Unified synthesis of mono/bis-arylated phenols via RhIII-catalyzed dehydrogenative coupling

Article abstract of DOI:10.1039/c6sc03169b

2,6-Bis-arylated phenols are rarely reported and are synthetically challenging. Directed C-H functionalization reactions, using a directing group (DG), might provide a convenient solution to their synthesis. However, this strategy usually results in partial cleavage of the directing group, preventing further/second C-H activation cascades. Herein we report a general strategy that allows for the precise control of the oxidation pathways so that directing groups can be either preserved or cleaved. We found that N-phenoxyacetamides could undergo ortho-arylation reactions with or without an external oxidant, yielding products with different oxidation states, notably the rare bis-arylated phenols. Notably, a unique rhodacycle intermediate was isolated, characterized by X-ray crystallography, and confirmed to be an active catalyst. Switching between internal and external oxidation could be a general strategy in diverse directed C-H functionalization reactions to realize bis-functionalized products.

Full text of DOI:10.1039/c6sc03169b

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Unified Synthesis of Mono/Bis-arylated Phenols via Rh(III)-
Catalyzed Dehydrogenative Coupling
Qian Wu,a Ying Chen,a Dingyuan Yan,b Muyue Zhang,b Yi Lu,b Wei-Yin Sun,b and Jing Zhao*ab
Received 00th January 20xx,
Accepted 00th January 20xx
2,6-Bis-arylated phenols are rarely reported and synthetically challenging. Directed C−H funcꢀonalizaꢀon reacꢀons, using a
directing group (DG), might provide a convenient solution to their synthesis. However, this strategy usually results in
partial cleavage of the directing group, preventing further/second C−H acꢀvaꢀon cascades. Herein we reported a general
strategy that allows for the precise control of the oxidation pathways so that directing groups can be either preserved or
cleaved. We found that N-phenoxyacetamides could undergo ortho-arylation reactions with or without an external oxidant,
yielding products with different oxidation states, notably the rare bis-arylated phenols. Notably, a unique rhodacycle
intermediate was isolated, characterized by X-ray crystallography, and confirmed to be an active catalyst. Switching
between internal and external oxidation could be a general strategy in diverse directed C−H funcꢀonalizaꢀon reacꢀons to
realize the bis-functionalized products.
DOI: 10.1039/x0xx00000x
www.rsc.org/
which complexed heterocyclic scaffolds were synthesized in
one step from N-phenoxyacetamides and alkynes with up to a
quadruple cascade2d. Wang and Yi made impressive progress
on O−NHAc-directed C−H reacꢀons using cyclopropenes3t and
carbenoids3x as coupling partners, respectively. In these
reactions, the N−O bond of the DG was cleaved and serves as
an internal oxidant, leading to the corresponding phenol
products. Notably, You group reported a pioneering C-H/C-H
Introduction
Transition metal-catalysed C−H acꢀvaꢀon reacꢀons directed
by a coordinative group has become one of the most efficient
and straightforward synthetic strategies for the direct
functionalization of inert C−H bonds1. The role of a DG extends
beyond a simple anchor for the selective cleavage of a
neighbouring C−H bond. DGs can oꢁen undergo further in situ
condensation reactions that yield products of great structural
diversity2. More recently, DGs containing an oxidative N−O or
N−N bond were introduced to replace the required external
oxidant to render the C−H funcꢀonalizaꢀon reacꢀons redox-
neutral2d,3. Among them, oxyacetamide (O−NHAc) is one of the
most versatile functionalities for directed C-H functionalization
reaction cascades (Fig. 1a). The reactions involving this unique
DG can be classified into two types:
cross-coupling
heteroarenes through
synthesize the
between
N-phenoxyacetamides
traceless directing strategy to
highly functionalized 2-(2-
and
a
hydroxyphenyl)azoles, which are novel optoelectronic
materials3v.
Type 1: Internal oxidation via N-O bond cleavage.
O
OH
[ M ]
O
O
or
N
+
Partner 1
H
Type 1: internal oxidation with N−O bond cleavage. Lu group
first reported O−NHAc as a superb directing group for redox-
neutral olefination reactions of phenol derivatives by coupling
with alkynes3l and alkenes3n. Subsequently, our group reported
a number of reaction cascades using rhodium catalysis in
Partner 1
H
Product:
OH
OH
NHAc
R1
R2
OH
O
O
Ph
N
N
OCH3
COOR1
COOR2
R2
X
Ph
O
R1
Lu, 2013
Me
Wang, 2014
Yi, 2015
You, 2015
Zhao, 2014
Type 2: External oxidation with N-O bond preserved.
O
Product:
Ac
O
N
O
O
O
[ M ]
OH
N
N
N
Partner 2
+
H
H
R
R
Zhao, 2014
Zhao, 2014
b
This work: ONHAc-directed, controllable switching of internal oxidation and external oxidation
Het2
OH
Internal oxidation
External oxidation
Fluorescent
Heteroarylated Products
O
Het1
O
H
N
X
N
+
H
H
O
O
DG Preserved
Heteroarylated Products
N
H
N-phenoxyacetamide
Heteroarenes
Het
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corresponding phenol product 3ba in 80% yield. The meta-
methyl analogue 1c afforded 72% yield, and the ortho-methyl
derivative 1d yielded 76% of product. The yield for meta-OMe
N-phenoxyacetamide 1j was noticeably less. No obvious
electronic effect was observed. Substrates bearing either
electron-donating (phenyl and methoxy) or electron-
withdrawing groups (ester), delivered their corresponding
heteroaryl phenols in comparable yields.
Figure 1 The O−NHAc groupꢀdirected C−H activation reactions.
Type 2: external oxidation with preservation of the N−O
bond. In contrast to type 1 reactions, we recently described a
series of oxidative C−H funcꢀonalizaꢀon reacꢀons. In the
presence of
a
stoichiometric external oxidant, N-
phenoxyacetamides could react with aldehydes or α,β-
unsaturated aldehydes using palladium4 and rhodium5
catalysis. In these reactions, the N−O bond of the DG remained
intact after the reaction and was retained in the products.
Table 1. Substrate scope of fluorescent monoꢀarylated products 3 a,b
.
We thought that the unification of type 1 and type 2 into a
single reaction would allow general access to products with
different oxidation states with controlling the cleavage of the
N−O bond. For example, based on an isolated rhodacycle
intermediate Inter I6 from our previous studies5, the O−NHAc-
directed cross dehydrogenative coupling reactions with simple
heteroarenes would be an attractive strategy to access diverse
heteroarylated phenol scaffolds (Scheme 1)1b, 1d, 7-9
.
[a]. Conditions: Nꢀpheoxyacetamine (0.2 mmol), heteroarenes (0.3 mmol),
[Cp*RhCl2]2 (5 mol%), AgNTf2 (25 mol%), CsOAc (2.5 eq.), DMSO (1 mL) at
85oC for 30 hours under an N2 atmosphere. [b]. Isolated yield. [c]. GC yield.
In the reaction of mono-heteroarylated product 3aa, a
Scheme 1. The proposed transformations based on an isolated organometallic
different product with a stronger fluorescence was isolated in
intermediate.
a
small quantity. This double arylation product was
determined as the bis-heteroarylated phenol by single crystal
X-ray crystallography and it became predominant when a silver
salt was introduced as the external oxidant along with 2.5 eq.
of benzothiazole. Treating 1a and benzothiazole with
[Cp*RhCl2]2 (5 mol%) and PivOH (3 eq.) in THF at room
temperature for 20 h afforded 4aa in 78% isolated yield. Highly
fluorescent bis-heteroarylated phenols were obtained in
moderate to high yields under the optimized reaction
conditions (Table 2). Para- and meta-substituents were well
Heteroarylated phenols, such as 2-(2-hydroxyphenyl)-
benzothiazole (HBT) and 2-(2-hydroxyphenyl) benzoxazole
(HBO), possess high fluorescence quantum yield and a large
Stokes shift due to the excited–state intramolecular proton
transfer effect (ESIPT) and are widely used in various
fluorescent probes and related fields10. If the redox activity of
the N−O bond could be tuned using a proper external oxidant,
enabling the switching between type 1 and 2 reactions and up
to three coupling products could be obtained selectively in a
unified fashion (Scheme 1). Based on this design, we set out to
explore the combination of different reaction parameters from
phenoxyacetamides to achieve a unified strategy.
tolerated (4ba, 4ea, 4ka and 4ja). In addition, benzoxazoles
reacted with comparable efficiency (4eb).
Table 2. Substrate scope of fluorescent bisꢀarylated products 4 a,b
.
O
O
H
[Cp*RhCl2]2 (5 mol%)
AgF (3 eq.)
N
OH
X
X
N
N
Results and discussion
+
R
H
H
X
N
PivOH (3 eq.)
THF, r.t., 20 h
We embarked on our design by investigating a reaction
between N-phenoxyacetamide (1a) and benzothiazole (2a). As
expected, the desired ortho-heteroarylated phenol (3aa) was
obtained as the main product in the absence of any external
oxidant. After careful condition optimization, [Cp*RhCl2]2 (5
mol%), AgNTf2 (25 mol%), and CsOAc (2.5 eq.) in 1 mL DMSO at
85 °C for 30 h under an N2 atmosphere were found as the best
reaction conditions for 3aa, affording an 85% isolated yield. A
non-coordinating counter ion (SbF6, NTf2, CO3, OTf) was
R
4
1
2
The X-ray structure of 4aa shows that
the distance between the hydrogen
atom of hydroxyl (H3) and the
nitrogen atom of the heterocycle (N5)
is 1.862 [A], indicating that an
intramolecular hydrogen bond exists.
This may explain the low solubility of
the fluorescent bis-heteroarylationed
products 4 in many types of solvents.
N
S
OH
S
N
CCDC 1426755
4aa, 78%
N
N
N
essential for the catalytic activity of Rh3d, 3k, 3t, 11
.
O
OH
S
S
OH
OH
O
N
S
N
S
N
A
series of substituted N-phenoxyacetamides were
R
OMe
4ba, R = Me, 71%
4ea, R = tBu, 65%
4ka, R = COOMe, 43%
4ja, 55%
4eb, 68%
examined for substrate scope (Table 1). For methyl-substituted
N-phenoxyacetamides, the para-methyl substrate 1b gave the
2 | J. Name., 2012, 00, 1-3
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[a]. Reaction conditions: Nꢀpheoxyacetamine (0.2 mmol), heteroarenes (0.5
mmol), [Cp*RhCl2]2 (5 mol%), AgF (3 eq.), PivOH (3 eq.), THF (1 mL) at room
temperature for 20 hours in air. [b]. Isolated yield.
attempted to synthesize more interesting hybrid bis-
heteroarylated phenols. Gratifyingly, hybrid product 6a was
obtained in high yield (Scheme 2, Eq. 2). This type 2-type 1
sequence enhanced the structural diversity of these
fluorescent bis-heteroarylated phenols and provided a general
strategy for devising better fluorescent probes.
Interestingly, subjecting product 3aa and benzothiazole to
various Rh catalysis conditions did not yield 4aa, with or
without silver oxidants. This result suggested that 3aa was not
one of the intermediates leading to 4aa. The formation of the
bis-heteroarylated phenol arose from the type 2 product B
(Figure 2). Based on this result, we decided to target this type
2 product. Despite previous reports that the N−O bond was
always broken in the presence a silver oxidant, we were
excited to isolate a small amount of the mono-heteroarylated
N-phenoxyacetamide 5aa (8% yield) by replacing PivOH with
excess triethylamine (10 eq.). The structure was
unambiguously confirmed by NMR, HRMS and single crystal X-
ray crystallography. Further condition optimization revealed
that [Cp*RhCl2]2 (5 mol%), AgF (3 eq.) and benzothiazole (2
H
O
[Cp*RhCl2]2 (5 mol%)
AgF (2 eq.)
CsOAc (1 eq.)
PivOH (2.5 eq.)
O
N
H
N
S
OH
N
N
Eq. 1
H
N
S
S
THF, r.t., 6 h
S
5aa
2a, 1.2 eq.
4aa, 95%
[Cp*RhCl2
]
2 (5 mol%)
H
O
R
AgF (2 eq.)
CsOAc (1 eq.)
PivOH (2.5 eq.)
O
O
OH
N
N
H
N
N
Eq. 2
H
R
N
S
THF, r.t., 6 h
O
S
6a, R = 6-Me, 82%
5aa
2b, 1.2 eq.
Fluorescent Bis-heteroarylationed
Hybrid Product
i
eq.) in a solvent mixture of PrOH: N,N-Diisopropylethylamine
CCDC 1442845
(DIPEA): H2O = 1:1:0.1 at room temperature for 18 h afforded
5aa in 20% yield. The starting material N-phenoxyacetamide
1aa remained largely intact. Adding AgF in small portions (2
eq. upon mixing and 1 eq. every six hours for 4 eq. total)
improved the yield of 5aa to 68%.
Scheme 2. The route to novel fluorescent bisꢀheteroarylated hybrid products.
To understand the mechanism of O−NHAc-directed C−H
activation reactions, we obtained the five-membered
rhodation intermediate Inter I in 85% yield. The structure was
confirmed by NMR spectroscopy, HRMS, and X-ray
crystallography (Figure S1). When Inter I was used as the
catalyst, our three reaction conditions led to three expected
heteroarylated products, suggesting the rhodation species
Inter I was the active intermediate (Scheme 3).
Table 3. Substrate scope of DGꢀpreserved monoꢀarylated products 5 a,b
.
O
O
NHAc
N
O
H
[Cp*RhCl2]2 (5 mol%)
AgF (4 eq.)
X
N
N
H
R
+
R
H
DIPEA : iPrOH : H2O = 1 : 1 : 0.1
r.t. , 18 h
X
1
2
5
5ba, R = Me, 58%
5ea, R = tBu, 47%
5ga, R = Cl, 55%
5ha, R = Br, 49%
5ia, R = Ph, 41%
5ka, R = COOMe, 72%
O
NHAc
N
O
S
NHAc
N
R
S
5aa, 68%
CCDC 1438599
O
NHAc
N
O
NHAc
N
O
NHAc
R
N
MeOOC
S
S
O
Cl
Cl
5kd, 37%
R = Me, 5ca, 30%
R = OMe, 5ja, 35%
5ac, 59%
[a]. Reaction conditions: Nꢀpheoxyacetamine (0.2 mmol), heteroarenes (0.4
mmol), [Cp*RhCl2]2 (5 mol%), AgF (4 eq., 2 eq. added for the first time and 1 eq.
every 6 hours for 2 times), DIPEA : iPrOH : H2O = 1 : 1 : 0.1 at room
temperature for 18 hours under air. [b]. Isolated yield.
The substrate scope for this type 2 heteroarylation reaction
was explored using the optimized conditions. Para-substituted
N-aryloxyacetamides afforded the corresponding products in
Scheme 3. The confirmation of catalytically active species Inter I.
You group had demonstrated that the reaction might start
from the cyclometalation of N-aryloxyacetamide rather than
the heteroarene by ortho-deuterium labelling experiments.
You group reported that the KIE value was 1.04 on N-
phenoxyacetamide substrate, while the KIE value was 2.89 on
moderate yields (5ba, 5ea, 5ga, 5ha, 5ia, 5ka in Table 3). The
yield for the para-COOMe substrate was particularly high
(72%), suggesting that an electron-withdrawing group
favoured the type 2 reaction. A lower yield was observed for
those carrying a meta-substituent. Meta-methyl and meta-
methoxy substrates gave the desired products in 30% and 35%
benzoxazole substrate, suggesting that the rate-limiting step
might involve in the C-H bond breaking of heteroarenes3v
yield, respectively (5ca,
and benzoxazoles worked equally well (5ac
5ja). Both substituted benzothiazoles
5kd).
, 12
.
,
Thus, we proposed a mechanism with two pathways of
internal oxidation and external oxidation to afford different
products (Figure S1). First, after the generation of real catalyst
of [RhIII] by the anion exchange of [NTf2]—, a five-membered
To explore the formation of bis-heteroarylated phenols,
phenol 5aa was subject to rhodium catalyst in the presence of
external oxidant AgF (2 eq.) and 4aa was obtained nearly
quantitatively (Scheme 2, Eq. 1). Encouraged by this result, we rhodation species Inter I which had been demonstrated as an
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active intermediate. Second, the heteroarene inserted to give convenient, one-step synthesis of a series of DG-preserved
the heteroaryl–RhIII–phenyl intermediate which would products could facilitate the continued generation of a library
undergo reductive elimination to a RhI complex (Inter III in of fluorescent probes. Switching between internal and external
Figure S1). Third, two pathways are possible depending on the oxidation could be a general strategy in other directed C−H
presence of the external oxidants: (1) in the absence of the functionalization reactions to realize the bis-functionalized
external oxidants, RhI complex Inter III would undergo an products.
internally oxidizing pathway to form a RhIII complex (Inter IV in
Figure S1) with O-N bond cleavage3l,3n. Protonation would
Acknowledgements
afford the mono-arylated product 3. (2) in the presence of the
external oxidants, the RhI complex Inter III could undergo an
Financial support was provided by the Shenzhen Government
(JCYJ20150331100341865 and JCYJ20140627145302109), the
Guangdong Government (S20120011226), the National
Science Foundation of China (21332005 and 21571098) and
the MOST of China (2014AA020512).
external oxidation pathway to form the O-N bond-preserved
mono-arylated product 5. As there was DG retained in product
5, it could subsequently react with another heteroarene to
afford the bis-arylated products 4.
Application
The fluorescent properties of mono- and bis-heteroarylated
phenols were evaluated (Figure 2). Considering the solvent
effect on ESIPT, a series of common organic solvents was
screened, and dichloromethane was chosen for measurement
(Figure S6)13. The fluorescence spectra of mono-substituted
HBTs showed a strong ESIPT emission band in the region of
480–540 nm. Both the λmax and the intensity of the absorption
were affected by the substituents. A methyl group in the para−
and meta-positions caused a bathochromic shift (~15 nm),
while the ortho−counterpart showed no obvious change.
Halogen substituents (−F, Cl, Br) led to increased fluorescence
intensity with small red shifts (~10 nm). Products with an extra
phenyl group resulted in the highest red shift (~25 nm), and
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8
The relative work of Rhodium-catalyzed (hetero)arylation of
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9
The relative work of the direct C-H bond functionalization of
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