Metal-free O-H/C-H difunctionalization of phenols by o-hydroxyarylsulfonium salts in water

Article abstract of DOI:10.1039/c6sc04504a

An environmentally benign method for C-H/O-H difunctionalization of phenols with sulfoxides under mild conditions has been developed. The reaction process is mediated by an electrophilic aromatic substitution and subsequent selective aryl or alkyl migrati

Full text of DOI:10.1039/c6sc04504a

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Metal-Free O–H/C–H Difunctionalization of Phenols by o-
Hydroxyarylsulfonium Salts in Water
Received 00th January 20xx,
Accepted 00th January 20xx
Dengfeng Chen, Qingyuan Feng, Yunqin Yang, Xumin Cai, Fei Wang and Shenlin Huang*
An environmentally benign method for C–H/O–H difunctionalization of phenols with sulfoxides under mild conditions has
been developed. The reaction process is mediated by an electrophilic aromatic substitution and subsequent selective aryl
or alkyl migration, involving C–S and C–O bond formations with broad substrate scope.
DOI: 10.1039/x0xx00000x
www.rsc.org/
reported.12
Introduction
a) Proposed difunctionalization of phenols
Carbon-heteroatom bond forming reactions are one of the
most important tools used in pharmaceutical chemistry,
medicinal chemistry, and material sciences.1 In particular,
transition metal catalyzed C–O or C–S cross-couplings,2 have
been widely used to construct important families of target
molecules, aryl ethers, sulfides and sulfoxides.3 Traditionally,
these reactions are carried out in organic solvents (e.g., THF,
CH3CN, DMSO, and DMF), which may cause potential health,
safety, and waste disposal issues.4 Therefore, transition-metal-
free C–O and C–S coupling reactions5 under greener conditions
are desirable, especially in the pharmaceutical industry.6
Herein, we report our efforts toward a novel environmentally
benign O–H/C–H difunctionalization of phenols by o-
hydroxyarylsulfonium salts in water (Scheme 1).
H
H
O
R2
Tf2O;
R2
S
S
R1
base
O
+
O
2 wt% TPGS-750-M/H 2O
R1
1
2
4
Tf2O
OTf
OTf
R2
R2
R2
base
S
S
S
O
OH
R1
R1
OTf
A
B
3
b) Selected important target molecules
N
PPh2
Our reaction design is detailed in Scheme 1. It is reasonable
to hypothesize that sulfoxides
2 may be activated using a
NH
S
S
O
CH3SO3H •
sulfonylating agent like trifluoromethanesulfonic anhydride
(Tf2O), inspired by the Pummerer reaction.7 The activated
OMe
H2N
OMe
sulfoxide
to o-hydroxyarylsulfonium
intermediates might then
rearrangement,8 thus affording versatile products o-
(phenoxy)aryl sulfides, , which could be readily converted to
A
could be attacked by phenols 1 at sulfur, giving rise
intermediates Such
undergo Smiles-like
3
.
chiral ligand (7)
urokinase inhibitor (8)
a
Scheme 1. Proposed difunctionalization of phenols
4
many important ligands9 and biologically active molecules10
(Scheme 1b). However, the following challenges in this
strategy were anticipated: (1) the activated sulfoxide can be
attacked by the phenolic hydroxyl group at the cationic sulfur,
according to previous studies;11 (2) it may require one
electron-withdrawing group (e.g., NO2) to activate the
migration of the aromatic ring in the Smiles rearrangement; (3)
such rearrangement of sulfonium salts has been seldom
Results and discussion
Our initial investigation began with 2-naphthol 1a and diphenyl
sulfoxide 2a in the presence of Tf2O in acetonitrile at 0 °C. To
our delight, the new sulfonium salt 3a was isolated in 95%
yield. Somewhat surprisingly, this type of sulfonium salt has
never been prepared by this electrophilic aromatic
substitution approach. The efficiency of this process was
briefly examined with various phenols and sulfoxides, as
shown in Table 1. Naphthols with electron-withdrawing
substituents (3b
electron-donating groups
sulfoxides and alkyl aryl sulfoxides led to the corresponding
-3e
)
worked equally well as those with
(
3f). In addition, both diaryl
products in excellent yields (3g
,
3h, and 3j). Interestingly, 3e
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Table 2. Optimization of the reaction conditionsa
and 3g were isolated as zwitterions. Unfortunately, though not
entirely unexpected, a poor yield was observed when 4-
hydroxyacetophenone was treated with diphenyl sulfoxide 2a
.
Table 1. Preparation of o-hydroxylarylsulfonium salts 3a
R'
TfO
R1
R
S
OH
Ph
Tf2O
O
S
OH
R1
+
CH3CN, 0 °C
R'
R
entry
1
base
NaHCO3
CsF
solvent
TPGS-750-M/H2Oc
TPGS-750-M/H2Oc
TPGS-750-M/H2Oc
TPGS-750-M/H2Oc
TPGS-750-M/H2Oc
TPGS-750-M/H2Oc
Triton X-100/H2Oc
SDS/H2O
T
Yieldb
78%
60%
88%
93%
70%
90%
76%
58%
75%
78%
86%
70%
1
2
70 °C
70 °C
70 °C
70 °C
60 °C
80 °C
70 °C
70 °C
70 °C
70 °C
70 °C
70 °C
3
Ph
Ph
2
Ph
Ph
S
Ph
S
S
3
Et3N
TfO
TfO
TfO
OH
Br
OH
Br
OH
4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
5
6
3a, 95%
3b, 92%
3c, 90%
7
Ph
S
Ph
Ph
Ph
S
Ph
Ph
8
S
TfO
9
CH3CN
TfO
O
OH
HO
OH
10
11
12
THF
DMF
NC
COOMe
H2O
3d, 87%
3e, 90%
3f, 81%
aConditions: 3a (0.375 mmol), base (1.125 mmol), solvent (0.75 M) for 24 h.
bYields were determined by HPLC analysis with nitrobenzene as an internal
standard. cUsing 2 wt% surfactant in water. SDS=sodium dodecyl sulfate.
Br
OTf
Ph
Ph
S
S
S
TfO
OH
O
OH
At this stage, we sought to develop a one-pot sequence.
Unfortunately, synthesis of the sulfonium salts either in
aqueous media or under solvent-free conditions was low
yielding. Hence, sulfonium salts were prepared in situ and used
directly for the rearrangement without chromatography.
Gratifyingly, the desired sulfide 4aa was produced in 86% yield
from the reaction of 2-naphthol 1a and diphenyl sulfoxide 2a
(Table 3).
O
3i, 26%
3g, 90%
3h, 93%
OTf
S
OH
With the optimized conditions in hand, we proceeded to
explore the substrate scope of phenols 1, as shown in Table 3.
3j (X-ray structure)
3j, 95%
Naphthols with different substituents proceeded smoothly
under the standard reaction conditions, leading to products in
aReaction conditions: phenol 1 (0.2 mmol), sulfoxide 2 (0.22 mmol), Tf2O (0.24
good to high yields (4aa-4fa). Importantly, these products
mmol), MeCN (0.1 M) at 0 °C for 3 h.
(
4ba-4ea) bearing Br and CN substituents could be potentially
With isolated salt 3a in hand, we sought to determine its further functionalized. Unfortunately, naphthol 1g with an
reactivity in a Smiles-like rearrangement, as a stepwise and amide functional group gave the corresponding product 4ga in
general methodology to o-(phenoxy)naphthyl sulfide 4aa only 15% yield. Besides naphthols, the substrate scope was
(Table 2). We first explored the choice of base under aqueous further expanded to phenol derivatives. Phenols 1h
-1j
micellar conditions13 at 70 °C (entries 1-3), utilizing TPGS-750- containing electron-donating groups reacted smoothly with
M, a “benign-by-design” surfactant developed by Lipshutz.14 sulfoxides 2a and 2b generating the corresponding sulfides
To our delight, in this medium, K3PO4 was identified to be the 4ha
most effective base for this rearrangement (entry 4). Lowering electron-withdrawing groups showed lower reactivity, leading
the reaction temperature led to incomplete conversion and, to products 4kb
4mb in moderate yields. Interestingly, for p-
-4ja and 4ib (X-ray) in good yields. Phenols bearing
-
therefore, a decreased yield (entry 5). No increase in yield was unsubstituted phenols 1j and 1l, the desired products 4ja and
observed at higher temperature (entry 6). Other surfactants 4lb were obtained selectively.
including Triton X-100 and SDS were screened, as were organic
The scope of sulfoxides was then evaluated in reactions with
solvents (CH3CN, THF, and DMF). All afforded inferior results 2-naphthol 1a (Table 4). Adducts 4ab and 4ac were obtained in
(entries 7-11). A control reaction “on water” (i.e., in the good yields from symmetrical diaryl sulfoxides 2b and 2c with
absence of TPGS-750-M) was conducted and a significantly different electronic properties. Furthermore, unsymmetrical
lower yield was observed (entry 12), confirming the diaryl sulfoxides 2d-2h were also investigated. Remarkably, the
importance of micellar catalysis in facilitating this use of sulfoxide 2d, simultaneously bearing p-tolyl and phenyl
transformation in aqueous media.
residues resulted in phenyl-migrated product 4ad, while the p-
tolyl-migrated isomer was not detected. A similar trend was
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Table 4. Scope of sulfoxidesa,b
observed for sulfoxides 2e-2h, and the more electron-deficient
aryl groups were selectively migrated delivering the
corresponding sulfides 4ae-4ah. These results suggest that the
rearrangement takes place by ipso-attack of a negatively
charged oxygen atom onto the more electron-deficient aryl
group.
Table 3. Scope of phenolsa,b
aSee Table 3 for conditions. bIsolated yields from phenol 1a.
Besides diaryl sulfoxides, alkyl aryl sulfoxides also
aConditions: 1 (0.2 mmol), 2 (0.22 mmol), Tf2O (0.24 mmol), MeCN (0.1 M) at 0 °C participated in this transformation, further expanding the
for 3 h; K3PO4 (0.88 mmol), 2 wt% TPGS-750-M/H2O (0.75 M) at 70 °C for 24 h.
range of participating substrates. Increasing the length of the
bIsolated yields from phenol 1. cHydrolysis product (carboxylic acid) was also
alkyl chain slightly decreased the reaction efficiency, as
isolated in 34% yield.
demonstrated by products 4ai-4ak. In addition, benzyl phenyl
sulfoxide 2l is also a competent reaction partner, providing 4al
in 78% yield. Moreover, the aryl group could also be modified
with different electronic properties, and the reactions
furnished the corresponding sulfides in good to high yields
(
4ak and 4am-4ar). Importantly, there was no significant effect
of substituents in the para, meta, or ortho positions. For
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example, p-bromophenyl methyl sulfoxide
(
2n), m-
We next sought to prepare sulfoxides and sulfones in one-
bromophenyl methyl sulfoxide
methyl sulfoxide (2p) all reacted uneventfully and gave sulfoxide 2a, sulfonium salt formation and then rearrangement
methyl-migrated sulfides 4an 4ap in useful yields. Notably, p- in water gave sulfide 4aa. Without isolation, the resulting
(2o), and o-bromophenyl pot (Scheme 3). Starting from 2-naphthol 1a and diphenyl
-
cyanophenyl methyl sulfoxide (2r) afforded cyanophenyl- sulfide could then be oxidized to sulfoxide 5aa and sulfone 6aa
migrated product 4ar exclusively, whereas aryl methyl in good overall yields, respectively (Scheme 3a). Sulfone 6eg
sulfoxides 2m
-
2q all provided methyl-migrated products 4am
-
was also obtained in high yield under similar reaction
4aq. These results suggest that stronger electron-withdrawing conditions. The selectivity of the Smiles-like rearrangement
groups on the aryl moiety can suppress alkyl migration, leading was unambiguously confirmed by single crystal X-ray analysis
to aryl migration instead.
of 6eg (Scheme 3b).
To understand whether the rearrangement step is
intramolecular or intermolecular, a crossover reaction of
naphthol 1a with diaryl sulfoxides 2b and 2c was conducted
under the standard reaction conditions (Scheme 4). It was
found that no cross-product was detected, which suggests that
this aryl migration is intramolecular. Furthermore, 4ab was
formed in 78% yield while 4ac was isolated in only 39% yield,
indicating that sulfoxide 2b was more reactive than sulfoxide
2c
.
Scheme 2. Application in late-stage construction and scaffold construction
To further explore the applicability of this approach, we
tested this new transformation on estrone 1n containing a
complex architecture. Accordingly, difunctionalization of
estrone proceeded smoothly, affording 4ni or 4nb in 68% yield
(Scheme 2a), demonstrating the mild and selective nature of
this overall transformation. In addition, a formal synthesis of
urokinase inhibitor
Scheme 2b.
8 was accomplished efficiently as shown in
Scheme 4. Application in late-stage construction and scaffold construction
Lastly, E Factors4d, 15 were calculated for the rearrangement
under aqueous micellar conditions, to assess the level of waste
generation. Initially, an E Factor of only 4 was obtained based
on organic solvent used, as illustrated in Table 5. An E Factor of
only 8.6, was realized even with water included in the
calculation. Moreover, the aqueous medium containing TPGS-
750-M can be recycled to generate a different sulfide 4am or
4ba efficiently. Since no additional water need be used in each
cycle, the associated E Factors dropped significantly.
Scheme 3. Oxidation of the products in water
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M) at 70 °C for 24 h. bIsolated yields.
Conclusions
In conclusion, we have developed an unprecedented approach
for C–H/O–H difunctionalization of phenols under mild
conditions. The process is mediated by an electrophilic
4
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aromatic
substitution
and
subsequent
Smiles-like
rearrangement, involving C–S and C–O bond formations.
Notably, this rearrangement proceeded under aqueous
micellar conditions, successfully addressing several
environmental issues. Further studies on related
rearrangements featuring multiple bond-formations in water
are currently in progress in our laboratories.
Acknowledgements
Financial support provided by the National Key Research and
Development Program (2016YFD0600801), the Natural Science
Foundation of China (21502094), the Jiangsu Specially-
Appointed Professor Plan, and the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD) is
warmly acknowledged with thanks.
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