Iodine(III)-catalyzed benzylic oxidation by using the (PhIO)n/Al(NO3)3 system

Article abstract of DOI:10.1080/00397911.2019.1707225

The first iodine(III)-based procedure for the benzylic oxidation of different arenes is described by using the (PhIO)_n/Al(NO₃)₃ system under catalytic conditions leading to the formation of the corresponding carbonyl derivatives. The method proceeds under mild, operationally simple, room temperature, short reaction times, and open flask conditions. In light of the organocatalysis relevance and the novelty of our protocol, we wish to communicate our initial results of this novel oxidation.

Full text of DOI:10.1080/00397911.2019.1707225

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Iodine(III)-catalyzed benzylic oxidation by using
the (PhIO)n/Al(NO3)3 system
Berencie Yahuaca-Juárez, Gerardo González, Marco A. Ramírez-Morales,
Calara Alba-Betancourt, Martha A. Deveze-Álvarez, Claudia L. Mendoza-
Macías, Rafael Ortiz-Alvarado, Kevin A. Juárez-Ornelas, César R. Solorio-
Alvarado & Keiji Maruoka
To cite this article: Berencie Yahuaca-Juárez, Gerardo González, Marco A. Ramírez-Morales,
Calara Alba-Betancourt, Martha A. Deveze-Álvarez, Claudia L. Mendoza-Macías, Rafael Ortiz-
Alvarado, Kevin A. Juárez-Ornelas, César R. Solorio-Alvarado & Keiji Maruoka (2020): Iodine(III)-
catalyzed benzylic oxidation by using the (PhIO)n/Al(NO3)3 system, Synthetic Communications,
DOI: 10.1080/00397911.2019.1707225
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1707225
Iodine(III)-catalyzed benzylic oxidation by using the
(PhIO)n/Al(NO3)3 system
a
b
b
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Berencie Yahuaca-Juarez , Gerardo Gonzalez , Marco A. Ramırez-Morales ,
b
b
b
ꢀ
ꢀ
Calara Alba-Betancourt , Martha A. Deveze-Alvarez , Claudia L. Mendoza-Macıas ,
Rafael Ortiz-Alvarado , Kevin A. Juarez-Ornelas , Cesar R. Solorio-Alvarado , and
a
b
b
ꢀ
ꢀ
Keiji Maruokac‡
a
ꢀ
ꢀ
ꢀ
Facultad de Quımicofarmacobiologıa, Universidad Michoacana de San Nicolas de Hidalgo, Morelia,
b
ꢀ
ꢀ
ꢀ
ꢀ
Michoacan, Mexico; Division de Ciencias Naturales y Exactas Departamento de Quımica, Universidad
de Guanajuato, Gto, Mexico; Department of Chemistry, Graduate School of Sciences, Kyoto University,
c
ꢀ
Sakyo, Kyoto, Japan
ABSTRACT
ARTICLE HISTORY
Received 8 September 2019
The first iodine(III)-based procedure for the benzylic oxidation of dif-
ferent arenes is described by using the (PhIO)n/Al(NO3)3 system
under catalytic conditions leading to the formation of the corre-
sponding carbonyl derivatives. The method proceeds under mild,
operationally simple, room temperature, short reaction times, and
open flask conditions. In light of the organocatalysis relevance and
the novelty of our protocol, we wish to communicate our initial
results of this novel oxidation.
KEYWORDS
Benzylic oxidation; Csp3-H
activation; iodine(III)-
catalysis; organocatalyst
GRAPHICAL ABSTRACT
Iodine(III)-catalyzed direct Csp3-H
benzylic Oxidation
O
H
H
organocatalyst
(PhIO)n
*
25 mol %
Al(NO3)3
+
PhIO
0.35 mol
MeCN, 23 ºC
O
O
Introduction
Benzylic oxidation is an excellent strategy for direct access to a wide range of benzylic
functional groups. Regarding the oxygen atom introduction at this position, different
strategies have been applied for the direct Csp3–H activation leading to the formation of
carbonyl derivatives as a final product. Protocols involving metal-catalyzed reactions
ꢀ ꢀ
Facultad de Quımicofarmacobiologıa, Universidad
CONTACT Rafael Ortiz-Alvarado
lnava14@udavinci.edu.mx
ꢀ
ꢀ
ꢀ
ꢀ
Michoacana de San Nicolas de Hidalgo, Morelia, Michoacan, Mexico; Kevin A. Juarez-Ornelas
ꢀ
ꢀ
ka.juarezornelas@ugto.mx; Cesar R. Solorio-Alvarado
csolorio@ugto.mx
Division de Ciencias Naturales y Exactas
ꢀ
ꢀ
Departamento de Quımica, Universidad de Guanajuato, Gto, Mexico; Keiji Maruoka maruoka@kuchem.kyoto-u.ac.jp
Department of Chemistry, Graduate School of Sciences, Kyoto University, Sakyo, Kyoto, Japan.
Both these authors have contributed equally to this article.
ꢀ
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
†School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
B. YAHUACA-JUÁREZ ET AL.
Scheme 1. Representative iodine(III) or (V)-base procedures for the benzylic oxidation.
using Cu(I)[1] and Cu(II),[2] Co(II),[3] Rh(II),[4] Fe(II)[5] and Fe(III),[6] Pd(II),[7]
Mg(II),[8] Mn(II)[9] or Ni(II)[10] has been described to carry out the benzylic oxidation.
On the other hand, metal-free oxidative procedures with persulfates and pyridine
(K2S2O8/Py),[11] molecular oxygen and n-octylphosphinate [PhP(O)H(Onoctyl)/O2][12]
R [13]
V
or potassium bromide and Oxone
were successfully applied. Finally, in the context
of this work, few protocols using hypervalent k3 or k5 iodanes have been described
under stoichiometric conditions. Among them, the procedure of Kita [(PhIO)n-KBr],[14]
Telvekar (PIDA-NaN3),[15] or Nicolaou (IBX-H2O)[16] are the most representatives
(Scheme 1).
All of these procedures showed the carbonyl formation at the benzylic position in
good yields. However, important disadvantages that are pointed out are high tempera-
tures or long reaction times; nevertheless, the biggest issue is the use of stoichiometric
amount of oxidant, which can be as far as ten times with respect to the benzylic alkane.
The former issue may indicate the generation of a soft-oxidant which needs a consider-
able excess to carry out the carbonyl formation. Herein, we present our initial findings
in the development of benzylic oxidation, catalyzed by monomeric iodosylbenzene
(PhIO) in redox-neutral conditions using the aluminum nitrate as an oxidant. To the
best of our knowledge, there are no catalytic procedures for the direct synthesis of

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SYNTHETIC COMMUNICATIONSV
3
Table 1. Optimization of the PhIO-catalyzed benzylic oxidation of 9 H-xanthene.a
Yield
(%)b
Entry
(PhIO)n (equiv)
Al(NO3)3 (equiv)
Solvent
1
2
3
4
5
6
7
8
0.25
0.20
0.30
0.25
0.25
0.25
0.25
0
0.35
0.35
0.35
0.40
0.30
0.35
0.35
0.35
0
MeCN
MeCN
MeCN
MeCN
MeCN
1,2-DCE
THF
MeCN
MeCN
MeCN
73
68
71
55
62
n.r.
Dec.
n.r.
n.r.
n.r.
9
0.25
0.25
10c
0.35
aReaction conditions: solvent (0.3 M), open flask;
bisolated yields;
cPhI was used instead of (PhIO)n.
benzylic carbonyls via the direct Csp3–H oxidation without a sequential route that
involves more than one step using iodine(III) reagents.
Results and discussion
Recently, our group discovered a novel reactivity of different iodine(III) reagents by
combining with different aluminum as well as ammonium halogen salts. This combin-
ation was applied for the chlorination, bromination, and iodination of arenes, mainly
phenols. This new reactivity is chemically supported on the direct coordination of the
aluminum with the ligand of the hypervalent iodine(III) reagent. The interaction gener-
ates an oxidative system such as PIFA-AlCl3,[17] PIDA-AlBr3,[18] or polymeric iodosyl-
benzene (PhIO)n–AlCl3/AlBr3,[19] which finally transfers the group bonded to the
aluminum to the arene. In the former study, we described the aluminum-mediated
depolymerization of (PhIO)n into its monomeric unit (PhIO) leading to the in situ for-
mation of the active species. Additionally, we observed the same depolymerization reac-
tion by using the corresponding ammonium chloride, bromide, and iodide salts.[20] In
line with this strategy, with regard to the combination of iodine(III)-AlX3 (X¼Cl, Br, I),
we described the nitration of different phenols using aluminum nitrate as the source of
nitro group.[21] During the development of this protocol, we tested the 9H-xhantene.
Surprisingly, we found the oxidative Csp3–H bond transformation to its corresponding
carbonyl group under the catalytic conditions at the Iodine(III) reagent. Thus, it was
decided to explore and optimize this unexpected transformation using the same sub-
strate (Table 1).
The first conditions involved 0.25 equivalent of polymeric iodosylbenzene and 0.35
equivalent of aluminum nitrate in acetonitrile to obtain the corresponding 9H-xanthen-
9-one in 73% of the yield (entry 1). Systematically, we started to vary the iodine(III)

4
B. YAHUACA-JUÁREZ ET AL.
Table 2. Scope of the iodine(III)/Al(NO3)3-catalyzed benzylic oxidation of arenes.a
aReaction conditions: MeCN (0.3 M), open flask and 2–4 h.
bIsolated yields.
source. Thus, by decreasing the iodosylbenzene equivalents to 0.2, a lower 68% of yield
was observed (entry 2). According to this experiment, we rationalized that less amount
of polymeric iodosylbenzene would continue decreasing the obtained yield. In conse-
quence, we increased to 0.3 equivalent of polymeric iodosylbenzene and we found again
a higher yield with almost the initial result (71%, entry 3). These experiments indicated
that 0.25 equivalents of polymeric iodosylbenzene is the optimal amount in the reaction.
Additionally, it is possible to mention that the reaction proceeds under catalytic condi-
tions at the iodine(III) reagent. On the other hand, the use of 0.4 or 0.3 equivalents of
aluminum nitrate (entries 4 and 5) showed lower 55 and 62% yields, respectively; by
comparing with entry 1. Other solvents such as 1,2-DCE or THF (entries 6 and 7) were
tested, however, no reaction or decomposition was found. To finish this optimization,
control experiments in the absence of polymeric iodosylbenzene, aluminum nitrate or
by using iodobenzene as a plausible oxidant precursor were carried out (entries 8–10),
nevertheless, no reaction was found.

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5
This series of experiments determined the conditions in entry 1 as the optimal for the
benzylic oxidation and carbonyl formation.
With the optimized conditions at hand, we proceeded to explore the scope of this
reaction (Table 2).
The scope exploration started with the oxidation of mono-annular phenols and phe-
nol-ethers such as 1-ethyl-4-methoxybenzene and 4-(methoxymethyl)phenol. Under our
optimized conditions, 78 and 62% (entries 1 and 2) of yield, respectively of the carbonyl
group formation at benzylic position was obtained for 1 and 2. In these two examples,
the arene moiety which contains the electron-donating groups (–OMe and –OH) did
not affect the expected reactivity, nevertheless, this former example is the electron-richer
which displays the lower yield. Other bis-annular arenes such as indene and isochro-
mane were successfully tested, giving rise to 3 and 4 in 66 and 82% of yield, respectively
(entries 3 and 4). Regarding these examples, it is worth to highlight that the oxidation
of the electron-neutral Csp3–H bonds proceeded in modest yield in the five-member
ring of the indene, while in the electron-rich Csp3–H bonds of the six-member ring of
the isochromane, the reaction proceeded in a much higher yield. The systematic scope
exploration continued with the oxidation of tris-annular arenes. In such a way, the
anthracen-9(10H)-one, 9H-fluorene, and the optimized 9H-xanthene were oxidized at
the benzylic position, leading to the formation of the corresponding benzylic carbonyls
5, 6, and 7 in 46, 58, and 73% of yield, respectively (entries 5–7). In these three experi-
ments, it is remarkable again, while the electron-richer is the benzylic Csp3–H bond, the
yield increases, thus a direct correlation between the electronic nature and the desired
reactivity at the benzylic position was observed.
Considering the results, it was decided to test other benzylic mono-annular deriva-
tives containing electron-donating groups such as phenyl and methoxyl at the arene
moiety and a benzylic methoxyl group. Surprisingly, the cleavage of the benzylic
methoxy group with concomitant over-oxidation to the corresponding carboxylic acid
group was observed giving rise to the compounds 8 and 9 in 34 and 38%, respectively
(entries 8 and 9). It is important to point out, these former examples contain the most
electron-rich benzylic Csp3–H bonds since they are bonded to two (entry 8) or three
(entry 9) electron-donating groups.
The results in this set of experiments clearly indicated that electron-rich arenes
mainly react toward the benzylic Csp3–H oxidation instead of Csp2–H nitration.[21]
However, the arenes strongly activated with electron-rich groups (ꢁOH, two ꢁOMe or
ꢁPh) will decrease the yield of the carbonyl formation or will show overoxidation to
the carboxylic acid. This implies an electron-dependent benzylic oxidation with a mod-
erate activation at the arene fragment.
Several other compounds containing electron-attracting substituents, electron-neutral
groups, saturated N-heterocycles at the arene moiety or the pyridine nucleus, did not
react under our catalytic oxidation conditions. In the case of the saturated N-hetero-
cycles and the pyridine, the absence of reactivity is attributed to the Lewis basicity of
the nitrogen. The rest of the tested substrates lack arene activation and in consequence,
no reaction was observed.
Finally, a mechanistic proposal of this novel oxidation is outlined (Scheme 2).

6
B. YAHUACA-JUÁREZ ET AL.
O2NO
Al
ONO2
start here
O2NO
Ph
O
Al(NO3)3
Ph
O
NO2
I
(O3N)2Al
(O3N)2Al
I
I
I
I
O
O
O
O
O
Ph
n
Ph
n
Al(NO2)3
I
Ph
(PhIO)n
(PhIO)n- Al(NO3)3
after three
cycles
I
O
O
O2N
O
O
NO2
I
II
Al
O
Ar
R
Ph
+
Ph
+
NO2
monomeric
NO2
NO2
III
H
H
IX
Ph
(OXN)2Al
H
O H
Ar
R
I
O
O
R
Ar
X
H
+
+
NO2
H
Ar
R
IV
NO2
Ph
Ph
(OXN)2Al
Al(NO3)2
O
I
O
O
O
H
H
O
I
IX
R
Ar
R
Ar
V
Path A
O2N
Ph
Ph
I=O
O
H
Path B
Al(NO3)2
H
(OXN)2Al
X= 2 or 3
+
I
O
O
I
O
O
R
H
(NO3)2(NO2)Al
+
Ar
Ph
II
VIII
R
Ar
NO2
VI
H
O
H
R
Ar
VII
Scheme 2. Proposed catalytic cycle for the PhIO-catalyzed benzylic oxidation.
The reaction started with the coordination of aluminum nitrate to polymeric iodosyl-
benzene to get the adduct (PhIO)n–Al(NO3)3. This depolymerizes giving rise to I. The
following radical fragmentation of I leads to the formation of II and III (radical nitrite).
The radical nitrite III promotes the hydrogen atom transfer (HAT) from the benzylic
arene leading to the formation of nitrous acid and the benzylic radical IV. Here, the
radicals IV and II gets coupled to form V. This intermediate reacts with the nitrite
anion formed, to produce the tetra-coordinated aluminum species VI. This intermediate
undergoes fragmentation giving rise to the corresponding benzylic alcohol, a mixed alu-
minum salt containing the nitrate and nitrite anions with concomitant formation of
monomeric iodosylbenzene. At this point, the mixed aluminum salt reacts with the
iodosylbenzene producing more nitrite radicals as well as those of type II which can
contain nitrate or nitrate and nitrite anions. In such a way, the catalytic cycle continues
following the path A or Path B. The path A gives rise to more benzylic alcohol; on the
other hand, the path B started by a second HAT from the benzylic alcohol with the
nitrite radical forming VIII that reacts with II producing the adduct IX. The following

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SYNTHETIC COMMUNICATIONSV
7
coordination to the aluminum center in IX gives rise to X which finally after fragmen-
tation, generates the mixed aluminum salt XI monomeric iodosylbenzene and yields the
observed carbonyl compound.
Additional theoretical and experimental details regarding this novel transformation
are currently been studied in our laboratory.
Conclusions
In summary, we have developed the first iodine(III)-catalyzed benzylic oxidation of elec-
tron rich-arenes which are moderately activated by electron-donating groups. The pro-
cedure is cost-economic since it needs only 0.35 equivalent of aluminum nitrate as an
oxidant. The protocol proceeds at room temperature and open flask conditions in
only 2–4 hours.
To the best of our knowledge, this is the first report describing an organocatalytic
benzylic oxidation, using hypervalent iodine(III) as a redox-neutral catalyst.
Experimental
General procedure
In a 25 ml dry round-bottom flask was suspended polymeric PhIO (0.25 equivalent) in
dry acetonitrile at 23 ꢂC. Then, Al(NO3)3 (0.35 equivalent) was added and stirred for
10 min, afterward, it was cooled to 0 ꢂC. The benzylic aryl derivative (1 equivalent) was
incorporated in one portion. The reaction was warmed to 23 ꢂC for a period of 2–4 h
until the starting material was fully consumed judging its advance by TLC. The reaction
mixture was quenched by the addition of NH4Cl saturated solution (25 mL) and then
extracted with EtOAc (3 ꢃ 10 mL). The organic extracts were collected, dried over
anhydrous sodium sulfate, filtered, and concentrated in vacuo to remove the solvent
and yield the crude of the reaction. The product was purified by flash column chroma-
tography on silica gel (100–200 mesh) with EtOAc/hexane system.
Selected spectroscopic data
Methyl 4-hydroxybenzoate (2)
White solid (62% yield); m.p. 126–128 ꢂC; 1H NMR (495 MHz, CDCl3) d 7.99 (d,
J ¼ 9.0 Hz, 1 H), 6.91 (d, J ¼ 9.0 Hz, 1 H), 3.88 (s, 2 H), 3.85 (s, 2 H); 13C NMR
(125 MHz, CDCl3) d.167.0, 163.4, 131.7, 122.7, 113.7, 55.5, 52.0. IR (neat) ꢀ/cmꢁ1: 3321,
2986, 1709, 1601, 1516, 1428, 798. HRMS (ESIꢁ) calcd. for C8H7O [M–H]: 151.0395,
found 151.0403.[14]
2,3-dihydro-1H-inden-1-one (3)
1
Light yellow solid (66% yield); m.p. 37–38 ꢂC; H NMR (495 MHz, CDCl3) d 7.77 (dd,
J ¼ 7.7, 0.5 Hz, 1H), 7.59 (td, J ¼ 7.6, 1.2 Hz, 1H), 7.52–7.46 (m, 1H), 7.40–7.35 (m, 1H),
3.15 (t, J ¼ 5.0 Hz, 2H), 2.73–2.67 (m, 2H); 13C NMR (125 MHz, CDCl3) d 207.3, 155.3,
137.2, 134.7, 127.4, 126.8, 123.9, 36.4, 25.9. IR (neat) ꢀ/cmꢁ1: 2952, 1655, 1560, 1475,

8
B. YAHUACA-JUÁREZ ET AL.
1230, 1333, 720. HRMS (ESIþ) calcd. for C9H9O [M þ H]: 133.0653,
found 133.0648.[14]
Isochroman-1-one (4)
1
Colorless oil (82% yield); H NMR (495 MHz, CDCl3) d 8.11 (dd, J ¼ 7.8, 0.5 Hz, 1H),
7.54 (td, J ¼ 7.5, 1.0 Hz, 1H), 7.44–7.36 (m, 1H), 7.30–7.25 (m, 1H), 4.74–4.44 (m, 1H),
3.07 (t, J ¼ 6.0 Hz, 1H); 13C NMR (125 MHz, CDCl3) d 165.2, 139.7, 133.8, 130.5, 127.8,
127.4, 127.4, 125.4, 67.4, 28.0. IR (neat) ꢀ/cmꢁ1: 1730, 1601, 1444, 1399, 1266, 1248,
1098, 1003, 918, 735, 675. HRMS (ESIþ) calcd. for C9H9O2 [M þ H]: 149.0603,
found 181.0674.[22]
9H-xanthen-9-one (5)
1
White solid (73% yield); m.p. 177–178 ꢂC; H NMR (495 MHz, CDCl3) d 8.35 (dd,
J ¼ 8.0, 1.7 Hz, 1H), 7.79–7.66 (m, 1H), 7.49 (dd, J ¼ 8.4, 0.8 Hz, 1H), 7.38 (ddd, J ¼ 8.0,
7.1, 1.0 Hz, 1H); 13C NMR (125 MHz, CDCl3) d 177.3, 156.3, 134.9, 126.9, 124.0, 122.0,
118.1. IR (neat) ꢀ/cmꢁ1: 2962, 1670, 1556, 1486, 14, 1323, 760. HRMS (ESIþ) calcd. for
C13H9O2 [M þ H]: 197.0603, found 187.0621.[22]
9H-fluoren-9-one (6)
1
Yellow solid (58% yield); m.p. 74–76 ꢂC; H NMR (495 MHz, CDCl3) d 7.68–7.64 (m,
1H), 7.54–7.45 (m, 2H), 7.29 (td, J ¼ 7.3, 1.2 Hz, 1H); 13C NMR (125 MHz, CDCl3) d
194.0, 144.6, 134.8, 134.3, 129.2, 124.5, 120.4. IR (neat) ꢀ/cmꢁ1: 1711, 1613, 1446, 1293,
955, 737, 658. HRMS (ESIþ) calcd. for C13H9O [M þ H]: 181.0653, found 181.0650.[22]
Anthracene-9,10-dione (7)
1
Yellow crystals (46% yield); m.p. 284–286 ꢂC; H NMR (495 MHz, CDCl3) d 8.32 (dd,
J ¼ 5.8, 3.3 Hz, 1H), 7.80 (dd, J ¼ 5.8, 3.3 Hz, 1H); 13C NMR (125 MHz, CDCl3) d 183.3,
134.3, 133.7, 127.4. IR (neat) ꢀ/cmꢁ1: 1677, 1566, 1346, 1278, 914, 756, 701. HRMS
(ESIþ) calcd. for C14H9O2 [M þ H]: 209.0603, found 181.0615.[22]
Acknowledgments
We acknowledge the facilities of the DCNyE, the Chemistry Department, the National
Laboratory UG-CONACyT (LACAPFEM) at the University of Guanajuato, the Department of
Chemistry, Graduate School of Sciences at Kyoto University and the laboratories of Prof. K.
Maruoka for full characterization. We also thank CONACyT for a fellowship to K.A.J.O.
Disclosure statement
The authors declare no competing financial interest.

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SYNTHETIC COMMUNICATIONSV
9
Funding
This work was partially supported by CONACyT [CB-2013/220836] and a JSPS KAKENHI grant
[JP26220803 and JP17H06450 hybrid catalysis].
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