerful and sustainable catalytic C–C and C–N bond forming reactions using simple alcohols. While transfer hydrogenation has become a powerful tool to shuttle hydrogen in organic synthesis, one could imagine that a diverse set of catalytic reversible transfer functionalization reactions could be developed through the catalytic shuttling of molecules beyond hydrogen (Scheme 1b). Such reactions involve the transfer of a chemical entity from a donor molecule to an acceptor molecule through shuttle catalysis. These reactions can be viewed as a particular subclass of group-transfer reactions wherein the transfer process effectively transforms the donor molecule into an acceptor molecule. In theory, these isodesmic reactions, in which all the bonds present on the reactant side are the same as on the product side
Scheme 1 – Transfer hydrogenation and schematic generalization of the shuttle catalysis concept
of the reaction, should be reversible. These reactions are thus distinct from other irreversible group-transfer reactions, such as epoxidation proceeding through metalmediated oxo transfer from an unstable reagent such as iodosylbenzene.5 In this reaction, the cleavage of a weak iodine-oxygen bond and the formation of an epoxide moiety make the reaction highly exergonic and, as a consequence, irreversible. In principle, many different chemical groups or compounds (henceforth described as the shuttled group) can be transferred between the donor and the acceptor either in the forward process, wherein a simple and inexpensive sacrificial donor molecule is used to functionalize acceptor molecules, or in the reverse process, wherein donor molecules are defunctionalized using a common sacrificial acceptor (Scheme 2). Shuttle catalysis is
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particularly attractive when the shuttled group is either a toxic or highly reactive compound (e.g. HCN, HF) or an unstable reactive intermediate (e.g. oxo, nitrene, carbene). Alkenes are obvious candidates for acceptor molecules but other systems that can accept a transferable group can also be employed.
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Another obstacle in developing these reactions is to find a catalytic system that can overcome all the kinetic barriers to shuttle the desired group, usually through a sequence of reversible elementary steps that proceed under mild reaction conditions. In particular, the development of these exchange reactions often rests on the ability to develop efficient elementary organometallic steps that can reversibly form stable bonds. For example, general and mild ways to mediate reversible oxidative additions and migratory insertions is essential for shuttle catalysis reactions to be developed.8 In the next section, we have selected examples from the literature that fit the criteria of shuttle catalysis,9 discussed key features of the reactions and unified them under a common umbrella to help the reader grasp the essential aspects of this topic. Although transfer hydrogenation would fall under the auspices of shuttle catalysis, it has been extensively reviewed and thus will not be discussed.2 Additionally, traditional exchange processes involving carbonyl derivatives, such as transacetalization, transesterification or transamidation10 will not be discussed.
Scheme 2 – Definition of forward and reverse shuttle catalysis reactions
In the forward reaction, shuttle catalysis enables the facile introduction of functional groups into substrates to prepare value added products. This feature is particularly attractive when polar functional groups can be introduced into simple starting hydrocarbon feedstocks, such as alkenes, and is likely to provide a useful accompaniment to other catalytic bond forming reactions.6 Alternatively, the reverse reaction, wherein the removal of a group is achieved through a bond breaking process, can contribute to the valorization of biomass or waste materials through defunctionalization.7 The reverse reaction also enables the use of organic functionalities as removable activating groups for the formation of challenging bonds in targetoriented synthesis. A challenge in shuttle catalysis is the control of the position of the equilibrium to favor either the forward or the reverse reaction to avoid obtaining mixtures of products and starting materials (Scheme 2). Similar to the strategies used in alkene metathesis,1 the formation and release of a gas, the release of ring strain or the formation of stabilized products (e.g. conjugated systems) can provide efficient driving forces. Alternative means of physical separation, such as the formation of an insoluble sideproduct (e.g. polymer), can also drive the reaction. Ideally, to improve the atom-economy of the reactions, the sacrificial donor or acceptor molecule should be a low molecular weight, inexpensive bulk chemical that can, if necessary, be used in excess to drive the reaction to high conversion. A prototypical example is the use of isopropanol as solvent and sacrificial reductant in the transfer hydrogenation of ketones.2
Selected Examples of Shuttle Catalysis Reactions Using metal-organic cooperative catalysis, Jun and coworkers were able to conduct the hydroacylation of alkenes using aldehydes11 and in an extension of this work they developed a related hydroacylation wherein an aldehyde group is formally transferred from a ketone (Scheme 3).12a The reaction facilitates the functionalization of monosubstituted and cyclic alkene acceptors employing ketones as sacrificial donors. Although an excess of the acceptor alkene is typically used, the polymerization of the styrene by-product or the release of ring strain are other productive driving forces.12
Scheme 3 – Metal-organic cooperatively catalyzed hydroacylation of alkenes by transfer of an aldehyde
This reaction is enabled by 2-amino-3-picoline which upon formation of the imine intermediate serves as a directing group for the challenging rhodium mediated
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C–C cleavage of the unstrained ketone (Scheme 4). β-hydride elimination produces the styrene by-product which undergoes ligand exchange with the acceptor alkene. The rhodium catalytic cycle is completed by insertion of first the hydride then the imino-acyl group. Hydrolysis of the imine yields the desired ketone product and also releases the 2-amino-3-picoline co-catalyst.
conditions (Scheme 5).15 As part of their strategy, they noted that the insertion of rhodium complexes into aldehyde C–H bonds is well documented and has been exploited for the catalytic hydroacylation of alkenes11 and the decarbonylation of aldehydes.16 As a consequence, they decided to encourage the desired retrohydroformylation by transferring the aldehyde group and β-hydrogen atom from the aldehyde to a sacrificial acceptor, in this case norbornadiene, and concurrently producing the corresponding alkene. Optimization studies revealed that the 3-methoxybenzoate counteranion plays a crucial role in both the efficiency of the reaction and the selectivity between the desired retrohydroformylation and the undesired decarbonylation.
Scheme 4 – Proposed mechanism for the hydroacylation of alkenes using sacrificial ketone donors (ligands omitted for clarity)
This transformation constitutes a seminal example of C–C bond activation and is one of the first examples of shuttle catalysis beyond hydrogen transfer. It has, though, found only limited use in organic synthesis because the shuttled group, an aldehyde, is a stable, innocuous and largely available class of chemical compound. There is thus no obvious synthetic advantage to using a ketone as an aldehyde donor as an alternative to traditional hydroacylation.11 Extension of this work to the reverse reaction, retro-hydroacylation, could find applications in the cleavage of carbon skeletons if the undesired isomerization of the resulting alkene could be prevented. While the catalytic hydroformylation reaction is well known and constitutes one of the most important industrial reactions,13 the reverse retro-hydroformylation has received scarce attention. However, lanosterol demethylase, a cytochrome P450 enzyme, is adept at performing this transformation14 and inspired by this Dong and co-workers pursued a rhodium-catalyzed retrohydroformylation procedure which operates under mild
Scheme 5 – Rhodium-catalyzed retrohydroformylation of aldehydes
A short semi-synthesis of (+)-yohimbenone was also realized by retro-hydroformylation of an aldehyde that could be prepared easily from commercially available (+)-yohimbine (Scheme 6). When the aldehyde was subjected to the reaction conditions, retrohydroformylation proceeded smoothly giving the desired regioselectivity for the formation of the alkene, presumably due to the absence of a syn β-hydrogen at the other position. Fortuitously, concomitant oxidation was also observed thus affording the desired (+)-yohimbenone without need for a further oxidation step. This highly efficient synthesis illustrates the power of shuttle catalysis as a tool for the degradation of complex molecules to generate value-added products. Following mechanistic studies, which highlighted the role of the benzoate counteranion, the authors proposed a catalytic cycle wherein the rhodium catalyst initially
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Scheme 6 – Application of the retro-hydroformylation reaction to the synthesis of Yohimbenone
Scheme 7 – Mechanism of the retro-hydroformylation reaction
inserts across the aldehyde C-H bond, generating a rhodi- um hydride species which undergoes reductive elimination with the benzoate ligand (Scheme 7). Subsequent, decarbonylation and β-hydride elimination leads to formation of the alkene product, which undergoes ligand exchange with norbornadiene. The microscopic reverse, namely hydride insertion, carbonylation, reassociation of the benzoic acid and subsequent reductive elimination gives the hydroformylated norbornadiene. This presumably acts as a driving force for the reaction due to the release of ring strain but the authors also propose that it helps to prevent catalyst poisoning by carbon monoxide. Kinetic control is observed in these reactions as the formation of the hydroformylated norbornadiene is likely to be irreversible because regeneration of the highly strained norbornadiene is unfavorable. Additionally, norbornadiene is probably more reactive than the product effectively preventing the formed alkene from reacting with the hydrido-carbonyl complex. Thus, kinetic control is possible under shuttle catalysis if the products of the reaction are unreactive under the reaction conditions. This process developed by Dong and co-workers offers a powerful and functional group tolerant tool for the synthesis of alkenes from aldehydes and is likely to emerge as a useful complement to hydroformylation processes. Extension of this concept to the forward shuttle reaction7 could be of interest because it could provide a convenient alternative to the use of syngas, paraformaldehyde and other carbon monoxide sources for laboratory-scale hydroformylation reactions.18
In a recent contribution, Morandi and co-workers reported a nickel-catalyzed transfer hydrocyanation reaction that efficiently interconverts alkenes and nitriles (Scheme 8).19 In the forward reaction, namely the transfer hydrocyanation of alkenes, this approach eludes the need to employ the highly toxic and volatile reagent hydrogen cyanide because the HCN molecule is directly transferred from the donor nitrile to the acceptor alkene. The method thus provides a safer approach to hydrocyanation reactions in comparison to the traditional approaches.20 This reaction is also a rare example of shuttle catalysis wherein the direction of the equilibrium can be fully controlled using simple driving forces to undergo either the functionalization or defunctionalization process with a broad set of structurally varied substrates.
Scheme 8 – Catalytic reversible transfer hydrocyanation
The forward process, alkene hydrocyanation, can be favored using a simple sacrificial donor molecule, isovaleronitrile, which is transformed into a volatile byproduct, isobutene, effectively driving the hydrocyanation
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of a range of alkene acceptors (Scheme 9). Styrene and heterostyrene substrates provided high yield of the corresponding alkyl cyanides with selectivity (4:1-20:1) for the formation of the linear product. This selectivity reflects the thermodynamic minimum of the system and is complementary to the Markovnikov selectivity typically obtained in traditional reactions using HCN gas or surrogates.20 The reaction also tolerated several aliphatic substrates, giving a broad range of structurally varied nitriles in excellent yields. The late-stage transfer hydrocyanation of bioactive starting materials was also demonstrated using terpene and amino acid derivatives. For larger scale applications, the inexpensive reagent butyronitrile can also be used as reagent and solvent.
Scheme 10 – Retro-hydrocyanation, the reverse process
Scheme 9 – Alkene transfer hydrocyanation, the forward process
The reverse process, retro-hydrocyanation, can be accomplished using norbornadiene as a sacrificial acceptor to drive the reaction to completion due to the release of its ring strain (Scheme 10). Under these conditions, the process is likely not reversible because of the high reactivity of norbornadiene as an acceptor, similar to Dong’s retro-hydroformylation process (vide supra). A range of styrene products could be produced through the retro-hydrocyanation of primary, secondary and even tertiary alkyl nitriles. The synthesis of aliphatic alkenes was also very effective, and most of the products were obtained from the corresponding nitriles with high selectivity. The potential of this defunctionalization reaction was also demonstrated in the use of the nitrile group as a removable activating group for the construction of C–C bonds. For example, a quaternary vinyl group could be installed by sequential radical C–C bond forming reaction with acrylonitrile21 followed by retrohydrocyanation (Scheme 10).
Scheme 11 – Mechanism of the transfer hydrocyanation (forward process shown)
Mechanistic experiments, including deuteriumlabelling, support the depicted reaction mechanism. For the forward reaction, initial oxidative addition of the C–CN bond in isovaleronitrile is followed by β-hydride elimination, and ligand exchange expelling the isobutene by-product (Scheme 11). The subsequent steps mirror the mechanism of the traditional hydrocyanation and are the reverse of the initial two steps, proceeding through migra-
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tory hydride insertion and reductive elimination. It is believed that both oxidative addition and reductive elimination might be facilitated through weakening of both C–CN and Ni–CN bonds by coordination to the Lewis Acid.22 Overall, the transformation enables the synthesis of a wide range of alkyl nitriles and alkenes using easily accessible reagents and should thus find broad use in synthesis. The undesired alkene isomerization observed in the forward hydrocyanation process remains a limitation that will have to be addressed. Inspired by earlier work from Cooper and Finkbeiner,23 Hayashi and co-workers developed a transfer hydromagnesiation reaction between alkyl Grignard reagents and alkenes (Scheme 12).24 Initially, they observed the isomerization of secondary alkyl Grignard reagents to the more stable primary Grignard reagents under Cu-Fe cooperative catalysis.25 They later exploited this reactivity in the shuttling of a HMgBr species between a donor Grignard reagent and a set of acceptor alkenes in the transfer hydromagnesiation.24 During the optimization of this process, they noticed that cyclopentyl magnesium bromide in particular was a highly efficient donor and only 1 equivalent was required to fully convert 1-hexene to the corresponding product. Using their optimized procedure, a range of terminal alkenes could be selectively converted to the primary Grignard reagent and subsequently trapped with an electrophile. Remarkably, a range of products not easily accessible through direct Grignard formation (i.e. from the insertion of elemental magnesium into C–halogen bonds) could be synthesized.
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Thus, a hydroxyl group and a chloride were tolerated by the transfer method. A diene also selectively reacted without any formation of the radical cyclization product, a challenging task using traditional approaches. The Grignard reagents accessed using this method were trapped efficiently in a one-pot process with silyl chlorides, CO2, aldehydes and iodine, thus showcasing the broad applicability of the reaction. In subsequent work, Thomas and co-workers harnessed this concept in the functionalization of styrene derivatives with carbon dioxide to obtain branched carboxylic acids selectively (Scheme 13).26 The system developed relies on a simple FeCl2 pre-catalyst in the presence of a tridentate bis(imino)pyridine ligand. A key step in the mechanism is the reversible transmetallation step between the Fe complex and the Grignard reagents. The driving force in the process is the release of a gas, ethylene. HMgBr
H Ar
+ BrMg
acceptor
shuttled group FeCl2 (1 mol%) ligand (1 mol%)
MgBr
THF, r.t. 15 examples 26-93%
sacrificial donor
H +
Ar
driving force gas evolution
Reaction mechanism LFeCl2
MgBr H
MgX2
Ph MgBr
Ph
H
Fe H
MgBr H
Ph
Ph Fe
Fe
H
H
RDS Ph Fe H
Scheme 13 – Catalytic transfer hydromagnesiation of styrenes and mechanism
Overall, these transfer hydromagnesiations show that reversible transmetallation can unlock an exchange reaction between alkenes and organometallic reagents. Extension of this strategy to other organometallic reagents could thus provide a facile preparation of synthetically versatile organometallic reagents and lead to powerful one-pot reactions. In a broader context, this transformation highlights the potential of shuttle catalysis to facilitate the preparation of highly reactive compounds. Scheme 12 – Catalytic transfer hydromagnesiation. a Yields after trapping with ClSiH2Ph
Following their initial work demonstrating silacyclopropanes as versatile reagents in organic synthesis,27 Woerpel and co-workers have developed a practical transfer silacyclopropanation reaction (Scheme 14). Initially,
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they disclosed a thermally induced silylene shuttle process between a donor silacyclopropane, cyclohexylsilacyclopropane, and a series of diverse alkene acceptors.28 Further research identified a silver catalyst that lowers the activation energy of the reaction and enables the silylene transfer at low temperature.29 A broad range of alkenes could be used as acceptors under these reaction conditions. The authors also took advantage of this process to develop a one-pot, two-step procedure to forge a new carbon-carbon bond through sequential transfer silacyclopropanation/formate insertion.29
Scheme 15 – Mechanism of the transfer silacyclopropanation
Scheme 14 – Catalytic transfer silacyclopropanation
The uncatalyzed transfer reaction is thought to proceed through reversible thermal cleavage of a silacyclopropane to form a reactive silylene species and an alkene. However, for the catalyzed process, the silver triflate initially cleaves the strained Si–C bond followed by β-silyl elimination to produce cyclohexene (Scheme 15).30 Silacyclopropanation of the acceptor alkene occurs by the reverse of the elementary steps and yields the product. Mechanistic investigations showed that while the catalyst can react with cyclohexylsilacyclopropane, it is unreactive towards the silacyclopropane products. The silver-catalyzed process is thus kinetically controlled. Overall, this silylene transfer reaction illustrates how reactive intermediates can be shuttled between substrates and this reaction should serve as an inspiration for the development of related oxo, nitrene or carbene shuttle reactions between epoxides, aziridines or cyclopropanes and alkenes. Conclusion In this perspective, we have discussed several examples that illustrate the potential of shuttle catalysis as a complementary, reversible approach to the functionalization of organic molecules. Conceptually, it can be regarded as
an extension of the transfer hydrogenation concept and offers new approaches for installing or removing functional groups. However, with the field very much in its infancy, there is considerable scope for extending this new theme in catalysis. One advantage of this type of system is that it can avoid the use of toxic or difficult to handle reagents (e.g. HCN, CO/H2, HMgX) and this feature could potentially be exploited in a wide range of alkene transfer functionalization reactions (e.g. HF, Cl2, H2S). The reverse process could potentially find applications for the defunctionalization of biomass and waste materials to value-added products. All the examples highlighted herein use alkenes as acceptors but an obvious extension would be to apply this strategy to carbonyls and imines or any other bonds capable of accepting a transferrable group. Woerpel’s transfer silacyclopropanation should serve as an inspiration to the transfer of other reactive intermediates, such as oxo, nitrene and carbene moieties, for the synthesis of epoxides, aziridines and cyclopropanes respectively without relying on the traditional, highly reactive reagents (i.e. hypervalent iodine reagents, azides, diazo compounds). Importantly, the development of new shuttle catalysis reactions critically relies on the development of mild and efficient reversible organometallic bond-forming reactions. Furthermore, we believe that the intrinsic reversibility of shuttle catalysis reactions lays the groundwork for the design of potentially exciting and unusual transformations beyond the simple transfer process mentioned above. Recently, the area of borrowing hydrogen catalysis4 has proved fruitful in the search for new efficient bond forming reactions and could provide a blueprint for the design of “borrowing functional group” reactions. Such reactions truly taking advantage of the reversibility of the process might greatly impact organic synthesis by giving synthetic chemists new retrosynthetic disconnections. Additionally, reversible bond forming reactions can enable metathesis reactions1 and are highly sought after for applications in dynamic combinatorial libraries.31 In summary, shuttle catalysis has already led to the development of safer reactions for laboratory-scale synthesis and provides a way to transfer unstable reactive interme-
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diates under mild reaction conditions. The reversibility of these reactions gives unprecedented flexibility in targetoriented synthesis when both the forward and reverse process can be performed under similar reaction conditions. We are thus confident that shuttle catalysis will open up new strategies for the construction and deconstruction of organic molecules with unprecedented synthetic flexibility.
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AUTHOR INFORMATION Corresponding Author *
[email protected] (11)
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Funding Sources Generous funding from the Max-Planck-Society/Max-PlanckInstitut für Kohlenforschung is acknowledged. BNB thanks the Leverhulme Trust for a fellowship.
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ACKNOWLEDGMENT Dr. Zachary K. Wickens is acknowledged for critically proofreading this manuscript.
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Grubbs, R. H. Handbook of Metathesis; Wiley-VCH Verlag GmbH & Co: Weinheim, Germany, 2003. (a) Wang, D.; Astruc, D. Chem. Rev. 2015, 115, 6621-6686. (b) Brieger, G.; Nestrick, T. J. Chem. Rev. 1974, 74, 567580. (a) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562-7563. (a) Hamid, M. H. S. A.; Slatford, P. A.; Williams, J. M. J. Adv. Synth. Catal. 2007, 349, 1555-1575. (b) Bower, J. F.; Kim, I. S.; Patman, R. L.; Krische, M. J. Angew. Chem. Int. Ed. 2009, 48, 34-36. (c) Guillena, G.; J. Ramón, D.; Yus, M. Chem. Rev. 2010, 110, 1611-1641. (d) Leonard, J.; Blacker, A. J.; Marsden, S. P.; Jones, M. F.; Mulholland, K. R.; Newton, R. Org. Proc. Res. Dev. 2015, 19, 1400-1410. (e) Dobereiner, G. E.; Crabtree, R. H., Chem. Rev. 2010, 110, 681-703. Oyama, S. T. Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis; Elsevier, 2008. Beller, M.; Seayad, J.; Tillack, A.; Jiao, H. Angew. Chem. Int. Ed. 2004, 43, 3368-3398. Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 24112502. Hartwig, J. F. Organotransition Metal Catalysis; Palgrave Macmillan, 2009. Some recent exciting catalytic methodologies proceeding through formal group-transfer from activated cyclohexadienes or dihydropyridines have not been included because the mechanism by which the donor is activated is likely not applicable to the reverse reaction. For transfer hydrosilylation see: (a) Simonneau, A.; Oestreich, M. Angew. Chem. Int. Ed. 2013, 52, 11905-11907. (b) Simonneau, A.; Oestreich, M. Nat. Chem. 2015, 7, 816-822.
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