Direct C(sp2)?H Hydroxylation of Arenes with Palladium(II)/Oxygen Using Sulfoximines as a Recyclable Directing Group

Article abstract of DOI:10.1002/cctc.201800064

The aerobic palladium(II) catalysis presented herein offers facile entry to various substituted phenols through site-selective C?H hydroxylation of arenes by using sulfoximines as a reusable directing group. The notable aspects of our method include the use of molecular oxygen (O₂) as the sole oxidant, the activation of molecular oxygen through aldehyde autoxidation, operational simplicity, and the mechanistic studies into the catalytic C?H hydroxylation process.

Full text of DOI:10.1002/cctc.201800064

Accepted Article
Title: Direct C(sp2)-H Hydroxylation of Arene with Pd(II)/O2 Using
Sulfoximine as Recyclable Directing Group
Authors: Prasenjit Das and Joyram Guin
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10.1002/cctc.201800064
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Direct C(sp2)-H Hydroxylation of Arene with Pd(II)/O2 Using
Sulfoximine as Recyclable Directing Group
Prasenjit Das and Joyram Guin*[a]
Abstract: The aerobic palladium(II) catalysis presented herein offers
facile entry to various substituted phenols through site selective C-H
hydroxylation of arenes using sulfoximine as reusable directing
group. Notable aspects of our method include: a) the use of
molecular oxygen (O2) as a sole oxidant, b) the activation of
molecular oxygen via aldehyde auto-oxidation, c) operational
simplicity and d) the mechanistic studies into the catalytic C-H
hydroxylation process.
installation of free hydroxyl group at arene C-H bond of
sulfoximine amide has been realized via a two-step reaction
procedure involving catalytic C-H acetoxy/benzoyloxylation
followed by hydrolysis of the ester functional group.[11] As a
result, the direct C-H hydroxylation of sulfoximine amides, which
would improve step- and atom-economy of the process, remains
challenging. Herein, we present Pd(II)-catalyzed direct C-H
hydroxylation of various N-sulfoximine amides using O2. In this
protocol, O2 plays a dual role as a green oxidant for Pd-catalysis
and as oxygen source for the hydroxyl group (Scheme 1).
The catalytic methods that allow selective conversion of
unactivated arene C-H bond to C-O bond are deemed as the
most straightforward approach to substituted phenols.[1] As a
result, numerous elegant transition metal catalytic processes for
the direct C-H hydroxylation of arenes have been developed.[1c, 2]
Among them, the oxidative palladium catalysis involving PdII/PdIV
catalytic cycle receives significant attraction recently.[3] However,
this catalytic C-H hydroxylation strategy requires the use of an
excess amount of chemical oxidant like oxone, K2S2O8, peroxide,
BQ, DDQ, PIDA and others.[3] In view of the increasing demand
toward green and sustainable chemical process, the use of
molecular oxygen (O2) as an oxidant is becoming increasily
important for organic synthesis.[4] As a result, the development
of an efficient palladium catalysis toward direct C-H
hydroxylation of arenes employing O2 as the sole oxidant would
be highly attractive.
Scheme 1. Different strategies for the synthesis of hydroxylated N-sulfoximine
amides.
Although the production of substituted phenol using Pd(II)/O2
catalytic system has been reported, the existing methods require
harsh reaction conditions,[5] additional oxidant and base[6] or
pyridine based directing group along with co-catalyst.[7] Recently,
our group has introduced a novel C-H hydroxylation of 2-
arylpyridines with O2 by simultaneous co-operation of aldehyde
auto-oxidation with palladium catalysis.[8] However, the use of
non-removable and non-modifiable pyridine moiety as directing
group has limited its application. We thus became interested in
expanding scope of the novel aerobic C-H hydroxylation method
with easily removable and reusable directing group.[9] Along this
line, we have examined different directing groups like
benzamide, 2-amino pyridine, 8-amino quinoline, oxime, 2-
amino pyrimidine, azo compound and sulfoximine amide in the
catalytic aerobic C-H hydroxylation process. Among the different
directing groups tested, the N-sulfoximine benzamide found to
be the most promising candidate (See Supporting Information).
Recently, sulfoximine has been introduced as an effective
and reusable directing group for the catalytic functionalization of
unactivated C-H bonds.[10] Despite these advancements, the
Being inspired by our earlier work on the aerobic
functionalization of unactivated C-H bonds,[12] we commenced
our studies by investigating the direct C-H hydroxylation of N-
sulfoximine benzamide 1a using Pd(II)-catalyst in the presence
of an aldehyde under O2 atmosphere. By performing the reaction
with 1a using 10 equiv. of n-butyraldehyde and 10 mol% of
Pd(OAc)2 in DCE at 80 °C under O2 atmosphere, the desired
phenol 2a was isolated in 67% yield (Eq. 1) (See Supporting
Information).
After identifying the optimal reaction conditions for the
aerobic C-H hydroxylation process, we decided to explore the
scope of the reaction with diversely decorated sulfoximine
amides (Table 1). Accordingly, we first tested different
substrates with varying substituent on the aromatic ring of the
benzamide unit. Substrates having benzamide as well as its
ortho, meta, and para alkyl derivatives were converted to the
corresponding ortho-hydroxylated products 2a-e in good yields
(53-73%). Likewise, the reaction proceeded well with sulfoximine
amides bearing methoxy (2f and 2g), benzyloxy (2h), O-phenyl
(2i), NH-acetyl (2j) and O-acetyl (2k) functional groups at 3- or
4-position of the benzamide unit. Substrates containing electron
[a]
P. Das and Dr. J. Guin*
Department of Organic Chemistry
Indian Association for the Cultivation of Science
2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032 (India)
E-mail: ocjg@iacs.res.in
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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deficient chloro, bromo, and ester groups at benzoyl moiety
were transformed to the corresponding products 2l-p with this
simple hydroxylation protocol, albeit with slightly low yield of the
desired products. Further substrate scope studies were carried
out with varying substituent in both the aromatic rings of
sulfoximine amide (Table 1). Amides bearing di-methyl and iso-
propyl substituents on the sulfoximine aromatic ring exhibited
good reactivity (products 2q and 2r). The reaction showed no
considerable electronic effect on yields of the hydroxylated
products 2s-y when electronic property of the aromatic rings of
sulfoximine units were altered by changing substituent from
electron rich (OMe) to electron deficient (F, Cl, Br, NO2)
functional groups. The reaction furnished good yield with ethyl
substituted sulfoximine benzamide (product 2z). However,
slightly low yield of the hydroxylated product 2za was obtained
with diphenyl sulfoximine benzamide presumably due to steric
reason. Notably, the reaction neither furnished C-H
hydroxylation of sulfoximine benzene ring nor bis-hydroxylated
product under the optimized reaction conditions.
The synthetic usefulness of the method was demonstrated
by the preparation of salicylic acid 3 in 68% yield via facile
exclusion of the sulfoximine auxiliary from the product 2a in a
traceless manner (Eq. 2). It is important to note that, unlike other
N-directing group, the sulfoximine can be recycled easily in this
hydroxylation process.
The aerobic direct C-H hydroxylation process was then
tested to enantioenriched N-sulfoximine amide 1a-ent. As
expected, the reaction afforded the hydroxylated product 2a-ent
with complete conservation of chirality (Eq. 3).
Table 1. Substrate scope.
To get insight into the reaction mechanism of the catalytic C-
H hydroxylation process, a series of control experiments were
carried out. Intermolecular competition experiment between
electronically non-equivalent substrates resulted in preferentially
C-H hydroxylation of electron rich substrate over the electron
deficient N-sulfoximine amide (Eq. 4). Likewise, difference in
O
O
O
O
O
O
S
S
S
S
N
N
N
HO
HO
HO
2a, 67%; 44% (1 g scale)b
2c, 73%
2b, 62%
O
O
O
O
O
O
S
S
N
N
N
HO
2e, 59% (30%)d
HO
OMe
HO
2f, 51%
2d, 53%
X=OMe, 2g, 52%;
O
O
O
O
X=OBn, 2h, 58%;
X=OPh, 2i, 51%c;
X=NHAc, 2j, 74%;
X=OAc, 2k, 44%;
X=Cl, 2l, 29%c (66%)d
reaction rate of the hydroxylation process was realized by
conducting the reaction using N-sulfoximine amide 1a and its
deuterated analogue. When the hydroxylation reaction was
studied using substrates 1a and 1a(d5) separately, otherwise
identical reaction conditions, the substrate with perdeuterated
benzamide moiety was sluggishly transformed to the
corresponding product. In line with this observation, the intra- as
well as intermolecular kinetic isotope effect (KIE) experiments
revealed kH/kD = 3.50 for intramolecular study (Eq. 5) and kH/kD =
2.12 for intermolecular study (Eq. 6). These results may indicate
that an intramolecular C−H metalation step is involved that could
also be the rate-limiting step of the process.[13]
S
X
S
N
N
HO
HO
Cl
2m, 31%c
O
O
O
O
O
N
O
S
Br
S
S
N
N
HO
HO
Br
HO
CO2Me
2p, 51%c
2n, 27%b
2o, 40%b
O
O
O
O
O
O
S
S
S
N
N
N
HO
HO
MeO
O2N
HO
2s, 59%
2q, 59%
2r, 56%
O
O
O
OH
O
S
Y=F, 2t, 52%;
S
Y=Cl, 2u, 50%;
Y=Br, 2v, 53%;
Y=NO2, 2w, 67%;
N
N
Y
HO
2t, 52%
2x, 77%
O
O
O
O
O
O
Ph
S
S
S
N
N
N
O2N
HO
HO
2za, 39%
HO
2y, 71%
2z, 55%
[a] Reaction conditions: sulfoximine amides (1.0 equiv), aldehyde (10-15
equiv), Pd(OAc)2 (10 mol%) and DCE (0.1 M) at 80 °C for 24 h under O2. [b]
Performed using 1g of substrate. [c] Run for 36 h. [d] Recovered starting
material.
We then performed the reaction using stoichiometric amount
of various chemical oxidants such as H2O2, m-CPBA, oxone and
(NH4)2S2O8 that are often used in Pd(II)-catalyzed C-H
hydroxylation process under anaerobic condition.[3] To our
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10.1002/cctc.201800064
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surprise, none of them afforded desired hydroxylated product
with N-sulfoximine benzamide 1a. The reaction also did not give
any hydroxylated product in the absence of either aldehyde or
O2. Additionally, only trace amount of the product 2a was
obtained with air. These observations suggest that both O2 and
n-butyraldehyde are essential for the success of the aerobic C-H
hydroxylation process (See Supporting Information).
is an independent process. Furthermore, the in situ generated
acylperoxy radical A is highly reactive species, which is known
to decompose to the corresponding carboxylic acid through
alternative reaction pathway.[15] As a result, slight excess of
aldehyde was required to achieve good yield of the desired
hydroxylated product.
Further mechanistic studies revealed that the reaction was
ineffective with either benzaldehyde or TBHP alone under O2
atmosphere(Scheme 2). Interestingly, by repeating the same
experiment in the presence of both the reagents afforded the
product 2a in 45% yield. These results indicate that
benzoylperoxy radical, which is known to form by the reaction of
TBHP and benzaldehyde in the presence of O2, could promote
the reaction.[14] In accordance with this observation, we
proposed that the acylperoxy radical generated in situ during
auto-oxidation of aldehyde[15] could be
a
key reactive
intermediate for the aerobic hydroxylation process. Furthermore,
the reaction was inhibited in the presence of a radical scavenger
like TEMPO. This observation may lead to the involvement of
radical intermediate in this process.
Scheme 3. Proposed reaction mechanism.
In summary, the first aerobic direct C-H hydroxylation of
arene using sulfoximine as reusable directing group with Pd-
catalysis is described. Abundant, safe and environmentally
benign molecular oxygen (O2) is used as the sole oxidant for the
catalytic process to afford various hydroxylated products in good
yields and functional group tolerance. Based on our mechanistic
studies, a cooperative mechanism between Pd(II)-catalysis and
aldehyde auto-oxidation is implicated for the hydroxylation
process. Our ongoing research in this area directs to expand the
scope and to study further mechanism investigations of this
aerobic hydroxylation reaction.
Scheme 2. Control experiments.
Finally, 18O labeling experiment proved that the oxygen atom
of the hydroxyl group is indeed originated from molecular
oxygen (Eq. 7), as hydroxylated product with 89% 18O
incorporation was obtained.
Experimental Section
Experimental details such as synthesis, control experiment and
spectroscopy can be found in supporting information.
Acknowledgements
Based on the results obtained in our mechanistic studies and
the literature reports,[16] a probable reaction mechanism is
presented in Scheme 3. We propose that the catalytic cycle
begins with the intramolecular sulfoximine assisted ortho C-H
palladation of substrate to form the complex B. Meanwhile, the
aerobic oxidation of aldehyde produces an active acylperoxy
radical intermediate A. The association of the radical A with the
complex B followed by single electron oxidation may lead to a
most likely pdIV-intermediate C. The reductive elimination via
putative pdIV-species is expected to provide the desired
hydroxylated product and regenerate the Pd(II)-biscarboxylate
catalyst. Alternatively, the involvement of a binuclear PdII/PdIII
catalytic cycle for the reaction cannot be excluded at this
point.[17] Additional mechanistic investigation would be required
to gain better inside of the reaction mechanism. The formation of
butyric acid was confirmed by GC/MS analysis. It is important to
mention that dioxygen activation via auto-oxidation of aldehyde
We gratefully acknowledge the generous financial support from
Science
and
Engineering
Research
Board,
India
(EMR/2016/006344) and CSIR for fellowship to PD.
Keywords: C-H hydroxylation • Pd-catalysis • sulfoximine •
molecular oxygen • aerobic oxidation
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Prasenjit Das and Joyram Guin*
Page No. – Page No.
Direct C(sp2)-H Hydroxylation of Arene
with Pd(II)/O2 Using Sulfoximine as
Recyclable Directing Group
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