Simple and facile benzylic C-H oxidation using (diacetoxyiodo)benzene and catalytic sodium azide

Article abstract of DOI:10.1080/00397911.2010.539756

A synthetic utility of hypervalent iodine reagent, (diacetoxyiodo)benzene, with a catalytic amount of sodium azide for oxidation of benzylic C-H to corresponding ketone compounds is described. The advantage of this protocol is characterized by mild reaction conditions and shorter reaction time to obtain moderate to good yields. Copyright Taylor & Francis Group, LLC.

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Simple and Facile Benzylic C-H Oxidation
Using (Diacetoxyiodo)benzene and
Catalytic Sodium Azide
a
a
Vikas N. Telvekar & Kulbhushan A. Sasane
a
Department of Pharmaceutical Sciences and Technology, Institute
of Chemical Technology, Matunga, Mumbai, India
Accepted author version posted online: 21 Oct 2011.Published
online: 11 Jan 2012.
To cite this article: Vikas N. Telvekar & Kulbhushan A. Sasane (2012): Simple and Facile Benzylic C-H
Oxidation Using (Diacetoxyiodo)benzene and Catalytic Sodium Azide, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:9, 1325-1329
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1
Synthetic Communications , 42: 1325–1329, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.539756
SIMPLE AND FACILE BENZYLIC C-H OXIDATION USING
DIACETOXYIODO)BENZENE AND CATALYTIC
SODIUM AZIDE
(
Vikas N. Telvekar and Kulbhushan A. Sasane
Department of Pharmaceutical Sciences and Technology, Institute of
Chemical Technology, Matunga, Mumbai, India
GRAPHICAL ABSTRACT
Abstract A synthetic utility of hypervalent iodine reagent, (diacetoxyiodo)benzene, with a
catalytic amount of sodium azide for oxidation of benzylic C-H to corresponding ketone
compounds is described. The advantage of this protocol is characterized by mild reaction
conditions and shorter reaction time to obtain moderate to good yields.
Keywords Benzylic C-H; (diacetoxyiodo)benzene; ketones; oxidation
INTRODUCTION
Hypervalent iodine reagents have attracted increasing interest during the past
decade because of their selective, mild, and ecofriendly properties as oxidizing agents
[1]
in organic synthesis. The relevance of this methodology stems from the fact that all
the aforementioned transformations are quite fundamental in nature and can be
easily applied to a multitude of synthetic strategies. Investigations from our labora-
tories have revealed a series of new paradigms for hypervalent iodine-mediated reac-
[2,3]
tions under mild conditions.
Benzylic oxidation is a fundamental transformation that is useful for the
conversion of alkylarenes into corresponding carbonyl compounds. Generally,
hypervalent iodine reagents such as bis(trifluroacetoxyiodo)benzene and 1-
hydroxy-1,2-benziodoxol-3(1H)-one are used in combination with tert-butyl hydro-
peroxide for the benzylic C-H oxidation. In this case, first the hypervalent iodine
reagent and tert-butyl hydroperoxide are reacted at a low temperature and bis
(
tert-butylperoxy)-iodane molecule is generated in-situ. With further increase in
Received September 15, 2010.
Address correspondence to Vikas N. Telvekar, Department of Pharmaceutical Sciences and
Technology, Institute of Chemical Technology, Matunga, Mumbai, India. E-mail: vikastelvekar@
rediffmail.com
1
325

1326
V. N. TELVEKAR AND K. A. SASANE
Scheme 1. Bezylic C-H oxidation using DIB and catalytic sodium azide.
reaction temperature, this bis(tert-butylperoxy)-iodane molecule decomposes into
the tert-butyloxidanyl radical, which is responsible for oxidation of benzylic
[
4,5]
C-H.
Thus, all these methods require lower reaction temperature initially for in
situ generation of bis(tert-butylperoxy)-iodane and higher temperature for oxidation
of benzylic C-H, as well as tedious workup procedure and long reaction time. There-
fore, a new method with mild reaction conditions is of great importance. Other than
these reactions, there is only one catalytic method reported: polymeric iodosoben-
[
6]
zene in the presence of catalytic KBr in water for benzylic oxidation.
(
Diacetoxyiodo)benzene (DIB) is a hypervalent iodine reagent that is readily
[
7,8]
available and frequently used in several oxidative transformations.
During the
course of our studies, we found that treatment of (diacetoxyiodo)benzene with
diphenylmethane in an acetonitrile=water mixture in the presence of a catalytic
amount of sodium azide at room temperature resulted in the formation of diphenyl-
methanone (Scheme 1). It was interesting to know that in the absence of sodium
azide no oxidation reaction was observed. Further solvent study indicated that
dichloromethane=water and chloroform=water are also suitable solvents for this
conversion.
We also carried out the reaction in water, but a slower reaction rate was
observed (4 h) because of heterogeneous nature of the reaction. In the case of dry
acetonitrile, no reaction was observed, and thus water is required for the reaction.
On the basis of these observations, we propose a plausible radical mechanism for
benzylic C-H oxidation (Scheme 2)
The mechanism involves the formation of an iodanyl radical by using sodium
azide. The water molecule is required to generate HN and secondary alcohol, which
is further oxidized into the product using another mole of (diacetoxyiodo)benzene.
3
Scheme 2. Plausible mechanism for benzylic C-H oxidation.

FACILE BENZYLIC C-H OXIDATION
1327
a
Table 1. Oxidation of benzylic C-H using (diacetoxyiodo)benzene and catalytic sodium azide
b
Yield (%)
Entry
Substrate
Product
Time (min)
1
20
85
2
3
20
20
87
85
4
5
20
25
84
80
6
7
25
25
80
85
8
9
30
30
30
85
84
85
1
1
0
1
30
85
85
1
1
2
3
30
c
—
120
NR
a
Reaction conditions: substrate (5.91 mmol), (diacetoxyiodo)benzene (2 equiv), and NaN
CH CN=H O (10 mL).
3
(0.1 equiv) in
3
2
b
Isolated yields after column chromatography and structures were confirmed by comparison of IR and
H NMR with authentic materials.
1
c
No reaction.

1328
V. N. TELVEKAR AND K. A. SASANE
RESULTS AND DISCUSSION
To explore the reaction scope, a variety of benzylic compounds were subjected
to this reaction condition and oxidized to the corresponding ketones in moderate to
good yields; the results are summarized in Table 1. It was found that five- and six-
member cyclic compounds successfully undergo oxidation reaction; 9H-flurene,
indan, and tetralin afforded the corresponding ketone compounds in good yields
(Table 1, entries 2–4). Heterocyclic compounds such as 9H-xanthene also converted
into corresponding 9H-xanthen-9-ones (Table 1, entry 5). The developed method is
also applicable for the preparation of anthacene-9,10-dione (Table 1, entry 6).
Further investigations indicated that linear alkylbenzenes also are suitable com-
pounds for this transformation; 1-ethylnaphthalene, ethylbenzene, (2-methylpropyl)-
benzene and propylbenzene successfully converted to corresponding ketones in a
short reaction time (Table 1, entries 7–10). Under these reaction conditions, no
hydrolysis of ether and ester groups was observed (Table 1, entries 11 and 12). It
was interesting to know that only benzylic compounds undergo this reaction trans-
formation, and thus no reaction was observed when toluene was subjected to these
reaction conditions (Table 1, entry 13).
In conclusion, a new reaction system using trivalent iodine reagent, (diacetox-
oyiodo)benzene, in combination with catalytic sodium azide has been developed that
is capable of converting various benzylic C-H compounds into corresponding
ketones at room temperature. The method is mild and gives moderate to good yields.
EXPERIMENTAL
Catalytic NaN (0.1 equiv) was added to a stirred solution of (diacetoxyiodo)-
3
benzene (2 equiv) in CH CN=H O (10 mL) at room temperature. The reaction mix-
3
2
ture was stirred for 2 min, followed by addition of diphenylmethane (1.0 equiv,
.91 mmol). After completion of the reaction (thin-layer chromatography, TLC),
the reaction mixture was diluted with H O (25 mL) and the resultant solution was
5
2
extracted with CH Cl (2 Â 25 mL). The combine organic layer was washed succes-
3
sively with 10% sodium bisulfate solution (2 Â 20 mL), 10% sodium bicarbonate
(
2 Â 15 mL), and water (2 Â 20 mL). The organic layer was dried over anhydrous
sodium sulfate and concentrated under reduced pressure to give the crude product.
Pure diphenylmethanone (85%) was obtained after silica-gel column chromato-
graphy (10% EtOAc-hexane).
ACKNOWLEDGMENT
K. A. S. thanks the Council of Scientific and Industrial Research (CSIR, New
Delhi) for providing a junior research fellowship.
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FACILE BENZYLIC C-H OXIDATION
1329
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