Half-Sandwich (η6-Benzene)Ruthenium(II) Complex of Picolyl Functionalized N-Heterocyclic Carbene as an Efficient Catalyst for Thioether Directed C?H Alkenylation of Arenes

Article abstract of DOI:10.1002/ejic.202100501

In this report, a half-sandwich (η⁶-benzene)Ru(II) complex of picolyl functionalized N-heterocyclic carbene was synthesized and efficiently used for the alkenylation arenes through thioether directed C?H bond activation. The thioether functionality of the substrate directed a selective ortho-vinylation through C?H bond activation. Moreover, reaction significantly works under mild reaction conditions than the previously reported ones which use the precious noble metal catalyst including the Pd(II), Rh(III), and Ir(III). Broad substrate scope using several benzyl thioethers and vinyl were found to be well tolerated for the present catalytic reaction to produces moderate to good yields of the desired products. Moreover, it is the first report where a Ru-based catalyst was used for the alkenylation of thioethers.

Full text of DOI:10.1002/ejic.202100501

European Journal of Inorganic Chemistry
Accepted Article
Title: Half-Sandwich (η6-Benzene)Ru(II) Complex of Picolyl Functional-
ized N-Heterocyclic Carbene as an Efficient Catalyst for
Thioether Directed C-H Alkenylation of Arenes
Authors: Sangeeta Kumari, Charu Sharma, Avinash K Srivastava,
Naveen Satrawala, Kamal N Sharma, and Raj Kumar Joshi
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100501
Link to VoR: https://doi.org/10.1002/ejic.202100501
01/2020

10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
Half-Sandwich (η6-Benzene)Ru(II) Complex of Picolyl Functional-
ized N-Heterocyclic Carbene as an Efficient Catalyst for Thioether
Directed C-H Alkenylation of Arenes
Sangeeta Kumari,+ Charu Sharma, Avinash K. Srivastava, Naveen Satrawala, Kamal N. Sharma,+ Raj
K. Joshi*
This manuscript is dedicated to Prof. Christian Bruneau, Institut des Sciences Chimiques - ISCR, Université de Rennes, France for
his contribution to catalysis.
Department of Chemistry, Malaviya National Institute of Technology, Jaipur – 302017, Rajasthan, INDIA. E-mail: rkjoshi.chy@mnit.ac.in
+ equal contribution
Supporting information for this article is given via a link at the end of the document. ((Please delete this text if not appropriate))
Abstract: In this report, a half-sandwich (η6-benzene)Ru(II) complex
of picolyl functionalized H-heterocyclic carbene was synthesized and
efficiently used for the alkenylation arenes through thioether directed
C-H bond activation. The thioether functionality of the substrate
directed a selective ortho-vinylation through C-H bond activation.
Moreover, reaction significantly works under mild reaction conditions
than the previously reported ones which use the precious noble
metal catalyst including the Pd(II), Rh(III), and Ir(III). Broad substrate
scope using several benzyl thioethers and vinyl were found to be
well tolerated for the present catalytic reaction to produces moderate
to good yields of the desired products. Moreover, it is the first report
where a Ru-based catalyst was used for the alkenylation of
thioethers.
Recently in 2019, Wang et. al. reported Ir catalyzed thioether-
directed mono and di-alkenylation of arene C-H bonds (Scheme
3) 26
.
Scheme 1. Pd catalyzed o-vinylation via C-H activation by Zhang et.al (2012)
Introduction
Scheme 2. Rh catalyzed ortho-vinylation via C-H activation by Shi et.al (2013)
C-H bond functionalization is a cost-effective and efficient
method for constructing complex molecules like natural or
pharmaceutical products, 1 which has attracted much attention in
recent years.2-4 Implementation of a directing group on the
substrate is an effective and robust approach for enhancing the
efficiency and regulating the selectivity of C-H activations.5
Literature indicates that Nitrogen6-8
,
Oxygen9-11, and
Scheme 3. Ir catalyzed mono and di-vinylation via C-H activation by Wang
et.al (2019)
Phosphorous based ortho directing groups are quite common
and frequently used, but, the sulfur base directing group has
received very less attention, despite the fact that sulfur-
containing moieties are building blocks for a large number of
drug compounds those are frequently used for pharmaceutical
or agrochemical applications.12-13 As per a recent revelation,
thioethers are an effective directing group for C-H alkenylation,
however, the use of thioether as a directing group is challenging
due to the tendency of poisoning of the transition metals.
Nevertheless, it is worthy to use thioether as a directing group,
as under reductive conditions these groups can be easily
removed.14 Furthermore, thioether could go through a variety of
useful transformations15-17, sulphur is also found in various
useful molecules including functional material, organocatalyst18-
These reports include the utilization of precious noble metal and
expensive additives, hence, a method that consists of much
economical metal catalyst and works under mild reaction
conditions for a range of substrates is strongly desired for the
functionalization of thioethers. Knowing the high catalytic
potential of Ru complexes27, and in the mindset to search a
feasible methodology, a half-sandwich Ru(II) complex of picolyl-
functionalized N-heterocyclic carbene was synthesized and
catalytically explored for the thioether directed C-H alkenylation
of arenes (Scheme 4). The Ru-catalyst along with copper
acetate (co-catalyst) is efficiently working at the mild conditions
for broad varieties of substrates. To the best of our knowledge,
the use of relatively inexpensive Ru(II) complexes for the C-H
alkenylation of arenes with benzyl thioethers using sulfur as a
directing group has not been reported so far. As a result, the
21,
and water reduction catalyst.22-23 Zhang and co-workers
reported a Pd catalyzed C-H bond alkenylation of thioethers in
2012 (Scheme 1).24 Furthermore, Shi reported an Rh catalyst for
C-H olefination of benzyl thioether (Scheme 2) in 2013.25
1
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10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
ultimate focus of the present research is to make our approach
more effective for late-stage activation while keeping it more
economical.
the carbons of pyridine and benzimidazole rings have been
appeared deshielded as compare to the other carbon atoms of
the complex, which further confirm the complexation of the
ligand with Ru(II). The presence of Ru in the proximity induces a
1
Present Work
high magnitude shift in the deshielded region in both H and 13
C
NMR.
Crystallographic analysis of Complex (1).
Complex 1 was found to be soluble in CH3CN, DMF, and DMSO
while sparingly soluble in CH2Cl2, CH3Cl, and CH3OH and
insoluble in diethyl ether and n-hexane. The single crystal
suitable for X-ray crystallographic analysis were grown in
CH3CN: CH3OH (3:1) solution under ambient conditions. The
analysis of crystallographic data resulted in the following
molecular structures of complex 1 (Figure 1). The bidentate
coordination mode of the ligand through carbene and nitrogen of
the pyridine resulted in a six-membered chelate ring with Ru(II)
metal center. The complex form a pseudo-octahedral half-
sandwich “piano stool” geometry at Ru center. The Ru−N bond
distance was observed at 2.119(4) Å, which is in close
agreement to the reported Ru(II) complex of picolyl
functionalized NHC.28 Key crystallographic distances and bond
angles are mentioned in Figure 1 and the summary of
crystallographic data and structural refinements are outlined in
Table S1 (supporting information).
Scheme 4. Ru-catalyzed thioether directed C-H vinylation of arenes.
Results and Discussion
Design, synthesis, and characterization of a Half-Sandwich
(η6-Benzene)Ru(II) Complex of Picolyl-Functionalized N-
Heterocyclic Carbene (1):
The methodology used for the synthesis of the Ru-complex (1)
is depicted in Scheme 5. 1-(2-Chloroethyl)-1H-benzimidazole (a)
and
1-(2-(phenylthio)ethyl)-1H-benzimidazole
(b)
were
synthesized by using a previously reported procedure.27
Scheme 5. Synthesis of picolyl/thioether functionalized benzimidazolium
bromide salt (c) and half-sandwich (η6-benzene)Ru(II) complex (1)
In the next step, the atom economy reaction of (2-
Figure 1. Molecular structure diagram (thermal ellipsoids at 30%
probability level) of half-sandwich (η6-benzene)Ru(II) complex (1);
Hexafluorophosphate anion and hydrogen atoms are not shown for
the sake of clarity; Key crystallographic distances [Å]: Ru(1)−C(1)NHC
2.034(5), Ru(1)−N(3) 2.119(4), Ru(1)−Cl(1) 2.4309(12); Bond angles
[°] N(3)−Ru(1)−Cl(1) 85.72(12), C(1)−Ru(1)−Cl(1) 88.73(13),
C(1)−Ru(1)−N(3) 84.24(18), N(1)−C(1)−Ru(1) 132.4(3),
N(2)−C(1)−Ru(1) 121.7(3), N(1)−C(1)−N(2) 105.8(4).[CCDC
2085434]
bromomethyl)pyridine
with
1-(2-(phenylthio)ethyl)-1H-
benzimidazole resulted in N-(1-thiophenyl-2-ethyl)-N'-(pyridine-
2-ylmethyl)benzimidazolium bromide (c). Thereafter, the Ru(II)
complex was synthesized by employing a silver carbene transfer
reaction in which c was first treated in CH2Cl2 with Ag2O in a N2
atmosphere at room temperature (in dark), followed by the
addition of a methanol suspension of [ƞ6(C6H6)RuCl(μ-Cl)]2. The
orange solid 1 was found to be stable and thermally robust. A
1
singlet observed at 11.02 ppm in the H NMR of c is related to
Catalytic investigation of complex 1 for thiol directed C-H
alkenylation of arenes:
the highly deshielded proton of precarbene carbon. The absence
of this signal in the 1H NMR spectrum of
1 suggests
The study started with the reaction of benzyl phenyl sulfane and
styrene in presence of RuCl3 or Ru3CO12 (Table 1, Entry 1,2) in
dichloroethane at 120°C, but no transformation was obtained.
The desired o-vinylation was observed when synthesized Ru(II)
complex (1) was used as a catalyst with Cu(OAc)2 as an oxidant
(Table 1, Entry 3-6). Other oxidants including the AgSbF6, TBHP,
TFA, AcOH were also screened in search of better productivity,
but none of them works for the reaction. The Cu(OAc)2 was
found to be an essential requisite for the reaction, as the
reaction under the complete absence of Cu(OAc)2 failed to
deprotonation of carbene carbon due to the coordination with
Ru(II). Furthermore, in comparison to the free ligand (c-HBr), the
coordination of ligand with Ru(II) produces deshielded NMR
signals. The two protons of each methylene group (C-8 and C-9)
are not equivalent (anisochronous), even they are not part of a
1
chelate ring, therefore, appeared as four multiplets in H NMR.
Due to the diastereotopic nature of C-16 methylene protons
which are part of the six-membered chelate ring, appeared as
two doublets in the proton NMR of 1. In the 13C NMR, C-16, and
2
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10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
produce the desired product. Moreover, 0.5 mmol of the
Cu(OAc)2 which bring 59% transformation was found to be an
ideal amount for the reaction, and increasing the quantity of
Cu(OAc)2 to 1 mmol and 1.5 mmol reduces yields to 49%, and
38%, respectively (Table 1, Entry 7-9). During the Ru-catalyst
(1) optimization, it was observed that a blank reaction does not
form the desired product (Table 1, entry 10), which indicates the
role of the Ru-catalyst in facilitating the present transformation.
During the optimizations of catalyst amount, it was noted that
increasing the amount of catalyst from 2 to 4 mol%, the yield of
desire product significantly increased from 23 to 59%
respectively, while marginal progress in yield (62% yield) was
observed with 6 mol% of catalyst (Table 1, entry 7 and 11-12).
Moving further to solvent optimization, below to average yield
(28%) was obtained in absence of solvent, but, surprising to us,
it decreased to 18% when toluene was used as a solvent. Other
solvents such as ethyl acetate, hexane, 1,4- dioxane, and water
were also failed to mimic the transformations (Entry 13-18).
The dichloroethane was the only solvent that shows a 59%
transformation with 4 mol% catalysts loading. Temperature
optimizations concluded that product formation was started at
80°C and the yield of the product was continues increases until
the reaction temperature reaches 120°C (Entry 19-20). The
product formation was observed in the initial 6h, but it takes
another 6 h to reach the maximum and no further significant
changes in the yield were obtained beyond 24h (Table 1, Entry
21).
Substrate Scope. After having the best-optimized reaction
conditions, the general scope of o-vinylation of arenes was
investigated (Table 2). The reaction of benzyl(phenyl)sulfane
and styrene produces the desired product in a good amount (2a,
59%). Whereas, a slight variation in the transformation was
observed when the electronic nature of the functional group
attached with styrene was varied. In the case of the electron-
donating group (p-methylstyrene) yield was slightly reduced
compared to the styrene containing an electron-withdrawing
group (2-bromostyrene) [2b and 2c].
Table 1. Optimization Table
Table 2. Substrate scope with respective isolated yields of
derivative of benzyl (phenyl) sulfane with various styrene’s.
S.
No.
Ru(II) Cat.
(mol%)
Cu(OAc)2
(mmol)
Temp
(°C)
Time
(h)
Yielda
(%)
Solvent
1.
2.
RuCl3 (4)
0.5
0.5
DCE
DCE
120
120
24
24
--
--
Ru3CO12
(4)
AgSbF6
(0.5)
3.
4.
5.
6.
4
4
4
4
DCE
DCE
DCE
DCE
120
120
120
120
24
24
24
24
--
--
--
--
TBHP
(0.5)
Acetic acid
(0.5)
TFA
(0.5)
7.
4
4
4
--
2
6
4
4
4
4
4
4
4
4
4
0.5
1
DCE
DCE
120
120
120
120
120
120
120
120
120
120
120
120
80
12
24
24
24
12
12
12
12
12
12
12
12
12
12
24
59
48
39
--
8.
9.
1.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
DCE
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
DCE
DCE
23
62
28
18
--
DCE
--
Toluene
EtOAc
Hexane
Water
Dioxane
DCE
--
--
trace
29
40
61
*Isolated yields, Condition. sulfane (0.5 Equiv), Styrene (1 Equiv), Cu(OAc)2
(0.5 mmol) , Ru Catalyst (4 mol%), Solvent: DCE, Temp 1200C, 12h
DCE
100
120
Furthermore, a slight variation in the transformation was also
experienced when the position of the functional group switched
from ortho- to meta- or para- position. In the case of chloro
substituted styrene, the highest 58% transformation was
DCE
aIsolated Yields, Condition. Benzylsulfane (0.5 Equiv), Styrene (1 Equiv)
3
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10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
obtained with p-chlorostyrene, and it reduced to 55% and 51%
with changing the substitution to meta- and ortho- position
respectively [2d-2f]. Functional groups present on benzyl group
also alter the rate of transformation and a good yield of 57% was
obtained for methyl substitution (2g), while 44% yield was
obtained for fluoro substitution (2h), and quite similar trends of
reactivity were noted for m-fluoro benzyl(phenyl) sulfane [2i]. A
significant coupling product (2j) was recorded by the reaction of
o-methylbenzyl(phenyl)sulfane and p-methylstyrene.
The substrate scope of the present alkenylation reaction was
further explored for acrylates and good yield of desire product
obtained with various benzyl(phenyl)sulfane and acrylates
(Table 3). A 64 % transformation was recorded by the reaction
of benzyl(phenyl)sulfane and methyl acrylate, and it increased to
67% with increasing the chain length of acrylates (3a-3c). All
three acrylates were found to be well-tolerated, and provide
fairly good yields, however, a slightly reduced transformation
was recorded for o-methylbenzyl(phenyl)sulfane [3d-3f]. While
examining the effect of o-flourobenzyl(phenyl)sulfane and m-
flourobenzyl(phenyl)sulfane with methyl acrylate, almost similar
transformations were obtained [3g and 3h]. Here, we found that
acrylate is a much compatible coupling partner for alkenylation
in comparison to styrene.
transformations of 71% and 73% (4c and 4d) respectively. A
highly rigid chloroalkyl functionalized 2-(benzylthio)aniline was
also investigated and it was found significantly reactive with
methylacrylate to form a good amount of desired alkenylated
product (58%, 4e). Here, the formation of 4e is quite
unprecedented due to dual transformation in a single step, the
C-H activations followed by C-N coupling or vice versa could be
the possible steps. Here, solvent dichloromethane participated in
the reaction to form the CN bond by the loss of a Cl- while
interacting with NH2.
Table 4. Substrate scope of various thioethers with various
acrylate’s
Table 3. Substrate scope of benzyl(phenyl)sulfane with
acrylate’s
*Isolated Yields, Condition. sulfane (0.5 Equiv), Acrylates (1 Equiv), Cu(OAc)2
(0.5 mmol) , Ru Catalyst (4 mol%), Solvent: DCE, Temp 1200C, 12h
Control Experiments.
Intramolecular competitive experiments were carried out to
evaluate the selectivity towards various alkenes and benzyl
thioethers. In this sequence, the very first experiment was
performed with a mixture of 2-flourobenzyl(phenyl)sulfane and 2-
methylbenzyl(phenyl)sulfane, however, both the sulfane formed
the desired product in almost equal amount, but a little more
selectivity was shown by 2- methylbenzyl(phenyl)sulfane
(Scheme 6). Furthermore, the selectivity trends were retained
when investigating the reactivity of aromatic/aliphatic substituted
sulfane towards the styrene (Scheme 7).
*Isolated Yields, Condition. sulfane (0.5 Equiv), Acrylates (1 Equiv), Cu(OAc)2
(0.5 mmol) , Ru Catalyst (4 mol%), Solvent: DCE, Temp 1200C, 12h
Scheme 6. Selectivity between 2-flouro/2-methylbenzyl(phenyl)sulfane with
styrene
The general scope of the reaction was further extended and
some new benzylsulfanes consisting of the aliphatic and
substituted arenes were synthesized by the reaction of benzyl
bromide with and thiol derivatives (Table 4). The
benzyl(ethyl)sulfane and methyl/s produced fairly good yields of
68%
and
70%
(4a
and
4b).
Moreover,
benzyl(cyclopentyl)sulfane was also found to be well tolerated
with methyl/butylacrylates and shows highly significant
4
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10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
Scheme
7.
Selectivity
between
benzyl(phenyl)sulfane
and
Conclusion
benzyl(ethyl)sulfane with styrene
In summary, we had designed, synthesized, and characterized a
Ru(II)-NHC half pincer complex and explored the catalytic
efficiency of this complex for alkenylations of arenes. Here, Ru
complex and Cu(OAc)2 enable the ortho-vinylation of thiol-
directed functionalized arenes. The Ru and Cu show excellent
functional group tolerance on either of the reactants and show
significant S-directed ortho-vinylation via C-H activations of
activated styrenes and acrylates. Good selectivity was obtained
for acrylates and benzyl(cyclopentyl)sulfane.
A significant change in the reactivity was experienced when
different alkenes were investigated with benzyl(phenyl)sulfane,
and
a
reasonably good selectivity was obtained for
methylacrylate (Scheme 8). Furthermore, the mixture of
benzyl(cyclopentyl)sulfane and benzyl(ethyl)sulfane, and styrene
shows slightly better selectivity towards benzyl(cyclopentyl)
sulfane (Scheme 9).
Supporting information
Experimental details, spectral data, all NMR spectrams and
crystal and refinement data along with CIF of complex 1 (CCDC
2085434), are provided as supporting information.
Scheme 8. Selectivity between acrylate and styrene
Acknowledgments
RKJ thanks CSIR for research funding [01(2996)/19/EMR-II]. SK
and CS thank MNIT for the research fellowships. Authors
acknowledge MNIT-MRC for characterization facilities.
Scheme
benzyl(ethyl)sulfane
9.
Selectivity
between
benzyl(cyclopentyl)sulfane
and
Plausible Mechanism. A plausible mechanism (Scheme 10) has
been formulated on the basis of available literature.26,29-32 The
catalytic process may be initiate by the loss of Cl from Ru-
complex (1) to forms an active catalytic species (a). Thioether
chelated with (a) and undergoes ortho- metalation–
deprotonation at the benzyl ring and resulted in a five-membered
ruthenacycle intermediate (c). The coordinative insertion of an
olefin into Ru-C metallacycle (d) forms an intermediate (e).
Finally, the β-hydride elimination in presence of Cu(OAc)2
resulted in the desire o-vinylated product and regenerates the
active ruthenium species for the next catalytic cycle.
Keywords: C-H Activation, O-alkenylation, Picolyl NHC
Thioether Ligand, Ru(II) Half Pincer Complex, S-Directing
Alkenylation
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10.1002/ejic.202100501
European Journal of Inorganic Chemistry
COMMUNICATION
Entry for the Table of Contents
Picolyl Functionalized N-Heterocyclic Carbene Thioether ligand-based Novel Half sandwich pincer (η6-Benzene)Ru(II) Complex was
synthesized. The prepared Ru(II) complex in presence of Cu(OAc)2 was identified as an excellent catalyst for O-Alkenylation via CH
activation through the S-directing group. Transformations under Mild reaction condition and tolerance for a wide range of functional
groups was observed.
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This article is protected by copyright. All rights reserved.