· (MW 220.31)
(precursor for 4-aryl-2,6-di-t-butylphenols;2 for 4-alkynyl-2,6-di-t-butylphenols;3 oxidation of amines to amides via oxygenation of 4-(N-arylmethyleneamino)-2,6-di-t-butylphenols4)
Physical Data: mp 65-67 °C.
Solubility: insol H2O; sol organic solvents.
Form Supplied in: yellow solid.
Analysis of Reagent Purity: TLC, NMR.
Preparative Method: Co(salen)-catalyzed oxygenation of 2,6-di-t-butylphenol in DMF.1
Purification: silica gel column chromatography; recrystallization from hexane.
Preparation of 4-Substituted 2,6-Di-t-butylphenols.
2,6-Di-t-butyl-1,4-benzoquinone (1) is conveniently utilized for the preparation of a variety of 4-substituted 2,6-di-t-butylphenols. The reaction of (1) with Grignard reagents derived from aryl bromides, alkynes, and Ethylmagnesium Bromide gives the corresponding 1,4-quinols (2) in >90% yield from aryl Grignard reagents2 and in 65-77% yield from RC&tbond;CMgBr (R = Ph, n-Bu, t-Bu).3 The reduction of (2) with Zinc/HCl in ethanol then affords the corresponding phenols (3) in 74-96% yield (eq 1).3
Oxidation of Amines to Amides.
Quinone (1) can conveniently be used for the selective oxidation of an a-methylene group in amines to give the corresponding amides, normally a difficult transformation. Treatment of (1) with ArCH2NH2 in ethanol at reflux first leads to 4-(N-arylmethyleneamino)-2,6-di-t-butylphenol (4) (eq 2) (yields: Ar = Ph (82%), 4-MeC6H4 (61%), 4-MeOC6H4 (72%), 4-ClC6H4 (49%), 3-pyridyl (62%), 2-furyl (42%)).4 Base-promoted oxygenation of (4) then affords (1) and the corresponding amide (eq 2). The yield of amide depends on the reaction conditions; oxygenation with Potassium t-Butoxide in t-BuOH provides the amide in 36-85% yield, but
the recovery of (1) is only 10% because (1) is unstable under the conditions. On the other hand, the use of Potassium Hydroxide in ethanol leads to the amide in 15-79% yield with better recovery of (1) (70%).4 The oxygenation of (5) with Co(salpr) in dichloromethane reveals that hydroperoxide (5) is a likely intermediate in this reaction.5
- 1. De Jonge, C. R. H. I.; Hageman, H. J.; Hoentjen, G.; Mijs, W. J. OSC 1988, 6, 412.
- 2. (a) Rieker, A.; Scheffler, K. LA 1965, 689, 78. (b) Nishinaga, A.; Itahara, T.; Matsuura, T.; Rieker, A.; Koch, D.; Albert, K.; Hitchcock, P. B. JACS 1978, 100, 1826.
- 3. Nishinaga, A.; Iwasaki, H.; Shimizu, T.; Toyoda, Y.; Matsuura, T. JOC 1986, 51, 2257.
- 4. Nishinaga, A.; Shimizu, T.; Matsuura, T. CC 1979, 970.
- 5. Nishinaga, A.; Shimizu, T.; Matsuura, T. TL 1980, 21, 1265.
Osaka Institute of Technology, Japan
Copyright © 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.