Selective reduction of aldehydes to alcohols using alumina with a catalytic amount of base under microwave irradiation

Article abstract of DOI:10.1081/SCC-200026248

Aldehydes are selectively reduced to the corresponding alcohols in very good yields with alumina containing a catalytic amount of base under microwave irradiation at 300 W for 8-15 min.

Full text of DOI:10.1081/SCC-200026248

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Selective Reduction of
Aldehydes to Alcohols Using
Alumina with a Catalytic
Amount of Base Under
Microwave Irradiation
Prasun K. Pradhan a , Parasuraman Jaisankar a
,
Bikash Pal a , Sumit Dey a & Venkatachalam S. Giri
a
a Department of Medicinal Chemistry , Indian
Institute of Chemical Biology , 4, Raja S. C.
Mullick Road, Calcutta, 700 032, India
Published online: 10 Jan 2011.
To cite this article: Prasun K. Pradhan , Parasuraman Jaisankar , Bikash Pal , Sumit
Dey & Venkatachalam S. Giri (2004) Selective Reduction of Aldehydes to Alcohols
Using Alumina with a Catalytic Amount of Base Under Microwave Irradiation,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 34:15, 2863-2872, DOI: 10.1081/SCC-200026248
To link to this article: http://dx.doi.org/10.1081/SCC-200026248
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 15, pp. 2863–2872, 2004
Selective Reduction of Aldehydes to
Alcohols Using Alumina with a Catalytic
Amount of Base Under
Microwave Irradiation
Prasun K. Pradhan, Parasuraman Jaisankar, Bikash Pal,
*
Sumit Dey, and Venkatachalam S. Giri
Department of Medicinal Chemistry, Indian Institute of Chemical
Biology, Calcutta, India
ABSTRACT
Aldehydes are selectively reduced to the corresponding alcohols in very
good yields with alumina containing a catalytic amount of base under
microwave irradiation at 300 W for 8–15 min.
Key Words: Alumina; Microwave irradiation; Reductions; Aldehydes;
Alcohols; Base; Catalyst.
*
Correspondence: Venkatachalam S. Giri, Department of Medicinal Chemistry, Indian
Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Calcutta 700 032, India;
Fax: þ91-33-24735197; E-mail: vsgiri@iicb.res.in.
2863
DOI: 10.1081/SCC-200026248
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Pradhan et al.
INTRODUCTION
Many organic reactions have been performed on porous solid materials,[1]
especially alumina.[2] Environment friendly solid supports render the possi-
bility of recycling them for repeat reactions. The use of solid supports immobi-
lized with reagents without solvents and in combination with microwave
irradiation, provides ideal conditions for many organic reactions e.g. reducing
the reaction time, easier work-up procedures and enhanced selectivity and
reactivity.[3] Thus microwaves have become one of the most useful tools for
many known and novel organic reactions.[4]
In continuation of our endeavours[5] to develop newer methodologies
using microwaves, we now report an interesting and useful microwave-
assisted selective reduction of aldehydes 1 to their corresponding alcohols 2
in good yields (Sch. 1). A solution of the aldehyde with a catalytic amount
of a base (10 mol % of DMAP or NEt3) adsorbed on neutral or basic aluminaa
followed by microwave irradiation at 300 W for 8–15 min afforded the corre-
sponding alcohols 2 in excellent yields (Table 1).
The present method eliminates the drawback observed in Posner’s
report.[6a,b] Moreover, the presence of water does not hamper the reaction.
We have successfully performed the reduction of aldehydes 1 to alcohols 2
using chloroform–water mixture in high yields (Table 2). There is no
reduction of aldehydes 1 to alcohols 2 in the absence of one of the following:
base (DMAP/Et3N), protic solvents (MeOH, BnOH, and H2O), and alumina.
The reaction was very sluggish in the absence of microwave irradiation
(Table 3). Substrates bearing phenolic OH or –COOH groups retard the
reduction protocol (1l–1n in Table 1). The acidic –OH/–COOH is strongly
held by the basic site of Al2O3 preventing MeOH from forming the alkoxide
and no reduction occurred. The above reduction when performed using benzyl
alcohol afforded benzaldehyde, which confirms, that methanol in the reaction
is oxidized to formaldehyde. Conventional reduction of the aldehydes 1 in
refluxing MeOH resulted in moderate yields of the alcohols 2, whereas the
use of microwave conditions afforded excellent yields of the products
(Table 3). All the products have been characterized by direct comparison
with authentic samples prepared by classical methods.
In conclusion, the present method developed by us offers an alternative
and efficient method for selective reduction of aldehydes in the presence of
other reducible functional groups.
aBefore using the alumina for reaction, the desired amount of alumina in a Petri-dish
was activated by irradiating in a domestic microwave oven at 800 W for 10 min.

Selective Reduction of Aldehydes to Alcohols Using Alumina
2865
Scheme 1.
EXPERIMENTAL
General Procedures
All melting points were determined in open capillaries on SPAC-N-
SERVICE (India) capillary melting point apparatus and are uncorrected. IR
spectra were recorded on a JASCO FT-IR Model 410 using samples as KBr
1
plates. H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded
on a Bruker DPX 300 NMR instrument using TMS as internal standard.
Optical rotations were recorded at 258C in P-1020 JASCO polarimeter.
Elemental analyses were carried out on a PERKIN ELMER 2400 Series II
CHNS/O analyzer. Mass spectra (ESI) were done in a Micromass Q-Tof
MicroTM Spectrometer. The aldehydes 1a–1n were prepared using standard
procedures. The physical data of compounds such as 1h, 2h, 1i, and 2i are
not available in literature and so are reported.
Typical Procedure for Reduction of Aldehydes 1 to Alcohols 2
A solution of aldehyde (1 mmol) and DMAP or Et3N (0.1 mmol) in
MeOH (2 mL) was adsorbed over pre-activatedb basic or neutral alumina
(2 g) in a Petri dish and was placed inside a domestic microwave oven
(KENSTAR, 1000 W capacity) and irradiated at 300 W for the appropriate
time listed in Table 1. The reaction was monitored using TLC (CHCl3–
MeOH in the ratio 4 : 1) and after completion, the alumina was extracted
with a 3 : 1 mixture of CHCl3–MeOH (15 mL). After removal of the
solvent, the residue was purified by column chromatography over silica gel
using an increasing concentration of CHCl3 in petroleum ether to afford the
pure product 2 in excellent yield.
2-Allyloxy-benzaldehyde (1h). Colourless liquid; IR (Neat): 1687,
1
1598, 1482, 1455, 1287, 1240 cm21; H NMR (CDCl3): d 4.65 (m, 2H),
bSee footnote a.

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Pradhan et al.

Selective Reduction of Aldehydes to Alcohols Using Alumina
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Pradhan et al.

Selective Reduction of Aldehydes to Alcohols Using Alumina
2869
Table 2. Solvent effect on the reduction.
Time
(min)
% Yield
of 2a
Substrate 1a
Solvent
MeOH
CHCl3
MW (W)
300
300–500
500
10
20
10
10
95
0
CHCl3–H2O
BnOH
92
90
300
5.32 (dd, J ¼ 9.1 Hz, 1.4 Hz, 1H), 5.45 (dd, J ¼ 15.6 Hz, 1.5 Hz, 1H), 6.02–
6.21 (m, 1H), 6.97 (d, J ¼ 8.4 Hz, 1H), 7.19 (t, J ¼ 7.2 Hz, 1H), 7.52 (td,
J ¼ 7.8 Hz, 1.8 Hz, 1H), 7.83 (dd, J ¼ 7.5 Hz, 1.8 Hz, 1H), 10.53 (s, 1H);
ESIMS (m/z): 185 [M þ Na]þ. Anal. calcd. for C10H10O2: C, 74.06; H,
6.21. Found: C, 73.93; H, 6.30.
2-Allyloxy-benzylalcohol (2h). Colourless liquid; IR (Neat): 3384,
1
1599, 1490, 1455, 1239, 999.9 cm21; H NMR (CDCl3): d 4.52 (m, 2H),
4.67 (s, 2H), 5.25 (dd, J ¼ 6.0 Hz, 1.4 Hz, 1H), 5.40 (dd, J ¼ 12.0 Hz,
1.5 Hz, 1H), 5.90–6.10 (m, 1H), 6.82 (d, J ¼ 10.5 Hz, 1H), 6.91
(t, J ¼ 6.0 Hz, 1H), 7.15–7.30 (m, 2H); ESIMS (m/z): 187 [M þ Na]þ.
Anal. calcd. for C10H12O2: C, 73.15; H, 7.37. Found: C, 73.04; H, 7.44.
4-Allyloxy-3-methoxy-benzaldehyde (1i). Colourless liquid; IR (Neat):
1
1682, 1589, 1509, 1268, 1134 cm21; H NMR (CDCl3): d 3.94 (s, 3H), 4.72
(d, J ¼ 1.5 Hz, 2H), 5.30–5.50 (m, 1H), 6.02–6.15 (m, 1H), 6.97
Table 3. Comparison of yields of the product alcohols by microwave irradiation and
conventional methods in refluxing MeOH.
%
Conventional
methodb
time (hr)
%
Substrate
1a
Time
(min)
Yield
of 2a
Yield
of 2a
Basea
MW (W)
—
DMAP
Et3N
300–500
300
300
20
10
10
0
95
80
24
16
16
0
51
40
aCatalytic amount (10 mol%).
bFurther refluxing did not improve the yields of the product.

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Pradhan et al.
(d, J ¼ 7.8 Hz, 1H), 7.42 (s, 1H), 7.44 (d, J ¼ 7.8 Hz, 1H), 9.85 (s, 1H);
ESIMS (m/z): 193 [M þ H]þ. Anal. calcd. for C11H12O3: C, 68.74; H, 6.29.
Found: C, 68.61; H, 6.38.
4-Allyloxy-3-methoxy-benzylalcohol (2i). Colourless liquid; IR (Neat):
1
3394, 1593, 1512, 1460, 1420, 1263, 1138 cm21; H NMR (CDCl3): d 3.89
(s, 3H), 4.60 (d, J ¼ 1.3 Hz, 2H), 4.62 (s, 2H), 5.28 (d, J ¼ 10.4 Hz, 1H),
5.40 (d, J ¼ 17.2 Hz, 1H), 6.01–6.14 (m, 1H), 6.85 (brs, 2H), 6.94 (s, 1H);
ESIMS (m/z): 217 [M þ Na]þ. Anal. calcd. for C11H14O3: C, 68.02; H,
7.27. Found: C, 67.93; H, 7.35.
ACKNOWLEDGMENT
The authors thank CSIR, New Delhi for the generous grant.
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Received April 6, 2004