Ruthenium-catalyzed selective aerobic oxidative ortho-alkenylation of substituted phenols with alkenes through C-H bond activation

Article abstract of DOI:10.1002/ejoc.201201463

The oxidative coupling of phenyl carbamates and acetates with alkenes in the presence of a catalytic amount of [RuCl₂(p-cymene)]₂, AgSbF₆, and Cu(OAc)₂·H₂O under air provided substituted alkene derivatives in a highly regio- and stereoselective manner. The catalytic reaction was compatible with various alkenes such as acrylates, vinyl sulfones, acrylonitrile, and substituted styrenes. The present catalytic reaction was also compatible with phenyl carbamates and acetates with various electron-rich, electron-deficient, and halogen substituents on the aromatic ring. Further examination of the catalytic reaction was done with the substituted phenyl acetates. By using LiOH·H₂O or K ₂CO₃ as a base, the substituted alkene derivatives were converted into the corresponding phenol derivatives. To account for the catalytic reaction, a possible mechanism is proposed that involves a six-membered ruthenacycle as the key intermediate. The oxidative coupling of substituted phenyl carbamates and acetates with alkenes in the presence of [RuCl₂(p-cymene)]₂, AgSbF₆, and Cu(OAc) ₂·H₂O provided substituted alkene derivatives in a highly regio- and stereoselective manner. In the presence of either LiOH·H₂O or K₂CO₃, ortho-alkenylated phenyl carbamates and acetates were converted into the corresponding phenols. Copyright

Full text of DOI:10.1002/ejoc.201201463

FULL PAPER
DOI: 10.1002/ejoc.201201463
Ruthenium-Catalyzed Selective Aerobic Oxidative ortho-Alkenylation of
Substituted Phenols with Alkenes through C–H Bond Activation
Mallu Chenna Reddy[a] and Masilamani Jeganmohan*[a]
Keywords: Synthetic methods / Cross-coupling / C–H activation / Ruthenium / Regioselectivity / Alkenylation
The oxidative coupling of phenyl carbamates and acetates
with alkenes in the presence of a catalytic amount of
[RuCl2(p-cymene)]2, AgSbF6, and Cu(OAc)2·H2O under air
provided substituted alkene derivatives in a highly regio-
and stereoselective manner. The catalytic reaction was com-
patible with various alkenes such as acrylates, vinyl sulfones,
acrylonitrile, and substituted styrenes. The present catalytic
reaction was also compatible with phenyl carbamates and
acetates with various electron-rich, electron-deficient, and
halogen substituents on the aromatic ring. Further examina-
tion of the catalytic reaction was done with the substituted
phenyl acetates. By using LiOH·H2O or K2CO3 as a base, the
substituted alkene derivatives were converted into the corre-
sponding phenol derivatives. To account for the catalytic re-
action, a possible mechanism is proposed that involves a six-
membered ruthenacycle as the key intermediate.
and other research groups have reported that the alkenyl-
ation at the ortho-C–H bond of heteroatom-substituted aro-
matics by alkenes occurred in the presence of a palladium
catalyst by a chelation-assisted metalation pathway.[6,7] Sub-
sequently, Miura and other research groups have shown
that a similar type of alkenylation at the ortho-C–H bond
of heteroatom-substituted aromatics can take place with
alkenes in the presence of a rhodium catalyst through a
chelation-assisted metalation pathway.[8] Recently, a meta-
selective alkenylation by alkenes at the C–H bond of substi-
tuted aromatics in the presence of palladium complex has
been described by Yu and Sanford.[9]
Very recently, because of its remarkable reactivity, com-
patibility, and selectivity, a less expensive ruthenium com-
plex has gained tremendous attention in this type of hetero-
atom-directed ortho-alkenylation reaction. Interestingly,
this type of ruthenium-catalyzed alkenylation reaction can
be conducted under air or even in the presence of water as
the solvent.[10–12] Several directing groups such as amine,
imine, oxime, amide, pyridyl, COOH, phenol, OH, alde-
hyde, ketone, ester, acetate, and carbamate functional
groups have been used in the metal-catalyzed C–H bond
activation reaction. However, C–H bond activation by using
weak directing groups such as aldehyde, ester, cyano,
ketone, nitro, acetate, and carbamate functional groups is
very challenging. Herein, we wish to report the oxidative
coupling of phenyl carbamates and acetates with alkenes in
the presence of a catalytic amount of [RuCl2(p-cymene)]2,
AgSbF6, and Cu(OAc)2·H2O. Under air, this reaction pro-
vides substituted alkene derivatives in good to excellent
yields in a highly regio- and stereoselective manner. The
catalytic reaction is compatible with various substituted
alkenes such as acrylates, sulfones, acrylonitrile, and styr-
Introduction
Transition-metal-catalyzed coupling of aromatic electro-
philes or organometallic reagents with alkenes is a key reac-
tion for constructing alkene derivatives in a highly stereo-
selective manner.[1] This type of alkenylation reaction has
been widely used to synthesize various organic materials,
natural products, and drug molecules.[2] Although this type
of coupling reaction is highly useful in organic synthesis, a
preactivated coupling partner such as a C–X or C–M moi-
ety in the starting material is usually required for coupling
with alkenes. Instead of using a preactivated species, a sim-
ilar type of coupling reaction that is done by C–H bond
activation would be even more useful in organic synthesis.[3]
This type of reaction is highly atom economical and envi-
ronmentally friendly. The Fujiwara group was first to de-
scribe that the alkenylation reaction of electron-rich aro-
matics with alkenes occurred through an electrophilic met-
alation pathway in the presence of a palladium catalyst.[4]
The Lewis group demonstrated that the ortho-C–H bond of
phenol can be ethylated by ethylene in the presence of a
ruthenium catalyst.[5a] The Murai group showed that a se-
lective alkylation at the ortho-C–H bond of carbonyl-substi-
tuted aromatics can take place with alkenes in the presence
of a ruthenium(0) catalyst through a chelation-assisted met-
alation pathway.[5b,5c] Following these studies, Yu’s, Cheng’s,
[a] Department of Chemistry, Indian Institute of Science
Education and Research,
Pune 411021, India
Fax: +91-20-25865315
E-mail: mjeganmohan@iiserpune.ac.in
Homepage: http://www.iiserpune.ac.in/~mjeganmohan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201463.
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Ruthenium-Catalyzed ortho-Alkenylation of Substituted Phenols
enes. By using LiOH·H2O or K2CO3, the corresponding out Cu(OAc)2·H2O (see Table 1, Entry 9), but in this case,
alkenylated phenyl carbamates and acetates are converted coupling product 3a was not produced. Then, the amount
into the corresponding phenols.
of the oxidant Cu(OAc)2·H2O was reduced from 2.20 mmol
to a catalytic amount (30 mol-%), and the reaction was con-
ducted under air. This catalytic reaction proceeded nicely
and gave coupling product 3a in 92% yield by NMR (see
Table 1, Entry 10). On the basis of these optimization stud-
ies, we chose [RuCl2(p-cymene)]2] (4 mol-%), AgSbF6
(20 mol-%), and Cu(OAc)2·H2O (30 mol-%) in DME at
100 °C for 12 h under air as the standard conditions for our
coupling reaction. Under these optimized reaction condi-
tions, product 3a was observed in 86% isolated yield. Inter-
estingly, only a catalytic amount of the oxidant Cu(OAc)2·
H2O (30 mol-%) was used in the present reaction. However,
in most of the rhodium-catalyzed chelation-assisted C–H
bond activation reactions, only a stoichiometric amount of
the oxidant was required with the reaction conducted under
nitrogen.[8]
Results and Discussion
Initially, the coupling reaction was carried out with 4-
methoxyphenol (1a) and methyl acrylate (2a) in the pres-
ence of [RuCl2(p-cymene)]2 (4 mol-%), AgSbF6 (20 mol-%),
and Cu(OAc)2·H2O (2.20 mmol) in 1,2-dichloroethane
(DCE) at 100 °C for 12 h [Equation (1)]. In this reaction,
alkenylation was expected at the ortho position of phenol
1a. However, no such coupling product was observed, and
starting material 1a was recovered. Next, the catalytic reac-
tion was tested using phenol derivatives such as a substi-
tuted phosphorodiamidate 1b, 4-methoxyphenyl diethyl-
phosphate (1c), 4-methoxyphenyl mesylate (1d), 4-meth-
oxyphenyl triflate (1e), and 4-methoxyphenyl dimethylsulf-
amate (1f), in which under similar reaction conditions, an
additional heteroatom such as P=O or S=O was incorpo-
rated as a directing group to assist in the ortho-C–H bond
activation. However, no coupling products were observed
in these reactions as well.
Table 1. Optimization studies.[a]
Interestingly, under similar reaction conditions, the reac-
tion between 4-methoxyphenyl diethylcarbamate (1g) and
methyl acrylate (2a) produced coupling product 3a in 65%
yield (see Table 1). To increase the yield of 3a, the catalytic
reaction was carried out in various solvents such as tetra-
hydrofuran (THF), dimethylformamide (DMF), toluene, di-
methyl sulfoxide (DMSO), tBuOH, tert-amyl alcohol, 1,2-
dimethoxyethane (DME), and CH3CN. Among the sol-
vents, DME was the best and provided coupling product 3a
in 93% yield (see Table 1, Entry 4). The remaining solvents
were either less effective or totally ineffective in the coupling
reaction. Next, we examined the influence of the additive
(20 mol-%). A variety of additives such as AgSbF6, AgBF4,
AgOTf, and KPF6 were investigated (see Table 1, Entries 4–
7). Among them, AgSbF6 was very effective and gave 3a in
93% yield (see Table 1, Entry 4). AgBF4, AgOTf, and KPF6
were less effective and afforded 3a in 35, 46, and 20% yield,
respectively (see Table 1, Entries 5–7). The catalytic reaction
was also tested without any additive (see Table 1, Entry 8).
Entry
Solvent
Cu(OAc)2·H2O Additive % Yield of 3a[b]
1
2
3
4
5
6
7
8
9
DCE
THF
2.20 equiv.
2.20 equiv.
2.20 equiv.
2.20 equiv.
2.20 equiv.
2.20 equiv.
2.20 equiv.
2.20 equiv.
–
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgBF4
AgOTf
KPF6
65
52
15
93
35
46
20
15
–
tBuOH
DME
DME
DME
DME
DME
DME
DME
–
AgSbF6
AgSbF6
10
30 mol-%
92[c]
[a] All reactions were carried out under the following conditions: 1g
(1.0 equiv.), 2a (3.0 equiv.), [RuCl2(p-cymene)]2 (4 mol-%), additive
(20 mol-%), and Cu(OAc)2·H2O in DME at 100 °C for 12 h under
N2. [b] Yields were determined by 1H NMR integration methods
with mesitylene as an internal standard. [c] Under air, 86% isolated
yield was observed.
To explore the scope of the coupling reaction, various
In this case, coupling product 3a was observed in only 15% substituted aromatic carbamates 1h–1s were examined un-
yield. Similarly, the catalytic reaction was also studied with- der the optimized reaction conditions (see Scheme 1). Thus,
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Scheme 1. Scope of substituted phenyl carbamates.
treating halogen-substituted aromatic carbamates such as 4- coupling products 3m and 3n in 35 and 46% yield, respec-
iodophenyl diethylcarbamate (1h), 4-bromophenyl diethyl- tively. Here, the ortho-I group of 1s is involved in both the
carbamate (1i), 4-chlorophenyl diethylcarbamate (1j), and oxidative addition as well as the ortho-C–H bond acti-
4-fluorophenyl diethylcarbamate (1k) with methyl acrylate vation.[1] The exact reason for double alkenylation occur-
(2a) provided alkene derivatives 3b–3e in excellent to mod- ring in the reaction between aryl iodide carbamate 1s and
erate yields of 89, 87, 75, and 42%, respectively. Aromatic alkene 2a is unclear. A possible explanation might be that
carbamates with electron-withdrawing group substituents the C–I bond reacts first and then favors a second alkenyl-
such as 4-cyanophenyl diethylcarbamate (1l), 4-nitrophenyl ation with respect to other substrates.
diethylcarbamate (1m), methylester carbamate 1n, and 4-
Next, under the optimized reaction conditions, the re-
formylphenyl diethylcarbamate (1o) also efficiently partici- gioselectivity of substituted carbamates 1t–1x was exam-
pated in the coupling reaction with methyl acrylate (2a) to ined (see Scheme 2). The meta-substituted phenyl carb-
give coupling products 3f–3i in 27, 23, 52, and 66% yield, amate 3-methoxyphenyl diethylcarbamate (1t) regioselec-
respectively. In these reactions, any remaining amount of tively coupled with 2a at the less hindered C–H bond of 1t
starting material was recovered. It is important to note that exclusively to give coupling product 3o in 82% yield. The
the catalytic reaction was highly regioselective. Alkenylation reaction between 2-naphthyl diethylcarbamate (1u) and 2a
occurred only at the C–H ortho to the carbamate substitu- regioselectively occurred at the less hindered C–H bond of
ent, and no alkenylation was found at the position ortho to 1u exclusively to yield coupling product 3p in 66% yield.
the CN, NO2, CO2Me, or CHO groups on the aromatic Similarly, 3,4-dimethoxyphenyl diethylcarbamate (1v) and
ring. It is well-known that CO2Me and CHO are also a 3,4-dimethylphenyl diethylcarbamate (1w) also underwent
good directing groups for a C–H bond activation reac- the coupling reaction regioselectively with 2a to provide
tion.[11] The present results show that Ru chelates with the alkene derivatives 3q and 3r in 81 and 70% yield, respec-
carbamate group better than with CO2Me and CHO. The tively. Both of these reactions exclusively occurred at the
reaction between 1-naphthyl diethylcarbamate (1p) and 2a sterically less hindered C–H bond of 1v and 1w. In contrast,
afforded coupling product 3j in 61% yield. In addition, the the reaction between sesamol carbamate 1x and 2a took
catalytic reaction proceeded very well with ortho-substi- place regioselectively and exclusively at the highly sterically
tuted aromatic carbamates such as phenyl-substituted carb- hindered C–H bond of 1x to give coupling product 3s in
amate 1q and 2-bromophenyl diethylcarbamate (1r), which 85% yield.
yielded coupling products 3k and 3l in 57 and 85% yield,
Under the optimized reaction conditions, the present alk-
respectively. However, the reaction between 2-iodophenyl enylation reaction was successfully extended to the various
diethylcarbamate (1s) and methyl acrylate (2a) afforded substituted alkenes 2b–2j (see Schemes 3 and 4). Ethyl
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Ruthenium-Catalyzed ortho-Alkenylation of Substituted Phenols
Scheme 2. Regioselective studies of unsymmetrical phenyl carbamates.
Scheme 3. Scope of substituted alkenes.
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M. C. Reddy, M. Jeganmohan
FULL PAPER
Scheme 4. Scope of substituted styrenes.
acrylate (2b), n-butyl acrylate (2c), cyclohexyl acrylate (2d), noteworthy that the present catalytic reaction is compatible
tert-butyl acrylate (2e), and 2-hydroxyethyl acrylate (2f) ef- with electron-donating groups, halogens, and electron-with-
ficiently underwent reaction with sesamol carbamate 1x to drawing groups as substituents on the phenyl carbamates.
give the corresponding coupling products 3t–3x in 81, 75,
To further explore the scope of the coupling reaction, the
72, 62, and 65% yield, respectively (see Scheme 3). How- ortho-alkenylation of substituted phenyl acetates was at-
ever, in the reaction of tert-butyl acrylate (2e) with 1x, the tempted (see Scheme 5). Treatment of sesamol acetate 1y
tert-butyl group was cleaved in the expected alkenylated with methyl acrylate (2a) in the presence of [RuCl2(p-
product, and acrylic acid derivative 3w was produced. In cymene)]2 (4 mol-%), AgSbF6 (20 mol-%), and Cu(OAc)2·
the meantime, the catalytic reaction between acrylic acid H2O (30 mol-%) in 1,2-dichloroethane at 100 °C for 12 h
and 1x was tested, but coupling product 3w was not ob- under air gave coupling product 5a in 76% yield. Similarly,
served. It is noteworthy that the sensitive OH group in 2- sesamol pivalate 1z underwent coupling with 2a to provide
hydroxyethyl acrylate (2f) did not affect the coupling reac- alkenylated compound 5b in 70% yield.
tion. Other activated alkenes such as phenyl vinyl sulfone
Carbamate is a versatile functional group that can be uti-
(2g) and acrylonitrile (2h) were also compatible in the reac- lized for various organic transformations.[13] It is well-
tion. Thus, phenyl vinyl sulfone (2g) underwent coupling known that the carbamate group is a nice directing group
with 1x to provide alkenylated product 3y in an excellent for metal/base-mediated C–H bond activation reac-
81% yield. However, acrylonitrile (2h) provided a stereoiso- tions.[13a–13d] Furthermore, the carbamate group can also
meric mixture of coupling products 3z in a 55% combined be used as an electrophile for cross-coupling reactions with
yield and approximately a 60:40 E/Z ratio. In addition, arylboronic acids or nucleophiles in the presence of a nickel
other activated alkenes such as acrolein, ethyl vinyl ketone, catalyst.[13e–13g] By using 5.0 equiv. of LiOH·H2O in THF/
and N,N-dimethylacrylamide were explored for use in the MeOH/H2O (4:1:1) at 80 °C for 12 h, both the ester and
catalytic reaction. In these cases, no coupling products were carbamate groups of 3c were successfully cleaved to give
observed.
substituted phenol derivative 6a in 92% yield (see
The catalytic reaction was also successful with substi- Scheme 6). Interestingly, in the presence of 1.0 equiv. of
tuted styrenes 2i and 2j (see Scheme 4). Thus, styrene (2i) LiOH·H2O, the ester of 3d was selectively cleaved to provide
and 4-chlorostyrene (2j) underwent an efficient reaction acrylic acid derivative 6b in 86% yield, and the carbamate
with 1x to give alkene derivatives 4a and 4b in 76 and 71% group of 6b remained intact. However, in the presence of
yield, respectively. THF was used as a solvent in these reac- LiOH (3.0 equiv.), the carbamate group of 6b was success-
tions, as the DME solvent was not suitable. The reactions fully cleaved to afford substituted phenol derivative 6c in
were also highly regioselective, and similar to the situation 91% yield. Surprisingly, in the presence of K2CO3 in MeOH
for product 3s, the C–H bond activation took place exclu- for 2 h at room temperature, the acetyl (COMe) and pival-
sively at highly sterically hindered C–H bond of 1x. It is oyl (CO-tBu) groups of 5a and 5b were selectively cleaved
Scheme 5. ortho-Alkenylation of phenyl acetate and phenyl pivalate with alkenes.
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Ruthenium-Catalyzed ortho-Alkenylation of Substituted Phenols
Scheme 6. Synthesis of phenols.
Scheme 7. Proposed mechanism.
to afford the corresponding phenol derivative 6d in 85 and oxidant. The remaining amount of Cu(OAc)2 is regenerated
92% yield, respectively. In these reactions, the ester group under oxygen or air from the reduced copper source
substituent on the alkene moiety of 5a and 5b remained CuOAc.
intact.
A possible reaction mechanism to account for the pres-
ent catalytic reaction involves a six-membered ruthenacycle
Conclusions
as the key intermediate (see Scheme 7). The first step in-
We have described a ruthenium-catalyzed aerobic oxidat-
volves removal of the chloride ligand from the [RuCl2(p- ive ortho-alkenylation of phenyl carbamates and acetates
cymene)]2 complex by the AgSbF6 salt to give cationic ru- with alkenes. In this reaction, substituted alkene derivatives
thenium species 7. The coordination of the carbonyl group were produced in good to excellent yields in a highly regio-
of carbamate 1 to the ruthenium cationic species 7 is fol- and stereoselective manner. Later, the substituted alkene de-
lowed by ortho-metalation to afford six-membered ruthena- rivatives were further converted quantitatively into the cor-
cycle 8.[3c] A coordinative insertion of alkene 2 into the Ru– responding phenol derivatives by using LiOH·H2O or
carbon bond of ruthenacycle 8 provides intermediate 9, K2CO3 as a base. Further extensions of the coupling reac-
which undergoes a β-hydride elimination in the presence of tion through C–H bond activation by other chelating
Cu(OAc)2 to give coupling product 3 and regenerate active group-substituted aromatics, subsequent functionalizations
ruthenium species 7 for the next catalytic cycle. In the reac- with π-components, and detailed mechanistic investigations
tion, only 30 mol-% of Cu(OAc)2 is used as the internal are in progress.
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FULL PAPER
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Experimental Section
General Procedure for the Coupling Reaction of Aromatic Carb-
amates 1 with Alkenes 2: [RuCl2(p-cymene)]2 (0.04 mmol, 4 mol-%),
AgSbF6 (0.20 mmol, 20 mol-%), and Cu(OAc)2·H2O (0.30 mmol,
30 mol-%) were added to a 15 mL pressure tube, which was
equipped with a magnetic stirrer and septum. (Note: AgSbF6 is
moisture-sensitive. Thus, AgSbF6 was handled inside a nitrogen
glove box.) To the tube were added by syringe carbamate 1
[4]
(1.0 equiv.), alkene
2 (3.0 equiv.), and 1,2-dimethoxyethane
(3.0 mL) as the solvent, and the reaction mixture was allowed to
stir at room temperature for 5 min. During this time, the tube was
covered with a septum. Then, the septum was removed, and the
reaction mixture was stirred under open air for an additional 5 min.
[Note: During this time, the nitrogen gas that was initially in the
pressure tube dispersed, and air entered the tube. In the reaction,
only 30 mol-% of Cu(OAc)2·H2O was used for the internal oxidant.
In fact, 2.20 equiv. of Cu(OAc)2·H2O was needed for the reaction.
It is strongly believed that the remaining amount of Cu(OAc)2·H2O
necessary for the reaction was regenerated under oxygen or air
from the reduced copper source CuOAc. Therefore, we conducted
the reaction under open air.] Next, the pressure tube was sealed
with a screw cap, and the reaction mixture was stirred at 100 °C
for 12 h. After cooling to ambient temperature, the reaction mix-
ture was diluted with CH2Cl2 and then filtered through Celite and
silica gel. The filtrate was concentrated, and the crude residue was
purified through a silica gel column (hexanes and ethyl acetate) to
give pure 3. For styrenes 2i and 2j, THF (3.0 mL) was used as the
solvent. A similar procedure was followed for the coupling of
phenyl esters 1y and 1z with 2a, but these reactions were conducted
at 100 °C for 12 h with 1,2-dichloroethane (3.0 mL) as the solvent.
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[7]
Supporting Information (see footnote on the first page of this arti-
cle): General procedure for the preparation of compounds 6a–6c,
procedure for the preparation of compound 6d, spectroscopic data
of 3a–3z, 4a, 4b, 5a, 5b, and 6a–6d, and copies of the H and 13C
1
NMR spectra of all compounds.
[8]
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Acknowledgments
We thank the Department of Science and Technology (DST) (SR/
S1/OC-26/2011), India for the support of this research. M. C. R.
thanks the Council of Scientific and Industrial Research (CSIR)
for a fellowship.
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Received: November 1, 2012
Published Online: January 10, 2013
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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