Mild and efficient method for oxidative deprotection of trimethylsilyl ethers mediated by tetraethylammonium superoxide

Article abstract of DOI:10.1080/00397910701228604

Oxidative deprotection of trimethylsilyl ethers to their corresponding carbonyl compounds has been achieved under the mild reaction conditions of in situ generated tetraethylammonium superoxide. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701228604

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Mild and Efficient Method for Oxidative
Deprotection of Trimethylsilyl Ethers
Mediated by Tetraethylammonium Superoxide
R. S. Raghuvanshi a & K. N. Singh a
a Department of Applied Chemistry, Institute of Technology, Banaras Hindu
University, Varanasi, India
Version of record first published: 13 Apr 2007.
To cite this article: R. S. Raghuvanshi & K. N. Singh (2007): Mild and Efficient Method for Oxidative
Deprotection of Trimethylsilyl Ethers Mediated by Tetraethylammonium Superoxide, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:8,
1371-1374
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Synthetic Communicationsw, 37: 1371–1374, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701228604
Mild and Efficient Method for Oxidative
Deprotection of Trimethylsilyl Ethers
Mediated by Tetraethylammonium
Superoxide
R. S. Raghuvanshi and K. N. Singh
Department of Applied Chemistry, Institute of Technology, Banaras
Hindu University, Varanasi, India
Abstract: Oxidative deprotection of trimethylsilyl ethers to their corresponding
carbonyl compounds has been achieved under the mild reaction conditions of in situ
generated tetraethylammonium superoxide.
Keywords: deprotection, phase-transfer catalyst, superoxide ion, trimethylsilyl ether
The selective introduction and removal of protecting groups is very important in
synthetic organic chemistry.[1,2] Trimethylsilylation of hydroxyl groups has been
recognized as a useful method for the protection of alcohols. The cleavage of tri-
methylsilyl ethers to the corresponding carbonyl compounds has found consider-
able attention during recent years.[3–11] In continuation of our research program
on superoxide research,[12] we describe herein our results on the reactivity of
in situ generated tetraethylammonium superoxide (Et4NO2) with different tri-
methylsilyl ethers under nonaqueous conditions at room temperature,
affording the corresponding carbonyl compounds (Scheme 1).
As an outcome, trimethylsilyl ethers (1a–j) were readily deprotected to
their corresponding carbonyl compounds (2a–j). However, the substrates
(1f–j) invariably provided a little amount of corresponding carboxylic acids
(3f–j) along with the carbonyl compounds (2f–j).
Received September 21, 2006
Address correspondence to K. N. Singh, Department of Applied Chemistry,
Institute of Technology, Banaras Hindu University, Varanasi 221 005, India. E-mail:
knsinghbhu@yahoo.co.in
1371

1372
R. S. Raghuvanshi and K. N. Singh
Scheme 1.
The reactions were accomplished by using a 2.0-fold molar excess of KO2
with respect to substrate (1) in anhydrous DMF. Generally, the substrate tri-
methylsilyl ether was allowed to react with in situ generated superoxide ion
for 2–5 h at room temperature. The reaction was then quenched with cold
brine solution and worked up to afford the products in excellent yield
(Table 1). The total disappearance of the starting material was checked by
thin-layer chromatography (TLC). All the oxidation products are known
compounds and are identified by the comparison of their physical data and
IR and NMR spectra with those of the authentic samples.
In conclusion, a mild and convenient method for the oxidative removal of
the trimethylsilyl protecting group, using in situ generated tetraethylammo-
nium superoxide, has been described.
EXPERIMENTAL
Melting points were measured in open capillaries and are uncorrected. IR
1
spectra were recorded on a Jasco FT/IR-5300 spectrophotometer. H NMR
Table 1. Oxidative deprotection of trimethylsilyl (TMS) ethers with in situ generated
tetraethylammonium superoxide
Entry
Substrate
Product(s)
Ph2CO (2a)
PhCOMe (2b)
Yield (%)
a
b
c
d
e
f
Ph2CHOTMS (1a)
PhCH(Me)OTMS (1b)
c-C6H11OTMS (1c)
87
85
82
76
85
80
11
76
12
73
12
73
14
72
14
Cyclohexanone (2c)
4-ClC6H4COMe (2d)
4-PhC6H4COMe (2e)
PhCHO (2f)
4-ClC6H4CH(Me)OTMS (1d)
4-PhC6H4CH(Me)OTMS (1e)
PhCH2OTMS (1f)
PhCOOH (3f)
g
h
i
2-MeOC6H4CH2OTMS (1g)
3-MeOC6H4CH2OTMS (1h)
4-MeOC6H4CH2OTMS (1i)
2-NO2C6H4CH2OTMS (1j)
2-MeOC6H4CHO (2g)
2-MeOC6H4COOH (3g)
3-MeOC6H4CHO (2h)
3-MeOC6H4COOH (3h)
4-MeOC6H4CHO (2i)
4-MeOC6H4COOH (3i)
2-NO2C6H4CH2OH (2j)
2-NO2C6H4COOH (3j)
j

Oxidative Deprotection of Trimethylsilyl Ethers
1373
spectra were run on a Jeol AL300 FTNMR spectrometer (chemical shift in
d ppm, relative to TMS as internal reference). Potassium superoxide and tetra-
ethylammonium bromide were procured from E. Merck, Germany and were
used as received. Dry N,N-dimethylformamide (DMF) from Aldrich, USA,
˚
was stored over molecular sieves (4 A) prior to use. Trimethylsilyl ethers
were prepared according to a literature procedure.[13]
Oxidative Deprotection of Trimethylsilyl Ethers by in situ Generated
Tetraethylammonium Superoxide Ion: General Procedure
A mixture of potassium superoxide (1.13 g, 16 millimol) and tetraethylammo-
nium bromide (1.68 g, 8 millemol) (weighted under nitrogen atmosphere
using an atmosbag) in dry dimethylformamide (25 ml) was stirred for
15 min, and then trimethylsilyl ether 1a–j (8 millimol) was added. After
the reaction was completed (2–5 h) as indicated by TLC, brine solution
(20 ml) was introduced followed by saturated sodium hydrogen carbonate
solution (20 ml). The reaction mixture was extracted with diethyl ether or
dichloromethane (3 Â 20 ml). The combined organic extract was washed
with water, dried over anhydrous Na2SO4, filtered, and evaporated to give
the product (2a–j). The aqueous phase was acidified with hydrochloric acid
and extracted with diethyl ether or dichloromethane (3 Â 20 ml) to isolate
the acidic product (3f–j).
ACKNOWLEDGMENT
The authors are thankful to Council of Scientific and Industrial Research
(CSIR), New Delhi, for financial support.
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