The N-heterocyclic carbene IBioxMe4 enacts selective single and double C–F bond activation of octafluorotoluene and hexafluorobenzene, respectively. The formation of the fluoroarene substituted, zwitterionic imidazoliumolate products is consistent with a mechanism involving nucleophilic aromatic substitution and subsequent oxazoline ring opening by liberated fluoride

Chaplin, Adrian B.

Emerson-King, Jack

Hauser, Simone A.

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     warwick.ac.uk/lib-publications      Original citation: Emerson-King, Jack, Hauser, Simone A. and Chaplin, Adrian B.. (2017) C–F bond activation of perfluorinated arenes by a bioxazoline-derived N-heterocyclic carbene. Organic and Biomolecular Chemistry, 15 (4). pp. 787-789.  Permanent WRAP URL: http://wrap.warwick.ac.uk/86368                                   Copyright and reuse: The Warwick Research Archive Portal (WRAP) makes this work of researchers of the University of Warwick available open access under the following conditions.  This article is made available under the Creative Commons Attribution 3.0 (CC BY 3.0) license and may be reused according to the conditions of the license.  For more details see: http://creativecommons.org/licenses/by/3.0/    A note on versions: The version presented in WRAP is the published version, or, version of record, and may be cited as it appears here.  For more information, please contact the WRAP Team at: wrap@warwick.ac.uk                Organic &Biomolecular ChemistryCOMMUNICATIONCite this: Org. Biomol. Chem., 2017,15, 787Received 22nd November 2016,Accepted 13th December 2016DOI: 10.1039/c6ob02556kwww.rsc.org/obcC–F bond activation of perﬂuorinated arenes by abioxazoline-derived N-heterocyclic carbene†Jack Emerson-King, Simone A. Hauser and Adrian B. Chaplin*The N-heterocyclic carbene IBioxMe4 enacts selective single anddouble C–F bond activation of octaﬂuorotoluene and hexaﬂuoro-benzene, respectively. The formation of the ﬂuoroarene substi-tuted, zwitterionic imidazoliumolate products is consistent with amechanism involving nucleophilic aromatic substitution and sub-sequent oxazoline ring opening by liberated ﬂuoride.In addition to finding diverse applications as ancillary ligandsin organometallic chemistry and catalysis, N-heterocyclic car-benes (NHCs) have attracted considerable attention in theirown right as organocatalysts and for their propensity forσ-bond activation.1 Of the latter, notable landmarks includethe cleavage of dihydrogen and insertion into the N–H bondsof ammonia.2 In recent years examples demonstrating the acti-vation of characteristically robust C–F bonds by NHCs havealso begun to appear and highlight a new and potentiallyfertile avenue for exploration in synthetic organic chemistry.Kuhn and co-workers were the first to describe C–F bondactivation by an NHC in 1998; reactions involving the nucleo-philic aromatic substitution of pentafluoropyridine by tetra-alkylimidazol-2-ylidenes (Scheme 1).3 This approach was laterextended to the C–F bond activation of fluoroarenes includinghexafluorobenzene, with products isolated by sequestration offluoride as [BF4]− by addition of BF3·OEt2.4 Conceptuallyrelated to transition-metal-based processes, Bertrand andco-workers went on to describe the formal oxidative addition ofa C–F bond to a cyclic alkyl amino carbene in 2015 (Scheme 1).5To the best of our knowledge the only other examples of C–Fbond activation reactions by NHCs are limited to very recentreports by Turner, Lee and Baker (Scheme 1).6–8 The former isnotable for its parallels to Bertrand’s work and for the remark-able double C–H/C–F or C–F bond activation of pentafluoro-benzene and hexafluorobenzene, respectively.As part of our work exploring the coordination chemistry ofbioxazoline-derived imidazol-2-ylidenes developed by Gloriusand co-workers,9 in particular the synthesis of low-coordinaterhodium and iridium complexes of the tetramethyl substitutedvariant IBioxMe4 in fluorinated solvents,10 we discovered thatthese NHCs can enact interesting C–F bond activation chem-istry. Whilst assessing the suitability of a range of fluoroarenesas solvents in situ using NMR spectroscopy, the chemicalScheme 1 Previous examples of C–F bond activation by NHCs. Mes =2,4,6-Me3C6H2; Dipp = 2,6-iPr2C6H3.†Electronic supplementary information (ESI) available: Full experimentaldetails, NMR spectra of isolated compounds, computational details and opti-mised geometries in xyz format. CCDC 1517965 for 1a. For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/c6ob02556kDepartment of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL,UK. E-mail: a.b.chaplin@warwick.ac.uk; http://go.warwick.ac.uk/abchaplin,http://www.twitter.com/chaplinlabThis journal is © The Royal Society of Chemistry 2017 Org. Biomol. Chem., 2017, 15, 787–789 | 787Open Access Article. Published on 22 December 2016. Downloaded on 02/03/2017 13:50:56.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issuenon-innocence of IBioxMe4 in neat perfluorinated benzeneand toluene became readily apparent by complete con-sumption of the NHC ligand and concomitant formation ofbrightly coloured red solutions within 30 minutes of dis-solution at RT. In contrast, no decomposition of IBioxMe4 wasapparent by 1H NMR spectroscopy after 12 h at RT using arange of partially fluorinated benzene solvents: fluorobenzene,1,2-difluorobenzene and 1,3,5-trifluorobenzene – solvents thatinstead resulted in homogeneous colourless solutions.Seeking to understand these observations, IBioxMe4 wasreacted with 5 equiv. of octafluorotoluene at RT, using 1,2-difluorobenzene as an inert solvent. Analysis in situ by 1H and19F NMR spectroscopy indicated the quantitative formation ofzwitterionic imidazoliumolate 1a,11 which was subsequentlyisolated as an air and moisture sensitive crystalline solid onaddition of cyclohexane and fully characterised (32% yield;Scheme 2, Fig. 1). Based on the literature precedents discussedabove,3,4,7,8 we account for the formation of 1a by a mechan-ism involving initial nucleophilic aromatic substitution ofoctafluorotoluene by IBioxMe4, followed by ring opening ofone of the oxazolines by the liberated fluoride anion.In the solid-state structure of 1a (Fig. 1) the fluoroareneadopts an approximately orthogonal arrangement with respectto the imidazoliumolate ring (102.99(5)°), which showsbroadly similar component bonding metrics toIBioxMe4·HOTf;9c for instance the C3–C4 bond distance(1.3845(16) vs. 1.354(3) Å) and N2–C1–N5 angle (106.58(11) vs.105.55(16)°). Moreover, the exocyclic C4–O11 bond distance(1.2492(14) Å) is in close agreement with related zwitterionicimidazoliumolates (ca. 1.25 Å).11 The structure deduced byX-ray diﬀraction is fully corroborated in solution by NMR spec-troscopy. Notably 1H and 13C NMR spectra demonstratereduced symmetry compared to IBioxMe4 (Cs vs. C2v), the newH2C–F linkage is characterised by resonances at δH 4.91 (2JFH =47.2 Hz), δC 86.1 (1JFC = 176 Hz), and δF −224.59 (2JFH =47.2 Hz), and the 19F{1H} NMR spectrum features only twoC(sp2)–F resonances at δF −133.05 and −138.64.On turning to the equivalent reaction between IBioxMe4and hexafluorobenzene (5 equiv., RT in 1,2-difluorobenzene),analysis of the reaction mixture in situ by 1H and 19F NMRspectroscopy indicated the selective and quantitative for-mation of 2, as a mixture of two rotamers (ca. 1 : 1 ratio),resulting from double C–F bond activation. The new bis-imida-zoliumolate crystallises from the reaction mixture on standing,enabling its isolation as an extremely air and moisturesensitive red microcrystalline solid (19% yield). The structureof 2 was established spectroscopically in solution, withthe double C–F bond activation of the fluoroarene confirmedby the presence of one 4F integral C(sp2)–F ̲ resonance atδF −133.71/−133.63 and one 2F integral C(sp3)–F̲ resonance atδF −223.44 (2JFH = 47.3 Hz)/−223.89 (2JFH = 47.3 Hz) for eachrotamer (i.e. Ci/C2v symmetry). Moreover, the NMR data for theimidazoliumolate groups show good agreement with those of1a. Despite repeated attempts we have been so far unsuccess-ful in determining a suitably high quality solid-state structureof 2 (see ESI† for optimised structures).The formation of 2 is suggested to proceed via a mechan-ism analogous to that of 1a, i.e. via intermediate formation of1b (Scheme 2). Similar double C–F bond activation of hexa-fluorobenzene was observed by Kuhn.4 The presence of anexcess of hexafluorobenzene during the formation of 2 impliesthat the imidazoliumolate group is significantly more acti-vating than a fluorine substituent alone. Although a moredetailed computation analysis is required, corroborating thissuggestion the calculated natural charge of the para-disposedfluoroarene carbon in 1b is more positive that the carbonatoms of hexafluorobenzene (+0.302 cf. +0.290; see ESI†).Scheme 2 Reactions and proposed mechanism associated with theC–F bond activation of octaﬂuorotoluene and hexaﬂuorobenzene byIBioxMe4.Fig. 1 Solid-state structure of 1a; minor disordered components (CH2Fand CF3 groups) and solvent omitted for clarify. Thermal ellipsoidsdrawn at the 50% probability level. Selected bond lengths (Å) andangles (°): C1–N2, 1.3359(17); C1–N5, 1.3670(14); C1–C17, 1.4704(18);C3–C4, 1.3845(16); C3–O6, 1.3635(17); C4–O11, 1.2492(14); C3–C4–O11, 133.64(14); N2–C1–N5, 106.58(11); LSplane(C1–N5) < LSplane(C17–C22), 102.99(5)°.Communication Organic & Biomolecular Chemistry788 | Org. Biomol. Chem., 2017, 15, 787–789 This journal is © The Royal Society of Chemistry 2017Open Access Article. Published on 22 December 2016. Downloaded on 02/03/2017 13:50:56.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineIn summary, we have demonstrated that while bioxazoline-derived imidazol-2-ylidene IBioxMe4 is stable in a range of par-tially fluorinated arene solvents (fluorobenzene, 1,2-difluoro-benzene and 1,3,5-trifluorobenzene), in the presence of octa-fluorotoluene and hexafluorobenzene C–F bond activationensues. These reactions result in the formation of zwitterionicimidazoliumolates presumably via a mechanism involvingnucleophilic aromatic substitution by the NHC ligand and sub-sequent oxazoline ring opening by liberated fluoride. In thecase of hexafluorobenzene, the substituted arene appears to bemore activated towards nucleophilic attack, ultimately leadingto double C–F bond activation. These reactions help furthersubstantiate the potential of NHCs to mediate challengingbond disconnections of contemporary interest.We gratefully acknowledge financial support from the EPSRC(J. E.-K.), Swiss National Science Foundation (S. A. H), RoyalSociety (A. B. C), and University of Warwick. Crystallographicdata was collected using a diﬀractometer purchased throughsupport from Advantage West Midlands and the EuropeanRegional Development Fund.Notes and references1 (a) D. M. Flanigan, F. Romanov-Michailidis, N. A. Whiteand T. Rovis, Chem. Rev., 2015, 115, 9307–9387;(b) M. N. Hopkinson, C. Richter, M. 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Glorius, Chem. – Asian J., 2009,4, 1786–1789.Organic & Biomolecular Chemistry CommunicationThis journal is © The Royal Society of Chemistry 2017 Org. Biomol. Chem., 2017, 15, 787–789 | 789Open Access Article. Published on 22 December 2016. Downloaded on 02/03/2017 13:50:56.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

C–F bond activation of perfluorinated arenes by a bioxazoline-derived N-heterocyclic carbene

IBioxMe4enacts selective single and double C–F bond activation of octafluorotoluene and hexafluorobenzene, respectively.</p

C–F bond activation of perfluorinated arenes by a bioxazoline-derived N-heterocyclic carbene

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