Preparation of aromatic ethers and dioxolanes under microwave irradiation

Article abstract of DOI:10.1081/SCC-100106043

Aromatic ethers and dioxolane derivatives were prepared in a domestic microwave oven adapted with a Dean-Stark collector. The reactions under microwave heating were shorter in duration than the reactions in conventional methods, confirming the potentiality of microwave energy in liquid chemical preparations.

Full text of DOI:10.1081/SCC-100106043

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PREPARATION OF AROMATIC ETHERS AND
DIOXOLANES UNDER MICROWAVE IRRADIATION
Gean V. Salmoria a , Ademir Neves b , Evandro L. Dall'Oglio b & César Zucco c
a Laboratoire de Electro-Thermia Microondes , ENSEEIHT, 2 Rue Camichel, Toulouse,
31000, France
b Departamento de Química , Universidade Federal de Santa Catarina , Florianópolis,
Santa Catarina, 88049-900, Brazil
c Departamento de Química , Universidade Federal de Santa Catarina , Florianópolis,
Santa Catarina, 88049-900, Brazil
Published online: 20 Aug 2006.
To cite this article: Gean V. Salmoria , Ademir Neves , Evandro L. Dall'Oglio & César Zucco (2001) PREPARATION OF
AROMATIC ETHERS AND DIOXOLANES UNDER MICROWAVE IRRADIATION, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 31:21, 3323-3328, DOI: 10.1081/SCC-100106043
To link to this article: http://dx.doi.org/10.1081/SCC-100106043
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SYNTHETIC COMMUNICATIONS, 31(21), 3323–3328 (2001)
PREPARATION OF AROMATIC
ETHERS AND DIOXOLANES UNDER
MICROWAVE IRRADIATION
Gean V. Salmoria,1 Ademir Neves,2
Evandro L. Dall’Oglio,2 andCe sar Zucco2,
*
1Laboratoire de Electro-Thermia Microondes,
ENSEEIHT, 2 Rue Camichel,
31000 Toulouse, France
2Departamento de Quımica, Universidade
Federal de Santa Catarina, Florianopolis,
88049-900 Santa Catarina, Brazil
ABSTRACT
Aromatic ethers and dioxolane derivatives were prepared in a
domestic microwave oven adapted with a Dean–Stark collec-
tor. The reactions under microwave heating were shorter in
duration than the reactions in conventional methods, confir-
ming the potentiality of microwave energy in liquid chemical
preparations.
*Corresponding author.
3323
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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SALMORIA ET AL.
The Williamson reaction is still the best general method for the
preparation of symmetrical and unsymmetrical ethers. The synthesis is a
nucleophilic substitution reaction of alkyl halides and, because it requires
a high concentration of a good nucleophile, it is an SN2 process. The reac-
tion can also be carried out with aroxide ion prepared from a substituted
phenol1 (Scheme 1).
Scheme 1.
Dioxolane derivatives are used as blocking or protecting groups
in many synthetic reactions.2–4 1,2-Diols easily form acetal and ketal
groups with aldehydes and ketones; these 1,3 dioxolane derivatives are
resistant to basic and nucleophilic reagents, but are readily removed by
aqueous acids.
Dioxolanes are usually prepared by refluxing the carbonyl compound
and excess ethylene glycol in benzene, in presence of acid catalysts and the
water by-product is continuously removed by slow collection of the con-
densate by a Dean–Stark phase separator (Scheme 2). The time required for
the liberation of the theoretically-expected quantity of water gives a mini-
mum qualitative indication of the carbonyl compound’s reactivity.5
Scheme 2.
The preparation of both substituted functionalized aromatic ethers
and dioxalanes in large scale, following general procedures described in
the literature,6 was, in our hands, a low yield reaction and time consuming.
Therefore, we turned our attention to the use of a microwave oven adapted
with a reflux system and a Dean–Stark phase separator in order to collect
the water from the reaction media.

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3325
The etherification reactions conducted under microwave irradiation
occurred at a 6 to 9 times faster rate than with conventional reflux (Table 1).
The yield is significant higher compared to the literature.6
A critical component in the 4-alkoxybenzaldehyde preparation by pri-
mary alkylation of 4-hydroxybenzaldehyde under basic conditions is the
solvent which dissolves both the phenolic salt and the alkylhalide.1 The
mixture DMF/benzene gave good results, even with the microwave process.
The solution absorbs well the microwave energy due to the DMF’s high
dielectric permittivity7 and to the medium polarity7–10 (Ohmic effect).
The dioxolane preparations conducted under microwave irradiation
showed a rate enhancement of 3.5 to 4.1 and gave better yield than conven-
tional reflux (Table 2).
The highest ratios, 4.0and 4.1, were for the most polar compound
reactions, 4-methoxybenzaldehyde and acetophenone. The reaction mixture
solution demonstrated a good absorption of microwave energy due to ethy-
lene glycol dielectric permittivity.
The microwave reaction system was practical, efficient and safe, rein-
forcing the use of microwave technology for reactions in solution and its
possibilities in industrial continuous processes.
General Procedure for Etherifications
A solution of 4-hydroxybenzaldehyde (10.0 g, 80 mmol) and KOH
(4.5 g, 80mmol) in 120mL of a 1 : 1 mixture of DMF-benzene (caution!
use fume hood) was refluxed until the refluxing solvent was no longer
cloudy. The alkyl bromide (80mmol) (Table 1) was added and reflux
Table 1. A Comparison of Reaction Times and Yields in Etherifications Using
Microwave and Conventional Heating
Heating*
Conventional
Microwave
Product
tc
Yield
tmw
Yield
tc/tmw**
4-n-Butoxybenzaldehyde
4-n-Hexoxybenzaldehyde
6 h
6 h
58%
62%
45 min
60min
86%
85%
9.0
6.0
*The results are the mean of three experiments.
**The ratio tc/tmw (tc and tmw mean the time that the reaction was left under thermal
and microwave heating, respectively), quantifies the microwave effect.

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SALMORIA ET AL.
A Comparison of Reaction Times and Yields in the Dioxolane
Preparations Using Microwave and Conventional Heating
Table 2.
Heating*
Conventional
Microwave
Carbonyl Compound
tc
Yield %
tmw
yield %
tc/tmw**
4.0
4-Methoxybenzaldehyde
4-n-Butoxybenzaldehyde
4-n-Hexoxybenzaldehyde
Acetophenone
4 h
5 h
5 h
4 h
72
602 h
58
1 h
79
2 h
86
3.5
75
3.5
min7050 90 4.1
*The results are the mean of three experiments.
**The ratio tc/tmw (tc and tmw mean the time that the reaction was left under thermal
and microwave heating, respectively), quantifies the microwave effect.
continued. The reaction was monitored by UV spectrometry at 250nm on
aliquots. The insoluble solid was filtered from the cooled reaction mixture.
Solvents were removed in vacuo. Distillation of remaining liquid gave the
products.
Analysis Data
4-n-Butyloxybenzaldehyde: 86% yield, b.p. 114–5ꢀC/2 mmHg (lit.11
147–9ꢀC/15 mmHg). IR (film): 2754, 1691 (intense), 1608, 1580 cmꢁ1. H1
NMR (CDCl3): d ¼ 9.85 (s, 1H), 7.67 (d, 2H, J ¼ 8 Hz), 6.85 (d, 2H,
J ¼ 7 Hz), 3.91 (t, 2H, J ¼ 6 Hz) and 2.02–0.70 (m, 7 H). MS: m/e (%)
51 (20), 65 (30), 77 (15), 93 (11), 121 (100), 178 (24).
4-n-hexyloxybenzaldehyde: 85% yield, b.p. 135–6ꢀC (lit.11 154–5ꢀC/
6 mmHg). IR (film): 2750, 1690 (intense), 1600, 1580 cmꢁ1. H1 NMR
(CDCl3): d ¼ 9.84 (s, 1H), 7.69 (d, 2H, J ¼ 8 Hz), 6.84 (d, 2H, J ¼ 8 Hz),
3.91 (t, 2H, J ¼ 6 Hz) and 2.02–0.70 (m, 7H). MS: m/e (%) 51 (12), 65 (23),
77 (17), 93 (09), 121 (100), 206 (28).
General Procedure for Dioxolane Preparations
A mixture of carbonyl compound (0.22 mol) (Table 2), ethylene glycol
(0.44 mol, 23.4 g), p-toluenesulfonic acid monohydrate (0.1 g) and benzene
(20mL) was refluxed under microwave and conventional heating until

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3327
aqueous phase no longer separated. After neutralization with NaOH 0.5 M,
the organic phase was subjected to fractional distillation giving the product.
Spectral Data
2-(4-Methoxyphenyl)1,3-dioxalane: 86% yield, b.p. 120–1ꢀC/2 mmHg
(lit.5 158–160/17 mmHg). IR (film): 2952, 2879, 1612–1515, 1246 and
1082 cmꢁ1. H1 NMR (CDCl3): d ¼ 8.2 (d, 2H, 8 Hz), 7.7 (d, 2H, J ¼ 8 Hz),
6.5 (s, 1H), 4.7 (s, 4H) and 3.8 (s, 3H). MS: m/e (%) 51 (24), 65 (28), 73 (80),
77 (31), 94 (100), 107 (30), 121 (52), 179 (89), 180 (34).
2-(4-n-Butoxyphenyl)1,3-dioxalane: 79% yield, b.p. 145–7ꢀC/1 mmHg.
IR (film): 2951, 2879, 1612–1516, 1245 and 1078 cmꢁ1. H1 NMR (CDCl3):
d ¼ 8.2 (d, 2H, 8 Hz), 7.8 (d, 2H, J ¼ 8 Hz), 6.5 (s, 1H), 4.8 (s, 4H), 2.7 (m,
2H), 2.1 (m, 4H) and 1.7 (m, 3H). MS: m/e (%) 51 (25), 65 (34), 73 (71), 77
(28), 94 (100), 107 (33), 121 (62), 135 (15), 149 (28), 165 (62), 221 (91), 222
(36).
2-(4-n-Hexoxyphenyl)1,3-dioxalane: 75% yield, b.p. 140–2ꢀC/
0.5 mmHg. IR (film): 2950, 2879, 1611–1515, 1245 and 1080 cmꢁ1
.
H1 NMR (CDCl3): d ¼ 8.2 (d, 2H, 8 Hz), 7.7 (d, 2H, J ¼ 8 Hz), 6.5 (s, 1H)
4.6 (s, 4H), 2.6 (m, 2H), 2.0(m, 8H) and 1.6 (m, 3H). MS: m/e (%) 51 (13),
65 (26), 73 (90), 77 (23), 94 (100), 107 (26), 121 (43), 135 (13), 149 (24), 165
(80), 178 (23), 249 (87), 250 (30).
2,2-Methylphenyl-1,3-dioxalane: 90% yield, m.p. 60ꢀC (lit.12 59.60). IR
(film): 2961, 2870, 1611–1510, 1245 and 1077 cmꢁ1. H1 NMR (CDCl3):
d ¼ 8.0(m, 5H, J ¼ 8 Hz), 4.6 (s, 4H) and 2.6 (s, 3H). MS: m/e (%) 51
(14), 65 (25), 77 (23), 91 (14), 104 (100), 164 (32).
ACKNOWLEDGMENTS
We thank CNPq and PRONEX for supporting, in part, this work.
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Received in the USA January 21, 2001

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