444
N. Nowrouzi, M. Zareh Jonaghani / Chinese Chemical Letters 23 (2012) 442–445
I
O
Ph
P
Ph
II
O
I
Cl
2 NaN3
2 RCO2H
I2
Ph2PCl
RCON3
Ph2P(O)OH
Ph2P
+
R
O
O
R
I
Scheme 2.
As can be seen from the results in Table 1, both aromatic and aliphatic carboxylic acids converted cleanly to azides
in high yields. As expected, the presence of the nitro group on aromatic ring (stronger aryl acids) is very effective in
activating the carboxylic group toward reaction with azide ion (entries 2 and 3) compared to weaker acids such as
benzoic and 4-methoxybenzoic acid (entries 1 and 4).
Based on the previous studies [24] and above results, the reaction may proceed via in situ generated
acyloxyphosphonium ion (II). The attack of the azide ion at the carbonyl carbon atom of this intermediate affords the
corresponding acyl azide along with diphenylphosphinic acid as shown in Scheme 2.
In conclusion, an efficient new one-pot direct method for the synthesis of various acyl azides from carboxylic acids
was found. Mild reaction conditions, simplicity and ease of work-up and low toxicity of the reagents in contrast to the
hazardous reagents reported in the literature are the advantages of our system.
1. Experimental
Typical procedure for the conversion of benzoic acid into benzoylazide: To a flask containing a stirring mixture of
Ph2PCl (1.0 mmol, 0.18 mL) and I2 (1.0 mmol, 0.253 g) in dichloromethane (5 mL), was added benzoic acid
(2.0 mmol, 0.244 g) at room temperature. Sodium azide (2.1 mmol, 0.136 g) was then added to the reaction mixture.
After 90 min, the organic layer was washed successively with saturated aqueous sodium carbonate (3Â 5 mL),
aqueous sodium thiosulfate (2Â 5 mL) and water (5 mL). The organic layer was dried with anhydrous Na2SO4 and
concentrated. Purification by a short silica gel column chromatography using n-hexane afforded the pure benzoyl
azide in 80% yield. IR (KBr) 2138 cmÀ1; 1H NMR (CDCl3, 500 MHz): d 7.43–7.49 (m, 2H), 7.60–7.66 (m, 1H), 8.05
(d, 2H, J = 7.8 Hz); 13C NMR (CDCl3, 125 MHz): d 128.7, 129.4, 130.6, 134.4, 172.6.
Acknowledgment
We thank the Persian Gulf University Research Council for generous partial financial support of this study.
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