Next Article in Journal
Neuronal Nitric Oxide Synthase and Post-Translational Modifications in the Development of Central Nervous System Diseases: Implications and Regulation
Next Article in Special Issue
Poly Caprolactam Supported Hexaethylene Glycolic Imidazolium Ionic Liquid as a Heterogeneous Promoter for Nucleophilic Fluorination
Previous Article in Journal
Construction and Evaluation of Alginate Dialdehyde Grafted RGD Derivatives/Polyvinyl Alcohol/Cellulose Nanocrystals IPN Composite Hydrogels
Previous Article in Special Issue
Access to 2-Fluorinated Aziridine-2-phosphonates from α,α-Halofluorinated β-Iminophosphonates—Spectroscopic and Theoretical Studies
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Molecules
Volume 28
Issue 18
10.3390/molecules28186693
molecules-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes

by
Domenique Herbstritt
,
Pooja Tomar
and
Thomas Braun
*
Department of Chemistry, Humboldt–Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(18), 6693; https://doi.org/10.3390/molecules28186693
Submission received: 24 August 2023 / Revised: 6 September 2023 / Accepted: 14 September 2023 / Published: 19 September 2023
(This article belongs to the Special Issue Advances in Modern Fluorine Chemistry)
Browse Figures
Versions Notes

Abstract

:
The greenhouse gas SF5CF3 was photochemically activated with SIMes (1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) to give 1,3-dimesityl-2,2-difluoroimidazolidine (SIMesF2), and 1,3-dimesitylimidazolidine-2-sulfide, as well as the trifluoromethylated carbene derivative 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine. CF3 radicals, as well as SF4, serve presumably as intermediates of the conversions. In addition, the photochemical activation of SF5CF3 was performed in the presence of triphenylphosphine. The formation of triphenyldifluorophosphorane and triphenylphosphine sulfide was observed.

Graphical Abstract

1. Introduction

The greenhouse gases SF5CF3 and SF6 are both chemically highly inert and have a long atmospheric lifetime [1,2,3]. Whereas the activation of SF6 has been well established in the last decade [4,5,6,7,8,9,10,11,12,13,14,15,16,17], the studies on SF5CF3 are rare. Dresdner et al. published the reaction of SF5CF3 with perfluoropropylene at temperatures of 425 °C–513 °C to give perfluoroethane and SF4 [18,19,20]. Huang et al. were able to decompose the greenhouse gases SF6 and SF5CF3 via photolysis in the presence of propene at 184.9 nm [21]. Another SF5CF3 activation was performed at the rhodium hydrido complex [{Rh(µ-H)(dippp)}2] (dippp = 1,3-bis(diisopropylphosphanyl)propane) to yield a binuclear rhodium compound bearing a bridging SCF3 ligand [16]. We previously reported on a photocatalytic reduction of SF5CF3 using [Ir(dtbbpy)(ppy)2]PF6 (4,4′-di-tert-butyl-2,2′-dipyridyl, ppy = 2-phenylpyridine) as the photocatalyst and NEt3 as the reductant. The generation of a CF3 radical led to the development of a process for the trifluoromethylation of aromatics [22]. With regard to the activation of SF6, the N-heterocyclic carbene SIMes was used to achieve a photolytic activation at 311 nm, yielding 1,3-dimesityl-2,2-difluoroimidazolidin (SIMesF2, 2) and 1,3-dimesitylimidazolidine-2-sulfide. It was also shown that alcohols can be subsequently fluorinated in situ [15]. Rotering et al. demonstrated that triphenylphosphine can be utilized to activate SF6 under irradiation to yield Ph3PF2 and Ph3P=S in a ratio of 3:1. The latter mixture was used in situ for the deoxyfluorination of carboxylic acids [23]. For both processes, SF6 was presumably initially reduced to give SF6−. The latter can generally transform into an SF5 radical and a fluoride, or into SF5− and a fluorine radical [17,24,25]. SF5CF3, however, produces CF3 radicals and the SF5− anion after reduction [26,27]. In this paper, we report on the thermal and photochemical activation of SF5CF3 by the N-heterocyclic carbene SIMes to result in fluorinated and trifluoromethylated heterocycles [28]. The photochemical activation process of SF5CF3 using triphenylphosphine was studied for comparison.

2. Results

2.1. Thermal Activation of SF5CF3 with SIMes

Heating a 1:1 mixture of SIMes and SF5CF3 at 90 °C for 190 min in toluene-d8 led to the formation of 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine (1), SIMesF2 (2) and 1,3-dimesitylimidazolidine-2-sulfide (3), with NMR yields of 18%, 31%, and 31% based on the amount of SF5CF3 (Scheme 1). The 19F NMR spectrum (Figure 1) of the mixture shows a signal at δ = −55.8 ppm for SIMesF2 (2), which is consistent with the literature [15,28,29], as well as a doublet at δ = −76.3 with a coupling constant 3JFF of 4.2 Hz, and a quartet at δ = −83.1 with a coupling constant 3JFF of 4.4 Hz for compound 1. The formation of 3 was further confirmed through comparing the 1H NMR spectrum with the data reported in the literature [15]. It was noted that traces of trifluoromethane were also observed according to the 19F NMR spectrum. Heating the sample further for one hour led to a decrease in the amount of 1 and SIMesF2 (2) and, instead, more trifluoromethane and trifluoromethane-d1 were observed. The latter can be formed due to the reaction of an intermediate CF3 radical (see below) via hydrogen or deuterium atom transfer. Attempts to achieve a similar transformation in benzene gave the considerably lower amounts of 1, 2, and 3, possibly due to the lower reaction temperature, which was limited by the boiling point of benzene.

2.2. Photolytic Activation of SF5CF3 with SIMes

When a mixture of SF5CF3 and 6.7 equivalents of SIMes was irradiated at 311 nm in benzene-d6 for 3 h, the formation of 62% of 1, and 105% of 2 and 3 was observed, as well as 1% of α,α,α-trifluorotoluene-d5 (Scheme 2). All yields are NMR yields based on the amount of SF5CF3. Further irradiation for 16 h led to the hydrolysis of 2 by adventitious water, to give a urea derivative and HF. The generation of 4 could then be due to the presence of HF. Compound 4 and the thiourea product 3 were observed in a ratio of 4:7. In addition, as described below, the trifluoromethylation of C6D6 will result in the generation of DF. DF can subsequently lead an FDF− derivative of 4. Compound 4 shows a singlet at −65.68 ppm in the 19F NMR spectrum for the CF3 group and a broad signal at −169.40 ppm, indicating the presence of an FHF− anion. The presence of the cation was also confirmed via ESI-MS. Independently synthesized 1,3-dimesityl-2-trifluoromethylimidazolinium tetrafluoroborate showed the same signal in the 19F NMR spectrum. The formation of α,α,α-trifluorotoluene-d5 with a yield of 22% (based on SF5CF3) was confirmed via 19F NMR spectroscopy and GC-MS. The irradiation of a benzene-d6 solution of SF5CF3 at 311 nm for 168 h without the presence of SIMes gave α,α,α-trifluorotoluene-d5 with a yield of 2% only. With toluene-d8 as a solvent, the photochemical activation of SF5CF3 with SIMes led to the formation of CD3C6D5CF3, although the reaction is not selective, and small amounts for unknown products can be detected in the 19F NMR spectrum. Trifluoromethylation proceeded at the ortho (9%, NMR yield based on the consumption of SF5CF3) and para (5%) position of toluene-d8, but also at the meta position (4%) [22,30,31].

2.3. Mechanisms for the Activation of SF5CF3

Compound 1 was then investigated regarding its ability to transfer a CF3 group to aromatics. Thus, the reaction mixture of 1, SIMesF2 (2) and 3 in C6D6, obtained for the thermal SF5CF3 activation, (Scheme 1) was degassed under a vacuum to remove any excess of SF5CF3 and irradiated afterwards at 311 nm. No formation of α,α,α-trifluorotoluene-d5 was observed. This suggests that 1 is not capable of transferring a CF3 group to aromatics. However, as mentioned above, the formation of compound 1 resembles a process known in the literature for the photochemical activation of SF6 by N-heterocyclic carbenes, which is initiated by an electron transfer [15,28]. Thus, a SET (single-electron transfer) after carbene excitation to SF5CF3 can be proposed to give a carbene radical cation and SF5CF3− radical anion (Scheme 3). The formation of an N-heterocyclic carbene radical cation as an intermediate has been proposed by Severin et al. in their discussion of the reaction between SIMes and [Ph3C][B(C6F5)4] to yield [SIMes-C6H5–CPh2]+ at −40 °C [32,33]. The SF5CF3− radical anion will then decompose to give SF5− and a CF3 radical. The latter transformation is consistent with low-temperature electron attachment experiments [27,34,35]. The formed CF3 radical recombines with the SIMes radical cation to form 5 bearing an SF5− anion. The SF5− anion can convert into SF4 and 6 [24,36,37]. Compound 6 then reacts to give the observed compound 1 via the nucleophilic attack of the fluoride. SF4 reacts further, yielding SIMesF2 (2) and the thiourea derivative 3, as was shown in independent experiments [15]. For the thermal activation of SF5CF3, a comparable transformation can be imagined, although electron transfer can be hampered by a kinetic barrier. In this regard, an incipient transition state or pre-interaction of the nucleophilic carbene with SF5CF3 seems to be conceivable [8,23,38,39]. It should be noted that an ion flow tube study shows that OH− reacts with SF5CF3, yielding CF3OH and SF5− [40]. As mentioned above, DF can be formed in association with the photochemical trifluoromethylation of the aromatic compounds. For this process, initially a cyclohexadienyl radical via reaction with a CF3 radical with C6D6 might be generated [22,30,41,42,43]. The cyclohexadienyl radical can then transfer an electron to the SIMes radical cation, giving a cyclohexadienyl cation. The latter reacts with a fluoride, which stems from SF5−, and forms α,α,α-trifluorotoluene-d5, as well as DF.
To confirm the presence of radical intermediates, TEMPO (2,2,6,6-tetramethylpiperidinyloxy) was added to a mixture of SIMes and SF5CF3 in C6D6. After 5 h of irradiation at 311 nm, signals for SIMesF2 (2) and TEMPO-CF3 (8) [44] in a ratio of 1:1 were observed in the 19F NMR spectrum (Scheme 4). The presence of the thiourea derivative 3 was confirmed via 1H NMR spectroscopy, as well as via GC-MS. It should be noted that the addition of TEMPO to the reaction of SF5CF3 with SIMes under non-photolytic conditions did not show the formation of TEMPO-CF3, which indicates that no CF3 radicals were formed.
Furthermore, 1,1-diphenylethylene was used as an additional trapping reagent for the CF3 radical. After irradiation at 311 nm for 4 h, the trifluoromethylated olefin 9 (0.5% based on the amount of 1,1-diphenylethylene), and the trifluoroalkane 10 (0.6% based on the amount of 1,1-diphenylethylene) were observed in a ratio of 2:3, as well as SIMesF2 2 (25% based on the amount of SIMes), 1 (10% based on the amount of SIMes), and the thiourea derivative 3, among traces of other compounds, such as trifluoromethane (Scheme 4). Mechanistically, the CF3 radical reacts with 1,1-diphenylethylene, and a trifluoromethylbenzyl radical is formed. Two molecules of the latter can generate the olefin 9 and the alkane 10 via hydrogen atom transfer. The formation of trifluoromethane can be explained by HAT from the trifluoromethylbenzyl radical to also yield 9.

2.4. Activation of SF5CF3 with Triphenylphosphine

The described reactivity of SIMes was compared with that of PPh3. Thus, the photolysis of PPh3 and SF5CF3 at 353 nm led to the formation of 11 with a yield of 10%, and 12 with a yield of 28% (based on the amount of PPh3, see Scheme 5). α,α,α-Trifluorotoluene-d5 was formed, as well. Additionally, traces of F3PPh2, Ph2PF4−, and O=PPh2F were observed, according to the 19F NMR spectra [45]. In contrast to the described reactivity with SIMes, the generation of a phosphorane containing a CF3 group was not observed. The products were identified via 31P NMR, 19F NMR spectroscopy, as well as ESI-MS, and the data are consistent with compounds known in the literature [23]. Irradiation at a wavelength of 375 nm led to the formation of only small amounts of phosphorane 12, and only traces of α,α,α-trifluorotoluene-d5. No thermal activation of SF5CF3 in toluene-d8 could be achieved via heating the reaction solution at 100 °C for 9 h. Notably, Buß et al. reported on the thermal activation of SF6 using strongly basic phosphines [8], but could not observe the thermal activation of SF6 with PPh3, due to the lower nucleophilicity of the latter [23].
A possible mechanism involves SET from the phosphine to the SF5CF3, resulting in the formation of a SF5CF3− radical anion and a PPh3+ radical cation (Scheme 6). The SF5CF3 radical anion then decomposes to give a CF3 radical and a SF5− anion [27,34,35]. The latter can either decompose to fluoride and SF4, or give a Ph3PF radical via reaction with PPh3+. PPh3 reacts with SF4, yielding F2PPh3 and SPPh3. The generated Ph3PF might become further fluorinated via intermediate sulfur fluorides or SF4, to yield PPh3F2. The CF3 radical reacts with the solvent C6D6, yielding α,α,α-trifluorotoluene-d5 and, presumably, DF, possibly via the re-oxidation of a cyclohexadienyl radical cation with PPh3+, and subsequent deprotonation with fluoride. It should be noted that Dielmann et al. also proposed, for the photochemical activation of SF6 with triphenylphosphine, a mechanism based on DFT calculations, in which an electron is initially transferred from a π orbital of an arene moiety of PPh3 to the delocalized σ* orbital of SF6 [23].

3. Materials and Methods

3.1. General Instruments, Methods, and Materials

All reactions were performed under an argon atmosphere, to exclude air and moisture. Chemicals were stored in an argon-filled glass apparatus, using the standard Schlenk-technique. SIMes was synthesized from 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-1-ium tetrafluoroborate, KOtBu, and NaH, all of which were heated at night under a vacuum prior to use. TEMPO, SF5CF3, and PPh3 were purchased from commercial sources, and used without further purification. 1,1-Diphenylethylene was stored over a molecular sieve (3 Å) before use. Toluene-d8, C6D6, and THF-d8 were stored over Solvona®. All solvents were distilled and degassed prior to use, and stored under argon over molecular sieves (3 Å). As the light source, an LED lamp with a peak wavelength of 375 nm from Innotas Produktions GmbH (Zittau, Germany) was used, as well as a photo Multirays reactor (Helios Italquartz, Cambiago, Italy) equipped with ten light sources (15 W), with an emission maximum of 311 nm or 353 nm. The NMR spectra were recorded at room temperature with a Bruker AV III 300 or Bruker DPX 300 spectrometer (Ettlingen, Germany). The chemical shifts in the 1H and 13C{1H} NMR spectra were calibrated to the residual solvent signal of the deuterated solvents. The 1H NMR spectra were referenced as C6D5H: δ = 7.16 ppm; toluene-d7: δ = 6.97 ppm; CHD2CN: δ = 1.94 ppm, and CHDCl2: δ = 5.32 ppm. The 13C{1H} NMR spectra were referenced as C6D6: δ = 128.06 ppm; toluene-d8: δ = 20.43 ppm; CD3CN: δ = 1.32 ppm and CD2Cl2: δ = 53.84 ppm. The 19F NMR spectra were referenced externally to CFCl3 at δ = 0.0 ppm. As an internal standard for the quantification, 1-fluoropentane was used with δ of −217.6 ppm in the 19F NMR spectrum. GC–MS measurements were conducted using an Agilent 6890N gas chromatograph with a capillary column (Agilent 19091S-433 Hewlett-Packard 5 MS: 30 m length, 0.25 mm inside diameter, 0.25 μm film thickness) and an Agilent 5973 Network mass selective detector. Helium (0.74 bar, 1.2 mL/min, 40 cm/s) was used as the carrier gas. The electron impact ionization was carried out with an ionization voltage of 70 eV. Mass spectra were measured with a Micromass Q-Tof-2 instrument, with a Linden LIFDI source (Linden CMS GmbH, Weyhe, Germany). ESI-MS spectra were recorded using an ADVION EXPRESSION CMS spectrometer, as an eluent CD3CN was used, and the sample was directly injected into the instruments. The data were analyzed using ADVION DATA EXPRESS Version 6.0.11.3. Caution: in some experiments, traces of HF were generated. Immediate access to procedures in case of contact with HF-containing solutions must be available.

3.2. Activation of SF5CF3 with SIMes by Heating

A PFA tube was filled with SIMes (0.016 g, 0.0525 mmol) and 1-fluoropentane (6 μL, 0.0525 mmol) as an internal standard. The tube was attached to a steel line, and C6D6 (0.4 mL) was condensed into the PFA tube. After the solvent was degassed, SF5CF3 (175 mbar, 0.0525 mmol) was condensed into the PFA tube, which was then flame-sealed under a vacuum. The reaction mixture was heated at 90 °C for 3 h. Compound 1 was detected with a yield of 18%, 2 was detected with a yield of 31%, and 3 was detected with a yield of 31%. All yields are NMR yields (internal standard: 1-fluoropentane) based on SF5CF3.
NMR data for 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine 1: 19F NMR (282.4 MHz, tol-d8): δ = −76.3 (d, 3F, 3JFF = 3.8 Hz, CF3), −83.1 (q, 1 F, 3JFF = 3.8 Hz, F) ppm.
NMR data for 1,3-dimesityl-2,2-difluoroimidazolidin 2: 19F NMR (282.4 MHz, tol-d8): δ = −55.8 (s) ppm. The obtained NMR data are consistent with those in the literature [15].
Analytical data for 1,3-dimesitylimidazolidine-2-sulfide 3: 1H NMR (300.1 MHz, tol-d8): δ = 2.09 (m, 6 H, p-CH3), 2.22 (m, 12 H, o-CH3), 3.21 (s, 4H, NCH2CH2N), 6.71 (s, 4H, HAr) ppm., GC-MS (tol-d8): calculated (m/z) for [3]: 338.18 experimental (m/z) for [3]: 338. The obtained NMR data are consistent with those in the literature [15].
Reaction products 1, 2, and 3 were degassed (freed of SF5CF3) and irradiated at 311 nm in the photochemical reactor at 311 nm. After irradiation for 16 h, no trifluoromethylated solvent was observed.

3.3. Photochemical Activation of SF5CF3 with SIMes

A PFA tube was filled with SIMes (0.032 g, 0.105 mmol) and 1-fluoropentane (6 μL, 0.0525 mmol) as an internal standard. The tube was attached to a steel line, and C6D6 (0.4 mL) was condensed on top. After the solvent was degassed, SF5CF3 (87 mbar, 0.026 mmol, 1 eq) was condensed into the solution, and the PFA tube was flame-sealed under a vacuum. The reaction mixture was irradiated in a UV reactor (311 nm) at room temperature for 16 h.
Analytical data for 4: 19F NMR (282.4 MHz, C6D6): δ = −170.1 (br s, 2H), −65.6 (s, 3F) ppm., ESI-MS (CD3CN): calculated (m/z) for [4]+: 375.2, experimental (m/z) for [4]+: 375.3.
Analytical data for α,α,α-trifluorotoluene-d5: 19F NMR (282.4 MHz, C6D6): δ = −62.4 ppm., GC-MS (C6D6): calculated (m/z) for [α,α,α-trifluorotoluene-d5]: 151, experimental (m/z) for [α,α,α-trifluorotoluene-d5]: 151. The obtained NMR data are consistent with those in the literature [22].

3.4. Experiments to Trap Radicals

3.4.1. Addition of TEMPO to Reaction Mixture

A PFA tube was filled with SIMes (0.015 g, 0.05 mmol) and TEMPO (0.034 g, 0.22 mmol, 4.4 eq). The tube was attached to a steel line, and C6D6 (0.4 mL) was condensed on top. After the solvent was degassed, SF5CF3 (300 mbar, 0.09 mmol, 1.8 eq) was condensed into the solution, and the PFA tube was flame-sealed under a vacuum. The reaction mixture was irradiated in a UV reactor (311 nm) at room temperature for 12 h. TEMPO-CF3, SIMesF2, and compound 1 (1,3-Bis(2,4,6-trimethylphenyl)-imidazolidin-2-sulfide) were observed via 19F and 1H NMR spectroscopy and GC-MS.
Analytical data for 8: 19F NMR (282.4 MHz, C6D6): δ = −56.5 ppm. The obtained NMR data are consistent with those in the literature [15,22].

3.4.2. Addition of 1,1-Diphenylethylene

A PFA tube was filled with SIMes (0.016 g, 0.0525 mmol) and 1,1-diphenylethylene (46 µL, 0.263 mmol, 5 eq). The tube was attached to a steel line, and C6D6 (0.4 mL) was condensed on top. After the solvent was degassed, SF5CF3 (175 mbar, 0.0525 mmol, 1 eq) was condensed into the solution, and the PFA tube was flame-sealed under a vacuum. The reaction mixture was irradiated in a UV reactor (311 nm) at room temperature for 3 h. Compounds 9 and 10 were observed via 19F NMR spectroscopy with a 0.5% and 0.6% NMR yield (compared to 1,1-diphenylethylene), and via GC-MS; SIMesF2 2 was observed via 19F NMR spectroscopy with 25% (NMR yield, based on the amount of SIMes), 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine 1 was observed with a yield of 10% (NMR yield, compared to the amount of SIMes), and a signal for compound 3 was observed via 1H NMR spectra and GC-MS.
Analytical data for 9: 19F NMR (282.4 MHz, C6D6): δ = −55.6 (t, 3JFF = 10.2 Hz) ppm, GC-MS (C6D6): calculated (m/z) for [9]: 248.08, experimental (m/z) for [9]: 248. The obtained NMR data are consistent with those in the literature [46,47].
Analytical data for 10: 19F NMR (282.4 MHz, C6D6): δ = −63.3 (t, 3JFH = 10.2 Hz) ppm., GC-MS (C6D6): calculated (m/z) for [10]: 250.10, experimental (m/z) for [10]: 250. The obtained NMR data are consistent with those in the literature [46,48].

3.5. Activation of SF5CF3 with PPh3

A Young flask was filled with PPh3 (0.5 g, 1.9 mmol) and 1-fluoropentane (100 μL, 0.875 mmol) as an internal standard. The mixture was dissolved in C6D6 (20 mL). The Young flask was attached to a steel line. After the solvent was degassed, the flask was filled with SF5CF3 (1.3 bar). The reaction mixture was irradiated in a UV reactor (353 nm) at room temperature for 43 h. Compounds 11 and 12 were observed via 19F and 31P NMR spectroscopy. A signal for α,α,α-trifluorotoluene-d5 was observed (0.065 mmol) via 19F NMR spectroscopy.
Analytical data for 11: 31P NMR (121.5 MHz, C6D6): δ = 42.28 (s)., ESI-MS (CD3CN): calculated (m/z) for [11 + 2Na+ + H−]+: 341.05, experimental (m/z) for [11 + 2Na+ + H−]+: 341.05. The obtained NMR data are consistent with those in the literature [13,23].
Analytical data for 12: 19F NMR (282.4 MHz, C6D6): δ = −39.0 (d, 1JFP = 664.44 Hz) ppm., 31P NMR (121.5 MHz, C6D6): δ = −55.21 (t, 1JFP = 663.42 Hz), ESI-MS (CD3CN): calculated (m/z) for [13 + K]+: 339.05, experimental (m/z) for [13 + K]+: 339.3. The obtained NMR data are consistent with those in the literature [13,23].

4. Conclusions

In conclusions, reaction routes for the activation of the greenhouse gas SF5CF3 with SIMes and PPh3 were developed. Photochemical processes presumably proceed by an initial electron transfer to the fluorinated substrate, and provide CF3 radicals. This is revealed via trapping experiments of a CF3 radical, and also the trifluoromethylation of C6D6. The studies complement efforts regarding the activation and degradation of fluorinated compounds [49,50,51,52,53,54,55,56].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28186693/s1, Figures S1–S10: Figure S1: 19F NMR spectra for 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine (1) and 1,3-dimesityl -2,2-difluoroimidazolidin (2) in toluene-d8 after heating of SIMes and SF5CF3 at 90 °C; Figure S2: 19F{1H} NMR spectra for 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine (1) and 1,3-dimesityl -2,2-difluoroimidazolidin (2) in toluene-d8 after heating of SIMes and SF5CF3 at 90 °C; Figure S3: 19F19F COSY NMR spectrum for 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine (1) in toluene-d8 after heating of SIMes and SF5CF3 at 90 °C; Figure S4: 19F NMR spectrum for 1,3-dimesityl-2-fluoro-2-trifluoromethylimidazolidine (1) and SIMesF2 (2) in C6D6 after irradiation of SIMes and SF5CF3 at 311 nm after 3 h; Figure S5: 1H NMR spectrum for compounds 3 and 4 in C6D6 after irradiation of SIMes and SF5CF3 at 311 nm after 16 h; Figure S6: 19F NMR spectrum for compound 4 in C6D6 after irradiation of SIMes and SF5CF3 at 311 nm after 16 h; Figure S7: 19F NMR spectrum for compound 8 in C6D6; Figure S8: 19F NMR spectrum for compounds 1, 2, 9 and 10 in C6D6 after irradiation of SIMes, SF5CF3 and 1,1-diphenylethylene at 311 nm for 3 h; Figure S9: 19F NMR spectrum for compound 12 and α,α,α-trifluorotoluene-d5 in C6D6; Figure S10: 31P NMR spectrum for compounds 11 and 12 in C6D6.

Author Contributions

Conceptualization, T.B.; methodology, D.H. and P.T.; investigation, D.H. and P.T.; writing—original draft preparation, D.H.; writing—review and editing, D.H. and T.B.; supervision, T.B.; funding acquisition, T.B. All authors have read and agreed to the published version of the manuscript.

Funding

We acknowledge financial support from the CRC1349 funded by the Deutsche Forschungsgemeinschaft (German Research Foundation; Gefördert durch die Deutsche Forschungsgemeinschaft (DFG) Projektnummer 387284271–SFB 1349), as well as the graduate school SALSA (School of Analytical Science Adlershof).

Informed Consent Statement

Not applicable.

Data Availability Statement

Supporting information containing NMR spectra is available.

Conflicts of Interest

There are no conflict of interest to declare.

Sample Availability

Not applicable.

References

  1. Sturges, W.; Wallington, T.J.; Hurley, M.D.; Shine, K.; Sihra, K.; Engel, A.; Oram, D.E.; Penkett, S.A.; Mulvaney, R.; Brenninkmeijer, C.A.M. A potent greenhouse gas identified in the atmosphere: SF5CF3. Science 2000, 289, 611–613. [Google Scholar] [CrossRef] [PubMed]
  2. Savoie, P.R.; Welch, J.T. Preparation and utility of organic pentafluorosulfanyl-containing compounds. Chem. Rev. 2015, 115, 1130–1190. [Google Scholar] [CrossRef] [PubMed]
  3. Lu, N.; Kumar, H.P.S.; Fye, J.L.; Sun Blanks, J.; Thrasher, J.S.; Willner, H.; Oberhammer, H. Synthesis of the Long Sought after Compound Pentafluoronitrosulfane, SF5NO2. Angew. Chem. Int. Ed. 2006, 118, 952–954. [Google Scholar] [CrossRef]
  4. Holze, P.; Horn, B.; Limberg, C.; Matlachowski, C.; Mebs, S. The Activation of Sulfur Hexafluoride at Highly Reduced Low-Coordinate Nickel Dinitrogen Complexes. Angew. Chem. Int. Ed. 2014, 126, 2788–2791. [Google Scholar] [CrossRef]
  5. Zámostná, L.; Braun, T. Catalytic Degradation of Sulfur Hexafluoride by Rhodium Complexes. Angew. Chem. Int. Ed. 2015, 127, 10798–10802. [Google Scholar] [CrossRef]
  6. Wozniak, M.; Braun, T.; Ahrens, M.; Braun-Cula, B.; Wittwer, P.; Herrmann, R.; Laubenstein, R. Activation of SF6 at a Xantphos-Type Rhodium Complex. Organometallics 2018, 37, 821–828. [Google Scholar] [CrossRef]
  7. Rueping, M.; Nikolaienko, P.; Lebedev, Y.; Adams, A. Metal-free reduction of the greenhouse gas sulfur hexafluoride, formation of SF5 containing ion pairs and the application in fluorinations. Green Chem. 2017, 19, 2571–2575. [Google Scholar] [CrossRef]
  8. Buß, F.; Mück-Lichtenfeld, C.; Mehlmann, P.; Dielmann, F. Nucleophilic Activation of Sulfur Hexafluoride: Metal-Free, Selective Degradation by Phosphines. Angew. Chem. Int. Ed. 2018, 130, 5045–5049. [Google Scholar] [CrossRef]
  9. Eder, T.; Buß, F.; Wilm, L.F.B.; Seidl, M.; Podewitz, M.; Dielmann, F. Oxidative Fluorination of Selenium and Tellurium Compounds using a Thermally Stable Phosphonium SF5− Salt Accessible from SF6. Angew. Chem. Int. Ed. 2022, 61, e202209067. [Google Scholar] [CrossRef]
  10. Huchenski, B.S.N.; Speed, A.W.H. Room-temperature reduction of sulfur hexafluoride with metal phosphides. Chem. Commun. 2021, 57, 7128–7131. [Google Scholar] [CrossRef]
  11. Weitkamp, R.F.; Neumann, B.; Stammler, H.-G.; Hoge, B. Non-Coordinated Phenolate Anions and Their Application in SF6 Activation. Chem. Eur. J. 2021, 27, 6460–6464. [Google Scholar] [CrossRef] [PubMed]
  12. Taponard, A.; Jarrosson, T.; Khrouz, L.; Médebielle, M.; Broggi, J.; Tlili, A. Metal-Free SF6 Activation: A New SF5 -Based Reagent Enables Deoxyfluorination and Pentafluorosulfanylation Reactions. Angew. Chem. Int. Ed. 2022, 61, e202204623. [Google Scholar] [CrossRef] [PubMed]
  13. Kim, S.; Khomutnyk, Y.; Bannykh, A.; Nagorny, P. Synthesis of Glycosyl Fluorides by Photochemical Fluorination with Sulfur(VI) Hexafluoride. Org. Lett. 2021, 23, 190–194. [Google Scholar] [CrossRef] [PubMed]
  14. Kim, S.; Nagorny, P. Electrochemical Synthesis of Glycosyl Fluorides Using Sulfur(VI) Hexafluoride as the Fluorinating Agent. Org. Lett. 2022, 24, 2294–2298. [Google Scholar] [CrossRef] [PubMed]
  15. Tomar, P.; Braun, T.; Kemnitz, E. Photochemical activation of SF6 by N-heterocyclic carbenes to provide a deoxyfluorinating reagent. Chem. Commun. 2018, 54, 9753–9756. [Google Scholar] [CrossRef] [PubMed]
  16. Zámostná, L.; Braun, T.; Braun, B. S-F and S-C activation of SF6 and SF5 derivatives at rhodium: Conversion of SF6 into H2S. Angew. Chem. Int. Ed. 2014, 53, 2745–2749. [Google Scholar] [CrossRef] [PubMed]
  17. Rombach, D.; Wagenknecht, H.-A. Photoredoxkatalytische α-Alkoxypentafluorosulfanylierung von α-Methyl- und α-Phenylstyrol mithilfe von SF6. Angew. Chem. Int. Ed. 2020, 132, 306–310. [Google Scholar] [CrossRef]
  18. Dresdner, R. A New Synthesis of Simple Fluorocarbon Tertiary Amines. J. Am. Chem. Soc. 1956, 77, 69–70. [Google Scholar]
  19. Dresdner, R. The Pyrolysis of Trifluoromethyl Sulfur Pentafluoride and its Reaction with Perfluoropropylene. J. Am. Chem. Soc. 1955, 77, 6633–6634. [Google Scholar] [CrossRef]
  20. Dresdner, R.D.; Mao, T.J.; Young, J.A. Some Thermal Reactions of Perfluoroalkyl Derivatives of SF6 with Fluorocarbon Olefins. J. Am. Chem. Soc. 1957, 80, 3007–3009. [Google Scholar] [CrossRef]
  21. Huang, L.; Shen, Y.; Dong, W.; Zhang, R.; Zhang, J.; Hou, H. A novel method to decompose two potent greenhouse gases: Photoreduction of SF6 and SF5CF3 in the presence of propene. J. Hazard. Mater. 2008, 151, 323–330. [Google Scholar] [CrossRef] [PubMed]
  22. Herbstritt, D.; Braun, T. Reduction of SF5CF3 via iridium catalysis: Radical trifluoromethylation of aromatics. Chem. Commun. 2023, 59, 3850–3853. [Google Scholar] [CrossRef] [PubMed]
  23. Rotering, P.; Mück-Lichtenfeld, C.; Dielmann, F. Solvent-free photochemical decomposition of sulfur hexafluoride by phosphines: Formation of difluorophosphoranes as versatile fluorination reagents. Green Chem. 2022, 24, 8054–8061. [Google Scholar] [CrossRef]
  24. Iakobson, G.; Pošta, M.; Beier, P. Reductive activation of sulfur hexafluoride with TEMPOLi: Addition of the pentafluorosulfanyl group and TEMPO to terminal alkenes. J. Fluor. Chem. 2018, 213, 51–55. [Google Scholar] [CrossRef]
  25. Akhgarnusch, A.; Höckendorf, R.F.; Beyer, M.K. Thermochemistry of the Reaction of SF6 with Gas-Phase Hydrated Electrons: A Benchmark for Nanocalorimetry. J. Phys. Chem. A 2015, 119, 9978–9985. [Google Scholar] [CrossRef] [PubMed]
  26. Kennedy, R.A.; Mayhew, C. A study of low energy electron attachment to trifluoromethyl sulphur pentafluoride, SF5CF3: Atmospheric implications. Int. J. Mass Spectrom. 2001, 206, vii-x. [Google Scholar] [CrossRef]
  27. Sailer, W.; Drexel, H.; Pelc, A.; Grill, V.; Illenberger, N.; Skalny, J.D.; Mikoviny, T.; Scheier, P.; Märk, T.D. Low energy electron attachment to SF5CF3. Chem. Phys. Lett. 2002, 351, 71–78. [Google Scholar] [CrossRef]
  28. Tomar, P. N-Heterocyclic Carbene Derivatives for the Activation of Sulfur Fluorides. Ph.D. Thesis, Humboldt-Universität zu Berlin, Berlin, Germany, 2021. [Google Scholar]
  29. Herbstritt, D.; Tomar, P.; Müller, R.; Kaupp, M.; Braun, T. A 2,2-Difluoroimidazolidine Derivative for Deoxyfluorination Reactions: Mechanistic Insights by Experimental and Computational Studies. Chem. Eur. J. 2023, e202301556. [Google Scholar] [CrossRef]
  30. Nagib, D.A.; MacMillan, D.W.C. Trifluoromethylation of arenes and heteroarenes by means of photoredox catalysis. Nature 2011, 480, 224–228. [Google Scholar] [CrossRef]
  31. Studer, A. A “Renaissance” in Radical Trifluoromethylation. Angew. Chem. Int. Ed. 2012, 124, 9082–9090. [Google Scholar] [CrossRef]
  32. Dong, Z.; Pezzato, C.; Sienkiewicz, A.; Scopelliti, R.; Fadaei-Tirani, F.; Severin, K. SET processes in Lewis acid-base reactions: The tritylation of N-heterocyclic carbenes. Chem. Sci. 2020, 11, 7615–7618. [Google Scholar] [CrossRef] [PubMed]
  33. Ramnial, T.; McKenzie, I.; Gorodetsky, B.; Tsang, E.M.W.; Clyburne, J.A.C. Reactions of N-heterocyclic carbenes (NHCs) with one-electron oxidants: Possible formation of a carbene cation radical. Chem. Commun. 2004, 9, 1054–1055. [Google Scholar] [CrossRef] [PubMed]
  34. Chim, R.; Kennedy, R.; Tuckett, R. The vacuum-UV absorption spectrum of SF5CF3; implications for its lifetime in the earth’s atmosphere. Chem. Phys. Lett. 2003, 367, 697–703. [Google Scholar] [CrossRef]
  35. Solovev, S.; Palmentieri, A.; Potekhina, N.D.; Madey, T.E. Mechanism for Electron-Induced SF5CF3 Formation in Condensed Molecular Films. J. Phys. Chem. C 2007, 111, 18271–18278. [Google Scholar] [CrossRef]
  36. Goettel, J.T.; Kostiuk, N.; Gerken, M. The Solid-State Structure of SF4: The Final Piece of the Puzzle. Angew. Chem. Int. Ed. 2013, 125, 8195–8198. [Google Scholar] [CrossRef]
  37. Kostiuk, N.; Goettel, J.T.; Gerken, M. Synthesis and Characterization of SF4 Adducts with Polycyclic Amines. Inorg. Chem. 2020, 59, 8620–8628. [Google Scholar] [CrossRef]
  38. Masiak, P.; Sobolewski, A.L. Theoretical study of the photophysics of SF5CF3. Chem. Phys. 2005, 313, 169–176. [Google Scholar] [CrossRef]
  39. Kivimäki, A.; Álvarez Ruiz, J.; Coreno, M.; Stankiewicz, M.; Fronzoni, G.; Decleva, P. Photoelectron spectroscopy of sulfur L levels in the SF5CF3 molecule. Chem. Phys. 2008, 353, 202–208. [Google Scholar] [CrossRef]
  40. Arnold, S.T.; Thomas, M.; Viggiano, A.A.; Mayhew, C.A. A temperature-dependent selected ion flow tube study of anions reacting with SF5CF3. Int. J. Mass Spectrom. IJMS 2003, 223–224, 403–409. [Google Scholar] [CrossRef]
  41. Kamigata, N.; Ohtsuka, T.; Fukushima, T.; Yoshida, M.; Shimizu, M. Direct perfluoroalkylation of aromatic and heteroaromatic compounds with perfluoroalkanesulfonyl chlorides catalysed by a ruthenium(II) phosphine complex. J. Chem. Soc. Perkin Trans. 1 1994, 10, 1339–1346. [Google Scholar] [CrossRef]
  42. Ouyang, Y.; Xu, X.-H.; Qing, F.-L. Trifluoromethanesulfonic Anhydride as a Low-Cost and Versatile Trifluoromethylation Reagent. Angew. Chem. Int. Ed. 2018, 57, 6926–6929. [Google Scholar] [CrossRef] [PubMed]
  43. Harris, C.F.; Kuehner, C.S.; Bacsa, J.; Soper, J.D. Photoinduced Cobalt(III)-Trifluoromethyl Bond Activation Enables Arene C-H Trifluoromethylation. Angew. Chem. Int. Ed. 2018, 57, 1311–1315. [Google Scholar] [CrossRef] [PubMed]
  44. Wang, Y.-F.; Lonca, G.H.; Chiba, S. PhI(OAc)(2) -mediated radical trifluoromethylation of vinyl azides with Me3SiCF3. Angew. Chem. Int. Ed. 2014, 53, 1067–1071. [Google Scholar] [CrossRef] [PubMed]
  45. Zhao, S.; Guo, Y.; Su, Z.; Wu, C.; Chen, W.; Chen, Q.-Y. Deoxyfluorination of Carboxylic, Sulfonic, Phosphinic Acids and Phosphine Oxides by Perfluoroalkyl Ether Carboxylic Acids Featuring CF2O Units. Chin. J. Chem. 2021, 39, 1225–1232. [Google Scholar] [CrossRef]
  46. Zhang, H.-R.; Feng, C.-C.; Chen, N.; Zhang, S.-L. Direct Arene Trifluoromethylation Enabled by a High-Valent CuIII -CF3 Compound. Angew. Chem. Int. Ed. 2022, 61, e202209029. [Google Scholar] [CrossRef] [PubMed]
  47. Zhang, H.-R.; Xiao, C.; Zhang, S.-L.; Zhang, X. Radical C−H Bond Trifluoromethylation of Alkenes by High-Valent Copper(III) Trifluoromethyl Compounds. Adv. Synth. Catal. 2019, 361, 5305–5310. [Google Scholar] [CrossRef]
  48. Wang, F.; Wang, D.; Mu, X.; Chen, P.; Liu, G. Copper-catalyzed intermolecular trifluoromethylarylation of alkenes: Mutual activation of arylboronic acid and CF3+ reagent. J. Am. Chem. Soc. 2014, 136, 10202–10205. [Google Scholar] [CrossRef] [PubMed]
  49. Kim, Y.; Lee, E. Activation of C-F bonds in fluoroarenes by N-heterocyclic carbenes as an effective route to synthesize abnormal NHC complexes. Chem. Commun. 2016, 52, 10922–10925. [Google Scholar] [CrossRef]
  50. Kolychev, E.L.; Bannenberg, T.; Freytag, M.; Daniliuc, C.G.; Jones, P.G.; Tamm, M. Reactivity of a frustrated lewis pair and small-molecule activation by an isolable Arduengo carbene-B{3,5-(CF3)2C6H3}3 complex. Chem. Eur. J. 2012, 18, 16938–16946. [Google Scholar] [CrossRef]
  51. Kronig, S.; Theuergarten, E.; Holschumacher, D.; Bannenberg, T.; Daniliuc, C.G.; Jones, P.G.; Tamm, M. Dihydrogen activation by frustrated carbene-borane Lewis pairs: An experimental and theoretical study of carbene variation. Inorg. Chem. 2011, 50, 7344–7359. [Google Scholar] [CrossRef]
  52. Kuhn, N.; Fahl, R.B.; Henkel, G. On the Reaction of 2,3-Dihydroimidazol-2-ylidenes with Pentafluoropyridine: Carbenes as Reactants in Nucleophilic Aromatic Substitution. Z. Naturforsch. 1998, 53, 881–886. [Google Scholar] [CrossRef]
  53. Leclerc, M.C.; Gorelsky, S.I.; Gabidullin, B.M.; Korobkov, I.; Baker, R.T. Selective Activation of Fluoroalkenes with N-Heterocyclic Carbenes: Synthesis of N-Heterocyclic Fluoroalkenes and Polyfluoroalkenyl Imidazolium Salts. Chem. Eur. J. 2016, 22, 8063–8067. [Google Scholar] [CrossRef]
  54. Paul, U.; Radius, U. Ligand versus Complex: C-F and C-H Bond Activation of Polyfluoroaromatics at a Cyclic (Alkyl)(Amino)Carbene. Chem. Eur. J. 2017, 23, 3993–4009. [Google Scholar] [CrossRef]
  55. Sen, S.; Roesky, H.W. Silicon-fluorine chemistry: From the preparation of SiF2 to C-F bond activation using silylenes and its heavier congeners. Chem. Commun. 2018, 54, 5046–5057. [Google Scholar] [CrossRef]
  56. Soleilhavoup, M.; Bertrand, G. Cyclic (alkyl)(amino)carbenes (CAACs): Stable carbenes on the rise. Acc. Chem. Res. 2015, 48, 256–266. [Google Scholar] [CrossRef] [PubMed]
Molecules 28 06693 sch001
Scheme 1. Activation of SF5CF3 at 90 °C in toluene-d8.
Scheme 1. Activation of SF5CF3 at 90 °C in toluene-d8.
Molecules 28 06693 sch001
Molecules 28 06693 g001
Figure 1. 19F NMR spectrum for SIMesF2 2 and 3.
Figure 1. 19F NMR spectrum for SIMesF2 2 and 3.
Molecules 28 06693 g001
Molecules 28 06693 sch002
Scheme 2. The irradiation of SIMes and SF5CF3 in C6D6 at 311 nm (the NMR yields are based on the amount of SF5CF3).
Scheme 2. The irradiation of SIMes and SF5CF3 in C6D6 at 311 nm (the NMR yields are based on the amount of SF5CF3).
Molecules 28 06693 sch002
Molecules 28 06693 sch003
Scheme 3. The proposed mechanism for the activation of SF5CF3 with SIMes in benzene-d6.
Scheme 3. The proposed mechanism for the activation of SF5CF3 with SIMes in benzene-d6.
Molecules 28 06693 sch003
Molecules 28 06693 sch004
Scheme 4. Experiments to trap CF3 radicals (a) via the addition of TEMPO, and (b) via the addition of 1,1-diphenylethylene (* NMR yield based on the amount of 1,1-diphenylethylene, ** NMR yield based on the amount of SIMes).
Scheme 4. Experiments to trap CF3 radicals (a) via the addition of TEMPO, and (b) via the addition of 1,1-diphenylethylene (* NMR yield based on the amount of 1,1-diphenylethylene, ** NMR yield based on the amount of SIMes).
Molecules 28 06693 sch004
Molecules 28 06693 sch005
Scheme 5. The reduction of SF5CF3 with triphenylphosphine (* based on the amount of PPh3).
Scheme 5. The reduction of SF5CF3 with triphenylphosphine (* based on the amount of PPh3).
Molecules 28 06693 sch005
Molecules 28 06693 sch006
Scheme 6. The proposed mechanism for the activation of SF5CF3 with triphenylphosphine.
Scheme 6. The proposed mechanism for the activation of SF5CF3 with triphenylphosphine.
Molecules 28 06693 sch006
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Herbstritt, D.; Tomar, P.; Braun, T. Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes. Molecules 2023, 28, 6693. https://doi.org/10.3390/molecules28186693

AMA Style

Herbstritt D, Tomar P, Braun T. Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes. Molecules. 2023; 28(18):6693. https://doi.org/10.3390/molecules28186693

Chicago/Turabian Style

Herbstritt, Domenique, Pooja Tomar, and Thomas Braun. 2023. "Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes" Molecules 28, no. 18: 6693. https://doi.org/10.3390/molecules28186693

APA Style

Herbstritt, D., Tomar, P., & Braun, T. (2023). Activation of SF5CF3 by the N-Heterocyclic Carbene SIMes. Molecules, 28(18), 6693. https://doi.org/10.3390/molecules28186693

Article Metrics

Back to TopTop