Reactions of bromotrifluoromethane and related halides. Part 12. Transformation of disulfides into perfluoroalkyl sulfides in the presence of sulfoxylate anion radical precursors

摘要

J. CHEM.SOC. PERKIN TRANS. 1 1992 Reactions of Bromotrifluoromethane and Related Halides. Part 12. Transformation of Disulfides into Perfluoroalkyl Sulfides in the Presence of Sulfoxylate Anion Radical Precursors Jean-Louis ClaveL8 Bernard Langlois,b Roland Nantermet/ Marc Tordeux' and Claude Wakselman*tC a Rhdne- Poulenc Recherches, Centre de recherches des Carrieres, 6.P. 62, 69192 Saint- Fons, France Universite Claude Bernard, Laboratoire de Chirnie Organique 3, associe au CNRS (URA 467), 43 bd. du I1 novernbre 1918, 69622 Villeurbanne, France CNRS-CERCOA, 2 rue Henri Dunant, 94320 Thiais, France Perfluoroalkyl sulfides are prepared by reaction of perfluoroalkyl halides with disulfides in the presence of sulfoxylate anion radical precursors. Aliphatic, aromatic and heteroaromatic disulfides bearing cyano, ester and amino functional groups have been employed; a variety of perhalogenoalkanes can also be employed, e.g.CF,(CF,),I, CF,Br, CF,Br,, CF,BrCI, CFCI, and CF,CICFCI,. The most convenient sulfoxylate anion radical precursor for this reaction is formed by a combination of sodium formate and su If ur dioxide. The high lipophilicity ' of the trifluoromethylthio group makes Results it useful in the pharmaceutical and agrochemical fields 2.3 and Initial experiments were performed with the sulfoxylate anion consequently, the synthesis of trifluoromethyl sulfides has radical precursors previously used for the perfluoroalkylation In connection with this, long-chain of electron-rich aromatic compounds: sodium dithionite 28929attracted much intere~t.~ perfluoroalkyl sulfides have been proposed as intermediates for Na,S,O,, sodium hydroxymethanesulfinate (Rongaliteo) the synthesis of fluorinated sulfonic acids.5 Trifluoromethyl NaO,SCH,OH, zinc hydroxymethanesulfinate (Decrolinea) sulfides have been prepared by a variety of methods, the initial Zn(02SCH20H),, or a mixture of sulfur dioxide with a one being chlorination of a methyl group followed by halogen reductant, such as zinc.Later on, sodium formate was preferred Other methods have included use of trifluoro- for the mild transformation of SO, into the sulfoxylate anion e~change.~.~ methanesulfenyl halides 7-' O or trifluoromethylthio deriv- radical SO,' -,30 dipolar aprotic solvents such as dimethylforma- atives of metals,' '-' photochemical trifluoromethylations of mide (DMF) being used.Since sodium dithionite was sparingly disulfidesl7 and thiols '831 by trifluoromethyl halides, and soluble in DMF, water was employed as co-solvent: the greater spontaneous perfluoroalkylation of thiolates by bromotri-solubility of sodium hydroxymethansulfinate allowed less water fluoromethane under slight pressure; 20*2' the last-mentioned to be used whilst with the corresponding zinc salt it was compound can also be used for the trifluoromethylation of unnecessary. With the readily soluble formate dry DMF was thiocyanates in the presence of zinc.22 A more sophisticated employed. In the presence of water sodium dithionite produced trifluoromethylating agent led also to trifluoromethyl sulfides.23 the sulfite anion,31 the reaction of which with the disulfide Recently, the trifluoromethylation of disulfides with sodium led to a polar by-product (Scheme 2).Use of the sodium trifluoromethanesulfnate under oxidative conditions has been formate-sulfur dioxide mixture in pure DMF limited this proposed.24 whilst long-chain perfluoroalkyl sulfides are avail- possible side-reaction. able by perfluoroalkylation of disulfides,' thiolates ,',,1,25 and thiocyanates.,2,26 HSO,-+ RSSR-RSS0,H + RS-Frequently, the preparation of these fluorinated sulfides involves the use of toxic or expensive reagents, harsh conditions Scheme 2 or sensitive salt derivatives. Consequently, we have looked for In one example, inorganic products were determined.Prepar- a mild and cheap chemical transformation compatible with ation of 13a from the disulfide 11(1 equiv.), CF,Br (ca. 13bar t)various functional groups. We report here the smooth per- sodium formate (2 equiv.) and sulfur dioxide (2 equiv.), was fluoroalkylation of aliphatic, aromatic and heteroaromatic performed following method 2 in the Experimental section. disulfides by easily available perfluoroalkyl halides in the Acidimetry showed the presence of CO, (28 in the gas phase presence of various precursors of the sulfoxylate anion radical calculated us. sodium formate). Ionic chromatography detected SO,'-(see Scheme 1) (for a preliminary communication, see F-(8 us. disulfide), Br- (95 us. disulfide), (HC02- 19 ref.27). from sodium formate), CF3S02 -(20 from SO,) and SO3, -RSSR + 2RFX SO,'-precursors (3 from SO,). Sulfur dioxide was detected by iodometry (32RSRF in the gas phase and 65 in solution from initial SO2). I 2 3 Consequently, this anhydride was recovered or transformed into sodium trifluoromethanesulfinate in a 91 total yield. It Scheme 1 appeared to be a mediator in the reaction. The ratios between the reactants and the yield of 13a (85) were in agreement with This method was based on the formation of perfluoroalkyl the stoichiometry written on Scheme 1, which involved the radicals under reductive conditions 28*29 and on the suscepti- bility of the weak sulfur-sulfur bond in disulfides to free radical at tack. ' t 1 bar = lo5 Pa.transformation of the two R groups of the disulfide. The major by-product was the perfluoroalkanesulfinate salt resulting from the direct reaction of the halide with the sulfoxylate anion radical 32-34 (Scheme 3). The importance of this side-reaction was dependent on the reactivity of the fluorinated halide 2. Perfluoroalkyl iodides CF,(CF,),I (n = 1, 3, 5) and 1,1,2-tri- chlorotrifluoroethane were more reactive than bromotrifluoro- methane and bromochlorodifluoromethane. Initially, simple aromatic and aliphatic disulfides 1 (R = Ph, Me, Bu, PhCH,) were examined. Various fluoroalkyl sulfides 4-10 were obtained from C4F,I, C,F, 71, C,F, ,I, CCl,FCClF, and CF,BrCl (see Scheme 4). Et02CCH2SCF,CI 10 Scheme 4 The example 10 showed that the reaction was compatible with the presence of an ester group.The synthesis of substituted pyrazoles 13 and 14, from disulfides 11 and 12 respectively, extended the scope of this method to heterocyclic derivatives bearing cyano and amino groups (see Scheme 5; Table 1). NC 13 14 11 X=CF3 8 =CF3,X=CF3 a RF=CF~,X=CI 12 XtCI 5 RF= CF*Br, X = CF3 b I+ = CFZBr, X t CI c = CFClBr, X = CF3 c RF= CFCIBr, X = CI d RF=CFC~,X=CF~ d =CFCh,X=CI Scheme 5 Various perhalogenomethanes (CF,Br,, CFBr,Cl, CFCl,, CF,Br) were used for the synthesis of these fluorinated pyrazoles, showing the versatility of the method. Discussion Evidence for the presence of an intermediate perfluoroalkyl radical in the reactions of perfluoroalkyl halides with sulfoxyl- ate anion radical precursors was provided by alkylation of J.CHEM. SOC. PERKIN TRANS. 1 1992 However, the ready reduction of disulfides to thiolates ions is well-known and the formation of trifluoromethyl sulfides from thiolates and CF,Br by an SR,l pathway has already been observed.,' Some light was shed on the process by the reaction of bromochlorodifluoromethane and sodium benzenethiolate to give, by an ionic pathway, bromodifluoromethyl phenyl sulfide (see Scheme 7).,' From the same halide and diphenyl disulfide, we have now obtained the chloro analogue 9. Form-ation of this compound agreed with a direct radical attack on the sulfur-sulfur bond (Scheme 8) and not with the ionic path- way (Scheme 7) resulting from initial reductive cleavage of this bond by the reductant.Scheme 7 SO,'-+ CF,BrCI -CICF,' + Br-+ SO, Scheme 8 The reaction of 1,1,2-trichlorotrifluoroethanewith diphenyl disulfide gave compound 8 having two vicinal chlorine atoms (see Scheme 9), cleavage of the carbon-chlorine bond having occurred on one of the two geminal chlorine atoms, in agreement with the known homolysis of 1,1,2-trichlorotrifluoro-ethane.36 In contrast, we had earlier observed the formation, by an ionic pathway from a reaction with benzenethiolate,,' of an isomer having two geminal chlorine atoms (Scheme 10). SO2'-+ CF2CICFCI2----+ CF,CICFCI' + C1-+ SO, Scbeme 9 C,H,S-+ CF2CICFC12-C,H,SCI + CICF,CFCI-CICF,CFCI--F,C=CFCI + C1-C,H,SCF,CFCI-+ CF,CICFCI, -C,HSSCF2CFCl2+ ClCF,CFCI-Scheme 10 Thus, the structures of the products formed from these two halides were in favour of a radical pathway (see Scheme 6).The existence of the sulfoxylate anion radical in the mix- ture of formic acid (or its sodium salt) with sulfur dioxide (or aromatic substrates introduced into the reaction medi~m.~~.~~ one of its associate base) in DMF was checked by the direct The present synthesis can be considered as an attack on formation of perfluoroalkanesulfinate salts (in the absence of a disulfides by these perhalogenoalkyl radicals (see Scheme 6). disulfide) (Scheme 3; Table 2). Formation of trifluoromethane- SO,'-+ RFX RF' + X-+ SO, R,' + RSSR -RFSR + RS' 2RS' -RSSR Scheme 6 sulfinate from bromotrifluoromethane, sodium formate and sulfur dioxide has also been observed.37 In the reaction with aromatic disulfides, no perfluoroalkyl- ation of the nucleus was detected.However, perfluoroalkyl radical attack on anilines and phenols has been ob~erved.~**~~ The weak sulfur-sulfur bond is the most reactive site in these disulfides. J. CHEM. SOC. PERKIN TRANS. 1 1992 3373 Table 1 Pyrazoles Analyses:Found (Required) ~~~ Compd. Yield () M.p. ("C) 6, Formula C H N 34.6 1.05 13.213. 75 169.5-170.5 -44.9 cl 2H4C12F6N4S (-63.8) (34.22 0.96 13.30) 13b 90 162.5-1 63.5 -24.3 C,,H,BrCl,F,N,S 30.0 0.75 11.4 (-63.8) (29.90 0.84 1 1.62) 13C 24 191.5-193 -18 C, ,H,BrCl,F,N,S 29.4 0.75 11.0 (-63.8) (28.91 0.8 1 11.24) 13d 84 190-193 -21.4 12H4C14F4N4S 3 1.6 0.85 12.4 (-63.8) (31.74 0.89 12.40) 14p 72 192-194 -45 c,,H,Cl,F3N,S 34.0 1.o 14.4 (34.08 1.04 14.45) 14b 53 163.5-165 -24.3 C, ,H,BrCl,F,N,S 29.5 0.9 11.9 (29.46 0.90 12.49) 14 71 192-193 -17.9 C, H,BrCl,FN,S 28.6 0.9 11.9 (28.42 0.87 12.05) 14 64 189- 19 1 -21.4 C, ,H,Cl,FN,S 3 1.65 0.9 13.1 (31.42 0.96 13.3 1) ~~~~ ~~ ~ a SRF and if necessary C6H2Cl,CF, in parentheses.Prepared by a classical method.' Tab 2 RFX Reductor Precursor Sulfinate TP ()" _____~ _____~~ ___~ C8F,,I HC0,Na SO2 C8F1 ,SO,Na 15 83 C,F,I HC0,Na NaHSO, C,F,SO,Na 16 55 C,F,I HC0,H NaHSO, 16 55 C,F,I HC0,H C,F,I HC0,HCFCl, HC0,Na CF,BrCl HC0,H Na,S,O, Na,SO, SO, NaHSO, 16 16 CFCl,SO,Na 17 CF,ClSO,Na 18 53 29 44 38 @ TP transformation percentage of the perhalogenoalkane.Isolated yield, transformation percentage 100. Conclusion Introduction of disulfides into the reaction medium used for the preparation of perfluoroalkanesulfinate salts (from perfluoro- alkyl halides and sulfoxylate anion radical precursors) gives perfluoroalkylsulfides, by a mild and safe method with wide scope using inexpensive reagents. Experimental 'H and 13C NMR spectra were recorded on a Bruker AM 300 or AC 200 spectrometers in ppm downfield from tetramethyl- silane. 19FNMR were obtained on Varian EM360L or AC 200 spectrometers and were recorded in ppm downfield from tri- chlorofluoromethane (solvent: CDCl,). J values are recorded in Hz. Bromotrifluoromethane was purchased from Setic Labo, iodotrifluoromethane from Fluorochem Ltd., dimethylforma- mide, pyridine and organic reactants from Aldrich (and distilled before use), zinc, sodium hydrogen phosphate from Prolabo, sodium hydroxymethanesulfinate and sodium dithionite from Fluka.Iodoperfluoroalkanes were generous gifts of ATO- CHEM. Pyrazolyl disulfides were prepared by a literature method,'* and diethyl dithioacetate by esterification of the corresponding acid. Benzyl PerfIuorobutyl Sulfide 4.-A mixture of perfluorobutyl iodide (3.5 g, 10 mmol), sodium hydroxymethanesulfinate (4 g, 26 mmol), dibenzyl disulfide (2.5 g, 10 mmol) in DMF (10 cm3) and water (0.5 cm3) was stirred at room temp. for 6 h. The mixture was diluted with water (100 cm3) and extracted with ether.The extract was washed with 5 aqueous hydrochloric acid (2 x 20 cm3) and 10 aqueous sodium carbonate, dried (MgS04) and evaporated to give the title compound (0.6 g, 17); 22 b.p. 92 "C/17 Torr *; ~5~7.3(3 H, s) and 4.2 (2 H, s);SF -82.5 (3 F), -88.8 (2 F), -121.5 (2 F) and -126.5 (2 F). Methyl Per-uorooctyl Sulfide 5.-A mixture of perfluoro- octyl iodide (5.5 g, 10 mmol), sodium dithionite (3 g, 17 mmol), sodium hydrogen phosphate (3 g, 21 mmol) and dimethyl disulfide (1 g, 10.6 mmol) was stirred in DMF (10 cm') and water (5 cm3) for 6 h to give after work-up the title compound (0.9 g, 20); 22 b.p. 44OC/lOTorr; SH2.4;SF(SCF2)-92.3. PerJuorohexyl Phenyl Sulfide 6.-A mixture of perfluoro-hexyl iodide (4.5 g, 10 mmol), sodium hydroxymethanesulfinate (4 g, 26 mmol) and diphenyl disulfide (2.2 g, 10 mmol) in DMF (10 cm') and water (0.5 cm3) was stirred for 12 h to give after work-up perfluorohexyl phenyl sulfide (1.7 g, 40); b.p.99 "C/18 Torr; S,(SCF,) -87.2. Butyl PerJIuorohexyl Sulfide 7.-The preceding experiment was repeated with dibutyl disulfide (1.5 g, 10 mmol) to give the title compound (0.9 g, 22); b.p. 65 OC/19 Torr; aH(SCH2)2.7 (m); G,(SCF,) -92.3. 1,2-Dichloro- 1,2,2-tr@uoroethyl Phenyl Sulfide 8.-A mixture of 1,1,2-trichlorotrifluoroethane(3.8 g, 20 mmol), diphenyl disulfide (4.4g, 20 mmol), sodium dithionite (7 g, 40 mmol) and disodium hydrogen phosphate (6 g, 40 mmol) was stirred in DMF (20 cm3) and water (10 cm3) for 6 h to give after steam distillation and work-up the title compound (2.7 g, 52);39 b.p.55 "C/0.4 Torr; SF -63.3 (2 F, d, J 14.2), -89 (t, 1 F). Chlorodfluoromethyl Phenyl Suljide 9.-A thick glass flask containing a mixture of sodium hydroxymethanesulfinate (Ron- galiteo) (15.5 g, 100 mmol) and diphenyl disulfide (5.5 g, 25 mmol) in DMF (30 cm') and water (2 cm3) was evacuated and thermostatted at 20 "C. The mixture was then stirred under a pressure (1.7 bar) of bromochlorodifluoromethane for 6 h. After this the mixture was diluted with water (100 cm3) and extracted with ether. The extract was washed with 5 hydrochloric acid (2 x 20 cm') and 10 aqueous sodium carbonate, dried * 1 Torr z 133 Pa. (MgSO,) and evaporated to give the title compound (3.5 g, 72);" b.p. 71 "C/25 Torr; d,(SCF,) -27.Ethyl (Chlorodijfuoromethy1thio)acetate 10.-The preceding experiment was repeated with diethyl 2,2'-dithiodiacetate (5.9 g, 25 mmol) to give the title compound (3.3 g, 65); b.p. 81 "C/25 Torr; a,., 4.23 (2 H, q, J 10.5), 3.75 (2 H, s) and 1.3 (3 H, t); d~(sCF2) -27; v,,,/cm-' 1718; m/z 204, 206 (M+), 169 and 1 19. 5-Amino-3-cyano- 1 -(2,6-dichloro-4-trijluoromethylphenyl)-4-trijluoromethylthio- 1 H-pyrazole 13a.-Method 1. To a solution of bis-5-amino-3-cyano- 1 -(2,6-dichloro-4-trifluoromethylphen-yl)pyrazol-4-y1 disulfide 11(2 g, 2.9 mmol) in DMF (120 cm3) in a Teflon-coated autoclave (500 cm3) was added a solution of disodium hydrogen phosphate =12H,O (3.05 g, 9 mmol), in distilled water (60 cm3) followed, with stirring, by sodium dithionite (1.48 g, 9.5 mmol).Following closure, the autoclave was pressurized with bromotrifluoromethane (12-1 3 bar). After being stirred (1000 rpm Rushton turbine) at this pressure for 2 h at 25 "C, the autoclave was opened and the stirred reaction mixture diluted with water (500 cm3) and ether (500 cm3). The ethereal phase was separated, dried (MgSO,) and filtered. Removal of ether under reduced pressure gave a yellow oil which was subjected to a vacuum pump at 100°C to remove further volatile substances. The residue was passed through a lightning chromatography column containing silica gel (320 g), eluting with dichloromethane, to give the title compound (1.8 g, 75); m.p.169.5-170.5"C. Method 2. The disulfide (4 g, 5.9 mmol), sodium formate (1.16 g, 17.1 mmol), DMF (20 cm3) and sulfur dioxide (1.45 g, 22.8 mmol) were successively introduced into an autoclave and the mixture was stirred and heated at 60°C. Bromotrifluoro- methane (12-13 bar) was introduced and the mixture stirred for 4 h at this pressure to give the title compound (4.1 g, 85). 5-Amino-4-bromodijluoromethylthio-3-cyano-1 -(2,6-dichloro- 4-trifruoromethy1phenyl)-1 H-pyrazole 13b.-A solution of sod- ium dithionite (0.78 g, 4.5 mmol) and disodium hydrogen phosphate (0.64 g, 4.5 mmol) in water (20 cm3) was added to a stirred solution of the disulfide 11 (1.94 g, 3 mmol) in DMF (75 cm3) and water (5 cm3). After addition of dibromodifluoro- methane (1.89 g, 9 mmol) the reaction mixture was stirred at ambient temperature for 17 h and then poured onto water (185 cm3). Work-up as described for 13a, gave the title compound (1.31 g, 90); m.p.162.5-163.5 "C; m/z (IE) mode: 482 (parent with 35C137C179Br and 35C1281Br) and 351 (parent 35C1281Br). 5-Amino-4-bromochlorojluoromethylthio-3-cyano-1-(2,6-di-chloro-4-trijluoromethylpheny1)-1 H-pyrazole 13c.-Sodium di-thionite (1.92 g, ll mmol) and disodium hydrogen phosphate (1.56 g, 11 mmol) followed by water (45 cm3) were added to a stirred solution of bis-5-amino-3-cyano-l-(2,6-dichloro-4-tri-fluoromethylphenyl)pyrazo1-4-yldisulfide 11 (5.1 1 g, 7.3 mmol) in DMF (115 cm3). Partial solubilization of the inorganic reactants occurred.Chlorodibromofluoromethane (4.96 g, 2 1.9 mmol) was then added in one portion, followed by an additional quantity of DMF (75 cm3) to give an homogeneous mixture. This was stirred at ambient temperature for 1.6 h and then poured onto water (450 cm3). Work-up as described for 13a gave the title compound (1.76 g, 24), m.p. 191.5-193 "C; m/z 498 (IE) (parent with 35C1,37C179Br and 35C138'Br) and 351 (parent 'C1, 7C179Br minus CF79Br3 7C1). 5-Amino-3-cyano-4-dichloroJluoromethylthio-1 -(2,6-dichloro- 4-trijluorornethylphenyl)-lH-pyrazole13d-Sodium dithionite (3.96 g, 23 mmol), disodium hydrogen phosphate (3.23 g, 22 mmol) and fluorotrichloromethane (3.9 g, 28 mmol) were added J. CHEM. SOC. PERKIN TRANS. 1 1992 with stirring to the disulfide 11 (4 g, 5.7 mmol) in DMF (178 cm3) and water (89 cm3).This mixture was stirred at 15-17 "C for 1 h and then poured into ice-water (1600 cm3) and stirred for a further 30 min. The white solid was filtered off, washed with water (800 cm3) and dried to give the title compound (4.34 g, 84), m.p. 190-193 "C. 5-Amino-4-bromodijluoromethylthio-3-cyano-1 -(2,4,6-tri- chloropheny1)- 1 H-pyrazole 14b.-Sodium dithionite (0.42 g, 2.4 mmol) and disodium hydrogen phosphate (0.34 g, 2.4 mmol), followed by water (5 cm3) and dibromodifluoromethane (1.01 g, 4.8 mmol) were added with stirring to a solution of disulfide 12 (1.0 g, 1.6 mmol) in DMF (10 cm3). Since the stirred mixture remained heterogeneous, further DMF (15 cm3) and water (5 cm3) were added.The resulting solution, containing a small quantity of semisolid was then stirred at ambient temperature for 2.7 h. Work-up as described for 13a gave the title compound as a pale-yellow solid (0.76 g, 53); m.p. 163.5-165 "C; m/z (IE) 448 (parent with 35C1,37C179Br and 35C1381Br) and 319 (parent 35C1237C179Brminus CF,79Br). 5-Amino-4-bromochloroJEuoromethylthio-3-cyano-1 -(2,4,6- tri- chloropheny1)- 1 H-pyrazole 14c.-Sodium dithionite (1.23 g, 7 mmol) and disodium hydrogen phosphate (1 g, 7 mmol) followed by water (30 cm3) and chlorodibromofluoromethane (3.19 g, 14.1 mmol) were added with stirring to a solution of the disulfide 12 (3.0 g, 4.7 mmol) in DMF (75 cm3). The resulting mixture was stirred at ambient temperature for 40 min after which it was poured into water (300 cm3).Initially the mixture was extracted with diethyl ether (300 cm3) and then with dichloromethane. Lightning chromatography on a silica gel column of the material obtained by evaporation under reduced pressure of the ethereal extract gave a relatively impure product. The dichloromethane extract gave a product con- taining DMF which was removed by rotary evaporation at a temperature of 90-100 "C in uacuo. The resulting residue was dissoved in dichloromethane and extracted with water. Concen- tration of dried organic phase and lightning chromatography on a silica gel column gave the bright yellow title compound (0.5 g), m.p. 192-193 "C; total product yield, including the first fraction was 71; m/z (IE) 464 (parent with 35C1337C179Br and 35C1,8 'Br), 319 (parent 35C1337C179Br minus CF79Br35C1).5-Amino-3-cyano-4-dichlorojluoromethylthio-1-(2,4,6-tri-chloropheny1)-1 H-pyrazole 14d.-Zinc powder (60 g, 0.92 mol) was added to a stirred solution of the disulfide 12 (150 g, 0.2 mol) in DMF (1 125 cm3) at ambient temp. To this mixture, was added a solution containing sulfur dioxide (60.6 g, 0.95 mol) in DMF (160 cm3), followed by fluorotrichloromethane (290 g, 2.1 mol) After ca. 30 min a slight exotherm was noted (maximum temp. 30 "C). The reaction mixture was stirred at ambient temperature overnight and then filtered and added dropwise over 2 h to ice-water (14 dm3). The resulting solid was collected, washed thoroughly with water and dried to yield a yellow-range solid (185 g) which after recrystallization from toluene-hexane gave the title compound (123 g, 64), m.p.187- 189 "C. Sodium PerJIuorooctanesulJinate 15.-Sodium formate (2 g, 29 mmol) was added to stirred DMF (50 cm3) containing sulfur dioxide (2 g, 31 mmol) to give a blue mixture. Iodoperfluoro- octane (5.5 g, 10 mmol) was then added to the mixture which became yellow-orange with gas evolution. After 3 h, the mixture was analysed by I9F NMR. The iodide was almost completely transformed into the sulfinate salt 15 (6, CF,SO,--132). After filtration, DMF was removed under reduced pressure and the residue diluted with water (20 cm3). The solids were filtered off and chlorine (1 dm3, 44 mmol) was bubbled through the J.CHEM. SOC. PERKIN TRANS. i 1992 filtrate to give perfluorooctane sulfonyl chloride (4.3 g, 83) 40 after decantation. Sodium Perjluorobutanesulfinate 16.-Sodium formate (1.2 g, 18 mmol), sodium hydrogen sulfite (2.1 g, 20 mmol) and iodoperfluorobutane (3.5 g, 10 mmol) were stirred in DMF (20 cm3) for 10 h when 19F NMR analysis showed a 55 transformation percentage of the iodide into the sulfinate salt (6, CF,SO,--131). After the usual treatment (see 15), perfluorobutanesulfonyl chloride4' (1.3 g) was obtained by decantation (yield 78). The experiment was repeated with various mixtures: formic acid (1.2 g, 26 mmol) and sodium hydrogen sulfite (2.1 g, 20 mmol) (transformation percentage 55); formic acid (1 g, 22 mmol) and sodium metabisulfite (1.9 g, 10 mmol) (transformation percentage 53); formic acid (1 g, 22 mmol) and sodium sulfite (2.5 g, 20 mmol) (transformation percentage 29). Sodium Dichlorojluoromethanesuljinate 17.-Sodium formate (1.4 g, 21 mmol) followed by fluorotrichloromethane (2.7 g, 20 mmol) were added to DMF (20 cm3) containing sulfur dioxide (2 g, 3 I mmol) with stirring.After 3 h, the mixture was analysed by 19FNMR: the transformation percentage of the chloride into the sulfinate salt (6, CFS02--66) was 44.4' Sodium Chlorodijluoromethanesuljjnate 18.-Bromochloro-difluoromethane (1.65 g, 10 mmol) was condensed into DMF (20 cm3) after which formic acid (1.2 g, 26 mmol) and sodium hydrogenosulfite (2.1 g, 20 mmol) were added to it.After 14 h of stirring, the mixture was analysed by "F NMR: the trans- formation percentage of the bromide into the sulfinate salt (6, CF,SO,--69) was 38.33 Acknowledgements We thank Ongar (UK) and Triangle Park (USA) Rhone-Poulenc Research Centres for their cooperation in the extension of the method to the pyrazole series. References 1 A. Leo, C. Hansch and D. Elkins, Chem. Rev., 1971,71,525. 2 R. 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