Fluorocarbanion chemistry. Tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane and companions

Journal of Fluorine Chemistry 102 (2000) 185±188

Fluorocarbanion chemistry. Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methane and companions Robert Filler*, August E. Fiebig Jr., Braja K. Mandal

Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, IL 60616-3793, USA Received 28 June 1999; received in revised form 19 July 1999; accepted 16 August 1999 Dedicated to Professor Paul Tarrant on the occasion of his 85th birthday.

Abstract The titled compound (2) is prepared by oxidation of tris(4-amino-2,3,5,6-tetra¯uorophenyl) methane with 98% H2O2 and tri¯uoroacetic anhydride. Compound 2, a strong carbon acid, forms long-persisting deep blue solutions of the anion on reaction with alkalis, amines, and alcohols. Oxidation of the anion of 2 with KMnO4 provides the corresponding triarylmethyl radical. Reaction of 2 with LiOCH3 in methanol gives the stable blue lithium salt. The 4-cyano analog of 2 has also been prepared. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methane; Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methide lithium salt; Tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methyl radical; Tris(4-cyano-2,3,5,6-tetra¯uorophenyl) methane

1. Introduction In an earlier paper [1] we reported the preparation and ion-pair acidities of the carbon acid tris(penta¯uorophenyl) methane (1) and its p-methyl and p-methoxy analogs. While 1 can be obtained by Friedel±Crafts alkylation of penta¯uorobenzene with CHCl3 [2], a more convenient route is the reaction of tris(penta¯uorophenyl) methanol [3,4] with PBr3 (Eq. (1)). Subsequent studies using dimsyl ion as base [5,6] con®rmed the remarkable enhancement of equilibrium acidities of triarylmethanes by poly¯uoroaryl substitution. Generally, the replacement of each phenyl group by C6F5- or XC6F4- increased the acidity by 5±6 pK units, resulting in the of pKa values between 13.4 and 16.2.

acids. While tris(4-nitrophenyl) methane is prepared directly, in low yield, from triphenylmethane and fuming nitric acid, 2 is best synthesized from 1 in a three step sequence (Scheme 1). Numerous attempts to prepare the precursor tris(4-amino-2,3,5,6-tetra¯uorophenyl) methane (4) by direct amination of 1 [7] gave inconsistent results, with poor yields. We abandoned this approach and converted 1 to the hydrazino compound 3, which was reduced to 4 with 48% HI [8]. Oxidation of 4 with 98% H2O2 in tri¯uoroacetic anhydride afforded 2 in 77% crude yield.

HBr

…C6 F5 †3 COH ‡ PBr3 ! ‰…C6 F5 †3 Cdÿ Brd‡ Š ! …C6 F5 †3 CH 70%

1

(1) 2. Results and discussion We now wish to report the synthesis and distinctive chemistry of tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methane (2), a much more acidic member of this series of carbon *

Corresponding author.

Scheme 1. Synthesis of tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane.sc1

0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 2 4 2 - 0

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While the large ®eld/inductive effect of C6F5- and XC6F4groups causes the pronounced exaltation in acidities [5], the added resonance effect of the nitro groups in the methide ion of 2 contributes a major enhancement in acidity, though not as great as might be anticipated. Previous studies [9] demonstrated that the ®rst p-NO2 group introduced into triphenylmethane sharply increases the acidity, while successive substitutions of the second and third p-nitro groups result in a marked attenuation, due to steric inhibition of resonance and saturation effects. Thus, 2 reacts with NaOH and amines to form long-persisting intense cobalt blue solutions of the anion (Eq. (2)). Moreover, 2 also readily protonates alcohols, including tri¯uoroethanol (pKa 11.3), but not penta¯uorophenol (pKa 5.5). By comparison, tris(4nitrophenyl) methane (pKa 12.7), [9] reacts with NaOH to form a blue solution of anion which fades in a short time. On the basis of these observations and the previous studies, the pKa of 2 is estimated to be in the range of 6±7. The remarkable stability of the anion suggested that 2 might form a stable free radical. Earlier, we developed a simple method for generating stable triarylmethyl radicals directly from triarylmethanes by alkaline permanganate oxidation of the corresponding carbanion (Eq. (2)) [10].

(2) The radical is extracted with oxygen-free toluene to form a yellowish-brown solution. In this way, 2 is easily converted to the radical 5, whose ESR spectrum is shown in Fig. 1. Hyper®ne coupling constants for 5 and its non¯uorinated analog are listed in Table 1. The reaction of 2 with lithium methoxide in methanol gives the stable deep blue lithium salt, lmax 608 nm (propylene carbonate). Further studies on this salt will be published separately. Finally, another interesting member of this series, tris(4cyano-2,3,5,6-tetra¯uorophenyl) methane (7) was prepared in low yield by reaction of the 4-bromo analog (6) with cuprous cyanide (Eq. (3)). Compound 7 behaves much like 2 in forming colored solutions (orange-brown) when treated with weak bases.

(3) 3. Experimental General: All starting materials and solvents were of the highest quality available. NMR spectra were measured on a Varian HA-60 spectrometer. Infrared spectra were recorded

on a Perkin-Elmer 257 grating spectrometer or a PerkinElmer 2000 FTIR spectrometer. Ultraviolet and visible spectra were measured on a Perkin-Elmer Lambda-19 spectrometer. Mass spectra were recorded on a Varian CH7 mass spectrometer. Electron spin resonance spectra were obtained with a standard Varian X-band esr spectrometer with 100 KHz ®eld modulation. 3.1. Tris(pentafluorophenyl) methane (1) Tris(penta¯uorophenyl) methanol (5.3 g, 0.0l mol) was placed in a 100 ml three-necked ¯ask ®tted with a re¯ux condenser and magnetic stirrer. Phosphorus tribromide (2.7 g, 0.0l mol) was added slowly over a period of 15 min. The mixture was heated at 1308C and stirred constantly for 3 h. After cooling, carbon tetrachloride and water were added to the oily reaction mixture. The mixture was stirred and the carbon tetrachloride layer was separated. After drying over magnesium sulfate, the carbon tetrachloride layer was evaporated to dryness in vacuo. A small amount of methanol was added to the dried mass to cause a white solid to separate immediately. The solid was recrystallized from methanol to give 3.6 g (70%) of pure white product, m.p. l58±l59.58C. Anal. Calcd. for C19HF15: C, 44.38; H, 0.19; Found: C, 44.19; H, 0.44; IR 3000 cmÿ1; 1H NMR d 6.20 (m). 3.2. Tris(4-hydrazino-2,3,5,6-tetrafluorophenyl) methane (3) (nc) A mixture of tris(penta¯uorophenyl) methane (8.04 g, 15.6 mmol), 7.14 g (15.9 mmol) of 99±100% hydrazine hydrate and 40 ml of p-dioxane was heated under re¯ux for 21.5 h and then poured into 100 ml of water. The resulting mixture was extracted several times with ether and the combined ethereal layers thoroughly washed with water. The ethereal layer was then extracted with 3 N hydrochloric acid and the combined acid layers were washed with ether to remove any neutral material. The acid layer was carefully made basic with sodium hydroxide solution and then extracted with ether. The resulting ethereal layer was washed with water, dried over anhydrous magnesium sulfate, and evaporated in vacuo to give a dark oil which solidi®ed upon treatment with benzene. The crude solid was crystallized twice from benzene using Darco G-60 charcoal for decolorization to give large golden needles. After drying in a vacuum desiccator, the material weighed 7.53 g (87.5%). This compound does not have a characteristic melting point since temperatures of melting or decomposition ranging from 728C to 1498C were obtained, depending on the rate of heating; IR (nujol): 3346 and 3262 (w, N±H), 1658 (m, aromatic C=C), 1497 (s, aromatic ring), 1467 (vs, aromatic ring), 1003 and 982 cmÿ1 (s, C±F); 1 H NMR (CH3CN): d 3.88 [broad (17.5 Hz wide at half height) s, 6, ±NH2), d 5.84 [broad (9 Hz wide at half height) s, 3, NH), and d 6.11 (s, 1, CH); Accurate mass (by high

R. Filler et al. / Journal of Fluorine Chemistry 102 (2000) 185±188

187

Fig. 1. Electron spin resonance spectrum of the tris(4-nitro-2,3,5,6-tetrafluorophenyl) methyl radical.

resolution mass 550.07747.

spectrometry):

550.076546.

Calcd.:

3.3. Tris(4-amino-2,3,5,6-tetrafluorophenyl) methane (4) from tris(4-hydrazino-2,3,5,6-tetrafluorophenyl) methane The hydrazine reduction method of Birchall and Haszeldine [8] was applied to this synthesis. A solution of 20.0 g (36.3 mmol) of tris(4-hydrazino-2,3,5,6-tetra¯uorophenyl) methane in 136 ml of 48% hydroiodic acid was re¯uxed with stirring for 44 h. The resulting solution was poured into 300 ml of water to yield an oil which solidi®ed. The solid was ®ltered off, washed with water, dried, and crystallized from methylcyclohexane to yield 10.7 g (58.2%) of product as light yellow leaves, m.p. 170± 1738C (lit. [7] m.p. 170±1728C); IR (CHCl3): 3484 (w, N±H), 3392 (m, N±H), 1668 (s, N±H), 1606 (w, aromatic C=C), 1517 (s, aromatic ring), 1503 (vs, aromatic ring), 1013 and 976 cmÿ1 (vw, C±F); 1H NMR (CDCl3): d 6.10 (s, 1, CH) and 4.00 (broad s, 6, NH2). Table 1 Hyperfine coupling constants for the tris(4-nitrotetrafluorophenyl)- and tris(4-nitrophenyl) methyl radicals Position (atom)

(4±O2NC6F4)3C. a (G)

(4±O2NC6H4)3C. a (G)

para (N) ortho (F or H) meta (F or H)

0.55 1.85 1.35

0.69 2.52 1.14

3.4. Tris(4-nitro-2,3,5,6-tetrafluorophenyl) methane (2) (nc) To a solution of 12 ml of tri¯uoroacetic anhydride in 40 ml of dichloromethane, was added dropwise, 5 ml of 98% hydrogen peroxide. (Caution! This powerful oxidant is hazardous and should be handled with great care.) After the exothermic reaction subsided, the mixture was re¯uxed for 40 min. A solution of 6.00 g (11.9 mmol) of tris(4-amino2,3,5,6-tetra¯uorophenyl) methane in 160 ml of dichloromethane was added to the re¯uxing oxidizing mixture over a period of 2.7 h. The reaction mixture turned lime-green after the start of the addition. 10 min after completion of the addition, 4 ml of tri¯uoroacetic anhydride was added. After an additional 2.6 h of re¯ux, 3 ml of 98% hydrogen peroxide, followed by 8 ml of tri¯uoroacetic anhydride was added slowly to the re¯uxing mixture. The green reaction mixture turned to a golden yellow. After an additional 3 h of re¯ux, 3.5 ml of 98% hydrogen peroxide, followed by 10 ml of tri¯uoroacetic anhydride was added dropwise to the mixture and the solution was allowed to re¯ux under nitrogen for 14.8 h. The mixture was then poured into 300 ml of water, the layers were separated and the aqueous phase extracted three times with dichloromethane. The combined organic phases were washed three times with water, dried over anhydrous magnesium sulfate, and evaporated in vacuo to a crude oil. Trituration of the oil with hexane and hexane±dichloromethane mixtures in the cold gave 5.48 g (77.5%) of crude yellow powder. A 1.00 g sample of this material was chromatographed on a 18.5  2 cm2 column (ca. 13 g) of Baker Analyzed silica

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gel powder No. 3405. The column was eluted with a 50:50 hexane±dichloromethane mixture, and the ®rst 300 ml of eluted solvent were evaporated in vacuo to give 0.136 g (13.6%) of material as a white powder, m.p. ca. 1758C. The next 475 ml of eluted solvent yielded 0.321 g (32.1%) of product as a slightly yellow solid, m.p. 186±1878C. Further elution with 2 l of the mixed solvent yielded only traces of dark sticky solids; FTIR(KBr): 2924 (C±H), l627 (m, aromatic C=C), 1556 (vs, aromatic ring or ±NO2),1500 (s, aromatic ring or NO2), l358 (s, ±NO2), and 1030 (s) and 1005 cmÿ1 (s, C±F); 1H NMR (CDCl3): d 6.39 (s, C±H); Anal. Calcd. for C19HF12N3O6: C, 38.34; H, 0.17; N, 7.06. Found: C, 37.93; H, 0.41; N, 6.57. 3.5. Tris(4-nitrophenyl) methane Triphenylmethane (60.0 g, 0.246 mol) was added in small portions over 45 min to 600 ml of well-stirred 90% fuming nitric acid maintained at ca. ÿ408C. The yellow reaction mixture was then stirred at ÿ388C to ÿ468C for 35 min and poured onto 2 l of crushed ice. The crude solid was ®ltered off, washed with water, with saturated aqueous sodium bicarbonate solution until the ®ltrate was basic, and then with water until the ®ltrate was neutral. The dry solid was crystallized four times from toluene (500±600 ml each time) to give 21.5 g (23.1%) of product as yellow leaves, m.p. 214±2168C (lit. m.p. 212±2138C [12]; IR (nujol): 3100 and 3067 (w, aromatic C±H), 1606 and 1596 (s, aromatic C=C), 1517 and 1348 (vs, NO2), and 842 and 747 cmÿ1 (s, NO2). 3.6. Tris(4-bromo-2,3,5,6-tetrafluorophenyl) methane (6) The Friedel-Crafts reaction developed by Beckert and Lowe [2] was applied, with modi®cation, to this synthesis. A 150-ml Hoke stainless steel gas-sampling cylinder equipped with a Hoke No. 343 valve was used as the reaction vessel. The bomb was dried overnight in a small oven at ca. 908C and cooled under a nitrogen stream. When cool, the bomb was charged with 17.5 g (0.131 mol) of anhydrous aluminum chloride powder, 10.02 g (43.74 mmol) of 2,3,5,6tetra¯uorobromobenzene and 1.74 g (14.6 mmol) of chloroform. The valve threads were wrapped with Te¯on pipe joint tape and the valve assembly was screwed down tight on the bomb. The bomb was inverted several times to mix the contents and then heated in an upright electric furnace at 84± l358C for 7.3 h. After the reactor cooled, the hydrogen chloride gas was bled off and the violet-colored solids were broken up and extracted with approximately 1 l of carbon tetrachloride. The carbon tetrachloride was evaporated in vacuo to leave a crude yellow solid which was crystallized twice (once using Darco G-60 charcoal) from hexane to give 2.27 g (22.4%) of product as a white microcrystalline solid,

m.p. 136±1398C (lit. [11] m.p. 139±139.58C); IR (CHCl3): 1637 (w, aromatic C=C), 1488 (vs, aromatic ring), 1464 (m, aromatic ring) 1013 and 962 cmÿ1 (w, C±F); 1H NMR (CDCl3): d 6.31 (s, C±H). 3.7. Tris(4-cyano-2,3,5,6-tetrafluorophenyl) methane (7) (nc) A mixture of 2.57 g (3.69 mmol) of tris(4-bromo-2,3,5,6tetra¯uorophenyl) methane, 1.32 g (14.8 mmol) of cuprous cyanide, and 100 ml of puri®ed N,N-dimethylformamide was re¯uxed for 10 h. A mixture of 6.5 g ferric chloride and 26 ml of 1 N hydrochloric acid was added to the blood red reaction mixture, which was heated under re¯ux for an additional hour. The reaction mixture was poured onto ca. 300 ml of crushed ice and the resulting solid was collected by ®ltration. The crude solid was dissolved in benzene, the solution was ®ltered and mixed with hexane to yield an orange solid. Three crystallizations from benzene±hexane (once using Darco G-60 charcoal) gave 0.209 g (10.6%) of product, m.p. 197±2028C (dec.); IR (CHCl3): 2250 (w, C>N), 1651 (w, aromatic C=C), 1496 (vs, aromatic ring) and 1014 cmÿ1 (w, C±F); 1H NMR (CDCl3): d 6.41 (s, C± H); Anal. Calcd for C22HF12N3: C, 49.37; H, 0.19. Found: C, 49.11; H, 0.44. Acknowledgements We thank Dr. Shrikant V. Kulkarni for conducting the ESR studies on tris(4-nitro-2,3,5,6-tetra¯uorophenyl) methyl radical. References [1] R. Filler, C.S. Wang, Chem. Commun. (1968) 287. [2] W.F. Beckert, J.U. Lowe Jr., J. Org. Chem. 32 (1967) 582. [3] R. Filler, C.S. Wang, M.A. McKinney, F.N. Miller, J. Am. Chem. Soc. 89 (1967) 1026. [4] S.V. Kulkarni, R. Schure, R. Filler, J. Am. Chem. Soc. 95 (1973) 1859. [5] F.G. Bordwell, J.C. Branca, J.E. Bares, R. Filler, J. Org. Chem. 53 (1988) 780. [6] F.G. Bordwell, X.-M. Zhang, R. Filler, J. Org. Chem. 58 (1993) 6067. [7] T.N. Gerasimova, E.G. Lokshina, V.A. Barkhash, N.N. Vorozhtsov Jr., J. Gen. Chem. USSR 376 (1967) 1232. [8] J.M. Birchall, R.N. Haszeldine, A.R. Parkinson, J. Chem. Soc. (1962) 4966. [9] F.G. Bordwell, J.-P. Cheng, A.V. Satish, C.L. Twyman, J. Org. Chem. 57 (1992) 6542. [10] S.V. Kulkarni, A.E. Fiebig, R. Filler, Chem. Ind. (1970) 364. [11] T.N. Gerasimova, V.A. Barkhash, N.N. Vorozhtsov Jr., J. Gen. Chem. USSR 383 (1968) 510. [12] K. Bowden, R. Stewart, Tetrahedron 21 (1965) 261.