Transition metal catalyzed oxidations. 13

✍ Scribed by Karsten Krohn; Jochen Küpke

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 27 KB
• Volume: 341
• Category: Article
• ISSN: 1615-4150
• DOI: 10.1002/(sici)1521-3897(199907)341:5<509::aid-prac509>3.0.co;2-2

✦ Synopsis

In connection with the zirconium alkoxide-catalyzed oxidation of primary aromatic [2] and aliphatic [3] amines to nitro compounds, we also investigated the reaction of secondary N-aryl-N-alkyl amines with the system Zr(O-t-Bu) 4 /tert-butyl hydroperoxide (TBHP). Surprisingly, and in contrast to most other oxidations of secondary amines, a C-N-cleavage was observed in the reaction of N-methylaniline (1), and nitrobenzene (6) was isolated in 62% yield. There are only few reports in the literature on this interesting oxidative degradation of secondary amines. Very strong oxidation reagents such as peroxotrifluoroacetic acid [4] are able to cleave the C-N bond of secondary amines such as 1 to nitrobenzene (84%). In tungsten-catalyzed oxidations with hydrogen peroxide only 11% nitrobenzene was detected in the oxidation of N-benzylaniline along with other degradation products [5].

To test the scope and limitations of the reaction, we selected N-alkylanilines with primary, secondary, tertiary, and benzylic N-alkyl groups in addition to the cyclic tetrahydroquinoline (7). The experiments showed in fact that the oxidative cleavage of secondary N-aryl-N-alkyl amines with the system Zr(O-t-Bu) 4 /TBHP was not limited to N-methylanilines. The corresponding Nethyl (2), N-2-propyl (3), and N-benzyl anilines (5) were oxidized to nitrobenzene in 60, 49, and 76% isolated yield, respectively (Scheme 1). The alkyl group was converted to the corresponding carbonyl compound and formaldehyde, acetone, and benzaldehyde were detected by gas chromatography in the crude reaction mixture. Interestingly, N-tert-butylaniline (4) was not cleaved to nitrobenzene. Tetrahydroquinoline (7), which is known to undergo easy dehydrogenation to an aro-Transition Metal Catalyzed Oxidations.