Tetrabutylammonium fluorochromate(vi) (TBAFC): a mild and efficient reagent for oxidation of organic substrates

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1 J. Serb. Chem. Soc. 70 (11) (2005) UDC : JSCS 3363 Original scientific paper Tetrabutylammonium fluorochromate(vi) (TBAFC): a mild and efficient reagent for oxidation of organic substrates SHAHRIARE GHAMMAMI a* and SEYED ABOLFAZL SEYED SAJADI b a Department of Chemistry, Faculty of Science, Azad Islamic University, Saveh, Iran ( shghammami@yahoo.com) and b Faculty of Chemistry, Iran University of Science and Technology, Tehran (Received 17 December 2004) Abstract: Orange solid tetrabutylammonium fluorochromate, (C 4 H 9 ) 4 NCrO 3 F, TBAFC, is easily synthesized by the reaction of tetrabutylammonium fluoride with CrO 3 in a 1:1 mole ratio. Tetrabutylammonium fluorochromate(vi) is a versatile reagent for the effective and selective oxidation of organic substrates, in particular of alcohols, under mild conditions. The facile oxidation of triphenylphosphine to triphenylphosphine oxide by TBAFC in CH 3 CN provides clear evidence for an oxygen-transfer reaction. Keywords: oxidant, chromium(vi) reagent, tetrabutylammonium fluorochromate, oxidation, organic substrates. INTRODUCTION The oxidation of organic substrates in aprotic solvents, under mild and neutral conditions, is important in modern organic synthesis. Therefore, the search for new oxidizing agents is of interest to synthetic organic chemists. Many such reagents have been developed in recent years with some success. 1 In particular, there is continued interest in the development of new chromium(vi) reagents for the effective and selective oxidation of alcohols under mild conditions. Of the large number of mild chromium based oxidizing agents available, many have proven to be impractical when the reactions are performed on a large (mol) scale, 2 although in recent years, significant improvements have been achieved by the use of new oxidizing agents. 3 5 Examples are: pyridinium chlorochromate, 6 pyridinium dichromate, 7 pyridinium fluorochromate, 8 2,2-bipyridinium chlorochromate, 9 tripropylammonium fluorochromate 10 and tetramethylammonium fluorochromate. 11 Tetrabutylammonium fluorochromate (TBAFC) was synthesized in the belief that this reagent could be used for the oxidation of organic substrates. There were two primary incentives for the selection of (C 4 H 9 )N + as the counter ion. Firstly, quater- * Corresponding author. Tel or Fax doi: /JSC G 1243

2 1244 GHAMMAMI and SAJADI nary ammonium ions are often used as phase transfer catalysts. This could make TBAFC a more efficient and stronger oxidizing agent. 12,13 Secondly, quaternary ammonium ions are used as crystal growth agents. Hence, using this counter ion improves the quality of the TBAFC crystals. 14 EXPERIMENTAL Materials and instruments CrO 3 (Merck, p. a) was used without further purification. Solvents were purified by standard methods. Infrared spectra were recorded as KBr disks on a Shimadzu model 420 spectrophotometer. The UV/Vis measurements were made on an Uvicon model 922 spectrometer. Proton, 13 C, 19 F-NMR were carried out on a Bruker AVANCE DRX 500 spectrometer at 500, 125, MHz, respectively. All the chemical shifts are quoted in ppm using the high-frequency positive convention; the 1 Hand 13 C-NMR spectra were referenced to external SiMe 4 and the 19 F-NMR spectra to external CFCl 3. Chromium was estimated iodometrically. In the case of the reduced product of the oxidant, chromium was determined after oxidizing with acidic peroxodisulfate (K 2 S 2 O 8 ) solution. The percent composition of carbon, hydrogen and nitrogen were obtained from the Microanalytical Laboratories of the Department of Chemistry, OIRC, Tehran. Preparation of tetrabutylammonium fluorochromate (TBAFC), C 16 H 36 CrFNO 3 Chromium(VI) oxide (0.383 g, 3.83 mmol) was dissolved in dry acetonitrile (25 ml) in a polyethylene beaker and a stoichiometric amount of tetrabutylammonium fluoride (1.0 g, 3.83 mmol) was added under stirring at room temperature. Within 5 min, a clear orange solution formed which upon refrigerating gave solid TBAFC, which was isolated by filtration. The solid was washed with dry isopropanol and diethyl ether, and dried under vacuum for 1 h. Satisfactory elemental analysis was obtained. Yield: 1.1g (80 %); mp, 212 o C. C 16 H 36 CrFNO 3 : Calcd. %C, 53.18; %H, 9.97; %N, Found: %C, 53.55; %H, 10.02; %N, IR. (KBr): 910 cm -1 1 (A 1 )or (CrO 3 ), 645 cm -1 2 (A 1 )or (Cr-F), 950 cm -1 4 (E) or (CrO 3 ). The UV/Vis, 13 CNMR, 1 HNMRand 19 F NMR spectra were all consistent with the TBAFC structure. The above procedure can be performed on a 250 g scale without any difficulty. General procedure for the oxidation of organic substrates In order to examine the oxidative ability of TBAFC, the following typical procedure was employed: In a small-scale experiment, TBAFC (1 mmol) was suspended in dichloromethane (ca. 2 ml) and an alcohol (1 mmol in 0.5 to 1.5 ml of CH 2 Cl 2 ) was rapidly added at room temperature and the resulting mixture stirred vigorously for the appropriate time (Table I). The molar ratio of substrate to oxidant was 1:1 in the case of alcohols (1) and 1:2 in the case of polycyclic arenes (3). The solution became homogeneous briefly before the black-brown reduced reagent precipitated. The progress of the reaction was monitored by TLC and UV/Vis spectrophotometry (at 350 nm). Analysis of the reaction mixture for the corresponding carbonyl compounds was accomplished by the procedure reported in previous papers Reaction of TBAFC with triphenylphosphine The reaction was performed under a nitrogen atmosphere in a dry 100 ml round-bottomed flask under efficient stirring. To a vigorously stirred solution of 1.50 g (5.70 mmol) of triphenylphospine in 35 ml of acetonitrile, 2.26 g (6.28 mmol) of TBAFC was added in two instalments within about one minute, maintaining the substrate: oxidant ratio at 1:1.1. An exothermic reaction commenced instananeously, and was completed within 5 min. The solution was cooled and the reduced TBAFC product separated by centrifugation. The mother liquor and washings were filtered through a short silica gel column (7 2cm 2 ). The contents of the column were thoroughly washed with ether (3 35 ml) and filtered. The combined filtrates were evaporated on a steam bath, and the product was char-

3 TBAFC IN OXIDATION OF ORGANIC SUBSTRATES 1245 acterized as triphenylphosphine oxide by elemental analyses and IR spectral studies. Yield: 1.55 g (98 %). mp. 155 C (Lit. 8 mp C). RESULTS AND DISCUSSION The results obtained with tetrabutylammonium fluorochromate are very satisfactory and show the new reagent to be a valuable addition to the existing oxidizing agents. Table I summarizes the products, yields and reaction times of TBAFC with various substrates. The oxidative effect of TBAFC on polycyclic rings appears more pronounced than on alcohols (Table I). This could be due to the interaction of the electrons with the quaternary cation, as suggested by Felder 15 and Pullman. 16 The probable importance of such interaction in biological recognition processes has been emphasized by Dougherty et al. on the basis of a number of observations made by their group and others on similar cations. 17 Granier et al. 18 have already linked the higher reactivities of MCrO 3 F(M =K, Rb, Cs and NH + 4 ), with the extent of symmetry about the Cr. They concluded that the inequality between the Cr O and the Cr F bonds is responsible for the higher reactivity. The reason for this inequality is due to the CH...F hydrogen bond which forms between the butyl hydrogen of the cation and the fluoride atom of the anion. This type of hydrogen bonding has been studied by Harmon. 19 TBAFC appears to be a stronger reagent than the others; this could be due to its lower symmetry. It has also been found that this reagent has certain advantages over similar oxidizing agents in terms of the amounts of oxidant and solvent required, and especially in the short reaction times required and in the higher yields of the product (Table I). 8,20,21 Due to the stability and solubility of TBAFC, reactions can be performed at room temperature and the separation of the products is facile. During the reactions, the color of the oxidant changes from orange to brown, providing visual means for ascertaining the progress of the oxidation. The mechanism for this oxidation is still unclear. However, it can be assumed that the mechanism of oxidation is similar to that of other fluorochromates. In addition, this oxidant and the oxidation conditions can be used for the synthesis of highly functionalized molecules. Tetrabutylammonium fluorochromate in dichloromethane oxidizes primary and secondary alcohols to, respectively, the corresponding aldehydes or ketones in high yields (Table I). Tetrabutylammonium fluorochromate in dichloromethane also oxidizes anthracene (3a) and phenanthrene (3b) to anthraquinone (4a) and phenanthraquinone (4b) in 95 % and 98 % yields, in 60 and 45 min, respectively. This reagent functions as efficiently as activated manganese dioxide or Collins reagent. 22 The reaction of triphenylphosphine (5a) with TBAFC (mole ratio of PPh 3 :TBAFC = 1:1.1) in acetronitrile was carried out at room temperature and triphenylphosphine oxide (6a) was obtained in quantitative yield. This provides a clear-cut example of an ox-

4 1246 GHAMMAMI and SAJADI TABLE I. Oxidation of alcohols and polycyclic arenes with TBAFC Substrate Substrate/ oxidant ratio Time min Product Yield/%

5 TBAFC IN OXIDATION OF ORGANIC SUBSTRATES 1247 ygen transfer reaction involving TBAFC, and the results may also be useful in defining other related reactions. Tetrabutylammonium fluorochromate is easily prepared in 80 % yield from chromium(vi) oxide, tetrabutylammonium fluoride in a molar ratio of 1:1. The bright orange crystalline reagent can be stored in polyethylene containers for long periods without decomposition. The chromium(vi) content may be easily determined iodometrically. The IR spectrum is similar to those of other fluorochromates. 22 Tetrabutylammonium fluorochromate is soluble in water, dimethylformamide, acetonitrile, dimethyl sulfoxide and dichloromethane; it is only sparingly soluble in benzene, carbon tetrachloride, chloroform and hexane. Acknowledgement: The authors thank Dr. A. R. Mahjoub and Mr. K. Ghoddamy for valuable discussion. IZVOD TETRABUTILAMONIJUM-FLUOROHROMAT(VI) (TBAFC): BLAG I EFIKASAN REAGENS ZA OKSIDACIJU ORGANSKIH SUPSTRATA SHAHRIARE GHAMMAMI 1a i SEYED SAJADI SEYED ABOLFAZL b 1 Department of Chemistry, Faculty of Science, Azad Islamic University, Saveh, Iran ( shghammami@yahoo.com) and 2 Faculty of Chemistry, Iran University of Science and Technology, Tehran Naranyasti ~vrsti tetrabutilamonijum-fluorohromat (C 4 H 9 ) 4 NCrO 3 F, TBAFC,se lako sintetizuje reakcijom tetrabutilamonijum-fluorida sa CrO 3 pri molskom odnosu 1:1. Tetrabutilamonijum-fluorohromat(VI) je raznovrstan reagens za efektivne i selektivne oksidacije organskih supstanci, naro~ito alkohola, u blagim uslovima. Laka oksidacija trifenilfosfina u trifenilfosfin-oksid sa TBAFC u CH 3 CN je jasan dokaz reakcije prenosa kiseonika. (Primqeno 17. decembra 2004) REFERENCES 1. L. F. Fieser, M. Fieser in Reagents for Organic Synthesis; Vol. 1-11, Wiley, New York, E. J. Corey, J. W. Suggs, Tetrahedron Lett. 16 (1975) G. Maier, H. R. Reisenauer, M. D. Marco, Angew. Chem. Int. Ed. Engl. 38 (1999) A. Bhandari, P. K. Sharma, K. K. Banerji, Indian J. Chem. Sec. A. 40A (2001) S. Meenahshisundaram, R. Soctaungam, Collect. Czech. Chem. Commun. 66 (2001) E. J. Corey, D. L. Boger, Tetrahedron Lett. 19 (1978) E. J. Corey, G. Schmidt, Tetrahedron Lett. 20 (1979) M. N. Bhattacharjee, M. K. Chauduri, H. S. Dasgupta, Synthesis (1982) F. S. Guziec, F. A. Luzzio, Synthesis (1980) S. Ghammamy, A. Hashemzadeh, Bull. Korean Chem. Soc. 25 (2004) A. R. Mahjoub, S. Ghammami, M. Z. Kassaee, Tetrahedron Lett. 44 (2003) C. A. Deakyne, M. J. Meot-Ner, J. Am. Chem. Soc. 107 (1985) J. March, Advanced Organic Chemistry, Wiley, New York, 1985, p. 320

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