In situ alcohol oxidation-protection reactions

Article abstract of DOI:10.1081/SCC-200058009

The one-pot conversion of primary and secondary alcohols into phenylhydrazones and 2,4-dintrophenylhydrazones is reported using chromium trioxide supported on silica gel and phenylhydrazines or 2,4- dintrophenylhydrazines under solvent-free conditions. This oxidation arylhydrazone formation reaction has been applied to a range of aliphatic and benzylic alcohols. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200058009

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In Situ Alcohol
Oxidation‐Protection Reactions
A. R. Kiasat a , F. Kazemi a & K. Nourbakhsh a
a Chemistry Department , College of Science, Shahid
Chamran University , Ahvaz, Iran
Published online: 17 Dec 2010.
To cite this article: A. R. Kiasat , F. Kazemi & K. Nourbakhsh (2005) In Situ Alcohol
Oxidation‐Protection Reactions, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 35:11, 1555-1558, DOI:
10.1081/SCC-200058009
To link to this article: http://dx.doi.org/10.1081/SCC-200058009
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Synthetic Communicationsw, 35: 1555–1558, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200058009
In Situ Alcohol Oxidation-Protection
Reactions
A. R. Kiasat, F. Kazemi, and K. Nourbakhsh
Chemistry Department, College of Science, Shahid Chamran University,
Ahvaz, Iran
Abstract: The one-pot conversion of primary and secondary alcohols into
phenylhydrazones and 2,4-dintrophenylhydrazones is reported using chromium
trioxide supported on silica gel and phenylhydrazines or 2,4-dintrophenylhydrazines
under solvent-free conditions. This oxidation arylhydrazone formation reaction has
been applied to a range of aliphatic and benzylic alcohols.
Keywords: Phenylhydrazone, oxidation, protection, chromium trioxide
Multistep syntheses produce considerable amounts of waste, in part because of
their complexity, but also because of a series of complex isolation procedures
that must be carried out after each step. Thus, the combination of steps into a
multistep, one-pot reaction sequence can be of great value, as long as overall
yield and efficiency are not negatively influenced.[1] In a series of recent
relevant papers, Taylor and coworkers developed a number of synthetically
useful tandem oxidation processes such as conversion of primary alcohols
to alkenes,[2] imines,[3] amines,[3] oxime ethers,[4] esters,[5] amides,[5] and
nitriles.[6] One of the main benefits of this methodology is the in situ
formation and consumption of the intermediate aldehydes.
Recently, studies of solvent-free organic reactions, especially with the
reagents immobilized on porous solid supports, have had an important
Received in the UK November 10, 2004
Address correspondence to A. R. Kiasat, Chemistry Department, College of
Science, Shahid Chamran University, Ahvaz 61357-4-3169, Iran. Fax: þ98611
3337009; E-mail: kiasat_a@cua.ac.ir
1555

1556
A. R. Kiasat, F. Kazemi, and K. Nourbakhsh
effect.[7] The remarkable ability of silica gel to promote the various reactions
under solvent-free conditions[8] together with the importance of combinatorial
chemistry as a preparative technique[9] gave us the impetus to test the
oxidation of primary and secondary alcohols followed by in situ protection
of the resulting carbonyl compounds as arylhydrazones in a one-pot process
using chromium trioxide supported on silica gel and arylhydrazine under
solvent-free conditions.
Chromium trioxide supported on silica gel was prepared by simply
grinding silica gel with CrO3 in the ratio 10:1 (w/w) in an agate mortar. In
this simple and efficient method, when the starting alcohol and two molar
equivalent of CrO3/SiO2 and a few drops of t-BuOH was mixed with
grinding by a pestle, the alcohol was consumed (TLC) within 10 min to
furnish carbonyl compound as the only product of the oxidation step. This
reaction was followed by in situ trapping of the resulting carbonyl
compound by an additional elimination reaction. For this purpose, the
reaction mixture was ground with 5 molar equivalent of arylhydrazine
(10 min).
As reported in Table 1, satisfactory results were obtained in the
oxidation–arylhydrazone formation reaction of variety of primary and
secondary alcohols with CrO3/SiO2 and arylhydrazine under solvent-free con-
ditions. The structure of all the products were settled from their analytical and
spectral (IR, 1H NMR) data and by direct comparison with authentic samples.
This one-pot methodology avoids the need to isolate the intermediate
carbonyl compounds, a particularly useful feature in the case of carbonyl
compounds, which are volatile, toxic, or highly reactive. It can be emphasized
that the reaction is clean, the workup is straightforward, and, from economical
and environmental points of view, the use of solvent-free conditions is
favorable.
We have demonstrated a rapid and solventless method for one-pot
conversion of alcohols to arylhydrazone. We believe that the present
procedure provides easy, mild, efficient, versatile, and general method-
ology for the oxidation–arylhydrazone formation of different classes of
alcohols.

In Situ Alcohol Oxidation-Protection Reactions
1557
Table 1. One-pot preparation of arylhydrazone by treatment of alcohols with
CrO3/SiO2 and phenylhydrazine or 2,4-dintrophenylhydrazine
Yield %a,b Ar:
phenyl/2,4-
dinitrophenyl
Entry
1
Substrate
Product
75/75
2
3
4
88/90
78/78
80/83
5
6
7
75/77
80/80
85/82
8
9
73/68
86/89
aYields refer to pure isolated products.
bProducts were characterized by comparison of their physical data and IR and NMR
spectra with known samples.
EXPERIMENTAL
General Experimental Procedure for the Conversion of
Alcohols to Arylhydrazone
A mortar was charged with SiO2 (2 g) and CrO3 (0.2 g, 2 mmol); the mixture
was ground with a pestle for 1 min. Then, alcohols (1 mmol) and several drops
of t-butanol were added to the mixture. The reaction mixture was ground for
about 10 min. Phenylhydrazine or 2,4-dinitrophenylhydrazine (5 mmol) was
added to the mixture and ground for 10 min. The progress of reaction was

1558
A. R. Kiasat, F. Kazemi, and K. Nourbakhsh
monitored by TLC using ether–CCl4. On completion of reaction, the reaction
mixture was mixed with ether. The mixture was filtered to remove silica gel
and then mixed with water and extracted. The organic layer was dried over
Na2SO4. The solvent was removed in vacuuo to give the product.
ACKNOWLEDGMENT
Authors acknowledge the partial support of this work by Shahid Chamran
Ahvaz University Research Council.
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1. Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025.
2. Blackburn, L.; Pei, C.; Taylor, R. J. K. Synlett 2002, 215.
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4. Kanno, H.; Taylor, R. J. K. Synlett 2002, 1287.
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6. McAllister, G. D.; Wilfred, C. D.; Taylor, R. J. K. Synlett 2002, 1291.
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