A novel and highly efficient method for the direct conversion of aryl aldehyde bisulfite adducts to their aryl trimethylsilyl ethers in a one-pot manner catalyzed by montmorillonite K-10

Article abstract of DOI:10.1080/104265090969009

A new, convenient, and chemoselective method has been developed for the one-pot conversion of aryl aldehyde bisulfite adducts to the corresponding trimethylsilyl ethers. Copyright Taylor & Francis LLC.

Full text of DOI:10.1080/104265090969009

Phosphorus, Sulfur, and Silicon, 181:93–97, 2006
Copyright © Taylor & Francis LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090969009
A Novel and Highly Efficient Method for the Direct
Conversion of Aryl Aldehyde Bisulfite Adducts to their
Aryl Trimethylsilyl Ethers in a One-Pot Manner Catalyzed
by Montmorillonite K-10
Mohammad M. Khodaei
Ahmad R. Khosropour
Jamshide Abbasi
Department of Chemistry, Razi University, Kermanshah, Iran
A new, convenient, and chemoselective method has been developed for the one-pot
conversion of aryl aldehyde bisulfite adducts to the corresponding trimethylsilyl
ethers.
Keywords bisulfites; montmorillonite K-10; direct conversion; sodium borohydride;
trimethylsilyl ethers
INTRODUCTION
Protection and deprotection of functional groups are important in
multistep organic synthesis. Direct transformation between protect-
ing groups is also of great importance due to their efficiency and
convenience.2 The interconversion of a protecting group of the hydroxy
or the carbonyl function into another has been investigated. However,
direct conversion of a protecting group of the carbonyl function into the
protected form of the hydroxy group has not been reported.
Aryl bisulfite adducts are used frequently as protecting forms of
aldehydes under basic conditions, and due to their highly crystalline
forms, they are extensively utilized for the purification of these car-
bonyl compounds.3 On the other hand, silyl ethers as a protect-
ing form of hydroxyl groups have been extensively utilized in or-
ganic synthesis.4 They show (a) resistance to oxidation, (b) good
thermal stability, (c) low viscosity, and (d) are easily recoverable
Received December 7, 2004; in final form January 6, 2005.
We are grateful to the Research Council of Razi University for partial financial sup-
port of this work.
Address correspondence to Mohammad Mehdi Khodaei, Razi University, Department
of Chemistry, Kermanshah, 67149, Iran. E-mail: mmkhoda@razi.ac.ir
93

94
M. M. Khodaei et al.
from the parent compounds.4b As a result, a variety of methods
have been examined for their formation, but all of them are lim-
ited to the utilization of alcohols or phenol as a precursor.5 There-
fore, the introduction of new methods for the synthesis of these
compounds with other substrates is important in multistep organic
synthesis.
Among the reducing agents, sodium borohydride is the most fre-
quently used in the reduction process. It is a mild, inexpensive, and
invaluable reagent with applications in a wide range of functional
groups.6 In addition, the extensive application of heterogeneous catal-
ysis in synthetic chemistry can help to achieve new reactions to lower
the waste product. Recently, more attention has been focused on mont-
´
morillonite K-10 due to its strong catalytic activity as a Brønsted acid
as a heterogeneous catalyst in fine organic synthesis.7 Although mont-
morillonite K-10 has been extensively used as a catalyst in organic
synthesis, a literature search clearly shows that there has been no in-
vestigation into the reduction–protection procedure in the presence of
this clay. Thus, we decided to study the one-pot transformation of the
bisulfite addition product of aldehydes into silyl ethers-protected alco-
hol by montmorillonite K-10.
RESULTS AND DISCUSSION
As shown in Table I, a series of aryl aldehyde bisulfide adducts re-
acted with sodium borohydride and hexamethyldisilazine (HMDS) in
acetonitrile to afford the corresponding TMS-ethers in the presence of
montmorillonite K-10 at room temperature in high to excellent yields
at short reaction times (Scheme 1).
SCHEME 1
The presence of electron-donating and electron-withdrawing groups
on the aromatic ring of the bisulfites, irrespective of their positions in
the ring, did not make any obvious difference in the rate of the reaction.
Only in nitro aromatic bisulfites (Entries 2, 3), due to the deactivation of
the benzylic carbon, reaction times were prolonged. The reaction condi-
tions were mild enough not to induce any isomerization for conjugated
aldehyde bisulfites (Entry 17), or to damage moieties such as methoxy

Aryl Aldehyde Bisulfite Adducts
95
TABLE I One-Pot Conversion of Aryl Bisulfite Adducts to the
Corresponding Trimethylsilyl Ethes Catalyzed by Montmorillonite
K-10
Entry
1
Aryl bisulfite
Product
Time (min)
12
Yield (%)
98
2
3
90
40
87
90
4
5
6
7
13
8
96
97
94
90
8
12
8
9
8
95
90
56
10
11
13
18
91
97
12
13
12
10
95
90
14
8
98
15
16
35
32
82
93
17
33
80

96
M. M. Khodaei et al.
(Entries 10, 11), which often undergo cleavage in strongly acidic reac-
tion media. Side product formation was not observed in these reactions.
It worth noting that hydroxyl groups in the aromatic ring remain
intact under similar conditions (Entries 8, 9). In order to evaluate the
selectivity of the present method, competition reactions were carried
out for the conversion of aryl bisulfite adducts in the presence of the
cyclic acetals or 1,1-diacetates (acylals) as other protecting aldehyde
forms (Scheme 2).
SCHEME 2
When a 1:1 mixture of these protecting forms was allowed to re-
act with NaBH4 and HMDS in the presence of montmorillonite K-10
(50 mg in lieu of 1 mmol of substrate) for about 60 minutes at room
temperature, TLC analysis of the reaction mixture indicated complete
conversion of aryl bisulfite to its TMS-ether, while the acetal or acylal
was still intact.
In conclusion, we demonstrated a novel and highly efficient proce-
dure for the one-pot conversion of aryl aldehyde bisulfite adducts to
their TMS-ethers. The mild experimental conditions, short reaction
times, high yields and excellent chemoselectivity in addition to the uti-
lization of an inexpensive and nontoxic catalyst, represent the notable
feature of this procedure.
EXPERIMENTAL
Products are known compounds and were characterized by comparison
of their spectral data (1H NMR, IR) with those reported in the literature.
Monitoring of the reactions was accomplished by TLC on precoated
silica gel 60 F254 sheets. All yields refer to isolated products.
General Procedure for the Transformation of Bisulfites
to Trimethylsilyl Ethers
To a stirred mixture of aryl aldehyde bisulfite adducts (1 mmol) and
sodium borohydride (1 mmol) in CH3CN (2 mL), HMDS (1.5 mmol)
and montmorillonite K-10 (50 mg) were added at 25◦C. The mixture
was stirred at room temperature for the appropriate time according to

Aryl Aldehyde Bisulfite Adducts
97
Table I. The end of the reaction was monitored by TLC. After comple-
tion of the reaction, the mixture was filtered and washed with Et2O (3
× 5 mL). The filtrate was dried with Na2SO4 and evaporation of the
organic solvent gave the crude product, which was purified by plate or
column chromatography on silica gel to afford the pure TMS-ethers in
80–98% yields.
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