Photoswitchable Cu(ii)/Cu(i) catalyses assisted by enzyme-like non-covalent interactions in Cu(ii)-melamine coordination polymers for installing CO2/CS2and CF3groups in heterocycles

Article abstract of DOI:10.1039/d0nj02154g

Despite concerns over the environmental impact of isoelectronic CO₂and CS₂, they remain useful C1 building blocks with industrial utility, and trifluoromethylation could enhance the lipophilicity and metabolic stability of agrochemic

Full text of DOI:10.1039/d0nj02154g

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Photoswitchable Cu(II)/Cu(I) catalyses assisted by enzyme-like
non-covalent interactions in Cu(II)-melamine coordination
polymer for installing CO2/CS2 and CF3 groups in heterocycles
Received 00th January 20xx
Accepted 00th January 20xx
Tiexin Zhang,*a Hanbin Zang,a Fangyuan Gai,b Zhi Feng,a Mochen Lia and Chunying Duan*ac
DOI: 10.1039/x0xx00000x
Despite concern over the environmental impact of isoelectronic CO2 and CS2, they remain useful C1 building blocks with
industrial utility. and trifluoromethylation could enhance the lipophilicity and metabolic stability of agrochemicals and
pharmaceuticals. Merging the CO2/CS2 moiety together with the CF3 group in one heterocyclic scaffold is both
environmentally and pharmaceutically valuable, but it is still challenging for the homogeneous Cu-catalytic protocols to
circumvent the issue of residual heavy metal, and the aggregation and sulfur-poisoning effect of Cu-species. In this report,
a highly durable Cu(II)-melamine coordination polymer Cu–TDPAT, assembled from melamine-based ligand H6TDPAT and
Cu(II) salt, was employed in the heterogeneous approach. The photoinduced electron transfer from the melamine-based
ligand to the Cu(II) node of Cu–TDPAT and the ground-state back electron transfer enabled the controllable Cu(II)/Cu(I)
switch. The enzyme-like non-covalent interactions not only enriched and activated CO2/CS2, allylamine substrate, and Togni’s
reagent II, the CF3 radical precursor, but also drew close distances between them to facilitate the reaction. These advantages
benefit from synergising the ground-state Cu(II)-triggered CO2 installation and the photogenerated Cu(I)-catalysed
trifluoromethylation steps within the construction of biointeresting heterocycles in a sustainable and recyclable manner. In
particular, this heterogeneous approach was applicable to the construction of a central nervous system agent derivative
when CS2 was employed instead of CO2, showing the superior durability of Cu–TDPAT against sulfur-poisoning compared
with its homogeneous counterpart.
lipophilicity and metabolic stability.11 Highly reactive CF3
radicals,12 which are generated through a redox process
between a CF3 radical precursor and a transition metal13 or
excited-state photosensitizer,14 can directly add to sp2 C–H
substrates without the need for prefunctionalization of the
reaction sites, which is necessary in traditional transition-
metal catalysed couplings.15
Introduction
Although carbon dioxide (CO2) is a greenhouse gas, it is also
a nontoxic, abundant, and otherwise environmentally
benign C1 building block.1 In contrast, its sulfur analogue,
carbon disulfide (CS2), presents a greater environmental
hazard, contributing to the formation of photochemical
tropospheric sulfur aerosols, smog, and acid rain.2
Nevertheless, CS₂ is a major low-cost chemical with
industrial3 and therapeutic applications.4 The catalytic
5
6
conversions of isoelectronic CO2 and CS2 into fine
chemicals under mild conditions are highly valuable in both
industry and academia. The Lewis acid-catalysed
9
cycloaddition of CO27,8/CS2 is a promising method for the
atom-economic construction of oxygen/sulfur-containing
heterocycles of potential pharmaceutical interest.
In
agrochemical
and
pharmaceutical
trifluoromethyl groups (CF3) are often incorporated enhance
agents,10
Scheme 1 Schematic illustration of Cu(II)-melamine coordination polymer, showing
its enzyme-like substrate manipulation and photoswitchable Cu(II)/Cu(I) catalytic
processes for assembling CO2/CS2, the CF3 group, and allylamines into 2-
oxazolidinones and 2-thiazolidinethiones.
a.State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116024, P. R. China. E-mail: zhangtiexin@dlut.edu.cn; cyduan@dlut.edu.cn;
Fax: +86-411-84986476; Tel: +86-411-84986476
It would be both environmentally and pharmaceutically
b.Advanced Institute of Materials Science, School of Chemistry and Biology,
Changchun University of Technology, Changchun, 130012, P. R. China.
c. Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
300071, China
21–24
valuable to merge CO216–20/CS2
and CF3 groups in one
molecular scaffold. Monovalent Cu(I) can generate CF3
radicals through a redox process with CF3-radical precursors
such as Togni’s reagent,25–27 and bivalent Cu(II) not only
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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provides Lewis-acidic activation for the cycloaddition of
CO2/CS2,7 but is also involved in redox processes with radical
intermediates.28 Thus, the synergy between the Cu(II)- and
Cu(I)-catalytic steps probably helps to achieve this goal. In
the pioneering work of Yu group, the entire “CO2” moiety
polymers toward oxidative guest molecuVleiesw.4A5r,t4ic6le OTnhlinies
suggests a possible photoswitchable Cu(II)/Cu(I) conversion
by means of LMCT in Cu–TDPAT, which could synergise the
cascade of Cu(II)-triggered CO2/CS2 cyclisation and Cu(I)-
DOI: 10.1039/D0NJ02154G
catalysed trifluoromethylation steps in
manner.
a sustainable
and
trifluoromethyl
group
were
simultaneously
incorporated into 2-oxazolidone derivatives by using the
monovalent [Cu(MeCN)4]PF6 as precatalyst.29 It should be
noted that high loadings of Cu salts are usually needed in
homogeneous catalytic protocols to balance the undesirable
complexation of Cu species in the presence of multiple
coordinative species.30 Moreover, the use of CS2 would likely
result in severe sulfur poisoning31 for transition metal ions
such as catalytic Cu species in solution. To alleviate the
undesirable depletion or deactivation of the Cu catalyst and
circumvent the possible issue of residual heavy metals in the
products, the immobilisation of Cu ions onto porous
materials might provide a practical approach for achieving
catalysis in a heterogeneous manner.
Recent progress in the development of crystalline porous
coordination polymers with structural tailorability has
allowed access to monodisperse, highly dense, and uniform
assemblies of redox-active metal ions and other functional
centres. Extensive studies have revealed the potential of
coordination polymers for the storage of CO2,32–34 although
the analogous handling of CS2 is still in its infancy.35 In these
coordination polymer assemblies, compared with the
strained coordinative sphere of Cu(I), the Lewis-acidic open
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Fig. 1 (a) Structural illustration of rht-type framework and cages of Cu−TDPAT. The
cub-Oh, T-Td, and T-Oh cages are highlighted in yellow, turquoise, and pink,
respectively. Comparison of the as-synthesized and simulated PXRD patterns of
Cu−TDPAT (b) and Zn−TDPAT (c). Illustrations of the largest T-Oh cages of Cu−TDPAT
(d) and Zn−TDPAT (e).
metal sites of Cu2(O2C)4 paddle-wheel nodes facilitate the
36,37
activation and transformation of CO2
and CS2.38
Furthermore, the Lewis-basic sites of the ligand not only
strongly interact with CO2, but are also involved in non-
covalent interactions with other substrates or reagents.39
Thus, the combination of Cu2(O2C)4 paddle-wheel metal sites
and Lewis-basic ligands is capable of enriching, spatially
arranging, and activating CO240/CS238 and other substrates or
reagents within confined local environments. Moreover, the
possibility of photoinduced ligand-to-metal charge transfer
(LMCT) can provide the sustainable switching of Cu(II) to
Cu(I),41 while maintaining the tight binding and highly
exposed character of the metal sites.
Based on its strong Lewis basicity and ability to form
hydrogen bonds, melamine (1,3,5-triazine-2,4,6-triamine)-
based coordination polymers exhibit superior performance
in the storage of CO2.42–44 Herein, the highly durable Cu(II)-
melamine coordination polymer Cu–TDPAT, assembled from
Results and discussion
The Cu(II)-melamine coordination polymer Cu–TDPAT, also
known as rht-MOF–7,44 was constructed by the solvothermal
reaction of C3-symmetric melamine-based ligand H6TDPAT
and Cu(NO3)2∙3H2O, following previously published
procedures.43 As a control catalyst without Cu(II) sites, Zn–
TDPAT, which possesses a topology similar to that of Cu–
TDPAT, was synthesized from H6TDPAT and Zn(NO3)2·6H2O.45
The crystallinity and phase purity of the two coordination
polymers were verified by powder X-ray diffraction (PXRD)
analysis, and were in accord with the literature values;44,45
the similar PXRD patterns proved the similar topologies of
the Cu–TDPAT and Zn–TDPAT for the comparative catalytic
study (Fig. 1b–1c). The carboxylate group of the TDPAT6-
ligand bridges Cu(II)/Zn(II) ions to form Cu2(O2C)4/Zn2(O2C)4
paddle-wheel nodes with easily accessible axial coordination
vacancies for binding and activating substrates or reagents.
Both Cu- and Zn-coordination polymers are characterised by
isomorphic (3,24)-connected rht-type networks (Fig. 1a),44
and both structures can be regarded as the packing of three
types of cages: cuboctahedron (cub-Oh), truncated
tetrahedron (T-Td), and truncated octahedron (T-Oh), with
internal diameters of ca. 12, 9, and 17 Å, respectively (Fig.
1a). Compared with the cub-Oh cage constructed from the
the
melamine-based
ligand
2,4,6-tris(3,5-
dicarboxylphenylamino)-1,3,5-triazine (H6TDPAT) and
a
Cu(II) salt,43 was employed as a heterogeneous catalyst to
incorporate CO2/CS2 and CF3 groups into value-added
heterocycles. In the Cu–TDPAT cage, enzyme-like multiple
non-covalent interactions were expected to increase the
local concentration of substrates/reagents and avoid mutual
interference among the multiple components. Furthermore,
the photoreducing ability of the melamine-based ligand has
been demonstrated by the remarkable fluorescence
responses of its corresponding d10-metallic coordination
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paddle-wheel nodes and the phenyl termini of the ligands,
the cages of both T-Td and T-Oh are surrounded by nodes and
ligand melamine moieties, especially the large cavity.
Furthermore, the enriched non-covalent interaction sites of
the T-Oh cage endow it with the potential to manipulate the
substrate/reagent therein (Fig. 1d–1e).
material are responsible for the intra-framework
View Article Online
fluorescence quenching.55 The exDciOteI:d10-s.1t0a3t9e/D0rNeJd0u2c1t5i4vGe
potential of Zn–TDPAT was further determined as −1.76 V,
based on the free energy change (E0-0, ca. 3.16 eV vs. SCE,
Fig. 3b) between the ground and vibrationally relaxed
excited states.56 Considering the well-maintained structure
and the similar arrangement pattern of the ligands in both
coordination polymers, the excited-state reductive potential
of the ligands in Cu–TDPAT should be approximately in
agreement with that of Zn–TDPAT, which is sufficiently
negative to the reduce Cu(II) sites of the nodes to Cu(I) under
photoirradiation (Cu(II)/Cu(I) redox potential, ca. 0.24 V vs.
SCE, Fig. 3a). Then, the in situ-generated Cu(I) species can
react with the CF3 radical precursor, Togni’s reagent II, and
trigger the trifluoromethylation step of the reaction.26
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Table 1. Reaction condition optimization and control experimentsa
Fig. 2 CO2 absorption isotherms of Cu−TDPAT and Zn−TDPAT at 298 K.
The CO2 adsorption isotherms of Cu–TDPAT and Zn–TDPAT
from 0 to 1 atm (101 kPa) are shown in Fig. 2. The CO2 uptake
of Cu–TDPAT at 1 atm and 298 K is ca. 5.0 mmol·g-1 (ca. 22
wt%),43 which is nearly 20 times higher than that of Zn–
TDPAT. This not only reflects the vital role of the
coordinatively unsaturated metal sites and Lewis-basic
melamine moieties in CO2 fixation,47–53 but also implies the
potential of Cu–TDPAT as a catalyst in CO2 conversion
processes at ambient temperatures and pressures.
Entr
Photocatalyst
(mol%)
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
Light
Base
Solven
t
MeCN
MeCN
MeCN
Yield
(%)b
69
n.d.
< 10
y
1
2
3
Source
Xe light
Xe light
Xe light
DBU
–
collidin
e
4
5
6
7
8
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
Cu–TDPAT (5)
H6TDPAT (1.67)
Cu(NO3)2∙3H2O (5)
Cu(NO3)2∙3H2O (5)
+
Xe light
Xe light
Xe light
Xe light
Xe light
Xe light
Xe light
Xe light
Xe light
Xe light
DIPEA
K2CO3
Cs2CO3
DBU
DBU
DBU
DBU
DBU
DBU
DBU
MeCN
MeCN
MeCN
DCM
THF
DMF
EtOH
MeCN
MeCN
MeCN
22
trace
< 10
29
26
33
9
10
11
12
13
10
trace
< 10
12
H6TDPAT (1.67)
Zn–TDPAT (5)
Cu(NO3)2∙3H2O (5)
+
14
15
Xe light
Xe light
DBU
DBU
MeCN
MeCN
trace
27
Fig. 3 (a) Cyclic voltammetry (CV) curves of Cu−TDPAT (using ferrocene as an internal
standard) and Zn−TDPAT; the redox peaks of the TDPAT.+/TDPAT ligand are
highlighted by the grey bar. (b) The UV-Vis absorption spectra of Cu−TDPAT and
Zn−TDPAT, and the fluorescence and free energy change E0-0 of Zn−TDPAT.
Zn–TDPAT (5)
–
Cu–TDPAT (2.5)
Cu–TDPAT (10)
Cu–TDPAT (5)
Cu–TDPAT (5)
16
17
18
19
20
Xe light
Xe light
Xe light
Dark
Xe light
(> 400
nm)
DBU
DBU
DBU
DBU
DBU
MeCN
MeCN
MeCN
MeCN
MeCN
n.d.
33
72
n.d.
77
Electrochemical analyses of both coordination polymers
displayed peaks at ca. 1.40 V (vs. SCE), corresponding to the
TDPAT·+ /TDPAT redox couple of the ligands (Fig. 3a) and
reflecting the preservation of melamine structure during
framework assembly. The UV-Vis absorption spectrum of
solid-state Cu–TDPAT shows bands covering a broad visible
range (400–500 nm, Fig. 3b), indicating a visible-light
harvesting ability superior to that of Zn–TDPAT or the free
ligand (Fig. S2, ESI†). This extended absorption range may be
attributable to the LMCT transition.54 The typical d-d
transition peaks of the dicopper paddle-wheels are observed
above 500 nm. As shown in Fig. 3b, the solid sample of Zn–
TDPAT exhibits strong photoluminescence with broad bands
(excited at 370 nm), which is attributed to the TDPAT6-
ligand. In contrast, the solid-state fluorescence is totally
quenched in Cu–TDPAT, suggesting that the Cu(II) sites in the
21
22
Cu–TDPAT (5)
LED
DBU
DBU
MeCN
MeCN
Trace
31
(455 nm)
Xe light
(> 400
nm)
Cu–TDPAT (5)
filtered off after 1h
a Reaction conditions: 1a (0.25 mmol, 1.0 equiv.), 1 atm CO2 (balloon), Togni’s
reagent II (0.30 mmol, 1.2 equiv.), base (0.5 mmol, 2.0 equiv.), degassed
solvent (4.0 mL), photocatalyst (specified amount calculated based on the
b
metal sites), r.t., 12 h. Isolated yields. Ph = phenyl, Bn = benzyl, DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene, DABCO
DIPEA N,N-diisopropylethylamine, MeCN
dichloromethane, DMF = N,N-dimethylformamide, THF = tetrahydrofuran.
=
1,4-diazabicyclo[2.2.2]octane,
=
=
acetonitrile, DCM =
In view of the importance of utilising CO2 to generate
pharmaceutically valuable heterocyclic 2-oxazolidones with
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the concomitant incorporation of CF3 groups in the presence
of a Cu(I) salt,29 we explored the oxytrifluoromethylation of
allylamines with CO2 and Togni’s reagent II as the probe
reaction to evaluate our design. The initial reaction was
carried out with the model substrate N-benzyl-2-
phenylprop-2-en-1-amine 1a (0.25 mmol, 1.0 equiv.), Togni’s
reagent II (0.3 mmol, 1.2 equiv.), basic additive DBU (0.5
mmol, 2.0 equiv.), and Cu–TDPAT (0.0125 mmol, 5 mol%) as
the photocatalyst in acetonitrile under CO2 atmosphere (1
atm) at room temperature (r.t.), using Xe light as the light
source. Fortunately, the desired product (2a) was obtained
in 69% isolated yield (Table 1, entry 1). Control experiments
demonstrated that the Cu-based coordination polymer, the
basic additive, and light irradiation were indispensable for
the reaction (Table 1, entries 2, 16, and 19). Among the
various types of bases screened (entries 3–6), the inorganic
bases were ineffective, while the organic base DBU
outperformed the other organic bases. Then, several
solvents with different polarities and Cu(II) affinities were
examined (entries 7–10), but all gave inferior conversions to
2a compared with acetonitrile. In particular, because alcohol
solvents can engage in hydrogen-bond formation with the
melamine moieties and are more highly coordinating with
the Cu(II) centres in the catalyst, the activation of CO2,
allylamine 1a, or Togni's reagent II was severely deteriorated
in EtOH.57
complexation between the Lewis-basic melamine moieties
and Cu(II) ions is believed to block the non-covalent
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DOI: 10.1039/D0NJ02154G
interaction sites of the ligands, diminishing their substrate
activation ability. Comparatively, Cu–TDPAT provides well-
dispersed Cu(II) sites and melamine moieties while
maintaining the electron transfer routes between them.
Next, the impact of the catalyst loading was considered.
Halving the catalyst load from 5 to 2.5 mol% results in a
slower reaction (entry 17), whereas doubling to 10 mol%
does not produce a remarkable improvement (entry 18).
After filtering off the UV wavelength range below 400 nm of
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the Xe light, the yield of
4 was markedly increases to 77%
(entry 20), implying the destructiveness of the higher-energy
UV irradiation. When irradiating the reaction mixture near
the absorption edge using a 455 nm LED, nearly no reaction
is observed (entry 21).
As displayed in Table 1, entry 22, there is no noticeable
further transformation after hot filtration of the Cu–TDPAT,
suggesting the heterogeneity of the photocatalysis. After the
reaction, the coordination polymer powders could be easily
recovered by simple filtration, solvent elution, and re-
activation under vacuum heating, and then could be reused
for at least 5 rounds without significantly decreased
reactivity (Fig. 4a). There were no significant changes in the
PXRD patterns of the Cu–TDPAT solids after the reactions
(Fig. 4b). These results not only indicate the retention of the
crystallinity of the coordination polymer catalyst, but also
reveal the vital role of the structural rigidity in durable
photocatalysis.
Table 2. Substrate scope of heterogeneous photocatalytic oxytrifluoromethylationa
Fig. 4 (a) Column chart showing the recyclability of the Cu–TDPAT photocatalyst. (b)
Comparison of the powder X-ray powder diffraction patterns of freshly prepared Cu–
TDPAT (black), the simulated pattern based on the reported single-crystal data
(red),44 and the recycled solid after 5 recycling rounds (blue).
Furthermore,
a series of control experiments were
performed to determine the structure–activity relationship
of the photocatalysis. Compared to Cu–TDPAT, the desired
product was not formed using the Zn–TDPAT or free ligand
H6TDPAT as the photocatalyst, although Togni’s reagent II
and allylamine 1a were consumed in both cases, resulting in
complex mixtures (Table 1, entries 11 and 14). This indicates
the necessity of Cu(II) sites in mediating the photoredox
process. The direct combination of a Cu(II) salt and the ligand
in solution generated 2a in low yield (12%), with the
concomitant formation of amorphous aggregated copper
complexes (entry 13). A mixture of a Cu(II) salt and Zn–TDPAT
delivered 2a in a yield of 27% (entry 15); the incorporation
of the Cu(II) salt into Zn–TDPAT was visible according to the
colour change of the Zn–TDPAT crystals after mixing them in
the solution. The dispersion of the Cu(II) ions within the
pores of Zn–TDPAT might prevent the aggregation of copper
species in the solution phase; however, the strong
a
The optimal reaction conditions are shown in Table 1, entry 20. Isolated
yields.
With the optimal reaction conditions in hand, we explored
the substrate scope of this heterogeneous photocatalytic
oxytrifluoromethylation (Table 2). Combinations of CO2,
Togni’s reagent II, and various allylamines were transformed
into the desired trifluoromethylated 2-oxazolidone products
in moderate to good yields. Both an electron-donating
methyl group and electron-withdrawing halogen groups on
the vinylic aromatic substituent were tolerated in the
reaction (Table 2, 2b
substrate 1e was also converted smoothly in 60% yield (2e).
Both N-benzyl (2b 2e) and N-butyl groups (2f) were suitable
–2d), and β-naphthyl substituted
–
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nitrogen substituents in the substrate. Not only allylamines
with easily accessible terminal double bonds, but also the
bulky endocyclic alkene moiety containing substrate 1g
participated in the reaction to afford the corresponding
spirocyclic product 2g in 64% yield with high
diastereoselectivity.
When the radical scavenger tetramethylpiperidine-N-oxyl
(TEMPO) was added to the reaction mixture under optimal
conditions (Table 1, entry 20), the conversion was
immediately suppressed, and the CF3 radical was trapped to
generate TEMPO–CF3 in ca. 16% yield by 19F-NMR (Fig. 5a
and S3, ESI†).58 This result together with the
photodependence of the reaction (Table 1, entry 19)
suggests a photoinduced radical mechanism. To gain a
deeper mechanistic understanding, the in situ electron
paramagnetic resonance (EPR) of Cu–TDPAT was monitored
under Xe light photoirradiation. The coordination polymer
suspension displays the signal characteristic of Cu(II) sites
with a g value of ca. 2.09,59 and the photoirradiation of the
N–H to the CO2 enriched in the cage. AccordinVgietwoAprtircelevOionluinse
62
results from Sodeoka57 and our laboratory, the Cu2(O2CR)4
DOI: 10.1039/D0NJ02154G
paddle-wheel units might participate in the electrophilic
activation of Togni's reagent. The IR spectrum of Cu–TDPAT
encapsulated with Togni’s reagent II exhibits a shift in the
C=O stretching vibration frequencies of the ester part of
Togni’s reagent II, and the C=N stretching peak of the
melamine moiety in Cu–TDPAT is blue-shifted from 1565 to
1589 cm-1 (Fig. 6c). These shifts reflect the synergistic non-
covalent interactions of Togni's reagent II with the ligand of
the framework together with the Cu(II) node (Fig. 6d). It was
expected that these enzyme-like interactions would help
enrich and activate the substrate/reagent to enhance the
reaction kinetics of each step and avoid mutual interference
among the labile intermediates.
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Cu–TDPAT sample under N2 atmosphere suffers
a
continuous decay of the Cu(II) signal (Fig. 5b). These
phenomena evidence the photoinduced charge transfer
from the melamine-based ligands to the Cu(II) sites in the
coordination polymer (Fig. 5c), with the in situ generation of
Cu(I) species that trigger the generation of CF3 radicals with
Togni’s reagent II.
Fig. 6 (a) Comparison of the IR spectra of Cu−TDPAT (red), 1a (black), and Cu−TDPAT
encapsulated with 1a (blue). (b) Schematic illustration of non-covalent interactions
between 1a and the cage of Cu−TDPAT. (c) Comparison of the IR spectra of
Cu−TDPAT (red), Togni’s reagent II (black), and Cu−TDPAT encapsulated with Togni’s
reagent II (blue). (d) Schematic illustration of non-covalent interactions between
Togni’s reagent and the cage of Cu−TDPAT.
Fig. 5 (a) Control experiment in the presence of TEMPO as a CF3 radical quencher.
(b) In situ EPR monitoring of the Cu(II) species under photoirradiation by Xe light. (c)
Schematic illustration of the photoinduced charge transfer from the TDPAT6- ligand
to the Cu(II) site of Cu−TDPAT.
Based on the above experimental results and previous
reports,29,43,57 a plausible reaction mechanism was proposed,
as shown in Scheme 2. On the one hand, the fixation and
transformation of CO2 occurs irrespective of light irradiation.
Due to the high adsorption capacity of Cu–TDPAT (Fig. 2),
CO2 is enriched within the Cu–TDPAT cage. Similarly, driven
by hydrogen-bonding interactions, allylamine substrate 1a is
also docked and preorganized in the coordination polymer
to form a melamine···1a complex (Figs. 6a–6b). In the
presence of the basic DBU, the spatial proximity between the
adsorbed CO2 and 1a (Scheme 2a) is believed to facilitate the
nucleophilic addition of deprotonated 1a to CO2, and the
The effective absorption of organic dyes46,60,61 and natural
products61 by melamine-based coordination polymers
implies the possibility of Cu–TDPAT for docking
substrate/reagent inside the pores. As shown in the infrared
(IR) spectra, after soaking Cu–TDPAT solids in an acetonitrile
solution of allylamine 1a, the C=N stretching peak position of
the melamine-based ligand of Cu–TDPAT obviously changes
from 1565 to 1602 cm-1, and both the stretching (shifted
from 3340 to 3326 cm-1) and bending (shifted from 1307 to
1320 cm-1) vibrations of the N–H bond of 1a are affected in
this host-guest system (Fig. 6a). Thus, IR analyses reveal the
fixation and activation of substrate 1a within the Cu–TDPAT
(Fig. 6b), which might enhance the nucleophilic addition of
resultant anionic carbamate
A further coordinates to a
nearby Cu(II) site (Scheme 2b) and cyclises to give complex
B
(Scheme 2c). In contrast, the incorporation of the CF3 moiety
is a photodependent process; the melamine-based ligand
absorbs visible-light energy from the Xe light to trigger a PET
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Scheme 2 Proposed mechanism of enzyme-like catalytic behaviours in Cu–TDPAT and conceptual illustration of photoinduced Cu(II)/Cu(I) switch for CO2 incorporation (a to c) and
trifluoromethylation (d to f) steps, respectively (middle inset: the iconic Taiji chart).
process, reducing the Cu(II) sites of the node to Cu(I) species
together with the formation of the radical cationic form of
the melamine moiety (Scheme 2c). The non-covalent
interactions draw Togni's reagent II near the catalytic sites,
which facilitates electron transfer from the in situ
photogenerated Cu(I) site to Togni's reagent II, favouring the
generation of the CF3 radical (Scheme 2d). The inner-sphere
CS2, the isoelectronic counterpart of CO2, into heterocycles
such as 2-thiazolidinethiones, which are common motifs in
agrochemicals66 and drugs.67 However, the possibility of
severe catalyst poisoning by CS2-derived sulfur species
remained a serious problem.68 After soaking Cu–TDPAT
crystals in an acetonitrile solution of CS2, the C=O stretching
peak (1621 cm-1, carboxylate moiety) and C=N stretching
peak (1565 cm-1, melamine moiety) of the ligands in Cu–
TDPAT were intensified and redshifted, and the C=O
vibration peak (1374 cm-1, carboxylate moiety) was
broadened (Fig. 7a). In comparison, after incubating the
crystals of the control catalyst Zn–TDPAT under the same
conditions, only a slight sharpening of the C=N stretching
peak (1559 cm-1, melamine moiety) and C=O vibration peak
(1369 cm-1, carboxylate moiety) of the ligand in Zn–TDPAT
were observed (Fig. S4). These results indicate that stronger
noncovalent interactions are exerted between the Cu–
TDPAT host framework and CS2 guest molecule than those in
the case of Zn–TDPAT, which might facilitate the conversion
of CS2 within the pores of Cu–TDPAT.
redox process between the CF3 radical and complex
B
(Scheme 2e) forms desired product 2a and generates a Cu(I)
site (Scheme 2f).63 The Cu(I) node either undergoes back-
electron transfer to the radical cationic melamine moiety to
recover the initial state of the Cu–TDPAT (Scheme 2a) or
directly enters the next round of the redox reaction with
Togni's reagent II. Another possible route, in which the
alkene of allylamine substrate 1a first reacts with the in situ-
generated CF3 radical and is then subjected to cyclisation
with CO2, cannot be excluded at this time.29,64,65
The photoswitchable Cu(II)/Cu(I) conversion and the non-
covalent-interaction-assisted
enzyme-like
catalytic
behaviours in the rigid Cu–TDPAT impose good control over
the combination of CF3 and CO2 groups into 2-oxazolidone
derivatives. This success inspired us to extend this
heterogeneous methodology to the incorporation of CF3 and
The optimal reaction conditions for installing CF3 and CO2
groups, as displayed in Table 1, entry 20, were adopted for
this extended application. The sulfur-trifluoromethylation of
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allylamine 1a proceeded smoothly, and target compound 3a
ARTICLE
, , 1c, , , , and
Allylamine substrates 1a 70 1b 29 1g 71 1d 72 1Vieew7A3rticle O1nlfin7e4
1
2
3
4
5
6
7
8
,
which possesses the same trifluoromethylated 2-
thiazolidinethione scaffold as a known central nervous
system (CNS) agent,69 was obtained in a moderate yield of
54% (Fig. 7). In comparison, a multi-step synthesis was
required to access this CNS agent by homogeneous methods
in the literature,69 to circumvent the incompatibility
between the CS2 cycloaddition and other steps. When using
a mixture of Cu(II) salt and free ligand as a photocatalyst, we
observed a complex reaction as well as the precipitation of
unassignable sulfur-containing solids, and only trace
amounts of the desired product could be isolated (Fig. 7cb),
which highlights the durability of the heterogeneous
photocatalytic system using Cu–TDPAT against sulfur-
poisoning. In addition, the zinc-based coordination polymer
hardly induced the utilization of CS2 in this reaction (Fig. 7cc).
This photocatalytic approach might be helpful for producing
a library of CNS agents for drug discovery in a sustainable and
orthogonal manner.
were prepared according to literatureDpOrIo: t1o0.c1o0l3s9./D0NJ02154G
NMR spectra were acquired on a Bruker Avance 500 WB or
Bruker Avance 400 WB spectrometer, and chemical shifts
were recorded in parts per million (ppm) relative to TMS.
Elemental analysis was conducted on a Vario EL III elemental
analyser. High-resolution mass spectra (HRMS) were
recorded on an LC/Q-TOF mass spectrometer (Micromass,
England) equipped with a Z-spray ionisation source.
Powder X-ray diffraction (PXRD) measurements were
performed with a PANalytical Empyrean X-ray powder
diffractometer (Cu Kα radiation, 40 kV, 40 mA). Fourier-
transform infrared (FT-IR) spectra were obtained using the
KBr pellet method on a JASCO FT/IR-430. Solid and liquid UV-
vis spectra were recorded on a Hitachi U-4100 UV-vis-NIR
spectrophotometer and a TU-1900 spectrophotometer,
respectively.
Cyclic voltammograms were obtained using a carbon-paste
working electrode; a well-ground mixture of each bulk
sample and carbon paste (graphite and mineral oil) was set
in the channel of a glass tube and connected to a copper
wire. A platinum-wire counter electrode and a Ag/AgCl
reference electrode were utilised. Measurements were
conducted using a three-electrode system in an acetonitrile
solution of ammonium hexafluorophosphate (0.1 M) at a
scan rate of 100 mV·s-1.
Solid and liquid fluorescence spectra were measured using
an Edinburgh FS920 instrument. The excitation and emission
slits were both 3 nm wide, and the wavelengths of excitation
and emission were chosen according to each specific case.
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CO2 sorption measurements
CO2 adsorption data were collected using a Micromeritics
ASAP 2020 surface area and porosity analyser, up to
saturated pressure at 298 K. Before the CO2 sorption
Fig. 7 (a) Comparison of the IR spectra of Cu−TDPAT (red) and Cu−TDPAT
encapsulated with CS2 (blue). (b) Schematic illustration of non-covalent interactions
between CS2 and the Cu−TDPAT cage. (c) Heterogeneous photocatalytic approach to
measurements, the crystals of Cu–TDPAT and Zn–TDPAT
were washed with methanol, treated by supercritical CO2
extraction, activated by a dynamic high vacuum heat
treatment for 24 h, and then stored under nitrogen
atmosphere.
a
the synthesis of an analogue of a central nervous system agent using Cu−TDPAT.
Isolated yield. b Photocatalysed by a mixture of Cu(NO3)2∙3H2O and H6TDPAT under
c
the reaction conditions shown in Table 1, entry 13. Zn−TDPAT as photocatalyst,
under the reaction conditions shown in Table 1, entry 14.
Substrate/reagent encapsulation experiments
Experimental section
Substrate-impregnated crystals were obtained by soaking
the crystals of Cu–TDPAT in
a
solution of the
Materials and methods
substrate/reagent (0.25 mmol) in acetonitrile (0.5 mL) for 12
h. Thereafter, the soaked Cu–TDPAT crystals were lightly
washed with acetonitrile to remove the residual substrate
attached to the surface; then, the solid samples were directly
used for IR examinations. The analogous manipulations were
made for Zn–TDPAT.
Unless otherwise stated, organic solvents were dried and
distilled prior to use in accordance with standard methods.
The other commercially available chemicals were of reagent-
grade quality and were used as received. Thin-layer
chromatography (TLC) was carried out on SiO2 (silica gel 60
F254, Merck), with visualization under UV light. Flash
chromatography was carried out on SiO2 (200–300 mesh).
Cu–TDPAT43 and Zn–TDPAT45 were prepared according to
literature procedures and then activated successively by
washing with methanol, supercritical carbon dioxide
treatment, and vacuum heating. Afterwards, both crystals
were stored in N2-filled Schlenk tubes away from light.
General procedure for photocatalysis by Cu–TDPAT
Cu–TDPAT crystals (0.0125 mmol) and Togni’s reagent II (0.3
mmol) were added to a predried Pyrex tube equipped with a
cooling water system and a rubber septum. This mixture was
subjected to three freeze-pump-thaw cycles using nitrogen.
After adding degassed anhydrous acetonitrile (4.0 mL),
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substrate (0.25 mmol), and DBU (0.50 mmol) by syringe, the
reaction mixture was stirred and irradiated by a Xe light
source (using 400 nm cut-off filter) under CO2 atmosphere
(balloon) for 12 h. The catalyst was recovered by
centrifugation and filtration, and the filtrate was
concentrated under reduced pressure. The product was
isolated via flash chromatography on silica gel from the
crude mixture. In the case of CS2 as the C1 source, the
procedure was similar, except that CS2 (0.50 mmol) was
added into the reaction mixture by syringe, just before the
addition of DBU. The photocatalytic reaction was then
performed for 12 h under nitrogen atmosphere.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21971031, 21890381, and U1608224).
View Article Online
DOI: 10.1039/D0NJ02154G
Notes and references
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biologically
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oxazolidinones
and
2-
thiazolidinethiones. In this report, a highly durable Cu(II)-
melamine coordination polymer, Cu–TDPAT, was employed
in
the
heterogeneous
photocatalytic
oxytrifluoromethylation of allylamine and CO2. In the ground
state, the Cu(II) site of Cu–TDPAT triggered CO2 cycloaddition
with allylamine. Under the photoirradiation by Xe light, Cu(II)
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possibilities with both environmental and industrial
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DOI: 10.1039/D0NJ02154G
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This study describes photoswitchable Cu(II)/Cu(I) catalyses and enzyme-like interactions in Cu–TDPAT for installing CO2/CS2 and
CF3 groups in heterocycles.
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