Chemoselective Oxidative Spiroetherification and Spiroamination of Arenols Using I+/Oxone Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b04324

We developed a chemoselective oxidative dearomative spiroetherification and spiroamination of arenols using I⁺/oxone catalysis. The intramolecular dearomative C-O and C-N couplings proceeded much more efficiently under slightly acidic condition

Full text of DOI:10.1021/acs.orglett.9b04324

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Chemoselective Oxidative Spiroetherification and Spiroamination of
Arenols Using I+/Oxone Catalysis
Muhammet Uyanik, Naoto Sahara, Outa Katade, and Kazuaki Ishihara*
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
S
* Supporting Information
ABSTRACT: We developed a chemoselective oxidative
dearomative spiroetherification and spiroamination of arenols
using I+/oxone catalysis. The intramolecular dearomative C−
O and C−N couplings proceeded much more efficiently
under slightly acidic conditions to give the corresponding
spiro adducts in higher yields compared with previous
methods using transition metal or hypervalent iodine catalysts.
Control experiments suggested that both hypoiodous acid and iodine might be active species for these reactions.
have been less explored,2,4 despite their usefulness for the
xidative dearomatization reactions are found in the
synthesis of core structures of many biologically important
Obiosynthesis of many biologically active compounds in
compounds.5 Conventionally, these reactions were best
performed using hypervalent iodine compounds as stoichio-
metric oxidants under acidic conditions.4 Recently, the
oxidative cyclization of arenols under stoichiometric
(R4N+[Br3]−/K2CO3)6 or catalytic (10 mol % RuCl3/
KBrO3)7 conditions has also been reported. On the other
hand, Ciufolini and colleagues developed hypervalent iodine-
catalyzed enantioselective oxidative spiroetherification and
spiroamination reactions as a transition-metal-free method
using m-chloroperoxybenzoic acid (m-CPBA) as an organic
oxidant (Scheme 1b).8 However, the substrate scope seemed
to be limited to 1- and 2-naphthols, and more importantly, the
chemical yield of the cyclic products was moderate because of
undesired competitive reactions, including intermolecular
coupling with m-chlorobenzoic acid (m-CBA), which was
generated from the oxidant used, due to the lower reactivity of
alcohols and amides compared with carboxylic acids as
intramolecular nucleophiles.
nature.1 In chemical synthesis, many oxidative methods
mediated by mainly transition metal or hypervalent iodine
compounds have been developed for the dearomatization of
arenols.2 In this context, dearomative spirolactonization of
arenols tethered to a carboxyl group as an intramolecular
nucleophile have been developed using iodine-based catalysis
(Scheme 1a).3 Compared with spirolactonization, oxidative
spiroetherification of arenols tethered to a hydroxy group and
spiroamination of arenols tethered to a secondary amido group
Scheme 1. Oxidative Dearomative Cyclization of Arenol
Derivatives Using Iodine-Based Catalysis
We recently reported the quaternary ammonium hypoio-
dite9-catalyzed dearomative spirolactonization of arenols using
oxone,10,11 a triple salt (2KHSO5·KHSO4·K2SO4), as an
environmentally benign, safe, and easy to handle inorganic
oxidant (Scheme 1a).12 Compared with our previous I+/(H2O2
or ROOH) systems,13 high-performance catalysis could be
realized under these slightly acidic conditions (the pKa of
KHSO4 is 2.014) (Scheme 1c). Here, we applied I+/oxone
catalysis to the oxidative spiroetherification and spiroamination
of arenols. We envisioned that chemoselective intramolecular
oxidative coupling with alcohols or amide tethers might be
achieved by acceleration of the reductive elimination of iodide
via hydrogen-bonding interactions15 under acidic conditions or
halogen-bonding interactions with I2,16 a catalytic species
Received: December 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04324
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
generated in situ,12 to enhance the positive charge on the
electrophilic partner (arenol), which might then be easily
trapped by an intramolecular nucleophile. The dearomative
C−O and C−N couplings proceeded much more efficiently
(TON and TOF up to 900 and 450 h−1, respectively) under
mild conditions to give the corresponding spiro adducts in
higher yields compared with previous methods using transition
metal7 or hypervalent iodine catalysts.8
oxidative conversion of 1a was remarkably decreased because
of the competitive acceleration of nonproductive pathways as
in spirolactonization reactions (entries 6 and 7).9a,b,18 These
results suggested that undesired side reactions might be
suppressed under acidic conditions. Interestingly, in contrast to
spirolactonization,12 oxidative spiroetherification of 1a pro-
ceeded under nonaqueous conditions in which oxone was not
dissolved, although a longer time was required (entry 8).
Organic solvents were screened briefly under biphasic
conditions (entries 9−13), which revealed that an extremely
rapid reaction was observed in acetonitrile (entry 13).
Additionally, spiroetherification of 1a proceeded smoothly
even at low catalyst loading (0.1 mol %, TON = 900) with the
use of an almost equimolar amount of oxone (1.2 equiv of
KHSO5 as a component of oxone) to give 2a in 90% yield
(entries 14 and 15). Moreover, inexpensive KI could also be
used as a catalyst under identical conditions (entry 16). Finally,
almost no oxidation reaction was observed at 2 h in the
absence of catalyst (entry 17).
We began our investigation using 1-naphthol 1a as a model
substrate for the oxidative spiroetherification reaction (Table
1).17 First, we investigated the oxidant in the presence of 10
a
Table 1. Oxidative Spiroetherification of 1a
conv. of 1a yield of 2a
Next we investigated the oxidative dearomative spiroamina-
tion reaction using 1-naphthol 3a tethered to a tosylamido
group as a model substrate (Table 2).17 The oxidation of 3a
b
b
entry oxidant (equiv)
solvent
t (h)
(%)
(%)
c
1
2
3
4
30% H2O2 (2)
TBHP (2)
CHP (2)
m-CPBA (2)
oxone (1)
30% H2O2 (2)
TBHP (2)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
toluene
toluene
toluene
toluene
toluene
toluene
toluene
24
24
24
1
45
>95
>95
>95
>95
10
8
<1
<1
10
c
c
c
a
Table 2. Oxidative Spiroamination of 3a
5
d
8
85
7
6
88
86
52
89
90
6
7
24
24
24
5
24
3
d
10
e
8
9
10
11
12
toluene
EtOAc
>95
>95
65
>95
>95
>95
>95
>95
>95
<1
t-BuOMe
CH3NO2
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
t
conv. of 3a
yield of 4a
b
b
entry oxidant (equiv)
solvent
(h)
(%)
(%)
c
3
1
2
3
4
5
6
7
8
9
30% H2O2 (2)
TBHP (2)
CHP (2)
toluene
toluene
toluene
toluene
toluene
EtOAc
t-BuOMe
CH3NO2
CH2Cl2
CH3CN
toluene
toluene
toluene
24
24
24
1
3
3
24
3
3
60
>95
>95
>95
>95
>95
80
>95
>95
>95
<10
40
<1
c
13
oxone (1)
0.3
0.5
2
0.5
2
90
h
<1
f g
14 ,
oxone (0.6)
oxone (0.6)
oxone (0.6)
oxone (1)
91
c
<1
i
j
c
15
16
90
91
<1
m-CPBA (2)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
oxone (1)
oxone (0.6)
oxone (0.6)
<1
d
82
k
17
70
48
48
58
60
<5
31
84
a
Unless otherwise noted, the reactions were performed using 1a (0.1
mmol) and Bu4NI (10 mol %) in solvent (0.1 M)/H2O (2:1).
b
c
1
Determined by H NMR analysis. A messy complex mixture was
d
e
obtained. KHSO4 (2 equiv) was added. In toluene (monophasic).
10
3
f
g
h
i
e
Bu4NI (1 mol %). 1a (1 mmol) was used. Isolated yield. Bu4NI
11
24
24
3
j
k
f
(0.1 mol %). KI (1 mol %). In the absence of catalysts. For details,
see the Supporting Information.
12
13
>95
a
Unless otherwise noted, the reactions were performed using Bu4NI
b
mol % tetrabutylammonium iodide as a catalyst in toluene−
water biphasic solvents at room temperature. Because the
substrate is highly reactive toward oxidation,3c,12 1a was
consumed with the use of either hydrogen peroxide, alkyl
hydroperoxides, or m-CPBA; however, this gave complex
mixtures of several unidentified and oligomeric products along
with the desired spiroether 2a in less than 10% yield (entries
1−4). In sharp contrast, not only high reactivity toward
oxidative dearomatization, as in our spirolactonization,12 but
also high chemoselectivity for the desired intramolecular
cyclization could be achieved by I+/oxone catalysis under
acidic conditions. Indeed, clean oxidation of 1a using oxone
proceeded smoothly to give spiroether 2a in 85% yield (entry
5). In view of the beneficial effects of acidity on the reactivity
and especially the chemoselectivity, the oxidation of 1a with
hydrogen peroxide or TBHP was re-examined under acidic
conditions in the presence of KHSO4 as an additive. Although
the chemoselectivity of both reactions was improved, the
(10 mol %) in solvent (0.1 M)/H2O (2:1). Determined by 1H NMR
c
d
analysis. A messy complex mixture was obtained. Isolated yield.
e
f
Bu4NI (1 mol %). KI was used instead of Bu4NI. For details, see the
Supporting Information.
using either hydrogen peroxide, alkyl hydroperoxides, or m-
CPBA as an oxidant gave a much messier reaction than the
spiroetherification of 1a, which indicated that spiroamination is
much more challenging than spiroetherification with respect to
chemoselectivity (entries 1−4). In contrast, to our delight
again, a chemoselective intramolecular dearomative cyclo-
amination reaction proceeded using oxone as an oxidant, and
the desired spiroamine 4a was obtained in 82% yield (entry 5).
In contrast to spiroetherification, toluene was optimal with
respect to chemoselectivity (entries 6−10). Additionally, a
sluggish reaction was observed with a low catalyst loading or
with the use of KI instead of Bu4NI (entries 11 and 12). On
B
DOI: 10.1021/acs.orglett.9b04324
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the other hand, the use of almost an equimolar amount of
oxone (1.2 equiv of KHSO5 as a component of oxone) was
enough to complete the reaction to give 4a in 84% yield (entry
13).
Several arenols 1 and 3, including 1- and 2-naphthols and
phenols, tethered to hydroxy or sulfonamido groups were
examined for oxidative spiroetherification or spiroamination,
respectively, under the optimized conditions (Scheme 2).
(4i and 4j) could be synthesized from the common arenol core
structures tethered to hydroxy or sulfonamido groups,
respectively. Spiroether 2k was too unstable to isolate, and
[4 + 2]-dimerized to form the corresponding dimer 5 with
perfect diastereoselectivity,19 albeit in moderate yield. In
contrast, the oxidation of 3k afforded a complex mixture,
and no desired spiroamine 4k or its dimer was obtained. On
the other hand, oxidative cyclization of secondary and tertiary
alcohols 1l and 1m afforded the corresponding spiroethers 2l
and 2m in good to excellent yields. Moderate diastereose-
Scheme 2. Oxidative Spiroetherification and Spiroamination
a
lectivity (4:1 d.r.), as in previous methods,7 was observed for
c
of Arenols 1 and 3
the oxidative cyclization of 1l. Moreover, six-membered
spiroether 2n could also be synthesized in good yield from
the oxidative cyclization of 1n. Notably, beside the tosyl group
as a protecting group for the amine tether, a methanesulfonyl
(Ms) or tert-butyloxycarbonyl (Boc) group could also be used
to afford the desired spiroamines 4aa and 4ea or 4ab,
respectively.
Some interesting features were observed in control experi-
ments (Scheme 3). In our previous spirolactonization reactions
using I+/oxone catalysis, Raman analysis revealed the in situ
generation of I2 under acidic conditions but neither [I3]− nor
higher-valent species (Scheme 3a).12 Although hypoiodous
acid, which would be in equilibrium with hypoiodite, could not
Scheme 3. Control Experiments To Probe the Active
Species
a
Unless otherwise noted, the reactions were performed using Bu4NI
b
(10 mol %) and oxone (0.6 equiv) in CH3CN/H2O. Bu4NI (1 mol
%). In toluene/H2O. Oxone (1 equiv). For details, see the
Supporting Information.
c
d
Notably, for most of the spiroamination reactions, CH3CN was
used as the organic solvent instead of toluene, which was the
optimal solvent for model substrate 3a, because of the low
solubility of most arenols 3 in toluene. In general, as expected
from the optimization studies shown in Tables 1 and 2, the
spiroetherification reaction proceeded within a shorter reaction
time even at low catalyst loadings. Both o- (2a−h) and p-
spiroethers (2i and 2j) as well as o- (4a−h) and p-spiroamines
a
Conversions of the substrate (1a or 3a) are shown in parentheses.
In toluene (0.02 M)/H2O (10:1 v/v).
b
C
DOI: 10.1021/acs.orglett.9b04324
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Letter
be detected, probably because of band overlap with oxone,
control experiments suggested that hypoiodous acid might be
the active species and iodine might be an inert but dormant
species under these optimized conditions for enantioselective
oxidation (Scheme 3a).12 On the basis of these findings, we
performed stoichiometric control experiments for spiroether-
ification and spiroamination reactions (Scheme 3b). As in our
previous spirolactonization reaction,12 while the oxidation of
both 1a and 3a proceeded using hypoiodite species generated
in situ from I2 and base additives9b (entries 1, 2, 8, and 9), no
reactions were observed with the use of Bu4N+[I3]− (entries 3
and 10).20 Notably, compared with strong alkali conditions
with Bu4NOH, cleaner reactions were observed under mild
basic conditions with K2CO3. In addition, the oxidation of 3a
was much messier than that of 1a (entry 8 versus entry 1).
These results were consistent with the catalytic reactions in
that spiroamination is much more challenging than spiroether-
ification with respect to chemoselectivity (vide supra). On the
other hand, both oxidative spiroetherification and spiroamina-
tion reactions proceeded with I2 regardless of the solvent used
(entries 4−6 and 11). These results suggested that both
hypoiodous acid and iodine might be active species for
spiroetherification and spiroamination reactions, which is
consistent with the high-performance catalytic activity (TON
and TOF up to 900 and 450 h−1, respectively), especially for
the spiroetherification reaction in acetonitrile. The low
conversion of these stoichiometric reactions with I2 might be
attributed to the generation of [I3]−, an inert species, from the
fast reaction of I2 with I−, which is generated as the reaction
progresses.21 In our previous work, oxidative spirolactonization
reactions did not proceed with I2 in toluene under diluted
conditions, which was optimal for enantioselective reactions.12
We reinvestigated spirolactonization reactions under the
conditions used for spiroetherification. Interestingly, spirolac-
tonization reactions also proceeded in toluene or especially in
acetonitrile under concentrated conditions, but with dimin-
ished efficiency.17 In addition, spiroetherification of 1a did not
proceed with I2 under the diluted conditions used for
spirolactonization reactions (entry 7). These results suggest
that I2 might be an active species for either spirolactonization,
spiroetherification, or spiroamination reactions in polar
solvents and/or under concentrated conditions. Nevertheless,
to investigate the influence of the tether on the reaction with
I2, we performed an additional control experiment. Oxidation
of 1-methyl-2-naphthol (6) using iodine in acetonitrile
proceeded with similar efficiency as that of 1a or 3a to give
dimer 722 along with a small amount of o-quinol 823 (Scheme
3c). These results suggested that the reaction of arenols with I2
might not be accelerated by hydroxy or amido group tethers.
Since the generation of anionic species from alcohols or
amides seems to be difficult under acidic conditions, these
nucleophiles might react in neutral form followed by
deprotonation. Indeed, in contrast to previous spirolactoniza-
tion reactions,12 we could not achieve any asymmetric
induction in the present spiroetherification or spiroamination
reactions with the use of chiral quaternary ammonium iodide
catalysts, perhaps because no chiral ammonium alkoxides or
amides were generated.17
corresponding spiro adducts in higher yields compared with
previous methods. Control experiments suggested that both
hypoiodous acid and iodine might be active species for
spiroetherification and spiroamination reactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04324.
Additional information, experimental procedures, char-
acterization of new compounds, and spectral data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ishihara@cc.nagoya-u.ac.jp.
ORCID
Muhammet Uyanik: 0000-0002-9751-1952
Kazuaki Ishihara: 0000-0003-4191-3845
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this project was partially provided by
JSPS KAKENHI (15H05755 and 15H05810 to K.I. and
15H05484 and 18H01973 to M.U.) and the Nagoya
University Graduate Program of Transformative Chem-Bio
Research (GTR). We gratefully acknowledge Mr. Takehiro
Kato (Nagoya University) for his assistance with control
experiments on spirolactonization.
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DOI: 10.1021/acs.orglett.9b04324
Org. Lett. XXXX, XXX, XXX−XXX