Bis(trimethylsilyl)chromate: An efficient reagent for cleavage of phenylhydrazones

Article abstract of DOI:10.1080/104265090969829

Phenylhydrazones are converted to their parent carbonyl compounds by silica-gel supported bis(trimethylsilyl) chromate. Copyright Taylor & Francis LLC.

Full text of DOI:10.1080/104265090969829

Phosphorus, Sulfur, and Silicon, 181:227–231, 2006
Copyright © Taylor & Francis LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090969829
Bis(Trimethylsilyl)Chromate: An Efficient Reagent
for Cleavage of Phenylhydrazones
Shahnaz Khaleghi
Majid M. Heravi
Fatemeh Drikvand
Department of Chemistry, Azzahra University, Vanak Tehran, Iran
Phenylhydrazones are converted to their parent carbonyl compounds by silica-gel
supported bis(trimethylsilyl) chromate.
Keywords Bis(trimethylsilyl)chromate; carbonyl compound; phenylhydrazones;
protection
Protection and deprotection of a functional group almost is inevitable in
1
multistage organic synthesis. Phenylhydrazones frequently are used
to protect carbonyl compounds. They also serve as important synthetic
intermediates and can be used preferably for isolation, purification,
2
and characterization of aldehydes and ketones. Because of these sig-
nificances, a plethora of reagents and methods, such as ammonium
3
persulfate–clay Zr(t-ButO)4, as a catalyst and tert-butylhydrope-
roxides as oxidant;4 benzyl-triphenylphosphonium peroxodisulfate;
5
6
prolinium chlorochromate; potassium ferrate under microwave irradi-
ation in solvent free conditions, hypervalent organoiodine, and thal-
lium acetate have been reported. These methods have their own merits
7
8
9
and drawbacks.
The application of bis(trimethylsilyl) chromate as catalyst in or-
ganic transformation has been developed in our laboratory.10−17 In
view of extending this research and in our interest in the cleavage
of phenylhydrazones,18 we now report a simple, efficient, and high-
yielding method for the conversion of phenylhydrazones to their cor-
responding carbonyl compounds.
Received January 6, 2004; in final form February 1, 2005.
The authors are thankful to the research council of Azzahra University for the partial
financial support. Their gratefulness is expanded to Mrs. M. Khosroshahi for her kind
cooperation.
Address correspondence to Shahnaz Khaleghi, Department of Chemistry, School of
Science, Azzahra University, Vanak, Tehran, Iran. E-mail: khaleghi@azzahra.ac.ir
227

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28
S. Khaleghi et al.
The reagent Bis(trimethylsilyl)chromate (BTSC) was prepared by
the treatment of hexamethyldisiloxane with CrO3 in CH2Cl2.
Initially, we treated the phenylhydrazone of benzaldehyde with this
reagent in refluxing CH2Cl2. The progress of reaction was monitored
by TLC. The reaction was sluggish and more seriously molten; BTSC
adhered to the walls of the vessel, making the work up cumbersome.
At this point, we contemplated to change the solvent and implement
a homogeneous catalysis. However, homogeneous catalysts generally
are connected with the problem of separation and wasted inorganics,
which are too difficult to reuse. In addition, order to prepare condi-
tions for homogeneous catalysis, we needed high boiling-point solvents.
These problems could be minimized principally by a heterogeneously
catalyzed reaction.
The use of supported reagent19 has attracted interest because of im-
proved selectivity, reactivity, and ease of manipulation.
Application of solid support for a heterogeneous catalysis of an or-
2
0
ganic reaction has been one of our research interests. To use BTSC for
the cleavage of phenylhydrazones, we examined various mineral sup-
ports such as montmorillonite K-10, zeolites, alumina, and silica gel
in different low boiling-point solvents. After some preliminary experi-
mentation, it was found that BTSC that was supported on silica gel in
a mixture of chloroform and water as solvents provided the best results
in terms of the formation of pure products for the cleavage of phenylhy-
drazones. In the absence of silica gel, the reaction is very slow, the yield
is very low, and the isolation of products from the ensuing residues
is difficult. Surprisingly, when CH2Cl2 or CHCl3 were used as a sol-
vent, the yield was lower than when using a mixture of chloroform and
water. The procedure involves the simple mixing of BTSC with silica
gel and refluxing this mixture with a phenylhydrazone in a mixture of
1:1 CHCl3 and H2O. To establish the generality of the method, various
phenylhydrazones were cleaved by the aforementioned procedure. No
over oxidation to carboxylic acid was observed.
In summary, the presented methodology offers an attractive, effi-
cient, and high-yielding method for the cleavage of phenylhydrazones
to the corresponding carbonyl compounds, using a silicon reagent.
EXPERIMENTAL
2
Phenylhydrazones were prepared according to the literature. All prod-
ucts were known and their physical data was compared with those of
authentic samples. Yields refer to GC analysis, an Agilent 6890 Series
GC System.

229

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30
S. Khaleghi et al.
1
0–18
PREPARATION OF BIS(TRIMETHYLSILYL)CHROMATE
SUPPORTED ON SILICA GEL
To a solution of hexamethyldisiloxane (6.4 mL, 0.03 mol) in dry CH2Cl2
(
20 mL), CrO3, (3 g, 0.03 Mol) was added. The reaction mixture was
◦
stirred in a 50 C bath. Solid CrO3 dissolved and the dark red mixture
became homogeneous. Silica gel (13 g) per-dried at 120 C overnight
◦
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 removed under reduced pressure to afford 19 g of the supported
BTSC oxidant.
REGENERATION OF CARBONYL COMPOUNDS
FROM PHENYLHYDRAZONES
General Procedure
Silica-gel supported BTSC (0.93 g, equivalent to 1.5 mmol) was re-
fluxed with a phenylhydrazone (1 mmol) in a mixture of CHCl3 and
H2O (10 mL) for the indicated time. Progress of the reaction was moni-
tored by TLC. Upon the completion of the reaction, the reaction mixture
was filtered. The organic layer was separated and evaporated to dryness
to afford the corresponding carbonyl compound (Table I).
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