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Communication

Synthesis and Structures of Trifluoromethylborates [pinB(Aryl)CF3]−: pinB = 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane

by
Yu-En Huang
and
Shigekazu Ito
*
School of Materials and Chemical Technology, Institute of Science Tokyo, 2-12-1-H113, Tokyo 152-8550, Japan
*
Author to whom correspondence should be addressed.
Molbank 2026, 2026(3), M2183; https://doi.org/10.3390/M2183
Submission received: 23 April 2026 / Revised: 22 May 2026 / Accepted: 29 May 2026 / Published: 2 June 2026
(This article belongs to the Section Structure Determination)
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Abstract

Fluoroalkyl-substituted organoboron compounds are valuable building blocks for organic synthesis and for the development of functional molecules in medicinal chemistry, agrochemicals, and materials science. Building on our previous work on difluoromethyl-substituted borates, we report the synthesis and structural characterization of trifluoromethylated borates, 2-aryl-4,4,5,5-tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uide salts ([pinB(Aryl)CF3]−). Treatment of pinB–Aryl boronates (pinB = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane) with trimethyl(trifluoromethyl)silane (Ruppert–Prakash reagent) in the presence of potassium tert-butoxide and 18-crown-6 ether (18-C-6) afforded the corresponding trifluoromethylated borates as isolable crystalline compounds. Compared with the related difluoromethylated borates, the CF3 substituent increases the tendency of [pinB(Aryl)CF3]− to exhibit hygroscopic behavior, as supported by a hydrated crystal structure and the formation of a hygroscopic product. The isolable trifluoromethylborates can serve as reservoirs of electrophilic trifluoromethyl radicals upon oxidation.

1. Introduction

Fluorinated alkyl groups have been attractive in chemistry and the related research fields because the specific characteristics of fluorine, such as large electronegativity, small size, and ability to form strong chemical bonds with many elements and groups [1,2], are advantageous for developing functional compounds in medicine [1,2,3,4,5,6], agrochemicals [1,2,7], and materials [1,2,8]. Our previous work on the synthesis of difluoromethyl-substituted boron compounds [9,10] related hybridized functionality with the biologically OH-mimicked character of the CF2H group [11] and the boron-containing frameworks [12]. Subsequently, we attempted to use the CF2H-borate species [pinB(Aryl)CF2H]− [9] for organic synthesis, yielding difluoromethylated organic compounds. Unfortunately, the attempted transition-metal-catalyzed cross-couplings with haloarenes were unsuccessful because the CF2H group on the boron was reluctant to undergo transmetallation. On the other hand, the CF2H-borate species [pinB(Aryl)CF2H]− functioned as precursors of difluoromethyl radical (HF2C·) and could be employed for synthesizing difluoromethylated phenanthridines [13] and benzene-fused γ-sultams [14] via chemical oxidation and photoredox-catalyzed system, respectively. Also, the CF2H-borate has recently been employed in Cu/photoredox dual catalysis for C(sp3)-CF2H cross-coupling [15].
The successful synthesis using difluoromethylborates for radical difluoromethylation prompted us to employ other fluoroalkyl groups on the borate platform. Accordingly, we focused on the trifluoromethyl group because the CF3 unit has been widely used in pharmaceuticals and agrochemicals [1,2,3,4,5,6,7].
This paper describes a successful synthesis and isolation of the trifluoromethylated borate species. We also attempted to use them for radical trifluoromethylation by generating an electrophilic trifluoromethyl radical [16,17,18]. As a result, the trifluoromethyl radical prefers interaction with the pinB-Aryl unit.

2. Results and Discussion

2.1. Synthesis of the Aryl-Containing CF3-Borates

The synthesis of the desired trifluoromethylborates was partly based on the previous reports on trimethoxy(trifluoromethyl)borate [(MeO)3B-CF3]− [19,20,21,22] and dimethoxy(phenyl)(trifluoromethyl)borate [Ph(OMe)2B-CF3]− [23]. In our study, we employed pinB-Aryl boronates 1 (Scheme 1). At first, we employed p-(diethylamino)phenylboronate 1a because the C6H4-NEt2-p unit promoted the most efficient radical difluoromethylation reactions through one-electron oxidation of the aryl(difluoromethyl)borate precursor [13,14]. Indeed, the synthetic protocol using potassium tert-butoxide and 18-C-6 [9] was appropriate, but the obtained trifluoromethylborate 2a was considerably hygroscopic, and the isolated yield was low. Such a hygroscopic characteristic was observed in [pinB(Aryl)CF2H]− borates bearing relatively electron-donating aryl substituents [13]. It is plausible that combining the highly electron-withdrawing character of CF3, compared with CF2H, and the considerably electron-donating C6H4-NEt2-p unit would promote the inclusion of water. Therefore, we controlled the electron-rich character of the aryl group in 1 by avoiding amino groups and employed boronates 1b–d. As we expected, the isolated trifluoromethylborates 2b–d were air-stable crystalline compounds and not hygroscopic.

2.2. Crystal Structures of the Stable Aryl(trifluoromethyl)borates

Single crystals of 2b and 2d were obtained from hexane/dichloromethane (1:1) at 298 K. Figure 1a shows a drawing of 2b with 50% probability level. As displayed in a schematic drawing of the crystal structure (Figure 1b), 2b involved two H2O molecules. Two pinB(CF3)Ph units are bridged at the para positions of the phenyl groups with the K(18-C-6), with K1···C(para) of 3.154 Å. On the other hand, two H2O molecules coordinate to another K(18-C-6), and the K1 and K2 atoms are separated by 12.6 Å. The B–CF3 distance of 1.651(2) Å is comparable with that in [(MeO)3B-CF3]− of 1.646(7) Å [22]. The hydrated 2b, in which the water molecules would come from the ambient environment, is consistent with the CF3-promoted hygroscopic character of 2a.
Figure 2 shows a drawing of 2d (50% probability level), indicating no inclusion of water. The molecular structure is almost identical to the corresponding difluoromethyl derivative [9]. One of the fluorine atoms (F1) is distinctly coordinated by the potassium, and the C–F1 distance is slightly elongated. Additionally, one of the oxygen atoms (O2) interacts with K1.

2.3. Trifluoromethyl Radical from the Aryl-Containing Trifluoromethylborates

The difluoromethyl(aryl)borates served as sources of difluoromethyl radicals upon oxidation and were useful for the synthesis of difluoromethyl-substituted phenanthridines [13] and benzene-fused γ-sultams [14]. On the other hand, radical trifluoromethylation with photoredox catalysts (PC), affording trifluoromethylated phenanthridine 4 from isonitrile 3 [24], indicates that trifluoromethylborates 2 are reluctant to produce the trifluoromethylated product (Scheme 2). Preliminary density-functional theory (DFT) calculations estimated that the highest occupied molecular orbital (HOMO) of [pinB(Ph)CF3]− is approximately 0.2 eV lower than that of [pinB(Ph)CF2H]− at the ωB97XD/6-311+G(d) level [25], and accordingly, the oxidation of 2 would be insufficient for producing trifluoromethyl radical. In addition, in all entries in Scheme 2, 18–28% of HCF3 was produced, indicating the instability of 2 under the reaction conditions. On the other hand, it was noteworthy that the considerably electron-abundant 3,5-dimethoxyphenyl group in 2d was ineffective, as indicated by the comparable yield of the phenyl-substituted derivative 2b. We speculated that the ArylBpin intermediate, formed by the release of ·CF3 radical [16,17,18], preferably captures it back and disturb the ·CF3 addition to 3. In other words, the pinB-Aryl boronates 1 are functional for producing the paramagnetic tetra-coordinated boron species [26,27]. In our previous report [13], the calculated structure of paramagnetic [pinB(CF2H)Aryl]·, where Aryl = C6H4-NEt2-p, indicated a paramagnetic tetra-coordinated boron species.

3. Materials and Methods

3.1. General Instrumentation

All experiments were conducted under an inert atmosphere (nitrogen or argon) unless otherwise noted. 1H NMR, 13C{1H} NMR, and 19F NMR spectra were measured on a Bruker Avance Neo (400 MHz) spectrometer (Bruker BioSpin, Rheinstetten, Germany). Chemical shifts of 1H NMR were expressed in parts per million downfield from CHCl3 as an internal standard (δ = 7.26) in CDCl3. 13C{1H} NMR chemical shifts were expressed in parts per million relative to the central line of the triplet (δ = 77.10) in CDCl3. 19F NMR chemical shifts were expressed in parts per million downfield from benzotrifluoride (δ = −63.24) as the internal or external standard in CDCl3. Mass spectra were measured on a JEOL JMS-T100LC spectrometer (JEOL Ltd., Tokyo, Japan). pinB-Aryl boronates 1 and 2-isocyano-4′-methoxy-1,1′-biphenyl 3 were prepared according to the literature [13]. Analytical thin-layer chromatography (TLC) was performed on a glass plate pre-coated with silica gel (Merck Kieselgel 60 F254, layer thickness 0.25 mm) (Merck KGaA, Darmstadt, Germany). Visualization was accomplished by UV light (254 nm) and anisaldehyde. Column chromatography was performed on KANTO Silica Gel 60 N (spherical, neutral) (Kanto Chemical Co., Inc., Saitama, Japan). X-ray diffraction data were collected on a Rigaku XtaLAB Synergy R diffractometer (Rigaku Co., Tokyo, Japan). The structures were solved by utilizing the Olex2 package [28]. SHELXL and SHELXT were employed for structure refinement and structure solution, respectively [29]. CCDC 2548130 and 2548131 contain crystallographic data for compounds 2b and 2d, respectively. The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed on 22 April 2026).

3.2. Synthesis of Potassium 2-Aryl-4,4,5,5-Tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uides 2

18-Crown-6 ether (529 mg, 2.0 mmol) and solid potassium butoxide (225 mg, 2.0 mmol) were dissolved in 4 mL THF and cooled to −50 °C. Then (trifluoromethyl)trimethylsilane (593 μL, 4.0 mmol) was added, and the mixture was stirred for 20 min. Arylboronic acid pinacol ester 1 (2.0 mmol) dissolved in 1.0 mL THF was added, and the solution was stirred at 0 °C for 1 h. The solution was poured into 20 mL of hexane and stirred at room temperature for 5 min. The suspension was filtered and washed with hexane (20 mL × 2). After the residue was dried in vacuo, it was purified by washing with Et2O to afford the trifluoromethylborate as a solid.
2-(4-(Diethylamino)phenyl)-4,4,5,5-tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uide 18-crown-6-ether complex (2a): Colorless solid (170.2 mg, 26%).1H NMR (CDCl3, 400 MHz) δ 7.52 (d, J = 8.1 Hz, 2H), 6.58 (d, J = 8.6 Hz, 2H), 3.57 (s, 24H), 3.26 (q, J = 7.0 Hz, 4H), 1.21 (s, 6H), 1.10 (t, J = 7.0 Hz, 6H), 1.00 (s, 6H); 19F NMR (CDCl3, 376 MHz): δ −67.4 (s, 3F). Due to the hygroscopic character and instability, the 13C NMR and HRMS could not be obtained.
Potassium 2-phenyl-4,4,5,5-tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uide 18-crown-6-ether complex (2b): Colorless solid (439.7mg, 76%).1H NMR (CDCl3, 400 MHz) δ 7.71 (d, J = 7.2 Hz, 2H), 7.12 (t, J = 7.2 Hz, 2H), 7.05–7.00 (m, 1H), 3.56 (s, 24H), 1.22 (s, 6H), 0.97 (s, 6H); 13C{1H} NMR (CDCl3, 100 MHz) δ 134.7, 132.3, 125.9, 124.1, 78.7, 69.9, 26.4, 26.0 (CF3 was unclear. Cipso was temporary observed.); 19F NMR (CDCl3, 376 MHz) δ −67.6 (s, 3F); HRMS (ESI--TOF) calcd for C13H17BF3O2− [M+-K(18-C-6)]: 273.1, found: 273.0.
Crystallographic data of 2b·2H2O: C25H43BF3O9K, MW = 594.50, crystal dimensions = 0.160 × 0.140 × 0.060 mm3, monoclinic, P21/n (#14), a = 8.8792(3), b = 17.9898(6), c = 18.8255 (6) Å, β = 93.773(3)°, V = 3000.57(17) Å3, Z = 4, T = 100.15 K, λ = 0.71073 Å, ρcalc = 1.316 g cm−3, µMoKa = 0.242 mm−1, F000 = 1264, −11 ≤ h ≤ 9, −25 ≤ k ≤ 23, −26 ≤ L ≤ 25, 22,014 total reflections (2θmax = 62.28°), 7705 unique (Rint = 0.0567), R = 0.0478 (σ > 2σ(I)), 0.0633 (all data), wR = 0.1253 (I > 2σ(I)), 0.1350 (all data), S = 1.038 (360 parameters).
Potassium 2-(4-methoxyphenyl)-4,4,5,5-tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uide 18-crown-6-ether complex (2c): Colorless solid (981.4 mg, 88%).1H NMR (CDCl3, 400 MHz) δ 7.62 (d, J = 8.3 Hz, 2H), 6.75–6.70 (m, 2H), 3.75 (s, 3H), 3.57 (s, 24H), 1.21 (s, 6H), 0.98 (s, 6H); 13C{1H} NMR (CDCl3, 100 MHz) δ 156.9, 133.3, 111.6, 78.6, 69.9, 54.9, 26.6, 26.2 (CF3 and Cipso were unclear); 19F NMR (CDCl3, 376 MHz) δ −67.8 (s, 3F); HRMS (ESI--TOF) calcd for C14H19BF3O3− [M+-K(18-C-6)]: 303.1, found: 303.0.
Potassium 2-(3,5-dimethoxyphenyl)-4,4,5,5-tetramethyl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uide 18-crown-6-ether complex (2d): Colorless solid (827.5 mg, 87%).1H NMR (CDCl3, 400 MHz) δ 6.95 (d, J = 2.4 Hz, 2H), 6.18 (t, J = 2.4 Hz, 1H), 3.76 (s, 6H), 3.56 (s, 24H), 1.22 (s, 6H), 1.00 (s, 6H); 13C{1H} NMR (CDCl3, 100 MHz) δ 159.0, 109.7, 97.6, 78.7, 69.9, 55.2, 26.5, 26.2 (CF3 and Cipso were unclear); 19F NMR (CDCl3, 376 MHz) δ −67.7 (s, 3F); HRMS (ESI--TOF) calcd for C15H21BF3O4− [M+-K(18-C-6)]: 333.1, found: 333.0.
Crystallographic data of 2d: C27H45BF3O10K, MW = 636.54, crystal dimensions = 0.180 × 0.180 × 0.110 mm3, orthorhombic, P212121 (#19), a = 10.3006(3), b = 12.3895(4), c = 24.7086(8) Å, V = 3153.29(17) Å3, Z = 4, T = 100.15 K, λ = 0.71073 Å, ρcalc = 1.341 g cm−3, µMoKa = 0.238 mm−1, F000 = 1352, −10 ≤ h ≤ 14, −17 ≤ k ≤ 12, −26 ≤ L ≤ 33, 20,330 total reflections (2θmax = 62.28°), 8221 unique (Rint = 0.0389), R = 0.0384 (σ > 2σ(I)), 0.0498 (all data), wR = 0.0799 (I > 2σ(I)), 0.0692 (all data), S = 1.033 (396 parameters).

3.3. Synthesis of 8-Methoxy-6-(trifluoromethyl)phenanthridine 4

To a suspension of 2b or 2d (0.1 mmol) in DMSO (1 mL), 3 (0.1 mmol), PC (5.0 µmol), potassium persulfate (0.1 or 0.2 mmol), and potassium carbonate (0.1 or 0.2 mmol) were added. The resulting suspension was stirred upon blue LED (23 W) irradiation under a nitrogen balloon for 24 h at room temperature. After 24 h, BTF was added, and the mixture was analyzed by 19F NMR spectroscopy. 4: 19F NMR (CDCl3, 376 MHz) δ −64.2 (s, F) [24].

4. Conclusions

In this study, we successfully synthesized 4,4,5,5-tetramethyl-2-aryl-2-(trifluoromethyl)-1,3,2-dioxaborolan-2-uides 2. The hydrated crystal structure of 2b is consistent with the hygroscopic character of 2a. Thus, the trifluoromethyl group would promote the capture of water molecules. On the contrary, the 3,5-dimethoxyphenyl group in 2d showed an almost identical structure to that of the previously reported difluoromethyl derivative [9]. In contrast to the difluoromethyl derivatives, the trifluoromethylborates 2 were reluctant to act as a source of the synthetically useful trifluoromethyl radical. The findings would be helpful to produce novel functional trifluoromethylborates.

Supplementary Materials

The following supporting information is available online. Figures S1–S11: Copies of NMR spectra of 2; DFT calculations data of [pinB(Ph)CF3]− and [pinB(Ph)CF2H]−. Reference [30] is cited in the supplementary materials.

Author Contributions

Conceptualization, Y.-E.H. and S.I.; methodology, Y.-E.H.; investigation, Y.-E.H. and S.I.; writing—original draft preparation, Y.-E.H.; writing—review and editing, Y.-E.H. and S.I.; supervision, S.I.; project administration, S.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by JSPS KAKENHI Grant number 22K19023.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

Nissan Chemical Corporation provided financial support. The authors thank Tetsuro Murahashi and Tsubasa Omoda of Institute of Science Tokyo (Science Tokyo) for supporting the X-ray crystallographic analyses. Masaya Morisaki of Science Tokyo supported the NMR measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molbank 2026 m2183 sch001
Scheme 1. Preparation of trifluoromethylborates 2a–d with isolated yields.
Scheme 1. Preparation of trifluoromethylborates 2a–d with isolated yields.
Molbank 2026 m2183 sch001
Molbank 2026 m2183 g001
Figure 1. (a) An ORTEP drawing (50% probability level) of 2b·2H2O. (b) A schematic drawing of the molecular structure of 2b·2H2O in the crystalline state.
Figure 1. (a) An ORTEP drawing (50% probability level) of 2b·2H2O. (b) A schematic drawing of the molecular structure of 2b·2H2O in the crystalline state.
Molbank 2026 m2183 g001
Molbank 2026 m2183 g002
Figure 2. An ORTEP drawing (50% probability level) of 2d. Hydrogen atoms are omitted for clarity. Bond lengths (Å): C28–F1 1.381(3), C28–F2 1.363(3), C28–F3 1.371(3), F1···K1 2.923(2), B1–O1 1.475(3), B1–O2 1.483(3), O2···K1 2.656(1).
Figure 2. An ORTEP drawing (50% probability level) of 2d. Hydrogen atoms are omitted for clarity. Bond lengths (Å): C28–F1 1.381(3), C28–F2 1.363(3), C28–F3 1.371(3), F1···K1 2.923(2), B1–O1 1.475(3), B1–O2 1.483(3), O2···K1 2.656(1).
Molbank 2026 m2183 g002
Molbank 2026 m2183 sch002
Scheme 2. (a) Attempted photoredox-catalyzed synthesis of trifluoromethylphenanthridine 4. (b) A plausible paramagnetic tetracoordinated boron species.
Scheme 2. (a) Attempted photoredox-catalyzed synthesis of trifluoromethylphenanthridine 4. (b) A plausible paramagnetic tetracoordinated boron species.
Molbank 2026 m2183 sch002
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Huang, Y.-E.; Ito, S. Synthesis and Structures of Trifluoromethylborates [pinB(Aryl)CF3]−: pinB = 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane. Molbank 2026, 2026, M2183. https://doi.org/10.3390/M2183

AMA Style

Huang Y-E, Ito S. Synthesis and Structures of Trifluoromethylborates [pinB(Aryl)CF3]−: pinB = 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane. Molbank. 2026; 2026(3):M2183. https://doi.org/10.3390/M2183

Chicago/Turabian Style

Huang, Yu-En, and Shigekazu Ito. 2026. "Synthesis and Structures of Trifluoromethylborates [pinB(Aryl)CF3]−: pinB = 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane" Molbank 2026, no. 3: M2183. https://doi.org/10.3390/M2183

APA Style

Huang, Y.-E., & Ito, S. (2026). Synthesis and Structures of Trifluoromethylborates [pinB(Aryl)CF3]−: pinB = 4,4,5,5-Tetramethyl-1,3,2-dioxaborolane. Molbank, 2026(3), M2183. https://doi.org/10.3390/M2183

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