Direct Oxidative Deprotection using Montmorillonite Supported Bis(trimethylsilyl)chromate

Article abstract of DOI:10.1039/a803237h

Direct oxidative deprotection of different trimethylsilyl ethers to their corresponding carbonyl compounds has been achieved using montmorillonite K-10 supported bis(trimethylsilyl)chromate in dichloromethane.

Full text of DOI:10.1039/a803237h

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620 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 620±621$
Direct Oxidative Deprotection using Montmorillonite
Supported Bis(trimethylsilyl)chromate$
M. M. Heravi,* D. Ajami, K. Tabar-Heydar and M. M. Mojtahedi
Chemistry & Chemical Engineering Research Center of Iran, Tehran, P.O. Box 14335-186, Iran
Direct oxidative deprotection of different trimethylsilyl ethers to their corresponding carbonyl compounds has been
achieved using montmorillonite K-10 supported bis(trimethylsilyl)chromate in dichloromethane.
The protection of functional groups is a useful and import-
ant method in organic synthesis. Hydroxy groups are one of
the most abundant functional groups in organic molecules
and its controlled manipulation is very important in multi-
step synthesis. One of the most useful and convenient
methods for protection of hydroxy groups is their trans-
formation to trialkylsilyl ethers.1,2
Substituted benzyl trimethylsilyl ethers (entries 2 and 3)
were similarly oxidized to their corresponding aldehydes
under the same conditions. No traces of benzoic acid were
observed even after prolonged re¯ux of benzaldehyde with
excess supported BTSC. Cinnamyl trimethylsilyl ether (entry
4) was also oxidatively deprotected to cinnamyl aldehyde in
Direct oxidation of trimethylsilyl ethers to the corre-
sponding carbonyl compounds has recently found much
attention.3 However some of the reported methods show
limitations such as the requirement for aqueous reaction
conditions,4,5 use of expensive reagents,6±8 long reaction
times, tedious work-up5 and low yields of products.9
Therefore introduction of new methods and inexpensive
reagents for such functional group transformations is still
in much demand.
Table 1 Oxidative deprotection of trimethylsilyl ethers with
BTSC supported on montmorillonite K10
Recently we have demonstrated the use of bis(trimethyl-
silyl)chromate supported on silica gel and montmorillonite
K-10 as an ecient and mild oxidizing agent under classical
heating10,11 or microwave irradiation.11,12 Now we wish
to report a new and ecient method for the oxidative
deprotection of trimethylsilyl ethers to their corresponding
carbonyl compounds using bis(trimethylsilyl)chromate
supported on montmorillonite K-10 in dichloromethane at
room temperature.
Heating chromic anhydride under re¯ux with a slight
excess of hexamethyldisoloxane in dichloromethane pro-
duces a nearly homogeneous solution. BTSC supported
on montmorillonite K-10 is conveniently prepared simply
by adding montmorillonite K-10 to a vigorously stirred
solution of BTSC.10,11 A dark brown free-¯owing solid was
obtained on evaporation of volatiles. This reagent can be
stored in a dark brown bottle without appreciable loss of
reactivity for at least three months.
In a typical reaction 2±2.2 equiv. of BTSC supported on
montmorillonite K-10 was added to a stirred solution of a
trimethylsilyl ether in dry CH2Cl2 (Scheme 1). The reaction
mixture was stirred at room temperature until the reaction
was complete, then it was ®ltered and washed with dichloro-
methane. Evaporation of the solvent gave the corresponding
carbonyl compounds. Di€erent types of trimethylsilyl ethers
were converted to their corresponding carbonyl compounds
in high yield. For example benzyl trimethylsilyl ether
(Table 1, entry 1) was oxidized to benzaldehyde in excellent
yield.
Scheme 1 Reagents and conditions: i, CrO3, TMSOTMS,
montmorillonite K-10, CH2Cl2, 25 8C, 25 min
*To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

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J. CHEM. RESEARCH (S), 1998 621
high yield. No benzaldehyde was detected in this reaction,
showing that benzylic double bonds are not prone to
cleavage by this method.
References
1 T. W. Greene and P. G. M. Wuts, Protective Groups in Organic
Synthesis, New York, 2nd edn., 1991.
2 M. Latonde and T. H. Chan, Synthesis, 1985, 81 and references
therein.
3 I. Mohammadpoor-Baltork and S. H. Pouranshirvani, Synthesis,
1997, 7, 756.
In conclusion, BTSC supported onto montmorillonite
K-10 is a simple and inexpensive reagent for one-pot
oxidative deprotection of trimethylsilyl ethers.
4 R. Baker, V. B. Rao, P. D. Ravenscroft and C. J. Swann,
Synthesis, 1983, 572.
Experimental
5 R. Mahrwald, F. Theil, H. H. Schick, S. Sehwartz, H. J. Palme
and G. Weber, J. Prakt. Chem., 1986, 328, 777.
6 H. Firouzabadi and I. Mohammadpoor-Baltork, Synth. Commun.,
1995, 24, 1065.
7 H. Firouzabadi and F. Shiriny, Synth Commun., 1996, 26, 423.
8 H. Firouzabadi and F. Shiriny, Synth Commun., 1996, 26, 649.
9 H. W. Pinnick and N. H. Lajis, J. Org. Chem., 1978, 43, 371.
10 M. M. Heravi, D. Ajami and K. Tabar-Heydar, Monatsch.
Chem., 1998, in press.
All products were known compounds and identi®ed by com-
parison with authentic samples. Yields refer to GC analysis.
Trimethylsilyl ethers were synthesized according to the reported
procedure.13 BTSC supported on montmorillonite K-10 was also
prepared according to a reported procedure.10±12
Oxidative Deprotection of Trimethylsilyl Ethers, a Typical Pro-
cedure.ÐIn
a round bottomed ¯ask (50 ml) equipped with a
magnetic stirrer and a condenser a solution of trimethylsilyl benzyl
ether (180 mg, 1 mmol) in CH2Cl2 (20 ml) was prepared. To this
solution pre-made BTSC supported onto montmorillonite K-10
(1.66 g equiv. to 2.4 mmol of CrO3) was added. The reaction
mixture was stirred at room temperature for 10 min. The progress
of the reaction was monitored by TLC [eluent: light petroleum±
ethyl acetate (8:2)]. The mixture was ®ltered and the solid material
was washed with CH2Cl2 (20 ml). The ®ltrate was evaporated to
dryness under reduced pressure and the resulting crude material
was puri®ed on a silica gel pad. After evaporation of solvent pure
benzaldehyde was obtained in 93% yield (Table 1).
11 M. M. Heravi, D. Ajami and K. Tabar-Heydar, Synth
Commun., 1998, submitted.
12 Preparation of Bis(trimethylsilyl)chromate Supported on Mont-
morillonite K-10.ÐTo
a
solution of hexamethyldisiloxane
(6.4 ml, 0.03 mol) in 20 ml of dry dichloromethane was added
chromic anhydride (3 g, 0.03 mol). The reaction mixture was
stirred in an oil-bath at 50 8C for 5 h. Solid chromic anhydride
dissolved and the dark red mixture became a homogeneous
solution. Montmorillonite K-10 (13 g) pre-dried in a 120 8C oven
overnight and activated in a microwave oven for 3 min was
added to the warm reaction mixture and the resulting mixture
was stirred for a further 5 h. The solvent and other volatile
components were distilled under reduced pressure to a€ord 19 g
of supported chromium oxidant.
Received, 29th April 1998; Accepted, 12th June 1998
Paper E/8/03237H
13 G. Maity and S. C. Roy, Synth Commun., 1993, 23, 1967.