Product selectivity in semiconductor-mediated dehydrazonation of benzophenone hydrazone

Article abstract of DOI:10.1080/00397911.2010.493630

Product selectivity in the dehydrazonation of benzophenone hydrazone by photocatalytic oxidation with various semiconductor photocatalysts has been investigated using ultraviolet-A light. TiO₂-P25 shows greater product selectivity of benzophenone formation with 93.9% conversion. Doping of metals on TiO₂ selectively enhances the formation of azine from hydrazone. Solvents such as dichloromethane, chloroform, and dichloroethane also enhance the formation of azine.

Full text of DOI:10.1080/00397911.2010.493630

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Product Selectivity in Semiconductor-
Mediated Dehydrazonation of
Benzophenone Hydrazone
B. Krishnakumar a , K. Selvam a & M. Swaminathan a
a Department of Chemistry , Annamalai University , Annamalainagar,
India
Published online: 02 May 2011.
To cite this article: B. Krishnakumar , K. Selvam & M. Swaminathan (2011) Product Selectivity in
Semiconductor-Mediated Dehydrazonation of Benzophenone Hydrazone, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:13, 1929-1937,
DOI: 10.1080/00397911.2010.493630
To link to this article: http://dx.doi.org/10.1080/00397911.2010.493630
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Synthetic Communications1, 41: 1929–1937, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.493630
PRODUCT SELECTIVITY IN SEMICONDUCTOR-
MEDIATED DEHYDRAZONATION OF BENZOPHENONE
HYDRAZONE
B. Krishnakumar, K. Selvam, and M. Swaminathan
Department of Chemistry, Annamalai University, Annamalainagar, India
GRAPHICAL ABSTRACT
Abstract Product selectivity in the dehydrazonation of benzophenone hydrazone by
photocatalytic oxidation with various semiconductor photocatalysts has been investigated
using ultraviolet-A light. TiO2-P25 shows greater product selectivity of benzophenone for-
mation with 93.9% conversion. Doping of metals on TiO2 selectively enhances the formation
of azine from hydrazone. Solvents such as dichloromethane, chloroform, and dichloroethane
also enhance the formation of azine.
Keywords Dehydrazonation; doped photocatalysts; product selectivity; titanium
dioxide; UV-A light
INTRODUCTION
Hydrazones, oximes, and semicarbazones are useful protecting groups[1] and
are widely utilized for purification and characterization of carbonyl compounds.
Synthesis of hydrazones, oximes, and semicarbazones from noncarbonyl com-
pounds[2] provides an alternative way of preparing aldehydes and ketones. Hence,
more interest has been focused on the development of mild techniques for the con-
version of oximes and hydrazones into their corresponding carbonyl compounds.[3–5]
So far, a number of methods have been developed for the regeneration of carbonyl
compounds from hydrazones.[6–15] However, most of these methods suffer some
disadvantages such as large reaction time, higher reflux temperature, difficulties
in isolation of products, and poor yield. The classical method for the cleavage of
oximes, phenyl hydrazones, p-nitrophenyl hydrazones, and semicarbazones to
Received December 6, 2009.
Address correspondence to Dr. M. Swaminathan, Department of Chemistry, Annamalai
University, Annamalainagar 608 002, India. E-mail: chemres50@gmail.com
1929

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1930
B. KRISHNAKUMAR, K. SELVAM, AND M. SWAMINATHAN
aldehydes and ketones involves acid hydrolysis, which is not suitable for acid-
sensitive compounds.[16]
On the other hand, photocatalysis using semiconductor particles is now an
emerging and promising technology in view of pursuing environmentally benign
or green synthesis, because it uses solar-driven photocatalysts whose chemistry is
initiated by the absorption of low-energy photons and occurs at atmospheric pres-
sure and room temperature. Recently, a review has been published on the use of
titanium dioxide–based photocatalysts for selective organic transformations.[17]
In our laboratory, we had carried out the synthesis of benzimidazoles[18] with
TiO2 and modified TiO2 using ultraviolet (UV) and solar light. During our study on
the dehydrazonation of benzophenone hydrazone with TiO2, we observed the forma-
tion of azine along with the ketone. Interestingly, we found that the formation of
azine was enhanced by metal-doped TiO2. In the present work, we report the product
selectivity in photocatalytic dehydrazonation of benzophenone hydrazone with
various photocatalysts in different solvents and its mechanism.
RESULTS AND DISCUSSION
Irradiation of a suspension of TiO2-P25 in acetonitrile containing benzo-
phenone hydrazone gave corresponding benzophenone in 86.3% yield with a small
amount of azine derivative of benzophenone (Scheme 1). In control experiments,
irradiation of an acetonitrile solution of hydrazone without catalyst led to no reac-
tion product. On the other hand, without irradiation in the presence of catalyst, no
reaction product was detected either.
The dehydrazonation of benzophenone hydrazone with TiO2-P25 was carried
out in various solvents, and the results are shown in Table 1. It was found that the
solvent played a remarkable role in the conversion of hydrazone and the yield of
other products. The conversion efficiency and product selectivity of ketone are high
in acetonitrile, whereas dichloromethane, chloroform, and dichloroethane favor the
formation of azine with the less conversion efficiency than acetonitrile. This shows
that the more polar solvents such as acetonitrile enhance both product selectivity
of ketone and conversion efficiency, whereas in less polar solvents, such as chloro-
form, dichloroethane, and dichloromethane, product selectivity of azine increases.
In nonpolar solvents such as hexane and benzene, conversion efficiency and product
selectivity decrease. Though benzophenone formation is greater in 2-propanol and
ethanol (runs 7 and 8), overall efficiency is less in alcoholic solvents. Alcoholic
Scheme 1. Photocatalytic dehydrazonation of benzophenone hydrazone.