Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air

Article abstract of DOI:10.1002/asia.202001087

A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot seq

Full text of DOI:10.1002/asia.202001087

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic
Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air
Authors: Anup Mandal, Bholanath Garai, Suman Dana, Ratnadeep
Bera, and Mahiuddin Baidya
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10.1002/asia.202001087
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Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic
Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air
Anup Mandal,[a] Bholanath Garai,[a] Suman Dana,[a] Ratnadeep Bera,[a] and Mahiuddin Baidya*[a]
In memory of Professor Dr. Rolf Huisgen
[a]
Anup Mandal, Bholanath Garai, Suman Dana, Ratnadeep Bera, and Dr. Mahiuddin Baidya
Department of Chemistry
Indian Institute of Technology Madras
Chennai – 600 036, Tamil Nadu, India.
E-mail: mbaidya@iitm.ac.in
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: A cross-dehydrogenative coupling of arene carboxylic
acids with olefins is reported with ruthenium(II) catalyst employing air
and water as green oxidant and solvent, respectively. It offers a
intermediates.
Aromatic carboxylic acids are shelf-stable building blocks in
organic synthesis. They represent the core structure of many
natural products and also acid functionality can be easily
transformed into a wide range of useful functional groups.
Consequently, the transition-metal-catalyzed C–H bond
activation and functionalization of aromatic acids is trending[9]
and in this regime, ruthenium(II) catalysis has received
considerable attention owing to its inexpensive nature in
comparison to other noble metal catalysis, durability under
aqueous reaction conditions, and unique reactivity profile under
the assistance of weak coordination.[9b] In 2011, Ackermann et al.
first demonstrated that Ru(II)-catalyzed oxidative cross-coupling
between aromatic acids and activated olefins such as acrylates
and acrylonitrile smoothly took place in water (Scheme 1a).[10a]
Latter, Cai et al. also showed that such reaction also proceeded
in PEG-400/water mixture as reaction medium.[10b] However,
both the protocols utilized over-stoichiometric amount of
Cu(OAc)2H2O as a terminal oxidant. Recently, Shi and Gooen
groups independently reported Ru(II)-catalyzed coupling of
aromatic acids with vinyl ketones or acrylic acid in H2O, where
the protocols favoured the hydroarylation reaction mode instead
of cross-dehydrogenative coupling to give C–H alkylation
robust synthesis of valuable phthalide molecules.
A one-pot
sequential strategy is also disclosed to access Heck-type products
that are apparently difficult to make directly from arene carboxylic
acids.
With the increasing awareness of green and sustainable
chemistry principles, devising straightforward catalytic protocols
to access high-value products with enriched molecular
complexity is highly significant in the organic chemistry
community.[1] In this scenario, the transition-metal-catalyzed C–
H bond activation concept that accounts direct utilization of
otherwise inert C–H bond of organic molecules as a synthetic
handle turned out very promising.[2] It renovates the synthetic
policies as a greener alternative to traditional cross-coupling
reactions by avoiding the need of pre-functionalized substrates
and thereby improving overall step- and atom-economy.[3]
Specifically, the cross-coupling reaction between two different
C(sp2)–H bonds represents a very powerful C–C bond-forming
technology as it constitutes a twofold C–H functionalization
manifold.[4] However, such oxidative cross-dehydrogenative
couplings very often demand the employment of stoichiometric
amounts of metal-based terminal oxidants such as Cu(II) or
Ag(I) salts, generating significant amount of metallic wastes in
conflict of the green chemistry principles.[5] Employment of
abundant molecular oxygen in lieu of these metal-based
oxidants would be a green asset, where water is the sole by-
product.[6] Further, a simple and cost-effective setup is expected
if air can be directly engaged for the same purpose. Another
critical environmental issue arises from the fact that these
transformations, in general, consider a huge amount of organic
solvent as compared to the other reagents and thus produce a
bulk quantity of chemical waste. On the other hand, solvents
play crucial roles in most of the organic transformations by
controlling the reaction equilibrium and rate of the reaction.[7]
Given the environmentally benign portfolio of water, a prompt
solution could be the use of water as the reaction medium.[8] It is
non-toxic, non-flammable, and naturally abundant. However,
swift adoption of water as reaction medium for the transition-
metal-catalyzed C–H bond activation reactions is challenging
because of the poor substrate solubility along with intrinsic
sensitivity of metal catalysts and reactive organometallic
Scheme 1. Ruthenium(II)-catalyzed C–H bond activation of aromatic acids
with olefins in water.
1
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products (Scheme 1a).[11] Despite these visible advancements,
to the best of our knowledge, Ru(II)-catalyzed cross-
dehydrogenative coupling of aromatic carboxylic acids with
olefins under environmentally benign conditions that use both air
as terminal oxidant and water as green solvent remains elusive.
Herein, we have addressed this unmet issue through the
development of regioselective cross-dehydrogenative coupling
of aromatic carboxylic acids with vinyl phosphonates and
concomitant annulation under inexpensive ruthenium(II)
catalysis, leading to functionalized phthalides (Scheme 1b).
Notably, phthalide framework is a recurrent motif in natural
products.[12] The mild reaction condition is also effective for other
alkenes such as vinyl sulfone, acrylates, and acrylonitrile, while
styrene can be accommodated with a modification in reaction
temperature led to substantial reduction in yield (entries 8–9).
Control experiments showed that all components, base, catalyst,
and air, are essential (entries 10–12). Reaction completely shut
down in the absence of any one of them.
With this benign reaction conditions (Table 1, entry 7), the
substrate generality of this protocol was surveyed (Scheme 2).
conditions.
A one-pot sequential protocol involving cross-
dehydrogenative annulative coupling and subsequent phthalide
ring-opening has also been delineated for a formal Heck-type
process (Scheme 1b).
In pursuit of greener conditions for cross-dehydrogenative
coupling, we initially examined the reaction of readily available
ortho-toluic acid 1a with diethyl vinyl phosphonate 2a (Table 1).
To our satisfaction, coupling process proceeded smoothly with
commercially available [Ru(p-cymene)Cl2]2 catalyst in presence
of air and KOAc base in water at 100 oC and the desired product
3a was isolated in 74% yield (entry 1). Screening of other bases
gave inferior yields (entries 2–5). When the loading of KOAc
base was increased, yield improved gradually and complete
conversion was observed for 2 equivalents of KOAc, furnishing
3a in quantitative yield (entries 6–7). A lower reaction
Table 1. Optimization of oxidative annulation reaction conditions.[a]
Entry
Base (x equiv)
Temperature. (oC)
Yield (%)[b]
1
2
KOAc (1 equiv)
NaOAc (1 equiv)
LiOAc (1 equiv)
CsOAc (1 equiv)
KOH (1 equiv)
KOAc (1.5 equiv)
KOAc (2 equiv)
KOAc (2 equiv)
KOAc (2 equiv)
-
100
100
100
100
100
100
100
80
74
43
30
62
0
3
4
5
6
85
99
62
78
0
7
8
9
90
10[c]
11[d]
12[e]
100
100
100
KOAc (2 equiv)
KOAc (2 equiv)
0
0
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), water (1 mL) for 24 h
under air. [b] Yields of isolated products are given. [c] Without KOAc. [d]
Without [Ru(p-cymene)Cl2]2. [e] Under nitrogen atmosphere.
Scheme 2. Scopes of the oxidative annulation reaction of benzoic acids with
vinyl phosphonates.
2
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The methodology can accommodate
a
wide range of
electronically and sterically diverse benzoic acid derivatives.
ortho-, meta-, and para-Substituted aromatic carboxylic acids
having alkoxy (3c and 3i), aryl (3d), trifluoromethyl (3e), alkyl
(3f–3h) functionalities and common protecting groups, such as
benzyl (3j) and methoxymethyl (MOM, 3k), delivered
corresponding phthalide products in high yields. Di- and tri-
functionalized benzoic acids including piperonylic acid (3l–3q)
underwent a smooth transformation under the current conditions.
Halogenated benzoic acids also effectively furnished products
3r–3u in good yields. Gratifyingly, presence of free hydroxyl and
amine functionalities did not hamper the reaction; salicylic acid
(3v), 4-hydroxybenzoic acid (3w and 3z), vanillic acid (3x), and
4-aminobenzoic acid (3y) readily rendered products in good
yields (52–96%). Product 3z prepared with dimethyl vinyl
phosphonate was also structurally characterized with single
crystal X-ray analysis.[13] Reactions with 1- and 2-naphthoic
acids were successful, forging polycyclic products 4a and 4b in
92% and 64% yields, respectively. Heteroarene carboxylic acids,
for instance, indole-5-carboxylic acid and N-ethylcarbazole-3-
carboxylic acid, are also good substrates for this reaction,
offering 4c and 4d in 66% and 73% yields, respectively. Of note,
for all these cases, the less sterically hindered C–H bond was
activated preferably via C–H ruthenation, while 1H and 13C NMR
spectra revealed opposite regioselectivity for products 3p, 3s,
and 4c, which is in line with the prior observations by Schlosser,
Chatani, and others.[14]
Scheme 4. Oxidative C–H olefination of aryl carboxylic acids with styrene.
when terminal alkyne such as phenyl acetylene was tested as a
coupling partner under the catalytic conditions.[15b,c] Also, when
styrene is employed as
a
coupling partner, cross-
dehydrogenative product 8a was not detected with the recovery
of ortho-toluic acid 1a, suggesting a modified reaction condition
is necessary (Scheme 4). When solvent was changed to ethanol,
olefinated product 8a was formed in 60% yield. Further
screening revealed KOH as the most effective base for
ruthenium catalysis, delivering 8a in 83% isolated yield in
o
ethanol at 85 C under air. Under these conditions, electron-rich
The directing efficacy of the acid functionality under the
current conditions is also intriguing. When aromatic acid
substrates bearing additional auxiliary (directing group) such as
ketone, acetanilide, sulfonamide or pyridyl group were examined,
weak O-coordination of carboxylic acid turned out responsible to
control the site-selectivity, giving phthalides 4e–4i in 65–72%
yields (Scheme 2). The scale-up was also compatible and
product 3a was prepared in gram scale with 91% yield (Scheme
2).
aromatic acids offered olefinated products 8b–8d in good yields,
however, poor conversion was observed for electron-deficient 4-
(trifluoromethyl)benzoic acid (Scheme 4). The protocol was also
effective to transform heteroaromatic carboxylic acid into
corresponding styrylated products 8f8g in moderate yields.
This ruthenium-catalyzed cross-dehydrogenative coupling
protocol is not restricted only to vinyl phosphonates. Under the
standard conditions, coupling with other olefins such as phenyl
vinyl sulfone, acrylic acid esters, and acrylonitrile rendered
corresponding phthalides 6a–6h in good to excellent yields
(Scheme 3). The present conditions also accommodate internal
alkyne for annulation, producing 6i in 84% yield (Scheme 3).[15a]
On the other hand, the desired annulative product did not form
Scheme 5. Scopes of one-pot sequential mono-olefination process.
Scheme 3. Further scope of the annulation reaction of aromatic acids. [a]
Diphenylacetylene (1.5 equiv) was used as a coupling partner.
Transition-metal-catalyzed C–H olefination reactions of
benzoic acids with activated olefins often lead to phthalide
3
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molecules owing to facile oxa-Michael addition onto electron-
deficient double bond of generated Heck-type intermediate.[16]
Therefore, a ring-opening strategy of phthalide products was
conceived to access Heck-type products in a sequential one-pot
operation (Scheme 5). Considering the high acidity of the proton
attached to -carbon of phosphonate functionality in 3a, we
envisioned to exploit an E1cB-type process. Accordingly, after
completion of the reaction of 1a with diethyl vinyl phosphonate
invalidating the involvement of any radical intermediate in the
reaction (Scheme 6b). Furthermore, while both the kinetic
isotope effect values obtained via intermolecular competitive
experiment (pH/pD = 1.55) and independent parallel experiment
(kH/kD = 1.71) are significant, the involvement of C–H bond
cleavage in the rate-determining step is not affirmative owing to
reversible nature of the C–H metalation step (Scheme 6c).
Further, intermolecular competition experiments revealed that
electronically different aromatic acids produced comparable
yields of respective products, favoring concerted-metalation-
deprotonation (CMD) mechanism for C–H bond activation
process (Scheme 6d).
Based on the above control experiments and literature
precedents,[6] a reaction mechanism has been proposed in
Scheme 7. In presence of Ru(II) catalyst and acetate base,
arene carboxylic acid 1 forms the metalacycle I. Subsequent
coordinative insertion of olefin (2/5/7) generates ruthenacycle III,
which experiences a -hydride elimination to give intermediate
IV. Then, reductive elimination produces alkenylated
2a
under
standard
conditions,
DBU
(1,8-
diazabicyclo[5.4.0]undec-7-ene) was introduced and reaction
mixture was stirred at room temperature for additional 12 h.
Analysis of crude reaction mixture showed the formation of
desired Heck-type product, which was isolated after
esterification in 91% yield. Following this sequence, other Heck-
type products 9b–9g were prepared in high yields directly from
arene carboxylic acids, highlighting the flexibility of this protocol
(Scheme 5). The protocol was also equally effective with
acrylates and acrylonitrile coupling partners, where Heck-type
products 9h–9l were isolated in the form of free aromatic acid,
which is useful for further synthetic manipulations. Compound 9k
was crystallized, and the structure was unambiguously
confirmed by X-ray analysis.[13]
A series of control experiments has been conducted to
penetrate the mechanistic insights (Scheme 6). When the
reaction was performed with D2O under the optimal conditions,
an excellent level of deuterium incorporation was observed in
the ortho-position of acid 1a, confirming the reversible nature of
the C–H ruthenation step (Scheme 6a). In the presence of
intermediate
V and Ru(0) species. The intermediate V
spontaneously undergoes intramolecular oxa-Michael addition,
providing the annulated coupling product (3/4/6) and the active
Ru(II) catalyst is regenerated from Ru(0) species in the
presence of air to continue the catalytic cycle.
Scheme
7. Plausible reaction mechanism for oxidative C–H
alkenylation/annulation protocol.
In summary, a greener cross-coupling reaction of arene
carboxylic acids with olefins has been developed based on
ruthenium(II)-catalyzed weak coordination-assisted site-selective
C–H bond activation strategy in water employing air as the sole
oxidant. This operationally simple protocol is scalable,
accommodates a variety of olefins, and tolerates a diverse set of
common functional groups including free alcohol and amine, and
offers valuable phthalide molecules in good to excellent yields.
Also,
a one-pot protocol involving cross-dehydrogenative
coupling and subsequent phthalide ring opening has been
accomplished to access Heck-type products that are apparently
unattainable with activated olefins via transition-metal-catalyzed
C–H bond activation of arene carboxylic acids. Further
development of environmentally benign catalytic routes for
otherwise inert C–H bond functionalization is ongoing in our
laboratory.
Scheme 6. Mechanistic investigations for cross-dehydrogenative annulation
reaction with diethyl vinyl phosphonate.
radical scavengers, such as 1,1-diphenylethylene, TEMPO, and
BHT, annulative product 3g was formed in significant amount,
4
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P. Annamalai, S.-C. Chuang, C.-H. Cheng, Green Chem. 2017, 19,
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Acknowledgements
We gratefully acknowledge CSIR India (02(0369)/19/EMR-II) for
financial support. A.M. acknowledges University Grants
Commission (UGC), S.D. acknowledges IIT Madras, and R.B.
acknowledges Council of Scientific & Industrial Research (CSIR)
for providing the corresponding research fellowships. M.B.
thanks IIT Madras for the support through Institute Research
Development Award (IRDA).
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Keywords: C–H activation • Cross-dehydrogenative coupling •
Ruthenium(II) catalysis • Green synthesis • Aromatic carboxylic
acids
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5
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10.1002/asia.202001087
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents
Ruthenium catalysis in water: a greener cross-dehydrogenative coupling reaction of arene carboxylic acids with olefins has been
developed based on ruthenium(II)-catalyzed weak coordination-assisted site-selective C–H bond activation strategy in water
employing air as the sole oxidant.
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This article is protected by copyright. All rights reserved.