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Electrochemical fluorination of aromatic compounds in liquid R4NF·MHF—Part II. Fluorination of di- and tri-fluorobenzenes
EkctrochimicaActa. Vol. 39. No. I. pp. 41-49.1994 F’rintedin Great Britain.
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COMPOUNDS IN LIQUID R4NF. mHF-PART II. FLUORINATION OF DI- AND TRI-FLUOROBENZENES KUNITAKAMOMOTA,*KA~WA KATo,~ MASAWKI MORITA~and YOSHIHARUMATSUDA~ Department of Research and Development, Morita Chemical Industries Co. Ltd., Higashi-mikuni, Yodogawa-ku, Osaka 532, Japan (Received 29 March 1993) Abstract-Di- and tri-fluorobenzenes were electrochemically fluorinated at a platinum anode at 2.5 V [vs. Ag/Ag+ (0.01 moldm-3)] in Et,NF.mHF (Et = C,H,, m = 4.0, 4.35). The fluorinated cyclohexadienes were obtained as the major final products in high yield, and neither deposition of a polymeric 6lm on the anode surface nor a coloration of the electrolyte solution was observed. Some 124-trilluorobenxcne @) or 1,2,3,Stetrafluorobenzene (@ was produced in the course of the fluorination of l$ditluorobenzene (2) or 1,3,StrilIuorobenxene @, respectively. These were produced chemically by the dehydrofluorination of 1,3,3,6tetrafuoro-l&cyclohexadiene (&) or 1,3,3,5,6-pentafluoro-ltlcyclohexadiene 0, which was produced by the anodic fluorination, and large portions of the resulting 4 and 6 were further fluorinated electrochemically to the corresponding fluorinated cyclohexadienes. T&e reaction paths to the major products were explained by the combination of electrochemical reactions, 1,4- or l&addition of two fluoride anions to substrate compounds by an ECEC mccha&m (which yielded the fluorinated cyclohexadienes) and a chemical reaction (the dehydrofluorination of some fluorinated cyclohexadienes) which yielded fluorobenzenes. Key words: anodic fluorination, fluorobenzenes, fluorinated cyclohexadienea, tetraethylammonium fluoride-hydrogen fluoride electrolyte, dehydrofluorination.
INTRODUCITON
electrochemical fluorination of benzene, chlorobenzene etc. at a Pt anode in Et,N .3HF. In the cases of electrolyses using these electrolytes, deposition of a polymeric film on the anode surface was observed together with a low fluorination current efIiciency owing to side-reactions and/or a low electric current density reflecting low electrolytic conductivity of electrolyte[17J. In a previous paper by the authors[18], a series of new electrolytes, non-viscous liquids of tetraalkylammonium salts (R*NF *mHF: R-Me, Et, Pr: Me-CH, , Et-C,H, , Pr=C,H,: m > 3.5) at room temperature, were found to give high electrolytic conductivity and high electrochemical stability, and the anodic fluorination of benzene, fluorobenzene and Wdifluorobenzene using EtdNF *mHF electrolytes were reported. 3,6,6_Trifluoro-l,Qcyclohexadiene @ and 3,3,6,6-tetralluoro-1,4cyclohexadiene @), which are electrochemically stable and whose oxidation potentials were as high as 3.OV (vs. Ag/Ag+) on the cyclic voltammograms, were obtained as the final major products in high current efficiency and in high yield as the results of the electrolyses at 2.5 V. The investigation was scarcely concerned with the fluorination of di- and tri-fluorobenzenes which included chemical fluorination with fluorine, high valency metal fluorides, eg cobalt trifluoride (CoF,), silver difluoride (AgF,) and &urn tetrduoride (CcF,) and xenon difluoride (XeF,). Only Zweig et ul.[19] reported the fluorination of 1,2- and 1,4difluorobenzcne with AgFz in hexane. In the present work, the anodic fluorination of di- and trifluorobenzenes has been studied further. From the
Partial fluorination of organic compounds in electrochemical processes is expected to produce useful fluorinated precursors for pharmaceutical and agricultural agents, raw materials of fine chemicals etc.[l, 23, since it makes possible the selective introduction of fluorine atoms into organic compounds[3,
41. Up to the present day, solutions of aprotic solvents [mainly acetonitrile (CH,CN) or dichloromethane (CH,Cl,)] containing some fluorides, eg fluoride (HF)[5], silver monohydrogen fluoride(AgF)[6], tetraethylammonium tetrtiuoroborate (E&N BF,)[fl, triethylamine Whydrogen fluoride) (Et,N .3HF)[8, 93, tetramethylammonium fluoride bis(hydrogen fluoride) (Me,NF - ZHF)[lO, 1l] and tetraethylammonium fluoride tris(hydrogen fluoride) (EtdNF *3HF)[12, 131, have been used as the electrolytes for this purpose. These fluorides behave as both the fluorine sources and as the supporting electrolytes. Further, Huba et aL[14] obtained 2-fluoropyridine as a by-product in the electrolysis of 29% pyridine-71% anhydrous HF electrolyte with platinum (Pt) and carbon anodes, and Meurs and co-workers[lS, 161 carried out the Author to whom correspondence should be addressed. t Permanent address: Chemistry Department, Goveml
ment Industrial Research Institute. Nagoya, Hir&-cho, Kita-ku, Nagoya 462. Jam. $ Pennan% address:-Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755, Japan. 41
K. MOMOTA et al.
42
results of the electrolyses, the reaction paths of the electrochemical fluorination of fluorobenxenes in R,NF -mHF without solvent was clarified, and the fluorination mechanism was investigated. The rates of dehydrofluorination for some fluorinated cyclohexadienes were examined to account for the distribution of products and the quantity of electricity
r I
,
passed.
EXPERIMENTAL b(c)
Materials
1,2-Difluorobenzene, lJ-difluorobenxene, 1,3,5-trifluorobenxene and trifluoromethylbe were obtained from Aldrich Chemical. Chlorobenxene, tetrachloromethane (Ccl,) and dichloromethane (CH,Cl,) were obtained from Wako Pure Chemical. Acetonitrile (MeCN: F grade, water content < 20ppm) was obtained from Mitsubishi Petrochemical. These reagents were used without further purification. 1,2,3-Tritluorobenxene was synthesized from 2,3,4trifluoroaniline (Morita Chemical Industries) by a deamination process[20] and was purified up to 99% assay by fractional distillation. 1,2,4-T+ fluorobenzene was synthesized from 3,4difluoroaniline (Morita Chemical Industries) via 3,4diflurobenz.enediaxonium hexafluorophosphate by the Schiemann reaction[21] and was purified up to 99% assay by fractional distillation. The electrolytes, E&NF. mHF (m = 4.0 and 4.35), were prepared by mixing Et,NF *2HF and anhydrous HF in a dried nitrogen atmosphere as described elsewhere[lfl. Tetraethylammonium tetrafluoroborate (Et,N BF,: Morita Chemical Industries) and silver perchlorate (AgClO,: Wako Pure Chemical) were dried for 18 h under a vacuum at 100°C before use. Cell components The schematic diagrams of the electric cell and the electrodes are shown in Fig. 1. The cell components, except for the electrodes (Pt and Ag), leading wires (2mm0, copper wire), silicon rubber caps and epoxy-resin, were made of fluoro-resins, poly(tetrafluoroethylene - co - pertIuoroalkylvinylether) (PFA) and polytetrafluoroethylene (PTFE), to protect from corrosion by free HF. The reference electrode was Ag/AgClO., (0.01 M: M = moldm-‘) in MeCN containing Et,N . BF, (0.1 M) as the supporting electrolyte. The electrolyte, Et,NF *mHF, without the substrate was used as a salt bridge. Method
In the glove box, filled with dried nitrogen gas, the cell parts were constructed and 30cm3 of the electrolyte, Et,NF *mHF, and 0.0123 mol of the substrate compound, di- or tri-fluorobenzene, were added into the cell. The electrolysis was carried out at a constant potential (2SV) by a function generator (Hokuto Denko: HB-104) connected to a potentio/ galvanostat (Hokuto Denko: HA-501) at an ambient temperature. The solution was continuously stirred magnetically with a PTFE coated stirring bar. The
/’ (Anode and cathode)
.n i
m
a
b
m (Reference (Beaker-type
electrode)
PFA ccl I>
Fig. 1. The schematic diagrams of a beaker-type PFA cell and the electrodes. (a) PFA cell case; (b) Anode (Pt, 2Omm x 20mm); (c) Cathode (Pt, 2Omm x 2Omm); (d) Reference electrode (Ag/Ag+); (eJ Electrolyte; (cd Salt bridge; (eJ 0.01 M AgClO, + 0.1 M Et,N-BFJMeCN; (f) PTFE plug; (g) PTEF cap; (h) Gaa outlet; (i) Thermometer shell; (i) PTFE coated magnetic stirring bar; (k) Ag wire (1 mm@); (l) Silicon rubber cap; (m) PTFE diaphragm; (n) PFA shell; (0) Pt wire (1 mm@); (p) Epoxy-resin; (q) Solder; (r) Cu wire (2mm0); (s) PFA tube (6mmlzr).
electric current and the cell voltage vs. time were recorded on a X-Y recorder (Yokogawa Elect&s: 302523). The electrolyses were continued until the current had fallen to 30mA (about two times the background current in Et,NF *mHF (ca. 15 mA) at 2.5 V) owing to decrease of the concentration of electrochemically active compounds; fluorobenxenes which were the substrates and the products. During the usual electrolysis, a small amount of the electrolyte solution (ca. 0.1 g) was sampled for each 0.5 or 1.0 Faraday per mol of substrate (Fmol-‘) passed, in order to monitor the variations in the amounts of substrate and products. After the sampling, the mass of the sample was precisely weighed, and was treated by adding water (1 cm3) and Ccl, solution (1 cm3) containing chlorobenzene or trifluoromethylbenxene as an internal standard material. Consequently, the substrate and the products were extracted with CCl, solution from the sample solution and Et,NF *mHF remained in the water layer. Thii procedure was necessary to prevent the dehydrofluorination reaction of some fluorinated
Electrochemical fluorination in R,NF *mHF-Part cylohexadienes; especially, 1,3,3,6-tetralluoro-1,4cylohexadiene (2a) and 1,3,3,5,6-pentafluoro-1,4cyclohexadiene (5al. The resulting CCl, solutions were analyzed by gas chromatography (GC, Shimadzu, gas chromatograph: GC-14A, Chromatopac: CR4A) with a G-300 or/and a G-450 column (40 m1.2 mm@. Kagakuhin Kensa Kyoukai) and GC-mass spectrometry (GC-MS, Shimadzu: GCMSQPlOOO). After the electrolysis, a small part of the electrolyte solution (ca. 2 cm3) was stored in a sealed PFA-resin bottle (5cm3), in order to measure the variation of the product component by the dehyrofluorination of some fluorinated cyclohexadienes with the storage time. Another part of the solution was treated by adding water (50cm3), and the products were extracted with two 15cm3 portions of Ccl,. The resulting Ccl, solution was dried over anhydrous sodium sulfate (Na,SO,), and was analyzed by GC-MS and “F NMR spectrometry. Further, some of the Ccl, solution (cu. 1Ocm’) was mixed with 5 N-potassium hydroxide (KOH) aqueous solution (20cm3) in a 50cm3-Erlenmayer flask setting with a stopper and a stirring bar, and the mixture was stirred vigorously over-night at an ambient temperature to complete the dehydrofluorination of chemically unstable fluorinated cyclohexadienes, and then the Ccl, layer was analyzed by GC-MS, and “F NMR spectrometry. In few cases, all of the electrolyte solution was treated by adding water (50cm3), and the products were extracted with three 15cm3 portions of CHzCl,. The combined CH,Cl, solution was concentrated and the residue was distilled at atmospheric pressure. Further, the distillate was separated by preparative GC (Varian GC-920: column, 3/ 8inch@25feet in length packed 15% Silicone OV-17 on Chromosorb WAW-DMCS 60/80) at 160°C to isolate some fluorinated cylcohexadienes, 1,3,3,6,6-pentafluoro-1,Ccyclohexadiene (4aJ,2,5,5,6, 6-pentafluoro-1,3-cyclohexadiene (4bJ and 1,3,3,5,6, 6-hexafluoro-1,4-cyclohexadiene (6al. Some of products, fluorobenzenes, were identified in comparison with authentic samples by GC-MS and “F NMR spectrometry. Fluorinated cyclohexadienes were characterized by GC-MS, ‘H and “F NMR spectrometry. ‘H and “F NMR spectra were measured on a Hitachi R-90F (84.68 MHz) instrument using tetramethylsilane for lH NMR and hexafluorobenzene for “F NMR as the internal standards. The anodic half-wave potential (Elj2) of the fluorobenzcnes (0.01 M) was measured on a platinum
II
43
wire electrode (1 mm&lOmm; rate; voltammetry (scan Me,NF *4HF.
0.32 cm2) by cyclic v = lOOmVs-r) in
RESULTS AND DISCUSSION The electrolyte, R,NF.mHF, has been usually selected by considering the solubility and the oxidation potential of the substrate compounds. From the view-point of the conductivity, the electrochemical stability and the cathodic polarization (hydrogen evolution), Me,NF - mHF seemed to be the best electrolyte for electrochemical fluorination. However, practical use is limited to some extent by the solubility of the substrate compound in it. On the other hand, Pr,NF *mHF has a high anodic stability and most organic compounds dissolved sufficiently in it, but the electrolytic currents were appreciably lower than those using Me,NF . mHF or Et.,NF *mHF owing to the comparatively lower electrolytic conductivity of Pr,NF.mHF. Et,NF.mHF has an appreciably high electrolytic conductivity and a high solubility of these organic substrates, but the anodic background current tends to increase steeply at 2.5 V (vs. Ag/Ag+) or above. Therefore, Et4NF.mHF has been used as the electrolyte and the electrolyses have been carried out at 2.5 V in this work. Fluorination of 1,2-dipuorobenzene (lJ The results for the electrochemical fluorination of 1,2difluorobenzene (1) are shown in Table 1. Two products, major 1,3,6,6-tetrafluoro-l+cyclohexadiene (la) and 5,5,6,6-tetra-fluoro-lJ-cyclohexadiene Ilb), and three minor products whose yields were of few percentages, 1,2,4_trifluorobenzene @) 1,3,3,6,6-pentafluoro-l&cyclohexadiene (4aJ and an unidentified compound, were obtained in the electrolyses. In a previous paper[18], the authors described that the electrochemically fluorinated cyclohexadienes, 3,3,6-trifluoro-l&yclohexadiene @ and 3,3,6,6-tetrafluoro-l&cylohexadiene @J, are electrochemically stable compounds; their oxidation potentials are 3.OV or above. Therefore, Ia_ &J and & are expected to a have high anodic stability, and their anodic currents could not be observed under 3.OV on the cyclic voltammograms. Figure 2 shows the variations in the yields of & lb_ 4 and & during the electrolysis of 4 in Et,NF *4.35 HF at a ambient temperature. The yrelds of a and fi and the conversion of 1 linearly increased with the quantity of electricity passed.
Table 1. Results for the electrochemical fluorination of 1,2difluorobenzene (l) Q*
Yield(%)
Run
Electrolyte
(F mol - ‘)
Duration Wn)
1 2
EtbNF. 4.0 HF Et&NF .4.35 HF
2.2 2.2
231 205
Cell voltage (Y)
Conversion(%) 1
la
lb
4
&
*
3.2-2.8 3.2-2.8
97.6 98.0
48.3 53.5
27.8 30.7
1.2 1.3
2.2 2.4
0.4 0.4
etc.7 3.6 4.2
* Quantity of electricity passed. t A kind of unidentified compound. & = 1,3,6,6-tetrafluoro-l&yclohexadiene, 4 = 1,2&trifluorobenzene, & = 5,5,6.6-tctrafluoro-l,~~clohcnadicne, & = 1,3,3,6,6-pentafluoro-l&cyclohexadiene, & = 2,5,5,6,6-pentafluoro-1,3_cyclohexadiene.
44
K.
The electrochemical
MOMOTA et al.
fluorination
mechanism in for by an ECEC mechanism. A probable mechanism for the fluorination of 1 is shown in Scheme 1 and consists of two charge transfer reactions and an addition of two fluoride anions as follows (1) the substrate, L is oxidized to the radical-cation, Ii, by the electron; (2) the radical-cation, 12, is quenched to the radical, lJ, by the addition of a fluoride anion; (3) the radical, lf’, is oxidized to the cation, If+, by the abstraction of an electron; and (4) the cation, If +, is quenched to & or & by the addition of a fluoride anion. Consa quently, 1,4- or 1,Zaddition of two fluorides to 1 occurred by the electrochemical fluorination processes, although the l&addition was dominant. The orientation of fluoride anion addition to the radical-cation and cation would be governed by the positive charge distributions on them. The distribution data on some radical-cations of fluorobenxene and some radicals were calculated using INDOZ and INDO calculation programs by Burdon and Parsons[22]. According to the data, the calculated charge densities were higher on the carbon atoms which bind to fluorine atoms. In the fluorination of 1, 3 was obtained as the result of dehydrofluorination of &. Further, a part of 4 was fluorinated to &g and 2,5,5,6,6pentatluoro1,3-cyclohexadiene (4& by an ECEC mechanism as well as described in Scheme 1. The rate of dehydrofluorination of la was much slower than that of g reported in a previous paper[18] and also shown in Fig. 4. Therefore, during the electrolysis, a small amount of & was converted into & and a large amount of & remained as it was. The anodic half-wave potential of 4 (E,,, = 1.96V) is the same as that of 1 (gill = 1.94V), and most of 4 produced was electrochemitally fluorinated with L during the electrolysis. Consequently, a small amount of & (ltl_addition to 3 was formed with a smaller amount of & (0.4% in yield, 1,Zaddition of 4J as the result of electrochemical fluorination of I. The reaction paths for la- lb_ 4 & and & from 1 are shown in Scheme 2. R,NF *mHF could be accounted
c
E
O/
F mol-’
Variations in the yield of products during the eleo trolysis of 1,2-ditluorobenxene(1) in Et,NF 4.35HF: (0) la;(A)&;@)4;and(O)4a Fig. 2.
The values of the 19F NMR chemical shift of h and B shown in Table 2 agreed very closely with the data reported by Zweig et aI.[19], although hexafluorobenxene was used in this work and Furon 11 was used in Zweig’s work as the internal standard compounds. The molar ratio of the fluorinated products, & and lb_ in Zweig’s work was 1:0.57 and ours were 1:O.S and 1:0.57. This coincidence means that the fluorination mechanisms with high valency metal fluorides and by the electrochemical oxidation in the presence of fluoride are basically similar to those reported by Burdon et aI.[23], whose mechanism involves the presence of radical-cations and cations as the intermediates. Fluorination of 1,3-difluorobenzene (2)
The results for the fluorination of 1.3difluorobenxene (2) are shown in Table 2. Both 1,3,3,
r-
F
FF
I_ -2s ZF-
1
FF
I
-HF -
6
%
b-i
v
F -2e. 2F-
FFb
F4
Scheme 1.
Scheme 2.
\
Table 2. Results for the electrochemical fluorination of 1,3_difluorobenxene(2)
Run
Electrolyte
3 4
Rt,NF .4.0 HF Et,NF .4.35 HF
Yield(%)
Q
Duration (mm)
Cell voltage 0
Conversion 2
P
4
4a
A!2
etc.*
2.1 2.7
333 276
3.5-3.0 3.5-3.0
98.2 98.1
51.5 45.4
3.3 3.0
17.6 20.5
2.8 3.1
7.2 7.8
(Fmol-t)
* Three kinds of unidentitied compounds. & = 1,3,3,6-tetratIuoro-l&yclohexadiene.
Electrochemical fluorination in R*NF. mHF-Part
II
45
Table 3. Results for the electrochemical fluorination of 1,2$trifluotobenzcneuorobenzene (1)
Yield(%) Q Run
Electrolyte
5
Et,NF .4.0 HF Et‘,NF .4.35 HF
6
(Fmol-‘) 2.4 2.3
Duration (min)
Cell voltage w
Conversion(%) 1
&
&
&
6
514 425
3.3-2.8 3.2-2.8
97.2 95.8
24.8 23.4
38.1 39.6
7.2 7.5
2.1 2.2
&
&
2.4 0.6 2.5 0.6
etc.* 12.1 12.7
Four kinds of unidentified compounds. jg = 1,3,5,6,6-pentafluoro-l&cyclohexadiene, 3 = 1,5,5,6,6_pentalluoro-l,fcyclohexadiene, & = lJ,3,3,6_pentafluoro-l&cyclohexadiene, 6 = lJ,3,5-tetratluorobenzene, @ = 1,3,3,5,6,6-hexafluoro-l&cyclohexadlene, & = 1,3,5,5,6, 6-hexatluoro-1,3qclohexadiene. l
6-tetralluoro-1,kyclohexadiene &) and & were obtained as the major final products in about 50 and about 20% yields, respectively. In addition, small amounts of five kinds of compounds, &, & and three unidentified compounds, were observed on the gas chromatograms. The variations in the yields of a $, & and 3 during the electrolysis of 2 in Et,NF *4.0HF is shown in Fig. 3. In the initial stage of the electrolysis, the yield of 4 was increased with the quantity of electricity passed. The maximum yield (cu. 8%) was obtained at about 1.5 Fmol- ’ passed and then the yield decreased with the electricity. Similar curves between the yield and electricity passed were obtained for fluorobenzene in the fluorination of benzene and for l+difluorobenzene in the fluorination of fluorobenzene[lS]. The anodic half-wave potential of 2. (E,,, = 2.03 V) was similar to that of 4 (E,,, = 1.97V). Therefore, most of 4 yielded during the electrolyses was electrochemically fluorinated further with 5 and changed into & and a. The reaction paths for &, $, & and @ are shown in Scheme 3. The molar ratio of & and e in the fluorination of ?_was the same as that in the fluorination of 1. In the results of the dehydrofluorination of both & and 2a_ 4 was obtained. However, the dehydrofluorination rate of 2a was faster than that of b as shown in Fig. 4. ATincrease in the amount of 4
yielded in the electrolyses of 1 and 2 would result in an increase in the electricity passed, because the resulting 4 was further fluorinated to & and a. In the reaction paths from 1 or 2 to & and &, the electricity passed is 4 Frni-‘. Therefore, the charges which were 2.2Fmol-’ in runs 1 and 2 and 2.7 Fmol-’ in runs 3 and 4 directly affected the yield of 4 -. Fluorination of 1,2,3&j?uorobenzene (3J The results for the electrochemical fluorination of 1,2,3-trifluorobenzene @) are shown in Table 3. 1,3,5, 6,6-Penta-fluoro-l&yclohexadiene (3aJand 1,5,5,6, 6-pentafluoro-1,3-cyclohexadiene (3l~) were obtained F
FF
HF
23
I-HF
FF
Scheme 3.
Fig. 4. Dehydrotluorination profile of some fluorinated
F mol-’ eyclohexadknes in the electrolyte solution at room temFig. 3. Variations in the yield of prkduetsduring the ole~~ perature,after the ektrolysis of each of fluorobenzenesin t&sis of 1,3difluorobcmene@ in Et,NF 4.OHF:(0) 2; EtJW.4.0HF: (0) 3,3,6-Tritluoro-l&eyelohexadiene(IL); O&(O)h;=d(O)&. (A) k; (A) zls; (0) &a;and (0) S O/
K. MOMOTA et al.
46
as the major tinal products, and 1,2,3,5-tetrafluorobenzene 0, 1,3,3,5,6,6-hexafluoro-l&cyclohexadiene &g) and 1,2,3,3,6-pentafluoro-1,4cyclohexadiene (3d which was formed by l&addition of two fluorines to 3 (but the positions differed from &), were obtained & small amounts. Also, four kinds of unidentified products which were identified as fluorinated cyclohexadienes and cyclohexenes on the mass spectra (each yield was less than 3%) were obtained. The fluorinated cyclohexadiene, k was identified on the mass spectrum showing a parent peak (m/e 170), though the authors could not determine it on the “F NMR chemical shifts of & and 1,2,3,4-tetrafluorobenzene was obtained from the dehydrofluorination of &. Figure 5 shows the variations in the yields of 3a 3b. 3c, 5 and $g during the electrolysis of 3 in Et,NF *4.35 HF. Although the dehydrofluorin&on of & and & occurred occasionally during the electrolyses, most & and & remained as they were. The reaction paths for & & 4 & and 1,3,5,5,6,6-hexafluoro-1,3-cyclohexadiene (6b) are shown in Scheme 4. The reaction paths in the fluorination of 3 were similar to those of 1. Fluorinationof 1,2&trifluorobenzene(4J The results of the 5uorination of l,Z&trifluorobenzene (4J are shown in Table 4. In these cases, both electrochemically and chemically stable compounds, & and 4b formed through the l&addition and l,Zaddi& to & respectively, were obtained in high yield, and the quantity of electricity (2.lFmol-‘) was nearly the theoretical value (2Fmol-‘). products
In addition, which were
Scheme 4.
fluoromethyl fluorocyclopentadienes and two fluorinated cyclohexenes from the mass spectra were obtained and each yield was less than 3%. The variations in the yields of & and & during the electrolysis of 4 in EtJW - 4.35 HF are shown in Fig. 6. The yields increased linearly with the electricity passed. The reaction paths for & and & are shown in Scheme 5. In the viewpoint that electrochemically and chemically stable compounds were obtained as the major products, the fluorination of 4 was similar to that of 1,4-difluorobenzene[l8] which yielded 3,3,
four kinds of unidentified identified as two tri-
Q/ F mol-’ Fig. 6. Variations in the yield of products during the ektrolysis of 1,2&trifluorobenzene &) in Et,NF .4.35 HF. (0) & and (A) &.
O/
F mol-’
Fig. 5. Variations in the yield of products during the clewtroiysis of l,S3-trifluorobenzeneQ) in Et, NF 4.35 HF: 10)
3a;(A)3b;(O)~;@)6;and(0)6a.
Fpb Scheme 5.
.-'
Table 4. Results for the electrochemical fluorination of 1,2&tritluorobenzene &) Yield(%) Run
Electrolyte
1
Et,NF *4.0 HF Et,NF - 4.35 HF
8 l
Q
(Fmol-‘) 2.1 2.1
Four kinds of unidcntihd compounds.
Duration (nW
Cell voltage 0
Conversion(%) 4
L
&
etc.*
258 260
3.5-3.0 3.5-3.0
ZZ::
72.0 75.2
12.4 11.4
7.2 6.6
Electrochemical fluorination in R,NF . mHF-Part II 6,6-tetrafluoro-1,4- cyclohexadiene (h) in 90% yield. However, in the case of 1,4difluorobenzene, 1,4addition mainly occurred and the 1,2-addition product was not obtained. This means that the fluorine atoms binding on the benzene ring play an important role as presented above, and the proportion of 1,2-addition to l&addition increased greatly when fluorine atoms exist on the ortho position on the benzene ring. It is noteworthy that the molar ratio of & and 3 produced on the fluorination of 4 in all cases (runs l-4 and runs 7 and 8) showed sir&u constant value (1:0.15-0.18). Fluorination of 1,3,5-trijhorobenzene (SJ The results for the fluorination of 1,3,5-t& fluorobenzene 0J are shown in Table 5. The electrolytic currents in the fluorination of 3 (runs 9 and 10) were lower than those in other cases presented above because the anodic half-wave potential of 3 (E,,, = 2.18V) is obviously higher than those of other diand tri-fluorobenzenes. 13 , ,3,5,bPenta!luoro-1,4cyclohexadiene 15a) and 1,3,3,5,6,6-hexafluoro-1,4cyclohexadiene (6aJ were obtained as the major final products, and 1,2,3,5-tetrafluorob @; E,,, = 2.08 V) and 1,3,5,5,6,6-hexafluoro-1,3-cyclohexadiene (6bJ were obtained in small amounts. Figure 7 shows the variations in the yields of Jg, & & and & during the electrolysis of 3 in Et,NF -4.35 HF. The maximum yield of 5 cu. 11%, was obtained at about 1.5 F mol- ’ passed. Although fi was obtained by the dehydrofluorination of both & and 3a_ the dehydrofluorination rate of & was much faster than that of 50
I
1
I
O/ F mol-’ Fig. 7. Variations in the yield of products during the electrolysis of 1,3,5_trihorob ci) in Et,NF*4.35HF: (0)5a;@)6;=d(O)6b.
47
Scheme 6. 3a_ as shown in Fig. 4. The reaction paths for Sa_ 5 &g and 6b are shown in Scheme 6. In this case, both the rea&n paths and the distribution of the products were very similar to those in the electrolysis of 2 d The dehydrofluorination rates of & la 2a. 3a and jg after the electrolyses in Et4NF. 4.0 HF (runs 1, 3, 5 and 9) are compared in Fig. 4. The rate obeyed a quasi-first-order decomposition. The rate constant tended to increase with m value in Et,NF *mHF[ 181, which is consistent with the results in Tables 2 and 5. The dehydrofluorination rates have been classified into two groups. The dehydrofluorination rate of one group, Group A; s & and 5a_ was faster than that of another group, Group B; la and 3a_ and large amounts of g, & and & were converted chemically into l&fluorobenzene, 2 and & respectively. In these cases, the variations in the yields of these fluorobenzenes, 4 and 6 during the electrolyses showed characteristic curves in Figs 3 and 7, ie their maximum yields were observed at cu. 1.5 F mol- ’ and most of them were fluorinated to the corresponding fluorinated cyclohexadienes at the 6nal stages of the electrolyses as shown in Figs 3 and 7. The compounds in the Group B had a structural feature and one or two fluorine atoms exist on the meta position against a saturated carbon which combined with a hydrogen atom, and the fluorine atoms would play an important role in their stabilization. The rates of dehydrofluorination during the electrolyses were much faster than that shown in Fig. 4. The catalytic action of the pt anode would contribute to it and the intermediate compounds were more unstable in the diffusion layer. However, in these processes the order of the dehydrofluorination would not change as shown in the result of the electrolyses. Further, for the explanation of the formation of highly fluorinated compounds, jg and & in Fig. 3, and, & and @ in Fig. 7, at the early stages of the electrolyses, some amounts of corresponded fluorobenzenes, eg 4 in Fig. 3 and 6 in Fig. 7, in the diffusion layer played an important role. These
Table 5. Results for the electrochemical fluorination of 1,3,Stri!luorob
Run
Electrolyte
9 10
Et,NF *4.0 HF Et,NF .4.35 HF
l
Q (Fmol-‘) 3.0 3.0
(i) Yield(%)
Duration (h)
cell voltage 0
conversion(%) i
sa
6
825 773
3.4-2.8 3.3-2.8
98.9 99.4
33.9 28.0
5.2 4.3
Four kinds of unidentiged compounds. & = 1,3,3,5,6-pentatIuor1&cyc1ohexadiene.
k
&
etc.*
29.1 34.1
5.2 6.1
6.8 6.4
K. M O M O T A
48
fluorobenzenes would mainly be formed by the dehydrofluorination of fluorinated cyclohexadiences at the surface and in the vicinity of anode, and the effect of diffusion of the fluorobenzenes from the bulk solution was negligible because their concentration in the bulk was very low at the early stages. On the other hand, with their increasing concentration in the bulk, the effect of their diffusion increased. The analyses of fluorobenzenes were carried out in comparison with authentic samples by GC-MS and t9F NMR. However, since the authentic samples of the fluorinated cyclohexadienes were not obtained commercially, the analyses of these compounds were carried out by GC-MS, tH and 19F NMR. Their 19F NMR spectral data are shown in Table 6. From the results of the electrochemical fluorination of mono-[18], di- and tri-fluorobenzenes in Et4NF'mHF, the reaction paths and the distribution of fluorinated products were clarified, The reactions during the electrolyses were both the electrochemical fluorination, which yielded fluorinated cyclohexadienes, and the dehydrofluorination of some fluorinated cyclohexadienes which yielded fluorobenzenes. The reactions are summarized in Scheme 7. The fluorinated cyclohexadienes that had a saturated carbon binding to a hydrogen atom, e0 la. 2a_ 3a. 5a and II in Scheme 7, were dehydrofluorinated.
et al.
On the other hand, the cylohexadienes that had no saturated carbon binding to a hydrogen atom, e0 3b, 4b. 6a. 6b and I in Scheme 7, were chemically stable. The quantity of electricity passed was controlled by considering the amounts of fluorobenzenes which were yielded on dehydrofluorination during the elec. trolyses because these were further electrochemically fluorinated. In the electrolyses, all fluorobenzenes were fluorinated into the fluorinated cyclohexadienes at 2.5V of the anode potential and the cylohexadienes were not fluorinated further.
F
(I)
-2e, 2FFF
FF H
(n=0~2)
F
F
Scheme 7.
Table 6. tgF NMR spectral data of fluorinated cyclohexadienes described in this study* Compound
Formula
Chemical shift (ppm)
J (Hz)t
Compound
Formula
•F H
eF
pd F
J (Hz)
CF
bF 1_.8
Chemical shift (ppm)
b 62.0 c
60.4
(m) on) (m) 38.7
39.8
(s)
d -17.0
F
lb
4a
bF °F
1~
b 70.4 c 37.4
(d) 5.0 on)
b 46.1
(d) 19.8 (m)
!TM
4b
P
c
57.2
F
i
i
~
F"
2a
aF t , F ~
H
bF
P
3n
a b c d
52.0 T'/.5 76.1 -27.7
a 31A b 48.9 c -16.2
on) (m) on) (m)
On) (m) (m) 38.7
bF a p ~ H 5a --
"F
°F
I~
, F ~ I
I~
~
6a
a 78.1 b 48.7 c -33.8
(m) on) (m)
a 73.6 b 44.4 c 33.5
(m) (t) 19.8
(In)
"F" " - - ~ -1~
'F •F
I~
bF b 30.4 c 32.8
3b •P
(d) 19.8 (m)
1~ 1~
6b
¢ 38.0 d 61.1
"F Fe
* Hexafinorobenzone was used as an internal standard in chloroform - (C]DCI~) solution. t Splitting and coupling constant: s=singlet, d,,doublet, t=triplet, m=multiplet.
(d) 17.4 (d) 19.8
(In) (in)
Electrochemicalfluorination in R,NF -mHF-Part II CONCLUSION In the anodic fluorination of mono-, di- and tri-fluorobenxenes in R,NF - mHF, the electrochemically stable corresponding fluorinated dyclohexadienes were obtained as the major products through 1,2- or l&addition of two fluorines to the substrate material in high yield. Therefore, the method is expected to be a useful preparation process of fluorocyclohexadienes which have been scarcely obtained as major products. Further, when relatively stable fluorinated cyclohexadienes, the Group B, were obtained in high yielded, the electrolyses will be utilized for the synthesis of some fluorobenxenes by dehydro0uorination with alkaline solution after the electrolyses. AcknowledgementsThe authors wish to thank Dr H. Kimoto, Government Industrial Research Institute, Nagoya, for advising us on the characterization of the products. This work was partially supported by Grant-in-Aid for Cooperative Research A (No. 04303007) from The Ministry of Edition, Science and Culture, Japan.
REFERENCES 1. M. Hudlicky, Chemistry of Organic Fluoride Compounds, 2nd edn. John Wiley, New York (1976). 2. R. E. Banks, Organofhwrine Chemicals and Their Industrial Applications. Ellis Horwood, Chichester, U. K. (1979). 3. T. Fuchigami, T. Hayashi and A. Konno, Tetrahedron L.&t.33, 3161 (1992). 4. K. Surowiec and T. Fuchigami, Tetrahedron L&t. 33, 1065 (1992).
49
5. H. Schmidt and H. D. Schmidt, J. Pm&t.Chim. 2, 250 (1955). 6. H. Schmidt and H. Meinert, Angew. Chem. 72, 109 (1960). 7. V. R. Koch and L. L. Miller, J. electroanal. Chem. 43, 318 (1973). 8. E. Laurent, H. Lefranc and R. Tardivel, Nouu. J. Chim. 8,345 (1984). 9. E. Laurent, B. Marquet and R. Tardivel, Tetrahedron &I,4431 (1989). 10. C. J. Ludman, E. M. McCarron and R. F. O’Mall9, J. electrochem. See. 119,874 (1972). 11. J. R. Ballinner and F. W. Teare. Electrochim. Acta 30. 1075 (1985).12. I. N. Rozhkov. Rus. Chem. Rev. 45.615 (1976). 13. I. N. Roahkov and I. Y. Alyev, .Tetr&dr& 31, 977 (1975). 14. F. Huha, E. B. Yeager and G. A. Olah, Electrochim. Acta 24,489 (1979). 15. J. H. H. M&us,. D. W. Sopher and W. Eilenherg, Anaew. Chem. Int. Ed. Encl. 28.927 (1989). 16. J. H. H. Meurs and W. &&erg, ?etraLdron 47, 705 (1991). 17. K. Momota, M. Morita and Y. Matsuda, Electrochim. Acta 38,619 (1993). 18. K. Momota, M. Morita and Y. Matsuda, Electrochim. Acta 38, 1123 (1993). 19. A. Zweig, R. G. Fisher and J. E. Lancaster, J. Org. cheat. 4s, 3597 (1980). 20. G. H. Goleman and W. F. Talhot. Org. Syn. Coil. Vol. 2, p. 592. New York (1966). 21. A. Roe. Oraanic Reactions. Vol. 5. D. 193. John Wilev. _I New York (11949). * 22. J. Burdon and I. W. Parsons, Tetrahedron 31, 2401 (1975). 23. J. Burdon, I. W. Parsons and J. C. Tatlow, Tetrahedron 28,43 (1972).
-w
-rrm-A_rrwlr
ELEL
0013-4686/w SW + o.00 CQ1993. Perpmon Ror Ud
m*-
1 KULHEMICAL
I1
-1
vTAm1.T.
r LUUK~NA
1 ium
mT-.*
ur Awl
1-n.
AKUMA
I
I__
i ic
COMPOUNDS IN LIQUID R4NF. mHF-PART II. FLUORINATION OF DI- AND TRI-FLUOROBENZENES KUNITAKAMOMOTA,*KA~WA KATo,~ MASAWKI MORITA~and YOSHIHARUMATSUDA~ Department of Research and Development, Morita Chemical Industries Co. Ltd., Higashi-mikuni, Yodogawa-ku, Osaka 532, Japan (Received 29 March 1993) Abstract-Di- and tri-fluorobenzenes were electrochemically fluorinated at a platinum anode at 2.5 V [vs. Ag/Ag+ (0.01 moldm-3)] in Et,NF.mHF (Et = C,H,, m = 4.0, 4.35). The fluorinated cyclohexadienes were obtained as the major final products in high yield, and neither deposition of a polymeric 6lm on the anode surface nor a coloration of the electrolyte solution was observed. Some 124-trilluorobenxcne @) or 1,2,3,Stetrafluorobenzene (@ was produced in the course of the fluorination of l$ditluorobenzene (2) or 1,3,StrilIuorobenxene @, respectively. These were produced chemically by the dehydrofluorination of 1,3,3,6tetrafuoro-l&cyclohexadiene (&) or 1,3,3,5,6-pentafluoro-ltlcyclohexadiene 0, which was produced by the anodic fluorination, and large portions of the resulting 4 and 6 were further fluorinated electrochemically to the corresponding fluorinated cyclohexadienes. T&e reaction paths to the major products were explained by the combination of electrochemical reactions, 1,4- or l&addition of two fluoride anions to substrate compounds by an ECEC mccha&m (which yielded the fluorinated cyclohexadienes) and a chemical reaction (the dehydrofluorination of some fluorinated cyclohexadienes) which yielded fluorobenzenes. Key words: anodic fluorination, fluorobenzenes, fluorinated cyclohexadienea, tetraethylammonium fluoride-hydrogen fluoride electrolyte, dehydrofluorination.
INTRODUCITON
electrochemical fluorination of benzene, chlorobenzene etc. at a Pt anode in Et,N .3HF. In the cases of electrolyses using these electrolytes, deposition of a polymeric film on the anode surface was observed together with a low fluorination current efIiciency owing to side-reactions and/or a low electric current density reflecting low electrolytic conductivity of electrolyte[17J. In a previous paper by the authors[18], a series of new electrolytes, non-viscous liquids of tetraalkylammonium salts (R*NF *mHF: R-Me, Et, Pr: Me-CH, , Et-C,H, , Pr=C,H,: m > 3.5) at room temperature, were found to give high electrolytic conductivity and high electrochemical stability, and the anodic fluorination of benzene, fluorobenzene and Wdifluorobenzene using EtdNF *mHF electrolytes were reported. 3,6,6_Trifluoro-l,Qcyclohexadiene @ and 3,3,6,6-tetralluoro-1,4cyclohexadiene @), which are electrochemically stable and whose oxidation potentials were as high as 3.OV (vs. Ag/Ag+) on the cyclic voltammograms, were obtained as the final major products in high current efficiency and in high yield as the results of the electrolyses at 2.5 V. The investigation was scarcely concerned with the fluorination of di- and tri-fluorobenzenes which included chemical fluorination with fluorine, high valency metal fluorides, eg cobalt trifluoride (CoF,), silver difluoride (AgF,) and &urn tetrduoride (CcF,) and xenon difluoride (XeF,). Only Zweig et ul.[19] reported the fluorination of 1,2- and 1,4difluorobenzcne with AgFz in hexane. In the present work, the anodic fluorination of di- and trifluorobenzenes has been studied further. From the
Partial fluorination of organic compounds in electrochemical processes is expected to produce useful fluorinated precursors for pharmaceutical and agricultural agents, raw materials of fine chemicals etc.[l, 23, since it makes possible the selective introduction of fluorine atoms into organic compounds[3,
41. Up to the present day, solutions of aprotic solvents [mainly acetonitrile (CH,CN) or dichloromethane (CH,Cl,)] containing some fluorides, eg fluoride (HF)[5], silver monohydrogen fluoride(AgF)[6], tetraethylammonium tetrtiuoroborate (E&N BF,)[fl, triethylamine Whydrogen fluoride) (Et,N .3HF)[8, 93, tetramethylammonium fluoride bis(hydrogen fluoride) (Me,NF - ZHF)[lO, 1l] and tetraethylammonium fluoride tris(hydrogen fluoride) (EtdNF *3HF)[12, 131, have been used as the electrolytes for this purpose. These fluorides behave as both the fluorine sources and as the supporting electrolytes. Further, Huba et aL[14] obtained 2-fluoropyridine as a by-product in the electrolysis of 29% pyridine-71% anhydrous HF electrolyte with platinum (Pt) and carbon anodes, and Meurs and co-workers[lS, 161 carried out the Author to whom correspondence should be addressed. t Permanent address: Chemistry Department, Goveml
ment Industrial Research Institute. Nagoya, Hir&-cho, Kita-ku, Nagoya 462. Jam. $ Pennan% address:-Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai, Ube 755, Japan. 41
K. MOMOTA et al.
42
results of the electrolyses, the reaction paths of the electrochemical fluorination of fluorobenxenes in R,NF -mHF without solvent was clarified, and the fluorination mechanism was investigated. The rates of dehydrofluorination for some fluorinated cyclohexadienes were examined to account for the distribution of products and the quantity of electricity
r I
,
passed.
EXPERIMENTAL b(c)
Materials
1,2-Difluorobenzene, lJ-difluorobenxene, 1,3,5-trifluorobenxene and trifluoromethylbe were obtained from Aldrich Chemical. Chlorobenxene, tetrachloromethane (Ccl,) and dichloromethane (CH,Cl,) were obtained from Wako Pure Chemical. Acetonitrile (MeCN: F grade, water content < 20ppm) was obtained from Mitsubishi Petrochemical. These reagents were used without further purification. 1,2,3-Tritluorobenxene was synthesized from 2,3,4trifluoroaniline (Morita Chemical Industries) by a deamination process[20] and was purified up to 99% assay by fractional distillation. 1,2,4-T+ fluorobenzene was synthesized from 3,4difluoroaniline (Morita Chemical Industries) via 3,4diflurobenz.enediaxonium hexafluorophosphate by the Schiemann reaction[21] and was purified up to 99% assay by fractional distillation. The electrolytes, E&NF. mHF (m = 4.0 and 4.35), were prepared by mixing Et,NF *2HF and anhydrous HF in a dried nitrogen atmosphere as described elsewhere[lfl. Tetraethylammonium tetrafluoroborate (Et,N BF,: Morita Chemical Industries) and silver perchlorate (AgClO,: Wako Pure Chemical) were dried for 18 h under a vacuum at 100°C before use. Cell components The schematic diagrams of the electric cell and the electrodes are shown in Fig. 1. The cell components, except for the electrodes (Pt and Ag), leading wires (2mm0, copper wire), silicon rubber caps and epoxy-resin, were made of fluoro-resins, poly(tetrafluoroethylene - co - pertIuoroalkylvinylether) (PFA) and polytetrafluoroethylene (PTFE), to protect from corrosion by free HF. The reference electrode was Ag/AgClO., (0.01 M: M = moldm-‘) in MeCN containing Et,N . BF, (0.1 M) as the supporting electrolyte. The electrolyte, Et,NF *mHF, without the substrate was used as a salt bridge. Method
In the glove box, filled with dried nitrogen gas, the cell parts were constructed and 30cm3 of the electrolyte, Et,NF *mHF, and 0.0123 mol of the substrate compound, di- or tri-fluorobenzene, were added into the cell. The electrolysis was carried out at a constant potential (2SV) by a function generator (Hokuto Denko: HB-104) connected to a potentio/ galvanostat (Hokuto Denko: HA-501) at an ambient temperature. The solution was continuously stirred magnetically with a PTFE coated stirring bar. The
/’ (Anode and cathode)
.n i
m
a
b
m (Reference (Beaker-type
electrode)
PFA ccl I>
Fig. 1. The schematic diagrams of a beaker-type PFA cell and the electrodes. (a) PFA cell case; (b) Anode (Pt, 2Omm x 20mm); (c) Cathode (Pt, 2Omm x 2Omm); (d) Reference electrode (Ag/Ag+); (eJ Electrolyte; (cd Salt bridge; (eJ 0.01 M AgClO, + 0.1 M Et,N-BFJMeCN; (f) PTFE plug; (g) PTEF cap; (h) Gaa outlet; (i) Thermometer shell; (i) PTFE coated magnetic stirring bar; (k) Ag wire (1 mm@); (l) Silicon rubber cap; (m) PTFE diaphragm; (n) PFA shell; (0) Pt wire (1 mm@); (p) Epoxy-resin; (q) Solder; (r) Cu wire (2mm0); (s) PFA tube (6mmlzr).
electric current and the cell voltage vs. time were recorded on a X-Y recorder (Yokogawa Elect&s: 302523). The electrolyses were continued until the current had fallen to 30mA (about two times the background current in Et,NF *mHF (ca. 15 mA) at 2.5 V) owing to decrease of the concentration of electrochemically active compounds; fluorobenxenes which were the substrates and the products. During the usual electrolysis, a small amount of the electrolyte solution (ca. 0.1 g) was sampled for each 0.5 or 1.0 Faraday per mol of substrate (Fmol-‘) passed, in order to monitor the variations in the amounts of substrate and products. After the sampling, the mass of the sample was precisely weighed, and was treated by adding water (1 cm3) and Ccl, solution (1 cm3) containing chlorobenzene or trifluoromethylbenxene as an internal standard material. Consequently, the substrate and the products were extracted with CCl, solution from the sample solution and Et,NF *mHF remained in the water layer. Thii procedure was necessary to prevent the dehydrofluorination reaction of some fluorinated
Electrochemical fluorination in R,NF *mHF-Part cylohexadienes; especially, 1,3,3,6-tetralluoro-1,4cylohexadiene (2a) and 1,3,3,5,6-pentafluoro-1,4cyclohexadiene (5al. The resulting CCl, solutions were analyzed by gas chromatography (GC, Shimadzu, gas chromatograph: GC-14A, Chromatopac: CR4A) with a G-300 or/and a G-450 column (40 m1.2 mm@. Kagakuhin Kensa Kyoukai) and GC-mass spectrometry (GC-MS, Shimadzu: GCMSQPlOOO). After the electrolysis, a small part of the electrolyte solution (ca. 2 cm3) was stored in a sealed PFA-resin bottle (5cm3), in order to measure the variation of the product component by the dehyrofluorination of some fluorinated cyclohexadienes with the storage time. Another part of the solution was treated by adding water (50cm3), and the products were extracted with two 15cm3 portions of Ccl,. The resulting Ccl, solution was dried over anhydrous sodium sulfate (Na,SO,), and was analyzed by GC-MS and “F NMR spectrometry. Further, some of the Ccl, solution (cu. 1Ocm’) was mixed with 5 N-potassium hydroxide (KOH) aqueous solution (20cm3) in a 50cm3-Erlenmayer flask setting with a stopper and a stirring bar, and the mixture was stirred vigorously over-night at an ambient temperature to complete the dehydrofluorination of chemically unstable fluorinated cyclohexadienes, and then the Ccl, layer was analyzed by GC-MS, and “F NMR spectrometry. In few cases, all of the electrolyte solution was treated by adding water (50cm3), and the products were extracted with three 15cm3 portions of CHzCl,. The combined CH,Cl, solution was concentrated and the residue was distilled at atmospheric pressure. Further, the distillate was separated by preparative GC (Varian GC-920: column, 3/ 8inch@25feet in length packed 15% Silicone OV-17 on Chromosorb WAW-DMCS 60/80) at 160°C to isolate some fluorinated cylcohexadienes, 1,3,3,6,6-pentafluoro-1,Ccyclohexadiene (4aJ,2,5,5,6, 6-pentafluoro-1,3-cyclohexadiene (4bJ and 1,3,3,5,6, 6-hexafluoro-1,4-cyclohexadiene (6al. Some of products, fluorobenzenes, were identified in comparison with authentic samples by GC-MS and “F NMR spectrometry. Fluorinated cyclohexadienes were characterized by GC-MS, ‘H and “F NMR spectrometry. ‘H and “F NMR spectra were measured on a Hitachi R-90F (84.68 MHz) instrument using tetramethylsilane for lH NMR and hexafluorobenzene for “F NMR as the internal standards. The anodic half-wave potential (Elj2) of the fluorobenzcnes (0.01 M) was measured on a platinum
II
43
wire electrode (1 mm&lOmm; rate; voltammetry (scan Me,NF *4HF.
0.32 cm2) by cyclic v = lOOmVs-r) in
RESULTS AND DISCUSSION The electrolyte, R,NF.mHF, has been usually selected by considering the solubility and the oxidation potential of the substrate compounds. From the view-point of the conductivity, the electrochemical stability and the cathodic polarization (hydrogen evolution), Me,NF - mHF seemed to be the best electrolyte for electrochemical fluorination. However, practical use is limited to some extent by the solubility of the substrate compound in it. On the other hand, Pr,NF *mHF has a high anodic stability and most organic compounds dissolved sufficiently in it, but the electrolytic currents were appreciably lower than those using Me,NF . mHF or Et.,NF *mHF owing to the comparatively lower electrolytic conductivity of Pr,NF.mHF. Et,NF.mHF has an appreciably high electrolytic conductivity and a high solubility of these organic substrates, but the anodic background current tends to increase steeply at 2.5 V (vs. Ag/Ag+) or above. Therefore, Et4NF.mHF has been used as the electrolyte and the electrolyses have been carried out at 2.5 V in this work. Fluorination of 1,2-dipuorobenzene (lJ The results for the electrochemical fluorination of 1,2difluorobenzene (1) are shown in Table 1. Two products, major 1,3,6,6-tetrafluoro-l+cyclohexadiene (la) and 5,5,6,6-tetra-fluoro-lJ-cyclohexadiene Ilb), and three minor products whose yields were of few percentages, 1,2,4_trifluorobenzene @) 1,3,3,6,6-pentafluoro-l&cyclohexadiene (4aJ and an unidentified compound, were obtained in the electrolyses. In a previous paper[18], the authors described that the electrochemically fluorinated cyclohexadienes, 3,3,6-trifluoro-l&yclohexadiene @ and 3,3,6,6-tetrafluoro-l&cylohexadiene @J, are electrochemically stable compounds; their oxidation potentials are 3.OV or above. Therefore, Ia_ &J and & are expected to a have high anodic stability, and their anodic currents could not be observed under 3.OV on the cyclic voltammograms. Figure 2 shows the variations in the yields of & lb_ 4 and & during the electrolysis of 4 in Et,NF *4.35 HF at a ambient temperature. The yrelds of a and fi and the conversion of 1 linearly increased with the quantity of electricity passed.
Table 1. Results for the electrochemical fluorination of 1,2difluorobenzene (l) Q*
Yield(%)
Run
Electrolyte
(F mol - ‘)
Duration Wn)
1 2
EtbNF. 4.0 HF Et&NF .4.35 HF
2.2 2.2
231 205
Cell voltage (Y)
Conversion(%) 1
la
lb
4
&
*
3.2-2.8 3.2-2.8
97.6 98.0
48.3 53.5
27.8 30.7
1.2 1.3
2.2 2.4
0.4 0.4
etc.7 3.6 4.2
* Quantity of electricity passed. t A kind of unidentified compound. & = 1,3,6,6-tetrafluoro-l&yclohexadiene, 4 = 1,2&trifluorobenzene, & = 5,5,6.6-tctrafluoro-l,~~clohcnadicne, & = 1,3,3,6,6-pentafluoro-l&cyclohexadiene, & = 2,5,5,6,6-pentafluoro-1,3_cyclohexadiene.
44
K.
The electrochemical
MOMOTA et al.
fluorination
mechanism in for by an ECEC mechanism. A probable mechanism for the fluorination of 1 is shown in Scheme 1 and consists of two charge transfer reactions and an addition of two fluoride anions as follows (1) the substrate, L is oxidized to the radical-cation, Ii, by the electron; (2) the radical-cation, 12, is quenched to the radical, lJ, by the addition of a fluoride anion; (3) the radical, lf’, is oxidized to the cation, If+, by the abstraction of an electron; and (4) the cation, If +, is quenched to & or & by the addition of a fluoride anion. Consa quently, 1,4- or 1,Zaddition of two fluorides to 1 occurred by the electrochemical fluorination processes, although the l&addition was dominant. The orientation of fluoride anion addition to the radical-cation and cation would be governed by the positive charge distributions on them. The distribution data on some radical-cations of fluorobenxene and some radicals were calculated using INDOZ and INDO calculation programs by Burdon and Parsons[22]. According to the data, the calculated charge densities were higher on the carbon atoms which bind to fluorine atoms. In the fluorination of 1, 3 was obtained as the result of dehydrofluorination of &. Further, a part of 4 was fluorinated to &g and 2,5,5,6,6pentatluoro1,3-cyclohexadiene (4& by an ECEC mechanism as well as described in Scheme 1. The rate of dehydrofluorination of la was much slower than that of g reported in a previous paper[18] and also shown in Fig. 4. Therefore, during the electrolysis, a small amount of & was converted into & and a large amount of & remained as it was. The anodic half-wave potential of 4 (E,,, = 1.96V) is the same as that of 1 (gill = 1.94V), and most of 4 produced was electrochemitally fluorinated with L during the electrolysis. Consequently, a small amount of & (ltl_addition to 3 was formed with a smaller amount of & (0.4% in yield, 1,Zaddition of 4J as the result of electrochemical fluorination of I. The reaction paths for la- lb_ 4 & and & from 1 are shown in Scheme 2. R,NF *mHF could be accounted
c
E
O/
F mol-’
Variations in the yield of products during the eleo trolysis of 1,2-ditluorobenxene(1) in Et,NF 4.35HF: (0) la;(A)&;@)4;and(O)4a Fig. 2.
The values of the 19F NMR chemical shift of h and B shown in Table 2 agreed very closely with the data reported by Zweig et aI.[19], although hexafluorobenxene was used in this work and Furon 11 was used in Zweig’s work as the internal standard compounds. The molar ratio of the fluorinated products, & and lb_ in Zweig’s work was 1:0.57 and ours were 1:O.S and 1:0.57. This coincidence means that the fluorination mechanisms with high valency metal fluorides and by the electrochemical oxidation in the presence of fluoride are basically similar to those reported by Burdon et aI.[23], whose mechanism involves the presence of radical-cations and cations as the intermediates. Fluorination of 1,3-difluorobenzene (2)
The results for the fluorination of 1.3difluorobenxene (2) are shown in Table 2. Both 1,3,3,
r-
F
FF
I_ -2s ZF-
1
FF
I
-HF -
6
%
b-i
v
F -2e. 2F-
FFb
F4
Scheme 1.
Scheme 2.
\
Table 2. Results for the electrochemical fluorination of 1,3_difluorobenxene(2)
Run
Electrolyte
3 4
Rt,NF .4.0 HF Et,NF .4.35 HF
Yield(%)
Q
Duration (mm)
Cell voltage 0
Conversion 2
P
4
4a
A!2
etc.*
2.1 2.7
333 276
3.5-3.0 3.5-3.0
98.2 98.1
51.5 45.4
3.3 3.0
17.6 20.5
2.8 3.1
7.2 7.8
(Fmol-t)
* Three kinds of unidentitied compounds. & = 1,3,3,6-tetratIuoro-l&yclohexadiene.
Electrochemical fluorination in R*NF. mHF-Part
II
45
Table 3. Results for the electrochemical fluorination of 1,2$trifluotobenzcneuorobenzene (1)
Yield(%) Q Run
Electrolyte
5
Et,NF .4.0 HF Et‘,NF .4.35 HF
6
(Fmol-‘) 2.4 2.3
Duration (min)
Cell voltage w
Conversion(%) 1
&
&
&
6
514 425
3.3-2.8 3.2-2.8
97.2 95.8
24.8 23.4
38.1 39.6
7.2 7.5
2.1 2.2
&
&
2.4 0.6 2.5 0.6
etc.* 12.1 12.7
Four kinds of unidentified compounds. jg = 1,3,5,6,6-pentafluoro-l&cyclohexadiene, 3 = 1,5,5,6,6_pentalluoro-l,fcyclohexadiene, & = lJ,3,3,6_pentafluoro-l&cyclohexadiene, 6 = lJ,3,5-tetratluorobenzene, @ = 1,3,3,5,6,6-hexafluoro-l&cyclohexadlene, & = 1,3,5,5,6, 6-hexatluoro-1,3qclohexadiene. l
6-tetralluoro-1,kyclohexadiene &) and & were obtained as the major final products in about 50 and about 20% yields, respectively. In addition, small amounts of five kinds of compounds, &, & and three unidentified compounds, were observed on the gas chromatograms. The variations in the yields of a $, & and 3 during the electrolysis of 2 in Et,NF *4.0HF is shown in Fig. 3. In the initial stage of the electrolysis, the yield of 4 was increased with the quantity of electricity passed. The maximum yield (cu. 8%) was obtained at about 1.5 Fmol- ’ passed and then the yield decreased with the electricity. Similar curves between the yield and electricity passed were obtained for fluorobenzene in the fluorination of benzene and for l+difluorobenzene in the fluorination of fluorobenzene[lS]. The anodic half-wave potential of 2. (E,,, = 2.03 V) was similar to that of 4 (E,,, = 1.97V). Therefore, most of 4 yielded during the electrolyses was electrochemically fluorinated further with 5 and changed into & and a. The reaction paths for &, $, & and @ are shown in Scheme 3. The molar ratio of & and e in the fluorination of ?_was the same as that in the fluorination of 1. In the results of the dehydrofluorination of both & and 2a_ 4 was obtained. However, the dehydrofluorination rate of 2a was faster than that of b as shown in Fig. 4. ATincrease in the amount of 4
yielded in the electrolyses of 1 and 2 would result in an increase in the electricity passed, because the resulting 4 was further fluorinated to & and a. In the reaction paths from 1 or 2 to & and &, the electricity passed is 4 Frni-‘. Therefore, the charges which were 2.2Fmol-’ in runs 1 and 2 and 2.7 Fmol-’ in runs 3 and 4 directly affected the yield of 4 -. Fluorination of 1,2,3&j?uorobenzene (3J The results for the electrochemical fluorination of 1,2,3-trifluorobenzene @) are shown in Table 3. 1,3,5, 6,6-Penta-fluoro-l&yclohexadiene (3aJand 1,5,5,6, 6-pentafluoro-1,3-cyclohexadiene (3l~) were obtained F
FF
HF
23
I-HF
FF
Scheme 3.
Fig. 4. Dehydrotluorination profile of some fluorinated
F mol-’ eyclohexadknes in the electrolyte solution at room temFig. 3. Variations in the yield of prkduetsduring the ole~~ perature,after the ektrolysis of each of fluorobenzenesin t&sis of 1,3difluorobcmene@ in Et,NF 4.OHF:(0) 2; EtJW.4.0HF: (0) 3,3,6-Tritluoro-l&eyelohexadiene(IL); O&(O)h;=d(O)&. (A) k; (A) zls; (0) &a;and (0) S O/
K. MOMOTA et al.
46
as the major tinal products, and 1,2,3,5-tetrafluorobenzene 0, 1,3,3,5,6,6-hexafluoro-l&cyclohexadiene &g) and 1,2,3,3,6-pentafluoro-1,4cyclohexadiene (3d which was formed by l&addition of two fluorines to 3 (but the positions differed from &), were obtained & small amounts. Also, four kinds of unidentified products which were identified as fluorinated cyclohexadienes and cyclohexenes on the mass spectra (each yield was less than 3%) were obtained. The fluorinated cyclohexadiene, k was identified on the mass spectrum showing a parent peak (m/e 170), though the authors could not determine it on the “F NMR chemical shifts of & and 1,2,3,4-tetrafluorobenzene was obtained from the dehydrofluorination of &. Figure 5 shows the variations in the yields of 3a 3b. 3c, 5 and $g during the electrolysis of 3 in Et,NF *4.35 HF. Although the dehydrofluorin&on of & and & occurred occasionally during the electrolyses, most & and & remained as they were. The reaction paths for & & 4 & and 1,3,5,5,6,6-hexafluoro-1,3-cyclohexadiene (6b) are shown in Scheme 4. The reaction paths in the fluorination of 3 were similar to those of 1. Fluorinationof 1,2&trifluorobenzene(4J The results of the 5uorination of l,Z&trifluorobenzene (4J are shown in Table 4. In these cases, both electrochemically and chemically stable compounds, & and 4b formed through the l&addition and l,Zaddi& to & respectively, were obtained in high yield, and the quantity of electricity (2.lFmol-‘) was nearly the theoretical value (2Fmol-‘). products
In addition, which were
Scheme 4.
fluoromethyl fluorocyclopentadienes and two fluorinated cyclohexenes from the mass spectra were obtained and each yield was less than 3%. The variations in the yields of & and & during the electrolysis of 4 in EtJW - 4.35 HF are shown in Fig. 6. The yields increased linearly with the electricity passed. The reaction paths for & and & are shown in Scheme 5. In the viewpoint that electrochemically and chemically stable compounds were obtained as the major products, the fluorination of 4 was similar to that of 1,4-difluorobenzene[l8] which yielded 3,3,
four kinds of unidentified identified as two tri-
Q/ F mol-’ Fig. 6. Variations in the yield of products during the ektrolysis of 1,2&trifluorobenzene &) in Et,NF .4.35 HF. (0) & and (A) &.
O/
F mol-’
Fig. 5. Variations in the yield of products during the clewtroiysis of l,S3-trifluorobenzeneQ) in Et, NF 4.35 HF: 10)
3a;(A)3b;(O)~;@)6;and(0)6a.
Fpb Scheme 5.
.-'
Table 4. Results for the electrochemical fluorination of 1,2&tritluorobenzene &) Yield(%) Run
Electrolyte
1
Et,NF *4.0 HF Et,NF - 4.35 HF
8 l
Q
(Fmol-‘) 2.1 2.1
Four kinds of unidcntihd compounds.
Duration (nW
Cell voltage 0
Conversion(%) 4
L
&
etc.*
258 260
3.5-3.0 3.5-3.0
ZZ::
72.0 75.2
12.4 11.4
7.2 6.6
Electrochemical fluorination in R,NF . mHF-Part II 6,6-tetrafluoro-1,4- cyclohexadiene (h) in 90% yield. However, in the case of 1,4difluorobenzene, 1,4addition mainly occurred and the 1,2-addition product was not obtained. This means that the fluorine atoms binding on the benzene ring play an important role as presented above, and the proportion of 1,2-addition to l&addition increased greatly when fluorine atoms exist on the ortho position on the benzene ring. It is noteworthy that the molar ratio of & and 3 produced on the fluorination of 4 in all cases (runs l-4 and runs 7 and 8) showed sir&u constant value (1:0.15-0.18). Fluorination of 1,3,5-trijhorobenzene (SJ The results for the fluorination of 1,3,5-t& fluorobenzene 0J are shown in Table 5. The electrolytic currents in the fluorination of 3 (runs 9 and 10) were lower than those in other cases presented above because the anodic half-wave potential of 3 (E,,, = 2.18V) is obviously higher than those of other diand tri-fluorobenzenes. 13 , ,3,5,bPenta!luoro-1,4cyclohexadiene 15a) and 1,3,3,5,6,6-hexafluoro-1,4cyclohexadiene (6aJ were obtained as the major final products, and 1,2,3,5-tetrafluorob @; E,,, = 2.08 V) and 1,3,5,5,6,6-hexafluoro-1,3-cyclohexadiene (6bJ were obtained in small amounts. Figure 7 shows the variations in the yields of Jg, & & and & during the electrolysis of 3 in Et,NF -4.35 HF. The maximum yield of 5 cu. 11%, was obtained at about 1.5 F mol- ’ passed. Although fi was obtained by the dehydrofluorination of both & and 3a_ the dehydrofluorination rate of & was much faster than that of 50
I
1
I
O/ F mol-’ Fig. 7. Variations in the yield of products during the electrolysis of 1,3,5_trihorob ci) in Et,NF*4.35HF: (0)5a;@)6;=d(O)6b.
47
Scheme 6. 3a_ as shown in Fig. 4. The reaction paths for Sa_ 5 &g and 6b are shown in Scheme 6. In this case, both the rea&n paths and the distribution of the products were very similar to those in the electrolysis of 2 d The dehydrofluorination rates of & la 2a. 3a and jg after the electrolyses in Et4NF. 4.0 HF (runs 1, 3, 5 and 9) are compared in Fig. 4. The rate obeyed a quasi-first-order decomposition. The rate constant tended to increase with m value in Et,NF *mHF[ 181, which is consistent with the results in Tables 2 and 5. The dehydrofluorination rates have been classified into two groups. The dehydrofluorination rate of one group, Group A; s & and 5a_ was faster than that of another group, Group B; la and 3a_ and large amounts of g, & and & were converted chemically into l&fluorobenzene, 2 and & respectively. In these cases, the variations in the yields of these fluorobenzenes, 4 and 6 during the electrolyses showed characteristic curves in Figs 3 and 7, ie their maximum yields were observed at cu. 1.5 F mol- ’ and most of them were fluorinated to the corresponding fluorinated cyclohexadienes at the 6nal stages of the electrolyses as shown in Figs 3 and 7. The compounds in the Group B had a structural feature and one or two fluorine atoms exist on the meta position against a saturated carbon which combined with a hydrogen atom, and the fluorine atoms would play an important role in their stabilization. The rates of dehydrofluorination during the electrolyses were much faster than that shown in Fig. 4. The catalytic action of the pt anode would contribute to it and the intermediate compounds were more unstable in the diffusion layer. However, in these processes the order of the dehydrofluorination would not change as shown in the result of the electrolyses. Further, for the explanation of the formation of highly fluorinated compounds, jg and & in Fig. 3, and, & and @ in Fig. 7, at the early stages of the electrolyses, some amounts of corresponded fluorobenzenes, eg 4 in Fig. 3 and 6 in Fig. 7, in the diffusion layer played an important role. These
Table 5. Results for the electrochemical fluorination of 1,3,Stri!luorob
Run
Electrolyte
9 10
Et,NF *4.0 HF Et,NF .4.35 HF
l
Q (Fmol-‘) 3.0 3.0
(i) Yield(%)
Duration (h)
cell voltage 0
conversion(%) i
sa
6
825 773
3.4-2.8 3.3-2.8
98.9 99.4
33.9 28.0
5.2 4.3
Four kinds of unidentiged compounds. & = 1,3,3,5,6-pentatIuor1&cyc1ohexadiene.
k
&
etc.*
29.1 34.1
5.2 6.1
6.8 6.4
K. M O M O T A
48
fluorobenzenes would mainly be formed by the dehydrofluorination of fluorinated cyclohexadiences at the surface and in the vicinity of anode, and the effect of diffusion of the fluorobenzenes from the bulk solution was negligible because their concentration in the bulk was very low at the early stages. On the other hand, with their increasing concentration in the bulk, the effect of their diffusion increased. The analyses of fluorobenzenes were carried out in comparison with authentic samples by GC-MS and t9F NMR. However, since the authentic samples of the fluorinated cyclohexadienes were not obtained commercially, the analyses of these compounds were carried out by GC-MS, tH and 19F NMR. Their 19F NMR spectral data are shown in Table 6. From the results of the electrochemical fluorination of mono-[18], di- and tri-fluorobenzenes in Et4NF'mHF, the reaction paths and the distribution of fluorinated products were clarified, The reactions during the electrolyses were both the electrochemical fluorination, which yielded fluorinated cyclohexadienes, and the dehydrofluorination of some fluorinated cyclohexadienes which yielded fluorobenzenes. The reactions are summarized in Scheme 7. The fluorinated cyclohexadienes that had a saturated carbon binding to a hydrogen atom, e0 la. 2a_ 3a. 5a and II in Scheme 7, were dehydrofluorinated.
et al.
On the other hand, the cylohexadienes that had no saturated carbon binding to a hydrogen atom, e0 3b, 4b. 6a. 6b and I in Scheme 7, were chemically stable. The quantity of electricity passed was controlled by considering the amounts of fluorobenzenes which were yielded on dehydrofluorination during the elec. trolyses because these were further electrochemically fluorinated. In the electrolyses, all fluorobenzenes were fluorinated into the fluorinated cyclohexadienes at 2.5V of the anode potential and the cylohexadienes were not fluorinated further.
F
(I)
-2e, 2FFF
FF H
(n=0~2)
F
F
Scheme 7.
Table 6. tgF NMR spectral data of fluorinated cyclohexadienes described in this study* Compound
Formula
Chemical shift (ppm)
J (Hz)t
Compound
Formula
•F H
eF
pd F
J (Hz)
CF
bF 1_.8
Chemical shift (ppm)
b 62.0 c
60.4
(m) on) (m) 38.7
39.8
(s)
d -17.0
F
lb
4a
bF °F
1~
b 70.4 c 37.4
(d) 5.0 on)
b 46.1
(d) 19.8 (m)
!TM
4b
P
c
57.2
F
i
i
~
F"
2a
aF t , F ~
H
bF
P
3n
a b c d
52.0 T'/.5 76.1 -27.7
a 31A b 48.9 c -16.2
on) (m) on) (m)
On) (m) (m) 38.7
bF a p ~ H 5a --
"F
°F
I~
, F ~ I
I~
~
6a
a 78.1 b 48.7 c -33.8
(m) on) (m)
a 73.6 b 44.4 c 33.5
(m) (t) 19.8
(In)
"F" " - - ~ -1~
'F •F
I~
bF b 30.4 c 32.8
3b •P
(d) 19.8 (m)
1~ 1~
6b
¢ 38.0 d 61.1
"F Fe
* Hexafinorobenzone was used as an internal standard in chloroform - (C]DCI~) solution. t Splitting and coupling constant: s=singlet, d,,doublet, t=triplet, m=multiplet.
(d) 17.4 (d) 19.8
(In) (in)
Electrochemicalfluorination in R,NF -mHF-Part II CONCLUSION In the anodic fluorination of mono-, di- and tri-fluorobenxenes in R,NF - mHF, the electrochemically stable corresponding fluorinated dyclohexadienes were obtained as the major products through 1,2- or l&addition of two fluorines to the substrate material in high yield. Therefore, the method is expected to be a useful preparation process of fluorocyclohexadienes which have been scarcely obtained as major products. Further, when relatively stable fluorinated cyclohexadienes, the Group B, were obtained in high yielded, the electrolyses will be utilized for the synthesis of some fluorobenxenes by dehydro0uorination with alkaline solution after the electrolyses. AcknowledgementsThe authors wish to thank Dr H. Kimoto, Government Industrial Research Institute, Nagoya, for advising us on the characterization of the products. This work was partially supported by Grant-in-Aid for Cooperative Research A (No. 04303007) from The Ministry of Edition, Science and Culture, Japan.
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49
5. H. Schmidt and H. D. Schmidt, J. Pm&t.Chim. 2, 250 (1955). 6. H. Schmidt and H. Meinert, Angew. Chem. 72, 109 (1960). 7. V. R. Koch and L. L. Miller, J. electroanal. Chem. 43, 318 (1973). 8. E. Laurent, H. Lefranc and R. Tardivel, Nouu. J. Chim. 8,345 (1984). 9. E. Laurent, B. Marquet and R. Tardivel, Tetrahedron &I,4431 (1989). 10. C. J. Ludman, E. M. McCarron and R. F. O’Mall9, J. electrochem. See. 119,874 (1972). 11. J. R. Ballinner and F. W. Teare. Electrochim. Acta 30. 1075 (1985).12. I. N. Rozhkov. Rus. Chem. Rev. 45.615 (1976). 13. I. N. Roahkov and I. Y. Alyev, .Tetr&dr& 31, 977 (1975). 14. F. Huha, E. B. Yeager and G. A. Olah, Electrochim. Acta 24,489 (1979). 15. J. H. H. M&us,. D. W. Sopher and W. Eilenherg, Anaew. Chem. Int. Ed. Encl. 28.927 (1989). 16. J. H. H. Meurs and W. &&erg, ?etraLdron 47, 705 (1991). 17. K. Momota, M. Morita and Y. Matsuda, Electrochim. Acta 38,619 (1993). 18. K. Momota, M. Morita and Y. Matsuda, Electrochim. Acta 38, 1123 (1993). 19. A. Zweig, R. G. Fisher and J. E. Lancaster, J. Org. cheat. 4s, 3597 (1980). 20. G. H. Goleman and W. F. Talhot. Org. Syn. Coil. Vol. 2, p. 592. New York (1966). 21. A. Roe. Oraanic Reactions. Vol. 5. D. 193. John Wilev. _I New York (11949). * 22. J. Burdon and I. W. Parsons, Tetrahedron 31, 2401 (1975). 23. J. Burdon, I. W. Parsons and J. C. Tatlow, Tetrahedron 28,43 (1972).