CeCl3/Sm promoted pinacolic coupling of aromatic aldehydes and ketones

Article abstract of DOI:10.1002/jccs.200300147

The pinacolic coupling of aromatic aldehydes and ketone was performed with cerium trichloride and samarium in THF at room temperature to afford the corresponding 1,2-diols in good yields.

Full text of DOI:10.1002/jccs.200300147

Journal of the Chinese Chemical Society, 2003, 50, 1043-1045
1043
CeCl3/Sm Promoted Pinacolic Coupling of Aromatic Aldehydes and
Ketones
Xue Li (
), Songlin Zhang* (
),
Xinying Zhang (
College of Chemistry and Environmental Science, Henan Normal University,
) and XiaoYang Wang (
)
Xinxiang, 453002, P. R. China
The pinacolic coupling of aromatic aldehydes and ketone was performed with cerium trichloride and sa-
marium in THF at room temperature to afford the corresponding 1,2-diols in good yields.
Keywords: Samarium; Cerium(III) chloride; Pinacol coupling.
INTRODUCTION
A very useful method for constructing a carbon-carbon
(Mg2+/Mg = -2.37V) and superior to that of zinc (Zn2+/Zn =
-0.71V). These properties encourage us and other workers to
use the more convenient and cheaper metallic samarium di-
rectly instead of samarium diiodide.15(c),16 In continuing our
research in this area, we investigated the CeCl3/Sm17 system
mediated reductive coupling of aromatic aldehydes and ke-
tones in THF-MeOH at room temperature that affords the
corresponding pinacols in moderate to good yields.
bond is the reductive coupling of carbonyl compounds to give
pinacols and olefins.1 The corresponding pinacols can be
used as intermediates for the preparation of ketones and ole-
ins.2 Generally, the reaction is performed by treatment of car-
bonyl containing compounds with an appropriate metal re-
agent to give the corresponding coupled product. A number
of different types of metal reagents have been used to carry
out the pinacol reaction, for instance, reactions with Ce-I2,3
Fe(CO)5/pyridine,4 NbCl3,5 SmI2,6 TiCl4/Zn,7 VCl3/Zn.8 Some
disadvantages of such procedures is that they have to be car-
ried out at low temperature since reaction with the reagents at
high temperature gives an olefin.
EXPERIMENTAL
IR spectra were obtained on a PE-683 infrared spectro-
photometer. 1H NMR spectra were recorded on a JEOL-90Q
1
NMR instrument. All H NMR samples were measured in
Recently, Cerium(III) chloride has emerged as a Lewis
acid imparting high regio- and chemoselectivity in various
chemical transformations,9 because this compound is a very
cheap, nontoxic, and water tolerant reagent.10 One of the re-
markable features of CeCl3 is its efficient activity in combi-
nation with sodium iodide,11 MeLi12 and n-BuLi13 to form an
organic cerium reagent which can be used as a selective or-
ganic magnesium reagent and the reductive system of NaBH4-
CeCl314 has been widely used in organic synthesis.
CDCl3 using TMS as internal standard. The solvent THF was
freshly distilled from sodium/benzophenone ketyl prior to
use. The reagents were purchased from a commercial source
and used without purification. The reactions were performed
in a Schlenk type glass apparatus under a nitrogen atmo-
sphere.
GENERAL PROCEDURE
The use of samarium metal in synthetic organic chemis-
try is rapidly increasing because of its several advantages
(availability, stability and cost) over the widely-used single-
electron transfer reagent samarium diiodide.15 Yanada has
studied extensively various reactions mediated by Lewis ac-
ids.15(c) Although SmI2 is a useful reagent, its sensitivity to air
limits its application in organic synthesis. On the other hand,
metallic samarium is stable in air and its strong reducing
power (Sm3+/Sm = -2.41V) is similar to that of magnesium
Under nitrogen atmosphere, metallic samarium powder
(3 mmol) and cerium trichloride (1 mmol) were placed in a
reaction flask and aromatic aldehyde or ketone (1 mmol) and
methyl alcohol (0.2 mL) were added with a syringe in one
portion. Afterwards, the mixture was magnetically stirred at
room temperature for the time indicated in the Table 1. Water
(10 mL) was added to quench the reaction and the mixture
was extracted with ethyl ether (10 mL ´ 3). The extracts were

1044 J. Chin. Chem. Soc., Vol. 50, No. 5, 2003
Li et al.
Table 1. CeCl3/Sm Induced Pinacolic Coupling Reactions
840, 750, 680, 650.
2b19 1,2-Di-(Fluorophenyl)-1,2-ethanediol (rac and
Entry
Ar
R
Time(h) Yield(%)a Rac:mesob
1
meso), H NMR (d, ppm) 6.60-7.50 (8H, m, Ar), 4.50 (rac)
a
b
c
d
e
f
C6H5
p-FC6H4
H
H
H
H
H
H
H
H
CH3
H
10
8
8
8
8
8
8
10
10
8
79
81
80
82
83
76
74
72
73
62
64:36
60:40
62:38
64:36
65:35
68:32
62:38
66:34
58:42
63:37
and 4.67 (meso) (2H, s, PhCH), 3.25 (2H, s, OH, this signal
disappeared by adding D2O); IR nmax (cm-1) 3100-3600.
2c20 1,2-Di-(4-Chlorophenyl)-1,2-ethanediol (rac and
meso), 1H NMR (d, ppm) 6.75-7.30 (8H, m, Ar), 4.45 (s, rac)
and 4.60 (s, meso) (2H, 2ArCH), 2.85 (2H, s, OH, this signal
disappeared by adding D2O); IR nmax (cm-1) 3200-3600, 3080,
3040, 2990, 2960, 2920, 2860, 1600, 1580, 1480, 1440, 1200,
1120, 1020, 900, 740, 640.
p-ClC6H4
p-BrC6H4
p-MeC6H4
p-MeOC6H4
m-BrC6H4
2,6-diClC6H3
C6H5
g
h
i
jc
p-MeC6H4
2d20 1,2-Di-(4-Bromophenyl)-1,2-ethanediol (rac and
meso), 1H NMR (d, ppm) 6.70-7.60 (8H, m, Ar), 4.55 (s, rac)
and 4.70 (s, meso) (2H, s, ArCH), 2.46 (2H, s, OH, this signal
disappeared by adding D2O); IR nmax (cm-1) 3100-3600, 3080,
3050, 3020, 2970, 2960, 2890, 1580, 1500, 1450, 1200, 1060,
1020, 900, 760, 690, 620.
a Isolated yields; b Ratios determined from the intensities of
benzylic protons in 1H NMR spectra (entry a-h,j), in which the
protons of rac isomer appeared at a higher magnetic field,
compared to the one of meso isomer and from the intensities of
methyl protons of methyl protons in 1H NMR spectra (entry i), in
which the methyl protons of rac isomer appeared at higher
magnetic field, compared to the one of meso isomer; c the control
experiment carried out without methyl alcohol.
The control experiment without the methyl alcohol afford the
same products, yet the yield is lower. And the Benzophenone did
not react under the conditions, only recovered the starting
material.
2e18 1,2-Di-(4-methylphenyl)-1,2-ethanediol (rac and
meso), 1H NMR (d, ppm) 6.75-7.30 (8H, m, Ar), 4.55 (s, rac)
and 4.69 (s, meso) (2H, 2ArCH), 2.65 (2H, s, OH, this signal
disappeared by adding D2O), 2.25 (s, rac) and 2.30 (s, meso)
(6H, 2CH3); IR n
(cm-1) 3100-3600, 3080, 3040, 2930,
max
1600, 1580, 1540, 1490, 1440, 1380, 1270, 1200, 1110, 1020,
850, 820, 770, 720, 640.
washed with brine, dried over anhydrous Na2SO4 and the sol-
vent was removed under reduced pressure. The residure was
then purified by preparative TLC on silica gel with petroleum
ether and dietheyl ether (3:1) as eluent to give the product.
2f18 1,2-Di-(4-methoxyphenyl)-1,2-ethanediol (rac and
1
meso), H NMR (d, ppm) 6.60-7.30 (8H, m, Ar), 4.55 (rac)
and 4.70 (meso) (2H, s, PhCH), 3.75 (6H, s, CH3O), 2.90 (2H,
s, OH, this signal disappeared by adding D2O); IR nmax (cm-1)
3100-3600, 3080, 3040, 2940, 1600, 1540, 1490, 1440, 1380,
1270, 1200, 1100, 1020, 850, 820, 770, 720, 640.
Scheme
2g21 1,2-Di-(3-Bromophenyl)-1,2-ethanediol (rac and
meso), 1H NMR (d, ppm) 6.80-7.50 (8H, m, Ar), 4.55 (s, rac)
and 4.73 (s, meso) (2H, 2ArCH), 2.95 (2H, s, OH, this signal
disappeared by adding D2O); IR nmax (cm-1) 3100-3600, 2080,
3040, 2940, 1600, 1580, 1480, 1450, 1200, 1070, 1000, 900,
780, 740, 690.
In conclusion, we have demonstrated that the CeCl3/Sm
system can be use for pinacol coupling reaction of aromatic
aldehydes and ketone, and that its experimental simplicity,
mild reaction conditions and good product yield provides a
novel way for using metallic samarium in organic synthesis.
2h22 1,2-Di-(2,6-dichlorophenyl)-1,2-ethanediol (rac
and meso), 1H NMR (d, ppm) 6.90-8.00 (m, 6H, ArH), 4.49
(s, rac) and 4.70 (s, meso), 2.74 (s, 2H, OH, this signal disap-
peared by adding D2O); IR n
(cm-1) 3100-3600, 3080,
max
3040, 2990, 2960, 2860, 1600, 1580, 1480, 1440, 1220, 1120,
1020, 900, 760.
DATA OF THE PRODUCTS
1
2i23 2,3-Diphenyl-2,3-butanediol (rac and meso), H
2a18 1,2-Diphenyl-1,2-ethanediol (rac and meso), H
1
NMR (d, ppm) 7.10-7.40 (10H, m, Ph), 2.47 (2H, s, OH, this
signal disappeared by adding D2O), 1.45 (s, rac) and 1.55 (s,
NMR (d, ppm) 7.10-7.35 (10H, m, Ph), 4.69 (s, rac) and 4.75
meso) (6H, 2CH3); IR n
(cm-1) 3100-3600, 3080, 3040,
(s, meso) (2H, 2PhCH), 2.49 (2H, s, OH, this signal disap-
max
peared by adding D2O); IR n
(cm-1) 3100-3500, 3080,
2990, 2940, 2890, 2860, 1600, 1500, 1480, 1420, 1380, 1200,
1100, 1020, 900, 850, 760, 630.
max
3040, 2920, 1600, 1500, 1460, 1390, 1280, 1020, 1000, 920,

CeCl3/Sm Promoted Pinacolic Coupling of Ar COR
J. Chin. Chem. Soc., Vol. 50, No. 5, 2003 1045
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ACKNOWLEDGEMENT
The authors are grateful to the Natural Science Founda-
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0111030600) for financial support.
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