Ruthenium-Catalyzed C-H Functionalization of Benzoic Acids with Allyl Alcohols: A Controlled Reactivity Switch between C-H Alkenylation and C-H Alkylation Pathways

Article abstract of DOI:10.1021/acs.orglett.8b02064

A highly selective and switchable ruthenium-catalyzed ortho C-H alkylation and C-H alkenylation of benzoic acids with allyl alcohols is reported. A complete switch in selectivity is achieved upon tuning the reactivity of the organometallic intermediate in the carboxylate-directed C-H activation to provide access to highly useful motifs such as 2-alkylbenzoic acids and phthalides.

Full text of DOI:10.1021/acs.orglett.8b02064

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ruthenium-Catalyzed C−H Functionalization of Benzoic Acids with
Allyl Alcohols: A Controlled Reactivity Switch between C−H
Alkenylation and C−H Alkylation Pathways
Gangam Srikanth Kumar, Tapasi Chand, Diksha Singh, and Manmohan Kapur*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal, MP
462066, India
S
* Supporting Information
ABSTRACT: A highly selective and switchable ruthenium-catalyzed ortho C−H alkylation and C−H alkenylation of benzoic
acids with allyl alcohols is reported. A complete switch in selectivity is achieved upon tuning the reactivity of the organometallic
intermediate in the carboxylate-directed C−H activation to provide access to highly useful motifs such as 2-alkylbenzoic acids
and phthalides.
Scheme 1. : Ruthenium-Catalyzed C−H Functionalization of
Benzoic Acids with Allyl Alcohols
ith outstanding levels of advances in the last two decades,
Wtransition-metal-catalyzed C−H functionalization has
witnessed a revolution.1 Among various directing groups
employed in arene C−H functionalization, carboxylic acids are
undoubtedly advantageous and are often considered to be ideal
directing groups. This is primarily due to their ubiquitous
availability, capability of acting as traceless directing groups, as
well as their ability to undergo cascade cyclization or annulations
via a C−H activation to construct complex molecular frame-
works.2,3 Recent advances in carboxylate-directed C−H
functionalization have been well-documented in the literature.4
However, despite its advantages, the weak coordinating ability of
the carboxylic acid group and control of the stability of the
metallacycle to regulate the outcome of the transformation is
still a major hurdle in the development of the selective
transformations.
Activated olefins such as α,β-unsaturated carbonyl com-
pounds are successfully used in C−H alkenylation5,6 as well as
C−H alkylation7−9 reactions. Most of the reports on hydro-
arylation of alkenes involve the use of strong chelating and non-
removable directing groups.9 In general, direct C−H alkenyla-
tion of arenes involves the formation of a metal alkyl
intermediate via migratory insertion of the metal aryl species
into the olefin partner (Scheme 1a).5−9 This elusive organo-
metallic intermediate can now choose between the β-hydride
elimination or protonolysis pathways, and it is the control of this
reactivity that is the key to selective access to C−H alkenylation
and C−H alkylated products.10 In this context, the development
of a mild and efficient catalytic system which enables a switch in
C−H alkenylation and hydroarylation pathways is highly
desirable.
coupling partners in C−H functionalization as they are capable
of selectively undergoing multiple reaction pathways to afford
diverse products.11−13 Although the oxidative coupling between
aryl carboxylic acids and alkenes to result in cyclic scaffolds is an
extensively studied topic,6 the use of allylic alcohols as coupling
partners with aryl carboxylic acids is still scarce. The lack of work
in this area can be explained by the fact that the direct coupling
of carboxylic acids with allylic alcohols leads to the formation of
β-aryl ketones or aldehydes via a decarboxylative Heck-type
reaction (Scheme 1b).14
Very recently, Goossen and co-workers reported the
ruthenium catalyzed C(sp2)−H allylation of substituted benzoic
acids using allyl alcohols as allylating agents (Scheme 1c).15
Tuning the reactivity of substrates which can undergo such
multiple reactive pathways brings diversity in synthesis. In this
regard, allylic alcohols are regarded as a privileged class of
Received: July 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02064
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
With our recent experience in exploring the reactivity of allyl
alcohols,16 we envisaged that by tuning the reactivity of allyl
alcohols in the C−H functionalization of substituted benzoic
acids, one can achieve selective formation of diverse products
such as aryl−alkyl motifs and phthalides by avoiding the use of
rather labile alkylating agents. Recently, Li and co-workers
developed a rhodium-catalyzed, carboxylate-directed synthesis
of phthalides by using enones.17 Later, the research groups of
Shi18a and Goossen18b independently reported the ruthenium-
catalyzed, C−H alkylation of benzoic acids with α,β-unsaturated
olefins. To the best of our knowledge, the oxidative coupling of
aryl carboxylic acids with allylic alcohols to afford phthalides or
C−H alkylated compounds is not known. Herein, we report
switchable ruthenium-catalyzed C−H functionalizations of
benzoic acids with allyl alcohols to access C−H alkylation
products and phthalides in a highly selective manner. The
highlights of the current protocol are the use of readily available,
bench-stable allyl alcohols as an alternative to the labile vinyl
ketones and a complete switch in selectivity to access the diverse
products (Scheme 1).
Scheme 2. Substrate Scope for C−H Alkylation
We initiated our studies by probing various conditions
including using commercially available [RuCl2(p-cym)]2 as the
catalyst (see the Supporting Information (SI) for details). To
our delight, the catalytic system comprising [RuCl2(p-cym)]2,
AgSbF6, and Cu(OAc)2·H2O delivered the desired phthalide
product. Systematic screening of the solvent system revealed
that THF is the optimal solvent for the formation of the
phthalide. Among a variety of oxidants, the best results were
obtained with Cu(OAc)2·H2O as the oxidant. Surprisingly, a
distinct dichotomy in product distribution was observed upon
the use of the KOAc in the transformation. A complete
selectivity switch was observed upon using a catalytic system
comprising [RuCl2(p-cym)]2, KOAc, and Cu(OAc)2 as it
delivered the hydroarylation product exclusively (see the SI
for further details). Switching to other acetate sources resulted in
inferior results. The use of KOAc may facilitate the formation of
the stable ruthenium−oxa-π-allyl intermediate that preferen-
tially undergoes protonolysis over the β-hydrogen elimination to
provide access to C−H-alkylated products exclusively.8e,9a,b
Among a variety of solvents screened, DCE was found to be the
solvent of choice for the C−H alkylation pathway. Extending the
reaction time did not significantly improve the yield or change
the selectivity of the transformation (see the SI for details).
Control reactions carried out in the absence of the Ru catalyst
did not yield the desired products.
a
Unless otherwise mentioned, all reactions were performed with 1
(0.3 mmol), 2 (0.6 mmol), catalyst (0.015 mmol), Cu(OAc)2 (0.6
mmol), and KOAc (0.45 mmol) in 1.5 mL of DCE followed by
b
K2CO3 (2.0 equiv) and MeI (5.0 equiv) in acetone (2 mL). Reaction
performed on 1 mmol scale. Based on recovered starting material
(ester). 4 equiv of allyl alcohol is used.
c
d
indicating that regioselectivity was controlled by the steric
nature of the substituents rather than electronic control (3t−v,
Scheme 2).
The reaction of p-methoxybenzoic acid afforded a mixture of
dialkylated and monoalkylated products (3w, Scheme 2).
Further, we successfully removed the carboxyl group via a
protodecarboxylation to afford meta-substituted alkylated
benzene (3ka, Scheme 2). The substrate scope for the
ruthenium-catalyzed synthesis of phthalides is depicted in
Scheme 3. The reaction worked well for most of the allyl
alcohols screened (4a−d, Scheme 3). The reaction clearly
displayed an electronic bias; the electron-withdrawing sub-
stituents hampered the reactivity of the carboxylic acids (4k, 4l
a
Scheme 3. Substrate Scope for the Synthesis of Phthalides
Subsequently, we explored the substrate scope of the
ruthenium-catalyzed oxidative C−H alkylation of substituted
benzoic acids with allyl alcohols (Scheme 2). The reaction
worked well with most of the benzoic acids as well as the allyl
alcohols screened for the substrate scope. Various allylic alcohols
bearing different alkyl substituents at the carbinol carbon
delivered the desired products with almost exclusive γ-selectivity
(3a−d, Scheme 2). Notably, the hydroarylation of aryl-
substituted allylic alcohols afforded the desired products in
moderate yields (3e−i, Scheme 2). The reaction was, however,
sensitive to steric crowding on the allyl alcohol; crotyl alcohol
failed to deliver the desired product (3x, Scheme 2). The
reaction worked well for benzoic acids containing electron-
donating as well as electron-withdrawing substituents (3j−n,
Scheme 2). Densely substituted aryl carboxylic acids also
delivered the hydroarylation products in moderate yields (3o−s,
Scheme 2). The reaction of meta-substituted benzoic acids
resulted in the alkylation at a less sterically hindered position,
a
All reactions were performed with 1 (0.3 mmol), 2 (0.6 mmol),
catalyst (0.015 mmol), AgSbF6 (0.12 mmol), and Cu(OAc)2·H2O
(0.6 mmol) in 1.5 mL of THF in a sealed tube. Based on recovered
starting material.
b
B
DOI: 10.1021/acs.orglett.8b02064
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3). The reaction proceeded efficiently with the electron-
rich benzoic acids to afford phthalides via a β-hydrogen
elimination pathway (4e−g). The reaction of m-toluic acid
proceeded regioselectivity at the less sterically hindered site (4h,
4i), whereas the coupling reaction of p-toluic acid afforded a
mixture of monosubstituted and disubstituted products (4j, 4j′,
Scheme 3).
To understand the origin of the observed selectivity, the
mechanism of the transformations was studied. To this end, we
performed reversibility experiments and kinetic isotope effect
(KIE) studies (Scheme 4). When the C−H alkylation and C−H
hydroxy functional group in the allylic alcohol was also not rate-
limiting (Scheme 4h). The reaction of 1-phenylprop-2-en-1-ol
(2e) in the absence of benzoic acid resulted the mixture of enone
2ea and ketone (2eb), thereby indicating that the oxidation of
the allylic alcohol was the initial step (see the SI for details).
On the basis of the above mechanistic investigations and
previously reported literature,5−9,16 plausible reaction mecha-
nisms for the switchable ruthenium-catalyzed C−H functional-
ization of benzoic acids with allyl alcohols are outlined in
Scheme 5. The reaction begins with the generation of an active
Scheme 5. Proposed Mechanisms
Scheme 4. Reversibility and Kinetic Studies
Ru(II) catalyst, followed by coordination of the carbonyl oxygen
and subsequent proximal C−H activation to result in the
ruthenacycle A. After that, coordination of this ruthenacycle A to
the in situ generated enone and subsequent 1,2-migratory
insertion delivers the ruthenacycle C.10b These ruthenacycles A
and C′ were detected in APCI-HRMS. The ruthenacycles C and
C′ also indicate that the oxidation of allylic alcohol to generate
the enone is the initial step in the transformation (see the SI for
details). Facile β-hydrogen elimination from the intermediate C
produces D. This intermediate D undergoes an intramolecular
Michael reaction to furnish phthalides (Scheme 5 (I)). The
regeneration of the active catalyst by Cu(OAc)2·H2O and
AcOH completes the catalytic cycle. However, the presence of a
base may facilitate the tautomerization of C-bound keto
intermediate (C) to the O-bound enol intermediate (C′),
which can prevent the β-H elimination process and undergo
preferential protonation to deliver the alkylated product
(Scheme 5 (II)).8e,9a,b
To conclude, we have developed a controllable ruthenium-
catalyzed C−H functionalization of substituted aryl carboxylic
acids with allyl alcohols to access C−H alkylation products and
phthalides in a highly selective manner. Control over the reactive
organometallic intermediate enables the switch between the
hydroarylation and C−H alkenylation reaction pathways. This
dichotomy in product distribution is highly dependent on the
reaction conditions employed.
alkenylation reactions were carried out in the presence of D2O, a
significant amount of the deuterium incorporation at the ortho
positions of both the recovered starting materials as well as the
products was observed. The high levels of deuterium
incorporation indicate that the formation of the ruthenacycle
is a reversible reaction (Scheme 4a−d).
A significant amount of deuterium incorporation was also
observed at the α-methylene positions of the carbonyl group in
the case of the C−H alkylation process, thereby indicating the
formation of the stable ruthenium−oxa-π-allyl intermediate
(Scheme 4b). In the case of the C−H alkenylation pathway, 20%
deuterium incorporation was observed, and it can be attributed
to the fact that the alkenylated product formed through β-
hydride elimination immediately undergoes an intramolecular
Michael reaction (Scheme 4d). When the reaction was
performed with isotopically labeled benzoic acid, a fast H/D
scrambling was detected in the presence as well as in the absence
of coupling partner (Scheme 4e (see the SI)). In both of the
transformations, the kinetic isotopic effect was found to be
approximately 1.0. Based on KIE studies and H/D-scrambling
studies, it can be concluded that the C−H ruthenation step is
not the rate-limiting step (Scheme 4e−g).8a Another KIE study
also indicated that the cleavage of the C−H bond bearing the
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02064.
Experimental procedures and full spectroscopic data for
all new compounds (PDF)
C
DOI: 10.1021/acs.orglett.8b02064
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Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mk@iiserb.ac.in.
ORCID
Gangam Srikanth Kumar: 0000-0002-6052-9913
Manmohan Kapur: 0000-0003-2592-6726
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding from SERB-India (SB/S1/OC-10/2014) and CSIR-
India (02(205)/14/EMR-II) is gratefully acknowledged. G.S.K.
thanks CSIR-India for a research Fellowship. T.C. and D.S.
thank IISER Bhopal for research fellowships. We also thank the
Director, IISER Bhopal, for funding and infrastructural facilities.
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