Chemoselective oxidation by electronically tuned nitroxyl radical catalysts

Article abstract of DOI:10.1002/anie.201302261

Electronic tuning: Nitroxyl radical 1 is shown to be an efficient catalyst for the oxidation of secondary alcohols, and promotes oxidation through an oxoammonium species which is highly reactive because of the adjacent electron-withdrawing ester groups. Chemoselective oxidation of benzylic alcohols in the presence of aliphatic alcohols is observed and is proposed to proceed by a rate-determining hydride transfer. Copyright

Full text of DOI:10.1002/anie.201302261

Angewandte
Chemie
Communications
Radical Reactions
S. Hamada, T. Furuta, Y. Wada,
T. Kawabata*
&&&& — &&&&
Chemoselective Oxidation by
Electronically Tuned Nitroxyl Radical
Catalysts
Electronic tuning: Nitroxyl radical 1 is
shown to be an efficient catalyst for the
oxidation of secondary alcohols, and
promotes oxidation through an oxo-
ammonium species which is highly reac-
tive because of the adjacent electron-
withdrawing ester groups. Chemoselec-
tive oxidation of benzylic alcohols in the
presence of aliphatic alcohols is observed
and is proposed to proceed by a rate-
determining hydride transfer.
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DOI: 10.1002/anie.201302261
Radical Reactions
Chemoselective Oxidation by Electronically Tuned Nitroxyl Radical
Catalysts**
Shohei Hamada, Takumi Furuta, Yoshiyuki Wada, and Takeo Kawabata*
Metal-free processes for the oxidation of alcohols to the
corresponding carbonyl compounds have been receiving
increasing attention in organic synthesis. Specifically, 2,2,6,6-
tetramethylpiperidyl-1-oxyl (TEMPO) has been widely
employed as a catalyst for the oxidation of alcohols under
environmentally benign conditions.[1] Although TEMPO has
been known to be useful for the selective oxidation of primary
alcohols in the presence of secondary alcohols,[2] it has
difficulties in catalyzing oxidation of sterically hindered
secondary alcohols. The difficulty in the oxidation of secon-
Scheme 2. Steric and electronic tuning of the activities of nitroxyl
dary alcohols by TEMPO is associated with the steric
radical oxidation catalysts.
environment around the active site.[3]
While the reactions of the sterically hindered secondary
alcohols with the catalytically active oxoammonium group
may be retarded by the steric interaction with the adjacent
tetrasubstituted carbon atoms (Scheme 1), the tetrasubsti-
sterically hindered secondary alcohols. This ability is ascribed
to the more favorable accessibility of alcohols to the
oxoammonium species derived from AZADO because of
the lack of the sterically demanding tetrasubstituted carbon
atoms adjacent to the N-oxyl group (Scheme 2, left).[5]
A
similar approach employing azbicyclo-N-oxyls was reported
by Onomura and co-workers.[6] Enhancement of the catalytic
activity may alternatively be achieved by the electronic
activation of the oxoammonium species. We envisaged that
the oxoammonium species derived from the nitroxyl radical
1 might be much more reactive than those derived from
TEMPO because of the electron-withdrawing ester groups
adjacent to the oxoammonium group (Scheme 2, right).[7,8]
Under these reaction conditions, secondary alcohols are
expected to readily react with the highly reactive oxoammo-
nium species, irrespective of the steric congestion around the
active site. In addition to the electronic activation, the ester
carbonyl groups are expected to facilitate the approach of the
alcohols through a hydrogen-bonding interaction.[9]
Scheme 1. Catalytic cycle of TEMPO-catalyzed oxidation of alcohols.
tuted carbon atoms are indispensable for the stability of the
catalyst.[4] Iwabuchi and co-workers have solved this problem
by virtue of Bredtꢀs rule in an adamantyl skeleton (Scheme 2).
2-Azaadanantane N-oxyl (AZADO) and its derivatives have
been found to be powerful catalysts for the oxidation of
The nitroxyl radical 1 and its meso isomer were prepared
by Einhorn and co-workers in 2000.[10] To the best of our
knowledge, however, the catalytic properties as an oxidation
catalyst have never been reported. The racemic 1 was
prepared according to the procedure reported by Einhorn
and co-workers, and the racemic nitoxyl radical 2, possessing
an ester group, was also prepared for estimating the effects of
the electron-withdrawing ester group (Figure 1; for the
preparation of 2 see the Supporting Information). The
redox potentials of the nitroxyl radicals 1, 2, PROXYL, and
TEMPO were measured by cyclic voltammetry, and found to
be + 761 mV, + 550 mV, + 339 mV, and + 330 mV, respec-
tively.[11] The increase in the number of electron-withdrawing
ester groups resulted in the increase of the oxidation
potentials. Thus, the nitroxyl radical with the higher oxidation
potential would generate the more reactive oxoammonium
species toward reductants (Scheme 2, red arrow) and Lewis
bases (Scheme 2, blue arrow).[12] With this information in
[*] S. Hamada, Dr. T. Furuta, Y. Wada, Prof. Dr. T. Kawabata
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011(Japan)
E-mail: kawabata@scl.kyoto-u.ac.jp
[**] We are grateful to Prof. Norihiro Tokitoh and Dr. Takahiro Sasamori
for generous assistance to the measurement of cyclic voltammetry.
This work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas “Advanced Molecular Transformation by Organo-
catalysts” from The Ministry of Education, Culture, Sports, Science
and Technology (Japan), and by Grant-in-Aid for JSPS Fellows to
S.H.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302261.
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Table 1: Catalytic properties of nitroxyl radicals TEMPO, PROXYL, 2, and
1 for the oxidation of secondary alcohols with PIFA as a cooxidant.[a]
Entry
Substrate
t
Yield [%][b]
TEMPO
PROXYL
10
2
1
1
2
25 min
25 min
20
44
62
100
Figure 1. Redox potentials of nitroxyl radicals.
29
93
100
mind, the properties of the nitroxyl radicals 1, 2, and
PROXYL as oxidation catalysts were investigated.
To investigate the catalytic properties of 1, we began with
the search for a cooxidant in the oxidation of 2-phenylethanol
(3) catalyzed by 1 (Scheme 3). After examining various
3
4
45 min
45 min
11
0
10
0
85
53
94
100
5
6
18 h
18 h
4
1
31
36
38
61
20
18
Scheme 3. Screening of the cooxidant in the oxidation of 3 catalyzed
by 1.
[a] The reactions were run at the substrate concentration of 0.09m.
[b] Yields determined by 1H NMR analysis of the crude reaction mixture
using sorbic acid as an internal standard.
oxidants, including phenyl iodonium diacetate (PIDA),
phenyl iodonium bis(trifluoroacetate) (PIFA), NaOCl,
mCPBA, NCS, NIS, trichloroisocyanuric acid (TCCA), and
the Koser reagent (TsOI(OH)Ph), PIFAwas found to be most
suitable for this purpose (see the Supporting Information).
Treatment of 3 with 10 mol% of 1, 1.3 equivalents of PIFA,
and 4 equivalents of K2CO3 in CH2Cl2 at room temperature
for 50 minutes gave phenylacetaldehyde in 72% yield.
tion). All of these data indicate that the catalytic efficiency of
the nitroxyl radicals parallels their oxidation potentials, and
are consistent with the electronic tuning hypothesis that
nitroxyl radicals with an adjacent electron-withdrawing group
would generate the oxoammonium species with enhanced
reactivity (Scheme 2).
We then examined the properties of the nitroxyl radicals
1, 2, PROXYL, and TEMPO as oxidation catalysts for various
secondary alcohols using PIFA as a cooxidant (Table 1).
Treatment of 4 with 1.3 equivalents of PIFA and 4 equivalents
of K2CO3 in the presence of 10 mol% of 1 in CH2Cl2 at room
temperature for 25 minutes gave bicyclo[2,2,1]hepta-2-one in
a quantitative yield (entry 1). TEMPO, PROXYL, and 2 were
used instead of 1 under the identical reaction conditions for
the transformation of 4, except for the source of the nitroxyl
radical, to give the corresponding ketone in 20, 10, and 62%
yield, respectively (entry 1). The catalytic activities were also
found to increase in the order of TEMPO ꢀ PROXYL < 2 < 1
for the oxidation of the secondary alcohols 5–7 (entries 2–4).
The corresponding ketones were obtained in almost quanti-
tative yields from the oxidation reactions of 5–7 catalyzed by
1. While the desired ketones were obtained only in moderate
yields (38–61%) by the oxidation of 8 and 9 catalyzed by 1,
a similar tendency of the catalytic activity was also observed
(entries 5 and 6). Kinetic studies for the oxidation of 7 in the
presence of nitroxyl radicals, PROXYL, 2, and 1, were
performed under pseudo-first-order conditions using
10 equivalents of PIFA. The relative rates for the oxidation
of 7 in the presence of PROXYL, 2, and 1 were found to be
about 0:1.0:2.7, respectively (see the Supporting informa-
Chemoselective oxidation of 10, possessing a benzylic
secondary hydroxy group and an aliphatic primary hydroxy
group, was then examined (Table 2). Reaction of 10 with
a TEMPO/PIDA system gave the aldehyde 12 exclusively for
the monooxidation products (71%), by the oxidation of the
primary hydroxy group, with concomitant formation of the
dioxidation product 13 (5%; entry 1). Use of PIFA as
a cooxidant increased the ratio of the oxidation of the
benzylic hydroxy group, while the oxidation of the aliphatic
primary hydroxy group was still predominant (entry 2).
AZADO showed the similar chemoselectivity to that of
TEMPO in the oxidation of 10 (entries 3 and 4). In contrast,
1 showed totally different chemoselectivity. The product 11
from the oxidation of bezylic secondary hydroxy group of 10
was obtained exclusively for the monooxidation products
(77%) with concomitant formation of 13 (6%; entry 5).
Reaction at the lower temperature (À158C) slightly improved
the mono/dioxidation ratio (81:4), maintaining the predom-
inant oxidation of the benzylic hydroxy group (entry 6).
Although it has been known that hypervalent iodine reagents
could promote chemoselective oxidation of benzylic alcohols
in the presence of aliphatic alcohols,[13,14] PIFA itself did not
promote oxidation of 10 in the absence of nitroxyl radical
catalysts (entry 7). Dess–Martin periodinate promoted che-
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Table 2: Chemoselectivity in the oxidation of 10.[a]
group by treatment with the 1/PIFA system in CH2Cl2 at
À158C. Notably, the diol 14 underwent chemoselective
oxidation of the benzylic alcohol to give 2-hydroxy-1-phenyl-
ethanone without oxidative cleavage of the vicinal diol.[15]
Allylic oxidation took place exclusively (> 99: < 1) for diols
18 and 19 in the presence of an aliphatic hydroxy group by
treatment with the 1/PIFA system. The diol 20 also underwent
chemoselective oxidation of the propargylic hydroxy group
(90:10).[16]
From the reaction scope in Table 3, the oxidation of 16
appeared to proceed faster than that of 17. It seemed to result
from the different electronic nature of the aromatic rings. We
then examined the competitive oxidation between 21 and 22
(Scheme 4). A 1:1 mixture of 21 and 22 was treated with
Entry Nitroxyl radi-
Cooxidant or oxi- 11 and 12
13
cal (10 mol%) dant (1.03 equiv) Yield [%][b,c]
Yield [%][b]
1
2
3
TEMPO
TEMPO
AZADO
PIDA
71 (<1:>99)
52 (29:71)
63 (9:91)
5
PIFA[d]
PIDA
14
14
16
6
4
<5
24
4
5
AZADO
PIFA[d]
PIFA[d]
PIFA[d]
PIFA[d]
DMP[f]
53 (50:50)
77 (>99:<1)
81 (>99:<1)
<5 (À)
1
1
–
–
6[e]
7
8
55 (75:25)
[a] Reactions were run at a substrate concentration of 0.05m. [b] Yields
1
determined by H NMR analysis of the crude reaction mixture using
sorbic acid as an internal standard. [c] Ratio of 11 to 12 given within
parentheses. [d] K2CO3 (4 equiv) was added. [e] Run at À158C for 4 h.
[f] Dess–Martin periodinate.
moselective oxidation of 10 to give 11 preferentially with
moderate selectivity and yield (entry 8).
Since highly selective oxidation of the benzylic secondary
hydroxy group of 10 was observed with a 1/PIFA system, the
reagent system was applied to the chemoselective oxidation
of various diols containing either a secondary benzylic, or,
allylic, or propargylic hydroxy group together with an
aliphatic hydroxy group (Table 3). Highly chemoselective
oxidation of the benzylic hydroxy group of the diols 14–17
proceeded in the presence of a primary aliphatic hydroxy
Scheme 4. a) Kinetic resolution of benzylic alcohols by chemoselective
oxidation discriminating the electronic nature of the substrates.
1.2 equivalents of PIFA in the presence of 10 mol% of 1, and
resulted in quantitative formation of the ketone 23 as the sole
oxidation product (Scheme 4a). In contrast, oxidation by an
AZADO/PIDA system afforded a mixture of almost equal
amounts of the ketones 23 and 24 (Scheme 4b). These
contrasting results indicate that 1 promotes oxidation by
discrimination of the electronic nature of the substrate, while
AZADO does so by discrimination of the steric environment
of the substrate. Thus, a 1/PIFA system enables oxidative
kinetic resolution of structurally similar but electronically
distinct benzylic alcohols.
Table 3: Chemoselective oxidation of various diols.[a]
Mechanistic aspects of the oxidation promoted by 1 were
then investigated. Since 1 could promote the oxidation of
benzylic alcohol in a highly chemoselective manner, mecha-
nistic investigation was focused on the oxidation of benzylic
alcohols. The kinetic isotope effect of the oxidation of
benzylic alcohol 7 with a 1/PIFA system was measured
under pseudo-first-order conditions, and determined to be kH/
kD = 4.3 (see the Supporting information).[17] Hammett plots
of the oxidation reactions of tert-butyl aryl carbinols (Fig-
ure 2a) and n-hexyl aryl carbinols (Figure 2b) catalyzed by
1 were performed (see the Supporting information). A linear
relationship was observed in both cases between log(kX/kH)
and substituent constants s, and the reaction contants were
found to be 1 = À3.1 and À3.2 for the former and the latter,
respectively. The negative and large reaction constants
indicate that the benzylic position is positively charged at
[a] The reactions were run at the substrate concentration of 0.05m.
[b] Combined yield of monooxidation products determined by 1H NMR
spectroscopy of the crude reaction mixture using sorbic acid as an
internal standard. [c] Ratio between the products from the oxidation of
C-OH to that from C-OH.
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Scheme 5. A possible catalytic cycle for the oxidation of benzylic
alcohols promoted by nitroxyl radical 1.
group of 1, it was found to be an effective oxidation catalyst
for various secondary alcohols. This was assumed to be due to
the high reactivity of the oxoammonium species generated
from 1 based on the electronic tuning effects. Chemoselective
oxidation of benzylic alcohols in the presence of aliphatic
alcohols and oxidative kinetic resolution of structurally
similar benzylic alcohols with different electronic nature
were also performed.
Received: March 17, 2013
Published online: && &&, &&&&
Figure 2. Hammett plots for the oxidation of alkyl aryl carbinols.
Keywords: chemoselectivity · electronic tuning · organocatalyst ·
oxidation · radicals
the transition state. This information indicates that the
hydrogen atom a to the OH group is transferred as a hydride
at the transition state.[18] From these data, we propose the
mechanism of the oxidation of benzylic secondary alcohols
with a
.
[1] For reviews, see: a) A. E. J. de Nooy, A. C. Besemer, H.
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Because of the high oxidation potential of 1, a strong
oxidant, PIFA, is required for the generation of oxoammo-
nium A. Since the oxoammonium group in A is expected to be
highly electron deficient because of the adjacent ester groups,
it readily undergoes reductive transformation. Substrate
approach may be assisted by the hydrogen-bonding inter-
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À
benzylic C H to the oxygen atom of the oxoammonium group
in A would produce a benzylic ketone and hydroxy amine B.
Oxidation of B with PIFA would regenerate A. This
mechanism seems limited only to the oxidation of secondary
benzylic, allylic, and propargylic alcohols. At the moment, we
have no definite view for the mechanism of the oxidation
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1/PIFA system.[19]
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In conclusion, we have disclosed intriguing catalytic
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Despite the steric congestion around the nitroxyl radical
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(Ref. [8]) have been reported to be more active catalysts
compared to the parent TEMPO and AZADO, respectively,
because of the electron-withdrawing substituents. Because the
electron-withdrawing substituents are located at a position
remote from the active site (nitroxyl group), the electronic
effects in these catalysts seems to be much smaller than the
present case. The extent of the electronic effects may be
suggested by the difference in their redox potentials: The
differences in E0 between TEMPO and the 4-OCOCF3 deriva-
tive (Ref. [7]), AZADO and the 5-F-AZADO (Ref. [8]), and
PROXYL and 1 (this work) were 121 mV, 177 mV, and 422 mV,
respectively.
[6] Y. Demizu, H. Shiigi, T. Oda, Y. Matsumura, O. Onomura,
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[7] Inokuchi and co-workers developed TEMPO derivatives pos-
sessing the OCOR group at the 4-position. These derivatives
were expected be more reactive catalysts than TEMPO because
of the inductive effects of the remote 4-OCOR groups, and have
been effectively used for the oxidation of secondary alcohols in
the presence of stoichiometric amount of Py HBr3. The differ-
ence in E0 between TEMPO and the 4-OCOCF3 derivative was
reported to be 121 mV. See: Z.-W. Mei, T. Omote, M. Mansour,
H. Kawafuchi, Y. Takaguchi, A. Jutand, S. Tsuboi, T. Inokuchi,
Tetrahedron 2008, 64, 10761 – 10766.
[8] 5-F-AZADO was claimed to be a more reactive catalyst than
AZADO because of the presence of an electron-withdrawing F
atom at the remote position. The difference in E0 between
AZADO and the 5-F-AZADO was reported to be 177 mV. See
Ref. [5c].
[9] We have proposed a hydrogen-bonding interaction between the
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[14] For a recent example of chemoselective oxidation of benzylic
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Ming, W. Hegeng, W. Yanguang, Chin. J. Chem. 2009, 27, 587 –
592.
[15] Vicinal diols have been known to undergo oxidative cleavage in
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[16] Recently, chemoselective oxidation of allylic and benzylic
alcohols with an N-hydroxyindole/CuI/air system has been
reported: S. Shen, V. Kartika, Y. S. Tan, R. D. Webster, K.
Narasaka, Tetrahedron Lett. 2012, 53, 986 – 990.
[17] The kinetic isotope effect of the oxidation of an aliphatic
alcohol, 3-phenylpropanol, with a 1/PIFA system was also
measured under pseudo-first-order conditions, and determined
to be kH/kD = 1.7 (for details see the Supporting Inforamtion).
[18] A hydride transfer mechanism has been proposed in TEMPO-
mediated oxidation of alcohols based on computational studies:
W. F. Bailey, J. M. Bobbitt, K. B. Wiberg, J. Org. Chem. 2007, 72,
4504 – 4509.
[10] J. Einhorn, C. Einhorn, F. Ratajczak, J-L. Pierre, Synth.
Commun. 2000, 30, 1837 – 1848.
[11] The reference electrode was Ag/Ag+C For details, see the
Supporting information.
[19] The Hammet plot for the oxidation of benzylic primary alcohols
(ArCH2OH) did not show linearity.
[12] Niroxyl radical catalysts such as TEMPO derivatives possessing
OCOR group at the 4-position (Ref. [7]) and 5-F-AZADO
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