Ultrasound-promoted magnesium-ammonium bromide-mediated pinacol coupling of aromatic aldehydes and ketones

Article abstract of DOI:10.1080/00397910500296828

Pinacol coupling of aromatic aldehydes and ketones by Mg powder in 0.1-mol/L ammonium bromide aqueous solution can lead to the corresponding pinacols in 22-90% yields within 1-3 h at rt under ultrasound irradiation. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500296828

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Ultrasound‐Promoted
Magnesium‐Ammonium
Bromide–Mediated Pinacol
Coupling of Aromatic
Aldehydes and Ketones
Ji‐Tai Li a b , Yan‐Xue Chen a & Tong‐Shuang Li a
a College of Chemistry and Environmental Science ,
Hebei University , Baoding, China
b Key Laboratory of Analytical Science and
Technology of Hebei Province , Baoding, China
Published online: 22 Aug 2006.
To cite this article: Ji‐Tai Li , Yan‐Xue Chen & Tong‐Shuang Li (2005)
Ultrasound‐Promoted Magnesium‐Ammonium Bromide–Mediated Pinacol Coupling of
Aromatic Aldehydes and Ketones, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 35:22, 2831-2837
To link to this article: http://dx.doi.org/10.1080/00397910500296828
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Synthetic Communicationsw, 35: 2831–2837, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500296828
Ultrasound-Promoted Magnesium-
Ammonium Bromide–Mediated Pinacol
Coupling of Aromatic Aldehydes and
Ketones
Ji-Tai Li
College of Chemistry and Environmental Science, Hebei University and
Key Laboratory of Analytical Science and Technology of Hebei
Province, Baoding, China
Yan-Xue Chen, and Tong-Shuang Li
College of Chemistry and Environmental Science, Hebei University,
Baoding, China
Abstract: Pinacol coupling of aromatic aldehydes and ketones by Mg powder in 0.1-
mol/L ammonium bromide aqueous solution can lead to the corresponding pinacols in
22–90% yields within 1–3 h at rt under ultrasound irradiation.
Keywords: aromatic aldehydes, aromatic ketones, pinacol coupling, ultrasound
irradiation
INTRODUCTION
Carbon–carbon bond formation is the essence of organic synthesis. Reductive
coupling of carbonyl compounds, which leads to vicinal diols, is one of the
Received in Japan March 23, 2005
Address correspondence to Ji-Tai Li, College of Chemistry and Environmental
Science, Hebei University, Hezuo Road No. 88, Baoding 071002, China. Fax: þ86-
312-5079628; E-mail: orgsyn@mail.hbu.edu.cn
2831

2832
J.-T. Li, Y.-X. Chen, and T.-S. Li
most important reactions for the formation of carbon–carbon bonds. Within
the past several years, the formation of 1,2-diol has been attempted using a
number of regents such as Al,[1] Mg,[2] Te,[3] Mn,[4] Sm,[5] TiBr2-Cu, and
TiI4-Cu.[6] However, some of these methods require absolutely anhydrous
conditions in an inert atmosphere. Moreover, some of these reagents are
costly, moisture-sensitive, and toxic. Among them, activated magnesium
has been developed to help initiate the reaction and has received more
attention because of its ready availability and cost. With economical and
environmental concerns, the use of water as a solvent for metal-mediated
carbon–carbon bond formations has also generated considerable interest.
Ultrasound has increasingly been used in organic synthesis in the past
three decades. Compared with the standard methods, this technique requires
special equipment but is easily controlled. A number of organic reactions,
especially ones involving metal, can be carried out in higher yields, shorter
reaction times, and milder conditions using ultrasound irradiation.[7–9] Lim
et al. reported the reaction of aromatic aldehydes with indium in neutral
aqueous media using sonication afforded the corresponding diols in
moderate to good yield.[10] Mecarova and Toma reported the pinacol-
coupling reaction in aqueous media under ultrasound irradiation and found
that ultrasound considerably accelerates the benzaldehyde conversion.[8]
For a long time, our laboratory has also reported the pinacol coupling of
aromatic aldehydes and ketones under ultrasound irradiation.[11–14] In this
article, we report the piancol coupling of aromatic aldehydes and ketones
using Mg-NH4Br-H2O under ultrasound irradiation (Scheme 1).
ˇ
´
RESULTS AND DISCUSSION
Some aromatic aldehydes resulted in good yield using Mg powder in aqueous
NH4Br under ultrasound irradiation (Table 1). The effect of Mg powder on the
pinacol coupling of C6H4CHO (1 mmol, 1a) has been investigated using
2 mmol or 20 mmol of Mg in 0.1 mol/L of aqueous NH4Br under 25 kHz of
ultrasound irradiation for 1 h; the yield of pinacol was 81% (2ad, Table 1)
and 83% (2ag) respectively. We also examined the effect of the concentration
of NH4Br on the reaction. When the concentration is 0.5 mol/L, the yield of
Scheme 1.

Aldehyde and Ketone Pinacol Coupling
2833
Table 1. Pinacol coupling of carbonyl compounds in aqueous NH4Br under
ultrasound irradiation
Isolated
yield, %/2
Isolated
yield, %/3
Entry
Substratesa
C6H5CHO
dl/meso
a
71b
73c
78d
81e
75f
83g
50h
80i
44
6
8
32/68
—
—
2
4
—
—
7
2
—
—
10
6
—
b
c
d
e
f
2-ClC6H4CHO
3-ClC6H4CHO
2,4-Cl2C6H3CHO
4-ClC6H4CHO
4-CH3C6H4CHO
4-CH3OC6H4CHO
25
9
53/47
61/39
40/60
30/70
42/58
84/16
—
73
22
35
13
2
68
84
g
90
3
60j
54
8
h
i
3,4-(OCH2O)C6H3CHO
PhCOCH3
PhCOPh
4
dl only
71/29
na
66
0
0
j
0
aThe amount of substrate is 1 mmol.
b–eThe amount of Mg is 2 mmol; the reaction time is 10 min, 20 min, 40 min, 1 h
respectively.
fThe ultrasound irradiation frequency is 40 kHz; the reaction time is 1 h.
gThe amount of Mg is 20 mmol; the reaction time is 1 h.
hThe amount of Mg is 2 mmol; 0.05 mol/L aqueous NH4Br; the reaction time is 1 h.
iThe amount of Mg is 2 mmol; 0. 5 mol/L aqueous NH4Br; the reaction time is 1 h.
jStirred without ultrasound for 12 h.
pinacol was 80% (2ai), which was similar to that in 0.1 mol/L (81%, 2ae).
When the concentration of NH4Br decreased to 0.05 mol/L, the yied of
pinacol decreased to 50%. When prolonging the reaction time from 10 min
to 1 h, the yield of pinacol increased from 71% (2ab) to 81% (2ae). Ultrasound
irradiation frequency has effect on this reaction; for example, the yields
pinacol coupling of C6H4CHO (1a) under 25 kHz or 40 kHz irradiation were
81% (2ae) and 75% (2af) respectively.
We did the experiment in the absence of ultrasound; the coupling of
4-CH3OC6H4CHO was carried out using stirring within 12 h in 60% yield
(2g). While under ultrasound irradiation, the corresponding pinacol was
obtained with 90% yield within 3 h. It is clear that the ultrasound can accele-
rate the pinacolization. The optimized reaction conditions are 5 mL (0.1 mol/L)
aqueous NH4Br, 25 kHz irradiation frequency, and 2 mmol of Mg powder.

2834
J.-T. Li, Y.-X. Chen, and T.-S. Li
Using this reaction system, we performed a series of experiments for pinacol
coupling of aromatic aldehydes and ketons under ultrasound irradiation.
The data in Table 1 indicated that high yields of piancols can be obtained
when the substrates possess an electron-donating group (1f, 1g compared to
1b, 1c, 1d, 1e). The coupling of acetophenone (1i) was also successful. On
the other hand, diphenyl ketone (1j) was inert under the reaction conditions.
The magnesium-mediated pinacol coupling in water appears strongly
affected by steric effects. The positions of substituents present on the
aromatic rings show a dramatic effect on the course of the reaction. For
example, when the substrates were 2,4-Cl2C6H3CHO (1d) and 3,4-
(OCH2O)C6H3CHO (1h), yield of pinacol decreased dramatically. The
presence of para-substituted (1e) and meta-substituents (1c) increased the
formation of reduced product compared to ortho-substituents (1b) aromatic
aldehydes.
From the results we can also find that high yields of piancols were
obtained for some aromatic aldehydes; for example, 1,2-di (3-chlorophe-
nyl)-1,2-ethanediol (2c) was previously prepared in 42% yield using Mg-
NH4Cl under stirring for 12–24 h,[2] whereas under ultrasonication, 1,2-di
(3-chlorophenyl)-1,2-ethanediol (2c) was obtained in 73% yield within 3 h.
When 4-CH3C6H4CHO (1f), 4-CH3OC6H4CHO (1g), and PhCOCH3 (1i) as
the substrates, the corresponding pinacol products were obtained in 84%,
90% and 66% yields respectively.
The pinacol coupling reaction can form dl and meso stereoisomers.
Improved diastereoselectivity has been observed in our system compared to
the other system,[2,8] in which the ratio of dl and meso of the 1,2-diols were
nearly 1 : 1. In the present process, higher dl stereoisomers are obtained (2g,
2h, 2i). It is noteworthy that 3,4-(OCH2O)C6H3CHO (1h) formed dl isomer
only.
EXPERIMENTAL
Magnesium powders were purchased from a Beijing chemical reagents
company and used directly as received. IR spectra were recorded on
Bio-Rad FTS-40 spectrometer (KBr). MS were determined on a VG-7070E
1
spectrometer (EI, 70 eV). H NMR spectra was measured on Bruker Avance
400 (400-MHz) spectrometer using TMS as internal standard and CDCl3 as
solvent. Sonication was performed in Shanghai Branson-CQX ultrasonic
cleaner (with a frequency of 25 kHz and a nominal power of 250 W) and
SK 250 LH ultrasonic cleaner (with a frequency of 40 kHz and 59 kHz and
a nominal power of 250 W; Shanghai Kudos Ultrasonic Instrument Co.,
Ltd.). The reaction flasks were located in 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.

Aldehyde and Ketone Pinacol Coupling
General Procedure
2835
A 50-mL Pyrexw flask was charged with 3-ClC6H4CHO (140 mg, 1 mmol), Mg
powder (48 mg, 2 mmol), and NH4Br (5 mL, 0.1 mol/L). The mixture was irra-
diated in the water bath of an ultrasonic cleaner under an air conditions at rt for
3 h. After the completion of the reaction, the resulting suspension was filtered to
remove the Mg powder. The filtrate was extracted with ethyl acetate
(3 ꢀ 15 mL). The combined organic layers were washed with saturated aqueous
NaHCO3 solution and brine, dried over anhydrous magnesium sulfate for 12 h,
and filtered. Ethyl acetate was evaporated under reduced pressure to give the
crude product, which was separated by column chromatography on silica (200–
300 mesh) and eluted with petroleum ether or a mixture of petroleum ether and
diethyl ether. All the products were confirmed by comparing their Rf value and
their IR, MS, 1H NMR spectral data with of the authentic samples.
2a: 1H NMR: d 2.22 (2H, s, OH, meso), 2.87 (2H, s, OH, dl), 4.73 (2H, s,
CH, dl), 4.86 (2H, s, CH, meso), 7.14–7.39 (20H, m, Ph-H). m/z (%): 214 (1),
180 (7.6), 167 (12.5), 149 (6.0), 107 (93.8), 79 (100), 77 (73.8). IR (KBr) nmax
3200–3480.
:
2b: 1H NMR: d 2.84 (2H, s, OH, meso), 3.49 (2H, s, OH, dl), 5.40 (2H, s,
CH, dl), 5.57 (2H, s, CH, meso), 7.09–7.68 (16H, m, Ph-H). m/z (%): 282 (1),
165 (47), 141 (89), 113 (13), 107 (14), 77 (100), 51 (38). IR (KBr) nmax
:
3100–3500.
2c: 1H NMR: d 2.79 (2H, s, OH, meso), 3.44 (2H, s, OH, dl), 4.57 (2H, s,
CH, dl), 4.75 (2H, s, CH, meso), 6.85–7.33 (16H, m, Ph-H). m/z (%): 263
(1.2), 251 (1.6), 178 (4.6), 165 (4.6), 141 (100), 113 (23.8), 77 (71.0). IR
(KBr) nmax: 3260–3318.
2d: 1H NMR: d 2.87(2H, s, OH, meso), 3.47(2H, s, OH, dl), 5.27(2H, s, OH,
dl), 5.55 (2H, s, CH, meso), 7.16-7.37 (12H, m, Ph-H). m/z (%): 352 (1), 305
(1.4), 233(10), 175(100), 145(10), 111(25), 77(15). IR(KBr)nmax: 3320–3400.
2e: 1H NMR: d 2.58 (4H, s, OH, meso), 3.15 (2H, s, OH, dl), 4.58 (2H, s,
CH, dl), 4.81 (2H, s, CH, meso), 6.98–7.31 (16H, m, Ph-H). m/z (%): 276
(14), 249 (32), 155 (100), 111 (8). IR (KBr) nmax: 3380–3420.
2f: 1H NMR: d 2.37(12H, s, CH3), 3.04 (4H, s, OH), 4.63 (2H, s, CH, dl),
4.73 (2H, s, CH, meso), 7.04–7.28 (16H, m, Ph-H). m/z (%): 242 (1.2), 195
(6), 121 (100), 107 (12), 77 (13). IR (KBr) nmax: 3280–3450.
2g: 1H NMR: d 2.22 (2H, s, OH meso), 2.97 (2H, s, OH dl), 3.78 (6 H, s,
OCH3, dl), 3.82 (6 H, s, OCH3, meso), 4.62 (2H, s, CH, dl), 4.74 (2H, s, CH,
meso), 6.76–7.28 (16H, m, Ph-H). m/z (%): 276 (14), 249 (32), 155 (100), 111
(8). IR (KBr) nmax: 3300–3600.
2h: 1H NMR: d 2.87 (4H, s, OH), 4.56 (4H, s, CH, dl), 5.95 (8H, s, CH2),
6.47–6.86 (12H, m, Ph-H). m/z (%): 302 (1), 284 (2.5), 268 (5.0), 255 (11.8),
151 (100), 123 (32), 93 (77.1), 65 (39.0). IR (KBr) nmax: 3100–3600.
2i: 1H NMR: d 1.46 (6H, s, CH3, dl), 1.53 (6H, s, CH3, meso), 2.55 (4H, s,
OH), 7.16–7.38 (20H, m, Ph-H). m/z (%): 225 (4), 206 (4), 181 (32), 165 (9),
121 (100), 105 (12), 77 (11), 43 (80). IR (KBr) nmax: 3100–3600.

2836
J.-T. Li, Y.-X. Chen, and T.-S. Li
CONCLUSION
In summary, we have provided an efficient and convient procedure for the
synthesis of pinacols from some aromatic aldehydes using Mg-NH4Br in
water under ultrasound irradiation.
ACKNOWLEDGMENTS
Educational Ministry of China, Educational Department of Hebei Province
(2001104), and Natural Science Foundation of Hebei Province (203107),
China, supported the project.
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