Synthesis of quinolines via Friedlaender reaction catalyzed by CuBTC metal-organic-framework

Article abstract of DOI:10.1039/c2dt11978a

Friedlaender condensation between 2-aminoaryl ketones and different carbonyl compounds, catalyzed by CuBTC was investigated by a combination of various experimental techniques and by density functional theory based modelling. CuBTC exhibiting hard Lewis a

Full text of DOI:10.1039/c2dt11978a

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PAPER
Synthesis of quinolines via Friedländer reaction catalyzed by CuBTC
metal–organic-framework
a,b
b
c
c
d
Elena Pérez-Mayoral, Zuzana Musilová, Barbara Gil, Bartosz Marszalek, Miroslav Položij,
d
b
Petr Nachtigall and Jiri Čejka*
Received 19th October 2011, Accepted 9th December 2011
DOI: 10.1039/c2dt11978a
Friedländer condensation between 2-aminoaryl ketones and different carbonyl compounds, catalyzed by
CuBTC was investigated by a combination of various experimental techniques and by density functional
theory based modelling. CuBTC exhibiting hard Lewis acid character showed highly improved catalytic
activity when compared with other molecular sieves showing high concentraion of Lewis acid sites, e.g.
in BEA and (Al)SBA-15. Polysubstituted quinolines were synthesized via a Friedländer reaction
catalyzed by CuBTC under the solvent-free conditions. High concentration of active sites in CuBTC
2+
together with the concerted effect of a pair of adjacent Cu coordinatively unsaturated active sites are
behind a very high quinoline yield reached within a short reaction time. Results reported here make
CuBTC a promising catalyst for other Lewis acid-promoted condensations, including those leading to
biologically active compounds with a particular relevance for the pharmaceutical industry. The
mechanism of a catalyzed Friedländer reaction investigated computationally is also reported.
1
. Introduction
CuBTC (Basolite™ C300). The presence of metals in the MOFs
structure acting as unsaturated coordination sites enables their
application as catalysts for oxidation–reduction reactions. In this
way, CuBTC and Fe(BTC) have been reported as efficient cata-
Metal–Organic-Frameworks (MOFs) are considered to be one of
the most fascinating groups of materials due to their structural
and chemical features and potential applications in areas like
8
lysts in the oxidation of p-benzoquinone, and benzylic com-
1
2
9
optoelectronic devices and sensors, storage and separation of
pounds. CuBTC is a 3D porous MOF with a zeolite-like
3
4
1
0
11
gases, and even medical imaging and drug delivery. Further-
more, crystalline MOFs, possess large specific surface areas and
uniform size distribution of the channels, which make them
promising for heterogeneous catalysis. Channel systems of
MOFs show a strictly ordered geometry, which could allow for
their use in size and shape selective catalysis, this type of cataly-
sis being up-to-now reported exclusively for zeolites. In contrast
to zeolites, MOFs are formed in principle from an “infinite” set
of building blocks and metals making them potentially more
versatile than zeolites.
structure, possessing a marked hard Lewis acid character.
CuBTC was reported as catalytically active in the cyanosilylation
12
of aldehydes, and recently in the cyclopropanation reaction of
olefins with diazoacetate esters.
5
13
More recently, we reported on Basolite™ C300, CuBTC, as
an efficient and environmentally-friendly catalyst for the Fried-
länder reaction between 2-aminobenzophenones and acetylace-
6
14
tone under mild reaction conditions. Our studies demonstrated
that the annulation is promoted by Lewis acid sites located over
metallic centers in CuBTC. This MOF was proposed as a novel
catalyst for this transformation with improved catalytic properties
when related to molecular sieves typically exhibiting Lewis
7
In the green chemistry context, we investigate the almost
unexplored catalytic behaviour of MOF materials, particularly
14,15
acidity, H-BEA and (Al)SBA-15.
Development of new and efficient catalysts for quinoline syn-
thesis via a Friedländer reaction is currently an enormously inter-
esting topic because this reaction is considered to be the most
efficient and simple synthetic approach to prepare nitrogen het-
erocycles, quinoline systems, with important pharmacological
a
Departamento de Química Inorgánica y Química Técnica, Facultad de
Ciencias, Paseo Senda del Rey 9, 28040 Madrid, Spain. E-mail:
eperez@ccia.uned.es; Fax: (+34) 3989766; Tel: (+34) 3989047
b
Department of Synthesis and Catalysis. J. Heyrovský Institute of
Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i.,
Dolejškova 3, 182 23 Prague 8, Czech Republic. E-mail: jiri.cejka@
jh-inst.cas.cz; Fax: (+)420-28658-2307; Tel: (+)420-26605-3795
16,17
activities.
Substitution in the quinoline ring system induces
c
different effects on their biological activity. As examples, com-
Department of Inorganic Chemistry, Faculty of Chemistry, Jagiellonian
18
pounds 1a–b were reported as antiparasitic agents while
tacrine-thiadiazolidinone hybrids are acetylcholinesterase
University, Ingardena 3, 30 060 Kraków, Poland. E-mail: gil@chemia.
uj.edu.pl; Fax: (+48)12- 634-0515; Tel: (+)48-12663-2016
2
d
Department of Physical and Macromolecular Chemistry, Faculty of
19
(AchE) inhibitors (Fig. 1).
Natural Sciences, Charles University, Albertov 2030, 120 00 Prague 2,
Czech Republic. E-mail: petr.nachtigall@molecular.cz; Fax: (+)420-
2
Among the heterogeneous catalysts reported for this
20
21–23
24,25
24919752; Tel: (+)420-221951289
annulation are Al O , H SO /SiO ,
2
3
2
4
2
NaHSO /SiO2
4
4036 | Dalton Trans., 2012, 41, 4036–4044
This journal is © The Royal Society of Chemistry 2012

(all-Lewis-sites system), TiO2 (mostly Lewis system) but is
absent in HPW heteroacid (all-Brønsted-sites system), as pre-
sented in Fig. 2a. This assignment is in agreement with the lit-
34
erature data.
Absorption coefficient of the 1069 cm− band was determined
in experiments in which small, measured doses of pyridine were
adsorbed in CuBTC activated at 423 K. Then, the dependence of
intensity of the 1069 cm− band vs. the concentration of the
absorbed pyridine is shown in Fig. 2b. The slope of this line was
1
Fig. 1 Substituted quinolines with biological activity.
1
26
27
HClO /SiO , silica gel-supported phosphomolybdic acid,
4
2
−1
2
8
29
taken as the absorption coefficient of the 1069 cm band. The
KAl(SO ) 12H O/SiO , sulfonated cellulose, silica-propyl-
4
2
2
2
30
31
average value for all experiments was 0.042 ± 0.0008 cm
sulfonic acid and AlKIT-5. In addition, we employed meso-
porous materials as novel and efficient catalysts for the
Friedländer condensation between 2-aminoaryl ketones and ethyl
acetoacetate selectively leading to the corresponding quinolin-2
−1
μmol . On the basis of the determined absorption coefficient
and intensity of the 1069 cm− band, the concentration of co-
ordinatively bonded pyridine was determined as equal to
2.30 mmol g− for CuBTC activated at 473 K. It is worth men-
tioning that concentration of the Lewis sites was calculated for
the weight of activated, partially dehydrated sample, determined
from the activation experiments performed on the sorption
balance. Fig. 3 shows that pyridine was not interacting with the
residual water or OH groups. The bands of organic linker were
also virtually unchanged after pyridine adsorption, indepen-
dently of the temperature the MOF was activated. Thus, it is safe
to assume that the stoichiometry of the pyridine interaction with
the active centres was: one coordinatively unsaturated metal
centre to one pyridine molecule.
1
1
32
(
1H)-ones. Synthesis of quinolines starting from aniline and
different aldehydes catalyzed by H-BEA zeolite, under vapour
3
3
phase conditions, was reported as well.
This contribution is targeted to characterization and catalytic
performance evaluation of CuBTC, in the synthesis of polysub-
stituted quinolines. It is a continuation of our preliminary
1
4,15
results
offering the reaction scope as well as accompanied
by the rationalization of the process by computational methods.
2
. Results and discussion
2
.1. Catalyst characterization
Table 1 shows the textural parameters of CuBTC. This MOF
exhibited a specific surface area, SBET, higher than the represen-
tative zeolite chosen for comparison. Furthermore, CuBTC
showed large pore diameters and volumes up to almost 1 nm.
This makes it a promising candidate as a catalyst for the syn-
thesis of interesting compounds with industrial potential.
2
.2. Lewis acidity of CuBTC
Lewis acidity of CuBTC arises from the presence of unsaturated
Cu(II) centres. Lewis acidity, by analogy to zeolites, may be
determined quantitatively by the adsorption of basic probe
molecules.
2
Fig. 2 A - IR spectra of pyridine adsorbed on HPW, MgO, TiO and
CuBTC, black lines – activated samples, red lines – after pyridine
adsorption and consecutive evacuation for 15 min. B - dependence of
the 1069 cm− band on the surface concentration of the adsorbed
pyridine.
1
At first, a new diagnostic band for coordinatively bonded pyri-
dine had to be selected as the one typically used, with a
−1
maximum around 1450 cm , overlapped with the native MOF
bands. As the diagnostic one we chose the band at 1069 cm−
assigned to ν18b of CC out-of-plane vibrations. This band
appears when pyridine is adsorbed on thermally treated MgO
1
Table 1 Textural parameters of catalysts under study
b
c
Si/Al
ratio
S
BET
V
pore
a
(m g−1)
2
c
(cm g−1)
3
Catalyst
Dpore (nm)
Basolite™
C300
H-BEA
n.a.
1499
0.35; 0.50;
0.90
0.70 × 0.67
0.642
12.5
674
0.224
a
b
Determined by X-ray fluorescence spectroscopy.
S
BET = BET surface
c
area.
Dpore and Vpore, pore diameter and volume, respectively,
calculated using BJH method.
Fig. 3 IR spectra of CuBTC activated at 373, 473 and 573 K before
(black spectra) and after (red spectra) pyridine adsorption at 373 K.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4036–4044 | 4037

Fig. 4 IR spectra of CO (A) and pyridine (B) adsorbed on CuBTC
activated at 373 (black), 473 (blue) and 573 K (red spectra).
Fig. 5 Time conversion plot for the Friedländer reaction between 2-
amino-5-chlorobenzophenone (3a) with acetylacetone (4) catalyzed by
CuBTC at (□) 363 K (■) 353 K and (▲) 343 K.
In reality, a significant amount of water is still present in the
activated sample. Moreover, water molecules are not being
reaction temperatures, 363 or 353 K, was almost in the same
degree, it was considerably higher at the shortest reaction times
when the reaction was carried out at 363 K. This feature can be
observed in Table 2 showing the reaction rates for the conden-
sation of 3a with 4, as a function of the temperature.
2
+
replaced by pyridine, thus at least a number of Cu cations are
shielded’. Adsorption of CO allows determination of the oxi-
‘
dation state of copper. In the case of CO adsorption in CuBTC,
several bands are arising. Activation at increasing temperatures
allows the assignment of two main maxima. The most intense
In order to compare the catalytic behaviour of CuBTC with
zeolites and other catalysts, we selected the most reactive
band at 2174 cm− can be assigned to the Cu –CO complex,
1
2+
the IR band rarely seen in zeolites, but observed in copper oxide
14
ketone, 2-aminobenzophenone (3b) as a model reactant. For
3
5
−1
systems. The broad maximum at ca. 2125 cm , appearing and
comparison, H-BEA was chosen due to our most recent results
−1
growing at the expense of the band at 2174 cm when the
sample is activated at increasing temperatures, can be assigned to
the Cu –CO moiety.
To determine whether in addition to Cu the Cu cations in
CuBTC may also be active as Lewis sites, pyridine was adsorbed
in samples activated at increasing temperatures (Fig. 3 and 4B).
The number of Cu centres, playing the role of Lewis acids,
also decreased with increasing activation temperature, therefore
demonstrating that it was the most efficient zeolite catalyzing the
15
Friedländer condensation. Fig. 6 shows the yield of compound
+
36
5
over H-BEA at 363 K. Additionally, we studied the conden-
2
+
+
sation of 3b with 4 catalyzed by Cu-BEA. Fig. 6 confirms that
CuBTC was the most efficient catalyst for the Friedländer reac-
tion between compound 3a and 4 as compared with H-BEA and
its Cu analogue.
The condensation reaction leading to 5 is almost quantitative
after 1.5 h when catalyzed by CuBTC while the same reaction
gives only 75 and 52% yields over H-BEA and Cu-BEA,
respectively.
2+
+
it seems probable that Cu sites are not interacting with pyridine.
2
.3. Catalytic performance
The catalytic activity of CuBTC was in the synthesis of quino-
line 1b by condensation of 2-amino-5-chlorobenzophenone (3a)
with acetylacetone (4) (Scheme 1). At first, we carried out the
condensation in the presence of CuBTC, under solvent-free con-
ditions using an excess of ketone 4, at 353 K. Under these
reaction conditions, compound 1b was isolated with 92% yield
after 2 h reaction time.
Table 2 Reaction rates for the condensation of 3a with 4.
Sample
CuBTC
Act. 373 K
2.44
Act. 473 K
2.16
Act. 573 K
1.40
1
4
Fig. 5 depicts a time dependence of conversion for the con-
densation between compound 3a and 4 catalyzed by CuBTC at
different reaction temperatures. The reaction proceeds already at
3
43 K yielding compound 1b in around 80% after 4 h. In
addition, although final yield of 1b when operating at the highest
Fig. 6 Kinetic profiles for the Friedländer reaction between 2-amino-
benzophenone (3b) with acetylacetone (4) catalyzed by (□) H-BEA (■)
Cu-BEA and (▲) CuBTC.
Scheme 1 Friedländer reaction between 2-aminoaryl ketones 3 and
carbonyl compounds affording quinolines.
4038 | Dalton Trans., 2012, 41, 4036–4044
This journal is © The Royal Society of Chemistry 2012

Scheme 2 Friedländer reaction between 2-aminoaryl ketones with
different ketones.
In this case, the catalyst was recovered and analyzed by X-Ray
Diffraction (XRD). Diffraction lines corresponding to the fresh
and used CuBTC were observed although with a lower intensity,
this circumstance is probably due to the small amount of the cat-
alyst and the presence of reactants/products in the channel
system. Therefore, CuBTC is a promising reusable catalyst.
We calculated the maximum catalyst TON (Table 3) for the
condensation of 3a with different ketones. Remarkably, TON
increases when the reaction between 3a and 4 was carried out
using higher amounts of ketone 3a (1 mmol) and a lower
amount of catalyst (25 mg) as depicted in Fig. 7. The maximum
values of TON and TOF were 2168 and 1084 h− , respectively.
These values together XRD results indicate no early deactivation
of the catalyst.
We also performed the condensation with dimedone (8) yield-
ing compound 9 with 41% yield (Table 2; entry 3). This low
yield might be caused by a lower solubility of this ketone in the
reaction medium. Hence 1,2-dichloroethane (1 mL) was used as
a solvent in order to facilitate the reaction. Finally, the reaction
of 3a with acetophenone (10) led to quinoline 11 with a moder-
ate yield of 57%. A lower yield was the result of a lower reactiv-
ity of this ketone as compared with the others (Table 2; entry 4).
To complete the study we carried out the Friedländer reaction
between acetylacetone (4) and 2-aminobenzophenones
(Scheme 2 and Table 4). The influence of functional groups (car-
bonyl and amine) on the substitution in the aromatic ring was
investigated (Table 4). Entries 1 and 2 in Table 4 have been pre-
viously commented on and the respective results shown in Fig. 6
and 7. On the other hand, the condensation of 12 with 4 yielded
quinoline 13 in 67% yield after 7 h of the reaction (Table 3;
entry 3). This result could be explained due to the weaker
nucleophilic character of the amine group in compound 12 as
compared with 2-aminoaryl ketone 3 with the nitro group
present. While chlorine at the 5-position in 3a produces a slight
decrease in the yield of compound 1b and hence a decrease in
the reactivity, the presence of a nitro group, as a strongly elec-
tron-withdrawing group, reduces dramatically the reactivity of
the compound 12. Furthermore, we performed the reaction
Fig. 7 Effect of catalyst amount in the condensation of 3a with 4 cata-
lyzed by CuBTC at 363 K.
Based on the molecular dimensions of the reactants and pro-
14
ducts, when using CuBTC which has large pores, the process
can take place inside or outside the channels, whereas in the case
of zeolites the condensation probably proceeds mostly on the
external surface. The highest concentration of Lewis acid sites in
CuBTC, located over metallic centres, could be responsible for
the great efficacy in the synthesis of quinoline 5. Obviously,
Brønsted acid sites in H-BEA may also be involved in the reac-
tion. However, in the MOF catalyst under study, no Brønsted
acid sites are present. To get a deeper insight, H-BEA zeolite
was exchanged with Cu(II) leading to Cu-BEA. In the activated
form of this zeolite, the copper atoms are usually Cu(I) as men-
tioned above. The lower yield of 5 in the reaction catalyzed by
Cu-BEA could indicate that Cu(I) is mostly inactive in this
reaction.
1
Therefore, we can infer that Lewis acid sites in CuBTC play a
remarkable role in the annulation between 3 and 4 whereas both
types of sites in zeolites, Brønsted and Lewis sites, contribute to
the overall activity in the reaction.
Following our ongoing investigations we evaluated different
catalyst amounts, in this case for the condensation of compounds
3
a and 4. As it is seen from Fig. 7, the use of CuBTC in 8%
(50 mg) and 4% (25 mg) afforded the compound 1b with excel-
lent yields, 92 and 90%, respectively. Decreasing amounts of the
catalyst led to lower conversions to 1b, therefore, the optimum
catalyst amount should be kept at ca. 4% (25 mg).
In order to generalize the results of this study we carried out
the Friedländer reaction of 3a with different ketones with active
methylene groups at the α-position (Scheme 2 and Table 3).
Thus, the reaction with cyclohexanone (6), a ketone less reac-
tive than 4, at 363 K over CuBTC, afforded quinoline 7 with
quantitative yield after 5 h of the reaction time (Table 3; entry 2).
Table 3 Friedländer reaction between 2-amino-5-chlorobenzophenone (3a) with different ketones, under solvent-free conditions and at 363 K,
catalyzed by CuBTC
1
2
3
4
Entry
R
R
R
R
Ketone
Time (h)
Quinoline
Yield (%)
TON
TOF
1
2
3
4
Ph
Ph
Ph
Ph
Cl
Cl
Cl
Cl
CH
3
COCH
3
4
6
8
10
2
5
4
15
1b
7
9
92
99
41
554
596
246
343
272
119
61
–(CH
–(CH
Ph
2
)
4
–
a
2
)–C(CH
3
2
) –CH
2
–CO
b
H
11
57
22
a
All the reactions were carried out using 0.5 mmol of 2-amino-5-chlorobenzophenone (3a). Reaction was carried out in the presence of solvent (1,2-
b
dichloroethane; 1 mL) at reflux and using the compounds 3a and 8 in 1 : 1 ratio. Reaction was performed at 393 K. Turnover number (TON) and
Turnover frequency (TOF) are defined as TON = conversion (%) (mol(substrate))/mol (catalyst) while TOF = TON/time (h).
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4036–4044 | 4039

Table 4 Friedländer reaction between different 2-aminoaryl ketones with acetylacetone
1
2
3
4
Entry
R
R
R
R
2-aminoaryl ketone
Time (h)
Quinoline
Yield (%)
1
2
3
4
Ph
Ph
Ph
Me
Cl
H
NO
H
CH
CH
CH
CH
3
3
3
3
COCH
COCH
COCH
COCH
3
3
3
3
3a
3b
12
14
2
1.5
7
1b
5
13
15
92
97
67
64
2
4
All of the reactions were carried out using 0.5 mmol of the corresponding 2-aminoaryl ketone.
Table 5 Friedländer reaction between 2-amino-4-chlorobenzonitrile 16
and ketones catalyzed by CuBTC
Entry
T/°C
Time (h)
Yield of 20a (%)
Yield of 20b (%)
a
1
2
3
4
90
5
4
5
5
39
—
—
42
51
100
100
100
83
58
49
Experimental conditions: 2-amino-4-chlorobenzonitrile 1 (0.5 mmol),
acetylacetone 2 (5 mmol) and CuBTC was heated at the corresponding
a
temperature during the time shown in table. Traces of 18 (2%) were
1
detected by H-NMR and MS.
Scheme 3 Synthesis of 3-acetyl-4-amino-7-chloro-2-metilquinoline
17) starting from 2-amino-4-chlorobenzonitrile (16).
(
carried out the reaction at 373 K and duration 4 h leading to the
corresponding quinoline in 83% yield. (Table 5; entry 2). We
have also investigated the synthesis of 9-chloroacridine 20a
because it is a useful building block for the synthesis of acetyl-
38
choline esterase inhibitors. The reaction of 16 with cyclohexa-
none (6) (at 373 K for 5 h) over CuBTC led to a mixture of
products being identified as compounds 20a (58%) and 20b
39
(42%), respectively (Scheme 4, Table 4; entry 3). Li et al.
reported this condensation employing stoichiometric ZnCl2
amounts, in DMF at reflux, isolating compound 20b as a major
product. The formation of compound 20b was rationalized as
occurring first through a Pinner reaction and a subsequent
Dimroth rearrangement. The CuBTC reported here appears as a
new catalyst for this process, operating under milder reaction
conditions and in a solvent-free environment, affording com-
pounds 20a–b in a 3 : 2 ratio.
We also carried out this reaction in the presence of Cu(NO3)2
leading to compounds 20a and 20b in 49% and 51% yield,
respectively, after 5 h of the reaction. These results are important
for the following reasons, i) the reaction leading to compounds
Scheme 4 Synthesis of tracrine derivatives.
between 2-aminoacetophenone (14) and acetylacetone (4) afford-
ing quinoline 15 in 64% yield for 4 h (Table 4; entry 4).
As mentioned, all MOFs consist of metallic centers and
organic ligands. Although the channels of CuBTC seem accessi-
ble to reactant 14, this substrate exhibited lower reactivity
towards acetylacetone (4) than 1a. We also observed this chemi-
cal behaviour when testing zeolites in the Friedländer reaction
2
0a–b is a catalytic process when using CuBTC or Cu(NO ) as
3 2
1
5
with both 2-aminoaryl ketones. It could be rationalized that in
the reactions with 2-aminobenzophenones, an increase in the
electrophilic character of the carbonyl group by π–π stacking
interaction of the phenyl group with organic ligand in CuBTC is
expected.
heterogeneous or homogeneous catalysts, respectively, and ii)
using the catalysts above mentioned it is possible to obtain qui-
noline 20a with increased yield and selectivity.
Finally, we performed the condensation of 2-amino-4-chloro-
benzonitrile 16 with 4 in a modified Friedländer reaction leading
to substituted 4-aminoquinolines (Schemes 3 and 4). This con-
densation is often useful for the synthesis of compounds with
pharmacological activity. Thus, the reaction of 16 with 4,
under solvent free conditions, at 363 K, catalyzed by CuBTC
afforded quinoline 17 with 39% yield after 5 h, with only traces
of compound 18 being detected by MS. Compound 18 was
formed by hydrolysis from the intermediate 19 (Table 5;
entry 1). In order to increase the yield for compound 17 we
2
.4. Mechanistic considerations.
A tentative mechanism for the Friedländer reaction has been also
investigated computationally using 2-aminobenzaldehyde (21)
and acetaldehyde (22k) (Scheme 5). Although the chemical be-
haviour of this type of carbonyl compound is slightly different to
that for the corresponding ketones, aldehydes were the substrates
of our choice for simplicity for the theoretical calculations.
Friedländer condensations between compounds 21 and 22k
could take place following two different effective pathways for
3
7
4040 | Dalton Trans., 2012, 41, 4036–4044
This journal is © The Royal Society of Chemistry 2012

Fig. 8 Energy profile of Friedländer reaction between 2-aminobenzal-
dehyde (21) and acetaldehyde (22) following the reaction path starting
with the aldolization step (Scheme 5). B3LYP energies calculated for
uncatalyzed and catalyzed reactions are reported.
Fig. 9 Adsorption complex of CuBTC with reactants 3b and 4 interact-
2
+
ing with adjacent Cu cus sites. Distance between atoms reacting in the
first aldolization step is depicted (in Å). The following colouring scheme
is adopted: Cu – orange, O – red, C – grey, N – blue, H – white.
the first reaction step i) aldolic condensation and ii) imination.
Both of them were investigated, aldolic condensation being
found to be energetically more favourable than the initial imine
formation; therefore, only the aldolization mechanism is dis-
cussed below. Stationary points on the reaction path obtained at
the B3LYP level for model reactants 21 and 22 are shown in
Fig. 8 for the following situations: gas phase reaction, Brønsted
acid catalysed (proton) reaction, and Lewis acid catalysed
adsorption complex of acetylacetone 4, characterized by −ΔE
ads
= 38 kJ mol− , is in its enol form (Fig. 9).
1
Considering the situation when reactants 3b and 4 interact
with adjacent cus sites, there are three possible arrangements of
reactants, all of them having the same energy (−ΔEads(3b + 4) =
79 kJ mol− ). One of the situations is depicted in Fig. 9; 2-ami-
nobenzophenone 3b interacts with cus via the N atom and acetyl-
acetone 4 in its enol form interacts with neighboring cus via the
O atom. Note that both reactants are ideally arranged for aldoli-
zation, the distance between reacting atoms (carbonyl carbon of
3b and central carbon of 4) is only 4.19 Å and mutual arrange-
ment of the two reactants is such that the reaction can proceed
without any steric hindrance. Similarly in the other two energeti-
cally preferable arrangements of 3b and 4 on adjacent cus sites
the orientation of reactants is favourable for the reaction.
1
(CuBTC cluster model) reaction. It is apparent that Brønsted
acid catalyst is the most efficient one, lowering barriers TS , TS ,
b
c
−1
and TS by about 60 kJ mol . The same catalytic effect is also
d
found for the Lewis acid catalyst (CuBTC), however, only for
the second reaction step (up to the ring closer, 23b). The barriers
for dehydration are lowered (with respect to the uncatalysed
−1
process) only by 5–25 kJ mol . It should be pointed out that
this small decrease in the barriers TS and TS can be due to a
small cluster model used to mimic the active site (see
Section 3.5.).
The purely energetic consideration presented above lead to the
conclusions that Brønsted acid catalysts should be more efficient
than Lewis acid ones. However, there is an additional, rather
peculiar, effect of CuBTC that should not be overlooked: there is
a large concentration of active sites in CuBTC (separated just by
Calculations on the periodic model of CuBTC thus show that
c
d
2
+
the concerted effect of a pair of neighboring Cu sites in
CuBTC enhances the catalytic effect compared to an isolated
2+
Cu Lewis acid center. The arrangement of reactants on neigh-
boring cus sites described above should result in an increase in
the pre-exponential factor of the reaction rate constant. In
addition, close proximity of active cus sites may lead to a further
decrease of TS and TS barriers due to the stabilization effect of
c
d
2+
8
.15 Å) that can act in a concerted manner. In order to further
the secondary Cu site interacting with the produced water
molecule.
increase our mechanistic understanding of the reactions investi-
gated here experimentally, the interaction of reactants 2-amino-
benzophenone (3b) and acetylacetone (4) with CuBTC rep-
resented by a periodic model was investigated. Both, 2-amino-
benzophenone and acetylacetone interact preferentially with the
Cu cus site (Fig. 9); the adsorption complexes via N- and
O- atoms are practically isoenergetic (−ΔEads = 39 kJ mol− ) in
the case of 2-aminobenzophenone 3b. The most stable
It should be also pointed out that the formation of Brønsted
3+
acid sites by water adsorbed on Cr sites in MIL-100 was repor-
40a
ted. However, the involvement of similar Brønsted acid sites
in CuBTC in the reaction investigated here is not likely. The
Brønsted site formed in MIL-100(Cr) is significantly weaker
than the Brønsted sites in H-zeolites (a shift of OH stretching fre-
quency of −200 and −300 cm− was reported for MIL-100(Cr)
1
1
Scheme 5 Friedländer reaction between 2-aminobenzaldehyde (21) and acetaldehyde (22, k = keto form; e = enol form) – reaction path starting with
aldolization step.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 4036–4044 | 4041

4
0b
and, e.g., H-FER,
respectively) and the corresponding
2-amino benzophenone (3a) (1 mmol) was added and the reac-
tion mixture was heated at 363 K, for 2 h. The reaction was fol-
lowed until total consumption of the starting material detected
by TLC (mixture of EtOAc/n-hexane in 1 : 2 ratio was used as
eluent). After cooling, reaction crude was treated with EtOH
(96%) and the catalyst was removed by centrifugation at 4000
rpm for 10 min. Finally, solvent was removed under vacuum.
2
+
Brønsted site formed by water adsorbed on Cu is even weaker.
Calculated interaction energies for the second water molecule
with the H O⋯Cu in CuBTC is less than 50 kJ mol , signifi-
cantly less than interaction energy of water with Brønsted sites
in zeolites. IR data also suggest that there is no interaction with
Brønsted centres in CuBTC, as proposed by Alaerts et al. The
−1
2
40c
−1
1
wide band of H O around 3300 cm does not decrease after
Conversions were determined by H NMR.
2
pyridine introduction, which proves that water is not replaced by
pyridine. Additionally, the binding energy of the pair pyridine +
1-(6-Chloro-2-methyl-4-phenyl-quinolin-3-yl)-ethanone (1b).
1
H NMR (300 MHz, CDCl ): δ 8.01 (1H, d J = 8.7 Hz), 7.65
3
4
1
H O was calculated by Watanabe and Sholl as 0.2 eV, exactly
(1H, dd J = 2.4, 9.0 Hz), 7.58–7.41 (4H, m), 7.37–7.31 (2H, m),
2.68 (3H, s), 2.00 (3H, s); MS (EI) m/z (%) 297(14), 296 (6),
2
the same as for the pair of H O molecules, therefore it is also
2
+
unlikely that pyridine would replace water in the coordination
295 (M , 44), 280 (100), 252 (21), 217 (52), 176 (47), 43 (61),
2+
sphere of Cu .
18 (87).
1
1
-(2-Methyl-4-phenyl-quinolin-3-yl)-ethanone (5). H NMR
(
=
300 MHz, CDCl ): δ 8.08 (1H, d J = 8.1 Hz), 7.72 (1H, app t J
6.9, 7.8 Hz), 7.62 (1H, d J = 8.1 Hz), 7.52–7.49 (3H, m),
3
3. Experimental
7
.48–7.42 (1H, m), 7.37–7.34 (2H, m), 2.70 (3H, s), 2.00 (3H,
3
.1. Specific surface area measurements
+
s); MS (EI) m/z (%) 261 (M , 42), 246 (100), 218 (61), 176
The surface area and void volumes of the zeolites under study
were determined from adsorption isotherms of nitrogen at 77 K
applying the BET method using a static volumetric apparatus
Micromeritics, ASAP 2020 (Table 1).
(41), 151 (20), 108 (18), 43 (28).
7-Chloro-9-phenyl-1,2,3,4-tetrahydro-acridine (7). H NMR
1
(300 MHz, CDCl ): δ 7.98 (1H, d J = 8.7 Hz), 7.57–7.49 (5H,
3
m), 2.28 (1H, d J = 1.8 Hz), 7.21 (1H, d J = 1.8, 8.1 Hz), 3.19
(2H, app t J = 6.3, 6.6 Hz), 2.59 (2H, app t J = 6.3, 6.6 Hz),
2
.00–1.92 (2H, m), 1.82–1.72 (2H, m); MS (EI) m/z (%) 295
3
.2. Acidity in MOFs
+
(36), 294 (32), 293 (M , 100), 292 (40), 258 (82), 120 (45).
7
-Chloro-3,3-dimethyl-9-phenyl-3,4-dihydro-2H-acridin-1-
The properties of CuBTC were investigated by adsorption of
pyridine and carbon monoxide as a probe molecule followed by
FTIR spectroscopy. CuBTC was prepared as self-supported
wafers and activated as described in the text prior to the adsorp-
tion of probe molecules. The adsorption temperatures were
1
one (9). H NMR (300 MHz, CDCl ): δ 8.00 (1H, d J = 0 9 Hz),
.69 (1H, dd J = 2.1, 9 Hz), 7.53–7.42 (3H, m), 7.46 (1H, d J =
.1 Hz), 7.17–7.14 (2H, m), 3.25 (2H, s), 2.57 (2H, s), 1.15 (6H,
s). MS (EI) m/z (%) 338 (4), 337 (26), 336 (37), 335 (M , 76),
34 (M+-1, 90), 279 (55), 216 (100), 189 (33), 120 (36).
6
CDCl ): δ 8.18 (2H, d J = 8.4 Hz), 7.86 (1H, d J = 7.5 Hz),
3
7
2
+
3
3
3
9
73 K for pyridine (POCh Gliwice, analytical grade, dried over
A molecular sieve) and 173 K for CO (Linde Gas Polska,
9.5% used without further purification).
1
-Chloro-2,4-diphenyl-quinoline (11). H NMR (300 MHz,
3
7
(
.65–7.40 (11H, m); MS (EI) m/z (%) 317 (30), 316 (45), 315
IR spectra were recorded with a Bruker Tensor 27 spec-
+
M , 92), 314 (93), 280 (41), 139 (100).
trometer equipped with an MCT detector and working with the
1
−1
1-(6-Chloro-2,4-dimethyl-quinolin-3-yl)-ethanone (15).
NMR (300 MHz, CDCl ): δ 8.00 (2H, m), 7.70 (1H, d J = 8.1
Hz), 7.56 (1H, m), 2.63 (3H, s), 2.59 (3H, s), 2.58 (3H, s); MS
EI) m/z (%) 199 (M , 42), 184 (86), 156 (100), 115 (47), 43
34).
H
spectral resolution of 2 cm . All spectra presented in this
work are normalized to the standard 10 mg pellet (density
3
−
2
3
.2 mg cm ).
+
(
(
3
.3. Product charaterization
1-(4-Amino-7-chloro-2-methyl-quinolin-3-yl)-ethanone (17).
1
H NMR (300 MHz, CDCl ): δ 12.68 (2H, s), 7.85 (1H, d J =
3
NMR spectra were recorded with a Varian Mercury 300 spec-
trometer. H chemical shifts (δ) in CDCl3 are given using
7
(
4
.8 Hz), 7.66 (1H, s), 7.40 (1H, d J = 7.8 Hz), 5.45 (1H, s), 2.06
1
+
3H, s), 2.04 (3H, s); MS (EI) m/z (%) 234 (M , 11), 219 (39),
internal tetramethylsilane as the internal standard. MS spectra
were recorded with a GS/MS ThermoElectron.
TLC chromatography was performed on DC-Aulofolien/Kie-
selgel 60 F245 (Merck) using mixtures of hexane/AcOEt or
CH Cl /EtOH as eluents.
All reagents and solvents for catalytic studies were obtained
from Aldrich. MOFs under study, CuBTC and FeBTC, were also
commercially available.
3 (100);. 1-(7-Chloro-4-hydroxy-2-methyl-quinolin-3-yl)-etha-
+
none (18). MS (EI) m/z (%)234 (M -1, 33), 219 (53), 18 (100).
1
6
-Chloro-1,2,3,4-tetrahydro-acridin-9-ylamine
(20a).
H
NMR (300 MHz, DMSO-d ): δ 8.18 (1H, d J = 9 Hz), 7.62 (1H,
6
2
2
d J = 1.8 Hz), 7.28 (1H, dd J = 9, 1.8 Hz), 6.51 (2H, br s), 2.8
2H, m), 2.58 (2H, m), 1.81-1.65 (4H, m).
(
6
-Chloro-2,2-pentamethylene-1,2-dihydroquinazolin-4(3H)-
1
one (20b). H NMR (300 MHz, CDCl ): δ 8.08 (1H, s), 7.54
3
(
(
1H, d J = 8.1 Hz), 6.93 (1H, s), 6.87 (1H, d J = 2.1 Hz), 6.62
1H, dd J = 8.1, 2.1 Hz), 1.71–1.23 (10H, m).
3
.4. Experimental procedures
Catalyst activation: CuBTC was dried, at 373 K, overnight
−1
3
.4.1. Synthesis of 1-(6-chloro-2-methyl-4-phenyl-quinolin-3-
yl)-ethanone (1b). To a mixture of the acetylacetone (4)
5 mmol) and activated CuBTC (0.083 mmol, 50 mg), 5-chloro-
(experimental conditions: 373 K, 5 K min , 900 min).
25
25,42
42
Spectroscopic data of compounds 1b, 5, 7–11,
15 and
39
(
20 are in accordance with those reported.
This journal is © The Royal Society of Chemistry 2012
4042 | Dalton Trans., 2012, 41, 4036–4044

3
.5. Theoretical calculations
preparation of AchE inhibitors via a modified Friedländer
reaction.
The mechanism of the Friedländer reaction taking place in a gas
phase without any catalyst, the reaction catalyzed by Brønsted
+
2+
acid (H ions) and the reaction catalyzed by Lewis acid (Cu
ions) was investigated using cluster models and Becke’s three
Acknowledgements
4
3,44
parameter exchange–correlation functional B3LYP
together
The research leading to these results has received funding from
the European Community’s Seventh Framework Programme
(FP7/2007-2013) under grant agreement n° 228862. MACADE-
MIA is a Large-scale Integrating Project under the Nanosciences,
Nanotechnologies, Materials and New Production Technologies
Theme (www.macademia-project.eu). PN and MP also acknowl-
edge the support of ME CR (grants 7E09111 and
MSM0021620857). This work has been also supported by
MICINN (CTQ2009-10478). The IR studies were carried out
with the equipment purchased thanks to the financial support of
the European Regional Development Fund in the framework of
the Polish Innovation Economy Operational Program (contract
no. POIG.02.01.00-12-023/08).
4
5
with 6-311G(2d,p) basis set. The active site in CuBTC MOF
was represented by the Cu(HCOO) cluster constrained at the
2
geometry obtained for Cu (HCOO) paddlewheel. Character of
2
4
all stationary points located along the reaction path was checked
by the frequency calculations performed within the harmonic
approximation. All energies were corrected for zero-point-
vibrational energy correction. Cluster model calculations were
4
6
performed with Gaussian09 program suite.
The Friedländer reaction taking place in CuBTC was further
investigated with the periodic DFT model, employing the PBE
47
functional. The periodic DFT calculations were performed for
the ferromagnetic case, using the rhombohedral primitive cell
2
+
containing 156 framework atoms (12 Cu cations) and cell par-
ameters optimized previously (a = b = c = 18.774 Å and α = β =
4
8
49
γ = 60°). The projector augmented wave (PAW) and kinetic
energy cutoff of 600 eV were used together with the Γ-point
sampling of the first Brillouin zone. Periodic DFT calculations
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