Res. Chem. Intermed.
, Vol. 30, No. 3, pp. 269– 278 (2004)
Also available online - www.vsppub.com
Resonance-stabilized phenoxy-like radicals formed by
NO-mediated mono-nitration of coumarins containing
a hydroxy group
L.-D. LEI, F. WANG, D.-S. YANG and L.-M. WU
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
Received 26 November 2003; accepted 12 December 2003
Abstract—Stable aminoxyls and an iminoxyl were observed by spin trapping and EPR techniques
during the nitration of coumarins containing a hydroxy group by nitric oxide. The trapped free radicals
are deduced to be the resonance stabilized phenoxy-like radicals. The mechanisms for the nitrations
: Nitration; nitric oxide; coumarins; aminoxyls; EPR; spin trapping.
Aromatic ring nitrations are among the most familiar and thoroughly studied organic
processes. Several issues on these reactions have been approached in previous
works. (a) They are classically carried out in concentrated nitric acid and often
catalyzed by strong acids such as sulfuric acid. The powerful Lewis acid, the
nitronium ion (
), acts as a powerful electrophile in aromatic nitrations [1– 4].
Consequently, stable nitronium salts are capable of nitrating the most unreactive
aromatic compounds. Perrin and Kochi [5, 6] proposed an electron transfer (ET)
mechanism for the nitronium-mediated nitration of all aromatic substrates with
an oxidation potential lower than toluene. The nitrous-acid-catalyzed aromatic
involves formation of aromatic and hetero-aromatic compounds
by nitrosonium (
NO) [3, 7– 11] by one-electron oxidation in the radical cation
). (b) The Kyotai-nitration of various kinds of aromatic compounds was
carried out with NO
at lower temperature . The reactions were
assumed to proceed via cation radicals formed from the oxidation of substrates
by nitrogen trioxide. (c) NO
is a useful aromatic nitration reagent [13 – 16].
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