LETTER
An Easy Access to Aromatic Azo Compounds under US/MW Irradiation
2607
(17) The disappearance of asymmetric and symmetric stretching
bands due to the N–O of NO2 (near 1520 cm–1 and 1345 cm–1
and the appearance of a strong band around 1460 cm–1 due
to N=N stretching was diagnostic. All the azo and azoxy
compounds were fully characterised by IR, mass and NMR
spectroscopy and when available compared with published
data.
(18) General Procedure: To a suspension of the nitro compound
(1 mmol) and Zn powder (4 mmol) in DMF (5 mL) or DMF–
H2O (95:5, 5 mL, substrates 4, 5, 7, 8) NH4Cl (2 mmol) was
added under stirring and the mixture was heated or irradiated
by US or/and MW as indicated before. After the reaction
(monitored by TLC or GC) was complete, the mixture was
filtered through a pad of celite and washed with H2O and
EtOAc. The organic layer was washed with brine, dried with
anhyd Na2SO4, concentrated under vacuum and purified
either by preparative TLC or by flash chromatography. (For
substrate 1, reaction at 80 °C: the product was precipitated
with 2 N HCl and the mixture was stirred until the Zn powder
had completely dissolved. The crude product was collected
on a Hirsch funnel and dried under vacuum.)
(19) GC analyses were carried out on a Shimadzu-14B gas
chromatograph, using SE52 capillary column (length 25 m;
internal diameter 0.25 mm; film thickness 0.25 mm). GC
conditions: injection split, 1:20; injector temperature, 250
°C; detector temperature, 300 °C; temperature program:
from 50 °C (1 min) to 300 °C at 10 °C/min (30 min); H2 as
carrier gas at 25 kPa. The analysed products had the
following retention time: 6: tR 16.2 min; azo-6: tR 27.3 min;
azoxy-6: tR 29.4 min; 7: tR 14.9 min; azoxy-7: tR 25.3 min; 8:
tR 14.4 min; azoxy-8: tR 24.2 min.
(20) 1,2-Bis{4-[2-(2,2-dichloroacetamido)-1,3-dihydroxy-
propyl]phenyl}diazene Oxide: Orange powder; mp 248.7 °C
(dec.); Rf 0.52 (CHCl3–MeOH, 8:2). IR (KBr): 3400 (OH,
NH), 2926, 1674 (C=O), 1541 (NCO), 1464 (N=NO), 1196,
1074, 810, 774 cm–1. 1H NMR (300 MHz, acetone-d6):
d = 8.24 (d, J = 8.7 Hz, 2 H, ArH-2,6), 8.15 (d, J = 8.7 Hz, 2
H, ArH-2¢,6¢), 7.65 (d, J = 8.7 Hz, 2 H, ArH-3,5), 7.59 (d,
J = 8.7 Hz, 2 H, ArH-3¢,5¢), 7.35 (br m, 2 H, CONH), 6.42
(s, 1 H, COCHCl2), 6.41 (s, 1 H, COCH¢Cl2), 5.28 (d, J = 2.4
Hz, 1 H, PhCHOH), 5.23 (d, J = 2.4 Hz, 1 H, PhCH¢OH),
5.19 (br, 1 H, PhCHOH), 5.08 (br, 1 H, PhCHOH¢), 4.23 (br,
2 H, CH2OH), 4.17 (m, 2 H, NHCH), 3.83 (m, 2 H, CH2OH),
3.74 (m, 2 H, CH¢2OH). 13C NMR (75 MHz, acetone-d6):
d = 164.27, 164.25 (CONH), 147.6 (ArC-1), 147.5 (ArC-1¢),
145.1 (ArC-4¢), 143.4 (C-4), 127.2 (ArC-3,5), 126.9 (ArC-
3¢,5¢), 125.5 (ArC-2¢,6¢), 122.1 (ArC-2,6), 70.6 (PhCHOH),
70.3 (PhC¢HOH), 67.1 (COCHCl2), 67.0 (COC¢HCl2),
61.3 (CH2OH), 57.7 (NHCH), 57.6 (NHC¢H). MS (ESI):
m/z = 621 [M+ + Na].
The outcome of the reaction was monitored by TLC and
GC-MS. Products were readily isolated by silica gel
column chromatography. Yields were determined by GC
analysis19 and confirmed by chromatographic separation
(Table 1).
In conclusion, we disclose an efficient procedure for the
preparation of azo and azoxy compounds from nitro-
arenes. The use of US and MW, separately or combined,
promotes the reactions and increases their yields. It is
noteworthy that no costly starting materials or toxic
reagents are required; moreover the metal needs no acti-
vation. Our future work will aim to exploit the synthetic
potential of the protocol and to improve the yields ob-
tained so far.
Acknowledgment
COST Action D32 is gratefully acknowledged. The present work
was supported by MIUR (project: ‘Sviluppo di processi sintetici
eco-compatibili nella sintesi organica’). We are indebted to Ing.
Cesare Buffa, Mr. Gabriele Omiccioli and Dr. Davide Garella for
their technical contribution.
References and Notes
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(21) 1,2-Bis[3,4-di(pentan-3-yl)phenyl]diazene: Orange powder;
mp 97.2 °C; Rf 0.71 (hexane–EtOAc, 99:1). IR (KBr): 2963,
2872, 1458 (N=N), 1375, 901, 839, 812 cm–1. 1H NMR (300
MHz, CDCl3): d = 7.76 (s, 2 H, H-2,2¢), 7.65 (d, 2 H, H-6,6¢),
7.34 (d, 2 H, H-5,5¢), 2.91 [m, 4 H, CH(CH2CH3)2], 1.92–
1.55 [m, 16 H, CH(CH2CH3)2], 0.88–0.79 [m, 24 H,
CH(CH2CH3)2]. 13C NMR (75 MHz, CDCl3): d = 151.1 (C-
1,1¢), 147.8 (C-4,4¢), 145.6 (C-3,3¢), 127.1 (C-5,5¢), 122.9
(C-2,2¢), 118.2 (C-6,6¢), 42.2 [CH(CH2CH3)2], 29.5, 29.2,
29.1 [CH(CH2CH3)2], 12.5, 12.4, 12.3 [CH(CH2CH3)2]. MS
(CI, isobutane): m/z (%) = 463 (100) [MH]+.
(22) 1,2-Bis[3,4-di(pentan-3-yl)phenyl]diazene Oxide: Yellow
powder; mp 114.2 °C; Rf 0.64 (hexane–EtOAc, 99:1). IR
(KBr): 2961, 2872, 1483, 1460 (N=NO), 1397, 928, 826
cm–1. 1H NMR (300 MHz, CDCl3): d = 8.20–8.06 (overlap,
3 H, H-2,6,6¢), 7.99 (s, 1 H, H-2¢), 7.31 (d, 2 H, H-5,5¢), 2.94
[br m, 4 H, CH(CH2CH3)2], 1.91–1.52 [m, 16 H,
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(16) Cravotto, G.; Omiccioli, G.; Buffa, C.; Dal Lago, G. Patent
VR 2004A000167 IT, 2004;.
Synlett 2006, No. 16, 2605–2608 © Thieme Stuttgart · New York