Covalent Organic Framework as a Heterogeneous Ligand for the Regioselective Oxidative Heck Reaction

Article abstract of DOI:10.1021/acs.orglett.0c00061

A simple imine-based covalent organic framework (COF) as heterogeneous ligand for Pd^II-promoted Heck reaction is reported. Good regioselectivity for a wide range of electronically unbiased olefins is obtained (linear/branched >100:1 in most cases). Related tests and density functional theory calculations are used to explore the reason underlying the high selectivity. This research opens a route for COF as an intriguing platform to control regioselectivity catalysis.

Full text of DOI:10.1021/acs.orglett.0c00061

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Letter
Covalent Organic Framework as a Heterogeneous Ligand for the
Regioselective Oxidative Heck Reaction
Jiyao Han, Xiaowei Sun, Xiao Wang, Qiong Wang, Shenghuai Hou, Xin Song, Yingqin Wei,*
Rongyu Wang, and Wenhua Ji*
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sı Supporting Information
ABSTRACT: A simple imine-based covalent organic framework (COF) as heteroge-
II
neous ligand for Pd -promoted Heck reaction is reported. Good regioselectivity for a
wide range of electronically unbiased olefins is obtained (linear/branched >100:1 in
most cases). Related tests and density functional theory calculations are used to explore
the reason underlying the high selectivity. This research opens a route for COF as an
intriguing platform to control regioselectivity catalysis.
ovalent organic frameworks (COFs), constructed by
Scheme 1. Different Products of the Heck Reaction
C
combining organic components via covalent bonds, are a
1
type of predetermined crystalline polymers. Benefiting from
large surface area, ordered channel, and tunable pore size,
COFs demonstrate superior potential in storage, separation,
2
sensing, catalysis, optoelectronics and drug delivery. Recently,
the research of COFs in organic synthesis has received
immense attention due to available active sites, high stability,
3
and good reusability. COFs can be used for the heterogeneous
catalysis through two strategies: (1) directly used as catalyst;
highly branched-selective Heck reaction have been developed
4
(
2) used as catalyst support. Due to the limitation of available
13
(
Figure 1, L1−L4). Sigman reported that high linear-
5
catalytic sites, COFs are less explored directly as catalysts. In
contrast, the introduction of catalytically active species to
COFs can expand the application range. Importantly, the
selective of electronically unbiased olefins can be produced
through incorporating a carbene ligand into homogeneous
3
,6
14
catalytic system (Figure 1, L5). Tang fabricated mononuclear
well-organized framework structure can regulate the specific
Pd-loaded porous organic polymer catalyst to achieve linear
5
15
reaction. Since the pioneering work of Pd-loaded COF to
products with high yield (Figure 1, L6). Zhan demonstrated
bipyridine-based and thiophene−alkyne-based conjugated
microporous polymers as heterogeneous ligands for high
7
catalyze cross-coupling reaction, research in this area is widely
performed on multiple stereoselectivity and chemoselectivity
16
catalysis, including C−H activation, coupling, oxidation,
selectivity oxidative Heck reaction (Figure 1, L7). Unfortu-
nately, both fussy and high-cost procedures to process
heterogeneous ligand limit the development of Mizoroki−
Heck reaction. Herein, a simple imine-based COF is designed,
8
reduction, etc. To our knowledge, however, COFs have no
well-developed in regioselective reactions.
The Mizoroki−Heck reaction provides a common strategy
for C−C bonds construction and has been widely used to
synthesize various chemically important and biologically active
II
synthesized, and employed as the support for the Pd -
promoted oxidative Heck reaction.
9
compounds. A key issue for this reaction is to confine the
COFs with hydroxyl groups has been used in the selective
10
8d
position where aromatic groups insert into olefins. For
oxidation. Thus, 3,3′-dihydroxybenzidine was chosen as one
electronically biased alkenes, high regioselectivity can be well
of monomers. The crystalline COF-BTDH can be obtained by
1
1
achieved. However, electronically unbiased alkenes, such as
aliphatic alkenes, can cause complex and inseparable
Received: January 6, 2020
1
2
mixtures, which limits the application of Mizoroki−Heck
reaction (Scheme 1).
Generally, ligands play a key role in regulating the
regioselectivity. Several homogeneous ligand-regulated and
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c00061
A
Org. Lett. XXXX, XXX, XXX−XXX

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pubs.acs.org/OrgLett
Letter
Figure 2. PXRD patterns.
Figure 1. Ligands for selective Heck reaction.
17
stacking pattern simulated by Materials Studio program. The
PXRD pattern of COF-BTDH remains unchanged after
treated with different solvents for 3 days (Figure S3), revealing
that the crystalline structure is stable. TG shows that COF-
BTDH and Pd/COF-BTDH maintain intact at temperature up
to 580 and 150 °C, suggesting excellent thermal stability
condensing 1,3,5-tricarbaldehyde and 3,3′-dihydroxybenzidine
in a mixture of 1,4-dioxane/n-butanol/3 M acetic acid (4:1:3,
v/v/v) at 120 °C for 72 h. (Table S1 and Scheme 2).
Scheme 2. Synthesis of COF-BTDH and Pd/COF-BTDH
(
Figure S4)
Energy-dispersive spectrometer mappings illustrate the
uniform distributions of Pd, C, O, and N in Pd/COF-BTDH
Figure S5). The Pd content was 5.13% analyzed by
(
1
3
inductively coupled plasma (ICP). Solid-state C NMR
spectrum of Pd/COF-BTDH peak at 181 ppm is assigned to
7
carbonyl group of Pd(OAc) (Figure S2). These results
2
confirm the load of Pd(OAc) on the frameworks. The FT-IR
2
−
1
spectrum of Pd/COF-BTDH at 1618 cm indicates the
structural preservation of imine-linking (Figure S6). SEM and
TEM images show that the morphology of Pd/COF-BTDH
can retain the same as that of COF-BTDH (Figure 3). N2
adsorption−desorption analysis of COF-BTDH and Pd/COF-
BTDH show similar hysteresis loops above P/P = 0.7 and the
0
sharp increase above P/P = 0.9 (Figure S7). Type IV isotherm
0
is fit for this sorption profile, which is feature of mesoporous
COF-BTDH and related Pd/COF-BTDH are characterized
by Fourier transform infrared (FT-IR) spectrum, elemental
analysis, thermogravimetry (TG), powder X-ray diffraction
13
(
PXRD), solid-state C NMR spectroscopy, transmission
electron microscopy (TEM), X-ray photoelectron spectrosco-
py (XPS), scanning electron microscopy (SEM), and N2
adsorption−desorption analysis. FT-IR spectrum at 1619
−
1
13
cm and solid-state C NMR spectrum peak at 157 ppm
(
Figures S1 and S2), both of which are for CN, suggest the
structural formation of imine linking. Elemental analysis results
(
[
C 75.37%, H 5.89%, N 9.66%) of COF-BTDH
(C H O N ) ] are well consistent with the calculated
1
9
16
2
2
n
values (C 74.98%, H 5.30%, N 9.20%). The PXRD pattern of
COF-BTDH displays an intense peak at 3.52° corresponding
to the (100) plane, revealing the good crystallinity (Figure 2).
Minor peaks are observed at 6.03°, 7.22°, 9.01° and 24.95°,
which are assigned to (110), (200), (210), and (001) planes.
The presence of the (001) plane is ascribed to π−π stacking
between layers, indicating ordered periodicity of COF-BTDH.
Additionally, the experimental data show a match with AA
Figure 3. SEM images of COF-BTDH (A) and Pd/COF-BTDH (B),
TEM images of COF-BTDH (C) and Pd/COF-BTDH (D).
B
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pubs.acs.org/OrgLett
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1
8
a
materials. Pore size distribution of COF-BTDH and Pd/
Table 1. Substrate Scope
COF-BTDH, calculated by using Barrett−Joyner−Halenda
BJH) method, show that the mesopores are mainly
(
distributed at 2−3 nm (Figure S8). This result is consistent
19
with the literature (2.4 nm). The appreciable decrease in
3
2
pore volumes (∼0.15 cm /g) and surface areas (∼26 m /g) for
20
Pd/COF-BTDH is ascribed to the loading of Pd(OAc) .
2
PXRD implies the sensibility of COF-BTDH to the condition
21
of N2 adsorption−desorption analysis (Figure S9). The
crystalline destruction can cause the poor surface area. XPS is
II
used to investigate the incorporation of Pd into COF-BTDH.
As shown in Figure S9, the 1.3 eV shift for N in C = N and 1.4
II
eV shift for O in O−H stem from the electron-effect of Pd ,
II
which is different from the incorporation of Pd between the
7
adjacent layers of COF-LZU1. Considering the presence of
−
OAc , the coordination interaction between Pd and COF-
BTDH is predicted as shown in Scheme 2. The PXRD pattern
reveals that the reserved crystal structure of Pd/COF-BTDH
II
(
Figure 2). We suggest that both the coordination of Pd with
imine nitrogen and hydroxy oxygen inside the holes and
ordered channel structure plays a vital role in regulating the
regioselectivity for the oxidative Heck reaction.
1
-Octene 1a and phenylboronic ester 2a are used as model
substrates. Excitedly, COF-BTDH as heterogeneous ligand
shows good ln/br selectivity of 50:1 and regioisomeric ratio
(
rr) of 7:1 with moderate conversion and yield (Table S1,
entry 1). In the absence of COF-BTDH, the lower ln/br
selectivity of 5:1 is obtained (Table S2, entry 2). Optimization
of reaction conditions is performed. Extending reaction time
improves the conversions and yields without diminishing the
selectivity (Table S1, entries 2 and 3). Decreased the molar
ratio of 1a and 2a results in reduced conversions and yields
due to self-coupling reaction of 2a (Table S1, entries 4 and
1
5
22
5
). Copper salt can effective facilitate transmetalation.
Among copper salts, 20% mmol Cu(OTf) is the best (Table
2
S1, entries 3 and 6−10). Performing the reaction under air
results in moderate conversion and poor yield (Table S1, entry
1
1). Further optimization in temperature (Table S1, entry 12)
and solvent (Table S1, entries 13 and 14) fails to obtain
desired yields.
Under the optimized conditions, various electronically
unbiased olefins and arylboronic esters are used to evaluate
the catalytic efficiency of the Pd(OAc) /COF-BTDH system
2
(Table 1). Heteroatom-free substrates such as aliphatic alkenes
are generally challenging substrates for Heck reaction owing to
the absence of chelation and inductive effects. In the presence
of COF-BTDH, aliphatic alkenes are delivered successfully to
the (E)-linear products in excellent ln/br selectivity (>33:1
ratio in most cases) with good yields (Table1, entries 1−10),
suggesting the selectivity under these conditions observed is
23
catalyst-controlled rather than chelate-controlled.
In addition, the catalytic system was certified to be tolerant
of a variety of functional groups, delivering the desired (E)-
linear products. For example, substrates containing a ketone
affords excellent regio- and E/Z stereoselectivities and good
yields with a wide range of arylboronic esters (Table 1, entries
a
Reaction conditions: olefins (0.8 mmol), arylboronic esters (2.4
mmol), Pd(OAc) (6 mol %), Cu(OTf) (20 mol %), COF-BTDH
2
2
(
20 mg), DMA (8 mL), 40 °C, O (1 atm), 48 h. The selectivity (rr)
2
b c
for desired product was >7:1. Isolated yield. Recovered starting
1
1−13). Allyl esters usually undergo oxidative addition in the
d
1
e
material. Determined by H NMR spectroscopy. 12 h.
existence of palladium catalyst to afford allyl-aryl coupling
24
product via β-acetoxy elimination. However, in the presence
of Pd(OAc) /COF-BTDH, allyl ester is a suitable substrate for
proceeds linear products with excellent selectivity and good
yields. Moreover, the catalyst system promotes the cross-
coupling of an allylic amine protected by two carbamate groups
with arylboronic esters to access the linear products with good
2
oxidative Heck reaction, converting the desired linear product
with excellent selectivity and yield (Table 1, entry 14). The
olefins bearing an ester (Table 1, entries 15 and 16) all
C
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pubs.acs.org/OrgLett
Letter
selectivity and excellent yields (Table 1, entries 17 and 18). It
is noteworthy that good yield and excellent selectivity are also
observed for an alkenyl epoxide, a class of substrate which is
generally suffered from nucleophilic attack through organo-
25
boron compounds (Table 1, entry 19). Submission an ether
also affords the desired product with excellent selectivity and
good yield (Table 1, entry 20). More intriguingly, the ratio of
linear to branched products achieves up to 100:1 for the
majority of vinylcyclohexanes and heteroatom olefins (Table 1,
entries 6−16 and 19−21), which is exceptionally improved
1
4−16
compared with previous studies.
15
26
Coordination pattern and ordered channel structure
show significant factors for controlling the selectivity. Thus,
7
COF-LZU1 (Figure S11) and amorphous COP-BTDH
(Figure S12) were synthesized. Compared with COF-BTDH,
a reducing in ln/br selectivity is obtained with COF-LZU1 in
which the coordination of Pd(OAc) to COF-LZU1 between
2
the adjacent layers (Table S3, entry 1). It is rational to suppose
that coordination in holes can effectively improve the
regioselective. Due to the diffusion limitation of reactants,
low regioselectivity and poor yield are observed in the presence
of COP-BTDH (Table S3, entry 2), illustrating that ordered
channel structure in COF-BTDH exhibits remarkable perform-
ance for regioselective catalysis.
The recyclability of Pd/COF-BTDH catalyst is examined in
the reaction of 1a and 2a. The results shows that Pd/COF-
BTDH can be reused nine times without loss of regioselectivity
and catalytic activity (Table S3). During the tenth run,
however, both selectivity and yield decrease significantly. The
recyclability of Pd/COF-BTDH can stand comparison with
1
5,16
reported catalysts (e.g., Pd@POP-9, CC@CMP-1).
II
XPS prove that Pd does not change before and after catalysis
(Figure S13). The Pd content was 4.97% with little loss of
quality as analyzed by ICP. The decrease of catalytic activity is
because of the crystalline destruction of COF-BTDH (Figure
S14). In addition, trace Cu2+ can be determined by XPS
Figure 4. DFT-predicted relative free energies for the alkene insertion
pathways.
analysis in recycling Pd/COF-BTDH (Figure S15), which
2
+
implies Cu can also be coordinated with the skeleton of
COF-BTDH.
To further explore mechanistic about the selectivity in
catalyst system, Density Functional Theory (DFT) calculations
is employed to furnish the pathways of 1-octylene inserting
Our findings suggest that COFs can open up a new route for
regulating regioselective catalysis.
II
into ligand-ligated Pd -phenyl species. Due to the complexity
ASSOCIATED CONTENT
sı Supporting Information
■
of heterogeneous catalytic system, a modified ligand is used to
II
*
substitute COF-BTDH. The coordination of 1-octylene to Pd
leads to two lowest-energy isomers Int1 and Int5. Sub-
sequently, the two most favorable olefins insertion transition
states TS1−2 and TS5−6 lead respectively to linear and
branched products. Note that TS1−2 is lower than TS5−6
about 2.7 kcal/mol in free energy. Additionally, the energy
barrier of linear process in rate-determining step is 5.8 kcal/
mol, which is lower than 9.6 kcal/mol of branched process
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00061.
Detailed synthetic procedures, FT-IR spectra, PXRD
patterns, gas adsorption, TG, SEM images, TEM images,
XPS spectra, GC−MS, analytical data for compounds,
NMR spectra (PDF)
(
Figure 4: Int1 → TS1−2; Int5 → TS5−6). Therefore, the
AUTHOR INFORMATION
Corresponding Authors
formation of branched isomer is less preferable than that of
linear regioisomer for both thermodynamics and kinetics, in
contrast to branched regioselectivity in neocuproine-ligated
■
Yingqin Wei − School of Chemistry and Pharmaceutical
Engineering, Qilu University of Technology (Shandong Academy
of Sciences), Jinan 250353, China; Email: wyq@qlu.edu.cn
Wenhua Ji − School of Pharmaceutical Sciences, Shandong
Analysis and Test Center, Qilu University of Technology
(Shandong Academy of Sciences), Jinan 250014, China;
orcid.org/0000-0002-4807-9091; Email: jwh519@
163.com
II
13b
Pd -catalyzed oxidative Heck reaction.
In summary, we developed a high selectivity oxidative Heck
II
reaction catalyzed by Pd in the present of imine-based COF-
BTDH. For a wide range of electronically unbiased olefins,
COF-BTDH shows excellent linear selectivity, mainly due to
the ordered channel structure and proper coordination pattern
II
(
Pd coordinate with imine nitrogen and hydroxy oxygen).
D
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Organic Letters
Authors
pubs.acs.org/OrgLett
Letter
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Xiaowei Sun − School of Pharmaceutical Sciences, Shandong
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(Shandong Academy of Sciences), Jinan 250014, China
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2
(
Shandong Academy of Sciences), Jinan 250014, China;
orcid.org/0000-0003-4236-2483
(
(
̀
Qiong Wang − College of Chemistry, Chemical Engineering and
Materials Science, Shandong Normal University, Jinan 250014,
China
Shenghuai Hou − School of Pharmaceutical Sciences, Shandong
Analysis and Test Center, Qilu University of Technology
(
Ed. 2012, 51, 5915. (b) Zheng, C.; Wang, D.; Stahl, S. S. J. Am. Chem.
Soc. 2012, 134, 16496. (c) Standley, E. A.; Jamison, T. F. J. Am. Chem.
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F. Angew. Chem., Int. Ed. 2014, 53, 1858.
(Shandong Academy of Sciences), Jinan 250014, China
Xin Song − School of Pharmaceutical Sciences, Shandong
Analysis and Test Center, Qilu University of Technology
(14) Werner, E. W.; Sigman, M. S. J. Am. Chem. Soc. 2010, 132,
1
(
3981.
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16) (a) Zhou, Y. B.; Wang, Y. Q.; Ning, L. C.; Ding, Z. C.; Wang,
(Shandong Academy of Sciences), Jinan 250014, China
Rongyu Wang − School of Pharmaceutical Sciences, Shandong
Analysis and Test Center, Qilu University of Technology
2
(
(Shandong Academy of Sciences), Jinan 250014, China
W. L.; Ding, C. K.; Li, R. H.; Chen, J. J.; Lu, X.; Ding, Y. J.; Zhan, Z.
P. J. Am. Chem. Soc. 2017, 139, 3966. (b) Li, R. H.; Ding, Z. C.; Li, C.
Y.; Chen, J. J.; Zhou, Y. B.; An, X. M.; Ding, Y. J.; Zhan, Z. P. Org.
Lett. 2017, 19, 4432.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00061
(
17) To clarify the lattice packing, AA and AB stacking patterns were
Notes
simulated by the Materials Studio program. AA stacking pattern was
The authors declare no competing financial interest.
constructed using space group p1 with the optimized parameters of a
=
1
(
30.245 Å, b = 29.944, and c = 3.600 Å, α = β = 90°, and γ =
19.928°, which show a good match with experimental data.
18) (a) Guan, X. Y.; Li, H.; Ma, Y. C.; Ming, X.; Fang, Q. R.; Yan,
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 31972145).
Y. S.; Valtchev, V.; Qiu, S. L. Nat. Chem. 2019, 11, 587. (b) Wu, X.
C.; Wang, B. W.; Yang, Z. Q.; Chen, L. G. J. Mater. Chem. A 2019, 7,
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