Indian Journal of Chemistry Vol. 408, December 2001 , pp. 1 234- 1235

Note

A new facile and rapid synthesis of acyl azides under solvent-free conditions

Shadpour E MaUakpour*, Abdol Reza Hajipour* & Ramin Vahabi

Organic Polymer Chemistry Research Laboratory, College of Chemistry, Isfahan University of Technology,

Isfahan, 841 56, IR, Iran E-Mail: Mallak@cc.iut.ac.ir

Received 1 1 February 2000; accepted (revised) 4 January 2001

A .rapid and simple method for the preparation of acyl azides is

described. The reaction is carried out under solvent-free conditions from the reaction of acid chlorides with sodium azide supported on silica gel.

Although most organic reactions have been studied in solution, but organic solid state reactions which are facilitated by supported reagents on various solid inorganic surfaces have received great interest in recent years '-4. The advantages of these methods over conventional homogenous reactions are that they provide enhanced reaction rates, less environmental pollutions, greater selectivity, cleaner products and manipulative simplicity.

Organic azides are important reactive intemediates which have a wide variety applicationss. Among these azides is acyl azide, which is usually prepared by reaction of sodium azide solution in water with an acid chloride solution in cold acetones. In continuation of our ongoing program to develop environmentally benign method using solid supports6, in this article we wish to report a simple method for the preparation of acyl azide under solvent-free conditions using silica gel as an inorganic support.

The reaction was carried out in a porcelain dish involving the reaction of supported sodium azide on silica gel with an appropriate acid chlorides (Scheme J). The process involves a simple mixing of supported sodium azide on silica gel and acid chloride in a mortar, followed by grinding the mixture for I min. The progress of the reaction was followed by

? Ar - C - CI

silica gel + Scheme I

TLC technique. The reactions are fast and only require grinding the reaction mixture for 1 min. The resulting acyl azides were extracted with ether or chloroform and the isolated yields are good and are in a range of 65-95% (Table I).

The products were characterized by IR analysis. The acid chlorides have characteristic strong absorp-. I tiOn around 1780 cm- for the carbonyl group. Disappearance of these strong peaks and appearance of peaks at 1 700 cm- ' for the carbonyl groups and peaks around 2100 cm" for N3 groups in the IR spectra of the resulting acyl azides confirmed the �omplete conversion of acid chlorides to correspondmg acyl azides. It is interesting to note that azides usually show bands in IR near 2140 cm- I suggestive of the resonance forms (-N=N+= N-¢:::> -N--N+=N) with cumulated double bonds.

The resulting acyl azides in entries 3, 5-8 and 10 are stable as long as they are in solution. When solvent is removed, they gradually will decompose. Thus their IR spectra (neat) were recorded immediately prior to decomposition.

In conclusion we have developed a rapid, efficient and environmental friendly method for the conversion of acid chlorides to corresponding acyl azides under solvent-free conditions. Some of these acyl azides are unstable as neat upon standing in air, but they are stable as long as they are in solution.

Experimental Section Chemicals were purchased from Fluka, Merck,

Riedel-dehaen AG and Aldrich chemicals companies. The acid chlorides were prepared by the reaction of acids with thionyl chloride and their purity was confirmed by IR spectra. Yields refer to isolated pure products. All products were identified by IR spectra.

General procedure. Into a mortar silica gel ( 1 00 mg) and sodium azide ( 1 .2 mmole) were added. The mixture was mixed well and 1 .0 mmol of acid chloride was added. The reaction mixture was ground with pestle in the mortar for 1 min. After 1 min. the TLC showed complete disappearance of acid chloride. Thw the reaction mixture was extracted with ether or chloroform (2 x 1 0 mL). The solvent was removed to gi ve the pure product.

NOTES 1 235

Table I-Synthesis of acyl azides from acid chlorides and sodium azide supported on silica gel

Entry

2

3b

4

5

6

7

8

9

10

Substrates

(Q)-�-a 0

02N-©-�-CI 0

(Q)--CH2-�-C1

O,N ��-CI O,N

0 Cr-Q--�-CI NO,

0 o.�-CI CI

CHJO 0 ;.0- II CHJ 0 CHrC-a

0 �-CI CI

0 H3C�-CI

Q 0 II CH-c-a

©

Product

0 (Q)--�-NJ

0 02N-©-�-NJ

0 (Q)-CH2-�-NJ

o.N ��-NJ 02N

0 Cr-Q--�-N3 NO,

0 o.��-N3 CI

CHJO 0 � II CHJ 0 CHrC-NJ

0 �-N3 CI

0 H3C-©-�-N3

Q 0 II dH-C-NJ

Yield m.p(°C) IR (N) IR (C=O) (%) (Lit. 7.M) cm · 1 cm · 1

65 oil (32) 2 120 1685

70 65-67 (65) 2 140 1690

70 oil (_ )c.d 2 1 20 1 7 10

75 94-96 (_)C 2140 1680

75 oil (_/.d 2140 1680

74 oil (_td 2120 & 1690 2250e

57 oil (_)c.d 2120 1 700

95 oil (_ )c.d 2 1 10 & 1690 2240e

70 32-33 (35) 2 1 10 1680

2 1 10 & oil (_)""d 1700 80 2220e

a Yields refer to pure isolated products obtained based on acid chlorides. b the product was extracted from chloroform; C melting points or boiling points are not available; d the acyl azides (neat) decompose upon standing in air; e this peak could be due to formation of some isocyanate.

Acknowledgement

Financial support for this work by the Research Affairs Division, Isfahan University of Technology (lUT), Isfahan, I .R. Iran, is gratefully acknowledged.

References I Toda F, Ace Chem Res, 28, 1995, 480. 2 Come lis P & Laszlo P, Synthesis, 1985, 909. 3 Abramovitch, R A, Ogr Prep Proc lilt , 23, 1991, 685. 4 Migos D M P & Baghurst D R, Chem Soc Rev, 20, 1991, I .

5 Scriven E F V & Turnbull K, Chem Rev, 88, 1988, 297. 6 (a) Hajipour A R, Mallakpour S E & Imanzadeh G. J Chem

Research (S) 1999, 228. (b) Hajipour A R, Mallakpour S E & Imanzadeh G, Chern Lett, 1999, 99. (c) Hajipour A R, Indian J Chem, 36B, 1997, 1 069. (d) Hajipour A R, Mallakpour S E & Afrusheh A, Tetrahedron, 55, 1999, 23 1 1 . (e) Hajipour A R, Mohammadpoor-Baltork I . , Nikbaghat K & Imanzadeh G., Synth. Commun. 26, 1999, 1697. (f) Hajipour A R & Islami F. lndian J Chem, 38B, 1999, 46 1 .

7 Yukawa Y & Tsuno Y , J Am Chem Soc, 79, 1957, 5530. 8 Hand-book of Chemistry and Physics, edited by R.C. Weast,

66th Edn, 1985-1986.