First pre-functionalised polymeric aromatic framework from mononitrotetrakis(iodophenyl)methane and its applications

Article abstract of DOI:10.1002/chem.201304163

Starting from mononitrotetrakis(iodophenyl)methane as monomer, we report the preparation of the first pre-functionalised porous aromatic frameworks (PAFs) and their application as supports for organometallic catalysts. Neutral coordinate imino-pyridine Schiff base (PAF-NPy) or chiral bis-amino (PAF-NPro) ligands were obtained by post-synthetic treatment of PAF-NH₂ and treated with copper(I) or rhodium(I) to yield the corresponding supported transition-metal catalysts. The as-prepared PAF-NN-M catalysts exhibited activity and selectivity similar to that of the corresponding homogeneous catalysts and were easily removed from reaction media and recycled without loss of activity or selectivity. New copper catalysts: The preparation of pre-functionalised porous aromatic frameworks (PAFs) and their application as supports for organometallic catalysts (see figure) is reported for the first time.

Full text of DOI:10.1002/chem.201304163

DOI: 10.1002/chem.201304163
Full Paper
&
Polymeric Aromatic Frameworks
First Pre-Functionalised Polymeric Aromatic Framework from
Mononitrotetrakis(iodophenyl)methane and its Applications
Ester Verde-Sesto,[a] Mercedes Pintado-Sierra,[b] Avelino Corma,[c] Eva M. Maya,[a]
Jose G. de la Campa,[a] Marta Iglesias,*[d] and Felix Sꢀnchez*[b]
Abstract: Starting from mononitrotetrakis(iodophenyl)me-
thane as monomer, we report the preparation of the first
pre-functionalised porous aromatic frameworks (PAFs) and
their application as supports for organometallic catalysts.
Neutral coordinate imino-pyridine Schiff base (PAF-NPy) or
chiral bis-amino (PAF-NPro) ligands were obtained by post-
synthetic treatment of PAF-NH2 and treated with copper(I)
or rhodium(I) to yield the corresponding supported transi-
tion-metal catalysts. The as-prepared PAF-NN-M catalysts ex-
hibited activity and selectivity similar to that of the corre-
sponding homogeneous catalysts and were easily removed
from reaction media and recycled without loss of activity or
selectivity.
Introduction
chemical environment in organic-solvent-mediated homogene-
ous catalysis.
High-surface-area polymeric compounds with a system of reg-
ular pores are promising materials for preparing solid catalysts
or for use as organic catalyst carriers. Porous organic polymers
(POPs)[1] with pore dimensions in the micro- and mesoporous
range, including porous aromatic frameworks (PAFs) with dia-
mond-like structures,[2] have attracted considerable attention
because they have high stability and high specific surface
areas. Moreover, they are easier to prepare and functionalise[3]
on a large scale under milder conditions than other porous
materials synthesised with expensive organic surfactants or
templates that frequently require thermal treatment for activa-
tion.[4] In addition, owing to the intrinsic nature of the organic
pores, the “cavities” of the organic polymers can mimic the
The properties of these porous materials make them good
candidates for applications in gas storage,[5] gas separation[6]
and heterogeneous catalysis.[7] Recently, the post-modification
of a PAF was reported to yield a bifunctional amino-sulfonated
system for application as an acid–base catalyst,[8] whereas
monofunctional sulfonated[9] or polyamine[10] derivatives have
also been employed as catalysts[11] or for CO2 uptake.
To the best of our knowledge, no work regarding the prepa-
ration of pre-functionalised PAFs has yet been reported, only
the synthesis of a family of not pure aromatic framework con-
jugated polymers by Sonogashira coupling has been de-
scribed.[12] Thus, we describe herein the synthesis of an amino-
functionalised PAF (PAF-NH2) starting from the corresponding
nitro-functionalised polymer (PAF-NO2) prepared from a new
monomer containing a nitro group. We also report the trans-
formation of the amino group into ligands for the construction
of organometallic catalysts on the pore walls. This has been
achieved through the reaction of PAF-NH2 1) with tBoc-proline
(Boc=tert-butoxycarbonyl) followed by deprotection to give
PAF-NPro and 2) by treatment with picolinaldehyde to form
PAF-NPy. Thus, we obtained two bidentate ligands that coordi-
nate with rhodium and copper to yield the corresponding
transition-metal catalysts covalently bonded to PAF (PAF-NPro-
M and PAF-NPy-M, Figure 1). Finally, the resulting heterogene-
ous catalysts have been successfully used for promoting vari-
ous reactions, for example, cyclopropanation, hydrogenation
and a multi-component reaction to give propargylamines with
high efficiency and excellent recyclability.
[a] E. Verde-Sesto, Dr. E. M. Maya, Prof. J. G. de la Campa
Departamento Quꢀmica Macromolecular Aplicada
Instituto de Ciencia y Tecnologꢀa de Polꢀmeros, CSIC
C/Juan de la Cierva, 3, 28006 Madrid (Spain)
[b] Dr. M. Pintado-Sierra, Prof. F. Sꢁnchez
Departamento Sꢀntesis, Estructura y Propiedades
de Compuestos Orgꢁnicos
Instituto de Quimica Organica General, CSIC
C/Juan de la Cierva 3, 28006 Madrid (Spain)
E-mail: felix-iqo@iqog.csic.es
[c] Prof. A. Corma
Instituto de Tecnologꢀa Quꢀmica, UPV-CSIC
Avda de los naranjos, sn., 46020 Valencia (Spain)
[d] Prof. M. Iglesias
Departamento Nuevas Arquitecturas en Quꢀmica de Materiales
Instituto de Ciencia de Materiales de Madrid, CSIC
C/Sor Juana Inꢂs de la Cruz, 3, Cantoblanco, 28049 Madrid (Spain)
Fax: (+34)913720623
E-mail: marta.iglesias@icmm.csic.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304163.
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Figure 1. Heterogenised PAF-transition-metal complexes.
Figure 2. FTIR spectra of PAF-NO2 and PAF-NH2.
Results and Discussion
Synthesis of functionalised monomer and functionalised
PAFs
tive and reproducible synthesis of the networks and to give
a porous structure. PAF-NO2 could also be prepared by post-
functionalisation of PAF by nitration with nitric acid in trifluoro-
acetic acid.[7] The nitro groups were reduced to the corre-
sponding amino groups by using SnCl2·2H2O, as verified by
FTIR spectroscopy (Figure 2). The band at 1529 cmÀ1 assigned
to the nitro groups of PAF-NO2 disappeared upon reduction
and two new bands emerged at 3500–3300 (w, NH) and
1615 cmÀ1 (m, NH), which have been assigned to the presence
of -NH2 groups in PAF-NH2.
The thermal stabilities of PAF-NO2 and PAF-NH2 were evalu-
ated by thermogravimetric analysis (TGA) in air (Figure 3). Both
functionalised polymers showed high thermal stability, decom-
position occurring at around 4008C (TGA of PAF-NO2 shows
a small step corresponding to the loss of -NO2 groups). Except
for the shoulder attributed to the loss of the nitro groups, the
thermograms and differential thermograms of both polymers
are practically identical, which indicates that the reduction of
the -NO2 groups does not affect the thermal properties to
a large extent.
4,4’,4’’-[(4-Iodo-3-nitrophenyl)methanetriyl]tris(iodobenzene) (3)
was synthesised in three steps following the procedure depict-
ed in Scheme 1. First, triphenylcarbinol was treated with o-ni-
troaniline in a Friedel–Crafts alkylation reaction to yield 2-nitro-
4-tritylaniline (1). Because only the melting point and elemen-
tal analysis have previously been reported for this product,[13]
1
we have also characterised this compound by H and 13C NMR
spectroscopy and mass spectrometry. In the second step, the
amine group was converted into an iodine by sequential di-
azotisation/iodination.[14] Finally, a mixture of 2, I2 and [bis(tri-
fluoroacetoxy)iodo]benzene under the standard conditions[15]
yielded the novel nitrotetraiodo monomer 3.
The synthesis of PAF-NO2 was achieved quantitatively by
a Suzuki coupling reaction of 4,4’,4’’-[(4-iodo-3-nitrophenyl)me-
thanetriyl]tris(iodobenzene) (3) with 1,4-phenylenediboronic
acid (Scheme 1) following a literature procedure.[16] Microwave-
assisted synthesis (1458C, 12 bar) was necessary for a quantita-
The materials were also char-
acterised by solid-state 13C cross-
polarisation magic-angle spin-
ning (CP/MAS) NMR spectrosco-
py and the spectra are dominat-
ed by resonances from the aro-
matic groups (see Figure S11 in
the Supporting Information).
The porosity of the new mate-
rials was characterised by nitro-
gen sorption after heating the
samples at 1008C. The shapes of
both isotherms (see Figure S15)
are characteristic of materials
with meso/macroporous struc-
tures and the specific surface
Scheme 1. Synthesis of monomer 3 and functionalised PAFs PAF-NO2 and PAF-NH2. Reagents and conditions:
i) HCl, AcOH; ii) NaNO2/KI; iii) I2, PhI(OCOCF3)2; iv) 1,4-phenylenediboronic acid, MW, 12 bar, 1458C, 10 min;
v) SnCl2·2H2O, THF, 908C.
areas and pore sizes obtained
are recorded in Table 1.
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Scheme 2. Synthesis of heterogenised PAF-ligands. Reagents and conditions:
i) picolinaldehyde, ethanol, reflux; ii) N-tBoc-l-proline, isopropyl chlorofor-
mate, triethylamine; iii) trifluoroacetic acid.
Figure 3. Thermograms of functionalised PAFs PAF-NO2 and PAF-NH2.
Table 1. Textural properties of functionalised PAFs PAF-NO2 and PAF-
NH2.
Polymer
Surface area [m2 gÀ1
]
Pore volume [cm3 gÀ1
]
PAF-NO2
PAF-NH2
302
248
0.23
0.22
Figure 5. Thermograms of PAF-NProBoc and PAF-NPro.
Figure 4. SEM images of PAF-NO2 and PAF-NH2.
PAF-NProBoc and PAF-NPro were studied by TGA to control if
the reaction had taken place. Thus, in Figure 5 a loss can be
observed at 2008C corresponding to the tert-butoxycarbonyl
group. This step was not observed in the thermogram of PAF-
NPro.
Both PAF-NPro and PAF-NPy were characterised by solid-
state 13C NMR (see Figures S16 and S17 in the Supporting In-
formation) and FTIR spectroscopy (Figures S22 and S23), TGA
(Figures S24 and S25), X-ray diffraction (Figures S26 and S27),
nitrogen sorption isotherms (Figures S28 and S29), SEM (Fig-
ure S30), elemental analysis (Table S3) and energy-dispersive X-
ray spectroscopy (EDX; Table S4).
The SEM images of both functionalised polymers (Figure 4)
show spherical structures and agglomerates with morpholo-
gies typical of porous cross-linked polymers.
Preparation of PAF ligands on the pore wall
The incorporation of different transition metals requires the
previous modification of PAF-NH2 with coordinating groups,
such as prolinamide or Schiff base and pyridine (imino-pyridine
compounds). PAF-NPy was prepared by the reaction of PAF-
NH2 with picolinaldehyde to yield the corresponding imino-
pyridine ligand (Scheme 2).
The FTIR spectrum of PAF-NPro shows a n(C=O) band at
1686 cmÀ1 characteristic of prolinamide groups, which were
also evident in the solid-state 13C NMR spectrum at d=
173 ppm (see Figure S17).
To introduce the proline derivative, PAF-NH2 was treated
with N-tBoc-l-proline, isopropyl chloroformate and triethyl-
amine to give PAF-NProBoc (Scheme 2). The removal of the
Boc group with trifluoroacetic acid gave rise to PAF-NPro.
The powder X-Ray diffraction patterns of PAF-NPro and PAF-
NH2 (Figure 6) show similar patterns, corresponding to an
amorphous structure.
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Scheme 4. Synthesis of heterogenised PAF-NN-Cu complexes.
complexes with a copper loading of 0.045–0.050 mmolgÀ1
(Scheme 4).
Figure 6. PXRD patterns of PAF-NH2 and PAF-NPro.
The presence of the characteristic functional groups of NPy
or NPro in the heterogenised materials PAF-NN-Cu was verified
by FTIR spectroscopy (see Figures S22 and S23 in the Support-
ing Information). The stretching vibration modes of the porous
framework appear at 701, 750, 810, 1487 (NPy), 708, 742, 814
and 1488 cmÀ1 (NPro), as in the parent PAF-NH2. The presence
of ligands is apparent from the n(C=N, C=O (NPro), C=C)
stretching modes at 1720–1570 cmÀ1 and the copper complex
exhibits bands at 1713, 1691 and 1596 cmÀ1. The n(C=N) bands
of the imine and pyridine moieties are observed at 1594 cmÀ1
.
Notably, this C=N bond absorption is redshifted in the catalyst
due to the strong interaction between nitrogen atoms in the
Schiff base and the metal. A weak band is also detected at
2036 cmÀ1 associated with the CꢀN moiety in the CH3CN li-
gands. The n(ClÀO) band appears at 1120 cmÀ1, and no split-
ting or broadening of n3 or n4 is observed in the spectra of
complexes containing ClO4 anions.
Scheme 3. Synthesis of ligands for the formation of soluble complexes. Re-
agents and conditions: i) picolinaldehyde, ethanol, reflux; ii) tBoc-l-proline,
isopropyl chloroformate, triethylamine; iii) CF3COOH.
The PAF-NNCu complexes were also characterised by solid-
state 13C CP/MAS NMR spectroscopy (see Figures S18 and S19).
The spectra are dominated by the resonances of the aromatic
groups between d=128 and 144 ppm. The characteristic sig-
nals of the pyridine group are indistinguishable from the rest
of the aromatic signals, whereas the proline moiety can be
clearly observed by the presence of signals between d=20
and 60 ppm.
The 13C CP/MAS NMR spectrum of PAF-NPy-Cu (see Fig-
ure S18) shows a weak signal at d=151 ppm corresponding to
the imine carbon (-HC=N-) and the 13C CP/MAS NMR spectrum
of PAF-NPro-Cu (see Figure S19) exhibits a signal at d=
169.3 ppm attributed to the CO group. The signals correspond-
ing to CH3CN in the copper complexes are observed at d=
161.8 (CN) and 30.0 ppm (CH3).
Preparation of ligands for homogeneous catalysis
Scheme 3 shows the synthesis of ligands for the formation of
soluble complexes (prepared for comparative purposes); we
used 4-tritylaniline as precursor to obtain the new ligands. This
precursor was treated with picolinaldehyde to obtain quantita-
tively (E)-N-(pyridin-2-ylmethylene)-4-tritylaniline (NPy). To in-
troduce the proline moiety, 4-tritylaniline was treated with N-
tBoc-l-proline, isopropyl chloroformate and triethylamine to
give NProBoc. The removal of the tBoc group with trifluoro-
acetic acid gave rise quantitatively to NPro. These ligands were
characterised by analytical and spectroscopic methods (see
Figures S1–S4 in the Supporting Information)
The porous properties of the heterogenised PAFs were also
investigated (Table 2). The results obtained show that incorpo-
ration of the metal slightly reduces the specific surface area
and pore volume, as has previously been observed after the in-
corporation of metal into other porous polymers.[1f]
Synthesis of heterogenised PAF-Cu complexes
The PAF-NPro and PAF-NPy ligands, with bidentate nitrogen-
binding sites, were treated with [Cu(CH3CN)4]ClO4 to yield
the copper-incorporated heterogenised catalyst PAF-NN-Cu
Thermogravimetric analyses (see Figures S24 and S25) re-
vealed that the introduction of the metal significantly reduces
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Table 2. Textural properties of PAF ligands and Cu complexes.[a]
Polymer
Surface area
Average pore size
[nm]
Pore volume
[cm3 gÀ1
]
[m2 gÀ1
]
PAF-NPro
PAF-NPro- 173
190
10.9
9.2
0.33
0.23
Cu
PAF-NPy
PAF-NPy-
Cu
216
174
5.7
5.0
0.19
0.18
[a] The BET and BJH methods have been developed for rigid inorganic
solids and have not been optimised for analysing pure organic porous
materials. We obtained slightly different values for several batches.
Scheme 5. Synthesis of soluble copper complexes.
occurs, n(ClÀO) appears at 1166 and 1080 cmÀ1, and signals
due to the NCCH3 group can be observed at 2253 cmÀ1.
Catalytic activity of PAF-NN-Cu and homogeneous copper
complexes in the cyclopropanation of alkenes
After the incorporation of copper into the porous polymers,
the reactivities of these new solid copper catalysts were inves-
tigated. The cyclopropanation of alkenes was used as probe re-
action to evaluate their catalytic performance. The reproduci-
bility of the catalyst preparation and reactivity was excellent.
The metal-catalysed cyclopropanation reaction between
diazo compounds and alkenes is one of the most important
methods for obtaining cyclopropane derivatives and has been
widely used in organic synthesis.[17] CuI complexes were ap-
plied here as catalysts for olefin cyclopropanation reactions be-
tween ethyl diazoacetate and styrene, and the results of the
study are presented in Table 3. GC and GC-MS analyses of the
reaction mixtures showed that cyclopropanes (as cis/trans mix-
tures) are formed as the main products of these reactions to-
gether with minor amounts of coupling products.
Figure 7. SEM images of PAF ligands and complexes
the thermal stability of both types of PAF. The weight loss is
high (around 60%), which indicates a generalised decomposi-
tion of the polymer, probably induced by the metal. The SEM
images of the functionalised polymer ligands and complexes
are similar to those of the precursors, showing typical porous
polymer morphology (Figure 7).
The influence of copper content on the catalytic yield of cy-
clopropane is shown in Table 3. The best yield of cyclopro-
panes was reached with 10 mol% of copper catalyst. In addi-
tion, the effects of solvent and reaction temperature on this re-
action were also investigated. No desired product was ob-
tained below 408C, instead mainly fumarate and maleate cou-
pling products were observed. When ClCH2CH2Cl was used as
solvent at 608C, higher selectivity towards cyclopropanes was
achieved. A higher temperature did not lead to an increase in
yield. When diazoacetate was added in one portion, self-cou-
pling products were obtained, whereas if the addition was car-
ried out over 4 h through an automatic syringe pump, the de-
sired products were obtained with improved yields and selec-
tivities.
Synthesis of reference copper complexes for homogeneous
catalysis
The experimental data allowed us to establish that both NPro
and NPy ligands act as chelating ligands. Diamagnetic cop-
per(I) complexes were obtained in high yields by reaction of
the corresponding ligands with [Cu(CH3CN)4]ClO4 (Scheme 5,
1
see the Supporting Information). All the signals in the H NMR
The reactivities of different alkene derivatives were studied
with PAF-NN-Cu as catalysts to verify the substrate scope of
the reaction. As shown in Table 3, the catalyst worked well
with different alkenes, giving the desired products in excellent
yields. Styrene was converted almost completely to give high
yields of cyclopropanecarboxylates (see entries 1–5, Table 3). b-
Methyl- (entries 6–8) and a-methylstyrene (entries 9–11) were
spectra are broad and shifted upfield in comparison with the
uncoordinated ligands and ESI-MS shows the mass for the mo-
lecular peak at m/z=546 for NPy-Cu ([(NPy)Cu(CH3CN)(H2O)])
and m/z=531 for NPro-Cu [(Npro)Cu(H2O)2]+). FTIR analysis of
NPro-Cu shows that n(C=O) is shifted to a lower frequency
(Dn=75 cmÀ1), which indicates that coordination to the metal
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(USY-Cu and MCM-41-Cu) was
made[19] (Table 4, entries 6 and
7). The silica oxide based or-
Table 3. Performances of PAF-NN-Cu as catalysts in cyclopropanation reactions.[a]
dered mesoporous
material
MCM-41 was covalently modified
to form the MCM-41-Cu catalyst.
This supported catalyst gave
a significantly lower yield than
PAF-NN-Cu, despite its higher
surface area. The USY-Cu catalyst
gave still poorer results. Compar-
ison with a porous organo-inor-
ganic hybrid polymer Cu-MOF
(Cu3(benzenetricarboxylate)2,
Catalyst ([mol.%])
Substrate
Conv. [%], time [h]
(selectivity [%])[b]
trans/cis
ee [%][c]
1
2
3
4
5
6
7
8
NPy-Cu (5)
PAF-NPy-Cu (10)
NPro-Cu (5)
PAF-NPro-Cu (10)
PAF-NH2-Cu (10)
NPy-Cu (5)
91, 23 (90)
82, 18 (50)
60, 18 (100)
90, 18 (95)
14, 24 (100)
57, 18 (60)
60, 18 (60)
92, 18 (50)
85:15
70:30
85:15
70:30
60:40
70:30
70:30
85:15
0
0
60
40[d]
0
0
30
40[d]
NPro-Cu (5)
PAF-NPro-Cu (10)
Cu3BTC2; Table 4, entry 5)[19] re-
vealed a similar trans/cis per-
formance to that of the support-
ed PAF-NPy-Cu catalyst. These
results support our hypothesis
that an organic-like microenvir-
9
10
11
NPy-Cu (5)
NPro-Cu (5)
PAF-NPro-Cu (10)
66, 2 (97)
72, 2 (97)
96, 18 (99)
55:45
55:45
55:45
0
3
<5
12
13
14
NPy-Cu (5)
NPro-Cu (5)
PAF-NPro-Cu (10)
91, 16 (97)
97, 2 (95)
96, 18 (80)
94:6
95:5
75:25
0
47
85[d]
onment
of
polymer-based
porous materials provide excel-
lent catalysts for organic trans-
formation in heterogeneous pro-
cesses and improved enantiose-
lectivity of cycloaddition reac-
tions.
[a] T=608C, ClCH2CH2Cl. [b] Selectivity towards cyclopropanes; remaining diazo compound was converted into
self-coupling products. [c] trans enantiomer determined by HPLC (chiralcel OD-H column, hexane/isopropanol:
95:5). [d] This value is the average of ten experiments.
also successfully cyclopropanated in good yields with moder-
ate diastereoselectivity. The PAF-NN-Cu-supported catalysts re-
quired longer reaction times to reach yields as high as the ho-
mogeneous catalysts and a decrease in the enantioselectivity
for styrene as substrate (trans isomer: ee=60 vs. 40%, entries 3
and 4, Table 3) was observed. Cyclopropanation of 3,4-dihydro-
2H-pyran led to a trans/cis ratio of 95:5 with the homogeneous
catalyst and a ratio of 75:25 for PAF-NPro-Cu. However, higher
enantioselectivity (ee) than for the homogeneous catalyst was
obtained with PAF-NPro-Cu (trans isomer: ee=47 vs. 85%, en-
tries 13 and 14, Table 3). The lower reactivity of the heteroge-
neous PAF-NN-Cu catalyst, relative to the corresponding ho-
mogeneous catalyst, is probably a result of poorer accessibility
of the reagents to the catalytic centre within the pores.
To verify whether the PAF-NN-Cu complexes behave as real
heterogeneous catalysts, these materials were separated from
the reaction media and the solutions employed in new cyclo-
propanation experiments. We observed that the reaction in
the original reaction media stopped and that no further con-
version was achieved. The concentration of copper in the solu-
Table 4. Results of catalysed cyclopropanation reactions involving sty-
rene and ethyl diazoacetate.
Entry
Catalyst[a]
Conv. [%], time [h]
(selectivity [%])[a]
trans/cis
ee [%]
1
2
3
4
5
6
7
NPro-Cu
60, 18 (100)
82, 3 (80)
74, 2 (49)
66, 20 (79)
68, 3 (98)
32, 20 (100)
60, 21 (100)
85:15
70:30
66:33
62:38
70:30
55:45
59:41
60
40
<5
<5
0
PAF-NPro-Cu
Cu–Schiff base-1[18]
Cu–Schiff base-2
The complex (S)-N-(4-tritylphenyl)pyrrolidine-2-carboxamide–
Cu (NPro-Cu catalyst) gave significantly improved trans/cis dia-
stereo- and enantioselectivities (Table 4, entry 1) compared
with other previously reported[18] chiral prolinamide homoge-
neous Schiff-base–copper complexes (Figure 8 and entries 3
and 4 in Table 4). The structures of these complexes (Figure 8)
show that multidentate ligands can have considerable flexibili-
ty in their conformations. The square-planar coordination of
the metal centre confers a reasonable planarity on the com-
plexes suitable for the formation of stacked arrangements with
potential intermetallic interactions that may be responsible for
the lack of chirality in the cyclopropanation reaction. One im-
portant aspect of the porous polymer-based solid-supported
catalysts is the organic-like microenvironment that differenti-
ates PAFs from inorganic porous catalysts. To verify this hy-
pothesis, a comparison with inorganic-based porous materials
[19]
Cu3BTC2
USY-Cu–Schiff base[18]
MCM-41-Cu[19]
<5
<5
[a] Selectivity towards cyclopropanes; remaining diazo compound was
converted into coupling products.
Figure 8. Molecular structures of Cu–Shiff-base complexes.[18]
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tion, as determined by inductively coupled plasma (ICP) analy-
sis, was less than 0.01 ppm. These results indicate that very
little copper leaching occurs during the cyclopropanation reac-
tion and that no homogeneous reaction can be detected in
the filtrate. PAF-NPro-Cu showed good reusability, and good
reactivity was observed even if the catalyst was recycled 10
times.
Scheme 6. Metal-catalysed three-component synthesis of propargylamines.
Table 5. Performances of Cu catalysts in an A3-coupling reaction involv-
ing piperidine, phenylacetylene and benzaldehyde.[a]
Entry
Catalyst
Yield [%][b]
Multi-component PAF-NN-Cu-catalysed reactions of alde-
hydes, terminal alkynes and amines
1
2
3
4
NPro-Cu
PAF-NPro-Cu
NPy-Cu
80
75–80
75
Propargylamines are high-value building blocks in organic syn-
thesis[20] and their structural motif has been found in various
natural products[21] as well as in important pharmaceutical[22]
and phytoprotective[23] compounds. Three-component reac-
tions for the synthesis of propargylamines are known, either as
such[24] or promoted by various transition metals.[25] However,
the use of these homogeneous catalysts has some disadvan-
tages as they are expensive, non-recyclable, and their separa-
tion from reaction mixtures is tedious. This led to the prepara-
tion of different heterogeneous, recyclable catalysts. A sup-
ported gold-metal–organic framework catalyst[26] has been re-
ported as being highly active and selective for domino-cou-
pling and cyclisation reactions in the liquid phase. Other
metal-based heterogeneous catalysts have also been used suc-
cessfully: AgI–tungstophosphoric acid,[27] CuI anchored on
a silica gel support[28] or on USY-zeolite,[29] a CuII salt on a hy-
droxyapatite support,[30] or N-heterocyclic carbene–CuI on
a silica support.[31] We have examined here the applicability of
both PAF-NN-Cu complexes as catalysts for the three-compo-
nent reaction, and it has been shown that they are highly
active, selective and recyclable catalysts.
PAF-NPy-Cu
40–50
[a] Reagents and conditions: aldehyde (0.19 mmol), piperidine
(0.22 mmol), phenylacetylene (0.28 mmol), catalyst (5 mol%), CHCl3
(2 mL), 708C, 24 h. [b] Yields of isolated product based on aldehyde; ee
value<5% in all cases.
fuged to separate the solid catalyst from the reaction mixture,
and then it was reused for at least six additional reaction
cycles without loss of activity.
Study of the applicability of the heterogenisation of PAFs
To examine the scope of applicability of the PAF as a support
for homogeneous catalysts, we also prepared the correspond-
ing RhI complexes and their homogeneous counterparts. The
PAF-ligands were treated with [Rh(cod)(thf)2]BF4 to give the
corresponding PAF-NN-Rh heterogenised complexes with
a rhodium loading of 0.15–0.22 mmolgÀ1 (Scheme 7). Full char-
acterisation data for the PAF derivatives are included in the
Supporting Information.
The behaviour of PAF-NN-Cu complexes compared with sim-
ilar soluble counterparts in MCRs (multi-component reactions)
was screened by using the above-mentioned synthesis of
propargylamines (Scheme 6). Piperidine, benzaldehyde and
phenylacetylene were chosen as model substrates in a ratio of
1:1.2:1.5 (see Table 5). The conditions chosen were 5 mol% of
catalyst, 2 mL of chloroform as solvent and reflux temperature
(708C). PAF-NPro-Cu (entry 2)
The catalytic properties of the PAF-NPro-Rh catalyst in the
hydrogenation of alkenes and imines (diethyl itaconate, diethyl
(E)-2-benzylidenesuccinate, a-methylstilbene, methyl (Z)-2-
benzamido-3-(4-methoxyphenyl)acrylate, ethyl (Z)-2-benzami-
do-3-phenylacrylate or (E)-N,1-diphenylethan-1-imine) were ex-
amined and compared with those of the corresponding homo-
geneous catalyst under conventional conditions (Table 6). Both
displayed a similar activity to
that of the corresponding solu-
ble catalyst (entry 1) whereas
PAF-NPy-Cu (entry 4) showed
a poorer performance than its
reference
soluble
complex
(entry 3). No conversion was
found in the absence of catalyst
under identical conditions.
To examine the recyclability of
the catalysts, the condensation
reaction between piperidine,
benzaldehyde and phenylacety-
lene was repeated several times
with the same supported cata-
lyst. After completion of each re-
action, the contents were centri- Scheme 7. Synthesis of heterogenised PAFs-NN-Rh complexes.
Chem. Eur. J. 2014, 20, 5111 – 5120
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Synthesis of 4,4’,4’’-[(4-iodo-3-
nitrophenyl)methanetriyl]-
tris(iodobenzene) (3)
Table 6. Performances of rhodium complexes in hydrogenation reactions.[a]
NPro-Rh
PAF-NPro-Rh
Conv. [%], time [h]
Entry
Substrate
Conv. [%], time [h]
96, 0.83
TOF[b]
7000
5400
TOF[b]
1303
2880
A mixture of compound 2 (5 g,
10 mmol), I2 (7.75 g, 30.5 mmol),
[bis(trifluoroacetoxy)iodo]benzene
(10.9 g, 25.4 mmol) and CCl4
(80 mL) was stirred at reflux for
18–20 h. The resulting precipitate
was filtered and the residue
washed with CCl4, sodium thiosul-
fate and diethyl ether. The residue
was then washed with sodium
thiosulfate and extracted with
chloroform. The organic phase was
dried over sodium sulfate and puri-
fied by column chromatograph
1
2
100, 3.5
99, 0.33
100, 0.33
3
4
88, 3.33
60, 18[c]
2364
450
90, 1
1950
624
90, 20
[a] Reagents and conditions: catalyst (0.1 mol%), H2 (2 bar), 408C, EtOH. [b] mmol substrate/mmol catalysth.
[c] Reagents and conditions: catalyst (1%), H2 (4 bar), 708C, toluene/ethanol (2:1).
with
heptane/dichloromethane
(gradient 9:1 to 5:5) as eluent.
1H NMR (300 MHz, CDCl3): d=7.90
(d, 1H; j-H), 7.70 (d, 1H; g-H), 7.62 (d, 6H; b-H), 7.05 (dd, 1H; k-H),
6.86 ppm (d, 6H; c-H); 13C NMR (75 MHz, CDCl3): d=152.8 (C-h),
147.6 (C-f), 143.9 (C-d), 140.6 (C-j), 137.5 (C-c), 135.6 (C-k), 132.1 (C-
complexes showed significant activities for the hydrogenation
reaction with TOF values of up to 7000 hÀ1 for a-methylstil-
bene. Although the catalytic activity is good, the enantioselec-
tivity obtained was very low, even in the presence of the phos-
phine 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP) as
chiral auxiliary.
b), 126.7 (C-g), 93.1 (C-a), 84.5 (C-i), 63.9 ppm (C-e); IR (KBr): n˜max
=
3056 (narCÀH), 1530, 1478, 1395, 1343 (narCÀC, nNO2), 1064, 1018, 1003,
820, 810, 758, 731 (nCN, darCÀH, darNO), 527, 490 cmÀ1 (nCÀI); MS (EI):
m/z (%): 742 [MÀ127]+ (4), 666 (70), 621 (42), 493 (50), 239 (100).
To determine the durability and stability of the rhodium cat-
alyst under the reaction conditions, the heterogeneous catalyst
was isolated and washed thoroughly by centrifugation and
then dried at 80 8C for 8 h under vacuum. Then the catalyst
was reused with a fresh charge of solvent and reactants under
the same reaction conditions, with the activity being main-
tained for at least five runs.
Microwave-assisted synthesis of PAF-NO2
Compound 3 (200 mg, 0.230 mmol), 1,4-phenylenediboronic acid
(2.2 equiv, 84 mg, 0.506 mmol), triphenylphosphine (30 mg,
0.115 mmol), sodium bicarbonate (85 mg, 1.31 mmol) and palladi-
um acetate (3 mg, 0.015 mmol) were suspended in a mixture of
DMF (2 mL) and water (0.5 mL). The mixture was degassed by bub-
bling N2. Microwave heating was performed in a computer-con-
trolled CEM Discover microwave with temperature and pressure
control. The initial heating was performed at a power input of
75 W. After the pressure reached around 12 bar, the heating was
stopped for 1 min. Then the heating was continued until a reaction
temperature of around 1458C was reached. The reaction was per-
formed at this temperature and a pressure of around 5–7 bar for
10 min. After cooling to room temperature, the mixture was fil-
tered and the crude product was washed for around 1 h with a so-
lution of water (50 mL), conc. HNO3 (1 mL) and conc. HCl (1 mL).
The mixture was filtered and the crude product washed with DMF
(3ꢂ30 mL), THF (3ꢂ30 mL), water (3ꢂ30 mL), CH2Cl2 (3ꢂ30 mL)
and diethyl ether (2ꢂ20 mL). After drying in vacuo a pale yellow
powder was obtained. The reaction yield was nearly quantitative
based on the molecular weight of the ideal, fully condensed prod-
ucts. Elemental analysis calcd (%) for C37H23NO2 (513): C 86.55, H
4.48, N 2.73; found: C 78.62, H 4.57, N 2.75.
Conclusion
A new type of porous organic-polymer-based catalyst has
been developed starting from a pre-functionalised monomer.
This porous polymer has been used as a support to heterogen-
ise copper and rhodium catalysts. The catalysts were readily
prepared on a large scale from cheap materials and were
highly effective in promoting organic reactions. Notably, these
catalysts exhibited catalytic efficiencies comparable to the cor-
responding homogeneous soluble catalysts in various organic
reactions. More importantly, they displayed strong durability,
could be easily removed from reaction media and recycled in
successive reactions, behaving similarly to catalysts bonded to
other solid supports. Other organometallic catalysts could also
be immobilised onto these porous polymer supports, which
may lead to more practical applications for immobilised homo-
geneous catalysts.
Synthesis of PAF-NH2
.
A mixture of SnCl2 2H2O (1.5 g, 6.66 mmol, previously pulverised in
a mortar) and PAF-NO2 (265 mg, 0.512 mmol) was dispersed in an-
hydrous THF (50 mL) in a three-necked flask equipped with a con-
denser. The reaction mixture was stirred at room temperature for
30 min and then heated at reflux for 48 h. After cooling to room
temperature, the solid obtained was filtered and washed with a so-
lution of 10 wt% NaOH (5ꢂ25 mL), water (5ꢂ30 mL), CH3CN (3ꢂ
30 mL) and diethyl ether (2ꢂ20 mL). The resulting beige powder
Experimental Section
Precursors 1 and 2 were obtained as described in the literature.[13]
Chem. Eur. J. 2014, 20, 5111 – 5120
www.chemeurj.org
5118
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
was dried under vacuum at 1008C for 12 h. The reaction yield was
near quantitative. Elemental analysis calcd (%) for C37H25N (483): C
91.93, H 5.99, N 2.87; found: C 81.73, H 5.41, N 2.69.
for 15 min. Then the solution of [Rh(cod)(L)]BF4 was added to the
mixture and stirred for 48 h. After this time, the solid was centri-
fuged and washed with THF (3ꢂ10 mL), CH2Cl2 (3ꢂ10 mL) and di-
ethyl ether (10 mL). Finally, the solid was dried under vacuum over-
night at 1008C to yield the corresponding rhodium heterogeneous
catalyst.
Preparation of PAF-NN ligands
Synthesis of PAF-NPy: PAF-NH2 (0.2 g) and absolute ethanol
(100 mL) were added to a round-bottomed flask equipped with
a magnetic stirrer and a condenser. Picolinaldehyde (0.10 mL,
0.13 g, 1.23 mmol) and a drop of formic acid were added to this
suspension. The reaction mixture was heated at reflux for 12 h.
After this time, the solid was centrifuged and washed with EtOH
(3ꢂ50 mL), CH2Cl2 (3ꢂ50 mL) and diethyl ether (50 mL). Finally, the
solid was dried under vacuum at 1008C overnight to yield PAF-
NPy quantitatively. Elemental analysis calcd (%) for C43H28N2 (572):
C 90.21, H 4.89, N 4.90; found: C 82.95, H 4.84, N 4.12.
PAF-NPy-Rh: ICP-OES analysis calcd for 14% functionalization: Rh
2.52; found: Rh 2.18.
PAF-NPro-Rh: ICP-OES analysis calcd for 8% functionalization: Rh
1.26; found: Rh 1.50.
Soluble ligands and complexes
Details for the preparation of the soluble ligands and their corre-
sponding copper and rhodium complexes are presented in the
Supporting Information.
Synthesis of PAF-NProBoc: A solution of tBoc-l-proline (0.35 g,
1.64 mmol) and anhydrous THF (15 mL) under nitrogen atmos-
phere and with vigorous stirring was cooled to À108C. Then 1m
isopropyl chloroformate in toluene (1.8 mL, 1.8 mmol) and triethyl-
amine (0.23 mL, 0.17 g, 1.64 mmol) were added dropwise at À108C
and the reaction mixture was stirred for 3 h at room temperature.
After this time, PAF-NH2 (0.5 g) was added slowly to the reaction
mixture, which was then stirred at room temperature for 12 h and
subsequently heated at reflux for 24 h. After completion of the re-
action, the salts formed were removed by washing with water. The
resulting solid was washed with EtOH (3ꢂ50 mL), CH2Cl2 (3ꢂ
50 mL) and diethyl ether (50 mL). Finally, the solid was dried under
vacuum at 1008C overnight to give PAF-NProBoc.
General procedure for the cyclopropanation reactions
In a 10 mL round-bottomed flask, the diazo compound (11 mmol)
was slowly added to the catalyst (0.04 mmol) and alkene (8 mmol)
in a dropwise manner through a syringe pump over 4 h, reaction
temperature 608C, reaction time 24 h. The evolution of gas was
observed along with a smooth change in colour. The reaction was
monitored by gas chromatography on an HP5890 II GC-MS chro-
matograph equipped with a cross-linked methyl silicone column
(SPB, 25 mꢂ0.2 mm x 0.33 mm; helium as carrier gas. 20 psi; injec-
tor temperature: 2308C; detector temperature: 2508C; oven pro-
gram for styrene: 708C (3 min), 158CminÀ1 to 2008C (5 min); reten-
tion times: ethyl diazoacetate 3.2 min, styrene 3.82 min, n-decane
5.47 min, diethyl maleate 7.84 min, diethyl fumarate 8.02 min, cis-
cyclopropanes 10.91 min, trans-cyclopropanes 11.41 min). When
the evolution of nitrogen had finished, the volatiles were removed
and the extract was purified by flash chromatography using a 1:9
mixture of ethyl acetate/hexane as eluent. For the filtration tests,
part of the reaction suspension was withdrawn at a certain conver-
sion at the reaction temperature, the catalyst was separated and
the supernatant was allowed to react further in a separate vial.
Synthesis of PAF-NPro: PAF-NProBoc (0.1 g) was added to a solu-
tion of 30% trifluoroacetic acid (9 mL, 13.32 g, 35.04 mmol) in di-
chloromethane (30 mL) and the reaction mixture stirred at room
temperature for 48 h. The solvent was then removed by filtration,
centrifuged and washed with CH2Cl2, a solution of 2n K2CO3, THF
and diethyl ether. Finally, the resulting solid was dried under
vacuum at 1008C overnight to yield PAF-NPro quantitatively. Ele-
mental analysis calcd (%) for C42H29N2O (577): C 87.37, H 5.02, N
4.85; found: C 72.89, H 4.53, N 3.10.
Preparation of heterogeneous catalysts—PAF-NN-metal
complexes
Acknowledgements
General procedure for the synthesis of PAF-NN-Cu: A suspension
of PAF-NN (100 mg) in acetonitrile (10 mL) was prepared in
a Schlenk flask under nitrogen atmosphere and stirred vigorously
We acknowledge the Direcciꢄn General de Investigaciꢄn Cien-
tꢅfica y Tꢆcnica (MINECO) Spain (Projects MAT2011-29020-C02–
02 and Consolider-Ingenio 2010-CSD-0050-MULTICAT) for finan-
cial support.
[32]
for 15 min. Then a solution of [Cu(CH3CN)4]ClO4 (15 mg in 5 mL
of acetonitrile) and acetonitrile (5 mL) was added to the mixture
and heated at 608C for 48 h. The resulting solid was centrifuged
and washed with CH3CN (3ꢂ10 mL) and diethyl ether (10 mL). Fi-
nally, the solid was dried overnight under vacuum at 1008C to
yield the corresponding copper heterogeneous catalyst.
Keywords: cyclopropanation
porous aromatic frameworks
catalysts
·
heterogeneous catalysts
·
·
recyclability supported
·
PAF-NPy-Cu: ICP-OES analysis calcd for 5% functionalization: Cu
0.54; found: Cu 0.45.
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Received: October 24, 2013
Revised: January 22, 2014
Published online on March 12, 2014
Chem. Eur. J. 2014, 20, 5111 – 5120
www.chemeurj.org
5120
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