LETTER
2039
Molecular Iodine as Efficient Co-Catalyst for Facile Oxidation of Alcohols
with Hypervalent(III) Iodine
M
N
olecular Iodine
a
.
s Efficient
C
N
o-Catalyst for
F
ac
.
ile
O
xidation
K
of
A
lcohols arade,* G. B. Tiwari, D. B. Huple
School of Chemical Sciences, Swami Ramanand Teerth Marathwada University, Nanded – 431606, Maharashtra, India
Fax +91(2462)229245; E-mail: nnkarade@rediffmail.com
Received 21 April 2005
gies require use of toxic transition metal complexes, ex-
pensive, less readily available co-catalysts, suffer from
long reaction times and have a relatively narrow applica-
Abstract: A simple and mild procedure for the facile oxidation of
alcohols to ketones and acids using a catalytic quantity of molecular
iodine in combination with (diacetoxyiodo)benzene in acetonitrile
is described. Direct oxidative methyl esterification of alcohols is tion scope for substrates. Thus, development of improved
also reported in methanol as solvent. Oxidation of alcohols is in- synthetic methods for the oxidation of alcohols with
duced by iodonium ion generated in situ by the chemical oxidation
of molecular iodine with (diacetoxyiodo)benzene.
PhI(OAc) still remains an active research area. Herein,
2
we present our preliminary findings in the development of
Key words: alcohols, hypervalent elements, iodine, oxidation
transition metal-free oxidation of alcohols with
PhI(OAc) using molecular iodine as efficient co-catalyst
2
(
Scheme 1).
The oxidation of alcohols to carbonyl compounds is a
fundamental transformation in organic synthesis and
numerous methods utilizing various reagents have been
1
reported. In the past decade, there has been increasing
interest by organic chemists in the oxidizing properties of Scheme 1
2
hypervalent iodine compounds. In general, pentavalent
3
iodine reagents such as Dess–Martin periodine and o-io-
To examine this process, we have selected the model oxi-
dation reaction of cyclohexanol. In a typical experiment to
a solution of 1.0 mmol of cyclohexanol and 0.1 mmol of
iodine in 10 mL of MeCN, 1.0 mmol of DIB was added
and the reaction mixture was stirred in an open vessel for
4
doxybenzoic acid have been extensively used for effi-
cient oxidation of alcohols to the carbonyl compounds.
However, despite their utility, iodine(V) reagents are po-
5
tentially explosive, cannot be stocked and the liberated
side product, iodine(III) species, are not effective for the
oxidation of alcohols. Therefore, the use of iodine(III) re-
agents in place of iodine(V) reagents for the oxidation of
alcohols has been long desired, but the facile, safe to use,
readily available and persistent iodine(III) oxidants such
2
hours at room temperature. The dark color of iodine al-
most disappeared within half an hour and after work up,
the formation of cyclohexanone as the exclusive product
with 96% yield was established. As a control experi-
12
ment, the same reaction was carried out in the absence of
molecular iodine at room temperature, when no oxidation
product was observed even after stirring for a longer peri-
od of time. The optimum yield of the product is obtained
when a molar ratio of DIB to molecular iodine of 1:0.1 is
used. Of the solvents tested for this reaction (CHCl3,
CH Cl , MeOH and MeCN), MeCN was found to be the
most efficient for maximum yield of the oxidation prod-
uct. Having succeeded with secondary alcohol, we then
investigated the oxidation of benzyl alcohol (1.0 mmol)
with molecular iodine (0.1 mmol) and DIB (1.0 mol) in
acetonitrile solvent. As anticipated, benzyl alcohol under-
went oxidation but benzoic acid was observed as the ex-
clusive product with 98% quantitative yield. When the
reaction solvent was changed to methanol, the oxidation
of benzyl alcohol did not yield benzoic acid, but instead
the methyl ester was obtained in 92% yield under similar
6
as iodosobenzene and most notably (diacetoxyiodo)ben-
7
zene, have, in comparison, not been used very much.
Consequently, only few selective catalytic oxidations
with PhI(OAc) are known, especially of alcohols to car-
2
bonyl products. In one early report, it was shown rutheni-
um catalyzes the oxidation of alcohols by iodosylbenzene,
as demonstrated for saturated aliphatic and benzylic alco-
2
2
8
hols. Recently, the catalytic oxidation of secondary allyl-
ic alcohols with PhI(OAc) mediated by chromium(III)
2
(
salen) complexes to afford the respective enones, has
9
been reported. The activation of 2,2,6,6-tetramethyl-1-
piperidinyloxyl (TEMPO) by DIB is used in the oxidation
of alcohols. Polymer-supported PhI(OAc) in combina-
1
0
2
tion with KBr in aqueous media can also bring about fac-
ile oxidation of alcohols but primary alcohols were further
1
1
oxidized to carboxylic acids. Even though these reported
methods are very efficient and provide good conversion
for the oxidation of alcohols, some synthetic methodolo-
13
experimental conditions. Additionally, we found that
other alcohols such as ethanol, 1-propanol, and 2-pro-
panol also provide their corresponding esters in excellent
yields, although oxidation in t-butyl alcohol furnished
benzoic acid as the sole product (Table 1, entries 9–14).
SYNLETT 2005, No. 13, pp 2039–2042
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Advanced online publication: 20.07.2005
DOI: 10.1055/s-2005-871955; Art ID: D10605ST
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Georg Thieme Verlag Stuttgart · New York