The synthesis and structure of pyridine-oxadiazole iridium complexes and catalytic applications: Non-coordinating-anion-tuned selective C–N bond formation

Article abstract of DOI:10.1016/j.cclet.2019.08.049

Several novel pyridine-oxadiazole iridium complexes were synthesized and characterized through X-ray crystallography. The designed iridium complexes revealed surprisingly high catalytic activity in C–N bondformation of amides and benzyl alcohols with the assistance of non-coordinating anions. In an attempt to achieve borrowing hydrogen reactions of amides with benzyl alcohols, N,N'-(phenylmethylene)dibenzamide products were unexpectedly isolated under non-coordinating anion conditions, whereas N-benzylbenzamide products were achieved in the absence of non-coordinating anions. The mechanism explorations excluded the possibility of “silver effect” (silver-assisted or bimetallic catalysis) and revealed that the reactivity of iridium catalyst was varied by non-coordinating anions. This work provided a convenient and useful methodology that allowed the iridium complex to be a chemoselective catalyst and demonstrated the first example of non-coordinating-anion-tuned selective C–N bond formation

Full text of DOI:10.1016/j.cclet.2019.08.049

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CCLET 5200 No. of Pages 5
Chinese Chemical Letters xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
The synthesis and structure of pyridine-oxadiazole iridium complexes
and catalytic applications: Non-coordinating-anion-tuned selective
CÀÀN bond formation
Wei Yaoa, Yilin Zhangb, Haiyan Zhua, Chenyang Gea, Dawei Wanga,
*
a
Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
China
b
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, WV 26506, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 4 July 2019
Received in revised form 10 August 2019
Accepted 27 August 2019
Available online xxx
Several novel pyridine-oxadiazole iridium complexes were synthesized and characterized through X-ray
crystallography. The designed iridium complexes revealed surprisingly high catalytic activity in CÀÀN
bondformation of amides and benzyl alcohols with the assistance of non-coordinating anions. In an
attempt to achieve borrowing hydrogen reactions of amides with benzyl alcohols, N,N'-(phenyl-
methylene)dibenzamide products were unexpectedly isolated under non-coordinating anion conditions,
whereas N-benzylbenzamide products were achieved in the absence of non-coordinating anions. The
mechanism explorations excluded the possibility of "silver effect" (silver-assisted or bimetallic catalysis)
and revealed that the reactivity of iridium catalyst was varied by non-coordinating anions. This work
provided a convenient and useful methodology that allowed the iridium complex to be a chemoselective
catalyst and demonstrated the first example of non-coordinating-anion-tuned selective CÀÀN bond
formation
Keywords:
Borrowing hydrogen
Dehydrogenation
Non-coordinating anion
Iridium
Bisamides
© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Gem-bisamides, which are bearing aminal functional groups,
are found in a wide variety of synthetic intermediates and natural
products. Many pharmaceutical compounds based upon the gem-
bisamides have exhibited characteristic bioactivities, for instance,
hypertension in peptidomimetic structure [1]. Methylidene
bisamide is another typical example and could be served as CB2
receptor inverse agonist. These compounds served as inverse
agonists with a significant CB1/CB2 selectivity (Ki(CB1)/Ki(CB2))
up to 235 folds. Similar molecule could show the osteoclast
achieve high atom efficiency in organic chemistry [4,5]. It has been
well known that catalysts play an important role during these
processes [6,7]. Several groups have made great efforts and
achieved significant progress in borrowing hydrogen area during
the past [8,9]. However, borrowing hydrogen and dehydrogenation
reactions with traditional catalysts remain infeasible for the
synthesis of gem-bisamides bearing aminal functional groups [10].
Our interest in developing new ligands to adjust catalytic
activity in borrowing hydrogen and dehydrogenation reactions has
led to the recent discoveries of copper, gold, ruthenium, iridium
catalysts [11]. However, when these triazole-based metal com-
plexes were employed as catalyst in the synthesis of gem-bisamide
derivatives from benzamide, it was disappointing that no desired
product was obtained, which might be caused by strong electron
withdrawing and coordinating properties of these triazoles. Since
it was known that benzamide is less reactive in borrowing
hydrogen strategy than conventional amine, the development of
novel and new catalysts for the preparation of gem-bisamide
derivatives are apparently highly necessary and desirable
(Scheme 1). Herein, we reported the synthesis and character-
izations of several new pyridine-oxadiazole iridium complexes,
which revealed excellent catalytic activity in CÀÀN bond formation
of benzamides with benzyl alcohols. The structures of these
formation with the IC72 of 0.1 mmol/L [2]. Despite that
gem-bisamides have important therapeutic use, to date, methods
achieving efficient synthesis of gem-bisamides are limited, which
are usually conducted employing various aldehydes in the
presence of strong Lewis acids or bases (sulphuric acid, SnCl4,
etc.) under harsh conditions. Development of efficient method for
gem-bisamides synthesis utilizing green reagents under mild
conditions has, therefore, remained an important challenge [3].
During the past several years, borrowing hydrogen and
dehydrogenation reactions have been recognized as mild tool to
* Corresponding author.
E-mail address: wangdw@jiangnan.edu.cn (D. Wang).
https://doi.org/10.1016/j.cclet.2019.08.049
1001-8417/© 2019 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: W. Yao, et al., The synthesis and structure of pyridine-oxadiazole iridium complexes and catalytic
applications: Non-coordinating-anion-tuned selective CÀÀN bond formation, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.
cclet.2019.08.049

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Scheme 1. Selective CÀÀN bond formation tuning by non-coordinating anions
through dual catalysis.
complexes have been confirmed by X-ray crystallography. The
mechanism explorations proved that the reactivity of iridium
catalyst was varied by non-coordinating anions.
Firstly, 2-picolinyl hydrazide was mixed with CS2 and KOH and
refluxed in EtOH for 11 h to achieve 5-(pyridin-2-yl)-1,3,
4-oxadiazole-2-thiol L1. After arylation of L1, Ir complexes were
successfully prepared in high yields by mixing [Cp*IrCl2]2 and
corresponding ligands L2 in a 1:2.1 molar ratio in MeOH at room
temperature under N2 atmosphere for 2 h. The exact structure of
iridium complex was confirmed by single-crystal X-ray diffraction
(Scheme 2).
Scheme 3. Reaction of benzamides with benzyl alcohols. Conditions: 2 (1.0 mmol),
3 (1.1 mmol), catalyst (1.0 mol%), Cs2CO3 (1.0 equiv.), toluene (1.5 mL), 12 h. Isolated
product.
With these iridium(III) complexes (1aÀ1c) in hand, their
catalytic activities in the borrowing hydrogen reaction of
benzamide with benzyl alcohol were subsequently examined.
Preliminary results showed that the reaction could proceed
smoothly and the desired product could be successfully isolated.
Table 1
The effect of additive on this reaction.a
After
a series of reaction conditions screening (Supporting
information for details), toluene and cesium carbonate were
proven to be the most efficient medium and base respectively. As
shown in Scheme 3, a wide range of functional groups or
substituents including methyl, methoxy, chloro, bromine, and
heterocycle at different positions of benzyl alcoholwere tolerated,
affording the target products in moderate to good yields. Similarly,
a variety of benzamides have shown good activity in this reaction.
N,N'-(Phenylmethylene)dibenzamides were another ubiqui-
tous class of heterocyclic derivatives and widely distributed in
alkaloids and natural products. They have demonstrated excellent
biological and pharmaceutical properties during the past decade.
Due to their significant applications, we next dedicated to achieve
the synthesis of N,N'-(phenylmethylene)dibenzamides utilizing
this catalytic system. A series of initial attempts were unable to
produce the desired dibenzamide (5a) with satisfied yields
(Table 1, entries 1–4). To our delight, when 1 equiv. of AgOTf
was employed as additive to this catalytic system, the dibenza-
mide was obtained in 23% yield (entry 5). Further investigations
revealed that several silver salts could effectively promote this
transformation. Notably, silver hexafluoroantimonate produced
the highest yield even with catalytic amount (2 mol%) loaded
(entry 10). However, the control experiment revealed that product
5a couldn’t be obtained when only silver additive was employed
Entry
Catalyst
Additive (equiv.)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
–
–
12
12
12
12
12
12
12
12
12
24
24
24
24
24
24
24
<5
<5
<5
<5
23
39
58
64
76
81
<5
<5
72c
<5d
73
K2CO3 (1.0)
KOH (1.0)
AgNO3 (1.0)
AgOTf (1.0)
AgNTf2 (1.0)
AgBF4 (1.0)
AgSbF6 (1.0)
AgSbF6 (0.1)
AgSbF6 (0.02)
AgSbF6 (0.02)
AgSbF6 (1.0)
AgSbF6 (0.02)
AgSbF6 (0.02)
AgSbF6 (0.02)
AgSbF6 (0.02)
9
10
11
12
13
14
15
16
–
1a
1a
1b
1c
68
a
Conditions: 2a (2.5 mmol), 3a (1.0 mmol), catalyst (1.0 mol%), additive
(0.02–1.0 mmol), toluene (2.5 mL).
b
Isolated product.
c
110 ꢀC.
d
refluxed in water.
(entries 11 and 12). It should be noted the reaction couldn’t take
place in water (entry 14).
Next, the reaction was applied in the synthesis of various N,
N'-(phenylmethylene)dibenzamides by the use of bimolecular
benzamide under oxygen conditions (Scheme 4). Obviously,
relatively high yields were obtained when benzyl alcohol with
para-substituted groups was employed, mainly because less steric
hindrance exists. Among those, substrate with electron donating
group gave higher yield. Interestingly, the reaction scope was also
successfully expanded to different substituted benzamide and
thienyl group, producing good yields.
To gain insights into the possible mechanism, several control
experiments were conducted. Initial studydisclosed that electron-
rich benzamide was preferentially consumed, which revealed-
thatbenzamidewith electron-donating group was much more
Scheme 2. The synthesis and X-ray of Ir catalysts.
Please cite this article in press as: W. Yao, et al., The synthesis and structure of pyridine-oxadiazole iridium complexes and catalytic
applications: Non-coordinating-anion-tuned selective CÀÀN bond formation, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.
cclet.2019.08.049

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3
Scheme 4. The reaction of benzamides with benzyl alcohols. Conditions:
(2.5 mmol), (1.0 mmol), 1a (1.0 mol%), AgSbF6 (2.0 mol%), toluene (2.5 mL).
Isolated product.
2
3
Scheme 5. The possible mechanism investigations.
Fig. 1. Hammett plot equation and kinetic plot studies.
reactive than that with electron-withdrawing group (Scheme 5A).
Subsequent efforts were made on trapping of key intermediate,
one of commonly used method to learn reaction pathway.
Surprisingly, benzaldehyde intermediate (3a') was detected during
this transformation, which was also detected by Massspectrom-
etry. Further exploration revealed that gem-bisamide (5a) could be
synthesized frombenzaldehyde (Scheme 5B), which could explain
the proposed mechanism. Additionally, we found that the gem-
bisamide could be converted to N-benzylbenzamide product
(Scheme 5C), which was consistent with previous result [10e].
Meanwhile, the Hammett plot equation was carried out to
further understand this transformation. Several substrates were
selected to build the equation and the plot was shown in Fig. 1. The
results revealed that the electronic effect had significant impact on
this reaction and a positive charge at the reaction center was
favorable in the transition state.
converted to N-benzylbenzamide with this iridium catalytic
system.
In the past several years, "silver effect" was found in silver-
involved transformations, which was a long-overlooked phenom-
enon [12]. It was disclosed that reactions involving silver are in fact
accompanied with silver-assisted metal or bimetallic catalysis.
Recently, several studies on "silver effect" were conducted by our
group and some preliminary results were acquired [13]. Here,
"silver effect" was also investigated for its impact on this reaction
system.
As shown in Table 2, experiments for silver salts effect was
conducted accordingly. Catalytic system (1a/AgSbF6) with silver
chloride salt removed by celite can also afford the gem-bisamide
only with slightly lower yield. Similar results were obtained
with other silver additives (Table 2). These experiments disclosed
that non-coordinating anions promoted the synthesis of N,
N'-(phenylmethylene)dibenzamides from benzamides with benzyl
alcohols, while silver salts did not play any role during this
process. This was another typical example for the effect of non-
coordinating anions and highlighted the application of non-
coordinating anions.
In addition, we carried out an intermolecular competition
reaction on the kinetically relevant elementary steps. The kinetic
isotope effect (KIE) value (2.26) was achieved through the first
order reaction plot between ln[3a] and ln[3a-d2], providing a clear
evidence that dehydrogenation of benzylic alcohol was the rate
determining step during this transformation (Fig. 1).
To further elucidate the reaction mechanism, kinetic inves-
tigations was conducted, which revealed that benzaldehyde is
the most convincing reaction intermediate (Fig. 2). Further
research proved that N,N'-(phenylmethylene)dibenzamide can be
In summary, we have accomplished the synthesis of pyridine-
oxadiazole iridium complexes and characterized the compounds
with X-ray crystallography. The obtained Ir catalyst was
successfully applied to CÀÀN bond formation of amides with
Please cite this article in press as: W. Yao, et al., The synthesis and structure of pyridine-oxadiazole iridium complexes and catalytic
applications: Non-coordinating-anion-tuned selective CÀÀN bond formation, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.
cclet.2019.08.049

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W. Yao et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
21861039), the Fundamental Research Funds for the Central
Universities (No. JUSRP 51627B), and the Ministry of Education
and the State Administration of Foreign Experts Affairs for the 111
Project (No. B13025).
Appendix A. Supplementary data
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at doi:https://doi.org/10.1016/j.cclet.2019.08.049.
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Please cite this article in press as: W. Yao, et al., The synthesis and structure of pyridine-oxadiazole iridium complexes and catalytic
applications: Non-coordinating-anion-tuned selective CÀÀN bond formation, Chin. Chem. Lett. (2019), https://doi.org/10.1016/j.
cclet.2019.08.049