Next Article in Journal
Electron-Induced (EI) Mass Fragmentation is Directed by Intra- molecular H-Bonding in Two Isomeric Benzodipyran Systems
Previous Article in Journal
Synthesis of Novel Azo Schiff Bases and Their Antibacterial and Antifungal Activities
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Synthesis and Reactions of N-Methylbenzylammonium Fluorochromate(VI) on Silica Gel, a Selective and Efficient Heterogeneous Oxidant

Department of Chemistry, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran
*
Author to whom correspondence should be addressed.
Molecules 2004, 9(10), 825-829; https://doi.org/10.3390/91000825
Submission received: 28 June 2004 / Revised: 11 June 2004 / Accepted: 15 June 2004 / Published: 30 September 2004

Abstract

:
N-Methylbenzylammonium fluorochromate(VI) (MBAFC) is easily synthesized by addition of N-methylbenzylamine to an aqueous solution of CrO3 and HF. MBAFC shows selectivity in the oxidation of aryl alcohols to their corresponding aldehydes and ketones under mild conditions. The durability, ease of work up and efficiency of MBAFC are considerably increased upon its absorption on silica gel.

Introduction

Chromium(VI) based reagents are widely used in modern organic synthesis for the oxidation of a variety of compounds under anhydrous and aprotic conditions, including primary and secondary alcohols. Extensive work has led to the development of a good number of these oxidants such as the Collins reagent [1], chromium trioxide-3,5-dimethylpyrazole complex [2], pyridinium chloro-chromate (PCC) [3], pyridinium dichromate (PDC) [4], 2,2'-bipyridinium chlorochromate (BIPCC) [5], pyridinium fluorochromate [6,7], quinolinium fluorochromate [8], quinolinium chlorochromate [9], 3,5-dimethylpyrazolium fluorochromate [10], 2,6-dicarboxypyridinium chlorochromate [11,12], N-methylpiperidinium chlorochromate [13], tetramethylammonium fluorochromate(VI) (TMAFC) [14] and N-methylbenzylammonium fluorochromate(VI) (MBAFC) [15]. These reagents may all be used for the oxidation of alcohols to corresponding aldehydes and ketones. This manuscript introduces N-methylbenzylammonium fluorochromate(VI) (MBAFC) absorbed on silica gel as a new promising reagent with improved efficiency, selectivity and durability, for the oxidation of aryl alcohols to their corresponding aldehydes and ketones, under mild conditions.

Results and Discussion

Different primary and secondary alcohols 1 were subjected to oxidations with N‑methylbenzylammonium fluorochromate(VI) (MBAFC) absorbed on wet SiO2 (50% w/w), in dichloromethane (Scheme 1). These oxidations take place under mild and completely heterogeneous conditions giving excellent yields (Table 1).
Scheme 1.
Scheme 1.
Molecules 09 00825 g001
The heterogeneous reaction mixtures are thoroughly stirred, at room temperature, for 10-33 minutes. The corresponding aldehyde and ketone products 2 can then be easily isolated by simple filtration and evaporation of the solvent. Yields obtained using MBAFC/silica gel are mostly higher than those reported in the literature, while both reagent ratios as well as reaction times are lower (Table 1).
Oxidations may also occur using only MBAFC, in the absence of wet SiO2, but considerable improvements of both the yields and the corresponding reaction times are observed in the presence of the absorbent. This implies that the wet SiO2 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 selectivity of MBAFC is well demonstrated through its oxidations of 4-chlorobenzyl alcohol (yield of product: 91 %), and/or benzyl alcohol (yield of product: 93%), in the presence of equimolar amounts of 2‑phenylethyl alcohol (no product detected). Neither carboxylic acids over-oxidation products nor other by-products are formed upon oxidation of alcohols via MBAFC (Table 1). Functional groups such as methoxy and methyl attached on the phenyl ring are inert to this reagent. In addition, we have shown that this reagent does not oxidize a variety of other substrates, including diphenyl sulfide, thiophenol, 3,5-dimethoxyphenol, 2-benzyl-3,4-dihydro-2H-pyran, benzyloxytrimethylsilane, N-methyl-2-phenylthioacetamide and sodium 2‑hydroxyimino-malonate.
Table 1. Oxidations via MBAFC/Silica gel, MBAFC, PFC a and IQFC b
Table 1. Oxidations via MBAFC/Silica gel, MBAFC, PFC a and IQFC b
AlcoholProduct c MBAFC/SilicagelMBAFCPFCIQFC
Molar Ratio ROH:OXTime (min)Yield c (%)Time (min)Yield (%)Time (min)Yield (%)Time (min)Yield (%)
Benzyl alcohol 1aBenzaldehyde 2a1:1 (1:1.25)a1093608545906091
p-Chlorobenzyl alcohol, 1bp-Chlorobenzaldehyde 2b1:115917589---NR
p-Methoxybenzylalcohol, 1cp-Methoxybenzaldehyde 2c1:1 (1:1.25)a179380805090-NR
p-Methylbenzylalcohol, 1dp-Methylbenzaldehyde 2d1:120927089---NR
p-Nitrobenzyl alcohol, 1ep-Nitrobenzaldehyde 2e1:123906082---NR
p-Bromobenzylalcohol, 1fp-Bromobenzaldehyde 2f1:130918075---NR
Cyclohexanol 1gCyclohexanone 2g1:1 (1:1.5)a249275652108924090
Cyclopentanol 1hCyclopentanone 2h1:119879570---NR
1-Phenylethanol 1lAcetophenone 2l1:1339018080---NR
3-Phenyl-2-propen-1-ol, 1m3-Phenylpropenal 2m1:1 (1:1.5)a268915080--30070
Benzoin 1nBenzil 2n1:1 (1:1.25)a309880801509818098
a Pyridinium fluorochromate used as the oxidant [7]b Isoquinolium fluorochromate used as the oxidant [16].c Products were characterized by comparison (NMR, IR, TLC and m.p./b.p.) with authentic samples.

Conclusions

A new reagent, N-methylbenzylammonium fluorochromate(VI) (MBAFC) absorbed on silica gel, is easily synthesized. It proves to be a low cost, readily available and highly selective oxidizing reagent for a variety of aromatic alcohols. Its advantages include higher yields, shorter reaction times, lower alcohol/oxidant molar ratios, application to pH sensitive molecules and ease of separation of products. Moreover, during the reaction, the color of the oxidant changes from orange to brown, thus providing a visual means for ascertaining the progress of the oxidation. Many functional groups are inert towards this oxidizing agent, including thiols, sulfides and phenols, enhancing the usefulness of the oxidant and the oxidation conditions for the synthesis of highly functionalized molecules.

Experimental

General

N-Methylbenzylamine and chromium trioxide were obtained from Fluka (Buchs, Switzerland). Melting points are measured on an Electrothermal 9100 apparatus and are uncorrected. Elemental analyses (C, H, and N) were performed using a Heraeus CHN-O-Rapid analyzer. IR spectra were recorded on a Bomen FT-IR-MB100 Spectrometer. 1H-NMR and 13C-NMR spectra were measured on a JNM-EX90A a BRUKER-DRX500 AVANCE instrument, respectively.

Preparation of N-methylbenzylammonium fluorochromate(VI)

Chromium(VI) oxide (CrO3; 1g, 0.01 mol) was dissolved in water (3 mL) in a polyethylene beaker and 40% hydrofluoric acid (0.65 mL, 0.015 mol) was added with stirring. After 5 min, the homogenous solution was cooled to 0°C. N-Methylbenzylamine (1.35 mL, 0.01 mol) was carefully added over 10 min. The resulting solution was stirred at 0°C for 30 minutes. A yellow-orange solid precipitated. The crystals were collected on a sintered glass funnel and dried under vacuum (yield: 85%); IR υ (KBr): 871 cm-1(m, Cr-O), 941 cm-1 (s, Cr-O), 640 cm-1 (m, Cr-F); Anal. Calcd. for C8H12NCrO3F: C, 39.8; H, 5.0; N, 5.8. Found: C, 40.1; H, 5.2; N, 5.5; 1H-NMR (D2O) δ: 2.71 (s, 3H, CH3), 4.20 (s, 2H, CH2), 4.99 (s, 2H, NH2), 7.4 (s, 5H, Ph); 13C-NMR (D2O) δ: 42.09 (C methyl), 52.18 (C benzyl), 129.25 (C para), 129.61 (C ortho), 129.81 (C meta), 130.96 (C ipso).

General Oxidation Procedure

A solution of the alcohol in the minimum amount of dichloromethane is added dropwise at room temperature to a stirred suspension of MBAFC/wet silica gel (0.5g MBAFC : 0.5g silica gel : 2 drops of H2O) in dichloromethane (typically 5 mL). The molar ratios of alcohol to the oxidant are either 1:1, 1:1.25 or 1:1.5 (Table 1). The progress of the reaction is monitored by TLC (solvent 6:1 hexane/ethyl acetate, v/v). After the completion of the reaction, the mixture is filtered through a short column of silica gel to give a clear solution. The solvent is evaporated and the crude products are purified by distillation, crystallization and/or column chromatography.

Acknowledgements

We wish to express our thanks to Dr. Mahmood Tajbakhsh (University of Mazandaran, Babolsar) and Ms. B.N. Haerizade for their helpful discussions. The technical assistance of Mr. M. Abbaspor at the NMR Laboratory of Tarbiat Modares University is fully appreciated.

References

  1. Collins, J.C.; W.W.; Franck, F.J. Dipyridine Chromium(VI) Oxide Oxidation of Alcohols in Dichloromethane. Tetrahedron Lett. 1968, 3363. [Google Scholar]
  2. Corey, E.J.; Fleet, G.W.J. Chromium Trioxide-3,5-Dimethylpyrazole Complex as a Reagent for Oxidation of Alcohols to Carbonyl Compounds. Tetrahedron Lett. 1973, 4499. [Google Scholar]
  3. Corey, E.J.; Suggs, J.W. Pyridinium Chlorochromate. An Efficient Reagent for Oxidatoin of Primary and Secondary Alcohols to Carbonyl Compounds. Tetrahedron Lett. 1975, 2647. [Google Scholar]
  4. Corey, E.J.; Schmidt, G. Useful Procedures for the Oxidation of Alcohols Involving Pyridinium Dichromate in Aprotic Media. Tetrahedron Lett. 1979, 399. [Google Scholar]
  5. Guziec, F.S.; Luzzio, F.A. The Oxidation of Alcohols Using 2,2'-Bipyridinium Chlorochromate. Synthesis 1980, 691. [Google Scholar]
  6. Bhattacharjee, M.N.; Chaudhuri, M.K.; Purkayastha, S. Some Aspects of Pyridinium Chlorochromate, C5H5NHCrO3F (PFC), Oxidations. Stoichiometry of Alcohols, Evidence for Oxygen Transfer, and Identity of the Reduced Chromium Species. Tetrahedron 1987, 43, 5389. [Google Scholar]
  7. Bhattacharjee, M.N.; Chaudhuri, M.K.; Dasgupta, H.S.; Roy, N. Pyridinium Fluorochromate; A New and Efficient Oxidant for Organic Substrates. Synthesis 1982, 588. [Google Scholar]
  8. Murugesan, V.; Pandurangan, A. Quinolinium Fluorochromate - A New Reagent for the Oxidation of Organic Compounds. Indian J. Chem. Sec. B 1992, 31B, 377. [Google Scholar]
  9. Srinivasan, R.; Ramesh, C.V.; Madhulatha, W.; Balasubramanian, K. Oxidation of Alcohols by Quinolinium Chlorochromate. Indian J. Chem. Sec. B. 1996, 35B, 480. [Google Scholar]
  10. Bora, U.; Chaudhuri, M. K. 3,5-Dimethylpyrazolium Fluorochromate(VI), C5H8N2H[CrO3F], (DmpzHFC): A Convenient New Reagent for Oxidation of Organic Substrates. Tetrahedron 2001, 57, 2445. [Google Scholar]
  11. Tajbakhsh, M.; Hosseinzadeh, R.; Yazdani-Niaki, M. Synthesis and Application of 2,6-Dicarboxypyridinium Chlorochromate as A New Oxidizing Reagent for Alcohols, Silyl ethers and THP ethers under Mild and Non-aqeous Conditions. J. Chem. Research (S) 2002, 2. [Google Scholar]
  12. Hosseinzadeh, R.; Tajbakhsh, M.; Yazdani-Niaki, M. 2,6-Dicarboxypyridinium Chloro-chromate: a Mild, Efficient and Selective Reagent for Oxidative Deprotection of Oximes to Carbonyl Compounds. Tetrahedron Lett. 2002, 43, 9413–9416. [Google Scholar]
  13. Tajbakhsh, M.; Heravi, M.M.; Mohanazadeh, F.; Sarabi, S.; Ghassemzadeh, M. N-Methyl-piperidinium Chlorochromate Adsorbed on Alumina: A New Deoximation Reagent. Monatsh. Chem. 2001, 132, 1229–1231. [Google Scholar]
  14. Kassaee, M.K.; Mahjob, A.R.; Ghammami, S. Tetramethylammonium fluorochromate(VI): a new and efficient oxidant for organic substrates. Tetrahedron Letters 2003, 44, 4555–45557. [Google Scholar]
  15. Kassaee, M.Z.; Sajjadi-Ghotbabadi, H.; Sayyed-Alangi, S.Z. N-Benzylmethylammonium Fluorochromate: A New Reagent for the Oxidation of Organic Compounds. The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittcon 2003, March 9-14. Orlando, Florida, Abstracts; 2003; p. 1200-7P. [Google Scholar]
  16. Srinivasan, R.; Stanley, Preethi; Balasubramanian, K. Isoquinolinium Fluorochromate: A New and Efficient Oxidant for Organic Substrates. Synth. Commun. 1997, 27, 2057–2064. [Google Scholar]
  • Sample availability: Available from the authors

Share and Cite

MDPI and ACS Style

Kassaee, M.Z.; Sayyed-Alangi, S.Z.; Sajjadi-Ghotbabadi, H. Synthesis and Reactions of N-Methylbenzylammonium Fluorochromate(VI) on Silica Gel, a Selective and Efficient Heterogeneous Oxidant. Molecules 2004, 9, 825-829. https://doi.org/10.3390/91000825

AMA Style

Kassaee MZ, Sayyed-Alangi SZ, Sajjadi-Ghotbabadi H. Synthesis and Reactions of N-Methylbenzylammonium Fluorochromate(VI) on Silica Gel, a Selective and Efficient Heterogeneous Oxidant. Molecules. 2004; 9(10):825-829. https://doi.org/10.3390/91000825

Chicago/Turabian Style

Kassaee, M. Z., S. Z. Sayyed-Alangi, and H. Sajjadi-Ghotbabadi. 2004. "Synthesis and Reactions of N-Methylbenzylammonium Fluorochromate(VI) on Silica Gel, a Selective and Efficient Heterogeneous Oxidant" Molecules 9, no. 10: 825-829. https://doi.org/10.3390/91000825

Article Metrics

Back to TopTop