Pinacol coupling of aromatic aldehydes using aluminium under ultrasound irradiation

Article abstract of DOI:10.1081/SCC-120003606

The pinacol coupling reaction of aromatic aldehydes and ketones was carried out in 26-98% yield with Al in aqueous NaOH-MeOH under ultrasound irradiation at r.t. within 30 min.

Full text of DOI:10.1081/SCC-120003606

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PINACOL COUPLING OF AROMATIC ALDEHYDES USING
ALUMINIUM UNDER ULTRASOUND IRRADIATION
Yan-Jiang Bian a , Shu-Ming Liu a , Ji-Tai Li a & Tong-Shuang Li a
a College of Chemistry and Environmental Science , Hebei University , Baoding, 071002,
China
Published online: 16 Aug 2006.
To cite this article: Yan-Jiang Bian , Shu-Ming Liu , Ji-Tai Li & Tong-Shuang Li (2002) PINACOL COUPLING OF AROMATIC
ALDEHYDES USING ALUMINIUM UNDER ULTRASOUND IRRADIATION, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 32:8, 1169-1173, DOI: 10.1081/SCC-120003606
To link to this article: http://dx.doi.org/10.1081/SCC-120003606
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SYNTHETIC COMMUNICATIONS, 32(8), 1169–1173 (2002)
PINACOL COUPLING OF AROMATIC
ALDEHYDES USING ALUMINIUM UNDER
ULTRASOUND IRRADIATION
*
Yan-Jiang Bian, Shu-Ming Liu, Ji-Tai Li,
and Tong-Shuang Li
College of Chemistry and Environmental Science,
Hebei University, Baoding 071002, China
ABSTRACT
The pinacol coupling reaction of aromatic aldehydes and
ketones was carried out in 26–98% yield with Al in aqueous
NaOH–MeOH under ultrasound irradiation at r.t. within
30 min.
Key Words: Pinacol coupling; Ultrasound; Aluminum reagent
Carbon–carbon bond formation is the essence of organic synthesis.
One of the most powerful methods for constructing carbon–carbon bond
is the pinacol coupling of aromatic aldehydes and ketones. In the last several
years, various metals such as Zn,1 Mn,2 Ln,3 Sm,4 and Mg5 have been
developed to mediate the pinacol coupling reaction. Recently, the pinacol
coupling of aromatic aldehydes and ketones has been reported to produce
1,2-diols using Al6,7 in the presence of potassium hydroxide. However, the
base reagent used is much more excessive (substrate/base ¼ 1/9, mole ratio),
which will lead to economical and environmental concerns.
*Corresponding author. E-mail: orgsyn@mail.hbu.edu.cn
1169
Copyright & 2002 by Marcel Dekker, Inc.
www.dekker.com

1170
BIAN ET AL.
Ultrasound has increasingly been used in organic synthesis.8,9 Many
metal-involved organic reactions have been accelerated under ultrasound.
Recently, we reported that ultrasonic irradiation could promote the pinacol
10
coupling of aromatic aldehydes and ketones by using Zn–ZnCl2 and
Mg–NH4Cl.11 This prompted us to study the possibility of pinacol coupling
aromatic aldehydes and ketones with Al using a small amount of sodium
hydroxide under ultrasound. Herein, we report the results of pinacol coup-
ling of aromatic aldehydes and ketones with Al in aqueous NaOH–MeOH
under ultrasound.
As shown in table and scheme, the coupling of aromatic aldehydes
gives pinacols in good yield using the Al–NaOH–MeOH system under
ultrasound. The data in the table show that ultrasonic irradiation is a
highly chemoselective method. For example, benzaldehyde (1a), p-methoxy
benzaldehyde (1e) and p-methyl benzaldehyde (1g) were reacted with
aluminium in aqueous NaOH–MeOH. The corresponding pinacol-
coupling products were obtained in 91% (2a), 89% (2e) and 88% (2g)
yield respectively, and 3-chlorobenzaldehyde (1d) and 4-chlorobenzalde-
hyde (1c) gave nearly quantitative yields of 1,2-diols (2d, 2c). The steric
hindrance around the carbonyl groupmay inhibit the coupling reaction,
for 2,4-dichlorobenzaldehyde gives only 61% pinacol. When aromatic
ketones such as p-methoxyacetophenone and p-chloroacetophenone were
used as substrates, fewer pinacols (2) and more alcohols (3) were obtained.
The reaction with cinnamaldehyde was unsuccessful under the same
conditions.
Scheme.
In conclusion, we have demonstrated that the Al–NaOH–MeOH
system can be used for pinacol coupling reactions of aromatic aldehydes
under ultrasound irradiation. Compared with classical methods, the main
advantage of the present procedure is that the base used is reduced
(substrate/base ¼ 1/3, mole ratio), thus, the environmental concerns are
also improved.

PINACOL COUPLING OF AROMATIC ALDEHYDES
Table.
Ultrasound*
1171
Pinacol Coupling Mediated by Al in Aqueous NaOH–MeOH Under
Isolated Yield, % (lit.6)
Reaction Time
(min)
Entry
Substrate
2
3
dl/meso
51 : 49
a
20
30
20
91 (87)
5
b
c
61
0
0
meso
98 (93)
71 : 29
d
20
98
0
60 : 40
e
f
20
30
20
89 (98)
60
8
10
8
54 : 46
66 : 34
71 : 29
g
88 (91)
h
30
70
6
–
i
30
30
30
30
26 (89)
41
50
66
0
–
–
–
j
k
*Isolated yield based on the substrate. Ratio of dl/meso was calculated by 1H NMR.
EXPERIMENTAL
Liquid substrates were purified by distillation prior to use. Melting
points were uncorrected. IR spectra were recorded on a Bio-Rad FTS-40
spectrometer (KBr). 1H NMR spectra were measured on a VXR-300S
spectrometer (300 MHz) by using CDCl3 as solvent and TMS as internal
standard. Mass spectra were determined on an AEI MS-50 SD90 spectrom-
eter (EI, 70 eV). Sonication was performed in a Shanghai Branson-CQX
ultrasonic cleaner with a frequency of 25 kHz and a nominal power 500 W.

1172
BIAN ET AL.
The reaction flask was located at the maximum energy area in the cleaner
and the temperature of the water bath was controlled by addition or removal
of water.
General Procedure
A 100 ml Pyrex flask was charged with the desired aldehydes or ketone
(1, 1 mmol), Al powder (0.1 g, 3.7 mmol), 10% aqueous NaOH (1.5 ml), and
methanol (3 ml). The mixture was irradiated in the water bath of an ultrasonic
cleaner at room temperature for 20–30 min. The reaction was quenched with
3 M aqueous HCl and extracted with ethyl acetate (3 ꢀ 15 ml). The combined
organic layers were washed with saturated aqueous NaHCO3 solution and
brine, dried over magnesium sulfate and filtered. The filtrate was concen-
trated in vacuum to give a crude material, which was separated by column
chromatography on silica (200–300 mesh), eluted with petroleum ether (b.p.
60–90ꢁC) or a mixture of petroleum ether and diethyl ether. The authenticity
of the product was established by their spectra.
2a: dH 2.30 (s, 2H, OH), 4.71 (s, dl) and 4.83 (s, meso) (2H, phCH-),
7.11–7.32 (m, 10H, Ar); m/z (%): 214 (1), 180 (7.6), 167 (12.5), 149 (6.0), 107
(93.8), 79 (100), 77 (73.8); nmax (KBr): 3400–3480 cmꢂ1
.
2b: dH 2.98 (s, 2H, OH), 5.49 (s, meso) (2H, phCH-), 7.00–7.26 (m, 6H,
Ar); m/z (%): 352 (1), 305 (1.4), 233 (10), 175 (100), 145 (10), 111 (25), 77
(15); nmax (KBr): 3320–3400 cmꢂ1
.
2c: dH 2.32 (s, 2H, OH), 4.63 (s, dl) and 4.85 (s, meso) (2H, phCH-),
7.02–7.32 (m, 8H, Ar); m/z (%): 282 (10), 235 (12), 165 (5.5), 141 (100), 107
(19), 77 (10); nmax (KBr): 3380–3420 cmꢂ1
.
2d: dH 2.33 (s, 2H, OH), 4.55 (s, dl) and 4.71 (s, meso) (2H, phCH-),
7.07–7.21 (m, 8H, Ar); m/z (%) 263 (1.2), 251 (1.6), 178 (4.6), 165 (4.6), 141
(100), 113 (23.8), 77 (71.0); nmax (KBr): 3260–3318 cmꢂ1
.
2e: dH 2.00 (s, 2H, OH), 3.76 (s, dl) and 3.80 (s, meso) (s, 6H, OCH3),
4.64 (s, dl) and 4.74 (s, meso) (2H, phCH-), 6.74–7.26 (m, 8H, Ar); m/z (%):
274 (1), 256 (2), 227 (30), 137 (100), 109 (9), 77 (8); nmax (KBr):
3300–3380 cmꢂ1
.
2f: dH 2.04 (s, 2H, -OH), 5.00 (s, dl) and 5.02 (s, meso) (2H,
fur-CH-), 6.27 (m, 2H, 3,4-fur-H), 7.26 (m, 1H, 5-fur-H). m/z (%): 196
(10), 178 (25), 152 (73), 137 (33), 98 (100), 84 (22), 49 (30); nmax (KBr):
3240–3300 cmꢂ1
.
2g: dH 2.31 (s, 6H, CH3), 2.51 (s, 2H, OH), 4.67 (s, dl) and 4.75
(s, meso) (2H, -CH-OH), 7.04–7.26 (m, 8H, Ar); m/z (%): 242 (1), 195
(6), 121 (100), 107 (12), 93 (19), 77 (13); nmax (KBr): 3280–3450 cmꢂ1
.

PINACOL COUPLING OF AROMATIC ALDEHYDES
1173
2h: dH 2.20 (s, 2H, OH), 4.57 (s, 2H, phCH-), 5.92 (s, 4H, CH2)
6.64–6.71 (m, 6H, Ar); m/z (%): 302 (1), 284 (2.5), 268 (5.0), 255 (11.8),
151 (100), 123 (32), 93 (77.1), 65 (39.0); nmax (KBr): 3450–3490 cmꢂ1
.
2k: dH 1.47 (s, dl) and 1.56 (s, meso) (6H, -CCH3), 2.2 (s, 2H, OH), 3.8
(s, 6H, OCH3), 6.7–7.3 (m, 8H, Ar), m/z (%): 302 (1), 151 (100), 135 (5), 109
(4), 77 (3), 43 (60); nmax (KBr): 3200–3600 cmꢂ1
.
ACKNOWLEDGMENTS
The project was supported by National Natural Science Foundation of
China (29872011), Educational Ministry of China and Educational
Department of Hebei Province (99014). The project was supported by
Natural Science Foundation of Hebei Province (299080).
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Received in the Netherlands May 12, 2001