Figure 1. Nitrodecarboxylation of 4-methoxycinnamic acid 1:
effect of nitric acid concentration on the yield of â-nitrostyrene 2.
(a) In the presence of oxygen; (b) without AIBN.
Figure 2. Nitrodecarboxylation of 4-methoxycinnamic acid 1:
effect of nitric acid concentration on the yield of 4-methoxy
benzaldehyde 2a. (a) In the presence of oxygen; (b) without AIBN.
reaction, provided meaningful insight into the mechanistic
course of the reaction. Reaction of 1 with 2 equiv of nitric
acid and additives such as acetic acid, sulfuric acid, or
nitronium salts afforded negligible yield of nitrodecarboxy-
lated product. In sharp contrast, facile nitrodecarboxylation
occurred once the nitric acid concentration was raised to 3
equiv or when catalytic AIBN was added in acetonitrile as
the solvent at 40-60 °C and under an inert atmosphere,
giving rise to the corresponding â-nitrostyrene 2 and 4-meth-
oxybenzaldehyde 2a. Henceforth, reactions were carried out
using 1 mM 1 in MeCN (2 mL) at the optimized temperature
of 50 °C and monitored for 2 and 2a by GC at regular time
intervals for 6 h.9 Major observations are summarized in
Figures 1-4.
along with 2a (16%) (Figures 1 and 2). The marked effect
of nitric acid concentration becomes obvious if one compares
the data for the first 30 min of reaction. Thus, for nitric acid
concentrations of 2, 3, 4, and 6 mM, the yields of 2 were
12, 55, 59, and 59, respectively, after 15 min. With 3 mM
nitric acid but under an oxygen atmosphere, the yield of 2
sharply decreased to 18%. A control experiment confirmed
the formation of aldehyde 2a to be due to the oxidative
decomposition of 2 by nitric acid.
The effect of other additives in the nitrodecarboxylation
was tested with 3 mM nitric acid in acetonitrile at 50 °C
(Figures 3 and 4). While lithium nitrate sharply decreased
the yield of 2 (15 min, 8%; 30 min, 20%), silver nitrate
remained noninfluential. Note that nitrate salts are known
to have a marked effect on nitration reactions proceeding
via ionic pathways.7,10 As stated earlier, AIBN was found to
A combination of 3 mM nitric acid along with 0.02 mM
AIBN (2%) provided the highest yield of 2 (75% after 4 h)
(8) (a) HNO3-H2SO4: Tinsley, S. W. J. Org. Chem. 1961, 26, 4723.
(b) C(NO2)4-Py: Penczek, S.; Jagur-Grodzinski, J.; Szwarc, M. J. Am.
Chem. Soc. 1968, 90, 2174. (c) Fuming HNO3: Shin, C.-G.; Masaki, M.;
Ohta, M. Bull. Chem. Soc. Jpn. 1970, 43, 3219. (d) AgNO2-PhSeBr-
HgCl2: Hayama, T.; Tomoda, S.; Takeuchi, Y.; Nomura, Y. Tetrahedron
Lett. 1982, 23, 4733. (e) NO2X-Et3N: Sy, W.-W.; By, A. W. Tetrahedron
Lett. 1985, 26, 1193. (f) AgNO2-I2: Jew, S.-S.; Kim, H.-D.; Cho, Y.-S.;
Cook, C. H. Chem. Lett. 1986, 1747. (g) NaNO3/KNO3-H3PO4: Grebenyuk,
A. D.; Ismailova, R. A.; Tokbolatov, R. B.; Ovodova, T. Zh. Org. Khim.
1990, 26, 680. (h) C(NO2)4-hν: Mathew, L.; Varghese, B.; Sankararaman,
S. J. Chem. Soc., Perkin Trans. 2 1993, 2399. (i) NaNO2-CAN: Hwu, J.
R.; Chen, K.-L.; Ananthan, S. J. Chem. Soc., Chem. Commun. 1994, 1425.
(j) NaNO2-CAN-sonication: Hwu, J. R.; Chen, K.-L.; Ananthan, S.; Patel,
H. V. Organometallics 1996, 15, 499. (k) NO2-O3: Suzuki, H.; Mori, T.
J. Org. Chem. 1997, 62, 6498. (l) Clayfen: Varma, R. S.; Naicker, K. P.;
Liesen, P. J. Tetrahedron Lett. 1998, 39, 3977. (m) NOx-I2: Navarro-
Ocana, A.; Barzana, D.; Lopez-Gonzalez, D.; Jimentez-Estrada, M. Org.
Prep. Proced. Int. 1999, 31, 117.
(9) GC Monitoring for Nitrodecarboxylation of 4-Methoxycinnamic
Acid 1. Nitric acid (21.35 M, 0.15 mL, 3 mM) was added to a stirred
solution of 1 (178 mg, 1 mM) in MeCN (2 mL) at room temperature under
an argon atmosphere. To this reaction mixture was added AIBN (3.3 mg,
2 mol %), and the mixture was stirred at 50 °C. An aliquot of 0.2 mL was
taken out at different time intervals and quenched with ice-cold water (0.2
mL) followed by triethylamine (0.03 mL), and a known amount of
â-nitrostyrene solution in chloroform (0.134 M) was added as a reference.
The organic layer was separated and directly used for GC analysis. The
yield of 4-methoxynitrostyrene 2 was found to be 75% after 4 h. It was
also isolated by column chromatography in 68% yield.
Figure 3. Nitrodecarboxylation of 4-methoxycinnamic acid 1:
effect of additives on the yield of â-nitrostyrene 2.
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Org. Lett., Vol. 4, No. 18, 2002