Reductive coupling of carbonyl compounds to pinacols with zinc in THF-saturated aqueous ammonium chloride

Article abstract of DOI:10.1007/s007060170083

In the presence of metallic zinc, aldehydes and ketones experience reductive coupling in tetrahydrofuran-saturated aqueous ammonium chloride (5:3) to afford the corresponding pinacols in moderate to high yields.

Full text of DOI:10.1007/s007060170083

Monatshefte f uÈ r Chemie 132, 689±691 (2001)
Reductive Coupling of Carbonyl Compounds to
Pinacols with Zinc in THF-Saturated Aqueous
Ammonium Chloride
1
;Ã
2
1
Rahim Hekmatshoar , Issa Yavari , Yahya S. Beheshtiha ,
and Majid M. Heravi1
1
Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran
2
Department of Chemistry, University of Tarbiat Modarres, Tehran, Iran
Summary. In the presence of metallic zinc, aldehydes and ketones experience reductive coupling in
tetrahydrofuran-saturated aqueous ammonium chloride (5:3) to afford the corresponding pinacols in
moderate to high yields.
Keywords. Carbonyl compounds; Pinacols; Zinc; Pinacolization; Carbon-carbon bond formation.
Introduction
The reductive coupling of carbonyl compounds to pinacols constitutes an important
method for the formation of vicinally functionalized carbon-carbon bonds [1]. This
reaction proceeds by dimerization of carbonyl radical anions formed by single
electron transfer from a variety of metals, metal salts, or metal complexes under
various conditions. There is a considerable variety of pinacolization procedures
[2±8] re¯ecting the continuous search for more ef®ciency and higher selectivity.
Results and Discussion
Recently, there has been considerable interest in performing organic reactions in
aqueous media [9±11]. We report here that reductive coupling of aldehydes and
ketones can be affected easily in tetrahydrofuran and saturated aqueous ammonium
chloride in the presence of zinc to afford the corresponding pinacolic alcohols in
moderate to fairly high yields. Our results are summarized in Table 1.
Stirring of a mixture of an aldehyde or ketone and zinc powder in THF-saturated
aqueous ammonium chloride proceeded smoothly at room temperature to give the
corresponding pinacols. The procedure was applied to benzaldehydes and cin-
namaldehyde as well as cyclohexanone, benzophenone, ¯ourenone, anthrone,
xanthone, and thioxanthone (Table 1). The reactions were in most cases completed
within 10±50 minutes as evidenced by disappearance of the metal. As shown in
à Corresponding author

690
R. Hekmatshoar et al.
Scheme 1
Table 1. Pinacolization of carbonyl compounds; the entries in the last column refer to isolated yields;
physical and spectroscopic (IR, H and 13C NMR) data of the products are in agreement with the
expected structures; the pinacols are formed as dl/meso mixtures which have been analyzed only for
benzaldehyde (dl:meso ˆ 40:60) and cinnamaldehyde (dl:meso ˆ 15:85) as educts
1
Entry
Carbonyl compound
Pinacol
Yield/%
1
2
C6H5±CHO
ꢀC6H5CHOH†
45
55
2
C6H5±CH=CH±CHO
ꢀC6H5CH=CH±CHOH†
2
3
45
4
5
6
7
p� Br� C6H4� CHO
p� Cl� C6H4� CHO
o� MeO� C6H4� CHO
m� MeO� C6H4� CHO
ꢀp� Br� C6H4� CHOH†
80
75
60
80
2
2
ꢀp� Cl� C6H4� CHOH†
ꢀo� MeO� C6H4� CHOH†
2
ꢀm� MeO� C6H4� CHOH†
ꢀC6H5� CꢀCH3†OH†2
ꢀꢀC6H5† � COH†
2
8
9
40
95
ꢀC6H5† C=O
2
2
2
10
11
12
13
X ˆ nil
X ˆ CH2
X ˆ S
X ˆ nil
85
92
90
95
X ˆ CH2
X ˆ S
X ˆ O
X ˆ O
Table 1, the reaction of substituted benzaldehydes (entries 1, 4±7), cinnamaldehyde,
or acetophenone gives dl/meso mixtures of pinacols. Due to its high yields and its
speed, the method is competitive with many other ways of pinacol formation [2±8].
Especially, the yields obtained in the coupling of aromatic aldehydes substituted by
Cl, Br, or OCH deserve interest.
3
In conclusion, the zinc-promoted reductive coupling of aldehydes and ketones
in aqueous media turned out to be remarkably effective in pinacolization. This
method is a relatively rapid, manipulatively simple, and inexpensive protocol when
compared with other methods which are disadvantageous concerning expensive
catalysts, long reaction times, and tedious work up.

Reductive Coupling of Carbonyl Compounds
691
Experimental
Melting points were measured with an Electrothermal 9100 apparatus and are uncorrected. IR
spectra were recorded on a Philips PU 9800 FT-IR instrument. H and 13C NMR spectra were
recorded on a JEOL EX-90A spectrometer at 90 and 22.6 MHz. All compounds have been reported
in the literature; their physical and spectroscopic data were in accordance with those of authentic
samples.
1
General procedure
3
Zn powder (4 mmol, 0.26 g) and carbonyl compound (2 mmol) were slurried in 3 cm saturated
3
aqueous NH4Cl and 5 cm THF. The mixture was stirred at room temperature. The temperature rose,
and after 10 min the Zn powder had disappeared. The solvents were evaporated to dryness, and the
product was extracted with diethyl ether. The etheral layer was separated and evaporated to dryness
to afford the pure pinacol.
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Received October 23, 2000. Accepted (revised) December 12, 2000