Studies on the lanthanum-induced pinacol coupling of aromatic aldehydes and ketones in aqueous media

Article abstract of DOI:10.1080/00397910600781307

The pinacol coupling of aromatic aldehydes and ketones was carried out in 16-71% yield using La-36%HAc-CH₂Cl₂ system with stirring at rt in 4 h. The reactions in the same system gave pinacols in 12-91% yield under ultrasound irradiation at rt in 2 h. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600781307

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Studies on the Lanthanum‐Induced Pinacol
Coupling of Aromatic Aldehydes and Ketones
in Aqueous Media
a
b
a
c
Yan‐Jiang Bian , Xu‐Guang Yu , Hong‐Wei Peng & Ji‐Tai Li
a
Department of Chemistry , Langfang Normal College , Hebei, Langfang,
China
b
Beijing General Research Institute of Mining and Metallurgy , Beijing,
China
c
College of Chemistry and Environmental Science , Hebei University ,
Baoding, China
Published online: 27 Oct 2006.
To cite this article: Yan‐Jiang Bian , Xu‐Guang Yu , Hong‐Wei Peng & Ji‐Tai Li (2006) Studies on the
Lanthanum‐Induced Pinacol Coupling of Aromatic Aldehydes and Ketones in Aqueous Media, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:17,
2513-2518, DOI: 10.1080/00397910600781307
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Synthetic Communications , 36: 2513–2518, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600781307
Studies on the Lanthanum-Induced Pinacol
Coupling of Aromatic Aldehydes and
Ketones in Aqueous Media
Yan-Jiang Bian
Department of Chemistry, Langfang Normal College, Hebei,
Langfang, China
Xu-Guang Yu
Beijing General Research Institute of Mining and Metallurgy,
Beijing, China
Hong-Wei Peng
Department of Chemistry, Langfang Normal College, Hebei,
Langfang, China
Ji-Tai Li
College of Chemistry and Environmental Science, Hebei University,
Baoding, China
Abstract: The pinacol coupling of aromatic aldehydes and ketones was carried out in
2 2
16–71% yield using La–36%HAc–CH Cl system with stirring at rt in 4 h. The
reactions in the same system gave pinacols in 12–91% yield under ultrasound
irradiation at rt in 2 h.
Keywords: Acid aqueous media, lanthanum, pinacol coupling, ultrasound irradiation
[
1]
1
,2-Diols are very useful synthons for a variety of organic syntheses and
have been used as intermediates for the construction of biologically
[2]
important natural product skeletons.
A lot of methods for pinacol
Received in Japan January 5, 2005
Address correspondence to Yan-Jiang Bian, Department of Chemistry, Langfang
Normal College, Hebei, Langfang 065000, China. E-mail: bianyanjiang@126.com
2
513

2
514
Y.-J. Bian et al.
[
3]
coupling described previously claimed utilization of metals such as Mg,
[
4]
[5]
Mn, Al, and In. Within the past several years, the use of lantha-
[6]
[
7–11]
nides
have been developed to mediate the pinacol coupling reaction.
However, anhydrous conditions and long reaction times were required to
get satisfactory yields of the reaction products.
Ultrasound has increasingly been used in organic synthesis. Sono-
chemistry is also becoming more and more important for a variety of
synthetic organic reactions as an energy source to generate radicals and
initiate the electron transfer process. Many metal-involved organic reactions
[12]
have been accelerated under ultrasound.
Ultrasound irradiation can
activate the surface of metal powder, reduce the particle size, and bring
about an effective surface modification. Recently, we have reported the ultra-
sonic irradiation–promoted pinacol coupling of aromatic aldehydes by using
[
13]
[14]
[15]
[16]
AI, Mg, Mn, and Mn in aqueous media. This prompted us to study
the possibility of the pinacol coupling reactions of aromatic aldehydes using
lanthanum under ultrasound irradiation. The main aim of this work is to
examine whether lanthanum can be as the mediator of the pinacol coupling
in aqueous media and to find out if ultrasound can affect these reactions.
Herein, we report the results of the pinacol coupling reaction of aromatic
aldehydes and ketones in aqueous media by La with strring or under ultra-
sound respectively (Scheme 1).
When the substrate is C H CHO, we did the experiment using reaction
4
6
system La–36%HAc–CH Cl under ultrasound irradiation. From the result
2
2
in Table 1, we found that the amount of 36%HAc has big effect on the
reaction yield: when the amount of 36%HAc increased from 1 mL to 2 mL,
the yield of pinacol increases from 72% to 90.8%. Prolonging the reaction
time from 1 h to 2 h, increases the yield of pinacol from 78% to 91%.
When increasing the amount of La from 1 mmol to 1.4 mmol, the yield of
pinacol is not increased. Using Table 1, we chose the reaction conditions
C H CHO (1 mmol), La (1.4 mmol), CH C1 , and 36%HAc (2 mL) with a
6
4
2
2
reaction time of 2 h. Using this reaction system, we did a series of experiments
for pinacol coupling of aromatic aldehydes and ketones under ultrasound
irradiation or using stirring.
As shown in Table 2, some aromatic aldehydes via pinacol coupling can
give pinacols in good yields in acidic aqueous media under ultrasound
irradiation. For example, when the substrates are 1a, 1b, 1e, and 1f, the
Scheme 1.

Lanthanum-Induced Pinacol Coupling
2515
Table 1. Optimization of pinacol coupling of C
under ultrasound irradiation
6
H
4
CHO in acidic aqueous media
C
6
H
4
CHO Metal (La) CH Cl
(mL)
2
2
36%HAc
(mL)
Reaction
time (h)
Isolated
yield (%)
Entry
(mmol)
(mmol)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
1
1
1
2
2
2
3
2
2
1
2
3
1
2
3
2
2
2
60
72
85
78
90.8
91
91
91
91
1
1
1
1
1
1
1.4
2
Notes: Ultrasound irradiation frequency 40 kHz; reaction temperature 25–308C.
corresponding pinacol product was obtained in 91, 79, 80, and 76% yield
respectively for 2 h of ultrasound irradiation; for stirring, the yield of
pinacol was 52, 50, 68, and 71% respectively for 4 h, indicated that lower
yield of pinacols can be obtained using stirring compared to ultrasound
[
2,7]
irradiation. It is well known
via a single electron-transfer mechanism.
(Scheme 2) that pinacol coupling proceeds
Accordingly, the sonication is in favor of the reaction via single-electron
transfer mechanism. In contrast, aromatic aldehydes have heigher reactivity
than aromatic ketones. The steric hinderance around the carbonyl group
inhibits the coupling reaction.
Table 2. Pinacol coupling of aromatic aldehydes and ketones in aqueous media under
ultrasound irradiation or stirring
Isolated yield (%)
dl/meso
Entry
Substrates
Benzaldehyde
Ultrasound Stirring
Ultrasound
Stirring
a
b
c
d
e
f
91
79
67
60
80
76
12
35
52
50
28
21
68
71
—
16
40/60
50/50
meso
34/66
46/54
20/80
—
38/62
46/54
meso
42/58
52/48
18/82
—
p-Chlorobenzaldehyde
Furfural
Cinnamaldehyde
Piperonal
p-Methoxybenzaldehyde
Benzophenone
Acetophenone
g
h
55/45
62/38
1
Notes: Ratio of dl/meso was calculated by H NMR; ultrasound irradiation time for
h; stirring time for 4 h.
2

2
516
Y.-J. Bian et al.
Scheme 2.
In summary, ultrasound irradiation can efficiently prompt pinacol
coupling of some aromatic aldehydes in acidic aqueous media. The main
advantages of the present procedure are the milder reaction conditions, the
lower amount of catalyst, and operational simplicity.
EXPERIMENTAL
Liquid substrates were distilled before use. IR spectra were recorded on Bio-
Rad FTS-40 spectrometer (KBr). MS spectra were determined on an AEI
1
MS-50 SD90 spectrometer (EI, 70 eV). H NMR spectra were measured
on VXR-300s spectrometer (300 MHz) using CDCl as solvent and TMS
3
as internal standard. Sonication was performed in a Shanghai SK8200LH
ultrasonic cleaner (with a frequency of 40 KHz and a nominal power 500 W;
Shanghai Kudos Ultrasonic Instrument Co., Ltd.). The reaction flask was
located at the maximum energy area in the cleaner, where the surface of
reactants is slightly lower than the level of the water. The reaction temperature
was controlled by addition or removal of water from ultrasonic bath.
General Procedure for the Pinacol Coupling of Aromatic Aldehydes
in Acidic Media under Ultrasound Irradiation
w
A 100-ml Pyrex flask was charged with the desired aldehyde or ketone
(1, 1 mmol), La powder (0.2 g, 1.4 mmol), 36%HAC (2 mL), and CH Cl
2 2
(
2 mL). The mixture was irradiated in the water bath of an ultrasonic cleaner
under air conditions at 25–308C for 2 h. After the completion of the
reaction, the resulting suspension was filtered to remove the La powder. The
filtrate was extracted with ethyl acetate (3 Â 15 mL). The combined organic
layers were washed with saturated aqueous NaHCO solution and brine,
3
dried over anhydrous magnesium sulfate for 12 h, and filtered. Ethyl acetate
was evaporated under reduced pressure to give the crude product, which was

Lanthanum-Induced Pinacol Coupling
2517
separated by column chromatography on silica (200–300 mesh) and eluted
with petroleum ether or a mixture of petroleum ether and diethyl ether. All
1
the products were confirmed by IR, MS, H NMR, and spectral data.
Data
1
a. H NMR: d 2.32 (s, 2H, OH), 4.84 (s, dl) and 4.92 (s, meso) (2H, PhCH–
2
H
)
, 7.20–7.35 (m, 10H, Ar); m/z (%): 214 (1), 180 (7.6), 167 (12.5), 149 (6.0),
2
1
.
1
07 (93.8), 79 (100), 77 (73.8); IR(KBr) v: 3400–3480 cm
1
b. H NMR: d 2.36 (s, 2H, OH), 4.68 (s, dl) and 4.90 (s, meso) (2H, PhCH–
2
H
)
, 7.10–7.38 (m, 8H, Ar); m/z (%): 282 (10), 235 (12), 165 (5.5), 141 (100),
2
1
1
07 (19), 77 (10); IR (KBr) v: 3380–3420 cm
.
1
c. H NMR: dH 2.10 (s, 2H, –OH), 5.11 (s, dl) and 5.16 (s, meso) (2H,
2
fur-CH–), 6.34 (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); IR (KBr) v:
2
1
3
240–3300 cm
.
1
d. H NMR: dH 1.92 (s, 2H, OH), 4.54 (s, meso) (2H, –CH–OH), 6.42
2
(
(
(
t, 2H, –CH 55 CH–), 6.78 (d, 2H, –CH 55 CH–); m/z (%): 282 (1), 266
15), 221 (12), 177 (24), 162 (26), 151 (30), 135 (23), 120 (70), 85 (38), 77
2
1
17), 57 (90); IR (KBr) v: 3360–3450 cm
.
1
e. H NMR: d 2.24 (s, 2H, OH), 5.62 (s, 2H, phCH–), 5.98 (s, 4H, CH ),
2
6
1
H
2
.71–6.89 (m, 6H, Ar); m/z (%): 302 (1), 284 (2.5), 268 (5.0), 255 (11.8),
2
1
51 (100), 123 (32), 93 (77.1), 65 (39.0); IR (KBr) v: 3450–3490 cm
.
1
f. H NMR: dH 3.08 (s, 2H, OH), 3.79 (s, dl) and 3.90 (s, meso) (s, 6H,
2
OCH ), 4.78 (s, dl) and 4.89 (s, meso) (2H, PhCH–), 6.98–7.38 (m, 8H,
3
Ar); m/z (%): 274 (1), 256 (2), 227 (30), 137 (100), 109 (9), 77 (8); IR
2
1
(KBr) v: 3300–3380 cm
.
1
h. H NMR: d 1.66 (s, dl) and 1.75 (s, meso) (6H, –CCH ), 2.26 (s, 2H,
2
OH), 7.33–7.46 (m, 10H, Ar); m/z (%): 225 (4), 206 (4), 181 (32), 165 (9),
H
3
2
1
1
21(100), 105 (12), 77 (11), 43 (80); IR (KBr) v: 3364–3494 cm
.
ACKNOWLEDGMENTS
The project was supported by Natural Science Foundation of Hebei Province
(
(
(
203107), Science and Technology Foundation of Hebei Province
04213041), and Science Foundation of Hebei Lang Fang Normal College
200517).

2
518
Y.-J. Bian et al.
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