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[
5b]
[17]
served in the structure of 4a–[18]C6. The hydroxy ligand is
engaged in strong hydrogen bonding with the bis(triflyl)imide
anion (H1ÀO10 1.819 ꢀ), which is probably due to its highly
2.1 equivalents of 3a–[18]C6 (entry 8). In this case, the use
of acetonitrile slightly increased the yield (entry 9). The oxida-
tion of thioanisole with 3a–[18]C6 or 4a–[18]C6 proceeded
smoothly in water and selectively afforded methyl phenyl sulf-
[
14]
acidic nature (pK =4.3). In addition, the ligated water mole-
a
[
18]
cule also interacts with two oxygen atoms of [18]C6 through
hydrogen bonds (O5ÀH2 and O7ÀH3). All of these close con-
tacts are responsible for the enhanced thermal stability of the
complexes. The coordination behavior of 4a–[18]C6 and 8–
oxide in high yields (entries 10 and 11).
No formation of
methyl phenyl sulfone was observed. Other thioanisoles 13b
and 13c, bearing electron-donating and electron-withdrawing
groups, were also selectively oxidized to sulfoxides 14b and
14c in a similar manner (entries 12 and 13).
[
18]C6 seemed to suggest that the structure of the iodosylben-
3
zene monomer in aqueous acid should be a tetracoordinate
square-planar hydroxy(phenyl)iodonium ion with two mole-
It is well known that hypervalent l -iodanes with two
heteroatom ligands can be used for oxidative 1,2- and/or 1,1-
[15]
[1a,19]
cules of water coordinated through hypervalent bonding.
difunctionalization of carbon-carbon double bonds.
How-
3
Since these hydroxy- and aquo(hydroxy)-l -iodane–[18]C6
complexes are fairly soluble in water, they serve as versatile ox-
idizing agents in aqueous media. The oxidation of phenols pro-
ceeds smoothly at below room temperature. Exposure of 4-
methylphenol (9a) to 3a–[18]C6 (1.2 equiv) in water at 08C,
followed by gradual warming to room temperature, resulted in
the formation of p-quinol 10a in 97% yield (Table 2,
ever, to the best of our knowledge, no reports of oxidative di-
functionalization of olefins in water have appeared in the liter-
ature, probably because of poor solubility of the substrates
3
and/or the l -iodanes in water. Our complexes could serve as
versatile oxidizing agents for various olefins in water (Table 3).
The oxidation of styrene (15) with 3a–[18]C6 or 4a–[18]C6 pro-
duced phenylacetaldehyde (16) in high yields (Table 3, en-
tries 1 and 2), and the oxidation of indene (17)
with 2 equivalents of 3a–[18]C6 gave homo-
[
15]
Table 2. Oxidation of phenols and sulfides with activated iodosylbenzene monomer–
phthalaldehyde (18) in 76% yield (entry 3). The
[
a]
[18]C6 complexes.
former oxidation presumably proceeds through
3
oxy-l -iodanylation followed by 1,2-rearrange-
ment of the phenyl group, whereas the latter ox-
idative cleavage of the double bond probably
proceeds through 1,2-dihydroxylation of the
olefin followed by glycol fission mediated by the
3
[1a,19]
l -iodane.
Other solvents also serve as good
nucleophiles. Thus, the use of methanol as sol-
vent afforded the rearranged acetal 19 (entries 4
and 5). In acetic acid, 17 gave trans-diacetate 20
with 90% stereoselectivity, probably through
[
b]
Entry
Substrate
Complex
Solvent
Product
Yield [%]
1
2
3
4
5
6
7
9a
9a
9b
9c
9c
9d
9d
11
3a–[18]C6
4a–[18]C6
3a–[18]C6
3a–[18]C6
4b–[18]C6
3a–[18]C6
3a–[18]C6
3 a–[18]C6
3 a–[18]C6
3a–[18]C6
4a–[18]C6
3a–[18]C6
3a–[18]C6
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
10a
10a
10b
10c
10c
10d
10d
12
97
87
92
85
68
0
79
68
75
98
93
90
89
facile S 2 reaction of a cyclic acetoxonium ion in-
N
[20]
termediate (entry 6).
a–[18]C6 functions as a selective oxygen
3
atom donor toward ketones. Reaction of b-ke-
toester 21 with 3a–[18]C6 in water at room tem-
perature quantitatively afforded the correspond-
ing a-hydroxy-b-ketoester 22 (Table 4, entry 1).
Treatment of acetophenone (23) with 3a–[18]C6
MeCN/H
H O
2
MeCN/H
2
2
O (3:1)
O (2:1)
[
c]
c]
8
9
1
[
11
12
[
d]
0
13a
13a
13b
13c
H
2
H
2
H
2
H
2
O
O
O
O
14a
14a
14b
14c
[
d]
11
[
d]
d]
1
1
2
3
[
in acetonitrile/H O (3:1) at 458C produced a-hy-
2
droxyacetophenone (24) in moderate yield
(entry 2). These results indicate that a keto–enol
equilibrium exists under the reaction conditions,
and that the in situ generated enol presumably
[
a] Unless otherwise noted, the reaction was carried out using 0.02m complex (1.2 equiv)
in water or in water/MeCN at 08C to room temperature for 3 h. [b] Isolated yields.
[c] 2.1 equiv. of 3a–[18]C6 was used. [d] Reactions were carried out at room temperature.
reacts with 3a–[18]C6 to give an a-iodanylke-
[
1a,16]
[21]
entry 1).
Use of complex 4a–[18]C6 also proved to be ef-
tone intermediate, which then undergoes S 2 reaction with
N
fective (entry 2). Phenols ortho-disubstituted with methyl and
water. Silyl enol ether 26 was found to be a better substrate.
Thus, oxidation of 26 in water with 3a–[18]C6 afforded 24 in
58% yield, accompanied by the coupling product, 1,4-diketone
bromo groups also served as good substrates. Thus, mesitol
(
9b) and 2,6-dibromo-4-methylphenol (9c) cleanly produced
the corresponding p-quinols in good to high yields (entries 3–
). Because of its poor solubility in water, 2,6-di-tert-butylphe-
[22]
27, in 24% yield (entry 3). The use of acetonitrile as a co-sol-
vent increased the yield of 24 (entries 4 and 5). It is interesting
to note that 1,4-diketone 27 was obtained in 62% yield as the
predominant product in dichloromethane (entry 6). Use of 8–
[18]C6, instead of 3a–[18]C6, increased the yield to 88%
(entry 7). Silyl enol ether 28 selectively afforded a-hydroxycy-
5
nol (9d) was inert under similar conditions (entry 6). However,
the use of MeCN as a co-solvent dramatically improved the
yield of p-quinol 10d to 79% (entry 7). 1-Naphthol (11) selec-
tively afforded 1,4-naphthoquinone when treated with
Chem. Eur. J. 2014, 20, 5447 – 5453
5450
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