Received: December 2017

DOI 10.17677/fn20714807.2018.01.02

Fluorine Notes, 2018, 116, 3-4

Synthesis of Polychlorofluoroarenes from Polyfluoroarenethiols, SOCl₂ and SO₂Cl₂

P. V. Nikul’shin, А. M. Maksimov, R.A.Bredikhin, and V. Е. Platonov

N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, pr. Academika Lavrent’eva 9, Novosibirsk, 630090, Russian Federation

e-mail: platonov@nioch.nsc.ru

Abstract. The thiol group in polyfluoroarenethiols was replaced by the chlorine atom, using SOCl₂ and SO₂Cl₂ as chlorinating reagents. By heating in ampules at 200–220°C polyfluoro- and polychlorofluoroarenethiols with SO₂Cl₂ monochloro- and dichloropolyfluoroarenes and also 1,2,4-trichlorotrifluorobenzene were synthesized. Dichloropolyfluoroarenes contain chlorine atoms in ortho- and para-positions.

Keywords: Thionyl chloride, sulfuryl chloride, thiol group, polyfluoroarenethiols, polychlorofluoroarenes

Polychlorofluoroarenes are important products for the synthesis of a large number of polyfluoroaromatic compounds with various functional groups. Selective participation of the C_Ar-Cl bond in the metallation reactions is one of the directions of transformation of chloropolyfluoroarenes. The Grignard reagents [1], polyfluoroaryl lithiums [1] and polyfluoroarylzinc compounds [2] formed from chloropolyfluoroarenes are versatile and convenient to use for the synthesis of a large number of functional derivatives of polyfluoroaromatic compounds. In this connection, in recent years, we have been developing a method of synthesis of chloropolyfluoroarenes consisting in substitution of the thiol group in polyfluoroarenethiols by chlorine atom. By this method a series of chloropolyfluoroaromatic compounds was synthesized in high yields [3, 4]. The process was realized at high temperature (~400°C) in a flow reactor as join pyrolysis of polyfluoroarenethiols with Cl₂ as well as with SOCl₂ and SO₂Cl₂ as chlorine sources [3]. In addition to the reactions in the flow system, we studied the conversion of polyfluoroarenethiols with PCl₅ in ampoules at ~ 200°C, which also led to the preparation of chloropolyfluoroarenes [4]. On the example of the reaction of pentafluorobenzenethiol 1 with Cl₂ and PCl₅, carried out under different conditions, the scheme for the replacement of the thiol group by the chlorine atom was suggested. This scheme includes the intermediate formation of pentafluorobenzenesulfenyl chloride 2 and its conversion under the action of Cl₂ and PCl₅ to chloropentafluorobenzene 3 with the participation of an intermediate radical σ- complex [3, 4].

Use in the gas phase process, along with Cl₂, thionyl chloride and sulfuryl chloride as chlorine sources, characterizes the reaction of substitution of the thiol group in polyfluoroarenethiols by the chlorine atom in the methodical and practical sense as a rather general way of synthesis of chloropolyfluoroarenes. The reactions of polyfluoroarenethiols with PCl₅, carried out in ampoules at a lower temperature (~200°C) [4], are additional evidence in favor of such conclusion. We have also studied the conversion of polyfluoroarenethiols with SOCl₂ and SO₂Cl₂ in ampoules under similar conditions to increase the possibilities of this type of reactions.

We have shown that when thiol 1 was heated with SOCl₂ or SO₂Cl₂ in ampoules at ~200°C, arene 3 was obtained with high yields. It was found that the reaction of thiol 1 with SOCl₂ requires a longer time to achieve a good yield of arene 3 than in the case of using SO₂Cl₂ at the same temperature of process (Scheme 1).

Scheme 1

When the reaction time of thiol 1 with SOCl₂ was reduced from 24 h to 5 h at ~200°C, the reaction mixture contained arene 3, the supposed decafluorodiphenylpolysulfanes and sulfenyl chloride 2 in a ratio of ~ 55: 34: 11, respectively (according to NMR ¹⁹F).

In the reaction of thiol 1 with SO₂Cl₂ at ~200 ° C for 2.5 h, the reaction mixture contained arene 3 in a larger amount than in the case of the reaction of thiol 1 with SOCl₂ for 5 h. In addition to arene 3, the supposed decafluorodiphenylpolysulfanes and sulfenyl chloride 2 were also present in the mixture. The ratio of these products was ~ 79: 18: 3, respectively (according to NMR ¹⁹F). However, a comparison of the results of the reactions of thiol 1 with SOCl₂ (5 h) and SO₂Cl₂ (2.5 h) is relative, since the mass yields of the reaction mixtures have not been evaluated.

The above results of the reactions of thiol 1 with SOCl₂ and SO₂Cl₂ may indicate that the process involving SO₂Cl₂ takes less time than using SOCl₂. Therefore, we further carried out the reactions of polyfluoroarenethiols with SO₂Cl₂.

Sulfenyl chloride 2 and decafluorodiphenylpolysulfanes formed in the reaction of thiol 1 with SO₂Cl₂ at ~ 200°C for 2.5 hours can be of interest to obtain arene 3 from them under the action of SO₂Cl₂. In this connection, the reaction of sulfenyl chloride 2 with SO₂Cl₂ (~ 200 ° C, 5 h) was carried out. As a representative of these polysulfanes, it seemed reasonable to carry out the reaction of decafluorodiphenyldisulfane 4 with SO₂Cl₂ in the reaction conditions of thiol 1 with SO₂Cl₂ (~200°C, 5 h).

As it turned out, when sulfenyl chloride 2 and disulfane 4 were heated with SO₂Cl₂, compound 3 was also obtained in high yields (Scheme 2).

Scheme 2

A similar process occurs in the case of para-substituted derivatives of thiol 1. Thus, when 2,3,5,6-tetrafluorobenzenethiol 5 was heated with SO₂Cl₂, 1-chloro-2,3,5,6-tetrafluorobenzene 6 was obtained. The reaction of 4-chloro-2,3,5,6-tetrafluorobenzenethiol 7 with SO₂Cl₂ gave 1,4-dichloro-2,3,5,6-tetrafluorobenzene 8. When 4-tifluoromethyl-2,3,5,6-tetrafluorobenzenethiol 9 was reacted with SO₂Cl₂, 4-chloroheptafluorotoluene 10 was obtained in a high yield (Scheme 3).

Scheme 3

At the same time, the attempt to obtain 1-bromo-4-chloro-2,3,5,6-tetrafluorobenzene 11 by chlorination of 4-bromo-2,3,5,6-tetrafluorobenzenethiol 12 at ~ 180°C showed that in this reaction the replacement of the bromine atom by chlorine occurs also to a large extent. With an increase in the reaction temperature to ~ 240°C, compound 8 was obtained with a small admixture of arene 11 (Scheme 4). We have previously shown that a similar process occurred in the reaction of thiol 12 with PCl₅ under similar conditions [4].

Scheme 4

5-Chlorononafluoroindan 15 and 5,6-dichlorooctafluoroindan 16 were obtained from 5-nonafluorindanthiol 13 and 6-chlorooctafluoroindan-5-thiol 14, respectively (Scheme 5).

Scheme 5

Heating of a mixture of dichlorotrifluorobenzenethiols with SO₂Cl₂ in an ampoule was used to synthesize practically individual 1,2,4-trichlorotrifluorobenzene 17 (Scheme 6)..

Scheme 6

The isomeric mixture of dichlorotrifluorobenzenethiols was prepared from a technical mixture of o-, m-, p-C₆Cl₂F₄ and KSH (90% yield) [5].
When thiol 1 is mixed with SO₂Cl₂ at room temperature, mainly sulfenyl chloride 2 is formed, along with a small amount of disulfane 4 (according to NMR ¹⁹F, Scheme 7).

Scheme 7

It is possible that compound 4 in the reaction with SO₂Cl₂ is converted to sulfenyl chloride 2 [6]. The latter, under the action of the chlorine atom formed from SO₂Cl₂ [7-9] (Equations 3 and 4, Scheme 8), is converted to arene 3, including the intermediate formation of the radical σ-complex A [4]. Probably, the reaction of thiol 1 with SO₂Cl₂ in the flow system at ~ 400°C, described by us earlier [3], proceeds in a similar manner.

Scheme 8

EXPERIMENTAL

Authors would like to acknowledge the Multi-Access Chemical Service Center SB RAS for spectral and analytical measurements.

The ¹⁹F and ¹H NMR spectra were recorded on a Bruker AV-300 [282.4 (¹⁹F) and 300 (¹H) МHz] spectrometer in CCl₄ with added (CD₃)₂СO, internal reference С₆F₆ and HMDS (0.04 ppm from TMS). Positive values of chemical shifts correspond to the signal downfield shift.

GC analysis was carried out on gas chromatograph LKhM-72 with a detector of thermal conductivity (TDC) and packed (stuffed) columns 2 m long and 4 mm inside diameter, which were filled with a solid inert carrier of Chromosorb W-AW-DMCS, impregnated with a liquid stationary phase (dimethyl polysiloxane BC-1 or dimethyltrifluoropropyl polysiloxane SCTF-50) in an amount of 15% of the mass of the carrier. The flow rate of helium through each of the columns is 60 ml/min. The temperature of the evaporator is 280ºC, the initial temperature of the column is 50ºC -1 min, 10 deg/min to 280ºC, the isotherm at 280ºC before the exit of all components of the sample, the temperature of TDC 280ºC.

Initial polyfluoroarenethiols were obtained by the method [10].

The formation of compounds 2 and 4 and the final products by reactions of the polyfluoroarenethenthiols chlorination was confirmed by comparing the chemical shifts and spin-spin coupling constants of the ¹⁹F and ¹H NMR spectra of these compounds with the literature data [3, 5, 11].

Method 1. Polyfluoroarenethiol was placed in an ampoule and SOCl₂ or SO₂Cl₂ was added by portions. After the end of the visible gas evolution, the ampoule was sealed, placed into a metal case, and heated. After the reaction completed, the ampoule was cooled, opened, its content was placed into a flask under a layer of water with ice (80–100 g), and subjected to steam distillation. The reaction product was separated, dried with CaCl₂, and analyzed by GC, ¹⁹F and ¹H NMR methods.

Similarly, the reactions of sulfenyl chloride 2 and disulfane 4 with SO₂Cl₂ were carried out. In this case, the ampoule was sealed immediately after the reagents were placed in it.

Reactions of pentafluorobenzenethiol 1 with SOCl₂. From 2.77 g (13.84 mmol) of compound 1 and 3.30 g (27.74 mmol) of SOCl₂ (200–202°C, 24 h), 2.52 g of compound 3 were obtained (GC content: 97.3%), yield: 88%.

The results of the reactions of polyfluoroarenethioles (1, 5, 7, 9, 13, 14, a mixture of C₆Cl₂F₃SH, and also compounds 2 and 4) with SO₂Cl₂ are given in Table 1.

Table 1. Synthesis of chloropolyfluoroarenes

0.15 g (0.75 mmol) of compound 1 was mixed in a flask with 0.21 g (1.57 mmol) of SO₂Cl₂ at room temperature. At the end of the gas evolution, the resulting mixture was compounds 2 and 4 in a ratio of ~ 95: 5, respectively (¹⁹F NMR data).

Method 2. Compound l was placed in an ampoule and SOCl₂ or SO₂Cl₂ was added by portions. After the end of the visible gas evolution, the ampoule was sealed, placed into a metal case, and heated. After the reaction completed, the ampoule was cooled, opened, its content was dissolved in ~ 2 ml of methylene chloride and analyzed by ¹⁹F NMR.

Heating of 0.12 g (0.60 mmol) of compound 1 with 0.16 g (1.35 mmol) of SOCl₂ at 202-204°C for 5 h gave a mixture of compounds 2, 3, 4 (NMR ¹⁹F) and also probably decafluorodiphenyltrisulfane (δ, ppm: 13.5 (F-4 and F- 4') [12]) and decafluorodiphenyltetrasulfane (δ, ppm: 13.4 (F-4 and F-4') ( [12]) contained in the methylene chloride solution. In these two polysulfanes, chemical shifts of meta-atoms of fluorine are in the region of 2.1÷2.6 ppm [12], ortho- atoms of fluorine are in the region of 31÷32 ppm [12]. The ratio of compounds 2, 3, 4 and a mixture of decafluorodiphenyltri- and -tetrasulfane was ~ 11: 55: 10: 24, respectively (¹⁹F NMR data).

Similarly, heating of 0.20 g (1.00 mmol) of thiol 1 with 0.27 g (2.02 mmol) of SO₂Cl₂ at 200-202°C for 2.5 h gave compounds 2, 3, 4 and a mixture of decafluorodiphenyltri- and -tetrasulfane in a ratio of ~ 3: 79: 3: 15, respectively (¹⁹F NMR data).

Reactions of 4-bromo-2,3,5,6-tetrafluorobenzenethiol (12) with SO₂Cl₂. From 0.32 g (1.23 mmol) of compound 12 and 0.34 g (2.54 mmol) of SO₂Cl₂ (180-182°C, 5 h), a mixture containing compounds 8 and 11 in a ratio of 69: 31, respectively,) was obtained (NMR ¹⁹F data). When the reaction temperature was increased to 238-240°C (5 h), heating of 0.27 g (1.03 mmol) of compound 12 and 0.30 g (2.24 mmol) of SO₂Cl₂ gave a mixture containing compounds 8 and 11 in a ratio of 97: 3, respectively (¹⁹F NMR data).

The work was supported by the Russian Foundation for Basic Research (Project No. 15-03-08869a).

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Recommended for publication by Prof. V. E. Platonov

Fluorine Notes, 2018, 116, 3-4