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Insights into reactivity patterns of Ag(II)SO4 with respect to fluoro- and trifluoromethyl-substituted aromatics
Journal of Fluorine Chemistry 218 (2019) 105–110
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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Insights into reactivity patterns of Ag(II)SO4 with respect to fluoro- and trifluoromethyl-substituted aromatics
T
P.J. Leszczyńskia, , A.K. Budniaka,b, M. Grzeszkiewicza, J. Gawraczyńskia,b, Ł. Dobrzyckib,c, ⁎ P.J. Malinowskia, T. Jarońa, M.K. Cyrańskib,c, P. Szareka, Z. Mazejd, W. Grochalaa, ⁎
a
Center of New Technologies, University of Warsaw, Żwirki i Wigury 93, 02089, Warsaw Poland Faculty of Chemistry, University of Warsaw, Pasteur 1, 02093, Warsaw, Poland c The Czochralski Laboratory of Advanced Crystal Engineering, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02089, Warsaw, Poland d Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, 1000, Ljubljana, Slovenia b
ARTICLE INFO
ABSTRACT
This work is dedicated to Professor Henryk Koroniak at his 70th birthday.
Direct oxidative functionalization of CeH bonds is an ongoing chemical challenge. Here we report results of the comparative tests of Ag(II)SO4 reactivity towards 24 selected fluoro- and/or trifluoromethyl-substituted aromatic hydrocarbons with ionization potentials (IP) varying in the 8.15–10.72 eV range. For many of those we observed the oxidative CeC coupling reactions and/or O-insertion in the CeH bonds. Observed differences in reactivity of organic substrates are explained on the basis of their IP values on one side, free Gibbs energy of the heterolytic CeH bond cleavage on the other, as well as impact of the steric hindrance effect of CF3- groups. The electron affinity of the surface of solid Ag(II)SO4 was estimated at ca. 9.3 eV. Interesting Brønsted acid-induced side reaction of the protolytic CeF bond activation, followed by conversion of the trifluoromethyl group to esters of hexafluoro-i-propanol or nonafluoro-t-butanol used as reaction medium, has been observed for 1-(trifluoromethyl)naphthalene.
Keywords: Fluoroaromatics Trifluoromethylaromatics Oxidative CeC coupling O atom insertion Silver(2+)
1. Introduction The oxidative CeC coupling reactions of aromatic and/or alkylaromatic compounds proceeded by direct oxidative CeH bond activation are valuable tools for the synthesis of various fascinating organic molecules, however functionalization of the substrates is usually necessary [1,2]. Transition metals compounds - especially Pd(II) and Cu (II) catalysed CeH bond activation has emerged as a successful synthetic methodology for this chemistry, but the use of Ag(II) - an open shell [Kr]4d9 system isolelectronic with Cu(II) has not been extensively investigated [3,4]. On the other hand, reactions involving AgF2 and other fluoro Ag(II) species, lead to exhaustive fluorination of organic compounds rather than CeC coupling [5]. Recently, we have enriched the list of reagents used for direct CeC coupling reactions by introducing a powerful Ag(II)SO4 oxidizer. Ag(II) SO4 synthesized a few years ago in our laboratories using fluorine chemistry methods [6], is a very strong one-electron transfer agent containing true divalent silver cation (E° ≈ +2 V vs. NHE), which is capable of activation CeH bonds in molecules that are unreactive in classical CeH bond activation protocols (followed by CeC coupling). The synthetic protocol does not require any prefunctionalization of ⁎
organic substrates and reactions proceed at a room temperature [7]. Considerable degree of spin density transfer between silver(II) cations and sulphate anions: Ag(II)%(SO42–)↔Ag(I)(SO4–%)
(1)
constitutes one unique property of Ag(II)SO4. [8] Reconnaissance of reactivity of Ag(II)SO4 oxidizer proved usability of this compound in binaphthyl derivatives formation via CeC coupling of the corresponding naphthalene moieties in a single-pot system albeit with rather low yields [7]. The first step of reactions is very fast and likely corresponds to outer-sphere electron transfer between organic molecule and surface of Ag(II)SO4, followed by deprotonation of organic radical cation. Wishing to learn whether applicability of Ag(II) SO4 might be extended to activate oxidation-resistant molecules, here we have studied a set of fluoro- and trifluoromethyl derivatives of naphthalene and benzene with their ionization potentials (IP) varying in the 8.15–10.72 eV range. The strongly electron-withdrawing nature of F atom results in large values of IP in these compounds (Schemes 1–3Scheme 1 respectively) [9]. Due to lack of solubility of silver(II) sulphate in any of organic solvents, the room temperature Ag(II) induced C–H bond activation
Corresponding authors. E-mail addresses: [email protected] (P.J. Leszczyński), [email protected] (W. Grochala).
https://doi.org/10.1016/j.jfluchem.2018.12.002 Received 29 September 2018; Received in revised form 7 December 2018; Accepted 7 December 2018 Available online 10 December 2018 0022-1139/ © 2018 Elsevier B.V. All rights reserved.
Journal of Fluorine Chemistry 218 (2019) 105–110
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Scheme 1. Ag(II)SO4-based reactions involving F-substituted aromatics. [3].
proceeds at the solid-liquid interface. The formation of the radical cation is possible only if ionization potential of the neutral fluoroorganic molecule is smaller than the electron affinity of the Ag(II)SO4 surface sites. This opens an additional possibility of determination of electron affinity of the surface of silver(II) sulphate crystallites using fluorinated compounds. In all performed experiments, reactivity of the Ag(II)SO4 have been examined using semi-quantitative tests with gc/ms identification of products in post-reaction mixtures, which is sufficient for our qualitative screening. It should be stressed that the purpose of this study
was to test reactivity of very strong novel oxidizer towards fluoroorganics and detect reactivity patterns rather than optimize synthetic protocols for high yields. 2. Results and discussion Selection of appropriate liquid medium for studies of reactivity of Ag(II)SO4 towards fluoroorganics is not trivial due to immense reactivity of inorganic substrate, as well as solubility problems for some 106
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subsequent formation of hydrogensulfate derivative (3b). While the mechanism of this process is unclear at present, it is supported by the formation of XRD-detectable amount of Ag(I)2SO4 in the solid residue, which points to the detachment of the entire sulfate moiety from Ag(II) SO4: Ag(II)(SO4) → Ag(I)2(SO4) + [SO4]
(2)
Simultaneously, the facile intramolecular charge transfer in 2,3,5,6tetrafluorotoluene radical cation; from π* C(sp2) to σ* C(sp3) orbital is a driving force of the oxidative C(sp3)-H bond activation that leads to formation of the 1,1′-ethane-1,2-diylbis(2,3,5,6-tetrafluorobenzene) (3a) molecule, according to the CeC coupling scenario which has been discussed recently [7]. Dramatic change of Ag(II)SO4 reactivity towards fluoroorganics from CeC coupling to O atom insertion clearly correlates with the number of the fluorine atoms or CF3 groups present in organic substrate molecules (Schemes 1,2Scheme 1 and 3 respectively). The reactivity change was noticeable in reactivity tests result towards compounds: 2 and 3, and 6, 7 and 15, with similar IP values in each group (9.16 and 9.33 eV respectively). Importantly, proton acidity varies within each group, as measured by free Gibbs energies of heterolytic CeH bond cleavage (Fig. 1) [9]. Lower free Gibbs energy of the heterolytic C–H bond cleavage and thus easier deprotonation of the 2,3,5,6-tetrafluorotoluene (3) in comparison to 1,2-difluorobenzene (2) explains the observed difference in their reactivity. Insertion of O atom may be connected with the formation of phenols or – via additional attachment of SO3 moiety – of the hydrogensulfate derivatives (C-OSO3H). Formation of the latter has been observed for following fluoroorganic systems: 2,3,5,6-tetrafluorotoluene (3), 1,2,4,5tetrafluorobenzene (6), 1,2,3-trifluorobenzene (10), 1,2,4-trifluorobenzene (11), 1,2,4,5-tetrafluorobenzene (12), 1,2,3,4-tetrafluorobenzene (13), pentafluorobenzene (14) and 1,3,5-tris(trifluoromethyl)benzene (24), which are characterized by moderately low values of the free Gibbs energy of heterolytic CeH bond cleavage (see Scheme 1,2,3 and Fig. 1). However, the steric hindrance effect of CF3groups in 24 might be responsible for formation of 2,4,6-tris(trifluoromethyl)phenol (24a) with no accompanying SO3 insertion. Determination of Ag(II)SO4–induced mechanism of formation of phenols and hydrogensulfates requires further research, both experimental and theoretical. The pathways of reactivity of Ag(II)SO4 towards fluoroorganics are jointly presented in Fig. 1. For systems with IP < 9.33 eV and with difficult heterolytic cleavage of CeH bond, the oxidative CeC coupling predominates the reactivity. For molecules with IP > 9.33 eV but facile deprotonation, O insertion usually takes place (sometimes followed by SO3 insertion, at no steric hindrance). The systems with large IP and difficult deprotonation are usually unreactive towards Ag(II)SO4. The reactions described so far (Fig. 1) proceeded in the absence of any solvent. However, during a preliminary screening of Ag(II) SO4–based reaction conditions [7] we have found out that rate of reactions as well as their yield often increase when hexafluoro-i-propyl alcohol (HFIP) or nonafluoro-t-butanol (NFTB) are used as reaction media. These effects are usually ascribed to the electron-withdrawing effects of the fluorine atoms which increase ionizing power, Brønsted acidity and hydrogen bond donation of solvent molecules in comparison with their nonfluorinated counterparts [12,13]. From a synthetic viewpoint, fluorinated alcohols are excellent candidates for reaction media when radical cation stabilization is required [13]. Therefore, we have conducted additional reactions between 18 and Ag(II)SO4 in HFIP or NFTB (with pKa of 9.3 and 4.4 respectively), as well as in nitromethane (NM) with pKa of 10.2, and their mixtures. Interestingly, aside from the oxidative CeC coupling, a Brønsted acid induced side reactions were observed of protolytic CeF bond activation, subsequent conversion of the trifluoromethyl group to acyl fluoride and alcoholysis of the acyl fluoride by fluorinated alcohols
Scheme 2. Ag(II)SO4-based reactions involving F- and CF3-substituted aromatic hydrocarbons. [3].
of organic compounds. Thus, our tests have been carried out in most cases without addition of any solvents which made the obtained results easier to interpret. Oxidative CeC coupling has been observed for: 1fluoronaphthalene (1), 1,2-difluorobenzene (2), 2,3,5,6-tetrafluorotoluene (3), 2-fluoro-1-(trifluoromethyl)benzene (15) and 1-(trifluoromethyl)-naphthalene (18) (see Schemes 1,2,3Scheme 1), with IP < 9.33 eV. The fate of other naphthalene derivatives with their IP < 9.33 eV was similar in our previous experiments7 (compounds 4 and 5 constituting two interesting exceptions). On the other hand, many molecules with their IP > 9.33 eV (e.g., 7, 8, 9, 16, 17, 19, 20, 21, 22, and 23) did not undergo any reaction. This results suggests that the electron affinity of the surface sites of Ag(II)SO4 is around 9.3 eV. In other words, the giant electron affinity of the naked divalent silver cation in the gas phase (21.50 eV) [8] is reduced to about 43% of its value due to ligation by (SO4)2– Lewis bases. Nevertheless, surface sites of Ag(II)SO4 remain potent oxidizers; note, that EA of F4-TCNQ organic “super-oxidizer” is as small as 5.2 eV [9]. Actually, it is the huge oxidizing power of Ag(II)SO4 which may be responsible for the low yields of reactions studied here; with EA of Ag(II)SO4 so large, at least some systems may correspond to inverted Marcus region [10]. It turns out that the CeC coupled “dimers” are not the only type of product observed in our tests. A proton-coupled electron transfer (PCET), which is a broad reaction class in which e– and H+ are transferred to one or more acceptors via concerted or stepwise mechanisms, is a common mode of CeH bond activation. For oxidative functionalization of CeH bonds, the two most typical scenarios are: concerted proton-electron transfer or stepwise electron transfer-proton transfer [11]. The presence of four strongly electron withdrawing F substituents in 2,3,5,6-tetrafluorotoluene (3) molecule significantly increases the acidity of CeH bond and thus decreases the reaction barrier for reactions involving CeH bond rupture. Consequently, 3 undergoes insertion of the SO4 group into the CeH bond with 107
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Scheme 3. Ag(II)SO4-based reactions involving CF3-substituted aromatic hydrocarbons. [3].
Fig. 1. Map of reactivity of AgSO4 with respect to organic compounds (including F- and/or CF3-substituted aromatic hydrocarbons) plotted in function of the free Gibbs energies of heterolytic C–H bond cleavage and their ionization potential. [3] Data for previously studied systems were also included. [8].
Fig. 2. The molecular structure of 2,2,2-trifluoro-1-(trifluoromethyl)ethyl 1naphthoate (18b). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as spheres of arbitrary radius.
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(solvents). These reactions do not proceed in the absence of Ag(II)SO4, which suggests that reactivity of the CF3 group is greatly increased for the radical cation as compared to the neutral substrate molecule. Although splitting of the CeF bond by Brønsted acids and protolytic defluorination of the trifluoromethyl-substituted aromatic hydrocarbons are known, this chemistry has not been studied in great detail [12]. Here, esters of the 1-naphthoic acid: 2,2,2-trifluoro-1-(trifluoromethyl)ethyl 1-naphthoate (18b) and 2,2,2-trifluoro-1,1-bis(trifluoromethyl)ethyl 1-naphthoate (18c) that formed in the course of reactions, remain in the solution, and they may be easily separated using extraction and preparative thin-layer chromatography, their chemical identity being confirmed using 1H, 13C, 19F and 1H vs. 13C HSQC NMR spectra, and IR spectroscopy (ESI), as well as powder or single crystal xrd analysis. The molecular structure of 2,2,2-trifluoro-1(trifluoromethyl)ethyl 1-naphthoate (18b) determined from single crystal x-ray diffraction is shown in Fig. 2, while crystal structure data for both compounds are given in ESI. It turns out that the relative yield of products for substrate 18 crucially depends on the reaction conditions. In the absence of any solvent, only 18a is formed in low yield < 1%. When not very acidic but polar NM is used as a solvent, yield increases to ca. 17%, but 18a is still the only product observed. However, when moderately Brønsted acidic HFIP is used as a reaction medium, total yield is ca. 11% but 18a and 18b form in nearly equal amounts. Finally, when most Brønsted acidic NFTB is used as a solvent, only hydrolysis product (18c) forms in 10% yield. This example shows that steering is possible of the reaction pathways by using solvent of a given Brønsted acidity. The surmise that formation of radical cations is the initial step of all these reactions (which may lead even to the detachment of CF3 group via CeC bond cleavage) is additionally confirmed by formation of trace amounts of traces of 1,5-bis(trifluoromethyl)naphthalene with additional CF3 moiety attached to 18. It should be noted that 18, 18a, 18b and 18c may additionally participate in homo-coupling or cross-coupling processes and trace products of such reactions have been observed here using gc/ms.
by McKillop et al. (CoF3 as an oxidant) and by Hirano-Miura (Cu(II) acetate as an oxidant) are both completely ineffective in comparative tests of usability towards 1-(trifluoromethyl)naphthalene (18), [14] and not even traces of the CC-coupled products are observed in gc/ms data. In forthcoming contributions we will describe applicability of Ag(II) SO4 and other Ag(II) salts for formation of C(sp2)–C(sp3) and C(sp3)–C(sp3) coupled aromatic and aliphatic products, as well as spectroscopic and mechanistic studies of the reaction mechanism. 4. Experimental ‡ Reactants and solvents were purchased from Sigma-Aldrich. Ag(II) SO4 was obtained using standard procedure. [8] The purity of organic substrates was checked using tandem gc/ms technique; identification of products in post-reaction mixtures used the same analytical methodology. Samples for gc/ms measurements were prepared under inert argon atmosphere in Labmaster® DP MBRAUN glovebox (O2 < 1.0 ppm; H2O < 1.0 ppm). Millipore Millex® FH 13 mm syringe driven filter unit with hydrophobic fluoropore™ (PTFE) 0.45 μm membrane were used for clarification of the post-reactions samples. Agilent 7890 A & 5975 spectrometer (EI ionization) with standard HP-5MS column and NIST 08 database was used in all cases. Mass spectra were recorded in the range of 10–800 amu. Heterogeneous reactions (0.1 mM of organic substrate, 0.2 mM Ag(II)SO4 and no or 2 ml of solvent) commence even at –35 °C; reaction mixture was allowed to warm up to room temperature and process was conducted for 72 h. The X-ray measurement of 18b single crystal was performed at 100(2) K on a Bruker D8 Venture Photon100 diffractometer equipped with a TRIUMPH monochromator and a MoKα fine focus sealed tube. Crystal structure of the products were determined with help of the software and procedures described in reference [15]. § Crystallographic data for 18b (deposition number CCDC 1476391): Pbcn, a = 19.3779(13) Å, b = 7.2187(5) Å, c = 18.7462(12) Å, Z = 8, V = 2622.3(3) Å3; 20803 reflections collected, 2322 independent, Rint = 0.0299; R1 = 0.0340 and wR2 = 0.0792 for 1950 reflections with I > 2σ(I). §§ Crystallographic data for 18c: P21/c, a = 29.0373(17) Å, b = 6.9782(2) Å, c = 22.8297(8) Å, β = 110.411(5)o, Z = 12. Quality of this single crystal was insufficient for deposition of its crystal structure in the structural database (cf. ESI). For lattice parameters of the room-temperature crystal structure see ESI.
3. Conclusions We have conducted qualitative screening of the reactivity of Ag(II) SO4 towards fluorinated and trifluoromethylated aromatics without any solvent or using NM, HFIP or NFTB as reaction medium. The main goal of this study was to gain insight into reactivity patterns rather than optimization of the reaction conditions and achieving high yields. The reactivity of Ag(II)SO4 is quite interesting but controlling the reactivity of this species is clearly challenging. Our preliminary study provides some insight into how to better utilize this reagent to enable a useful synthetic transformation. Three major reactivity patterns are seen: (i) oxidative CeC coupling, (ii) O atom insertion leading to phenols, followed sometimes by insertion of SO3 group and resulting in aromatic hydrogensulfates, and (iii) protolytic activation of CF3 group in Brønsted acidic solvents, which results in formation of esters for HFIP and particularly for NFTB. These reactivity patterns may be rationalized using the values of the first ionization potential of organic substrate molecule, the free enthalpy of heterolytic CeH bond rupture, as well as pKa values of respective solvents. The electron affinity of the sites on the surface of Ag(II)SO4 was estimated at ca. 9.3 eV as judged from comparative study for a range of F– and CF3–derivatives with varying IP values. Despite significant reduction from the value of 21.50 eV for naked Ag(II) cation in the gas phase, silver(II) sulfate still possess a high oxidizing power. Ag(II)SO4 turns out to be the only oxidizer currently known which enables direct one-pot oxidative aromatic coupling of naphthalene substituted with electron withdrawing CF3 group. Our results (not shown) indicate that the classical protocols of C(sp2)–H bond activation in aromatic systems:
Conflict of interest There are no conflict to declare. Acknowledgements P.J.L. thanks the National Science Centre of the Republic of Poland (NCN) for OPUS grant (UMO-2015/19/B/ST5/02863). Mr Maciej Sojka (IChO PAN, Warsaw) is thanked for performing of the gc/ms analyses. Z.M. acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1–0045 Inorganic Chemistry and Technology). A.K.B. thanks Warsaw Consortium of Academic Chemistry for KNOW scholarship. P.M. thanks the Biopolymers Laboratory, Faculty of Physics, University of Warsaw, for the access to Agilent Supernova X-ray single-crystal diffractometer (compound 18c) co-financed by the European Union within the ERDF Project POIG.02.01.0014-122/09. Raman and IR measurements were carried out with the use of CePT infrastructure financed by the European Union - the European Regional Development Fund within the Operational Programme "Innovative economy" for 2007-2013 (POIG.02.02.00-14-024/08-00).
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jfluchem.2018.12.002. Details of crystal structures for 18b and 18c, and their NMR and IR spectra.
[6] [7]
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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Insights into reactivity patterns of Ag(II)SO4 with respect to fluoro- and trifluoromethyl-substituted aromatics
T
P.J. Leszczyńskia, , A.K. Budniaka,b, M. Grzeszkiewicza, J. Gawraczyńskia,b, Ł. Dobrzyckib,c, ⁎ P.J. Malinowskia, T. Jarońa, M.K. Cyrańskib,c, P. Szareka, Z. Mazejd, W. Grochalaa, ⁎
a
Center of New Technologies, University of Warsaw, Żwirki i Wigury 93, 02089, Warsaw Poland Faculty of Chemistry, University of Warsaw, Pasteur 1, 02093, Warsaw, Poland c The Czochralski Laboratory of Advanced Crystal Engineering, Faculty of Chemistry, University of Warsaw, Żwirki i Wigury 101, 02089, Warsaw, Poland d Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, 1000, Ljubljana, Slovenia b
ARTICLE INFO
ABSTRACT
This work is dedicated to Professor Henryk Koroniak at his 70th birthday.
Direct oxidative functionalization of CeH bonds is an ongoing chemical challenge. Here we report results of the comparative tests of Ag(II)SO4 reactivity towards 24 selected fluoro- and/or trifluoromethyl-substituted aromatic hydrocarbons with ionization potentials (IP) varying in the 8.15–10.72 eV range. For many of those we observed the oxidative CeC coupling reactions and/or O-insertion in the CeH bonds. Observed differences in reactivity of organic substrates are explained on the basis of their IP values on one side, free Gibbs energy of the heterolytic CeH bond cleavage on the other, as well as impact of the steric hindrance effect of CF3- groups. The electron affinity of the surface of solid Ag(II)SO4 was estimated at ca. 9.3 eV. Interesting Brønsted acid-induced side reaction of the protolytic CeF bond activation, followed by conversion of the trifluoromethyl group to esters of hexafluoro-i-propanol or nonafluoro-t-butanol used as reaction medium, has been observed for 1-(trifluoromethyl)naphthalene.
Keywords: Fluoroaromatics Trifluoromethylaromatics Oxidative CeC coupling O atom insertion Silver(2+)
1. Introduction The oxidative CeC coupling reactions of aromatic and/or alkylaromatic compounds proceeded by direct oxidative CeH bond activation are valuable tools for the synthesis of various fascinating organic molecules, however functionalization of the substrates is usually necessary [1,2]. Transition metals compounds - especially Pd(II) and Cu (II) catalysed CeH bond activation has emerged as a successful synthetic methodology for this chemistry, but the use of Ag(II) - an open shell [Kr]4d9 system isolelectronic with Cu(II) has not been extensively investigated [3,4]. On the other hand, reactions involving AgF2 and other fluoro Ag(II) species, lead to exhaustive fluorination of organic compounds rather than CeC coupling [5]. Recently, we have enriched the list of reagents used for direct CeC coupling reactions by introducing a powerful Ag(II)SO4 oxidizer. Ag(II) SO4 synthesized a few years ago in our laboratories using fluorine chemistry methods [6], is a very strong one-electron transfer agent containing true divalent silver cation (E° ≈ +2 V vs. NHE), which is capable of activation CeH bonds in molecules that are unreactive in classical CeH bond activation protocols (followed by CeC coupling). The synthetic protocol does not require any prefunctionalization of ⁎
organic substrates and reactions proceed at a room temperature [7]. Considerable degree of spin density transfer between silver(II) cations and sulphate anions: Ag(II)%(SO42–)↔Ag(I)(SO4–%)
(1)
constitutes one unique property of Ag(II)SO4. [8] Reconnaissance of reactivity of Ag(II)SO4 oxidizer proved usability of this compound in binaphthyl derivatives formation via CeC coupling of the corresponding naphthalene moieties in a single-pot system albeit with rather low yields [7]. The first step of reactions is very fast and likely corresponds to outer-sphere electron transfer between organic molecule and surface of Ag(II)SO4, followed by deprotonation of organic radical cation. Wishing to learn whether applicability of Ag(II) SO4 might be extended to activate oxidation-resistant molecules, here we have studied a set of fluoro- and trifluoromethyl derivatives of naphthalene and benzene with their ionization potentials (IP) varying in the 8.15–10.72 eV range. The strongly electron-withdrawing nature of F atom results in large values of IP in these compounds (Schemes 1–3Scheme 1 respectively) [9]. Due to lack of solubility of silver(II) sulphate in any of organic solvents, the room temperature Ag(II) induced C–H bond activation
Corresponding authors. E-mail addresses: [email protected] (P.J. Leszczyński), [email protected] (W. Grochala).
https://doi.org/10.1016/j.jfluchem.2018.12.002 Received 29 September 2018; Received in revised form 7 December 2018; Accepted 7 December 2018 Available online 10 December 2018 0022-1139/ © 2018 Elsevier B.V. All rights reserved.
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Scheme 1. Ag(II)SO4-based reactions involving F-substituted aromatics. [3].
proceeds at the solid-liquid interface. The formation of the radical cation is possible only if ionization potential of the neutral fluoroorganic molecule is smaller than the electron affinity of the Ag(II)SO4 surface sites. This opens an additional possibility of determination of electron affinity of the surface of silver(II) sulphate crystallites using fluorinated compounds. In all performed experiments, reactivity of the Ag(II)SO4 have been examined using semi-quantitative tests with gc/ms identification of products in post-reaction mixtures, which is sufficient for our qualitative screening. It should be stressed that the purpose of this study
was to test reactivity of very strong novel oxidizer towards fluoroorganics and detect reactivity patterns rather than optimize synthetic protocols for high yields. 2. Results and discussion Selection of appropriate liquid medium for studies of reactivity of Ag(II)SO4 towards fluoroorganics is not trivial due to immense reactivity of inorganic substrate, as well as solubility problems for some 106
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subsequent formation of hydrogensulfate derivative (3b). While the mechanism of this process is unclear at present, it is supported by the formation of XRD-detectable amount of Ag(I)2SO4 in the solid residue, which points to the detachment of the entire sulfate moiety from Ag(II) SO4: Ag(II)(SO4) → Ag(I)2(SO4) + [SO4]
(2)
Simultaneously, the facile intramolecular charge transfer in 2,3,5,6tetrafluorotoluene radical cation; from π* C(sp2) to σ* C(sp3) orbital is a driving force of the oxidative C(sp3)-H bond activation that leads to formation of the 1,1′-ethane-1,2-diylbis(2,3,5,6-tetrafluorobenzene) (3a) molecule, according to the CeC coupling scenario which has been discussed recently [7]. Dramatic change of Ag(II)SO4 reactivity towards fluoroorganics from CeC coupling to O atom insertion clearly correlates with the number of the fluorine atoms or CF3 groups present in organic substrate molecules (Schemes 1,2Scheme 1 and 3 respectively). The reactivity change was noticeable in reactivity tests result towards compounds: 2 and 3, and 6, 7 and 15, with similar IP values in each group (9.16 and 9.33 eV respectively). Importantly, proton acidity varies within each group, as measured by free Gibbs energies of heterolytic CeH bond cleavage (Fig. 1) [9]. Lower free Gibbs energy of the heterolytic C–H bond cleavage and thus easier deprotonation of the 2,3,5,6-tetrafluorotoluene (3) in comparison to 1,2-difluorobenzene (2) explains the observed difference in their reactivity. Insertion of O atom may be connected with the formation of phenols or – via additional attachment of SO3 moiety – of the hydrogensulfate derivatives (C-OSO3H). Formation of the latter has been observed for following fluoroorganic systems: 2,3,5,6-tetrafluorotoluene (3), 1,2,4,5tetrafluorobenzene (6), 1,2,3-trifluorobenzene (10), 1,2,4-trifluorobenzene (11), 1,2,4,5-tetrafluorobenzene (12), 1,2,3,4-tetrafluorobenzene (13), pentafluorobenzene (14) and 1,3,5-tris(trifluoromethyl)benzene (24), which are characterized by moderately low values of the free Gibbs energy of heterolytic CeH bond cleavage (see Scheme 1,2,3 and Fig. 1). However, the steric hindrance effect of CF3groups in 24 might be responsible for formation of 2,4,6-tris(trifluoromethyl)phenol (24a) with no accompanying SO3 insertion. Determination of Ag(II)SO4–induced mechanism of formation of phenols and hydrogensulfates requires further research, both experimental and theoretical. The pathways of reactivity of Ag(II)SO4 towards fluoroorganics are jointly presented in Fig. 1. For systems with IP < 9.33 eV and with difficult heterolytic cleavage of CeH bond, the oxidative CeC coupling predominates the reactivity. For molecules with IP > 9.33 eV but facile deprotonation, O insertion usually takes place (sometimes followed by SO3 insertion, at no steric hindrance). The systems with large IP and difficult deprotonation are usually unreactive towards Ag(II)SO4. The reactions described so far (Fig. 1) proceeded in the absence of any solvent. However, during a preliminary screening of Ag(II) SO4–based reaction conditions [7] we have found out that rate of reactions as well as their yield often increase when hexafluoro-i-propyl alcohol (HFIP) or nonafluoro-t-butanol (NFTB) are used as reaction media. These effects are usually ascribed to the electron-withdrawing effects of the fluorine atoms which increase ionizing power, Brønsted acidity and hydrogen bond donation of solvent molecules in comparison with their nonfluorinated counterparts [12,13]. From a synthetic viewpoint, fluorinated alcohols are excellent candidates for reaction media when radical cation stabilization is required [13]. Therefore, we have conducted additional reactions between 18 and Ag(II)SO4 in HFIP or NFTB (with pKa of 9.3 and 4.4 respectively), as well as in nitromethane (NM) with pKa of 10.2, and their mixtures. Interestingly, aside from the oxidative CeC coupling, a Brønsted acid induced side reactions were observed of protolytic CeF bond activation, subsequent conversion of the trifluoromethyl group to acyl fluoride and alcoholysis of the acyl fluoride by fluorinated alcohols
Scheme 2. Ag(II)SO4-based reactions involving F- and CF3-substituted aromatic hydrocarbons. [3].
of organic compounds. Thus, our tests have been carried out in most cases without addition of any solvents which made the obtained results easier to interpret. Oxidative CeC coupling has been observed for: 1fluoronaphthalene (1), 1,2-difluorobenzene (2), 2,3,5,6-tetrafluorotoluene (3), 2-fluoro-1-(trifluoromethyl)benzene (15) and 1-(trifluoromethyl)-naphthalene (18) (see Schemes 1,2,3Scheme 1), with IP < 9.33 eV. The fate of other naphthalene derivatives with their IP < 9.33 eV was similar in our previous experiments7 (compounds 4 and 5 constituting two interesting exceptions). On the other hand, many molecules with their IP > 9.33 eV (e.g., 7, 8, 9, 16, 17, 19, 20, 21, 22, and 23) did not undergo any reaction. This results suggests that the electron affinity of the surface sites of Ag(II)SO4 is around 9.3 eV. In other words, the giant electron affinity of the naked divalent silver cation in the gas phase (21.50 eV) [8] is reduced to about 43% of its value due to ligation by (SO4)2– Lewis bases. Nevertheless, surface sites of Ag(II)SO4 remain potent oxidizers; note, that EA of F4-TCNQ organic “super-oxidizer” is as small as 5.2 eV [9]. Actually, it is the huge oxidizing power of Ag(II)SO4 which may be responsible for the low yields of reactions studied here; with EA of Ag(II)SO4 so large, at least some systems may correspond to inverted Marcus region [10]. It turns out that the CeC coupled “dimers” are not the only type of product observed in our tests. A proton-coupled electron transfer (PCET), which is a broad reaction class in which e– and H+ are transferred to one or more acceptors via concerted or stepwise mechanisms, is a common mode of CeH bond activation. For oxidative functionalization of CeH bonds, the two most typical scenarios are: concerted proton-electron transfer or stepwise electron transfer-proton transfer [11]. The presence of four strongly electron withdrawing F substituents in 2,3,5,6-tetrafluorotoluene (3) molecule significantly increases the acidity of CeH bond and thus decreases the reaction barrier for reactions involving CeH bond rupture. Consequently, 3 undergoes insertion of the SO4 group into the CeH bond with 107
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Scheme 3. Ag(II)SO4-based reactions involving CF3-substituted aromatic hydrocarbons. [3].
Fig. 1. Map of reactivity of AgSO4 with respect to organic compounds (including F- and/or CF3-substituted aromatic hydrocarbons) plotted in function of the free Gibbs energies of heterolytic C–H bond cleavage and their ionization potential. [3] Data for previously studied systems were also included. [8].
Fig. 2. The molecular structure of 2,2,2-trifluoro-1-(trifluoromethyl)ethyl 1naphthoate (18b). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as spheres of arbitrary radius.
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(solvents). These reactions do not proceed in the absence of Ag(II)SO4, which suggests that reactivity of the CF3 group is greatly increased for the radical cation as compared to the neutral substrate molecule. Although splitting of the CeF bond by Brønsted acids and protolytic defluorination of the trifluoromethyl-substituted aromatic hydrocarbons are known, this chemistry has not been studied in great detail [12]. Here, esters of the 1-naphthoic acid: 2,2,2-trifluoro-1-(trifluoromethyl)ethyl 1-naphthoate (18b) and 2,2,2-trifluoro-1,1-bis(trifluoromethyl)ethyl 1-naphthoate (18c) that formed in the course of reactions, remain in the solution, and they may be easily separated using extraction and preparative thin-layer chromatography, their chemical identity being confirmed using 1H, 13C, 19F and 1H vs. 13C HSQC NMR spectra, and IR spectroscopy (ESI), as well as powder or single crystal xrd analysis. The molecular structure of 2,2,2-trifluoro-1(trifluoromethyl)ethyl 1-naphthoate (18b) determined from single crystal x-ray diffraction is shown in Fig. 2, while crystal structure data for both compounds are given in ESI. It turns out that the relative yield of products for substrate 18 crucially depends on the reaction conditions. In the absence of any solvent, only 18a is formed in low yield < 1%. When not very acidic but polar NM is used as a solvent, yield increases to ca. 17%, but 18a is still the only product observed. However, when moderately Brønsted acidic HFIP is used as a reaction medium, total yield is ca. 11% but 18a and 18b form in nearly equal amounts. Finally, when most Brønsted acidic NFTB is used as a solvent, only hydrolysis product (18c) forms in 10% yield. This example shows that steering is possible of the reaction pathways by using solvent of a given Brønsted acidity. The surmise that formation of radical cations is the initial step of all these reactions (which may lead even to the detachment of CF3 group via CeC bond cleavage) is additionally confirmed by formation of trace amounts of traces of 1,5-bis(trifluoromethyl)naphthalene with additional CF3 moiety attached to 18. It should be noted that 18, 18a, 18b and 18c may additionally participate in homo-coupling or cross-coupling processes and trace products of such reactions have been observed here using gc/ms.
by McKillop et al. (CoF3 as an oxidant) and by Hirano-Miura (Cu(II) acetate as an oxidant) are both completely ineffective in comparative tests of usability towards 1-(trifluoromethyl)naphthalene (18), [14] and not even traces of the CC-coupled products are observed in gc/ms data. In forthcoming contributions we will describe applicability of Ag(II) SO4 and other Ag(II) salts for formation of C(sp2)–C(sp3) and C(sp3)–C(sp3) coupled aromatic and aliphatic products, as well as spectroscopic and mechanistic studies of the reaction mechanism. 4. Experimental ‡ Reactants and solvents were purchased from Sigma-Aldrich. Ag(II) SO4 was obtained using standard procedure. [8] The purity of organic substrates was checked using tandem gc/ms technique; identification of products in post-reaction mixtures used the same analytical methodology. Samples for gc/ms measurements were prepared under inert argon atmosphere in Labmaster® DP MBRAUN glovebox (O2 < 1.0 ppm; H2O < 1.0 ppm). Millipore Millex® FH 13 mm syringe driven filter unit with hydrophobic fluoropore™ (PTFE) 0.45 μm membrane were used for clarification of the post-reactions samples. Agilent 7890 A & 5975 spectrometer (EI ionization) with standard HP-5MS column and NIST 08 database was used in all cases. Mass spectra were recorded in the range of 10–800 amu. Heterogeneous reactions (0.1 mM of organic substrate, 0.2 mM Ag(II)SO4 and no or 2 ml of solvent) commence even at –35 °C; reaction mixture was allowed to warm up to room temperature and process was conducted for 72 h. The X-ray measurement of 18b single crystal was performed at 100(2) K on a Bruker D8 Venture Photon100 diffractometer equipped with a TRIUMPH monochromator and a MoKα fine focus sealed tube. Crystal structure of the products were determined with help of the software and procedures described in reference [15]. § Crystallographic data for 18b (deposition number CCDC 1476391): Pbcn, a = 19.3779(13) Å, b = 7.2187(5) Å, c = 18.7462(12) Å, Z = 8, V = 2622.3(3) Å3; 20803 reflections collected, 2322 independent, Rint = 0.0299; R1 = 0.0340 and wR2 = 0.0792 for 1950 reflections with I > 2σ(I). §§ Crystallographic data for 18c: P21/c, a = 29.0373(17) Å, b = 6.9782(2) Å, c = 22.8297(8) Å, β = 110.411(5)o, Z = 12. Quality of this single crystal was insufficient for deposition of its crystal structure in the structural database (cf. ESI). For lattice parameters of the room-temperature crystal structure see ESI.
3. Conclusions We have conducted qualitative screening of the reactivity of Ag(II) SO4 towards fluorinated and trifluoromethylated aromatics without any solvent or using NM, HFIP or NFTB as reaction medium. The main goal of this study was to gain insight into reactivity patterns rather than optimization of the reaction conditions and achieving high yields. The reactivity of Ag(II)SO4 is quite interesting but controlling the reactivity of this species is clearly challenging. Our preliminary study provides some insight into how to better utilize this reagent to enable a useful synthetic transformation. Three major reactivity patterns are seen: (i) oxidative CeC coupling, (ii) O atom insertion leading to phenols, followed sometimes by insertion of SO3 group and resulting in aromatic hydrogensulfates, and (iii) protolytic activation of CF3 group in Brønsted acidic solvents, which results in formation of esters for HFIP and particularly for NFTB. These reactivity patterns may be rationalized using the values of the first ionization potential of organic substrate molecule, the free enthalpy of heterolytic CeH bond rupture, as well as pKa values of respective solvents. The electron affinity of the sites on the surface of Ag(II)SO4 was estimated at ca. 9.3 eV as judged from comparative study for a range of F– and CF3–derivatives with varying IP values. Despite significant reduction from the value of 21.50 eV for naked Ag(II) cation in the gas phase, silver(II) sulfate still possess a high oxidizing power. Ag(II)SO4 turns out to be the only oxidizer currently known which enables direct one-pot oxidative aromatic coupling of naphthalene substituted with electron withdrawing CF3 group. Our results (not shown) indicate that the classical protocols of C(sp2)–H bond activation in aromatic systems:
Conflict of interest There are no conflict to declare. Acknowledgements P.J.L. thanks the National Science Centre of the Republic of Poland (NCN) for OPUS grant (UMO-2015/19/B/ST5/02863). Mr Maciej Sojka (IChO PAN, Warsaw) is thanked for performing of the gc/ms analyses. Z.M. acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1–0045 Inorganic Chemistry and Technology). A.K.B. thanks Warsaw Consortium of Academic Chemistry for KNOW scholarship. P.M. thanks the Biopolymers Laboratory, Faculty of Physics, University of Warsaw, for the access to Agilent Supernova X-ray single-crystal diffractometer (compound 18c) co-financed by the European Union within the ERDF Project POIG.02.01.0014-122/09. Raman and IR measurements were carried out with the use of CePT infrastructure financed by the European Union - the European Regional Development Fund within the Operational Programme "Innovative economy" for 2007-2013 (POIG.02.02.00-14-024/08-00).
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jfluchem.2018.12.002. Details of crystal structures for 18b and 18c, and their NMR and IR spectra.
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