Oxidation of Diols Using Trialkylammonium fluorochromates(VI), R₃NH[CrO₃F], (R= CH₃, C₂H_5, C₃H₇ and C₄H₉) Supported on Silica Gel and Alumina

Shahriare Ghammamy^*a Ali Moghimi^b and Behnam Kohestani^c

^aDepartment of Chemistry, Faculty of Science, Imam Khomeini International University Qazvin,

^bDepartment of Chemistry Islamic Azad University Varamin, Varamin , Iran

^cDepartment of Chemical Engineering, Islamic Azad University, Arak Branch, Arak, Iran

Iran; Fax: (+98) 281-3780040 Correspondence should be addressed; E-mail: kamran9537@yahoo.com or shghamami@ikiu.ac.iror shghamami@yahoo.com

Abstract

A number of new chromium(VI) supported reagents Trialkylammonium fluorochromates(VI), R₃NH[CrO₃F], (R= CH₃, C₂H_5, C₃H₇ and C₄H₉) (TriRAFC) has been developed by reacting trialkylamine with CrO₃ and aqueous HF. These reagents are versatile reagents for the effective and selective oxidation of organic substrates, in particular for diols, under mild and heterogeneousconditions on silica gel and alumina.

Keywords: Trialkylammonium fluorochromates(VI); oxidation; diols; heterogeneous oxidants; silica gel; alumina

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 success¹. In particular; there is continued interest in the development of new chromium(VI) reagents for the effective and selective oxidation of organic substrates, under mild conditions. Of the large number of “mild” chromium based oxidizing agents available, many prove 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,⁷pyridinium fluorochromate⁸, 2, 2'–bipyridinium chlorochromate⁹, tripropylammonium fluorochromate¹⁰ and tetramethylammonium fluorochromate (TMAFC)¹¹. In continuation of our ongoing work on development of highly efficient oxidation process, we observed that the oxidation of diols with solid state supported Trialkylammonium fluorochromates(VI) R₃NH[CrO₃F], (R= CH₃, C₂H_5, C₃H₇ and C₄H₉) (TriRAFC) at room temperature could be carried out much more convenient.

Results and Discussion

Our supported reagents are remarkably effective in oxidation diols in high yields. Diols were subjected to oxidations with Trialkylammonium fluorochromates(VI), R₃NH[CrO₃F], (R= CH₃, C₂H_5, C₃H₇ and C₄H₉) (TriRAFC) absorbed on SiO₂and Al₂O₃. These oxidations take place under mild and completely heterogeneous conditions giving excellent yields (Scheme 1)(Table 1 and 2).

 (Scheme 1)

Table 1.Oxidation via TriRAFC Supported on Silica gel

Table 2.Oxidation via TriRAFC Supported on Alumina

The heterogeneous reaction mixtures are thoroughly stirred, at room temperature. Oxidations may also occur using only TriRAFC, in the absence of SiO₂ and Al₂O₃ but considerable improvements are observed in the presence of the absorbent. This implies that the SiO₂ and Al₂O₃ may act as a reaction medium, providing an effective heterogeneous surface area for the oxidation and at the same time making the work-up much more convenient.

The nature of the solvent does not appear to be particularly critical. Hydrocarbons, benzene, ethers and chlorinated hydrocarbons are equally effective, the practical choice being oriented by the solubility of the products and the desired reaction temperature. The chromium (VI) contents easily determined iodometrically.

The IR spectra of TriRAFC are similar to that of other fluorochromates¹². TriRAFC is soluble in water, dimethylformamide, acetonitrile, acetone and dichloromethane; they are only sparingly soluble in benzene, carbon tetrachloride, chloroform and hexane.

Experimental Section

CrO₃ (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/Visible measurements were made on an Uvicon model 922 spectrometer. Proton, ¹³C, ¹⁹F NMR were carried out on a Bruker AVANCE DRX 500 spectrometer at 500, 125, 470.66 MHz. All the chemical shifts are quoted in ppm using the high-frequency positive convention; ¹H and ¹³C NMR spectra were referenced to external SiMe₄ and ¹⁹F 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 (K₂S₂O₈) solution. The relative concentrations of carbon, hydrogen and nitrogen were obtained from the Microanalytical laboratories, Department of Chemistry, OIRC, Tehran. Melting points were measured on Electrothermal 9100 melting point apparatus.

A 1g (10 mmol) sample of chromium (VI) oxide, CrO₃, and 0.9ml (20 mmol) of 40% hydrofluoric acid were added to 20 mlof water in a 100 ml polyethylene beaker with stirring. After 7 min the homogeneous solution was cooled to ca.-2 ° C. To the resultant orange solution, trimethylamine (0.01mol) with hydrofluoric aside was added drop wise with stirring over a period of 0.5 h and stirring was continued for 0.5 h at 2 ^oC. The precipitated clear-orange liquid was isolated by filtration on a polyethylene funnel, washed with petroleum ether (3 × 60 ml) and dried in vacuum for 2 h at room temperature Yield: (86%); mp 126 °C, C₁₂H₂₈CrFNO₃: Calc. C, 20.11; H, 5.58; N, 7.82 Found: C, 20.08; H, 5.64; N, 7.69. I.R. (KBr): 912 cm^-1 ν₁(A₁) or ν(CrO₃), 636 cm^-1 ν₂(A₁) or ν(Cr-F), 950 cm^-1 ν₄(E) or ν(CrO₃), Electronic absorption at 22026 cm^-1, corresponded to ¹A₂¹E (ε= 174 M^-1cm^-1); 28735 m^-1 to ¹E¹E (ε= 664 M^-1cm^-1) and 36231 cm^-1 to ¹A₂¹E(ε= 1248 M^-1cm^-1). UV/Visible, ¹³C NMR, ¹H NMR and ¹⁹F NMR were all consistent with the TriMAFC structure. The above procedure can be scaled up to larger quantities, if desired. The pH of 0.01 M solution of TriMAFC in water was 2.9.

A 1g (10 mmol) sample of chromium (VI) oxide, CrO₃, and 0.9 ml (20 mmol) of 40% hydrofluoric acid were added to 20 mlof water in a 100 ml polyethylene beaker with stirring. After 5 min the homogeneous solution was cooled to ca. 0-2 ° C. To the resultant clear orange solution, triethylamine (1.400 ml, 10 mmol) was added dropwise with stirring over a period of 0.5 h and stirring was continued for 0.5 h at -4 °C. The precipitated yellowish-orange solid was isolated by filtration on a polyethylene funnel, washed with petroleum ether (3 × 60 ml) and dried in vacuum for 2 h at room temperature Yield: (88%); mp 132 °C, C₆H₁₆CrFNO₃: Calc. C, 32.57; H, 7.23; N, 6.33 Found: C, 32.08; H, 7.64; N, 6.44. I.R. (KBr): 904 cm^-1 ν₁(A₁) or ν(CrO₃), 648 cm^-1 ν₂(A₁) or ν(Cr-F), 948 cm^-1 ν₄(E) or ν(CrO₃), Electronic absorption at 22321 cm^-1, corresponded to ¹A₂¹E (ε= 270 M^-1cm^-1); 28735 m^-1 to ¹E¹E (ε= 845 M^-1cm^-1) and 36231 cm^-1 to ¹A₂¹E(ε= 1233 M^-1cm^-1). UV/Visible, ¹³C NMR, ¹H NMR and ¹⁹F NMR were all consistent with the TriEAFC structure. The above procedure can be scaled up to larger quantities, if desired. The pH of 0.01 M solution of TriEAFC in water was 3.45.

A 1g (10 mmol) sample of chromium (VI) oxide, CrO₃, and 0.9 ml (20 mmol) of 40% hydrofluoric acid were added to 20 ml of water in a 100 ml polyethylene beaker with stirring. After 5-7 min the homogeneous solution was cooled to ca. 3-5 ° C. To the resultant clear orange solution, tripropylamine (1.4ml, 10 mmol) was added dropwise with stirring over a period of 0.5 h and stirring was continued for 0.5 h at -4 °C. The precipitated yellowish-orange solid was isolated by filtration on a polyethylene funnel, washed with petroleum ether (3 × 60 ml) and dried in vacuum for 2 h at room temperature Yield: (98%); mp 142 °C, C₉H₂₂CrFNO₃: Calc. C, 41.05; H, 8.35; N, 5.31 Found: C, 41.22; H, 8.46; N, 5.02. I.R. (KBr): 904 cm^-1 ν₁(A₁) or ν(CrO₃), 647 cm^-1 ν₂(A₁) or ν(Cr-F), 949 cm^-1 ν₄(E) or ν(CrO₃), Electronic absorption at 22321 cm^-1, corresponded to ¹A₂¹E (ε= 140 M^-1cm^-1); 28725 m^-1 to ¹E¹E (ε= 667 M^-1cm^-1) and 35971 cm^-1 to ¹A₁¹E(ε= 1287 M^-1cm^-1). UV/Visible, ¹³C NMR, ¹H NMR and ¹⁹F NMR were all consistent with the TriPAFC structure. The above procedure can be scaled up to larger quantities, if desired. The pH of 0.01 M solution of TriPAFC in water was 3.3.

A 1g (10 mmol) sample of chromium (VI) oxide, CrO₃, and 0.9ml (20 mmol) of 40% hydrofluoric acid were added to 20 mlof water in a 100 ml polyethylene beaker with stirring. After 5-7 min the homogeneous solution was cooled to ca. 0-2 ° C. To the resultant clear orange solution, tributylamine (2.35ml, 10 mmol) was added dropwise with stirring over a period of 0.5 h and stirring was continued for 0.5 h at -4 °C. The precipitated yellowish-orange solid was isolated by filtration on a polyethylene funnel, washed with petroleum ether (3 × 60 ml) and dried in vacuum for 2 h at room temperature Yield: (84%); mp 134 °C, C₁₂H₂₈CrFNO₃: Calc. C, 47.20; H, 9.24; N, 4.58 Found: C, 46.92; H, 9.64; N, 5.20. I.R. (KBr): 914 cm^-1 ν₁(A₁) or ν(CrO₃), 634 cm^-1 ν₂(A₁) or ν(Cr-F), 950 cm^-1 ν₄(E) or ν(CrO₃), Electronic absorption at 22321 cm^-1, corresponded to ¹A₂¹E (ε= 177 M^-1cm^-1); 28735 m^-1 to ¹E¹E (ε= 701 M^-1cm^-1) and 35971 cm^-1 to ¹A₁¹E(ε= 1314 M^-1cm^-1). UV/Visible, ¹³C NMR, ¹H NMR and ¹⁹F NMR were all consistent with the TriBAFC structure. The above procedure can be scaled up to larger quantities, if desired. The pH of 0.01 M solution of TriBAFC in water was 3.15.

For supporting of the TriRAFC on silica gel or alumina, in synthesizing process silica gel or alumina (mesh 65-250) were added before the trialkylamine addition step. The supported reagents separated and washed by cold water and acetone. The capacity of the supported reagents were determined by stirring overnight 0.5 g of the supported reagents with 10 ml of 2 N aqueous potassium hydroxide, filtering off and titrating iodometrically the obtained chromate solution. The determined average capacity of the dried supported for the above reagents are 1-1.8 mmol of TriRAFC per 1 g of silica gel or alumina. The supported reagents so obtained did not noticeably lose their activities neither on storing in air at room temperature for several weeks nor on refluxing for 5 h in benzene or hexane.

To a stirred suspension of trialkylammonium fluorochromate absorbed on silica gel or alumina in dichloromethane (generally 5 cm³), a solution of the substrate in the minimum amount of dichloromethane were added dropwise. The completion of the reaction was followed by TLC using ether/petroleum ether (60/40) as eluant. The mixture is diluted with ether (1:1 v/v) and filtered through a short column of alumina to give a clear solution. The solution was evaporated and the residual product purified by distillation, recrystallization or column chromatography. The separated compounds characterized and it is found the product in that only one of the hydroxyl groups oxidized is main product. The progress of the reactions was also monitored and checked by UV/Visible spectrophotometry. The amount of the oxidant during the reaction was measured spectrophotometrically at 363 nm. A very small magnetic stirrer was designed at the cell (10 mm quartz cell) compartment just in the bottom of sample cell in the spectrophotometer to stir up the solution under study in cell. The reaction mixtures remained homogenous in the solvent system used.

The authors thank Dr. A. R. Mahjoub and Mrs. S. khorsandtabar for valuable discussions.

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