Synthesis of Novel α-Trifluorothioanisole Derivatives Containing Phenylpyridine Moieties with Herbicidal Activity

Abstract

To discover novel herbicidal compounds with favorable activity, a range of phenylpyridine-moiety-containing α-trifluorothioanisole derivatives were designed, synthesized, and identified via NMR and HRMS. Preliminary screening of greenhouse-based herbicidal activity revealed that compound 5a exhibited >85% inhibitory activity against broadleaf weeds Amaranthus retroflexus, Abutilon theophrasti, and Eclipta prostrate at 37.5 g a.i./hm², which was slightly superior to that of fomesafen. The current study suggests that compound 5a could be further optimized as an herbicide candidate to control various broadleaf weeds.

1. Introduction

Organosulfur compounds are widely present in animals and plants due to their physiological activities, such as diallyl disulfide compounds in garlic, isothiocyanate compounds in cruciferous vegetables and some fruits, and sulfur-containing amino acids and vitamins. These molecules play a vital role in maintaining the metabolism of organisms and various life activities. Therefore, many researchers have conducted in-depth research on these compounds and found that some molecules containing thioheterocycles, thioureas, sulfonamides, thioethers, sulfoxides, sulfones, and other structures show biological activity [1,2,3,4,5,6,7,8]. For instance, Wang et al. [9] reported a class of sulfonylurea compounds with high herbicidal activities against Echinochloa crusgalli and Digitaria sanguinalis for pre- and post-emergence treatment and noted that they were safe for peanut as post-emergence treatment. A range of 5-substituted sulfonylurea derivatives discovered by Li et al. [10] showed good herbicidal activities against Amaranthus retroflexus and Brassica campestris for pre-emergence treatment. It is worth noting that compounds containing sulfide, sulfoxide, and sulfone structures have been widely used in materials [11,12], medicine [13,14,15], and pesticides [16,17,18,19,20,21,22] in the past few decades (Figure 1). The pre-emergence herbicide pyroxasulfone, discovered by Todoroki et al. [23], exhibited excellent herbicidal activity and crop safety, and it is currently used in the field.

Substituted 2-phenylpyridines discovered by Schaefer et al. exhibited good inhibition activity against weeds [24,25]. Substituted 3-(pyridin-2-yl)benzenesulfonamide derivatives disclosed by Liu et al. showed excellent inhibitory activity against a variety of weeds [26,27,28]. Du et al. also reported that a range of kresoxim-methyl derivatives containing phenylpyridine moieties exhibited higher inhibitory activities against broadleaf weeds than mesotrione [29,30].

Herein, 18 novel α-trifluorothioanisole derivatives were synthesized through introducing phenylpyridine moieties into α-trifluorothioanisole. The inhibitory activities of the resultant compounds against broadleaf and grass weeds were determined.

2. Results and Discussions

2.1. Chemistry

The synthesis procedures for the target compounds used in this work are outlined in Scheme 1. Intermediates 3a–3f were obtained according to the literature [31]. The target compounds 5a–5f were prepared via nucleophilic substitution reaction from intermediates 3a–3f and compound 4. In addition, compounds 5a–5f were oxidized to yield compounds 6a–6f and 7a–7f using 3-chloroperbenzoic acid or hydrogen peroxide as oxidants according to previously disclosed methods [32,33]. After synthesis, all target compounds were characterized via HRMS and NMR. X-ray diffraction crystallography was further used to confirm the structure of compound 5a (Figure 2). The crystal data of compound 5a and the NMR spectrum of all target compounds are shown in the Supplementary Materials.

2.2. Greenhouse Herbicidal Activity Assays

As can be seen from Table 1, some target compounds exhibited excellent inhibitory activity against the tested dicotyledonous weeds but proved ineffective against monocotyledonous weeds. Of these, at 150 g a.i./hm², compounds 5a, 5f, 6b, and 7a exhibited >80% inhibitory activity when used for the post-emergence treatment of the broadleaf weeds AT, AR, and EP, while compounds 6f and 7f exhibited >80% activity against AT and AR. Furthermore, compound 5a also effectively suppressed the growth of DS and SV, which was slightly better than the positive control fomesafen. Other compounds exhibited varying levels of general herbicidal activity. Further analysis revealed that compound 5a exhibited >85% inhibition against AT, AR, and EP for post-emergence treatment at 37.5 g a.i./hm², which was slightly superior to the inhibition ability of fomesafen, whereas compound 5f also exhibited >70% inhibition against these three weeds.

From Table 1, we can see that the herbicidal activities of compound 5 were slightly better than those of compounds 6 and 7. According to the SAR of compound 5 in the field of herbicidal activity, when R₁ and R₂ were both substituted by hydrogen atoms (5a), the activities of compound 5 against three broadleaf weeds were better than those of other compounds. For compound 6, when R₁ was substituted by a hydrogen atom and R₂ was substituted by a fluorine atom (6b), the herbicidal activities of compound 6 were optimal. For compound 7, the optimal herbicidal activities were observed when R₁ and R₂ were substituted by either hydrogen atoms or fluorine atoms (7a, 7f).

3. Materials and Methods

3.1. Instrumentation

All reagents and other materials were purchased from commercial sources and used without additional purification unless otherwise noted. A B-545 melting point instrument was used to determine melting point without calibration. A Bruker AV-400 or AV-500 MHz spectrometer was used to generate NMR spectra with DMSO-d₆ or CDCl₃ serving as solvents. An Agilent 6545 Q-TOF LCMS spectrometer was used for mass spectrometry. A Bruker D8 Venture diffractometer was utilized to collect crystallographic data.

3.2. Synthesis

The synthesis methods of the title compounds are outlined in Scheme 1.

3.2.1. General Approach to the Synthesis of Compounds 3a–3f

2,3-Dichloro-5-(trifluoromethyl)pyridine 1 (5 mmol), triphenylphosphorus (0.5 mmol), K₂CO₃ (10 mmol), substituted p-hydroxybenzeneboronic acid 2a–2f (5.5 mmol), and palladium(II) acetate (0.25 mmol) were mixed and stirred for 6 h with CH₃CN (10 mL) and CH₃OH (5 mL) at 50 °C under N₂. Thereafter, the mixture was extracted using ethyl acetate (30 mL × 3), rinsed using brine, and concentrated. The remaining residue was then recrystallized using ethanol and water as solvents at 70 °C to obtain compounds 3a–3f [31].

3.2.2. General Approach to the Synthesis of Compounds 5a–5f

Substituted phenylpyridines 3a–3f (2 mmol), N,N-dimethylformamide (10 mL), and NaH (3 mmol, 0.12 g) were mixed and stirred at 20 °C for 30 min under N₂. Next, 4-trifluoromethylthiobenzyl bromide (2.4 mmol) was added and stirred for 8 h at 60 °C. The mixture was extracted thrice using ethyl acetate (30 mL × 3), rinsed using brine, and concentrated. Residues were then purified via silica gel column chromatography using ethyl acetate (EA) and petroleum ether (PE) (V_EA:V_PE=1:15) to obtain compounds 5a–5f [31].

3.2.3. General Approach to the Synthesis of Compounds 6a–6f

Compounds 5a–5f (0.5 mmol), 85% 3-chloroperbenzoic acid (0.5 mmol), and dichloromethane (10 mL) were mixed and stirred at 20 °C for 6 h. The mixture was evaporated to remove the solvent. Residues were then purified via silica gel column chromatography using ethyl acetate (EA) and petroleum ether (PE) (V_EA:V_PE=1:7) to obtain compounds 6a–6f [32].

3.2.4. General Approach to the Synthesis of Compounds 7a–7f

Compounds 5a–5f (0.4 mmol), 30% hydrogen peroxide (1.6 mmol), and trifluoroacetic acid (2 mL) were mixed and stirred at 60 °C for 6 h. Thereafter, the mixture was made alkaline using a sodium hydroxide solution, extracted thrice using ethyl acetate (30 mL × 3), rinsed using Na₂SO₃, and concentrated. Residues were then purified via silica gel column chromatography using ethyl acetate (EA) and petroleum ether (PE) (V_EA:V_PE=1:10) to obtain compounds 7a–7f [33].

3-chloro-5-(trifluoromethyl)-2-(4-((4-((trifluoromethyl)thio)benzyl)oxy)phenyl)pyridine (5a): White solid, yield 78.8%, m.p. 93.2–95.4 °C. ¹H NMR (500 MHz, DMSO-d₆) δ: 8.99 (s, 1H), 8.51 (s, 1H), 7.76 (dd, J = 8.4, 3.4 Hz, 4H), 7.64 (d, J = 8.1 Hz, 2H), 7.17 (d, J = 8.8 Hz, 2H), 5.29 (s, 2H). ¹³C NMR (126 MHz, DMSO-d₆) δ: 159.17, 158.79, 144.27 (q, J = 4.0 Hz), 140.63, 136.27, 135.71 (q, J = 3.1 Hz), 131.02, 129.57 (q, J = 308.4 Hz), 129.37, 129.17, 128.82, 124.39 (q, J = 33.0 Hz), 122.86 (q, J = 273.4 Hz), 122.29 (q, J = 2.0 Hz), 114.37, 68.43. HRMS (ESI): calculated for C₂₀H₁₃ClF₆NOS [M+H]⁺ 464.0305, 466.0276, found 464.0308, 466.0277.

3-chloro-2-(3-fluoro-4-((4-((trifluoromethyl)thio)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (5b): White solid, yield 85.9%, m.p. 63.5–65.0 °C. ¹H NMR (500 MHz, DMSO-d₆) δ: 9.01 (d, J = 0.9 Hz, 1H), 8.55 (s, 1H), 7.79 (d, J = 8.1 Hz, 2H), 7.70–7.64 (m, 3H), 7.61 (d, J = 8.6 Hz, 1H), 7.41 (t, J = 8.7 Hz, 1H), 5.38 (s, 2H). ¹³C NMR (126 MHz, DMSO-d₆) δ: 157.57, 150.96 (d, J = 244.7 Hz), 147.15 (d, J = 10.6 Hz), 144.33 (q, J = 3.9 Hz), 140.06, 136.34, 135.89 (q, J = 3.3 Hz), 129.79 (d, J = 6.7 Hz), 129.58 (q, J = 308.3 Hz), 129.42, 128.92, 126.27 (d, J = 3.2 Hz), 124.85 (q, J = 33.1 Hz), 122.80 (q, J = 273.4 Hz), 122.59 (q, J = 1.9 Hz), 117.22 (d, J = 20.0 Hz), 114.71, 69.36. HRMS (ESI): calculated for C₂₀H₁₀ClF₇NOS [M−H]⁻ 480.0065, 482.0036, found 480.0064, 482.0043.

3-chloro-2-(3-chloro-4-((4-((trifluoromethyl)thio)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (5c): White solid, yield 94.0%, m.p. 82.0–83.5 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.02 (s, 1H), 8.57 (s, 1H), 7.88 (s, 1H), 7.78 (t, J = 10.5 Hz, 3H), 7.67 (d, J = 7.2 Hz, 2H), 7.40 (d, J = 8.3 Hz, 1H), 5.41 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.49, 154.37, 144.38 (q, J = 3.9 Hz), 140.14, 136.38, 135.89 (q, J = 3.1 Hz), 130.94, 130.25, 129.68, 129.60 (q, J = 309.1 Hz), 129.45, 128.62, 124.87 (q, J = 33.4 Hz), 122.81 (q, J = 274.1 Hz), 122.48, 121.21, 113.62, 69.28. HRMS (ESI): calculated for C₂₀H₁₀Cl₂F₆NOS [M−H]⁻ 495.9770, 497.9740, found 495.9767, 497.9740.

3-chloro-2-(3-nitro-4-((4-((trifluoromethyl)thio)benzyl)oxy)phenyl)-5 (trifluoromethyl)pyridine (5d): White solid, yield 86.5%, m.p. 114.5–116.5 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.06 (s, 1H), 8.63 (s, 1H), 8.35 (d, J = 2.2 Hz, 1H), 8.11 (dd, J = 8.8, 2.2 Hz, 1H), 7.80 (d, J = 8.1 Hz, 2H), 7.65 (d, J = 8.1 Hz, 2H), 7.61 (d, J = 8.9 Hz, 1H), 5.51 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 156.76, 151.71, 144.63 (q, J = 3.6 Hz), 139.64, 138.99, 136.51, 136.12 (q, J = 3.5 Hz), 135.49, 129.74, 129.66 (q, J = 308.8 Hz), 129.27, 128.63, 126.42, 125.33 (q, J = 33.3 Hz), 122.83 (q, J = 274.1 Hz), 122.66 (q, J = 1.9 Hz), 115.28, 69.85. HRMS (ESI): calculated for C₂₀H₁₀ClF₆N₂O₃S [M−H]⁻ 507.0010, 508.9981, found 507.0010, 508.9983.

3-chloro-5-(trifluoromethyl)-2-(3-(trifluoromethyl)-4-((4 ((trifluoromethyl)thio)benzyl)oxy)phenyl)pyridine (5e): White solid, yield 93.4%, m.p. 111.6–113.5 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.07 (s, 1H), 8.62 (s, 1H), 8.13 (d, J = 8.8 Hz, 1H), 8.10 (s, 1H), 7.83 (d, J = 8.0 Hz, 2H), 7.67 (d, J = 8.1 Hz, 2H), 7.55 (d, J = 8.7 Hz, 1H), 5.52 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.42, 156.64, 144.47 (q, J = 3.6 Hz), 139.94, 136.39, 135.93 (q, J = 3.5 Hz), 135.35, 129.60 (q, J = 308.7 Hz), 129.51, 129.06, 128.33, 128.15 (q, J = 5.1 Hz), 125.02 (q, J = 33.3 Hz), 123.45 (q, J = 273.6 Hz), 122.80 (q, J = 274.1 Hz), 122.48 (q, J = 1.8 Hz), 117.04 (q, J = 30.7 Hz), 113.76, 69.21. HRMS (ESI): calculated for C₂₁H₁₀ClF₉NOS [M−H]⁻ 530.0033, 532.0004, found 530.0031, 532.0010.

3-chloro-2-(2,3-difluoro-4-((4-((trifluoromethyl)thio)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (5f): White solid, yield 80.2%, m.p. 79.5–82.0 °C. ¹H NMR (500 MHz, CDCl₃) δ: 8.84 (d, J = 1.1 Hz, 1H), 8.06 (d, J = 1.5 Hz, 1H), 7.71 (d, J = 8.2 Hz, 2H), 7.53 (d, J = 8.3 Hz, 2H), 7.19–7.15 (m, 1H), 6.92–6.89 (m, 1H), 5.26 (s, 2H). ¹³C NMR (126 MHz, CDCl₃) δ: 155.42, 149.29 (dd, J = 252.8, 11.0 Hz), 149.28 (dd, J = 8.2, 2.8 Hz), 144.45 (q, J = 3.6 Hz), 141.76 (dd, J = 249.8, 14.4 Hz), 139.03, 136.78, 134.93 (q, J = 3.2 Hz), 132.36, 129.68 (q, J = 308.6 Hz), 128.26, 127.20 (q, J = 33.9 Hz), 124.84 (t, J = 3.6 Hz), 124.65, 122.74 (q, J = 273.6 Hz), 120.22 (d, J = 12.2 Hz), 110.36 (q, J = 2.6 Hz), 70.97. HRMS (ESI): calculated for C₂₀H₉ClF₈NOS [M−H]⁻ 497.9971, 499.9942, found 497.9969, 499.9949.

3-chloro-5-(trifluoromethyl)-2-(4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)pyridin (6a): White solid, yield 41.7%, m.p. 123.3–124.7 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.00 (s, 1H), 8.53 (s, 1H), 7.94 (d, J = 8.1 Hz, 2H), 7.82 (d, J = 8.3 Hz, 2H), 7.76 (d, J = 8.7 Hz, 2H), 7.19 (d, J = 8.8 Hz, 2H), 5.35 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 159.17, 158.87, 144.40 (q, J = 3.7 Hz), 143.20, 135.85 (q, J = 3.4 Hz), 134.70 (q, J = 1.4 Hz), 131.14, 129.50, 129.28, 128.67, 126.26, 124.83 (q, J = 337.5 Hz), 124.47 (q, J = 32.9 Hz), 122.96 (q, J = 273.7 Hz), 114.45, 68.47. HRMS (ESI): calculated for C₂₀H₁₃ClF₆NO₂S [M+H]⁺ 480.0254, 482.0225, found 480.0254, 482.0227.

3-chloro-2-(3-fluoro-4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (6b): White solid, yield 80.3%, m.p. 113.0–115.0 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.02 (s, 1H), 8.57 (s, 1H), 7.97 (d, J = 7.8 Hz, 2H), 7.84 (d, J = 8.0 Hz, 2H), 7.69 (dd, J = 12.2, 1.5 Hz, 1H), 7.62 (d, J = 8.2 Hz, 1H), 7.43 (t, J = 8.6 Hz, 1H), 5.45 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.59, 150.96 (d, J = 245.2 Hz), 147.10 (d, J = 10.4 Hz), 144.36 (q, J = 3.7 Hz), 142.56, 135.91 (q, J = 3.2 Hz), 134.94 (q, J = 1.0 Hz), 129.88 (d, J = 6.6 Hz), 129.44, 128.68, 126.31, 126.25, 124.86 (q, J = 33.1 Hz), 124.77 (q, J = 337.7 Hz), 122.81 (q, J = 273.9 Hz), 117.26 (d, J = 19.9 Hz), 114.75, 69.38. HRMS (ESI): calculated for C₂₀H₁₀ClF₇NO₂S [M−H]⁻ 496.0014, 497.9985, found 496.0015, 497.9993.

3-chloro-2-(3-chloro-4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (6c): White solid, yield 50.1%, m.p. 119.3–120.3 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.01 (s, 1H), 8.58 (s, 1H), 7.96 (d, J = 8.1 Hz, 2H), 7.87–7.83 (m, 3H), 7.76 (dd, J = 8.6, 1.9 Hz, 1H), 7.40 (d, J = 8.7 Hz, 1H), 5.46 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.55, 154.35, 144.47 (q, J = 3.9 Hz), 142.68, 135.98 (q, J = 3.3 Hz), 134.86 (q, J = 1.0 Hz), 131.02, 130.37, 129.78, 129.53, 128.44, 126.33, 124.93 (q, J = 33.2 Hz), 124.83 (q, J = 337.6 Hz), 122.88 (q, J = 274.1 Hz), 121.23, 113.67, 69.30. HRMS (ESI): calculated for C₂₀H₁₀Cl₂F₆NO₂S [M−H]⁻ 511.9719, 513.9689, found 511.9718, 513.9692.

3-chloro-2-(3-nitro-4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (6d): Yellow solid, yield 96.2%, m.p. 116.0–118.0 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.09 (s, 1H), 8.66 (s, 1H), 8.41 (d, J = 2.1 Hz, 1H), 8.17 (dd, J = 8.8, 2.1 Hz, 1H), 8.01 (d, J = 8.1 Hz, 2H), 7.87 (d, J = 8.2 Hz, 2H), 7.67 (d, J = 8.9 Hz, 1H), 5.61 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 156.67, 151.67, 144.54 (q, J = 3.6 Hz), 142.05, 138.94, 136.03 (q, J = 3.5 Hz), 135.44, 134.97, 129.67, 129.31, 128.30, 126.38, 126.28, 125.30 (q, J = 33.3 Hz), 124.77 (q, J = 337.8 Hz), 122.76 (q, J = 274.1 Hz), 115.28, 69.86. HRMS (ESI): calculated for C₂₀H₁₀ClF₆N₂O₄S [M−H]⁻ 522.9959, 524.9930, found 522.9961, 524.9921.

3-chloro-5-(trifluoromethyl)-2-(3-(trifluoromethyl)-4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)pyridine (6e): White solid, yield 51.1%, m.p. 117.9–120.1 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.05 (s, 1H), 8.60 (s, 1H), 8.09 (d, J = 9.8 Hz, 2H), 7.98 (s, 2H), 7.82 (d, J = 5.1 Hz, 2H), 7.54 (d, J = 6.4 Hz, 1H), 5.55 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.40, 156.58, 144.48 (q, J = 2.7 Hz), 142.44, 135.95, 135.37, 134.86 (q, J = 1.5 Hz), 129.53, 129.15, 128.20, 128.09, 126.29, 125.04 (q, J = 33.2 Hz), 124.77 (q, J = 339.3 Hz), 123.44 (q, J = 273.4 Hz), 122.80 (q, J = 274.3 Hz), 117.06 (q, J = 30.8 Hz), 113.78, 69.23. HRMS (ESI): calculated for C₂₁H₁₀ClF₉NO₂S [M−H]⁻ 545.9983, 547.9953, found 545.9985, 547.9960.

3-chloro-2-(2,3-difluoro-4-((4-((trifluoromethyl)sulfinyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (6f): White solid, yield 48.4%, m.p. 122.4–24.4 °C. ¹H NMR (500 MHz, CDCl₃) δ: 8.79-8.75 (m, 1H), 7.99 (d, J = 1.5 Hz, 1H), 7.78 (d, J = 8.2 Hz, 2H), 7.66 (d, J = 8.5 Hz, 2H), 7.13–7.10 (m, 1H), 6.86–6.83 (m, 1H), 5.25 (s, 2H). ¹³C NMR (126 MHz, CDCl₃) δ: 154.15, 148.13 (dd, J = 253.0, 11.6 Hz), 147.93 (dd, J = 8.1, 3.1 Hz), 143.30 (q, J = 3.9 Hz), 140.90, 140.59 (dd, J = 250.4, 14.4 Hz), 134.73 (q, J = 1.5 Hz), 133.79 (q, J = 3.5 Hz), 131.19, 127.08, 126.08 (q, J = 33.9 Hz), 125.42, 123.76 (t, J = 4.0 Hz), 123.66 (q, J = 335.8 Hz), 121.56 (q, J = 273.6 Hz), 119.28 (d, J = 12.4 Hz), 109.16 (d, J = 3.1 Hz), 69.60. HRMS (ESI): calculated for C₂₀H₉ClF₈NO₂S [M−H]⁻ 513.9920, 515.9891, found 513.9916, 515.9887.

3-chloro-5-(trifluoromethyl)-2-(4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)pyridine (7a): White solid, yield 40.1%, m.p. 132.7–134.8 °C. ¹H NMR (500 MHz, DMSO-d₆) δ: 9.02–8.97 (m, 1H), 8.53 (d, J = 1.5 Hz, 1H), 8.19 (d, J = 8.4 Hz, 2H), 7.94 (d, J = 8.5 Hz, 2H), 7.78–7.75 (m, 2H), 7.21–7.18 (m, 2H), 5.45 (s, 2H). ¹³C NMR (126 MHz, DMSO-d₆) δ: 158.92, 158.78, 147.74, 144.32 (q, J = 3.8 Hz), 135.76 (q, J = 3.5 Hz), 131.12, 131.03, 129.65, 129.23, 128.92, 128.58, 124.46 (q, J = 32.9 Hz), 122.88 (q, J = 273.3 Hz), 119.41 (q, J = 326.7 Hz), 114.42, 68.10. HRMS (ESI): calculated for C₂₀H₁₁ClF₆NO₃S [M−H]⁻ 494.0058, 496.0028, found 494.0061, 496.0035.

3-chloro-2-(3-fluoro-4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (7b): White solid, yield 48.4%, m.p. 111.8–114.0 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.02 (s, 1H), 8.58 (s, 1H), 8.23 (d, J = 8.2 Hz, 2H), 7.95 (d, J = 8.4 Hz, 2H), 7.71 (dd, J = 12.3, 1.9 Hz, 1H), 7.62 (d, J = 8.5 Hz, 1H), 7.41 (t, J = 8.7 Hz, 1H), 5.54 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.57, 150.93 (d, J = 245.4 Hz), 147.12, 146.91 (d, J = 10.5 Hz), 144.37 (q, J = 3.9 Hz), 135.92 (q, J = 3.4 Hz), 131.11, 130.09 (d, J = 6.8 Hz), 129.47, 128.96, 128.79, 126.34 (d, J = 3.2 Hz), 124.90 (q, J = 33.3 Hz), 122.81 (q, J = 274.0 Hz), 119.41 (q, J = 327.6 Hz), 117.33 (d, J = 19.8 Hz), 114.76, 68.99. HRMS (ESI): calculated for C₂₀H₁₀ClF₇NO₃S [M−H]⁻ 511.9964, 513.9934, found 511.9968, 513.9937.

3-chloro-2-(3-chloro-4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (7c): White solid, yield 68.9%, m.p. 142.6–144.8 °C. ¹H NMR (500 MHz, DMSO-d₆) δ: 9.04–9.00 (m, 1H), 8.57 (d, J = 1.5 Hz, 1H), 8.24 (d, J = 8.4 Hz, 2H), 7.97 (d, J = 8.5 Hz, 2H), 7.90 (d, J = 2.2 Hz, 1H), 7.78 (dd, J = 8.6, 2.2 Hz, 1H), 7.40 (d, J = 8.7 Hz, 1H), 5.57 (s, 2H). ¹³C NMR (126 MHz, DMSO-d₆) δ: 157.43, 154.12, 147.14, 144.36 (d, J = 3.7 Hz), 135.86 (d, J = 3.3 Hz), 131.07, 130.97, 130.50, 129.71, 129.45, 128.74, 128.66, 124.91 (q, J = 33.3 Hz), 122.78 (q, J = 273.4 Hz), 121.23, 119.39 (q, J = 326.9 Hz), 113.64, 68.94. HRMS (ESI): calculated for C₂₀H₁₀Cl₂F₆NO₃S [M−H]⁻ 527.9668, 529.9639, found 527.9664, 529.9640.

3-chloro-2-(3-nitro-4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (7d): Yellow solid, yield 24.8%, m.p. 129.1–131.6 °C. ¹H NMR (500 MHz, DMSO-d₆) δ: 9.06 (d, J = 0.9 Hz, 1H), 8.62 (d, J = 1.4 Hz, 1H), 8.40 (d, J = 2.3 Hz, 1H), 8.25 (d, J = 8.4 Hz, 2H), 8.15 (dd, J = 8.8, 2.3 Hz, 1H), 7.95 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 8.9 Hz, 1H), 5.67 (s, 2H). ¹³C NMR (126 MHz, DMSO-d₆) δ: 156.61, 151.50, 146.52, 144.51 (q, J = 3.8 Hz), 138.85, 136.00 (q, J = 3.6 Hz), 135.50, 131.09, 129.65, 129.47, 128.86, 128.61, 126.43, 125.30 (q, J = 33.3 Hz), 122.72 (q, J = 273.5 Hz), 119.38 (q, J = 326.8 Hz), 115.26, 69.51. HRMS (ESI): calculated for C₂₀H₁₀ClF₆N₂O₅S [M−H]⁻ 538.9909, 540.9879, found 538.9905, 540.9882.

3-chloro-5-(trifluoromethyl)-2-(3-(trifluoromethyl)-4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)pyridine (7e): White solid, yield 35.2%, m.p. 111.2–113.4 °C. ¹H NMR (400 MHz, DMSO-d₆) δ: 9.05 (s, 1H), 8.61 (s, 1H), 8.26 (d, J = 8.1 Hz, 2H), 8.13 (d, J = 9.0 Hz, 1H), 8.09 (s, 1H), 7.93 (d, J = 8.2 Hz, 2H), 7.53 (d, J = 8.7 Hz, 1H), 5.65 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ: 157.39, 156.37, 146.94, 144.49 (q, J = 3.6 Hz), 135.95 (q, J = 3.4 Hz), 135.44, 131.17, 129.55, 129.34, 128.77, 128.46, 128.22 (q, J = 5.3 Hz), 125.07 (q, J = 33.0 Hz), 123.44 (q, J = 273.5 Hz), 122.80 (q, J = 274.1 Hz), 119.41 (q, J = 327.3 Hz), 117.04 (q, J = 30.7 Hz), 113.77, 68.90. HRMS (ESI): calculated for C₂₁H₁₀ClF₉NO₃S [M−H]⁻ 561.9932, 563.9902, found 561.9935, 563.9913.

3-chloro-2-(2,3-difluoro-4-((4-((trifluoromethyl)sulfonyl)benzyl)oxy)phenyl)-5-(trifluoromethyl)pyridine (7f): White solid, yield 37.7%, m.p. 102.8–104.8 °C. ¹H NMR (500 MHz, CDCl₃) δ: 8.77 (d, J = 1.1 Hz, 1H), 8.02 (d, J = 8.3 Hz, 2H), 7.99 (d, J = 1.6 Hz, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.15–7.11 (m, 1H), 6.87–6.82 (m, 1H), 5.29 (s, 2H). ¹³C NMR (126 MHz, CDCl₃) δ: 155.22, 149.30 (dd, J = 253.0, 11.3 Hz), 148.83 (dd, J = 8.2, 3.1 Hz), 145.53, 144.47 (q, J = 3.8 Hz), 141.75 (dd, J = 250.2, 14.3 Hz), 134.96 (q, J = 3.5 Hz), 132.35, 131.42, 131.27, 128.14, 127.29 (q, J = 33.9 Hz), 124.99 (t, J = 3.9 Hz), 122.70 (q, J = 273.6 Hz), 119.92 (q, J = 326.4 Hz), 120.71 (d, J = 12.5 Hz), 110.29 (d, J = 3.1 Hz), 70.38. HRMS (ESI): calculated for C₂₀H₉ClF₈NO₃S [M−H]⁻ 529.9869, 531.9840, found 529.9869, 531.9824.

3.3. Herbicidal Activity Test

Levels of herbicidal activity for compounds 5a–5f, 6a–6f, and 7a–7f against the monocotyledonous weeds Digitaria sanguinalis (DS), Echinochloa crusgalli (EC), and Setaria viridis (SV), and the dicotyledonous weeds Abutilon theophrasti (AT), Amaranthus retroflexus (AR), and Eclipta prostrate (EP) were determined using previously disclosed methods [29,30,31,34].

All target compounds were dissolved in N,N-dimethylformamide, diluted to an appropriate dose with distilled water containing 0.1% Tween-80 emulsifier, and used for seedlings at the three-leaf stage with three replicates. Seedlings treated with N,N-dimethylformamide and distilled water containing 0.1% Tween-80 emulsifier served as blank controls, with fomesafen-treated seedlings serving as positive controls. The application rates were 150, 75, and 37.5 g a.i./ha. Herbicidal activity was assessed visually after a 20-day period, with the results being listed in Table 1.

4. Conclusions

In conclusion, 18 novel α-trifluorothioanisole derivatives containing phenylpyridine moieties were prepared as candidate herbicides. The preliminary herbicidal activity assay results show that some target compounds exhibited good herbicidal activities against broadleaf weeds at 150 g a.i./hm². Among these, compound 5a possessed excellent activity (>85%) against AT, AR, and EP at 37.5 g a.i./hm², which was slightly superior to fomesafen. Furthermore, compound 5f possessed good activity (>70%) against AT, AR, and EP at 37.5 g a.i./hm², which was equivalent to that of fomesafen. Thus, compound 5a may be a lead compound for further structural optimization.

Author Contributions

Z.C. and W.Z. carried out experimental work; Z.C. prepared the manuscript; X.D. designed the material and supervised the project; and Z.Y. and X.D. revised the paper. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Samples of the compounds are not available from the authors.

Acknowledgments

We are grateful to Yong-Hua Li (Zhejiang Research Institute of Chemical Industry) for assistance with the bioactivity assay.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures, Scheme and Table

Figure 1. Structures of reported sulfur-containing compounds.

Scheme 1. Overview synthesis methods of α-trifluorothioanisole derivatives.

Figure 2. The X-ray crystal structure of compound 5a.

Table 1

The structures and herbicidal activities of α-trifluoroanisole derivatives for post-emergence treatment in a greenhouse assay setting.

Compound	Chemical Structure		Dosage/ (g a.i./hm2)	Weed a
	R1	R2		EC	DS	SV	AT	AR	EP
5a	H	H	150	10 ± 2	90 ± 5	80 ± 3	100 ± 0	100 ± 0	90 ± 2
			75	0 ± 0	20 ± 3	0 ± 0	95 ± 1	100 ± 0	90 ± 5
			37.5	0 ± 0	0 ± 0	0 ± 0	90 ± 3	100 ± 0	85 ± 4
5b	H	F	150	0 ± 0	50 ± 4	30 ± 5	60 ± 2	70 ± 5	85 ± 1
5c	H	Cl	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	100 ± 0	40 ± 5
5d	H	NO2	150	0 ± 0	0 ± 0	0 ± 0	20 ± 4	20 ± 2	100 ± 0
5e	H	CF3	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	20 ± 3	100 ± 0
5f	F	F	150	20 ± 4	75 ± 2	60 ± 5	100 ± 0	95 ± 1	100 ± 0
			75	0 ± 0	0 ± 0	0 ± 0	100 ± 0	100 ± 0	95 ± 1
			37.5	0 ± 0	0 ± 0	0 ± 0	70 ± 5	90 ± 2	80 ± 4
6a	H	H	150	0 ± 0	0 ± 0	0 ± 0	30 ± 1	90 ± 4	30 ± 3
6b	H	F	150	0 ± 0	0 ± 0	0 ± 0	100 ± 0	100 ± 0	80 ± 5
6c	H	Cl	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0	0 ± 0
6d	H	NO2	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	80 ± 5	0 ± 0
6e	H	CF3	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	80 ± 2	0 ± 0
6f	F	F	150	0 ± 0	0 ± 0	0 ± 0	90 ± 3	80 ± 4	30 ± 3
7a	H	H	150	0 ± 0	0 ± 0	0 ± 0	90 ± 4	85 ± 3	80 ± 1
7b	H	F	150	0 ± 0	0 ± 0	0 ± 0	20 ± 2	95 ± 1	70 ± 5
7c	H	Cl	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	30 ± 3	0 ± 0
7d	H	NO2	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	80 ± 4	0 ± 0
7e	H	CF3	150	0 ± 0	0 ± 0	0 ± 0	0 ± 0	60 ± 2	0 ± 0
7f	F	F	150	0 ± 0	0 ± 0	0 ± 0	100 ± 0	90 ± 5	70 ± 2
fomesafen	/		150	68 ± 4	76 ± 3	78 ± 5	98 ± 2	100 ± 0	76 ± 5
			75	63 ± 1	69 ± 5	67 ± 3	92 ± 2	95 ± 3	67 ± 4
			37.5	47 ± 3	44 ± 2	56 ± 4	90 ± 1	87 ± 5	55 ± 2

^aEC, Echinochloa crusgalli; DS, Digitaria sanguinalis; SV, Setaria viridis; AT, Abutilon theophrasti; AR, Amaranthus retroflexus; EP, Eclipta prostrate.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27185879/s1, Table S1: Crystal data and structure refinement parameters of compound 5a; Table S2: Bond angles of compound 5a; Table S3: Atomic coordinates (×10⁴) and equivalent isotropic displacement parameters (Ų × 10³) of the nonhydrogen atoms; Table S4: Hydrogen coordinates (× 10⁴) and isotropic displacement parameters (Ų × 10³); Table S5: Bond lengths of compound 5a; Figure S1–S36:¹H NMR and ¹³C NMR spectra of target compounds 5a-5f, 6a-6f, and 7a-7f.

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