· 3-Benzylthiazolium Bromide
· (MW 256.16)
(in combination with base is a redox catalyst for the oxidation of aldehydes coupled with the reduction of organic compounds2)
Physical Data: mp 155-156 °C.
Solubility: freely sol methanol; sol ethanol, DMSO, and DMF; sparingly sol chloroform, dichloromethane, and acetone; insol THF, benzene, and ethyl acetate.
Form Supplied in: white columnar crystals.
Analysis of Reagent Purity: 1H NMR (DMSO-d6, 270 MHz) d 5.82 (s, 2H, CH2), 7.41-7.53 (m, 5H, C6H5), 8.37 (dd, 1H, H-5, J = 3.7 and 2.4 Hz), 8.61 (dd, 1H, H-4, J= 3.7 and 1.8 Hz); 10.40 (dd, 1H, H-2, J = 2.4 and 1.8 Hz).
Purification: recrystallized from 5:1 benzene/ethanol.
Handling, Storage, and Precautions: the reagent is somewhat hygroscopic and should be dried before use.
Oxidation of Aldehydes to Methyl Esters.
3-Benzylthiazolium bromide, in methanol containing Triethylamine, catalyzes a redox reaction in which aldehydes are oxidized to methyl esters, and certain organic compounds such as acridine, phenazine, azobenzene, and nitrobenzene are reduced.2 The redox reactions are carried out as follows. Triethylamine (5n mmol) and the substrate (4n mmol) are added to a frozen solution of a mixture of aldehyde (6n mmol) and 3-benzylthiazolium bromide (n mmol) in degassed methanol (10n mL). The solution, after melting, is stirred under argon at rt for a 20-h period. To increase the yield in the redox reaction, an excess of aldehyde is added. The presence of air in methanol results in the production of methyl esters by oxidation of aldehydes.
The mechanism of this process is outlined in eq 1. The thiazolium salt is first transformed into an ylide by the action of triethylamine3 and the ylide then reacts with the aldehyde to form an adduct (
active aldehyde)4 which undergoes a redox reaction with the organic substrate in methanol to give a methyl ester and the reduced form of the substrate (eq 1).
Acridine is reduced by benzaldehyde, 2-furaldehyde, and heptanal in the presence of 3-benzylthiazolium bromide and triethylamine in degassed methanol to give 9,9´-biacridan in satisfactory yields, while aldehydes are oxidized to the corresponding methyl esters (eq 2).2 The excess aldehydes are converted into a-hydroxy ketones by benzoin condensation.5 The redox reaction is thought to proceed via the transfer of electrons from the active aldehyde to acridine.
Phenazine and azobenzene are reduced, under similar conditions, to 5,10-dihydrophenazine (eq 3) and hydrazobenzene (eq 4), respectively; the latter is converted into benzidine on treatment with hydrochloric acid after the reaction.2 Nitrobenzene is reduced to aniline and N-benzoyl-N-phenylhydroxylamine (eq 5).2,6
- 1. (a) Ugai, T.; Tanaka, S.; Dokawa, S. J. Pharm. Soc. Jpn. 1943, 63, 269. (b) Metzger, J. V. Thiazole and its Derivatives; Interscience: New York, 1979; Part 1. (c) Metzger, J. V. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R.; Rees, C. W., Eds.; Pergamon: Oxford, 1984; Vol. 6, p 235.
- 2. Inoue, H.; Higashiura, K. CC 1980, 549.
- 3. Katritzky, A. R.; Lagowski, J. M. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R.; Rees, C. W., Eds.; Pergamon: Oxford, 1984; Vol. 5, p 63.
- 4. Stetter, H. AG(E) 1976, 15, 639.
- 5. (a) Hassner, A.; Rai, K. M. L. COS 1991, 1, 541. (b) Laird, T. In Comprehensive Organic Chemistry; Barton, D. H. R.; Ollis, W. D., Eds.; Pergamon: Oxford, 1979; Vol. 1, p 1142. (c) Breslow, R. JACS 1958, 80, 3719.
- 6. Inoue, H.; Tamura, S. CC 1985, 141.
University of Osaka Prefecture, Japan
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