Synthesis and characterization of N-heterocyclic carbene–PdCl2–1H-benzotriazole complexes and their catalytic activities toward Mizoroki–Heck and Sonogashira reactions

Article abstract of DOI:10.1080/00958972.2016.1272676

Four heteroleptic palladium complexes containing both N-heterocyclic carbenes and 1H-benzotriazole were synthesized and characterized. The solid-state structures show mononuclear carbene palladium complexes with each palladium coordinated by an NHC, the 3-position nitrogen of 1H-benzotriazole and two trans chlorides. The catalytic performance of the complexes for Mizoroki–Heck and Sonogashira reactions were further investigated. The results reveal that the complexes show high catalytic activities for coupling of aryl bromides with alkenes and alkynes.

Full text of DOI:10.1080/00958972.2016.1272676

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis and characterization of N-heterocyclic
carbene–PdCl2–1H-benzotriazole complexes and
their catalytic activity toward Mizoroki-Heck
and Sonogashira reaction
Jin Yang
To cite this article: Jin Yang (2016): Synthesis and characterization of N-heterocyclic
carbene–PdCl2–1H-benzotriazole complexes and their catalytic activity toward
Mizoroki-Heck and Sonogashira reaction, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1272676
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1272676
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Date: 20 December 2016, At: 20:01

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1272676
Synthesis and characterization of N-heterocyclic
carbene–PdCl2–1H-benzotriazole complexes and their catalytic activity toward
Mizoroki-Heck and Sonogashira reaction
JIN YANG*
Department of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P R China
Four heteroleptic palladium complexes containing both N-heterocyclic carbenes and
1H-benzotriazole were synthesized and characterized. The solid-state structures show
mononuclear carbene palladium complexes with each palladium coordinated by an NHC, the
3-position nitrogen of 1H-benzotriazole and two trans chlorides. The catalytic performance of
the complexes for Mizoroki–Heck and Sonogashira reactions were further investigated. The
results reveal that the complexes show high catalytic activities for coupling of aryl bromides with
alkenes and alkynes.
Keywords: N-Heterocyclic carbene; 1H-Benzotriazole; Palladium complexes; Mizoroki–Heck
reaction; Sonogashira reaction
1. Introduction
Palladium-catalyzed cross-coupling is one of the most powerful methods for C–C bond
construction [1]. Considerable efforts have been made to develop more active catalysts for
Suzuki-Miyaura, Mizoroki–Heck and Sonogashira reactions [2, 3]. In order to achieve high
efficiency of the reaction, air-sensitive, toxic and expensive phosphine ligands are usually
*Corresponding author. Email: yangjinlz@163.com

introduced to facilitate the reaction [4, 5]. As an alternative, N-heterocyclic carbene (NHC) has
been widely used in palladium-catalyzed coupling reactions. The NHC possesses the advantage
of being stronger σ-donor and weaker π-acceptor than phosphine ligands, which can increase the
stability and catalytic activity of the complexes. Since the initial presentation of NHC-Pd
catalysts for the Mizoroki–Heck reaction by Herrmann in 1995 [6], enormous effort has been
devoted to the development of new NHCs and their palladium complexes for the Mizoroki–Heck
reaction [7-12]. On the other hand, Sonogashira coupling based on NHC-Pd catalysts has also
been widely investigated [13-16].
Recently, a particularly successful family of well-defined NHC-Pd complexes was
developed by Organ, (NHC)PdCl2(3-chloropyridine) [17, 18]. Besides the skeleton of the NHC
[19-23], additional nitrogen-donors have been investigated. For example, Navarro developed
well-defined complexes (NHC)PdCl2(triethylamine), which exhibited high catalytic activities in
Mizoroki–Heck reactions [24]. Shao reported a 1-methylimidazole stabilized NHC-Pd complex
as an active catalyst for the Mizoroki–Heck reaction [25]. A large number of nitrogen-donors
have been introduced into the coordination with NHC–Pd complexes [26-33].
In the family of nitrogen-donor stabilized NHC-Pd complexes, azole ligands have
basically identical coordination behaviors with pyridine. 1H-Benzotriazole, as an inexpensive
commercial reagent, has been widely applied in construction of metal-organic frameworks [34].
The nitrogen of 1H-benzotriazole has an electron-withdrawing effect, which results in high
acidity or lower basicity of 1H-benzotriazole. The combination of NHC with 1H-benzotriazole in
NHC-Pd complexes can adjust the electronic properties of palladium, which could influence
catalytic activity. To the best of our knowledge, the heteroleptic NHC-Pd complexes based on
1H-benzotriazole have not been reported. In this work, four 1H-benzotriazole stabilized NHC-Pd
complexes were synthesized by direct cleavage of [Pd(μ-Cl)(Cl)(NHC)]2 with 1H-benzotriazole
(scheme 1). The application of the resulting complexes in Mizoroki–Heck and Sonogashira
reactions are also described.

Scheme 1. Synthetic route of the [(NHC)PdCl2(1H-benzotriazole)] complexes.
2. Experimental
2.1. Materials and measurements
All reactions and manipulations were carried out under air. The chemicals were purchased from
commercial suppliers and used without purification. NMR spectra were recorded at 400 MHz
(for 1H NMR) and 100 MHz (for 13C NMR) on a Bruker Avance 400 NMR spectrometer. C, H
and N analyses were performed with a Vario El III Elemental Analyzer. IR spectra were recorded
on a Bruker IFS 120HR spectrometer using KBr disks. The dimeric palladium complexes
[Pd(μ-Cl)(Cl)(NHC)]2 were prepared according to literature method [35].
2.2. General procedure for preparation of the NHC-Pd complexes (1a–1d)
A mixture of [Pd(μ-Cl)(Cl)(NHC)]2 (0.10 mmol) and 1H-benzotriazole (0.20 mmol) was stirred
in CH2Cl2 (5.0 mL) at room temperature for 1 h to give a yellow precipitate; addition of DMSO
gave a yellow transparent solvent. Single crystals for X-ray diffraction analysis were obtained by
slow diffusion of n-hexane into their CH2Cl2/DMSO solvents.
2.2.1. [(IMes)PdCl2(1H-benzotriazole)] (1a). The procedure yielded 98 mg (82%) of pure 1a as
a yellow powder. 1H NMR (DMSO-d6, 400 MHz) δ (ppm) = 7.88 (br, 1H, NH), 7.67–7.65 (m,
2H), 7.53–7.46 (m, 2H), 7.31–7.27 (m, 1H), 7.12 (s, 4H), 7.04 (s, 1H), 2.39 (s, 6H, p-CH3), 2.26
(s, 12H, o-CH3). 13C NMR (DMSO-d6, 100 MHz) δ (ppm) = 151.3 (Ccarbene), 141.7, 138.4, 136.1,
135.8, 135.2, 128.6, 128.0, 124.9, 124.6, 118.5, 111.5, 20.6 (p-CH3), 18.5 (o-CH3). IR (KBr,
cm−1): 3291, 2956, 1608, 1488, 1470, 1409, 1356, 1230, 1065, 902. Anal. Calc. for

[(IMes)PdCl2(1H-benzotriazole)] (C27H29Cl2N5Pd): C, 53.97; H, 4.86; N, 11.66%. Found: C,
53.76; H, 5.05; N, 11.38%.
2.2.2. [(IPr)PdCl2(1H-benzotriazole)] (1b). The procedure yielded 116 mg (85%) of pure 1b as
a yellow powder. 1H NMR (DMSO-d6, 400 MHz) δ (ppm) = 7.92 (br, 1H, NH), 7.82 (s, 2H),
7.73–7.71 (m, 1H), 7.64–7.60 (m, 3H), 7.47–7.45 (m, 4H), 7.40–7.38 (m, 1H), 7.32–7.28 (m,
1H), 3.14 (sept, J = 6.4 Hz, 4H, CH(CH3)2), 1.36 (d, J = 6.0 Hz, 12H, CH(CH3)2), 1.10 (d, J =
6.8 Hz, 12H, CH(CH3)2). 13C NMR (DMSO-d6, 100 MHz): δ = 153.5 (Ccarbene), 146.5, 146.3,
141.5, 135.2, 129.8, 127.8, 126.2, 124.5, 123.5, 118.5, 111.4, 28.2 (CH(CH3)2), 26.0 (CH(CH3)2),
22.8 (CH(CH3)2). IR (KBr, cm−1): 3291, 2956, 2917, 1606, 1486, 1410, 1355, 1229, 1165, 1130,
1061, 1047, 856, 702. Anal. Calc. for [(IPr)PdCl2(1H-benzotriazole)] (C33H41Cl2N5Pd): C, 57.86;
H, 6.03; N, 10.22%. Found: C, 57.58; H, 6.18; N, 10.51%.
2.2.3. [(SIMes)PdCl2(1H-benzotriazole)] (1c). The procedure yielded 102 mg (85%) of pure 1c
as a yellow powder. 1H NMR (DMSO-d6, 400 MHz) δ (ppm) = 7.88 (br, 1H, NH), 7.66–7.64 (m,
1H), 7.37–7.35 (m, 1H), 7.24–7.20 (m, 1H), 7.04 (s, 4H), 6.96 (s, 1H), 4.02 (s, 4H, NCH2CH2N),
2.48 (s, 12H, o-CH3), 2.34 (s, 6H, p-CH3). 13C NMR (DMSO-d6, 100 MHz) δ (ppm) = 182.9
(Ccarbene), 141.6, 137.5 137.2, 136.9, 135.1, 128.9, 127.9, 124.4, 118.6, 111.3, 50.5, 20.6 (p-CH3),
18.8 (o-CH3). IR (KBr, cm−1): 3292, 2920, 1612, 1486, 1470, 1410, 1377, 1246, 1232, 1090, 954,
856. Anal. Calc. for [(SIMes)PdCl2(1H-benzotriazole)] (C27H31Cl2N5Pd): C, 53.79; H, 5.18; N,
11.62%. Found: C, 53.99; H, 5.02; N, 11.41%.
2.2.4. [(SIPr)PdCl2(1H-benzotriazole)] (1d). The procedure yielded 120 mg (88%) of pure 1d
as a yellow powder. 1H NMR (DMSO-d6, 400 MHz) δ (ppm) = 7.88 (br, 1H, NH), 7.67–7.64 (m,
1H), 7.47–7.45 (m, 3H), 7.36–7.34 (m, 4H), 7.29–7.27 (m, 1H), 7.23–7.19 (m, 1H), 4.06 (s, 4H,
NCH2CH2N), 3.55 (sept, J = 6.4 Hz, 4H, CH(CH3)2), 1.40 (d, J = 6.0 Hz, 12H, CH(CH3)2), 1.18
(d, J = 6.8 Hz, 12H, CH(CH3)2). 13C NMR (DMSO-d6, 100 MHz): δ = 185.3 (Ccarbene), 147.4,

147.2, 141.4, 135.4, 128.9, 127.7, 124.4, 124.0, 118.5, 111.3, 53.6 (NCH2CH2N), 28.1
(CH(CH3)2), 26.5 (CH(CH3)2), 23.8 (CH(CH3)2). IR (KBr, cm−1): 3290, 2965, 1636, 1477, 1456,
1385, 1260, 1130, 1054, 801. Anal. Calc. for [(SIPd)PdCl2(1H-benzotriazole)] (C33H43Cl2N5Pd):
C, 57.69; H, 6.31; N, 10.19%. Found: C, 57.45; H, 6.33; N, 10.47%.
2.3. Crystallography
Data collection was performed on a Bruker-AXS SMART CCD area detector diffractometer at
296 K using ω rotation scans with a scan width of 0.5° and Mo-Kα radiation (λ = 0.71073 Å).
Multi-scan corrections were applied using SADABS [36]. Structure solutions and refinements
were performed with the SHELX-97 package [37]. All non-hydrogen atoms were refined
2
anisotropically by full-matrix least-squares on F . Hydrogens on carbons were included in
idealized geometric positions with thermal parameters equivalent to 1.2 times those of carbons.
2.4. General procedure for Mizoroki-Heck reaction
In a reaction vessel, aryl halide (0.50 mmol), alkene (0.60 mmol), K2CO3 (1.0 mmol) and
(NHC)PdCl2(1H-benzotriazole) (0.1–1% mol) were mixed in DMF (2.0 mL). The mixture was
stirred at 100 °C for 6 h. After cooling to room temperature, the filtrate was concentrated and the
residue was subjected to purification via flash column chromatography to give the product.
2.5. General procedure for Sonogashira reaction
In a reaction vessel, aryl halide (0.50 mmol), phenylacetylene (0.60 mmol), K2CO3 (1.0 mmol)
and (NHC)PdCl2(1H-benzotriazole) (1% mol) were mixed in DMSO (2.0 mL). The mixture was
stirred at 80 °C for 6 h. After cooling to room temperature, brine was added to the solution. The
resulting mixture was extracted with ethyl acetate three times, the filtrate was concentrated and
the residue was subjected to purification via flash column chromatography to give the product.

3. Results and discussion
3.1. Synthesis and characterization of the complexes
To assess the coordination mode of 1H-benzotriazole towards NHC-Pd complexes, herein,
reaction of [Pd(μ-Cl)(Cl)(NHC)]2 with 1H-benzotriazole was done and yellow precipitates were
formed when two compounds were mixed in CH2Cl2. Single crystals of
[(NHC)PdCl2(1H-benzotriazole)] (1a–1d) were obtained by slow diffusion of n-hexane into their
CH2Cl2/DMSO solutions. Complexes 1a–1d are insoluble in common solvents and only
sparingly soluble in DMSO. NMR spectra of the complexes were thus recorded in DMSO-d6. 1H
NMR spectra of 1a–1d show distinctive proton resonances of NHC and 1H-benzotriazole,
supporting the formation of the heteroleptic complexes with 1:1 ratio. In addition, the
characteristic peaks of the carbene carbons (151.3 ppm for 1a, 153.5 ppm for 1b, 182.9 ppm for
1c and 185.3 ppm for 1d) are found in their 13C NMR spectra.
3.2. Structural analyses
The solid-state structures are unambiguously determined by X-ray diffraction (figure 1). In
agreement with the spectral data, the crystal structures of 1a–1d show mononuclear complexes
with each palladium center coordinated by NHC, the 3-position nitrogen of 1H-benzotriazole and
two chlorides in square planar geometry. As expected, the carbenes and 1H-benzotriazole are
found trans to each other. The PdCNCl2 coordination planes are oriented approximately
perpendicularly to the carbene ring planes with dihedral angles ranging from 72.97 to 75.13°,
and are twisted from 1H-benzotriazole planes by 34.99–35.50°. Furthermore, the
1H-benzotriazole planes are tilted by 38.48–40.34° relative to the carbene ring planes. As usual
in NHC-bearing complexes, the Pd–C bonds in 1a and 1b (1.966(3) and 1.975(5) Å) are in the
range found in (IPr)PdCl2(3-chloropyridine) (Pd–C: 1.969(3) Å), however, this trend is not
followed by the Pd−C distances for 1c and 1d (1.968(3) and 1.964(3) Å), which are similar with
the unsaturated NHC-Pd complexes 1a and 1b, but are significantly shorter than that found in
their analogue, (SIPr)PdCl2(3-chloropyridine) (1.990(3) Å). Furthermore, the Pd–N bond

distances for 1a–1d (2.079(3)–2.086(4) Å) are slightly shorter than their 3-chloropyridine
analogues, which indicated the stronger Pd–N bonds due to the greater π-acceptor character of
1H-benzotriazole.
3.3. Catalytic reaction
With the isolation and characterization of this series of heteroleptic
[(NHC)PdCl2(1H-benzotriazole)] complexes, we were able to test and compare their catalytic
activities in C–C coupling reactions. Initially, the Mizoroki-Heck reaction of 4-bromoanisole
with acrylic acid methyl ester was carried out with various bases and solvents. The solvent effect
was examined and high yields are obtained when the reactions are performed in DMF and DMA
(table 1, entries 1 and 2), whereas DMSO and 1,4-dioxane afforded moderate yields (entries 3
and 4). After screening a series of bases (entries 6–12), K2CO3 and Cs2CO3 were found to be the
most effective bases (entries 1 and 6) and K3PO4, Na2CO3 and KOAc showed comparable results
(entries 7–9). Other bases such as NaOH, KOH and KOtBu afforded the coupled product in
moderate yields (entries 10–12). The amount of palladium catalyst was also screened, after
varying the catalyst stoichiometry; 0.1 mol% of the catalyst loading was chosen for subsequent
runs.
Mizoroki-Heck reactions of a variety of aryl halides with alkenes catalyzed by 1a–1d
were carried out in the presence of K2CO3 as base in DMF at 100 °C and the results are given in
table 2. The scope of this catalytic system with vinyl substrates including methyl acrylate, ethyl
acrylate, n-butyl acrylate and t-butyl acrylate were examined. High yields were obtained when
reactions of the above vinyl substrates with 4-bromoanisole were carried out (table 2, entries
1–12).
Since the complexes were effective catalyst precursors for the reactions of acrylic acid
alkyl ester with aryl bromides, they were subjected to other substrates in order to examine their
versatility.

Styrene was then used as substrate to react with aryl bromides. In general, all the
catalysts displayed moderate to good outcomes in coupling reaction of aryl bromides under the
increased catalyst loading, but the catalytic activities of the complexes were significantly
different. As can be seen by comparing the catalytic outcomes provided by different catalysts, the
sterically hindered IPr or SIPr-based complexes 1b and 1d are more efficient than their Mes or
SIMes-based analogues (1a and 1c). The possible reason was that the sterically bulky ligands
facilitate reductive elimination (or transmetalation) of the catalytic process, which makes the
sterically bulkier catalysts show higher conversion to product. Furthermore, activated aryl
chloride, 4-chlorobenzonitrile also can be coupled with styrene and moderate yields were
obtained. However, the reaction should be carried out under increased temperature and need for
equivalents of extra tetrabutylammonium bromide (TBAB) (entries 25–28). TBAB, as a
phase-transfer agent, can enter the catalytic cycle and assist oxidative addition and reductive
elimination during a Pd(0)/Pd(II) catalytic cycle. According to the literature report, the para
electron-withdrawing group of the aryl chloride is important for the coupling reaction, applying
this optimized condition to aryl chloride without an electron-withdrawing group resulted in
incomplete reactions. Attempt to couple styrene with chlorobenzene was unsuccessful and
highest yields of 18% were observed. Finally, a comparison of the catalytic activities between the
above catalysts and the dimeric palladium complexes [Pd(μ-Cl)(Cl)(NHC)]2 was also
investigated. As evident from table 2 (entries 25, 26, 35 and 36), under the same conditions, the
catalytic activities of 1c and 1d are similar to those of the dimer complexes. The complexes do
not display superiority in reaction activities under the same amount of metal loading, but
establish a platform for comparison.
The Sonogashira reaction, involving coupling of a terminal alkyne and an aryl halide to
access arylated alkynes, is useful in organic synthesis. The catalytic results of 1a–1d in the
Sonogashira reaction are summarized in table 3. According to the general reaction conditions,
coupling reactions were directly performed in DMSO in the presence of K2CO3 at 80 °C. The
coupling reactions of phenylacetylene with aryl halides, such as bromobenzene and

4-bromoanisole, were investigated. The results indicated that (NHC)PdCl2(1H-benzotriazole)
exhibited good catalytic activities for Sonogashira of aryl bromides, but the coupling of aryl
chlorides (4-chloroanisole) afforded very low yields (<5%). As can be seen by comparing the
catalytic outcomes provided by the different catalysts, comparable reactivities in catalytic
performance between 1a–1d were observed.
4. Conclusion
Four heteroleptic palladium complexes containing both N-heterocyclic carbenes and
1H-benzotriazole were synthesized and characterized. The catalytic behavior of the complexes
was investigated in Mizoroki-Heck and Sonogashira reactions and exhibited high catalytic
activities for aryl bromides. Also, the NHC-Pd complexes with sterically hindered 2,6-isopropyl
groups exhibit much higher activity than the related NHC-Pd complexes with 2,4,6-trimethyl
group in the Mizoroki-Heck reaction. 1H-Benzotriazole as a “throw-away” ligand in the catalytic
process own the similar role with the pyridyl groups in the Pd-PEPPSI-(NHC) catalysts. As
analogues of Pd-PEPPSI-(NHC) catalysts, this kind of NHC-Pd complexes would have potential
applications in organic synthesis and is currently being examined in our laboratory.
Acknowledgements
Financial supports from the National Natural Science Foundation of China (No. 21301061) and
the Anhui Provincial Natural Science Foundation (No. 1408085QB36) are gratefully
acknowledged.
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Table 1. Influence of base and solvent on Mizoroki-Heck reaction of
4-bromoanisole with acrylic acid methyl ester [a].
Entry
1
[Pd] [%]
Solvent
DMF
Base
Yield (%) [b]
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.1%)
1a (0.05%)
1b (0.1%)
1c (0.1%)
1d (0.1%)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
Na2CO3
KOAc
NaOH
KOH
93
90
80
82
75
92
90
92
94
80
76
70
65
94
94
96
2
DMA
DMSO
1,4-dioxane
toluene
DMF
3
4
5
6
7
DMF
8
DMF
9
DMF
10
11
12
13
14
15
16
DMF
DMF
DMF
KOtBu
K2CO3
K2CO3
K2CO3
K2CO3
DMF
DMF
DMF
DMF
[a] Reaction conditions: 4-bromoanisole (0.50 mmol), acrylic acid methyl
ester (0.60 mmol), NHC-Pd (0.1–0.5% mol), base (1.0 mmol), solvent
(2.0 mL) at 100 °C for 6 h. [b] Isolated yield.

Table 2. Mizoroki-Heck reaction of aryl halides with alkenes [a].
Entry
1
Catalyst (mol %)
1a (0.1)
Aryl halide
Alkene
Yield (%) [b]
92
2
1b (0.1)
94
3
1c (0.1)
95
4
1d (0.1)
95
5
1a (0.1)
90
6
1b (0.1)
92
7
1c (0.1)
92
8
1d (0.1)
94
9
1a (0.1)
94
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27[c]
28[c]
29 [c]
1b (0.1)
95
1c (0.1)
94
1d (0.1)
96
1a (1.0)
72
1b (1.0)
94
1c (1.0)
75
1d (1.0)
96
1a (1.0)
70
1b (1.0)
92
1c (1.0)
74
1d (1.0)
94
1a (1.0)
65
1b (1.0)
88
1c (1.0)
68
1d (1.0)
90
[Pd(μ-Cl)(Cl)(IPr]2 (0.5)
[Pd(μ-Cl)(Cl)(SIPr]2 (0.5)
1a (1.0)
80
85
54
1b (1.0)
82
1c (1.0)
50
13

30[c]
31[c]
32[c]
33 [c]
34[c]
35[c]
36[c]
1d (1.0)
85
<5
15
<5
18
10
15
1a (1.0)
1b (1.0)
1c (1.0)
1d (1.0)
[Pd(μ-Cl)(Cl)(IPr]2 (0.5)
[Pd(μ-Cl)(Cl)(SIPr]2 (0.5)
[a] Reaction conditions: aryl halide (0.50 mmol), alkene (0.60 mmol), NHC-Pd (0.1–1 mol%),
K2CO3 (1.0 mmol), DMF (2.0 mL) at 100 °C for 6 h. [b] Isolated yield. [c] TBHB (1.0 mmol)
was added and the reaction was performed at 140 °C.
14

Table 3. Sonogashira reaction of aryl halides with phenylacetylene [a].
Entry
Catalyst (mol %) Aryl halide
Phenylacetylene
Yield (%) [b]
1
2
3
4
5
6
7
8
1a (1)
1b (1)
1c (1)
1d (1)
1a (1)
1b (1)
1c (1)
1d (1)
82
86
85
88
87
90
88
92
[a] Reaction conditions: aryl halide (0.50 mmol), alkene (0.60 mmol), NHC-Pd
(1 mol%), K2CO3 (1.0 mmol), DMSO (2.0 mL) at 80 °C for 6 h. [b] Isolated
yield.
15

16

Graphical abstract
17