Enhancing Ru(II)-Catalysis with Visible-Light-Mediated Dye-Sensitized TiO2 Photocatalysis for Oxidative C?H Olefination of Arene Carboxylic Acids at Room Temperature

Article abstract of DOI:10.1002/asia.201901718

Erythrosine B sensitized TiO₂ photocatalysis has been combined with Ru(II)-catalysis to accomplish an oxidative olefination/annulation of benzoic acids with activated olefins under mild conditions that tolerates useful functionalities, such as

Full text of DOI:10.1002/asia.201901718

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Enhancing Ru(II)-Catalysis with Visible-Light Mediated Dye-
Sensitized TiO2 Photocatalysis for Oxidative C–H Olefination of
Arene Carboxylic Acids at Room Temperature
Authors: Suman Dana, Purusattam Dey, Siddappa A. Patil, and
Mahiuddin Baidya
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Enhancing Ru(II)-Catalysis with Visible-Light Mediated Dye-Sensitized TiO2
Photocatalysis for Oxidative C–H Olefination of Arene Carboxylic Acids at
Room Temperature
[
a]
[a]
[b]
[a]
Suman Dana, Purusattam Dey, Siddappa A. Patil, and Mahiuddin Baidya*
Abstract: Erythrosine B sensitized TiO
2
photocatalysis has been
2
-ncy of TiO is maximum in the UV-region due to its large band gap
combined with Ru(II)-catalysis to accomplish an oxidative
olefination/annulation of benzoic acids with activated olefins under
mild conditions that tolerates useful functionalities such as halides,
free hydroxy, acetamido etc. The morphology of the photocatalyst is
unaffected during the reaction and it can be reused. Mechanistic
studies favor the involvement of a photo-induced single electron
transfer process.
(~3.2 eV), surface-modification with organic or inorganic dyes can
shift the light absorption from UV to visible region. Recently, this
manoeuvre has been perceived as a strong impetus to execute
organic
reactions
under
visible-light,
sustainable means for organic
synthesis. Nevertheless, to the best of our knowledge, dye-
sensitized TiO has been rarely explored for redox manipulation of
as
heterogeneous
photocatalysis embodies
a
[
9]
2
the active transition-metal catalyst in CH activation reactions.
Transition-metal catalyzed direct CH bond functionalization
reactions have reigned supreme as a sustainable complementary
approach to conventional synthetic processes due to their step- and
[
1-2]
atom-economy.
However, in most of the cases, stoichiometric
amounts of sacrificial oxidants are necessary for manipulating the
oxidation-state of the active transition-metal catalyst to enable the
[
3,4a-b]
crucial redox events in the catalytic cycle.
Recently, visible-light
photo-redox catalysis has emerged as an alternative catalytic mode
[
4]
of redox manipulation and the groups of Sanford, Rueping, and
others have showcased its utility as catalytic internal as well as
[
5,6]
terminal oxidants in CH bond activation reactions.
Generally,
transition-metal based polypyridyl complexes, due to their high redox
potentials and long excited-state life-times, are found effective to
accomplish redox manipulation with high precision. However, less
abundance and high cost of these catalysts are the major issues
towards their broad utility. Organic dyes are also found ineffective
[
5c,d,f,g,h]
and only sporadic examples are reported in this direction.
Thus, the introduction of readily available inexpensive photo-redox
catalysts with good chemical stability can fulfil the available synthetic
space in metallaphotoredox catalyzed CH bond activation reactions.
Meanwhile, metal-oxide semiconductors, such as TiO
2
, have
found widespread application in visible-light induced water-splitting,
[
7,8]
dye-degradation reactions, and synthetic transformations.
We
questioned whether TiO nanoparticles, which is well recognized for
2
its oxygen reduction properties, could be engaged as an efficient
photo-redox catalyst in metallaphotoredox catalyzed CH activation
reactions. The features of TiO
2
complements suitably with the
necessities of an ideal photocatalyst. It is a low-cost material with
high chemical stability and reusable. Although photoreduction efficie-
Scheme 1. Ru(II)/photo-redox catalyzed CH bond activation.
With our continuous interest in carboxylate-assisted Ru(II)-
[
10]
catalysis, we herein envisaged to demonstrate dye-sensitized TiO
2
[
[
a]
b]
S. Dana, P. Dey, Dr. M. Baidya
Department of Chemistry
Indian Institute of Technology Madras
Chennai 600 036, Tamil Nadu (India)
E-mail: mbaidya@iitm.ac.in
Dr. S. A. Patil
Centre for Nano and Material Sciences
Jain University
Ramanagara District - 562112, Bangalore Rural
Karnataka, India
as state-of-the-art photocatalytic oxidant for transition-metal
a
catalyzed CH bond activation reactions and considered the cross-
dehydrogenative olefination of arene carboxylic acids as a model
reaction, where dye-sensitized TiO2 can be utilized as a terminal
oxidant. Recently, Rueping et al. reported Weinreb amide directed
[
6a]
Rh(III)-catalyzed olefination of arenes
pyridine directed Ru(II)-catalyzed olefination of phenols
Ir(ppy) (bpy)PF (Scheme 1). During our investigation, the same
group has also disclosed that these reactions can also be achieved
using Ru(bpy)
3
(PF
6
)
2
and
[
6b]
using
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
2
6
1
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using metal-oxide semiconductor materials as photocatalytic
entry 1). Control experiments clearly indicated that all the
components, Ru(II)-catalyst, KOAc base, visible-light, and
[
6c]
oxidants.
never been examined in this scenario. We hypothesized that the dye
adsorbed on the surface of the TiO can absorb visible-light and
goes to its excited electronic state. The excited dye molecule can
donate one electron to the conduction band of TiO , forming a
However, weakly coordinating carboxylic acids have
2
erythrosine B-modified TiO photocatalyst, are crucial to achieve
2
high yield (entries 25). In the absence of photocatalyst, 3a was
isolated only in 44% yield (entry4). Similar diminished yield of 3a
2
[a]
Scheme 2. Scope of the oxidative annulation with olefins
radical cation of the dye. Then the electron in the conduction band
can be donated to oxygen molecule forming superoxide anion
radical (Scheme 1). Simultaneously, arene carboxylic acid can
undergo Ru(II)-catalyzed CH bond activation followed by migratory
insertion to generate intermediate B. After -hydride elimination, the
intermediate B delivers the vinylated product C and Ru(0) species.
Finally, Ru(II)-catalyst can be regenerated from Ru(0) species
through electron transfer with the superoxide anion radical and
cationic dye radical to continue the catalytic cycle.
To verify our hypothesis, we performed the bench-mark reaction
between commercially available 2-toluic acid (1a) and phenyl vinyl
sulfone (2a). Delightfully, when the mixture of 1a and 2a in methanol
[
a]
Table 1. Optimization of reaction conditions
entry
deviation from final conditions
none
3a (%)
84
1
2
3
4
5
6
7
8
9
without [Ru(p-cymene)Cl
2
]
2
–
without KOAc
trace
44
without ErB-TiO
without light (without PC)
with Eosin Y-TiO instead of ErB-TiO
2
[
b]
47(58)
2
2
65
DMF instead of MeOH
MeCN instead of MeOH
trace
21
b
2 2
Cu(OAc) .H O instead of KOAc
19
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7 W White LED instead of 23 W CFL
7 W Blue LED instead of 23 W CFL
58
91
90
45
38
29
36
TiO
only ErB (2.5 mol %)(without TiO
ZnO instead of TiO
BiVO instead of TiO
air instead of O balloon
2
(6 mg) and ErB (1 mol %)
2
)
2
4
2
[
a]
2
Reaction conditions:
cymene)Cl (5 mol %), ErB (1 mol %), TiO
mL), oxygen balloon under 7 W blue LED irradiation at room temperature;
1
(0.15 mmol, 1.0 equiv),
2 (2.0 equiv), [Ru(p-
]
2 2
2
(50 mol %, 6 mg), MeOH (0.5
[
a]
Reaction conditions: 1a (0.15 mmol, 1.0 equiv), 2a (2.0 equiv), [Ru(p-
cymene)Cl (5 mol %), KOAc (1.0 equiv), ErB-TiO (50 mol %, 6 mg), MeOH
0.5 mL), oxygen balloon at room temperature for 24 h. Reaction was
perfomed under reflux conditions. Catalytic turn over number (TON) based on
entry 11 = 1.8.
[
b]

Reactions were performed at 50 C with two 7 W blue LEDs. Unreacted
]
2 2
2
[
c]
[
b]
aromatic acids were recovered; Reaction time was 36 h and unreacted
aromatic acids were also recovered.
(
was obtained in the absence of light source even under reflux
conditions (entry 5). These experiments clearly demonstrate that
was irradiated under 23 W CFL at room temperature in presence of
[
Ru(p-cymene)Cl
2
]
2
(5 mol %), KOAc (1.0 equiv), and erythrosine B
ErB-TiO
responsible for the regeneration of the Ru(II)-active catalyst. When
Eosin Y-sensitized TiO was used, 3a was isolated in 65% yield
(entry 6). Change of reaction solvent from MeOH to DMF or MeCN
2
is the active photocatalyst (not ruthenacycle intermediates)
(
ErB) modified TiO
2
for 24 h, the oxidative Heck-coupling followed by
oxa-Michael addition proceeded smoothly and the desired
2
[
11]
phthalide product 3a was obtained in 84% isolated yield (Table 1,
2
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COMMUNICATION
and replacement of KOAc with Cu(OAc)
2
.H
2
O gave inferior results
inhibited the reactivity, suggesting the involvement of a single
electron transfer process (Scheme 4c). Intriguingly, in the case of
1,1-diphenylethylene, we also observed the formation of
benzophenone, which clearly signified the superoxide radical
formation during the process (Scheme 4d). Further, light on-off
experiment showed that the product formation enhances significantly
in the presence of light, while small changes in reaction yields in the
absence of light account for the slow background reaction (Scheme
4e).
(
entries 79). Altering the light source to white LED displayed a
deleterious effect, whereas blue LED provided improved yield of
1% (entries 1011). Interestingly, the use of TiO and ErB instead
of the preformed ErB-TiO was equally effective to promote the
9
2
2
transformation (entry 12). As expected, ErB alone did not show any
impact in the reaction outcome (entry 13) and the product yield was
similar to the reactions performed without light/photocatalyst (entries
4
45). Other semiconductor materials such as ZnO and BiVO were
inefficient to provide increased yields (entries 1415). The use of
ambient air instead of oxygen balloon also gave diminished yield of
the desired product (entry 16).
With the optimal conditions in hand, we next intended to explore
the breadth of substrate compatibility (Scheme 2). The protocol was
found quite general with a series of benzoic acids consisting of
electron-donating or -withdrawing substituents. Benzoic acids with
aryl and alkyl substitutions underwent smooth transformation,
producing 3b–e in 5584% yields. Reaction of electron-rich 4-
methoxy benzoic acid proceeded efficiently, bestowing annulated
product 3f in 68% yield. The oxidative annulation process was also
fruitful with various difunctionalized benzoic acids (3gk). However,
sterically hindered 3,5-disubstituted benzoic acid delivered the
desired product 3j in 36% yield. We were also able to synthesize the
phthalide of 2-naphthoic acid in decent yield (3l). Electron-
withdrawing halogen containing benzoic acids were viable
substrates (3mo), where sensitive bromo and iodo functionalities
remained untouched under the catalytic conditions. Delightfully,
catalytic conditions proved efficient in the presence of coordinating
free-hydroxy functionality in ortho- as well as para-positions and the
corresponding phthalides 3p and 3q were obtained in good yields,
o
albeit an increase of temperature to 50
C was necessary.
Acetamido functionality, a well-known directing group in CH bond
activation reactions, was also compatible to dispense 56% yield of 3r.
To this end, we attempted to delve into the consonance of other
coupling partners under the current conditions. Delightfully, ethyl
acrylate was also well-suited under the catalytic conditions,
[
12a]
furnishing 3su in good yields (Scheme 2).
The protocol was also
competent to perform an oxidative annulation between benzoic acid
and alkyne at room temperature to produce isocoumarin 5 in 76%
[
12b]
yield (Scheme 3).
Scheme 4. Mechanistic studies.
One prime advantage of heterogeneous photocatalyst over
homogeneous photocatalyst is its reusability. To establish the
reusable nature of the current photocatalyst, we first recovered the
Scheme 3. Ruthenium/photo-redox catalyzed oxidative annulation of 2-toluic
acid and alkyne.
TiO
2
nanoparticles after the reaction and then employed for
successive annulation reactions under standard conditions.
Gratifyingly, we observed almost similar outcome with the recycled
catalyst (Figure 1a, see SI for details). We have also compared the
morphology of the fresh catalyst with the recycled catalyst. Analysis
of FESEM pattern of both the catalysts implied that the morphology
of the catalyst was intact after the reaction (Figure 1b, see SI for
To understand the mode of action of the catalytic system, we next
performed a set of control experiments. Deuterium exchange
experiment disclosed that a reversible metalation process is involved
in this case (Scheme 4a). Kinetic isotope effect (KIE) studies
indicated that the metalation step is kinetically relevant with k
.7 (Scheme 4b). Addition of superstoichiometric amounts of radical
scavengers such as BHT and 1,1-diphenylethylene significantly
H D
/k =
2
[
13]
details).
3
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COMMUNICATION
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2
017, 19, 2111; (h) S. S. Shah, A. Paul, M. Bera, Y. Venkatesh, N.
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[
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a
olefination/annulation strategy between arene carboxylic acid and
olefins. The reaction is operationally simple and delivers diverse
sulfonated phthalides in good yields. The photocatalyst is
recoverable and reusable. Study of the recovered photocatalyst
indicated an identical morphology as with the initial catalyst. Further
use of such heterogenous photocatalysts for visible-light mediated
organic transformations are ongoing in our laboratory.
2
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Acknowledgements
We gratefully acknowledge SERB (CRG/2019/001003), India for the
financial support. S.D. acknowledges IIT Madras for providing HTRA
Fellowship. M.B. thanks IIT Madras for the support through
Exploratory Research Project (ERP) and Institute Research
Development Award (IRDA).
1
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Keywords: Heterogeneous Photocatalysis • Ru(II)-Catalysis •
Cooperative Catalysis • C-H Activation • Weak Coordination
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(
4
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[
[
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[
13] Details of FESEM studies have been provided in Supporting
Information (pages S8-10).
5
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Entry for the Table of Contents
A merger of visible-light heterogeneous photocatalysis with ruthenium catalysis is reported for C–H olefination/annulation of benzoic
acids and activated olefins at room temperature to forge functionalized phthalides.
Institute and/or researcher Twitter usernames: @MBaidyaGroup and @sc2dana
6
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