6558
M. M. Reddy et al. / Tetrahedron Letters 52 (2011) 6554–6559
2-
HO- I+
+
SO4
In conclusion, we have developed an efficient new protocol for
-
HSO5
+
I-
the selective
a-monoiodination of the carbonyl compounds using
NH4I and OxoneÒ without metal or mineral acid catalyst. Various
H
ketones and b-keto esters can be iodinated by this reagent system
O+
OH
HO
under mild conditions affording the corresponding
a-iodo prod-
ucts in moderate to high yields. The present method, which is more
attractive than the existing methods, offers several advantages,
such as commercial availability of the reagents, simple reaction
conditions, no evolution of hydrogen iodide, high yields, economi-
cal process with easier setup/work-up procedures, and environ-
mentally friendly nature making our method more valuable from
preparative point of view.
I
R
R
HO- I+
- H2O
O
Acknowledgments
O
I
R
M.M.R. and M.A.K. acknowledge the financial support from
CSIR, India in the form of fellowships. P.S. acknowledges the finan-
cial support from UGC, India in the form of fellowship.
R
Scheme 2. Plausible reaction mechanism.
Supplementary data
methoxyethanone) instead of the expected
a-iodinated product
(1,2-diphenyl-2-iodoethanone) in low yield at room temperature
and excellent yield at reflux temperature (Table 1, entry 21). Fur-
Supplementary data associated with this article can be found, in
thermore, 1- and 2-acetonaphthones also led to the respective
iodinated products in excellent yields (Table 1, entries 23 and 24).
The nonmethyl ketone, 1-tetralone was also effectively -iodin-
a-
References and notes
a
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ated using this reagent system and afforded the excellent yield of
desired product (Table 1, entry 25). In order to generalize these re-
sults, further we selected a wide range of substituted 1-tetralone
derivatives, bearing on aromatic ring activated by methyl, methoxy
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(Table 1, entries 26–28). Methyl and methoxy groups present on
the aromatic ring direct the ring iodination. But in this case alpha
iodination achieved in high yields without forming ring iodination
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halenone and 5-methoxy-3,4-dihydro-1(2H)-naphthalenone were
iodinated to 2-iodo substituted products in high yields. 2-Hydro-
xy-1,4-naphthoquinone showed good reactivity with this reagent
system and produced the 2-hydroxy-3-iodo-1,4-naphthoquinone,
which is used in the synthesis of naphtho[2,3-b]furan-4,9-dione
natural product31 (Table 1, entry 29).
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(alkyl and cyclic ketones) and b-keto esters and the results are pre-
sented in Table 2. When cyclic ketones such as cyclohexanone and
cycloheptanone are treated in the same reaction conditions as
above corresponding
yields (Table 2, entries 1 and 2). In the case of unsymmetrical ali-
phatic ketones -iodination proceeded smoothly at room tempera-
a-iodo ketones are obtained in moderate
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a
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respectively.
ble products in this reaction conditions and resulted in a complex
mixture of unidentified products. Whereas -monosubstituted-b-
keto esters (Table 2, entries 6 and 7) furnished a mixture of corre-
sponding -iodo and -hydroxy products, respectively.
a-Unsubstituted-b-keto esters provided highly unsta-
a
a
a
In a blank experiment, no reaction occurred between acetophe-
none and NH4I without OxoneÒ (oxidant) under similar reaction
conditions. Thus the role played by the OxoneÒ is justified. Con-
cerning the reaction mechanism, it is assumed that OxoneÒ effi-
H
ciently oxidizes the I (NH4I) to IÈ(HOI), which further reacts
with enol form of carbonyl compound to afford the corresponding
25. Yang, D.; Zhang, C. J. Org. Chem. 2001, 66, 4814–4818.
a-iodinated product (Scheme 2).