Microwave assisted solid reaction: Reduction of esters to alcohols by potassium borohydride-lithium chloride

Article abstract of DOI:10.1081/SCC-100104338

Esters can be successfully reduced to the corresponding alcohols with potassium borohydride/lithium chloride under microwave irradiation without solvent. The reactions are generally completed in 2-8 minutes, with the yields varying from 55% to 95%.

Full text of DOI:10.1081/SCC-100104338

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MICROWAVE ASSISTED SOLID REACTION: REDUCTION
OF ESTERS TO ALCOHOLS BY POTASSIUM
BOROHYDRIDE-LITHIUM CHLORIDE
Jun-Cai Feng a , Bin Liu a , Li Dai a , Xiao-Liang Yang a & Shu-Jiang Tu b
a Department of Chemistry, Nanjing University, Nanjing, 210093, P.R. China
b Department of Chemistry, Xuzhou Normal University, Xuzhou, 221009, P.R. China
Version of record first published: 16 Aug 2006.
To cite this article: Jun-Cai Feng , Bin Liu , Li Dai , Xiao-Liang Yang & Shu-Jiang Tu (2001): MICROWAVE ASSISTED
SOLID REACTION: REDUCTION OF ESTERS TO ALCOHOLS BY POTASSIUM BOROHYDRIDE-LITHIUM CHLORIDE, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:12, 1875-1877
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SYNTHETIC COMMUNICATIONS, 31(12), 1875–1877 (2001)
MICROWAVE ASSISTED SOLID
REACTION: REDUCTION OF ESTERS TO
ALCOHOLS BY POTASSIUM
BOROHYDRIDE-LITHIUM CHLORIDE
Jun-Cai Feng,* Bin Liu, Li Dai, Xiao-Liang Yang,
and Shu-Jiang Tu#
Department of Chemistry, Nanjing University, Nanjing,
210093, P.R. China
ABSTRACT
Esters can be successfully reduced to the corresponding alco-
hols with potassium borohydride/lithium chloride under
microwave irradiation without solvent. The reactions are gen-
erally completed in 2–8 minutes, with the yields varying from
55% to 95%.
Lithium aluminum hydride and sodium borohydride have been widely
used for reduction of carbonyl compounds. As a reducing reagent, they are
somewhat more convenient to use on a small scale in the research labora-
tory, but sodium borohydride is a less vigorous and more selective reagent
than lithium aluminum hydride. It’s known, that sodium borohydride
will not usually reduce carboxylic esters under ordinary conditions.
* Corresponding author.
#
Current address: Department of Chemistry, Xuzhou Normal University, Xuzhou
221009, P.R. China.
1875
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

ORDER
REPRINTS
1876
FENG ET AL.
However, the reducing power of sodium borohydride could be increased1 by
(1) the proticsolvents ( 2) the presence of activating substituents (3) the
metal salt as additives. Paul and Joseph2 first reported the successful reduc-
tion esters by potassium borohydride-lithium chloride in 1952. Little atten-
tion has been paid toward it since then. In our laboratory, by taking
advantage of both dry reaction conditions and microwave irradiation, we
now wish to report here a facile reduction of esters by potassium boro-
hydride-lithium chloride under microwave irradiation. The reactions are
finished in 2–8 mins with the yields 55–95%.
The equation is shown as follows:
NaBH4ꢀLiCl
R1COOR2ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ! R1CH2OH þ R2OH
Microwave Irradiation
(1)
(2)
1a: R1 ¼ C6H5, R2 ¼ Et
1b: R1 ¼ o-ClC6H4, R2 ¼ Et
1d: R1 ¼ o-O2NC6H4, R2 ¼ Et
1f: R1 ¼ m-CH3C6H4, R2 ¼ Et
1c: R1 ¼ p-ClC6H4, R2 ¼ Et
1e: R1 ¼ p-BrC6H4, R2 ¼ Et
1g: R1 ¼ p-CH3C6H4, R2 ¼ Et 1h: R1 ¼ C6H5CH ¼ CH, R2 ¼ Et
1i: R1 ¼ 4-pyridyl, R2 ¼ Et
1j: R1 ¼ C6H5CH2, R2 ¼ Me
1k: R1 ¼ 3,4-(phCH2O)2C6H3, R2 ¼ Et
1l: R1 ¼ p-EtOCOC6H4, R2 ¼ Et
Table 1. The reduction of esters to alcohols by NaBH4-LiCl
Time
(min)
Yield
(%)
Entry
Product
C6H5CH2OH
o-ClC6H4CH2OH
p-ClC6H4CH2OH
o-O2NC6H4CH2OH
p-BrC6H4CH2OH
m-CH3C6H4CH2OH
p-CH3C6H4CH2OH
C6H5CH=CHCH2OH
4-pridylcarbinol
C6H5CH2CH2OH
3,4-(phCH2O)2C6H3CH2OH
p-HOCH2C6H4CH2OH
mp. or bp. (lit.)(ꢁC)
a
b
c
d
e
f
g
h
i
5
3
5
2
3
5
5
5
4
3
6
5
75
92
91
90
95
61
95
55
97
80
90
85
198/760
70–2
71–2
(205/760)
(69–71)
(70–2)
71–2
76–8
(69–70)
(75–7)
219/760
56–8
240/760
55–6
(215/740)
(59–61)
(33–5)
(57–9)
j
k
l
223/760
62–4
115–7
(219/750)
(66–8)
(117–9)
Yield are determined by HPLC [column/spherisorb C18 10 mm (4.6 ꢀ 200 mm) col-
umn temperature: 35ꢁC; mobile phase: methanol: water ¼ 70:30(v/v); flow rate: 1 ml/
min; ꢀ ¼ 272 nm].

ORDER
REPRINTS
MICROWAVE ASSISTED SOLID REACTION
1877
As shown in the table 1, esters of aromatic and heterocyclic acids were
reduced to the primary alcohols in good yield.
The application of microwave in solid reaction offers a very quick and
environmentally benign method for the reduction of esters. The operational
simplicity, high yields in significantly very short reaction time make this
procedure a useful and attractive alternative to be currently available
methods.
EXPERIMENTAL
Melting points were determined on a Yanaco MP-500 apparatus and
are uncorrected. Microwave irradiation was carried out with a commercial
microwave oven (2450 MHz, 500 W) under atmospheric pressure. IR
Spectra were obtained on a Nicolet FT-IR50DX instrument.
General procedure: Potassium borohydride 1.0 g (20 mmol), anhydrous
lithium chloride 0.8 g (20 mmol) were thoroughly mixed in a mortar and
transferred to a flask (100 ml) connected with refluxing equipment, then
dry THF 10 ml was added and the mixture was heated to reflux for 1 h.
After cooling, the ester (10 mmol) was added and stirred for 0.5 h at room
temperature, then the THF was removed under reduced pressure. After the
mixture was irradiated by microwave for 2–8 minutes, the mixture was
cooled to room temperature, water 20 ml was added, extracted with ether
(3 ꢂ 15 ml), dried with magnesium sulfate, and evaporated to give the crude
product, which was purified by crystallization, distillation or column chro-
matography.
REFERENCES
1. (a) Brown, H.C.; Krishnamurthy, S. Tetrahedron 1979, 35, 572.
(b) Meschino, J.A.; Bond, C.H. J. Org. Chem. 1963, 28, 3129.
(c) Brown, M.S.; Rappoport, H. J. Org. Chem. 1963, 28, 3261.
(d) Santaniello, E.; Manzochi, A.; Sozzeni, P. Tetrahedron Lett.
1979, 4581.
2. Paul, R.; Joseph, N. Bull. Soc. Chim. Fr. 1952, 550.
Received in the USA September 1, 2000

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