Pinacol coupling of aromatic aldehydes and ketones using TiCl 3-Al-EtOH under ultrasound irradiation

Article abstract of DOI:10.1081/SCC-200039396

Titanium trichloride in EtOH can be reduced by Al to the corresponding low-valent titanium complexes. This can reduce some aromatic aldehydes and ketones to the corresponding pinacols in 40-82% yields within 30-90 min at r.t. under ultrasound irradiation.

Full text of DOI:10.1081/SCC-200039396

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Pinacol Coupling of Aromatic
Aldehydes and Ketones
Using TiCl ‐Al‐EtOH Under
3
Ultrasound Irradiation
a
a
a
Ji‐Tai Li , Zhi‐Ping Lin , Na Qi & Tong‐Shuang
a
Li
a
College of Chemistry and Environmental Science,
Hebei University, Key Laboratory of Analytical
Science and Technology of Hebei Province, Hezuo
Road No. 88, Baoding, 071002, P.R. China
Published online: 10 Jan 2011.
To cite this article: Ji‐Tai Li , Zhi‐Ping Lin , Na Qi & Tong‐Shuang Li (2004):
Pinacol Coupling of Aromatic Aldehydes and Ketones Using TiCl ‐Al‐EtOH Under
3
Ultrasound Irradiation, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 34:23, 4339-4348
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w
SYNTHETIC COMMUNICATIONS
Vol. 34, No. 23, pp. 4339–4348, 2004
Pinacol Coupling of Aromatic Aldehydes
and Ketones Using TiCl -Al-EtOH Under
Ultrasound Irradiation
3
*
Ji-Tai Li, Zhi-Ping Lin, Na Qi, and Tong-Shuang Li
College of Chemistry and Environmental Science, Hebei University,
Key Laboratory of Analytical Science and Technology of Hebei Province,
Baoding, P.R. China
ABSTRACT
Titanium trichloride in EtOH can be reduced by Al to the corresponding
low-valent titanium complexes. This can reduce some aromatic aldehydes
and ketones to the corresponding pinacols in 40–82% yields within
30–90 min at r.t. under ultrasound irradiation.
Key Words: Pinacol coupling; Pinacol; Synthesis; Low-valent titanium;
Ultrasound irradiation.
*
Correspondence: Ji-Tai Li, College of Chemistry and Environmental Science,
Hebei University, Key Laboratory of Analytical Science and Technology of Hebei
Province Hezuo Road No. 88, Baoding 071002, P.R. China; Fax: þ86-312-5079628;
E-mail: orgsyn@mail.hbu.edu.cn.
4
339
DOI: 10.1081/SCC-200039396
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

4
340
Li et al.
INTRODUCTION
One of the most powerful methods for constructing a carbon–carbon
bond is the reductive coupling of carbonyl compounds giving olefins and
1
1
,2-diols. Of these methods, the pinacol coupling, which was described in
859, is still a useful tool for the synthesis of vicinal diols.
[1,2]
1,2-Diols
obtained in the reaction were very useful synthons for a variety of organic
syntheses and were also used as intermediates for the construction of biologi-
cally important natural product skeletons and asymmetric ligands for catalytic
[
3]
asymmetric reaction. In particular, pinacol coupling has been employed as a
[4]
key step in the construction of HIV-protease inhibitors. Recent efforts have
focused on the development of new reagents and reaction systems to improve
the reactivity of the reagents and diastereoselectivity of the products.
Low-valent titanium is a highly reactive reagent and is becoming increas-
[
5]
ingly attractive for carbonyl-coupling reactions. In 1973, Mukaiyama first
reported that TiCl -Zn reduced aromatic aldehydes and ketones to produce
4
[
6]
the corresponding 1,2-diols in high yield, but the stereoselectivity was not
reported. In 1982, Clerici et al. reported the pinacol coupling of aromatic alde-
[7]
hydes and ketones promoted by aqueous titanium trichloride in basic media.
The reaction was completed in a few minutes, but the reducing power of the
3
þ
4þ
Ti /Ti system is strongly pH dependent, and the method has some limit-
ations with respect to some aromatic aldehydes and ketones. Clerici et al.
again reported pinacolization of aromatic aldehydes mediated by titanium
[
8]
trichloride in dichloromethane in 1996. The reaction was completed in
high dl-stereoselectivity, but aromatic aldehydes bearing an electron-donating
group showed lower reactivity. In 2001, Yamamoto et al. reported diastereo-
selective pinacol coupling of aldehydes promoted by monomeric titanocene
[
9]
(
III) complex Cp TiPh. Five aromatic aldehydes gave desired pinacol in
2
5
obtained in pinacol coupling mediated by TiCl -Mg with a high stereoselec-
4–96% yields within 1–4 h. In 2000, Li et al. reported that 1,2-diols were
4
[10]
tivity.
However, in spite of their potential utility, some of the reported
methods suffer from drawbacks, such as longer reaction times, expensive
catalysts, lower yields, and cumbersome product isolation procedures.
Ultrasound has increasingly been used in organic synthesis in the last three
decades. Compared with traditional methods, this technique is more convenient
[
11]
and easily controlled. A large number of organic reactions, especially many
metal-involved reactions, can be carried out in higher yields, shorter reaction
[
12–14]
times and milder conditions under ultrasound irradiation.
reported the pinacolization mediated by TiCl -THF-Zn or TiCl -THF-Al at
Recently, we
4
4
[
15]
room temperature under ultrasound irradiation.
Eight pinacols were
obtained in 33–98% yield within 4–35 min under ultrasound irradiation. All
of the results stated above prompted us to study the possibility of the pinacol

Pinacol Coupling of Aromatic Aldehydes and Ketones
4341
coupling of aromatic aldehydes mediated by TiCl -Al-EtOH system under
3
ultrasound (Scheme 1).
RESULTS AND DISCUSSION
The effect of the reaction conditions on pinacolization under ultrasound
irradiation was summarized in Table 1. When the molar ratio of 4-ClC H
CHO : TiCl : Al was 1 : 2 : 3, 1,2-di(4-chlorophenyl)-1,2-ethanediol (2a) was
6
4-
3
obtained in 79% yield. Lowering the molar ratio to 1 : 1.5 : 3 lowers the
yield of the 1,2-diol to 50%. The most important discovery was that changing
the 4-ClC H CHO : TiCl : Al molar ratio to 1 : 2 : 4 lowers the yield of the
6
4
3
1,2-diol to 70%. Increasing this ratio to 1 : 2 : 8 caused a decrease in the
yield to 52% (entry c, f). This finding is of interest especially from the environ-
mental point of view. Increasing the 4-ClC H CHO : TiCl : Al molar ratio to
3
6
4
1 : 3 : 6, 1 : 3 : 8, and 1 : 4 : 8 increases the yield of the 1,2-diol to 67%, 80%,
and 83% (entry d, e, g). The results showed that changing the molar ratio of
TiCl with Al powder had a significant effect on the yield of the 1,2-diol.
3
We also observed the effect of different frequencies of ultrasound
irradiation on the reaction. When the frequency was 25 kHz, the coupling of
benzaldehyde resulted in the desired product in 75% yield within 60 min
(entry h). Under 40 kHz and 59 kHz ultrasound irradiation, the coupling of
benzaldehyde was completed in 60 min with 74% and 55% yields, respecti-
vely (entry k, l). It was shown that a lower frequency of ultrasound irradiation
improved the yield of pinacol coupling. The reason may be that the lower
[11,16]
frequency irradiation produces better cavitations.
Therefore, all other
couplings were carried out under 25 kHz ultrasound irradiation.
Moreover, when prolonging the reaction time from 60 min to 120 min, the
yield of 1,2-diphenyl-1,2-ethanediol decreased from 75% to 73% (entry h, i).
Additionally, after presonicating the Al powder for 60 min, the other reaction
reagents were added in one portion, and benzaldehyde gave the desired 1,2-
diol in 80% yield within 60 min (entry j).
In the absence of ultrasound, the coupling of 3-ClC H CHO mediated
6
4
by TiCl -Al-EtOH was carried out in 43% yield (entry n) by using stirring
3
Scheme 1.

4
342
Li et al.
Table 1. The effect of the reaction conditions on pinacolization under ultrasound
irradiation.
TSH Isolate
yield (%)
Molar ratio
of ArCHO/ Frequency Time
dl/
meso
ꢀ
Entry
Substrate
TiCl
3
/Al
(kHz)
(min)
2
3
a
b
c
d
e
f
4-ClC
4-ClC
4-ClC H CHO
6
H
4
CHO
1 : 2 : 3
1 : 1.5 : 3
1 : 2 : 4
1 : 3 : 6
1 : 3 : 8
1 : 2 : 8
1 : 4 : 8
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
1 : 2 : 3
25
25
25
25
25
25
25
25
25
25
40
59
25
30
30
40
20
20
40
20
60
120
60
60
60
40
40
40
79
50
70
67
80
52
83
75
73
80
74
55
79
43
56
12
20
7
66 : 34
H CHO
6 4
6
4
4-ClC H CHO
10
10
6
6
6
6
4
4
4
4-ClC
4-ClC
H
H
CHO
CHO
47 : 53
g
h
i
4-ClC H CHO
6
14
17
9
4
C
C
6
H
H
5
CHO
CHO
6
5
a
j
C H CHO
9
6
6
6
5
5
5
k
l
C
C
H
H
CHO
CHO
14
3
m
n
o
3-ClC H CHO
17
6
60 : 40
6
6
6
4
4
4
b
3-ClC
3-ClC
H
H
CHO
CHO
Stir
Stir
c
28
a
After presonicating the Al powder for 60 min, the other reaction reagents were added
in one portion.
b
Stirred without ultrasound irradiation.
After presonicating the reaction mixture for 10 min, the mixture was stirred without
c
ultrasound for 40 min.
*
1
The ratio of dl/meso is determined by H NMR.
at 208C within 40 min. After presonicating the reaction mixture for 10 min,
under stirring at 208C for 40 min, 1,2-di(3-chlorophenyl)-1,2-ethanediol
was obtained with 56% yield (entry o). While under 25 kHz ultrasound
irradiation, 1,2-di(3-chlorophenyl)-1,2-ethanediol was obtained with 79%
yield within 40 min (entry m). It is clear that the ultrasound can accelerate
the pinacolization. The reason may be that ultrasonic irradiation can cause
Al particle rupture, decrease particle size, and increase the surface area avai-
[
11,16]
3þ
lable for reaction,
plexes rapidly.
From the results above, the reaction conditions we chose are as follows:
which can reduce Ti to low-valent titanium com-
aldehyde (1 mmol), TiCl (2 mmol), Al (3 mmol)/EtOH (4 mL). Using this
3
reaction system, we did a series of experiments for pinacol coupling of aro-
matic aldehydes under 25 kHz ultrasound irradiation. The results are summa-
rized in Table 2.

Pinacol Coupling of Aromatic Aldehydes and Ketones
4343
Table 2. Pinacolization mediated by TiCl
3
-Al-EtOH under ultrasound irradiation.
Isolated yield (%)
Time
(min)
dl/
meso
ꢀ
Entry
Substrate
2
3
a
b
c
d
e
f
C
3-BrC H CHO
6
H
5
CHO
60
35
30
40
45
30
80
80
50
50
50
90
90
75
82
17
14
12
17
23
21
6
63 : 37
60 : 40
66 : 34
60 : 40
20 : 80
21 : 79
91 : 9
8 : 92
66 : 34
59 : 41
57 : 43
—
6
4
4-ClC
3-ClC
6
6
H
4
CHO
CHO
79
79
H
4
2-ClC H CHO
6
68
71
4
2,4-Cl C H CHO
2 6 3
g
h
i
4-CH
4-CH
3,4-(OCH O)C H CHO
3
C
6
H
4
CHO
CHO
69
62
3
OC
6
H
4
19
22
18
14
0
69
58
2
6 3
j
Furaldehyde
k
l
Cinnamaldehyde
PhCOPh
PhCOCH
40
0
Trace
m
3
—
—
1
*
The ratio of dl/meso is determined by H NMR.
As shown in Table 2, the coupling of some aromatic aldehydes was
carried out in good yields using TiCl -Al in EtOH under ultrasound
irradiation. For example, 1,2-di(4-methylphenyl)-1,2-ethanediol (2g) and
3
1
yields, respectively, using TiCl -CH Cl under stirring for 30min. Under
,2-diphenyl-1,2-ethanediol (2a) were previously prepared in 35% and 65%
[8]
3
2
2
ultrasonication, 1,2-di(4-methylphenyl)-1,2-ethanediol (2g) and 1,2-diphenyl-
1
,2-ethanediol (2a) were obtained with 69% and 75% yields, respectively.
From the data given in Table 2, the TiCl -Al-EtOH system can be seen to
3
have high chemoselectivity for some aromatic aldehydes, especially for
-BrC H CHO (1b), and 4-ClC H CHO (1c) gave the desired 1,2-diols
with 82% (2b) and 79% (2c) yields, respectively, within 30–35 min. In the
3
6
4
6 4
[
17]
Tyrlik report,
aliphatic carbonyls to pinacols in moderate yields. In addition, in 1989,
TiCl -Mg, gave alkenes with aromatic systems but coupled
3
[5]
McMurry et al. reported that the titanium-induced coupling reaction gave
pinacols at 08C, but at solvent reflux temperature, it gave alkene through
deoxygenation. In our system, the coupling was carried out at room tempera-
ture and gave pinacol in high yields. Besides, the competing Cannizzaro reac-
tion, giving alcohol and carboxylic acid, was not observed, and there was no
olefin formation arising from McMurry reactions.
Improved diastereoselectivity has been observed in our system
[9]
compared to the analogous process in THF at room temperature. When
-CH C H CHO (1g), 4-CH OC H CHO (1h) were used as a substrate, the
4
3
6
4
3
6 4

4
344
Li et al.
ratio of dl and meso of the 1,2-diols is 74 : 26 and 72 : 28, respectively, in the
Yamamoto et al. report. In the present system, the ratio of dl and meso of the
corresponding 1,2-diols is 91 : 9 and 8 : 92, respectively.
As shown in Table 2, some aromatic aldehydes via pinacol coupling
can give pinacols in good yields under ultrasound irradiation. The electron-
withdrawing substituents in the benzene ring (1b–1d) can increase the
rapidity of the reaction. In contrast, the electron-donating substituents in the
benzene ring (1g–1i) show less reactivity. In addition, the substituents
in the benzene ring have some effects on the ratio of dl and meso. When
4
Cl C H CHO (1f) are as used substrates, the ratio of dl and meso of the
-ClC H CHO (1c), 3-ClC H CHO (1d), 2-ClC H CHO (1e), and 2,4-
6 4 6 4 6 4
2
6 3
1
,2-diols is about 1/1–1/4; when 4-CH C H CHO (1g) is used as a substrate,
3 6 4
the ratio of dl and meso of the 1,2-diols is about 10/1; in contrast, when
4
1
-CH OC H CHO (1h) is used as a substrate, the ratio of dl and meso of the
3 6 4
,2-diols is about 1 : 10. It is shown that the aromatic aldehydes with electron-
donating substituents in the benzene ring have higher stereoselectivity than
those with electron-withdrawing substituents in the benzene ring.
On the other hand, we found that aliphatic aldehydes and aromatic
ketones show less reactivity in the pinacol coupling (1k–1m).
We have done the experiments using reducing agents Mg, Zn, or Mn
in combination with the TiCl -EtOH system. When the substrate was
3
[18]
4
respectively.
-ClC H CHO, the yields of pinacol were 92%,
91%, and 70%,
6
4
It is noteworthy that the reagents used are readily available, inexpensive,
and stable to air oxidation, and the method is easier and more convenient com-
[
9,10]
pared with those so far reported.
CONCLUSION
In summary, we have found an efficient and convenient method for
the preparation of pinacols from some aromatic aldehydes by using TiCl3-
Al-EtOH under ultrasound. The main advantages of the present procedure
are the milder reaction conditions, inexpensive catalyst, and operational
simplicity.
EXPERIMENTAL
Liquid aldehydes were distilled prior to use. Infrared (IR) spectra were
recorded on a Bio-Rad FTS-40 spectrometer (KBr). Mass spectra (MS)
1
were determined on a VG-7070E spectrometer (EI, 70 eV). H NMR spectra
were measured on a Bruker AVANCE 400 (400 MHz) spectrometer using

Pinacol Coupling of Aromatic Aldehydes and Ketones
4345
tetramethylsilane (TMS) as the internal standard and CDCl as a solvent. Soni-
3
cation was performed in Shanghai Branson-CQX ultrasonic cleaner (with a
frequency of 25 kHz and a nominal power 250 W) and SK 250 LH ultrasonic
cleaner (with a frequency of 40 kHz, 59 kHz, and a nominal power 250 W;
Shanghai Kudos Ultrasonic Instrument Co., Ltd.). The reaction flask was
located in the maximum energy area in the cleaner, where the surfaces of reac-
tants are slightly lower than the level of the water. The reaction temperature
was controlled by adding or removing of water from the ultrasonic bath.
General Procedure
A 50 mL two-neck round flask was charged with a 15% TiCl solution in
3
dilute HCl (2 mmol) and 4 mL EtOH under a nitrogen atmosphere. After 2 min
of irradiation, Al powder (4 mmol) was added. When the reaction mixture
turned into a dark color, a solution of the desired aldehyde (1, 1 mmol) in
1
mL EtOH was added. The mixture was irradiated in the water bath of
the ultrasonic cleaner for a period as indicated in Table 1 [the reaction was
followed by thin-layer chromatography (TLC)]. After the completion of the
reaction, the resulting suspension was quenched with 1 mL H O and filtered
2
to remove the Al powder. The filtrate was extracted with ethyl acetate
(
3 ꢁ 15 mL). The combined organic layers were washed with saturated
aqueous NaHCO solution and brine, dried over anhydrous magnesium
3
sulfate for 12 h, and filtered. Ethyl acetate was evaporated under reduced
pressure to give the crude product, which was separated by column chromato-
graphy on silica (200–300 mesh) and eluted with petroleum ether or a mixture
of petroleum ether and diethyl ether. The authenticity of the product was
1
13
established by their H NMR, C NMR, MS, and IR spectral data.
1
a: H NMR: d 2.32 (2H, s, OH, meso), 2.96 (2H, s, OH, dl), 4.72 (2H, s,
2
1
3
CH, dl), 4.84 (2H, s, CH, meso), 7.15–7.32 (20H, m, Ph-H) ppm.
NMR: 78.45, 79.50, 127.39, 127.53, 128.30, 128.46, 140.16, 140.29;
C
m/z (%): 214 (1), 180 (7.6), 167 (12.5), 149 (6.0), 107 (93.8), 79
.
21
(100), 77 (73.8). IR (KBr) nmax: 3200–3480 cm
1
2
2
b: H NMR: d 2.45 (2H, s, OH, meso), 3.04 (2H, s, OH, dl), 4.63 (2H, s,
CH, dl), 4.79 (2H, s, CH, meso), 6.96–7.41 (16H, m, Ph-H) ppm. m/z
(%): 325 (6), 186 (16), 157 (8), 107 (7), 77 (100), 51 (13). IR (KBr)
nmax: 3200–3500 cm
2
1
.
1
c: H NMR: d 2.32 (2H, s, OH, meso), 2.89 (2H, s, OH, dl), 4.63 (2H, s,
CH, dl), 4.84 (2H, s, CH, meso), 7.02–7.33 (16H, m, Ph-H) ppm.
1
3
C NMR: 78.82, 128.75, 129.00, 134.14, 134.20, 138.16, 138.37; m/z

4
346
Li et al.
(
%): 276 (14), 249 (32), 155 (100), 111 (8). IR (KBr) nmax:
21
380–3420cm .
3
1
d: H NMR: d 2.35 (2H, s, OH, meso), 2.92 (2H, s, OH, dl), 4.67 (2H, s,
2
1
3
CH, dl), 4.83 (2H, s, CH, meso), 6.93–7.30 (16H, m, Ph-H) ppm.
NMR: 78.68, 125.62, 127.34, 128.72, 129.85, 134.64, 141.94; m/z (%):
63 (1.2), 251 (1.6), 178 (4.6), 165 (4.6), 141 (100), 113 (23.8), 77
C
2
2
1
(71.0). IR (KBr) nmax: 3260–3318 cm
.
1
2
2
e: H NMR: d 2.64 (2H, s, OH, meso), 2.75 (2H, s, OH, dl), 5.39 (2H, s,
CH, dl), 5.63 (2H, s, CH, meso), 7.17–7.34 (16H, m, Ph-H) ppm. m/z
(%): 282 (1), 165 (47), 141 (89), 113 (13), 107 (14), 77 (100), 51 (38).
IR (KBr)nmax: 3100–3500 cm
2
1
.
1
f: H NMR: d 2.67 (2H, s, OH, meso), 2.75 (2H, s, OH, dl), 5.28 (2H, s,
1
3
CH, dl), 5.56 (2H, s, CH, meso), 7.16–7.65 (12H, m, Ph-H) ppm.
C
NMR: 71.71, 73.02, 127.26, 127.64, 128.74, 129.58, 130.12, 130.51,
1
33.53, 134.20, 134.53, 134.85, 135.05, 136.00; m/z (%): 352 (1),
3
nmax: 3320–3400 cm
05 (1.4), 233 (10), 175 (100), 145 (10), 111 (25), 77 (15). IR (KBr)
2
1
.
1
g: H NMR: d 2.32 (6H, s, CH , dl), 2.37 (6H, s, CH , meso), 4.70 (2H, s,
2
3
3
1
3
CH, dl), 4.77 (2H, s, CH, meso) 7.07–7.28 (16H, m, Ph-H) ppm.
NMR: 21.59, 21.62, 78.38, 79.18, 127.33, 127.49, 129.20, 129.35,
37.38, 137.43, 137.82, 138.11; m/z (%): 242 (1.2), 195 (6), 121
C
1
2
1
(100), 107 (12), 77 (13). IR (KBr) nmax: 3280–3450 cm
.
1
2
2
h: H NMR: d 3.79 (6H, s, CH O, dl), 3.83 (6H, s, CH O, meso), 4.65
3 3
(2H, s, CH, dl), 4.76 (2H, s, CH, meso), 6.77–7.28 (16H, m, Ph-H)
ppm. m/z (%): 302 (1), 284 (2.5), 268 (5.0), 255 (11.8), 151 (100),
23 (32), 93 (77.1), 65 (39.0). IR (KBr) nmax: 3100–3600 cm
2
1
1
.
1
i: H NMR: d 4.60 (2H, s, CH, dl), 4.67 (2H, s, CH, meso), 5.95 (4H, s,
CH , dl), 5.98 (4H, s, CH , meso), 6.56–6.88 (12H, m, Ph-H) ppm. C
NMR: 40.28, 40.49, 77.57, 78.15, 101.38, 108.02, 108.09, 108.34,
1
3
2
2
1
3
08.52, 121.31, 121.45, 137.18, 138.04, 146.64, 147.28; m/z (%):
02 (1), 284 (2.5), 268 (5.0), 255 (11.8), 151 (100), 123 (32), 93
2
1
(77.1), 65 (39.0). IR (KBr) nmax: 3100–3600 cm
.
1
j: H NMR: d 4.98 (2H, s, CH, dl), 5.02 (2H, s, CH, meso), 6.22–6.33
2
2
(
12H, m, furyl-H) ppm. m/z (%): 196 (10), 178 (25), 152 (73), 137 (33),
2
1
9
8(100), 84(22), 49 (30). IR (KBr) nmax: 3240–3300 cm
.
1
k: H NMR: d 2.29 (2H, s, OH, meso), 2.52 (2H, s, OH, dl), 4.31 (2H, d,
CH, dl), 4.47 (2H, d, CH, meso), 6.32 (2H, m, –CH 55 CH–), 6.74 (2H,
m, –CH 55 CH–), 7.18–7.42 (20H, m, Ph-H) ppm.

Pinacol Coupling of Aromatic Aldehydes and Ketones
4347
1
m: H NMR: d 1.53 (6H, s, CH , dl), 1.61 (6H, s, CH , meso), 2.29 (2H,
2
3
3
1
3
s, OH, meso), 2.59 (2H, s, OH, dl), 7.22–7.29 (20H, m, Ph-H) ppm.
NMR: 25.36, 25.52, 79.01, 79.27, 127.31, 127.34, 127.56, 127.79,
43.83, 144.20; m/z (%): 225 (4), 206 (4), 181 (32), 165 (9), 121
C
1
2
1
(100), 105 (12), 77 (11), 43 (80). IR (KBr) nmax: 3100–3600 cm
.
ACKNOWLEDGMENTS
We thank the Educational Ministry of China and Natural Science Foun-
dation of Hebei Province (203107), China, for financial support.
REFERENCES
1
. Robertson, G.M. In Comprehensive Organic Synthesis; Trost, B.M., Ed.;
Pergamon: New York, 1991.
2
. Rieke, R.D.; Kim, S.H. New Reagent for Reductive Coupling of Carbonyl
and Imine Compounds: Highly Reactive Manganese-Mediated Pinacol
Coupling of Aryl Aldehydes, Aryl Ketones, and Aldimines. J. Org.
Chem. 1998, 63 (15), 5235–5239.
3
. Seyden-Penne, J. Chiral Auxiliaries and Ligands in Asymmetric Syn-
thesis; John Wiley & Sons: New York, 1995.
4
. (a) Kammermeier, B.; Beck, G.; Jacobi, D.; Jendralla, H. Vanadium(II)-
and Niobium(III)-Induced Diastereoselective Pinacol Coupling of Homo-
chiral Peptide Aldehydes to C -Symmetic HIV-Protease Inhibitors.
2
Angew. Chem. 1994, 106 (6), 719–721; (b) Reetz, M.T.; Griebenow, N.
Synthesis and Stereoselective C–C Bond Forming Reactions of Peptide
Aldehydes. Liebigs Ann. 1996, (3), 335–348.
5
6
. McMurry, J.E. Carbonyl-Coupling Reactions Using Low-Valent Tita-
nium. Chem. Rev. 1989, 89 (7), 1513–1524.
. Mukaiyama, T.; Sato, T.; Hanna, J. Reductive Coupling of Carbonyl
Compounds to Pinacols and Olefins Using Titanium(IV) Chloride and
Zinc. Chem. Lett. 1973, (10), 1041–1044.
7
8
9
. Clerici, A.; Porta, O. Pinacolic Coupling of Aromatic Aldehydes and
Ketones Promoted by Aqueous Titanium Trichloride in Basic Media-
Tetrahedron Lett. 1982, 23 (34), 3517–3520.
. Clerici, A.; Clerici, L.; Porta, O. A Highly dl-Stereoselective Pinacoli-
zation of Aromatic Aldehydes Mediated by Titanium Trichloride in
Dichloromethane. Tetrahedron Lett. 1996, 37 (17), 3035–3038.
. Yamamoto, Y.; Hattori, R.; Miwa, T.; Nakagai, Y.I.; Kubota, T.;
Yamamoto, C.; Okamoto, Y.; Itoh, K. Diastereoselective Inter- and

4
348
Li et al.
Intramolecular Pinacol Coupling of Aldehydes Promoted by Mono-
meric Titanocene(III) Complex Cp TiPh. J. Org. Chem. 2001, 66 (11),
2
3
0. Li, T.Y.; Wang, L.P.; Zhang, T.; Li, Y.X.; Wang, Z.M. Studies on the
865–3870.
1
Stereoselectivity of Pinacol Coupling Reaction Mediated by TiCl -Mg.
4
Chem. J. Chin. Univ. 2000, 21 (9), 1391–1394.
1. Mason, T.J. Practical Sonochemistry; Ellis Horwood Limited: New York,
1
1
1
1
1
1
991.
2. Bian, Y.J.; Li, J.T.; Li, T.S. Applications of Ultrasound in Organic Syn-
thesis Using Metal Reagent. Chin. J. Org. Chem. 2002, 22 (4), 227–232.
3. Mecarova, M.; Toma, S. Optimization of Pinacol Coupling in Aqueous
Media. Green Chem. 1999, 1 (6), 257–259.
4. Mecarova, M.; Toma, S.; Babiak, P. Pinacol Synthesis in Aqueous Media.
Chem. Pap. 2001, 55 (5), 302–307.
5. Li, J.T.; Yang, J.H.; Li, T.S. Reductive Coupling of Aromatic Aldehydes
Using Low Valent Titanium Complexes Under Ultrasound Irradiation.
Chin. J. Org. Chem. 2003, 23 (12), 1428–1431.
1
1
6. Sprich, J.D.; Lewandos, G.S. Sonochemical Removal of Absorbed Water
and Alcohol from Magnesium Surfaces. Inorg. Chim. Acta. 1983, 76(4),
2
41–242.
7. Tyrlik, S.; Wolochowicz, I. Application of Transition Metal Complexes
with Low Oxidation States in Organic Synthesis I. New Synthesis of
Olefins from Carbonyl Compounds. Bull. Soc. Chim. Fr. 1973, 6 (Part
2
8. Li, J.-T.; Lin, Z.-P.; Li, T.-S. Pinacol Coupling of Aromatic Aldehydes
), 2147–2148.
1
and Ketones using TiCl -Mg under Ultrasound Irradiation. Ultrason.
3
Sonochem., (in press).
Received in Japan June 16, 2004