Metal-Free Trifluoromethylthiolation of Arylazo Sulfones.

A visible-light-driven protocol for the synthesis of aryl trifluoromethyl thioethers under photocatalyst- and metal-free conditions has been pursued. The procedure exploits the peculiar properties of arylazo sulfones (having electron-rich or electron-poor substituents on the (hetero)aromatic ring) as photochemical precursors of aryl radicals and S-trifluoromethyl arylsulfonothioates as easy-to-handle trifluoromethylthiolating agents.

The formation of an Ar–SCF3 bond is important in life sciences for the synthesis of bioactive molecules. In fact the SCF3 group, when present in an aromatic compound, strongly affected its physicochemical properties, mostly its lipophilicity. The latter is a key parameter in drug design since the bioavailability of the drug is enhanced when lipophilicity increases.[1] In this case, a combined effect of the presence of fluorine atoms[2] with heteroatoms imparts a good lipophilicity as witnessed by the high values of the Hansch parameters (πR = 1.44).[3] Some drugs, such as the antiprotozoal agent Toltrazuril and the stimulant amphetamine Tiflorex, contain the Ar–SCF3 moiety. Accordingly, a more reliable synthetic procedure in the forging of the Ar–S bond could increase a wider application of trifluoromethylthiolated aromatics in medicinal chemistry. In the last years, several synthetic protocols for the introduction of a SCF3 group in a (hetero)aromatic core have been developed.[4] An interesting approach is the direct trifluoromethylthiolation of (hetero)arenes, but only certain electron-rich derivatives led to the desired product in a clean fashion, avoiding the concomitant formation of undesired regioisomers (Scheme a).[5] Thus, several approaches have been developed by replacing an aromatic substituent with a SCF3 group making use of a metal catalyst (e.g., Cu,[6] Ni,[7] Pd,[8] and Ag[9]). The reaction is, in most cases, an ipso-substitution that starts from aryl diazonium salts,[6a,6d,6h] aryl halides,[6c,7a,7c,7d,9] aryl boronic acids,[6b−6g] aryltrifluoroborates,[6e] di(hetero)aryl-λ3-iodanes,[6f] aryl sulfonates,[7b] and arylmercaptodifluoroacetic acids (Scheme b).[8a] Metal-free alternatives, however, are limited to the alkylation of aryl sulfides by using CF3I under basic conditions[10] (Scheme c) except the trifluoromethylation of diaryl sulfides with trifluoromethyltriisopropylsilane (TIPSCF3)[11] and the radical trifluoromethylthiolation of arenediazonium salts with Me4NSCF3.[12] In recent years, a handful of examples were reported where photochemistry was applied for the synthesis of ArSCF3 thanks to the easy photogeneration of aryl radicals from aryl diazonium salts and ensuing reaction with a S-trifluoromethyl arylsulfonothioate[13a,13b] or 1,2-bis(trifluoromethyl)disulfane (CF3S)2 (Scheme d).[13c] Unfortunately, the latter approach requires the presence of a photocatalyst (PC) and a sensitive aryl radical precursor complicated by the special caution required on the use and storage of arenediazonium salts.[14]

Scheme 1

We then envisioned that a straightforward route to Ar–SCF3 could involve again aryl radicals but was generated by visible-light irradiation of arylazo sulfones (ArN2SO2Me, Scheme e). Such derivatives exhibit a wavelength selective behavior[15] and release an aryl radical upon visible-light irradiation[16] by cleavage of the N–S bond followed by nitrogen loss of the resulting aryl diazenyl radical.[17] The thus generated aryl radicals were recently applied for the forging of various Ar–C and Ar–heteroatom bonds.[18] In view of these premises, we planned to use such sulfones for the easy arylation of a SCF3-containing derivative to form the desired Ar–SCF3. As the trifluoromethylthiolating agent, we exclude the use of (CF3S)2 due to its difficult handling,[13c] focusing our attention on S-trifluoromethyl arylsulfonothioates.

Preliminary experiments were carried out on compound 1a, and the obtained results are summarized in Table . Irradiation of 1a (0.125 M, in 1,2-dichloroethane, DCE) in the presence of two equivalents of S-(trifluoromethyl) 4-difluorobenzenesulfonothioate (2a) by means of a 21 W green LED afforded the arylated compound 3a in low yields (21%, entry 1). Moving to blue light (entries 2 and 3) resulted in an improvement of the efficiency of the process, and a 53% yield was reached with a 21W blue LED lamp. The use of a different arylazo sulfone (1a′, entry 4) as well as the adoption of either electron-rich (2b) or difluorinated (2c) benzenesulfonothioates (entries 5 and 6) did not afford better results, while, among the different media tested (entries 7–9), DCE furnished the most satisfactory performance. Hydrodeaminated biphenyl can be competitively formed as the main product by changing the reaction medium (e.g., MeCN). Gratifyingly, when doubling the concentration of the reactants, 3a was isolated in 63% yield (entry 10), and this yield slightly decreased by using a 0.5 M amount of 1a (entry 11). Notably, increasing the loading of the trifluoromethylthiolating agent 2a from 2.0 equiv to 4.0 equiv improved the yield up to 75% (entry 12). The formation of 3a was completely inhibited in the absence of light (entry 13). Dedicated on–off experiments confirmed that the reaction did not proceed in the dark (see Figure S1).

Table 1

Optimization of the Reaction Conditionsa

entry	1a (conc)	2 (equiv)	solvent	light source (LED)	yield (%)
1	1a, 0.125 M	2a (2)	DCE	21 W green	21
2	1a, 0.125 M	2a (2)	DCE	9 W blue	39
3	1a, 0.125 M	2a (2)	DCE	21 W blue	53
4	1′a, 0.125 M	2a (2)	DCE	21 W blue	49
5	1a, 0.125 M	2b (2)	DCE	21 W blue	49
6	1a, 0.125 M	2c (2)	DCE	21 W blue	52
7	1a, 0.125 M	2a (2)	toluene	21 W blue	20
8	1a, 0.125 M	2a (2)	DMF	21 W blue	19
9	1a, 0.125 M	2a (2)	MeCN	21 W blue	<5%b
10	1a, 0.25 M	2a (2)	DCE	21 W blue	63
11	1a, 0.5 M	2a (2)	DCE	21 W blue	60
12	1a, 0.25 M	2a (4)	DCE	21 W blue	75
13c	1a, 0.25 M	2a (2)	DCE	21 W blue	trace

Reaction time = 12–36 h.

Biphenyl was observed as the main product;

In the dark.

The conditions described in entry 12 have been thus adopted to investigate the scope of the synthetic protocol.

Compound 3a was also synthesized on a gram scale and isolated in 45% yield (see Table ). We then investigated the scope of the protocol, as shown in Table . The desired trifluoromethylthiolated products have been isolated in discrete to satisfactory yields, and the process showed a good tolerance to both electron-donating and electron-withdrawing substituents present on the aromatic ring, including (thio)alkoxy groups (see products 3c–e, 3p), halogens (3i,n), and carbonyls (3k–m). In some cases, good results were obtained in the presence of only 2 equiv of 2a (as in the synthesis of 3f and 3l). The process was found to be also suitable to prepare naphthyl derivative 3q, whereas heteroaryl trifluoromethyl thioethers (3r–x) were mainly obtained in a lower yield. In the latter cases, the hydrodeamination product (see the case of 3u and 3x) was detected as the main byproduct (up to 30% yield).

Table 2

Trifluoromethylthiolation of Arylazo Sulfonesa

A solution of 1a–x in DCE in the presence of 2a (4 equiv) irradiated for 36 h by means of a 21 W Blue Light LED.

Reaction carried out on 10 mmol 1a, 48 h irradiation (gram scale = 1.15 g).

2 equiv of 2a employed.

Quinoline (30% yield) was observed as the byproduct via GC–MS analyses.

Benzothiazole (24% yield) was observed as the byproduct via GC–MS analyses.

A tentative mechanism is shown in Scheme . Sulfones 1a–x possess a weak nπ band between 400 and 500 nm (ε ca. 500 M–1) sufficient to allow the absorption in the visible region (465–470 nm blue LEDs have been used as inexpensive light source).[16] A direct photolysis of 2a is safely excluded since its absorption at the wavelength used is negligible (see Figure S2). The labile S–N bond underwent a smooth photocleavage (path a) liberating the desired aryl radical Ar• that, as previously described,[13a,13b] gave a substitution reaction with 2a (path b) to form derivatives 3a–x along with the stable 4-fluorophenylsulfonyl radical. Hydrogen atom abstraction of Ar• from the solvent to form the hydrodeaminated product Ar–H (path c) is the main competitive path.[18c] The intermediacy of an aryl radical has been further confirmed by the formation of adduct 4k that was isolated in 30% yield when the reaction of 1k with 2a was carried out in the presence of TEMPO (4 equiv, path d).

Scheme 2

Proposed Mechanism for the Thiotrifluoromethylation of Arylazo Sulfones

In summary, we proposed a radical trifluoromethylthiolation reaction for the formation of aryl–SCF3 bonds via simple visible-light activation of bench-stable arylazo sulfones under both metal-free and (photo)catalyst-free conditions. The process employed S-(trifluoromethyl) 4-difluorobenzenesulfonothioate as the trifluoromethylthiolating agent and was found suitable for the preparation of both electron-rich and electron-poor SCF3-substituted aromatics in discrete to satisfactory yields. In analogy with other metal-free[12] or photocatalyzed[13a−13c] trifluoromethylthiolation protocols, a low efficiency in the synthesis of heteroaryl trifluoromethyl thioethers was observed, with the only exception of benzofuran 3r.

Experimental Section

General Remarks

All solvents were distilled prior to use. For chromatography, 200–300 mesh silica gel (Qingdao, China) was employed. 1H, 13C, and 19F NMR spectra were recorded at 400, 100 and 375 MHz with a Bruker ARX 400 spectrometer. Chemical shifts are reported in ppm using tetramethylsilane as an internal standard. HRMS was performed on an Thermo Scientific LTQ Orbitrap XL (ion trap) or Bruker Solarix XR FTMS (Q-TOF) mass instrument. All arylazo sulfones were prepared from the corresponding arenediazonium tetrafluoroborates according to literature procedures.[18e−18g] The light-promoted reactions were carried out by using a standard blue LED lamp with 20 one-light-emitting diodes (12–28 V, 21 W, 465–470 nm). The distance from the light source to the irradiation vessel was 3 cm.

Typical Procedure for the Preparation of Arenediazonium Tetrafluoroborates

Method A

In a 100 mL round-bottom flask, the aromatic amine (20 mmol) was dissolved in a mixture of H2O (10 mL) and HBF4 (48% aq, 6 mL). After the mixture was stirred for 15 min, a solution of NaNO2 (1.5 g, 1.1 equiv, in 4 mL of H2O) was added dropwise at 0 °C (in an ice bath). The mixture was stirred for another 30 min at 0 °C. Then, the arenediazonium tetrafluoroborate was removed by filtration and washed with diethyl ether (2 × 10 mL). The crude product was dried in vacuo for 20 min and was then directly used without further purification.

Method B

In a 100 mL round-bottom flask, the aromatic amine (20 mmol) was dissolved in a mixture of ethanol (8 mL) and HBF4 (48% aq, 6 mL). Subsequently, tert-butyl nitrite (4.7 mL, 2.0 equiv) was added dropwise to the solution at 0 °C. The reaction mixture was stirred for 30 min at 0 °C, and anhydrous diethyl ether (20 mL) was added to precipitate the corresponding arenediazonium tetrafluoroborate. The solid was filtered off and washed with diethyl ether (2 × 10 mL). The product was then dried in vacuo for 20 min and used without further purification.

Typical Procedure for the Preparation for Arylazo Sulfones

In a 100 mL round-bottom flask, the suspension of the freshly prepared arenediazonium tetrafluoroborate (20 mmol) in CH2Cl2 (20 mL) was stirred at 0 °C; then sodium methanesulfinate (2.24 g, 1.1 equiv) was added to the reaction system in one portion. The temperature was risen to rt; the mixture was stirred overnight and then filtered, and the obtained solution evaporated. The crude product was thus purified by dissolution in CH2Cl2 and precipitation by adding cold petroleum ether. The obtained arylazo sulfones were filtered and dried in vacuo for 30 min.

Characterization Data for the Arylazo Sulfones 1a–t

1-([1,1′-Biphenyl]-4-yl)-2-(methylsulfonyl)diazene (1a).[18d]

Yellow solid (3.6 g, 69%). Mp (dec): 107–109 °C. 1H NMR (400 MHz, CDCl3): δ 8.03 (d, J = 8.6 Hz, 2H), 7.80 (d, J = 8.6 Hz, 2H), 7.68–7.66 (m, 2H), 7.52–7.48 (m, 2H), 7.46–7.42 (m, 1H), 3.24 (s, 3H).

1-([1,1′-Biphenyl]-4-yl)-2-((4-nitrophenyl)sulfonyl)diazene (1a′)

Yellow solid (4.0 g, 54%). Mp (dec): 100–101 °C. 1H NMR (400 MHz, CDCl3): δ 8.45 (d, J = 8.88 Hz, 2H), 8.19 (d, J = 8.92 Hz, 2H), 7.89 (d, J = 8.64 Hz, 2H), 7.75 (d, J = 8.68 Hz, 2H), 7.64–7.62 (m, 2H), 7.51–7.41 (m, 3H). 13C {1H} NMR (100 MHz, CDCl3): δ 151.3, 148.7, 147.9, 139.4, 138.9, 131.8, 129.1, 129.0, 128.2, 127.3, 125.5, 124.1. IR (neat, υ cm–1): 3413, 1616, 1526, 1352, 1149, 1074, 952, 858, 620, 517. The compound was found to decompose upon HRMS analysis.

4-(4-((Methylsulfonyl)diazenyl)phenyl)morpholine (1b)

Orange solid (2.1 g, 39%). Mp (dec): 139–140 °C. 1H NMR (400 MHz, CDCl3): δ 7.86 (d, J = 9.2 Hz, 2H), 6.90 (d, J = 9.2 Hz, 2H), 3.86 (t, J = 4.8 Hz, 4H), 3.46 (t, J = 5.1 Hz, 4H), 3.14 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 155.7, 140.9, 127.8, 113.1, 66.3, 46.8, 35.0. IR (neat, υ cm–1): 3413, 1607, 1379, 1318, 1234, 1141, 1067, 619, 533. HRMS (ESI) m/z: [M + H]+ calcd for C11H16N3O3S, 270.0907; found, 270.0904.

1-(Methylsulfonyl)-2-(4-phenoxyphenyl)diazene (1c)

Brown solid (4.4 g, 80%). Mp (dec): 110–111 °C. 1H NMR (400 MHz, CDCl3): δ 7.94 (d, J = 9.0 Hz, 2H), 7.47–7.43 (m, 2H), 7.27 (t, J = 7.4 Hz, 1H), 7.13–7.01 (m, 4H), 3.20 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 164.3, 154.5, 144.0, 130.2, 127.1, 125.4, 120.6, 117.7, 34.8. IR (neat, υ cm–1): 3413, 1615, 1486, 1338, 1243, 1137, 949, 853, 773, 617, 502. HRMS (ESI) m/z: [M + H]+ calcd for C13H13N2O3S, 277.0641; found, 277.0641.

(Benzyloxy)phenyl)-2-(methylsulfonyl)diazene (1d)

Pale yellow solid (3.0 g, 52%). Mp (dec): 150–151 °C. 1H NMR (400 MHz, CDCl3): δ 7.95 (d, J = 9.8 Hz, 2H), 7.45–7.36 (m, 5H), 7.11 (d, J = 9.1 Hz, 2H), 5.19 (s, 2H), 3.19 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 164.8, 143.3, 135.5, 128.8, 128.5, 127.5, 127.3, 115.7, 70.7, 34.9. IR (neat, υ cm–1): 3413, 1614, 1485, 1338, 1137, 949, 852, 617, 502. HRMS (ESI) m/z: [M + H]+calcd for C14H15N2O3S, 291.0795; found, 291.0798.

1-(Methylsulfonyl)-2-(4-(methylthio)phenyl)diazene (1e)

Yellow solid (2.9 g, 63%). Mp (dec): 130–132 °C. 1H NMR (400 MHz, CDCl3): δ 7.87 (d, J = 8.8 Hz, 2H), 7.35 (d, J = 8.0 Hz, 2H) 3.20 (s, 3H), 2.58 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 150.0, 145.8, 125.4, 124.9, 34.8, 14.7. IR (neat, υ cm–1): 3413, 1614, 1486, 1337, 1137, 949, 617, 502. HRMS (ESI) m/z: [M + H]+ calcd for C8H11N2O2S2, 231.0257; found, 231.0256.

1-(4-(tert-Butyl)phenyl)-2-(methylsulfonyl)diazene (1f).[18d]

Yellow solid (2.6 g, 55%). Mp (dec): 78–80 °C. 1H NMR (400 MHz, CDCl3): δ 7.88 (d, J = 8.8 Hz, 2H), 7.59 (d, J = 8.8 Hz, 2H), 3.20 (s, 3H), 1.36 (s, 9H).

Ethyl 3-(4-((E)-(Methylsulfonyl)diazenyl)phenyl)acrylate (1g).[18e]

Yellow solid (5.5 g, 98%). Mp (dec): 98–100 °C. 1H NMR (400 MHz, CDCl3): δ 7.95(d, J = 8.5 Hz, 2H), 7.68–7.72 (m, 3H), 6.57 (d, J = 16.1 Hz, 1H), 4.28 (q, J = 7.1 Hz, 2H), 3.22 (s, 3H), 1.34 (t, J = 7.1 Hz, 3H).

1-(Methylsulfonyl)-2-(4-((triisopropylsilyl)ethynyl)phenyl)diazene (1h).[18e]

Yellow solid (2.7 g, 37%). Mp (dec): 63–64 °C. 1H NMR (400 MHz, CDCl3): δ 7.89 (d, J = 8.5 Hz, 2H), 7.65 (d, J = 8.5 Hz, 2H), 3.22 (s, 3H), 1.16–1.12 (m, 21H).

1-(4-Bromophenyl)-2-(methylsulfonyl)diazene (1i).[18f]

Yellow solid (3.2 g, 60%). Mp (dec): 131–133 °C. 1H NMR (400 MHz, CDCl3): δ 7.83 (d, J = 8.8 Hz, 2H), 7.74 (d, J = 8.8 Hz, 2H), 3.22 (s, 3H).

4-((Methylsulfonyl)diazenyl)benzonitrile (1j).[18h]

Yellow solid (3.3 g, 79%). Mp (dec): 114–115 °C. 1H NMR (400 MHz, CDCl3): δ 8.04 (d, J = 8.4 Hz, 2H), 7.89 (d, J = 8.4 Hz, 2H), 3.26 (s, 3H).

Ethyl 4-((Methylsulfonyl)diazenyl)benzoate (1k).[18d]

Yellow solid (3.7 g, 73%). Mp (dec): 83–84 °C. 1H NMR (400 MHz, CDCl3): δ 8.25(d, J = 8.6 Hz, 2H), 7.99 (d, J = 8.6 Hz, 2H), 4.44 (q, J = 7.1 Hz, 2H), 3.25 (s, 3H), 1.43 (t, J = 7.1 Hz, 3H).

1-(4-((Methylsulfonyl)diazenyl)phenyl)ethanone (1l).[18d]

Yellow solid (3.6 g, 80%). Mp (dec): 119–120 °C. 1H NMR (400 MHz, CDCl3): δ 8.13 (d, J = 8.5 Hz, 2H), 8.00 (d, J = 8.5 Hz, 2H), 3.24 (s, 3H), 2.67 (s, 3H).

(4-((Methylsulfonyl)diazenyl)phenyl)(phenyl)methanone (1m)

Yellow solid (4.3 g, 75%). Mp (dec): 130 °C. 1H NMR (400 MHz, CDCl3): δ 8.01 (dd, J = 8.6, 31.4 Hz, 4H), 7.81 (d, J = 7.04 Hz, 2H), 7.67–7.62 (m, 1H), 7.52 (t, J = 7.88 Hz, 2H), 3.27 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 195.1, 150.6, 142.8, 136.4, 133.2, 131.0, 130.0, 128.5, 124.2, 34.8. IR (neat, υ cm–1): 3413, 1659, 1617, 1340, 1276, 1159, 1069, 954, 861, 700. HRMS (ESI) m/z: [M + H]+ calcd for C14H13N2O3S, 289.0638; found, 289.0641.

1-(3-Bromophenyl)-2-(methylsulfonyl)diazene (1n).[18e]

Yellow solid (2.5 g, 47%). Mp (dec): 95–96 °C. 1H NMR (400 MHz, CDCl3): δ 8.06 (s, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.47 (t, J = 8.0 Hz, 1H), 3.22 (s, 3H).

1-(Methylsulfonyl)-2-(3,4,5-trimethoxyphenyl)diazene (1o)

Pale yellow solid (2.7 g, 49%). Mp (dec): 122–124 °C. 1H NMR (400 MHz, CDCl3): δ 7.26 (s, 2H), 4.00 (s, 3H), 3.94 (s, 6H), 3.22 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 153.5, 144.4, 102.5, 61.1 (d, J = 6.4 Hz), 56.3 (d, J = 5.4 Hz), 34.9. IR (neat, υ cm–1): 3413, 1614, 1486, 1338, 1137, 949, 617, 502. HRMS (ESI) m/z: [M + H]+calcd for C10H14N2O5S, 275.0696; found, 275.0696.

1-(Benzo[d][1,3]dioxol-5-yl)-2-(methylsulfonyl)diazene (1p)

Dark green solid (0.91 g, 20%). Mp (dec): 113–114 °C. 1H NMR (400 MHz, CDCl3): δ 7.69 (dd, J = 2.0, 8.2 Hz, 1H), 7.34 (d, J = 1.9, Hz, 1H), 6.99 (d, J = 8.2 Hz, 1H), 6.14 (s, 2H), 3.18 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 154.4, 149.5, 144.8, 129.3, 108.4, 102.8, 98.8, 34.9. IR (neat, υ cm–1): 3414, 1617, 1477, 1414, 1152, 618. HRMS (EI) m/z: [M + H]+ calcd for C8H9N2O4S, 229.0278; found, 229.0281.

1-(Methylsulfonyl)-2-(naphthalen-2-yl)diazene (1q).[18e]

Yellow solid (2.2 g, 47%). Mp (dec): 108–109 °C. 1H NMR (400 MHz, CDCl3): δ 8.56 (s, 1H), 8.03–8.01 (m, 1H), 7.92–7.89 (m, 3H), 7.67 (t, J = 7.5 Hz, 1H), 7.61 (t, J = 7.5 Hz, 1H), 3.27 (s, 3H).

Ethyl 5-((Methylsulfonyl)diazenyl)benzofuran-2-carboxylate (1r).[18e]

Yellow solid (4.6 g, 78%). Mp (dec): 117–118 °C. 1H NMR (400 MHz, CDCl3): δ 8.35 (d, J = 2.0 Hz, 1H), 8.05 (dd, J = 2.0, 9.0 Hz, 1H), 7.73 (d, J = 9.0 Hz, 1H), 7.64 (d, J = 0.8 Hz, 1H), 4.47 (q, J = 7.1 Hz, 2H), 3.24 (s, 3H), 1.44 (t, J = 7.1 Hz, 3H).

2-Methyl-5-((methylsulfonyl)diazenyl)benzo[d]thiazole (1s).[18e]

Brown solid (3.9 g, 77%). Mp (dec): 123–124 °C. 1H NMR (400 MHz, CDCl3): δ 8.50 (d, J = 1.9 Hz, 1H), 7.95 (d, J = 8.6 Hz, 1H), 7.92 (dd, J = 1.9, 8.6 Hz, 1H), 3.25 (s, 3H), 2.89 (s, 3H).

Methyl 5-((Methylsulfonyl)diazenyl)nicotinate (1t).[18e]

Yellow solid (4.1 g, 85%). Mp (dec): 103–105 °C. 1H NMR (400 MHz, CDCl3): δ 9.46 (d, J = 1.9 Hz, 1H), 9.38 (d, J = 2.3 Hz, 1H), 8.73 (t, J = 2.1 Hz, 1H), 4.02 (s, 3H), 3.29 (s, 3H).

7-((Methylsulfonyl)diazenyl)quinoline (1u)

Red solid (3.1 g, 66%). Mp (dec): 140–142 °C. 1H NMR (400 MHz, CDCl3): δ 9.06 (dd, J = 1.5, 4.2 Hz, 1H), 8.79 (s, 1H), 8.25 (d, J = 8.2 Hz, 1H), 7.99–7.92 (m, 2H), 7.59–7.56 (m, 1H), 3.31 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 152.1, 149.0, 148.1, 136.0, 133.7, 132.0, 129.8, 123.8, 116.2, 34.8. IR (neat, υ cm–1): 3440, 3045, 3020, 1500, 1463, 1328, 1151, 946, 867, 772, 746, 616, 566, 443. HRMS (ESI) m/z: [M + Na]+ calcd for C10H9N3O2SNa, 258.0308; found, 258.0308.

4-Methyl-7-((methylsulfonyl)diazenyl)-2H-chromen-2-one (1v)

Orange solid (2.3 g, 43%). Mp (dec): 102–104 °C. 1H NMR (400 MHz, CDCl3): δ 7.88–7.85 (m, 2H), 7.80 (d, J = 9.0 Hz, 1H), 6.46 (d, J = 1.3 Hz, 1H), 3.27 (s, 3H), 2.51 (d, J = 1.3 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 159.04, 153.9, 151.1, 150.2, 126.0, 125.0, 118.9, 118.0, 113.6, 35.0, 18.7. IR (neat, υ cm–1): 3440, 3045, 3006, 2920, 1715, 1624, 14110, 1340, 1259, 1158, 979, 903, 558. HRMS (ESI) m/z: [M + H]+ calcd for C11H11N2O4S, 267.0434; found, 267.0434.

9-Ethyl-3-((methylsulfonyl)diazenyl)-9H-carbazole (1w)

Brown solid (4.9 g, 81%). Mp (dec): 145–147 °C. 1H NMR (400 MHz, CDCl3): δ 8.70 (d, J = 1.9 Hz, 1H), 8.12 (d, J = 7.8 Hz, 1H), 8.08 (dd, J = 1.9, 8.9 Hz, 1H), 7.58–7.54 (m, 1H), 7.48–7.45 (m, 2H), 7.37–7.33 (m, 1H), 4.41 (q, J = 7.2 Hz, 2H), 3.24 (s, 3H), 1.49 (t, J = 7.3 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 144.3, 142.3, 140.9, 127.3, 123.7, 123.3, 122.2, 121.0, 120.5, 111.4, 109.6, 109.2, 38.1, 35.0, 13.8. IR (neat, υ cm–1): 3442, 3032, 2975, 1593, 1498, 1418, 1119, 958, 835, 489. HRMS (ESI) m/z: [M + H]+ calcd for C15H16N3O2S, 302.0958; found, 302.0958.

5-((Methylsulfonyl)diazenyl)benzo[d]thiazole (1x)

Yellow solid (1.5 g, 31%). Mp (dec): 152–154 °C. 1H NMR (400 MHz, CDCl3): δ 9.16 (s, 1H), 8.74 (d, J = 1.8 Hz, 1H), 8.12 (d, J = 8.1 Hz, 1H), 8.02 (dd, J = 1.9, 8.7 Hz, 1H), 3.28 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 156.6, 153.8, 147.7, 140.7, 123.4, 122.9, 118.2, 34.9. IR (neat, υ cm–1): 3438, 3049, 1477, 1326, 1151, 865, 823, 531. HRMS (ESI) m/z: [M + H]+ calcd for C8H8N3O2S2, 242.0052; found, 242.0052.

Synthesis of Compound 2c

Compound 2c was obtained by following a procedure previously reported for the synthesis of compounds 2a,b.[19] A mixture of 3,5-difluorobenzenesulfinate sodium salt (20 mmol, 1 equiv), N-[(trifuoromethyl)thio] aniline (1 equiv),[20] and paratoluenesulfonic acid (2.5 equiv) in DCE (130 mL) was stirred at room temperature. After the completion of the reaction, indicated by TLC, the mixture was filtered with a sand core funnel with silica gel, washed with CH2Cl2, and dried over Na2SO4. After concentration, the residue was purified by flash column chromatography to obtain the final product 2c (light yellow oil, 3.8 g, 68%). 1H NMR (400 MHz, CDCl3): δ 7.55 (dd, J = 1.92, 5.68 Hz, 2H), 7.19 (tt, J = 2.28, 8.20 Hz, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ 164.1 (d, J = 11.52 Hz, 1C), 161.5 (d, J = 11.36 Hz, 1C), 147.0 (t, J = 9.46 Hz, 1C), 120.7 (q, J = 311.66 Hz, 1C), 111.3 (d, J = 29.06 Hz, 1C), 111.3 (d, J = 11.31 Hz, 1C), 110.9 (t, J = 24.89 Hz, 1C). 19F NMR (375 MHz, CDCl3): δ −38.1 (s, 3F), −103.4 (s, 2F). The present compound was found to decompose upon HRMS analysis.

Synthesis of Thiotrifluoromethyl Arenes (3a–x)

A solution of the chosen arylazo sulfone (1a–x, 0.25 M) in DCE was placed in a 10 mL microwave tube, and then S-(trifluoromethyl) 4-fluorobenzenesulfonothioate (2a, 1 mmol, 2.0–4.0 equiv, see Table ) was added. The resulting solution was irradiated under stirring at room temperature for 36 h by means of a 21 W blue LED, and then the reaction mixture was concentrated under reduced pressure to evaporate the solvent. The residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate mixture as the eluent).

Gram-Scale Preparation of 3a

1-([1,1′-Biphenyl]-4-yl)-2- (methylsulfonyl) diazene (1a, 10 mmol, 2.6 g) was placed in a round-bottom flask and dissolved in DCE (100 mL) under aerated conditions. Compound 2a (20 mmol, 2.0 equiv) was added, and the resulting solution was irradiated under stirring for 48 h by means of a 21 W blue LED. The reaction mixture was concentrated under reduced pressure to evaporate the solvent, and the residue was purified by silica gel column chromatography (petroleum ether as the eluent) to obtain [1,1′-biphenyl]-4-yl (trifluoromethyl) sulfane 3a (1.15 g, 45%) as a white solid.

Characterization Data for the Isolated Products (3a–x)

[1,1′-Biphenyl]-4-yl(trifluoromethyl)sulfane (3a).[13]

After purification by silica gel column chromatography (PE), compound 3a was isolated as a white solid (50 mg, 75%). R (PE) = 0.8. 1H NMR (400 MHz, CDCl3): δ 7.74 (d, J = 8.3 Hz, 2H), 7.66–7.60 (m, 4H), 7.49 (t, J = 7.4 Hz, 2H), 7.42 (d, J = 7.4 Hz, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ 143.8, 139.7, 136.7, 129.6 (q, 1J(C,F) = 306.4 Hz), 128.9, 128.1, 127.2, 123.1 (q, 2J(C,F) = 1.9 Hz). 19F NMR (375 MHz, CDCl3): δ −42.7 (s, 3F).

4-(4-((Trifluoromethyl)thio)phenyl)morpholine (3b).[9]

After purification by silica gel column chromatography (PE/EA = 30:1), compound 3b was isolated as a white solid (39 mg, 60%). R (PE/EA = 20:1) = 0.3. 1H NMR (400 MHz, CDCl3): δ 7.52 (d, J = 8.3 Hz, 2H), 6.88 (d, J = 8.3 Hz, 2H), 3.85 (t, J = 5.1 Hz, 4H), 3.23 (t, J = 5.0 Hz, 4H). 13C{1H} NMR (100 MHz, CDCl3): δ 152.9, 137.9, 129.7 (q, 1J(C,F) = 306.3 Hz), 115.2, 112.5, 66.6, 48.0. 19F NMR (375 MHz, CDCl3): δ −44.1 (s, 3F).

(4-Phenoxyphenyl)(trifluoromethyl)sulfane (3c).[13]

After purification by silica gel column chromatography (PE), compound 3c was isolated as a white solid (39 mg, 58%). R (PE) = 0.55. 1H NMR (400 MHz, CDCl3): δ 7.60 (d, J = 8.7 Hz, 2 H), 7.40 (t, J = 7.6 Hz, 2H), 7.22 (t, J = 7.4 Hz, 1H), 7.07 (d, J = 7.7 Hz, 2H), 7.00 (d, J = 8.8 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 160.4, 155.6, 138.3, 129.6 (q, 1J(C,F) = 306.3 Hz), 130.0, 124.5, 120.1, 118.6, 117.3 (q, 2J(C,F) = 2.0 Hz). 19F NMR (375 MHz, CDCl3): δ −43.5 (s, 3F).

(4-(Benzyloxy)phenyl)(trifluoromethyl)sulfane (3d).[21]

After purification by silica gel column chromatography (PE), compound 3d was isolated as a white solid (37 mg, 52%). R (PE) = 0.6. 1H NMR (400 MHz, CDCl3): δ 7.59 (d, J = 8.8 Hz, 2H), 7.45–7.34 (m, 5H), 7.01 (d, J = 8.9 Hz, 2H), 5.09 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 161.0, 138.3, 136.2, 129.6 (q, 1J(C,F) = 306.2 Hz), 128.7, 128.2, 127.5, 115.8, 115.2 (q, 2J(C,F) = 2.0 Hz), 70.2. 19F NMR (375 MHz, CDCl3): δ −43.8 (s, 3F).

Methyl(4-((trifluoromethyl)thio)phenyl)sulfane (3e).[13]

After purification by silica gel column chromatography (PE), compound 3e was isolated as a yellow oil (26 mg, 46%). R (PE) = 0.6. 1H NMR (400 MHz, CDCl3): δ 7.55 (d, J = 8.4 Hz, 2H), 7.25 (d, J = 8.8 Hz, 2H), 2.50 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 143.3, 136.6, 129.5 (q, 1J(C,F) = 306.4 Hz), 126.4, 119.7, 15.0. 19F NMR (375 MHz, CDCl3): δ −43.3 (s, 3F).

(4-(tert-Butyl)phenyl)(trifluoromethyl)sulfane (3f).[13]

After purification by silica gel column chromatography (PE/EA = 100:1), compound 3f was isolated as a colorless oil (35 mg, 60%). R (PE/EA = 50:1) = 0.6. 1H NMR (400 MHz, CDCl3): δ 7.58 (d, J = 8.0 Hz, 2H), 7.43 (d, J = 8.4 Hz, 2H), 1.33 (s, 9H). 13C{1H} NMR (100 MHz, CDCl3): δ 154.4, 136.1, 129.7 (q, 1J(C,F) = 306.0 Hz), 126.6, 120.9 (q, 2J(C,F) = 1.8 Hz), 34.9, 31.1. 19F NMR (375 MHz, CDCl3): δ −43.0 (s, 3F).

Ethyl (E)-3-(4-((Trifluoromethyl)thio)phenyl)acrylate (3g)

After purification by silica gel column chromatography (PE/EA = 50:1), compound 3g was isolated as a white solid (22 mg, 32%). R (PE/EA = 40:1) = 0.4. 1H NMR (400 MHz, CDCl3): δ 7.68–7.65 (m, 3H), 7.55 (d, J = 8.3 Hz, 2H), 6.49 (d, J = 16.0 Hz, 1H), 4.28 (q, J = 7.2 Hz, 2H), 1.34 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 166.4, 142.7, 136.9, 136.5, 129.4 (q, 1J(C,F) = 306.6 Hz), 128.7, 126.2 (q, 2J(C,F) = 2.0 Hz), 120.6, 60.8, 14.3. 19F NMR (375 MHz, CDCl3): δ −42.3 (s, 3F). HRMS (EM) m/z: [M – e]+calcd for C12H11F3O2S, 276.0432; found, 276.0417.

Triisopropyl((4-((trifluoromethyl)thio)phenyl)ethynyl)silane (3h)

After purification by silica gel column chromatography (PE), compound 3h was isolated as a colorless oil (40 mg, 45%). R (PE) = 0.8. 1H NMR (400 MHz, CDCl3): δ 7.59 (d, J = 8.2 Hz, 2H), 7.51 (d, J = 8.2 Hz, 2H), 1.13 (s, 21H). 13C{1H} NMR (100 MHz, CDCl3): δ 136.0, 134.0, 132.9, 124.4 (q, 1J(C,F) = 306.4 Hz), 126.4, 124.2 (q, 2J(C,F) = 2.1 Hz), 105.5, 94.1, 18.6, 11.3. 19F NMR (375 MHz, CDCl3): δ −42.6 (s, 3F). HRMS (EM) m/z: [M – e]+calcd for C18H25F3SSi, 358.1398; found, 358.1385.

(4-Bromophenyl)(trifluoromethyl)sulfane (3i).[13]

After purification by silica gel column chromatography (PE), compound 3i was isolated as a colorless oil (36 mg, 56%). R (PE) = 0.85. 1H NMR (400 MHz, CDCl3): δ 7.57 (d, J = 8.8 Hz, 2H), 7.52 (d, J = 8.5 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 137.7 132.8 129.2 (q, 1J(C,F) = 306.6 Hz), 126.0, 123.4 (q, 2J(C,F) = 2.1 Hz). 19F NMR (375 MHz, CDCl3): δ −42.7 (s, 3F).

4-((Trifluoromethyl)thio)benzonitrile (3j).[13]

After purification by silica gel column chromatography (PE/EA = 100:1), compound 3j was isolated as a white solid (27 mg, 53%). R (PE/EA = 50:1) = 0.35. 1H NMR (400 MHz, CDCl3): δ 7.77 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 136.0 132.9, 130.6 (q, 2J(C,F) = 2.1 Hz), 129.0 (q, 1J(C,F) = 307.0 Hz,), 117.6, 114.7. 19F NMR (375 MHz, CDCl3): δ −41.5 (s, 3F).

Methyl 4-((Trifluoromethyl)thio)benzoate (3k).[13]

After purification by silica gel column chromatography (PE/EA = 50:1), compound 3k was isolated as a yellow oil (52 mg, 83%). R (PE/EA = 50:1) = 0.5. 1H NMR (400 MHz, CDCl3): δ 8.08 (d, J = 8.5 Hz, 2H), 7.71 (d, J = 8.0 Hz, 2H), 4.40 (q, J = 7.1 Hz, 2H), 1.40 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 165.5, 135.5, 132.6, 130.8, 129.7 (q, 2J(C,F) = 2.0 Hz), 129.3 (q, 1J(C,F) = 306.5 Hz), 61.5, 14.2. 19F NMR (375 MHz, CDCl3): δ −41.9 (s, 3F).

1-(4-((Trifluoromethyl)thio)phenyl)ethanone (3l).[13]

After purification by silica gel column chromatography (PE/EA = 50:1), compound 3l was isolated as a yellow soil (36 mg, 66%). R (PE/EA = 50:1) = 0.25. 1H NMR (400 MHz, CDCl3): δ 7.98 (d, J = 8.4 Hz, 2H), 7.74 (d, J = 8.3 Hz, 2H), 2.63 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 197.0, 138.5, 135.7, 130.0 (q, 2J(C,F) = 2.0 Hz), 129.3 (q, 1J(C,F) = 306.6 Hz), 129.1, 26.7. 19F NMR (375 MHz, CDCl3): δ −41.8 (s, 3F).

Phenyl(4-((trifluoromethyl)thio)phenyl)methanone (3m).[13]

After purification by silica gel column chromatography (PE/EA = 100:1), compound 3m was isolated as a white solid (48 mg, 68%). R (PE/EA = 50:1) = 0.4. 1H NMR (400 MHz, CDCl3): δ 7.84–7.76 (m, 6H), 7.63 (t, J = 7.4 Hz, 1H), 7.51 (t, J = 8.4 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 195.5, 139.5, 136.8, 135.5, 133.0, 130.6, 130.7, 129.3 (q, 1J(C,F) = 306.7 Hz), 129.1 (q, 2J(C,F) = 2.0 Hz), 128.5. 19F NMR (375 MHz, CDCl3): δ −41.8 (s, 3F).

(3-Bromophenyl)(trifluoromethyl)sulfane (3n).[22]

After purification by silica gel column chromatography (PE), compound 3n was isolated as a white solid (31 mg, 48%): R (PE) = 0.8. 1H NMR (400 MHz, CDCl3): δ 7.82 (s, 1H), 7.64–7.59(m, 2H), 7.31 (t, J = 8.0 Hz, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ 138.7, 134.7, 134.0, 130.7, 129.3 (q, 1J(C,F) = 306.5 Hz), 126.3 (q, 2J(C,F) = 2.1 Hz), 122.9. 19F NMR (375 MHz, CDCl3): δ −42.4 (s, 3F).

(Trifluoromethyl)(3,4,5-trimethoxyphenyl)sulfane (3o).[23]

After purification by silica gel column chromatography (PE/EA = 40:1), compound 3o was isolated as a yellow solid (30 mg, 44%). R (PE/EA = 40:1) = 0.35. 1H NMR (400 MHz, CDCl3): δ 6.86 (s, 2H), 3.88(s, 9H). 13C{1H} NMR (100 MHz, CDCl3): δ 153.4, 140.6, 129.6 (q, 1J(C,F) = 306.4 Hz), 118.5 (q, 2J(C,F) = 2.1 Hz), 113.7, 60.9, 56.3. 19F NMR (375 MHz, CDCl3): δ −43.0 (s, 3F).

5-((Trifluoromethyl)thio)benzo[d][1,3]dioxole (3p).[21]

After purification by silica gel column chromatography (PE), compound 3p was isolated as a white solid (16 mg, 29%). R (PE) = 0.6. Mp = 113–114 °C. 1H NMR (400 MHz, CDCl3): δ 7.17 (dd, J = 1.72, 8.08 Hz, 1H), 7.09 (d, J = 1.56 Hz, 1H), 6.84 (d, J = 8.04 Hz, 1H), 6.04 (s, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 150.3, 148.3, 131.6, 129.5 (q, 1J(C,F) = 306.3 Hz), 116.2, 116.0 (q, 2J(C,F) = 2.1 Hz), 109.0. 19F NMR (375 MHz, CDCl3): δ −43.9 (s, 3F).

Naphthalen-2-yl(trifluoromethyl)sulfane (3q).[10c]

After purification by silica gel column chromatography (PE), compound 3q was isolated as a white solid (27 mg, 48%). R (PE) = 0.9. 1H NMR (400 MHz, CDCl3): δ 8.21 (s, 1H), 7.89–7.87 (m, 3H), 7.67 (d, J = 8.5 Hz, 1H), 7.61–7.55 (m, 2H). 13C{1H} NMR (100 MHz, CDCl3): δ 137.0, 133.9, 133.4, 131.8, 129.7 (q, 1J(C,F) = 306.6 Hz), 129.2, 128.2, 127.9, 127.8, 127.0, 121.5 (q, 2J(C,F) = 2.3 Hz). 19F NMR (375 MHz, CDCl3): δ −42.5 (s, 3F).

Ethyl 5-((Trifluoromethyl)thio)-2,3-dihydrobenzofuran-2-carboxylate (3r)

After purification by silica gel column chromatography (PE/EA = 50:1), compound 3r was isolated as a white solid (49 mg, 67%). R (PE/EA = 50:1) = 0.4. Mp = 102–103 °C. 1H NMR (400 MHz, CDCl3): δ 8.03 (s, 1H), 7.73–7.70 (m, 1H), 7.64–7.62 (m, 1H), 7.53 (s, 1H), 4.46 (q, J = 7.16 Hz, 2H), 1.43 (t, J = 7.1 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 159.0, 156.6, 147.2, 135.3, 131.8, 129.5 (q, 1J(C,F) = 306.5 Hz), 128.2, 119.5 (q, 2J(C,F) = 2.1 Hz), 113.5, 113.2, 61.8, 14.3. 19F NMR (375 MHz, CDCl3): δ −43.3 (s, 3F). HRMS (EM) m/z: [M −e]+ calcd for C12H9F3O3S, 290.0224; found, 290.0213.

2-Methyl-5-((trifluoromethyl)thio)benzo[d]thiazole (3s)

After purification by silica gel column chromatography (PE/EA = 20:1), compound 3s was isolated as a white solid (19 mg, 30%). R (PE/EA = 20:1) = 0.33. Mp = 87–88 °C. 1H NMR (400 MHz, CDCl3): δ 8.26 (d, J = 1.36 Hz, 1H), 7.87 (d, J = 8.28 Hz, 1H), 7.61 (dd, J = 1.48, 8.32 Hz, 1H), 2.86 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 168.9, 153.9, 138.7, 131.8, 130.5, 129.6 (q, 1J(C,F) = 306.4 Hz), 122.1, 131.7, 20.2 (q, 1J(C,F) = 4.4 Hz). 19F NMR (375 MHz, CDCl3): δ – 42.9 (s, 3F). HRMS (EI) m/z: [M + H]+ calcd for C9H7F3NS2, 249.9967; found, 249.9970.

Methyl 5-((Trifluoromethyl)thio)nicotinate (3t).[24]

After purification by silica gel column chromatography (PE/EA = 10:1), compound 3t was isolated as a white solid (25 mg, 43%). R (PE/EA = 5:1) = 0.5. 1H NMR (400 MHz, CDCl3): δ 9.33 (s,1H), 9.01 (s, 1H), 8.59 (s, 1H), 4.00 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 164.4, 159.0, 152.5, 144.3, 128.9 (q, 1J(C,F) = 307.1 Hz), 126.9, 122.4, 52.9. 19F NMR (375 MHz, CDCl3): δ −42.0 (s, 3F).

7-((Trifluoromethyl)thio)quinoline (3u)

After purification by silica gel column chromatography (PE/EA = 30:1), compound 3u was isolated as a yellow oil (17 mg, 30%). R (PE/EA = 10:1) = 0.3. 1H NMR (400 MHz, CDCl3): δ 9.00 (d, J = 2.8 Hz, 1H), 8.48 (s, 1H), 8.21 (d, J = 8.2 Hz, 1H), 7.87 (d, J = 8.5 Hz, 1H), 7.75 (d, J = 8.5 Hz, 1H), 7.52–7.49 (s, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ 151.5, 147.8, 137.8, 136.0, 132.3, 129.5 (q, 1J(C,F) = 306.8 Hz), 129.0, 128.9, 125.9 (q, 2J(C,F) = 1.8 H), 122.7. 19F NMR (375 MHz, CDCl3): δ −41.9 (s, 3F). HRMS (ESI) m/z: [M + H]+ calcd for C10H7F3NS, 230.0246; found, 230.0246.

4-Methyl-7-((trifluoromethyl)thio)-2H-chromen-2-one (3v).[25]

After purification by silica gel column chromatography (PE/EA = 10:1), compound 3v was isolated as a white solid (19 mg, 29%). R (PE/EA = 10:1) = 0.2. 1H NMR (400 MHz, CDCl3): δ 7.66–7.64 (m,2H), 7.55 (dd, J = 1.6, 8.2 Hz, 1H), 6.38 (d, J = 1.2 Hz,1H), 2.46 (d, J = 1.2 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 159.6, 153.3, 151.3, 130.8, 129.2 (q, 1J(C,F) = 306.8 Hz), 128.3 (q, 2J(C,F) = 2.1 Hz), 125.4, 123.9, 121.7, 116.7, 18.5. 19F NMR (375 MHz, CDCl3): δ −41.8 (s, 3F).

9-Ethyl-3-((trifluoromethyl)thio)-9H-carbazole (3w).[6d]

After purification by silica gel column chromatography (PE), compound 3w was isolated as a colorless solid (19 mg, 25%). R (PE) = 0.4. 1H NMR (400 MHz, CDCl3): δ 8.39 (d, J = 1.6 Hz, 1H), 8.12 (d, J = 7.8, Hz, 1H), 7.72 (dd, J = 1.6, 8.5 Hz, 1H), 7.55–7.51 (m, 1H), 7.45–7.72 (m, 2H), 7.31–7.27 (m, 1H), 5.14 (q, J = 7.2 Hz, 2H), 1.46 (t, J = 7.2 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 141.0, 140.3, 133.7, 129.9 (q, 1J(C,F) = 306.4 Hz), 129.6, 126.6, 123.9, 122.2, 120.7, 119.8, 112.5 (q, 2J(C,F) = 2.0 Hz), 109.2, 108.8, 37.7, 13.7. 19F NMR (375 MHz, CDCl3): δ −44.1 (s, 3F).

5-((Trifluoromethyl)thio)benzo[d]thiazole (3x)

After purification by silica gel column chromatography (PE/EA = 50:1), compound 3x was isolated as a colorless oil (12 mg, 20%). R (PE/EA = 50:1) = 0.5. 1H NMR (400 MHz, CDCl3): δ 9.09 (s,1H), 8.47 (d, J = 1.6 Hz, 1H), 8.02 (d, J = 8.4 Hz, 1H), 7.71 (dd, J = 1.6, 8.4 Hz, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ 155.7, 153.7, 136.7, 132.6, 131.9, 129.5 (q, 1J(C,F) = 306.4 Hz), 122.7, 122.2 (q, 2J(C,F)= 2.1 Hz). 19F NMR (375 MHz, CDCl3): δ −42.7 (s, 3F). HRMS (ESI) m/z: [M + H]+ calcd for C8H5F3NS2, 235.9810; found, 235.9811.

Irradiation of 1k in the Presence of TEMPO

A solution of arylazo sulfone 1k (0.25 M) in DCE (1 mL) was placed in a 10 mL microwave tube, and then S-(trifluoromethyl) 4-fluorobenzenesulfonothioate (2a, 1 mmol, 4.0 equiv) and TEMPO (4 equiv) were added. The resulting solution was irradiated under stirring at room temperature for 36 h by means of a 21 W blue LED, and then the reaction mixture was concentrated under reduced pressure to evaporate the solvent. Purification by silica gel column chromatography (PE/EA = 30:1), afforded ethyl 4-((2,2,6,6-tetramethylpiperidin-1-yl)oxy)benzoate 4k as a white solid (23 mg, 30% yield). R (PE/EA = 30:1) = 0.4. 1H NMR (400 MHz, CDCl3): δ 7.93 (d, J = 9.1 Hz, 2H), 7.21 (s, 2H), 4.33 (q, J = 7.2 Hz, 2H), 1.64–1.56 (m, 5H), 1.44–1.41 (m, 1H), 1.36 (t, J = 7.2 Hz, 3H), 1.23 (s, 6H), 0.99 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3): δ 167.4, 166.5, 131.0, 122.4, 113.7, 60.6, 60.4, 39.7, 32.4, 20.4, 17.0, 14.4.