336 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 336±337$
Reductive Coupling of Aromatic Carbonyl Compounds
to Pinacols Using Zinc Powder in Aqueous Media$
Lei Wang, Xinghua Sun and Yongmin Zhang*
Department of Chemistry, Hangzhou University, Hangzhou, 310028, P.R. China
Zinc-mediated reductive coupling of aromatic carbonyl compounds occurs to give corresponding 1,2-diols in moderate
to good yields in saturated NH4Cl(aq)±THF solution at room temperature.
1,2-Diols are very useful synthons for a variety of organic
syntheses.1 The reductive coupling of carbonyl compounds
is an important method for the formation of 1,2-diols,2 and
is usually carried out under anhydrous conditions. On the
other hand, many reagents such as TiCl3,3 Zn±ZnCl2,4
Al(Hg),5 Cp2TiCl6 and Mn7 have been developed for
pinacolic coupling reactions in aqueous media because
organic reactions in aqueous media oer a number of
advantages over conventional organometallic reactions in
organic solvents.8 As a cheap and ecient reagent, metallic
zinc powder has been used for the preparation of homo-
allylic alcohols by the coupling of allylic halides with car-
bonyl compounds in aqueous media.9 To the best of our
knowledge there are no literature examples of pinacolic
coupling using zinc powder in saturated NH4Cl(aq)±THF
solution. We therefore report herein that zinc-mediated
reductive coupling of aromatic carbonyl compounds to
pinacols can be carried out in aqueous media at room tem-
perature. The results are summarized in Table 1.
to good yields at ambient temperature. Except for that of
2 g, the DL :meso ratio of diols (entries a±f, h) is nearly 1:1
(1H NMR). The reason for the eect of the tri¯uoromethyl
group in the 4-position of the aromatic ring (entry g) on the
DL:meso ratio is not clear. Unfortunately, the aromatic
ketone (entry h) gives the pinacolic coupling product in
poor yield.
We have tried to use anhydrous THF, THF±water (4:1)
or water in place of THF±NH4Cl(aq) (4:1) respectively.
However, no product was formed under these conditions.
Also, other metal powders such as tin and indium in place
of zinc gives no reaction.
In summary, zinc is a useful metal to mediate the reduc-
tive coupling of aromatic carbonyl compounds in aqueous
media at room temperature. The advantages of this reaction
are mild and neutral reaction conditions, simple operation
and good yields.
Experimental
1H NMR spectra were recorded on
a JEOL PMX 60 SI
instrument. All NMR samples were measured in CDCl3 using TMS
as internal standard. IR spectra were obtained on a Perkin-Elmer
683 spectrophotometer as KBr plates.
General Procedure for the Preparation of Pinacols.ÐThe aromatic
carbonyl compound (1.0 mmol) and metallic zinc powder
(1.2 mmol) were added to saturated aqueous NH4Cl±THF (1:4,
5 ml) solution. The resulting mixture was stirred at room tempera-
ture for 8 h, then dilute hydrochloric acid (0.5 mol l 1, 2 ml) was
added to quench the reaction and the mixture was extracted with
diethyl ether (20 ml Â2). The extracts were washed with brine,
Table 1 shows that aromatic aldehydes undergo coupling
in the presence of zinc powder in THF±NH4Cl(aq) solution
to give pinacolic coupling products (1,2-diols) in moderate
Table 1 Zinc-mediated reductive coupling of aromatic carbonyl compounds
1
Entry
Ar
R
H
Yield(%)a
DL : mesob
1H NMR(d)
IR(ꢀ~/cm )
a
C6H5
65
55: 45
3.78 (2 H, s, OH),c 4.40 (s, DL) and 4.60 (s, meso)
(2 H, 2 PhCH), 6.50±7.30 (10 H, m, Ph)
3100±3600(s)
3200±3600(s)
3100±3600(s)
3150±3620(s)
3100±3500(s)
3100±3600(s)
3100±3600(s)
3100±3600(s)
b
c
d
e
f
p-ClC6H4
p-BrC6H4
p-FC6H4
m-BrC6H4
o-BrC6H4
p-CF3C6H4
C6H5
H
77
76
73
70
68
82
32
58: 42
50: 50
55: 45
52: 48
50: 50
74: 26
51: 49
2.85 (2 H, s, OH),c 4.40 (s, DL) and 4.63 (s, meso)
(2 H, 2 PhCH), 6.60±7.30 (8 H, m, Ar)
H
2.46 (2 H, s, OH),c 4.50 (s, DL) and 4.60 (s, meso)
(2 H, 2 PhCH), 6.60±7.60 (8 H, m, Ar)
H
3.15 (2 H, s, OH),c 4.55 (s, DL) and 4.70 (s, meso)
(2 H, 2 PhCH), 6.60±7.50 (8 H, m, Ar)
H
3.33 (2 H, s, OH),c 4.33 (s, DL) and 4.50 (s, meso)
(2 H, 2 PhCH), 6.70±7.40 (8 H, m, Ar)
H
2.90 (2 H, s, OH),c 4.55 (s, DL) and 4.65 (s, meso)
(2 H, 2 PhCH), 6.70±7.50 (8 H, m, Ar)
g
h
H
3.04 (2 H, s, OH),c 4.45 (s, DL) and 4.66 (s, meso)
(2 H, 2 PhCH), 6.65±7.50(8 H, m, Ar)
1.42 (s, DL) and 1.55 (s, meso) (6 H, 2 CH3), 2.40
CH3
(2 H, s, OH),c 6.80±7.30 (10 H, m, Ph)
aIsolated yields. bRatios determined from the intensities of benzylic protons in 1H NMR spectra(entries a±g), in which the protons of
the DL isomer appeared at a higher magnetic field compared to that of the meso isomer, or from the intensities of methyl protons in
1H NMR spectra(entry h), in which the methyl protons of the DL isomer appeared at higher magnetic field compared to that of the meso
isomer.10 cIn all examples this signal disappeared on adding D2O.
*To receive any correspondence.
dried over anhydrous Na2SO4 and the solvent was removed under
reduced pressure. The residue was then puri®ed by preparative TLC
on silica gel with light petroleum±ether as the eluent to give the
product.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Wang, Lei
