Microwave assisted condensation of aromatic methyl groups with aromatic aldehydes

Article abstract of DOI:10.1081/SCC-120038503

Aromatic aldehydes are condensed with 2,4-dinitro-1,5-dimethylbenzene in a Siegrist type coupling. The reported method features microwave irradiation, solvent-free conditions, and a catalytic amount of piperidine to give a high yield of the corresponding

Full text of DOI:10.1081/SCC-120038503

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Microwave Assisted Condensation of Aromatic Methyl
Groups with Aromatic Aldehydes
Kasper Moth‐Poulsen a b , Theis Reenberg a , Thomas Bjørnholm b & Jørn B. Christensen a
a Chemical Laboratory II, Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK‐2100, Copenhagen, Denmark
b Nano‐Science Center, University of Copenhagen, Denmark
Published online: 17 Aug 2006.
To cite this article: Kasper Moth‐Poulsen , Theis Reenberg , Thomas Bjørnholm & Jørn B. Christensen (2004): Microwave
Assisted Condensation of Aromatic Methyl Groups with Aromatic Aldehydes, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 34:12, 2215-2221
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 12, pp. 2215–2221, 2004
Microwave Assisted Condensation
of Aromatic Methyl Groups with
Aromatic Aldehydes
Kasper Moth-Poulsen,1,2 Theis Reenberg,1
Thomas Bjørnholm,2 and Jørn B. Christensen1,
*
1Chemical Laboratory II, Department of Chemistry and
2Nano-Science Center, University of Copenhagen, Denmark
ABSTRACT
Aromatic aldehydes are condensed with 2,4-dinitro-1,5-dimethylbenzene
in a Siegrist type coupling. The reported method features micro-
wave irradiation, solvent-free conditions, and a catalytic amount of
piperidine to give a high yield of the corresponding 1,3-bisstyryl-4,6-
dinitrobenzenes within a few minutes.
Key Words: Microwave irradiation; Solvent-free conditions; Distyryl
benzenes.
*
Correspondence: Jørn B. Christensen, Chemical Laboratory II, Department of
Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen,
Denmark; E-mail: jbc@kiku.dk.
2215
DOI: 10.1081/SCC-120038503
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Moth-Poulsen et al.
INTRODUCTION
Conjugated aryl alkenes have over the last decade proven their usefulness
as new materials for molecular electronics and non-linear optics.[1–4] In
connection with an ongoing project, a number of substituted bisstyryl dinitro-
benzenes were needed. The p-methoxystyryl member of this series was a
known compound. The classical synthesis proceeds under solvent-free
conditions by heating a mixture of anisaldehyde 2a and dinitroxylene 1 in
the presence of a catalytic amount of piperidine.[5] However, the long reaction
times and the somewhat low yields made a more efficient synthesis desirable.
Microwave-assisted organic synthesis is a technique, which has been shown
to be highly useful for thermal reactions.[6–8] The homogenous nature of micro-
wave heating allows a fast heating of a reaction mixture without local overheating
at the walls of the reaction flask. This can be a problem especially in the case of a
viscous reaction mixture. There is, however, still an ongoing debate in the literature
concerning whether a specific, non-thermal “microwave effect” exists or not.[9]
RESULTS AND DISCUSSION
The use of microwave heating on the neat reaction mixture of 1,5 dinitro-
m-xylene 1 and a number of aldehydes (2a–h) afforded the corresponding
bisstyryl benzenes (3a–h) in good to acceptable yields (36–90%) (Sch. 1).
Typically, the reaction were heated at 350 W for 3 min and then allowed to
cool for 15 min, the heating was repeated 1–3 times. The crude reaction mix-
ture was purified by dry column chromatography.[10] This procedure gave
(E,E)-1,3-bis(4-methoxystyryl)-4,6-dinitrobenzene, 3a in 90% yield (the use
of conventional heating as reported by Ruggli Zimmermann,[5] afforded typi-
cally 40%). The condensation of aldehydes 2b–h afforded the new bisstyryl
benzenes 3b–h in 32–82% yield (Table 1). The condensation of aldehyde
2h yielded additional 27% of the mono adduct (E)-1-(dimethylaminostyryl)-
3-methyl-4,6-dinitrobenzene 3i (Sch. 2).
Scheme 1.

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Microwave Assisted Condensation of Aromatic Methyl Groups
2217
Table 1. Chemical yields of dinitro bisstyryl benzenes upon reaction of 1,5 dinitro-m-
xylene with different aldehydes (2a–h).
Irradiation
time (min)a
Yield
(%)
Entry
Aldehyde
Dinitro bisstyryl benzene
a
3 ꢀ 3
90
82
54
b
c
2
2 ꢀ 3
d
3 ꢀ 3
75
e
f
3 ꢀ 3
3 ꢀ 3
54
82
g
3 ꢀ 3
3 ꢀ 3
73
h
32b
aThe reaction mixture was typically heated for 3 ꢀ 3 min and allowed to cool for
15 min between each heating.
bThe condensation of aldehyde 2h yielded additional 27% of the mono adduct (E)-1-
(dimethylaminostyryl)-3-methyl-4,6-dinitrobenzene 3i (Sch. 2).

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Moth-Poulsen et al.
Scheme 2.
CONCLUSION
A general method for the condensation of nitroxylenes with aromatic
carbonyl compounds by microwave irradiation has been developed. The reac-
tion times are considerably shorter than under the classical conditions and the
yields are satisfactory.
EXPERIMENTAL
Melting points were recorded on a hot stage calibrated using a set of
Reichert calibration substances. NMR spectra were recorded on a Bruker
AM250 or a Varian Unity 400 spectrometer. Mass spectra were recorded on
a 4 sector Jeol HX110/HX110. CHN analysis was recorded on a Carlo Elba
FLASH 1112 apparatus. The microwave irradiation was carried out using a
converted domestic microwave oven (OBH type 7510) operated at 350 W
(2450 MHz). The reaction flask was linked to an outside refluxing column
as described by Baghurst and Mingos.[7]
General Procedure
A 2 mmol (0.392 g) of 1,5-dinitro-m-xylene (1), 4 mmol of aldehyde
2a–h (all commercially available except 2f, which were prepared according
to published procedure[11]) and three drops of piperidine were typically heated
for 3 ꢀ 3 min at 350 W (allowed to cool for 15 min between each heating).
The resulting material was purified using dry column chromatography. The
resulting product was recrystallised from toluene.
(E,E)-1,3-bis(4-Methoxystyryl)-4,6-dinitrobenzene (3a). Yield: 90%.
1
Solid, m.p. 179.6–180.68C (litt.[5] 1798C); H NMR (CDCl3, 250 MHz) d:
3.8 (s, 6H), 6.9 (d, J ¼ 8.3 Hz, 4H), 7.2 (d, J ¼ 16.6 Hz, 2H), 7.5
(d, J ¼ 8.3 Hz, 4H), 7.55 (d, J ¼ 16.6 Hz, 2H), 8.0 (s, 1H), 8.64 (s, 1H); 13C
NMR (CDCl3, 62.9 MHz) d: 55.3, 114.4, 119.7, 122.8, 127.0, 128.5, 129.0,
136.7, 137.9, 144.6, 160.8.
EIMS: (m/z, relative intensity): 432 (Mþ, 98), 279 (40), 160 (50), 135
(64), 121 (100). UV-VIS (acetonitrile): 1387 ¼ 3.1 ꢀ 104 mol21 cm21
.

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Microwave Assisted Condensation of Aromatic Methyl Groups
2219
(E,E)-1,3-bis(2,4-Dimethoxystyryl)-4,6-dinitrobenzene (3b). Yield:
82%. Solid, m.p. 207.1–208.78C; 1H NMR (CDCl3, 250 MHz) d: 3.79 (s, 6H),
3.84 (s, 6H), 6.43 (d, J ¼ 2.4 Hz, 2H), 6.5 (dd, J1 ¼ 2.4 Hz, J2 ¼ 9.0 Hz, 2H),
7.48 (d, J ¼ 16.2 Hz, 2H), 7.50 (d, J ¼ 8.5 Hz, 2H), 7.61 (d, J ¼ 16.2 Hz, 2H),
8.02 (s, 1H), 8.61 (s, 1H). 13C NMR (CDCl3, 62.9 MHz) d: 55.4, 55.5, 98.4,
105.3, 118.1, 120.1, 122.7, 127.1, 128.9, 132.0, 138.5, 144.3, 158.9, 162.0.
EIMS m/z (relative intensity): 492 (Mþ, 100), 309 (20), 165 (30). HRMS
(Mþ) calcd. 492.1533. Found 492.1544. Anal. calcd. for C26H24N2O8: C
63.41; H 4.91; N 5.69. Found C 63.32; H 4.83; N 5.73. UV-VIS (acetonitrile):
1403 ¼ 2.7 ꢀ 104 mol21 cm21
.
(E,E)-1,3-bis(3,5-Dimethoxystyryl)-4,6-dinitrobenzene (3c). Yield:
54%. Solid, m.p. 197.0–199.38C; 1H NMR (CDCl3, 250 MHz) d: 3.78
(s, 12H), 6.43 (t, J ¼ 2.3 Hz, 2H), 6.67 (d, J ¼ 2.3 Hz, 4H), 7.17
(d, J ¼ 16.0 Hz, 2H), 7.60 (d, J ¼ 16.1 Hz, 2H), 7.98 (s, 1H), 8.66 (s, 1H).
13C NMR (CDCl3, 62.9 MHz) d: 55.4, 101.7, 105.4, 122.5, 122.6, 128.0,
137.2, 137.5, 145.1, 161.1. FABþMS (m/z, relative intensity): 493 (MHþ,
4), 460 (3), 307 (20), 154 (100). HRMS (MHþ) calcd. 493.1611, found
493.1613. UV-VIS (acetonitrile): 1341 ¼ 2.7 ꢀ 104 mol21 cm21
.
(E,E)-1,3-bis(3,4,5-Trimethoxystyryl)-4,6-dinitrobenzene (3d). Yield:
75%. Solid, m.p. 122–1238C. 1H NMR (CDCl3, 250 MHz) d: 3.68 (s, 6H), 3.70
(s, 12H), 6.72 (s, 4H), 8.54 (d, J ¼ 8 Hz, 2H), 8.56 (s, 1H), 8.67 (d, J ¼ 8 Hz,
2H,), 9.80 (s, 1H). 13C NMR (CDCl3, 62.9 MHz) d: 39.4, 56.1, 56.3, 104.7,
105.0, 105.3, 120.5, 122.3, 131.0, 132.6, 137.2, 138.1, 139.4, 153.4.
Anal. calcd. for C28H28N2O10: C 60.87; H 5.11; N 5.07, found C 60.91; H
5.16; N 4.99. UV-VIS (acetonitrile): 1381 ¼ 1.4 ꢀ 104 mol21 cm21
.
(E,E)-1,3-bis(2,4,6-Trimethoxystyryl)-4,6-dinitrobenzene (3e). Yield:
1
56%. Solid, m.p. 189–1908C. H NMR (CDCl3, 250 MHz) d: 3.82 (s, 6H),
3.84 (s, 12H), 6.08 (s, 4H), 7.66 (d, J ¼ 16 Hz, 2H), 7.92 (s, 1H), 7.97
(d, J ¼ 16 Hz, 2H), 8.57 (s, 1H). 13C NMR (CDCl3, 62.9 MHz) d: 21.1,
55.3, 55.7, 90.5, 107.0, 121.4, 122.2, 128.7, 131.2, 138.4, 139.8, 145.4,
160.4, 162.1. Anal. calcd. for C28H28N2O10: C 60.87; H 5.11; N 5.07.
Found: C 60.93; H 5.05; N 5.06. UV-VIS (acetonitrile): 1417 ¼ 2.8 ꢀ
104 mol21 cm21
.
(E,E)-1,3-bis(2,5-Dimethoxy-4-methyl-styryl)-4,6-dinitrobenzene
(3f). Yield: 82%. Solid, m.p. 161–1638C. 1H NMR (CDCl3, 250 MHz) d: 2.10
(s, 6H), 3.80 (s, 12H), 6.65 (s, 2H), 7.0 (s, 2H), 7.50 (d, J ¼ 16 Hz, 2H), 7.60
(d, J ¼ 16 Hz, 2H), 8.00 (s, 1H), 8.60(s, 1H). 13C NMR (CDCl3, 62.9 MHz)
d: 16.6, 55.8, 56.1, 108.6, 114.4, 121.2, 121.6, 122.4, 122.6, 127.6, 128.9,
130.1, 132.2, 138.3, 144.5, 151.7, 152.0. HRMS (Mþ) calcd. 520.1846,
found 520.1842. UV-VIS (acetonitrile): 1413 ¼ 1.7 ꢀ 104 mol21 cm21
.
(E,E)-1,3-bis(2,3,4,5,6-Pentamethylstyryl)-4,6-dinitrobenzene (3g).
1
Yield: 73% Solid. m.p. 152–1538C. H NMR (CDCl3, 250 MHz) d: 2.10

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Moth-Poulsen et al.
(m, 30H), 6.9 (d, 2H J ¼ 8 Hz), 7.3 (s, J ¼ 8 Hz, 2H), 8.1 (s, 1H) 8.7 (s, 1H).
13C NMR (CDCl3, 62.9 MHz) d: 15.1, 15.6, 15.9, 16.6, 17.0, 121.3, 127.3,
127.7, 130.5, 131.9, 132.6, 134.1, 137.3, 144.1. HRMS (Mþ) calcd.
512.2675, found 512.2675. UV-VIS (acetonitrile): 1338 ¼ 1.5 ꢀ
104 mol21 cm21
.
(E,E)-1,3-bis(-Dimethylaminostyryl)-4,6-dinitrobenzene (3h). Yield
1
32%. Solid, m.p. 170–1738C. H NMR (CDCl3, 400 MHz) d: 2.98 (s, 12H),
6.64 (d, J ¼ 9.2 Hz, 4H), 7.17 (d, J ¼ 15.3 Hz 2H), 7.42 (d, J ¼ 9.2 Hz,
4H), 7.46 (d, J ¼ 15.3 Hz, 2H), 7.97 (s, 1H), 8.59 (s, 1H). 13C NMR
(CDCl3, 62.9 MHz) d: 40.1, 111.9, 116.9, 123.1, 123.9, 125.7, 129.0, 137.4,
138.2, 143.5, 151.2. FABþMS (m/z, relative intensity): 458 (Mþ, 10), 307
(30). Anal. calcd. for C26H26N4O4: C 68.11; H 5.72; N 12.22, found C
68.15; H 5.62 N 12.13. UV-VIS (acetonitrile): 1467 ¼ 3.6 ꢀ 104 mol21 cm21
.
(E)-1-(Dimethylaminostyryl)-3-methyl-4,6-dinitrobenzene (3i). Yield
27% (byproduct). Solid, m.p. 185.0–185.68C. 1H NMR (CDCl3, 400 MHz) d:
2.74 (s, 3H), 3.05 (s, 6H), 7.21 (d, J ¼ 16.1 Hz, 1H), 7.47 (d, J ¼ 7.88 Hz, 2H),
7.48, (d, J ¼ 16.1 Hz, 1H), 7.72 (s, 1H), 8.69 (s, 1H). 13C NMR (CDCl3,
62.9 MHz) d: 21.2, 40.1, 111.9, 115.7, 122.5, 123.6, 129.1, 131.0, 138.1,
138.6, 144.5, 145.4, 151.3 (13 of 14 expected signals after 19,100 scans).
FABþMS (m/z, relative intensity): 327 (Mþ, 25). Anal. calcd. for
C17H17N3O4: C 62.38; H 5.23; N 12.84, found C 62.25; H 5.12, N 12.70.
UV-VIS (acetonitrile): 1459 ¼ 2.2 ꢀ 104 mol21 cm21
.
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Received in the UK November 11, 2003

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