Eco-friendly catalytic systems based on carbon-supported magnesium oxide materials for the friedl?nder condensation

Article abstract of DOI:10.1002/cctc.201402602

Carbon-supported MgO materials are excellent and sustainable catalysts for the synthesis of N-containing heterocyclic compounds by the Friedl?nder condensation under mild, solvent-free conditions. The results reported herein indicate that MgO is the most active catalytic species that accelerates the reaction compared with the catalytic behavior observed for the carbon material Norit RX3. On the basis of DFT calculations, a reaction mechanism that involves dual activation of the reacting structures by the catalyst is proposed. Oxide, outside: MgO supported on carbon is able to catalyze the synthesis of interesting N-containing heterocyclic compounds efficiently under mild conditions. MgO on the carbon surface is responsible for the catalytic behavior. These carbon materials are environmentally friendly catalysts for the Friedl?nder reaction.

Full text of DOI:10.1002/cctc.201402602

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DOI: 10.1002/cctc.201402602
Eco-Friendly Catalytic Systems Based on Carbon-
Supported Magnesium Oxide Materials for the Friedlꢀnder
Condensation
[
a]
[a]
[a]
Marina Godino-Ojer, Antonio J. Lꢀpez-Peinado, Rosa M. Martꢁn-Aranda,
[b]
[a]
[c]
Jacek Przepiꢀrski, Elena Pꢂrez-Mayoral,* and Elena Soriano*
Carbon-supported MgO materials are excellent and sustainable
catalysts for the synthesis of N-containing heterocyclic com-
pounds by the Friedlꢃnder condensation under mild, solvent-
free conditions. The results reported herein indicate that MgO
is the most active catalytic species that accelerates the reaction
compared with the catalytic behavior observed for the carbon
material Norit RX3. On the basis of DFT calculations, a reaction
mechanism that involves dual activation of the reacting struc-
tures by the catalyst is proposed.
Introduction
The Friedlꢃnder condensation, which proceeds through double
condensation between 2-aminoaryl carbonyl components with
other carbonyl compounds with active methylene groups, is
one of the most cited and useful reactions in organic synthe-
[
1]
sis. It is the method of choice for the synthesis of a large vari-
ety of N-containing heterocyclic compounds such as quino-
Figure 1. Biologically active N-containing heterocyclic compounds.
[
2]
[3]
[4]
lines, naphthyridines, and acridones. These N-containing
heterocyclic systems are present in many naturally occurring
products and synthetic drugs associated with several versatile
pharmacological activities, which make them interesting target
compounds for use in medicine and other fields. The structures
of some biologically active heterocyclic rings are shown in
Figure 1; compound 1 derived from 2-aminobenzaldehyde is
sation has been explored preferentially starting from the corre-
sponding 2-aminoaryl ketones in an acidic medium.
Our research group works on the development of new, envi-
ronmentally friendly, and efficient catalytic materials for appli-
cations in green processes, and the Friedlꢃnder condensation
occupies an important place in our investigations. In this
sense, several catalysts have been reported that differ in com-
[
5]
a known histone acetyltransferase inhibitor, whereas naph-
[
6]
[4]
[11]
thyridine 2 and acridone 3 exhibit antimicrobial and anti-
cancer activity, respectively.
position, structure, and porosity, which include zeolites,
[12]
acidic, basic, and bifunctional mesoporous silicas, metal–or-
ganic frameworks, particularly copper-1,3,5-benzenetricarboxy-
Several heterogeneous catalytic systems have been reported
for the synthesis of this type of N-heterocyclic compounds by
[13]
[14]
late, and carbon materials. Our studies often concern in-
teresting mechanistic revelations that result from the combina-
tion of experimental results with computational methods.
Our latest results indicated that the condensation of 2-ami-
noaryl aldehydes 4, which are substrates that are barely inves-
tigated in heterogeneous catalysis, proceeds regioselectively.
Thus, in the presence of amino-functionalized T/MCF (T=Nb
or Al, MCF=mesostructured cellular foams), which probably
[
7]
the Friedlꢃnder reaction such as Al O , acids supported on
2
[9]
3
[
8]
[10]
silica gel, silica propylsulfonic acid, and AlKIT-5. Notably,
most of these have acidic properties, whereas the investigation
of basic materials is almost neglected. In addition, this conden-
[a] M. Godino-Ojer, Prof. A. J. Lꢀpez-Peinado, Prof. R. M. Martꢁn-Aranda,
Dr. E. Pꢂrez-Mayoral
[12c–d]
Departamento de Quꢁmica Inorgꢃnica y Quꢁmica Tꢂcnica
Universidad Nacional de Educaciꢀn a Distancia, UNED
Paseo Senda del Rey 9, 28040 Madrid (Spain)
E-mail: eperez@ccia.uned.es
acts as a bifunctional catalytic system (Scheme 1),
the cor-
responding quinolones 5b and e are formed.
As a continuation of our studies, we report herein the first
basic hybrid carbon materials as efficient heterogeneous cata-
lytic systems for the synthesis of quinolines, naphthyridines,
and acridones by the Friedlꢃnder condensation using 2-amino-
aryl aldehydes and different carbonyl components as sub-
strates. Additionally, we have investigated the reaction mecha-
nism computationally and focused mainly on the catalytic
steps that take place during the synthesis of 5.
[b] Dr. J. Przepiꢀrski
Institute of Chemical and Environmental Engineering
West Pomeranian University of Technology
Pulaskiego 10, 70-322 Szczecin (Poland)
[c] Dr. E. Soriano
Instituto de Quꢁmica Orgꢃnica General, CSIC
C/Juan de la Cierva 3, 28006 Madrid (Spain)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402602.
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lꢃnder condensation of 2-amino-
5
-chlorobenzaldehyde (4a) and
1
2
acetylacetone
(R =R =Me)
under solvent-free conditions at
room temperature (293 K). Thus,
the condensation reaction cata-
lyzed by PET/MAG 30:70 and
PET/MAG 50:50 led to the corre-
Scheme 1. Friedlꢃnder condensation of 2-aminobenzaldehydes 4.
sponding
quinolone 5a
in
Results and Discussion
almost 90% yield after 4 h (Scheme 1; Table 2, entries 2–3). The
conversion to 5a depends on the PET/MAG ratio, and PET/
MAG 70:30 is the least efficient catalyst (Figure 2). Besides the
observed differences in the textural parameters, the carbon-
supported MgO samples under study showed different sizes of
MgO crystallites as a function of their MgO loadings (Table 1).
In this sense, the observed activity of the catalysts does not
only correlate with the MgO loading but also with the in-
creased number of active catalytic sites over the MgO crystal
surface. Notably, the carbons that include large MgO crystalli-
tes (PET/MAG 50:50 and PET/MAG 30:70) and the highest MgO
loadings afforded 5a with increased yields, whereas PET/MAG
70:30, in which the size of the MgO crystals is considerably
smaller, led to decreased yields to 5a. Although the mean crys-
tallite sizes of MgO of the hybrid
Catalysts were synthesized by using the synthetic methodolo-
[15]
gy reported previously by Przepiorski and co-workers. Thus,
carbon-supported MgO materials were prepared by the calci-
nation (923 K) of homogeneous mixtures of poly(ethylene ter-
ephthalate) (PET) and magnesite (MAG), which served as the
carbon source and MgO precursor, respectively.
The textural parameters of the commercial Norit RX3 and
the hybrid carbon-MgO materials under study together with
the MgO loadings are shown in Table 1. Compared to the
hybrid materials, the MgO-free activated carbon has a predomi-
nantly microporous character. In addition, it shows a considera-
bly higher specific surface area than the other materials, which
concerns both micropores and mesopores. However, the com-
materials PET/MAG 50:50 and
Table 1. Characterization of the investigated basic carbon materials.
PET/MAG 30:70 were compara-
ble, but slightly higher for PET/
[
a]
[b]
[b]
[b]
Catalyst
S
BET
2
S
total
2
S
ext
S
micro
2
MgO loading Mean MgO crystallite size pHPZC
À1
À1
2
À1
À1
MAG 30:70, the improved yield
of 5a in the process catalyzed
by PET/MAG 30:70, at the begin-
ning of the reaction, is probably
because of the high available
surface of MgO crystals. Howev-
er, in both cases, compound 5a
was obtained with similar con-
versions after prolonged reaction
[
m g
]
[m g
]
[m g
]
[m g
]
[wt%]
[nm]
PET/MAG 70:30
PET/MAG 50:50
PET/MAG 30:70
Norit RX3
370
240
206
480
279
223
160
136
115
191
320
143
68
53
65
78
–
13.0
23.5
24.6
–
10.16
10.21
10.23
[
c]
1306
1468
1277
7.4
[
a] Poly(ethylene terephthalate) (PET) and Magnesite (MAG). The values that follow correspond to the PET/MAG
ratios. [b] S is the surface area calculated by using the a-method. See Ref. [15]. [c] See Ref. [14].
bined materials are rich in basic MgO and reveal a bimodal, mi-
croporous–mesoporous character. In general, the pore struc-
ture parameters calculated for these porous systems tend to
decrease with the MgO loading, and the relative contribution
of microporosity to the total surface in these materials shows
the reverse order.
Table 2. Friedlꢃnder reaction between 2-aminoaryl aldehydes (4) and 1,3-
dicarbonyl compounds.
Entry
Catalyst
Product
Time [min]
Yield to 5 [%]
1
2
3
4
5
6
7
8
9
PET/MAG 70:30
PET/MAG 50:50
PET/MAG 30:70
PET/MAG 30:70
PET/MAG 30:70
5a
5a
5a
5a
5a
5a
5b
5c
5d
240
240
240
240
240
240
60 (15)
15
60
79
88
89
[
16]
As reported previously, the mean size of MgO crystallites
included in the hybrid materials tends to decrease with PET/
MAG ratio. The results of XRD examinations reported else-
[
a]
75
76
39
[
b]
[
c]
Norit RX3
[
15]
where confirmed a small amount of impurities, mainly silica,
which originates from the MAG.
[d]
[d]
PET/MAG 30:70
PET/MAG 30:70
PET/MAG 30:70
71 (99)
[
e]
97
98
[
d]
We studied the acid–base properties of the hybrid carbon-
MgO materials by determining the pHPZC values (pH at the
point of zero charge) following the experimental protocol re-
[a] Catalyst amount: 50 mg. [b] Catalyst amount: 12 mg. [c] See Ref. [11].
[d] Reaction temperature 323 K. [e] Reaction temperature 347 K.
[
17]
ported by Valente Nabais and Carrot. As a result of the pres-
ence of MgO, the hybrid materials under study exhibited basic
properties with a pHPZC value in the range of 10.16–10.21, con-
siderably higher than that of the carbon Norit RX3 as expected
times; this effect could be because of the diffusion of reagents
and product through the channels.
We also compared the catalytic performance of MgO-con-
taining carbons with that exhibited by a commercial carbon,
(Table 1).
[14]
The catalytic performance of porous carbon/MgO hybrids
Norit RX3. The yield of 5a versus time for the reaction cata-
with different MgO contents was first investigated in the Fried-
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1
2
1
2
Figure 2. Friedlꢃnder reaction between 4a and acetylacetone (R =R =Me)
Figure 3. Friedlꢃnder reaction between 4a and acetylacetone (R =R =Me)
under solvent-free conditions catalyzed by PET/MAG 30:70 at 293 ( ), 303
), and 323 K (~).
under solvent-free conditions at room temperature catalyzed by PET/MAG
&
30:70 (
&
), PET/MAG 50:50 (
&
), PET/MAG 70:30 (~), and Norit RX3 (ꢅ).
(
&
2
À1
a high surface area (SBET; 1306 m g ) and a pH around 7.4,
PZC
to give 5a with an expected low yield is shown in Figure 2,
and the results are also summarized in Table 2, entry 6. This cir-
cumstance could be attributed to the lower basicity of this
carbon but also to a change in the operative pathway and
hence in the reaction mechanism. Importantly, the conversions
[
12c–d]
to 5a on using amino-functionalized MCF materials
was
almost negligible after prolonged reaction times despite the
higher temperatures (323 K).
These results suggest that the MgO content is a critical
factor in the catalytic behavior of the hybrid carbon/MgO ma-
terials under study. We observed an increased yield to 5a if
the total surface area and the microporosity of the materials
are notably decreased; these features were probably caused
by the presence of increased amounts of MgO. The presence
of MgO in these materials accelerates the reaction considerably
and, hence, could be considered as the catalytically active spe-
cies that plays a crucial role in the efficient formation of 5a.
With these results in mind, further experiments were per-
formed by using the carbon material with the highest MgO
loading, PET/MAG 30:70, under the same experimental
conditions.
Figure 4. Influence of the catalyst amount on the Friedlꢃnder reaction be-
tween 4a and acetylacetone (R =R =Me) under solvent-free conditions at
room temperature catalyzed by PET/MAG 30:70 after 4 h of reaction time.
1
2
under mild reaction conditions to afford 5a in excellent yields
even if a small amount of the solid is used.
To explore the scope of the methodology reported herein,
we studied the Friedlꢃnder reaction using different 2-aminoaryl
aldehydes and other carbonyl components with enolizable H
atoms. In this sense, the corresponding naphthyridine 5d was
also prepared, in the presence of PET/MAG 30:70 at room tem-
perature in 60% yield after 4 h. The yield to 5d increased if
the temperature was increased to 323 K to obtain almost pure
5d in only 1 h (Figure 5; Table 2, entry 9).
In addition, we performed a study of the effect of the reac-
tion temperature in the condensation between 4a and acetyla-
cetone. The yield to 5a improved considerably at higher tem-
peratures as expected (Figure 3). Thus, it was possible to
obtain 5a in quantitative yield after 2.25 h and 15 min at 303
and 323 K, respectively.
However, the condensation between 4a and an asymmetric
1
2
carbonyl compound, the ethyl acetoacetate (R =Me; R =OEt),
allowed us to investigate the regioselectivity of the reaction
(Scheme 1). Compound 1 (related to 5b) has been reported as
a building block for the synthesis of lavendamycin ana-
We also examined the influence of the catalyst amount for
PET/MAG 30:70 in the reaction of 4a and acetylacetone
[18]
(Table 2; entries 3–5). The condensation proceeds even in the
logues. Thus, the reaction catalyzed by PET/MAG 30:70 yield-
ed quinolone 5b exclusively in 71% (1 h) and in a quantitative
yield (15 min) at room temperature and at 323 K, respectively
(Table 2, entry 7).
presence of a small amount of PET/MAG 30:70 (12 mg) to
afford 5a in 75% yield after 4 h. However, if we used more cat-
alyst (50 mg), the yield to 5a was maintained (Figure 4). The
experimental data then indicate that there is an optimum con-
centration of the catalyst (25 mg) to lead to 5a in the highest
yield after 4 h. This confirms an additional advantage of using
MgO-containing porous carbons in the efficient synthesis of N-
containing heterocycles that is minimum waste production.
We then demonstrated that the basic carbons under study,
particularly that with the highest MgO loading, PET/MAG
Finally, acridone 5c (related to biologically active 3) was syn-
1
2
thesized by the reaction of 4a and dimedone (R –R =
CH C(Me) CH ). The reaction was performed in the presence of
2
2
2
PET/MAG 30:70 at 347 K, as both starting materials are solid
compounds, to lead to 5c in 97% yield after only 15 min
(Table 2, entry 8).
The catalytic behavior of MgO-containing carbons in the
condensation of 4a with ethyl acetoacetate was quite similar
30:70, are efficient catalysts for the Friedlꢃnder condensation
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adsorbed or it can be dissociated chemically into two frag-
ments on the surface of MgO crystals. However, all our efforts
to locate the transition structure for the first adsorption mode
were unsuccessful. In contrast, it can be envisaged that the di-
carbonyl compound, which bears three O atoms, could be ad-
sorbed dissociatively by three different modes, namely,
through 1) one of the carbonyl groups (R0a and R0b) or
2) the carboxylic O atom (R0c). The calculations for all three
studied scenarios suggest that the first adsorption mode drives
the reaction to the most stable reactant complex with adsorp-
À1
Figure 5. Friedlꢃnder reaction between 4d and acetylacetone catalyzed by
tion free energies of À17.2 and À15.9 kcalmol , respectively,
PET/MAG 30:70 under solvent-free conditions at room temperature (
23 K ( ).
&
) and
whereas the interaction through the ester O atom leads to
3
&
À1
a metastable complex that is 2.5 kcalmol less stable than the
isolated reactants. That is, dissociative chemisorption to form
R0a and R0b is energetically favorable. These results and the
absence of transition structures have been confirmed by ex-
haustive relaxed scan calculations of the potential energy sur-
face (PES).
to that observed for [3-(2-aminoethylamino)propyl]trimethoxy-
silane/MCF, the most efficient amino-grafted mesoporous silica
[
12c]
reported previously for this transformation.
In this sense,
the basic carbons reported herein are cheap and environmen-
tally friendly alternative catalysts with remarkable thermal
stability.
Subsequently, the second reactant species, the aldehyde,
binds to the complex (Figure 6) through the Lewis acid proper-
ties of the catalyst through the carbonyl O atom lone pair in-
The results presented above have been rationalized by
using computational methods. Although the carbon-based
support has a slight influence in the condensation reaction,
the investigation of the reaction mechanism was performed
using only MgO as the predominant active catalytic species.
The lower activity found for Norit RX3 could be rationalized by
the p–p stacking interactions between the aromatic rings of
the carbon material and 4a, which probably modify the elec-
trophilic and nucleophilic character of the ÀCHO and ÀNH2
substituents. This approach has been proposed as an alterna-
À1
teraction with a coadsorption energy of 37.5–43.0 kcalmol
(R1a–c). The carbonyl bond length of the aldehyde increases
from 1.219 to 1.246–1.249 ꢆ.
According to the three possibilities, the nucleophilic addition
to the aldehyde can proceed through three transition struc-
tures (TS1a–c), the evolution of which lead to the intermedi-
ates I1a–c, respectively. The transition structure TS1a shows
the shortest distance for the formation of the CÀC bond
(1.984 ꢆ), whereas the largest value is found for TS1c
(2.123 ꢆ). The CÀC bond is fully formed in I1a–c (1.616–
1.625 ꢆ). During this step, the adsorbed proton remains bound
strongly to the catalyst (OÀH bond 0.980–1.007 ꢆ) and forms
[19]
tive to the classical acid catalysis described by Versꢂes et al.
It has been reported that p–p stacking interactions can alter
[
20]
[21]
the acid–base properties of pyridine and phenol.
The oxides of alkaline earth metals such as MgO,
BaO, and CaO are known for their basic properties,
related mainly to the strong Lewis basicity of the sur-
À
[22]
face O2 anions. It is believed that several reactions
of catalytic interest comprise primarily the rupture of
a heterolytic bond in which the basic character of an
À
O2 anion combines with the acid character of
2+
[23]
a Mg cation. Nevertheless, the acidic character of
Mg cations predominates in the case of molecular
[
24]
adsorption, in which the molecule resides on the
cationic site. These properties of MgO make it
a good catalyst for several catalytic reactions, such as
[
25]
the dehydrogenation of alcohols, aldol condensa-
[
26]
[22]
tion, hydrogenation of olefins, and transesterifi-
[
27]
cation of alkyl esters to produce biodiesel.
[
28]
[29]
[30]
[31]
Several molecules (alcohols, H , CH4, H O,
2
2
[
32]
[33]
[34]
[35]
[36]
À
NO2, H S, HCl, CO, CO and SO , O2 , and
2
2
2
À[37]
CO ) have been considered regarding the adsorp-
tion equilibrium geometry and the possibility of an
eventual molecular dissociation. Thus, the first step
of the most basic reaction is a deprotonation process,
which leads to a dissociated structure (Scheme 2).
Initially, we studied the adsorption of the reactants
Scheme 2. Reaction mechanism for the Friedlꢃnder reaction between 4a and ethyl ace-
1
2
on the oxide surface. The enolic dicarbonyl can be toacetate (R =Me; R =OEt) catalyzed by carbon-supported MgO materials.
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À1
Figure 6. Free-energy profile [kcalmol ] for the condensation reaction between 4a and ethyl acetoacetate catalyzed by MgO.
a strong H bond with the O atom of the carbonyl acceptor
1.737–1.766 ꢆ for the transition structures TS1, 1.496–1.554 ꢆ
for the intermediates I1). Moreover, the OÀMg interaction
through the carbonyl group is stronger than that through the
ester O atom.
TS3bW, which shows the advanced breaking of the CÀOH
(
bond (2.226 ꢆ) and the initial leaving of the amine proton. This
À1
exothermic step (16.2 kcalmol ) proceeds with a moderate ac-
tivation barrier and drives the reaction to the unsaturated bicy-
cle I3bW and three molecules of water.
This step is endothermic and proceeds with moderate to
high activation barriers. The formation of I1a is slightly favored
kinetically over the formation of I1b but very favored over the
formation of I1c, which suggests a lower catalytic effect
through the OÀMg interaction through the ester O atom.
Hence, we focus only on the former modes in the following
discussion.
At this stage, I3bW should be desorbed from the active site
to undergo a second dehydration step. Thus, the product de-
sorption is a barrierless step, as the relaxed scan of the PES
suggests, which generates the protonated adduct I4b by the
breaking of the HÀO(catalyst) bond. This step requires a free
À1 [38]
energy of 19.3 kcalmol . Once desorbed, the bicycle under-
goes water-assisted dehydration to form the quinolone 5b.
That the carbonyl O atom is bonded to Mg in the interme-
diates I1a and b, which has a critical impact on the regioselec-
tivity as the following heterocyclization step should take place
between the unbound carbonyl moiety and the amine group.
First, we describe the heterocyclization of I1b. This step can
proceed through TS2b, in which the NÀC bond (1.657 ꢆ) and
OÀH bond (1.388 ꢆ) are partially formed. It drives the reaction
to the heterocyclic intermediate I2b (NÀC=1.448, OÀH=
This step takes place with a free-energy barrier of 18.5 kcal
À1
mol via TS4b, which drives the reaction to the quinoline in
a strongly exothermic step because of the formation of
a highly stable aromatic product.
If we focus on the adsorption mode through the other car-
bonyl group, R1a, the nucleophilic attack of the amine in I1a
takes place on the ester carbonyl C atom to form the bicyclic
amide and ethanol. The transition structure TS2a (NÀC=1.616,
0
4
.966 ꢆ). This process involves a very high activation barrier of
CÀOEt=1.862 ꢆ) involves
a
very high activation barrier
À1
À1
À1
9.8 kcalmol above R1b. Previous calculations on this step
(39.6 kcalmol ), which is 59.1 kcalmol higher than that of
showed that traces of water can act as a bifunctional acid–
base catalyst. Moreover, we found that, besides this strong ki-
netic effect, the presence of two molecules of water enhanced
the regioselectivity for the formation of the quinoline frame-
R1a. Alternatively, the water catalysis reduces the Gibbs
À1
[12d]
energy by only 1.9 kcalmol (via TS2aW).
Hence, these re-
sults indicate that this cyclization is strongly disfavored over
the alternative route from a kinetic point of view and could ac-
count for the regioselectivity found at room temperature.
An inspection of the molecular orbitals provides further de-
tails. On the basis of Frontier molecular orbital theory, the
smaller the energy difference between the LUMO level of the
carbonyl acceptor and the HOMO level of amine in I1, the
easier the electron transfer during nucleophilic attack and het-
erocyclization proceeds. For I1a, the HOMO–LUMO gap is cal-
culated to be 4.12 eV, whereas for I1b it is decreased to
[
12d]
work.
In line with these results, our current calculations
reveal that the free-energy barrier for the cyclization catalyzed
by two molecules of water (TS2bW) is reduced by 24.5 kcal
À1
mol and shows an activation enthalpy, DH, of only 8.5 kcal
À1
mol . The cyclized intermediate formed in this step, I2bW,
then undergoes the elimination of water, which can be mediat-
ed by the two water molecules to alleviate the steric strain in-
volved in the “dry” system. This elimination proceeds through
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Figure 7. Comparison of the alternative reaction pathways computed for the condensation of 4 and ethyl acetoacetate to form 5b under MgO catalysis (solid
line) and the uncatalyzed system (dashed line). The reaction path through the initial uncatalyzed Knoevenagel condensation is shown by a solid gray line.
3
.53 eV. Furthermore, the LUMO coefficient at the carbonyl C
a similar value to that of the catalyzed system but higher in
À1
atom is 0.232 for the former and 0.914 for the latter, so the or-
bital overlap should be more effective in I1b.
energy (TS3bW’ is 27.7 kcalmol
TS3bW).
higher in energy than
We have also performed an equivalent simulation in the un-
catalyzed gas-phase system for comparison. In this case, the
These results suggest that the presence of a framework envi-
ronment that envelops and supports the reacting species
(MgO) not only stabilizes the reacting species but also provides
a number of reaction-promoting factors, such as adsorption
stability, the reduction of unfavorable entropic effects, the en-
hancement of collision chances in the correct orientation, and
recollision until the accumulating energy rises above the acti-
vation barrier.
À1
first step involves a barrier of 8.7 kcalmol (TS0, Figure 7) and
À1
leads to I1b’ with an energy release of 18.8 kcalmol . Hence,
the uncatalyzed aldolization is disfavored from kinetic and
thermodynamic viewpoints. The catalyst acts as a Lewis acid
by decreasing the LUMO energy (by 0.48 eV) and increasing
the LUMO coefficient of the acceptor carbonyl (from 0.691 to
0
.993 in the uncomplexed aldehyde and in R1b, respectively),
Finally, an alternative path that involves dehydration in I1b
before heterocyclization (Knoevenagel condensation) has been
ruled out as the calculations reveal a transition structure,
as expected. However, the catalyst also activates the dicarbon-
yl molecule by proton abstraction and formation of the eno-
late structure, as suggested by the charges computed by natu-
ral population analysis (À0.708 on the donor in R1b). This dual
activation leads to a reduced HOMO–LUMO gap upon com-
plexation, from 4.25 to 2.90 eV in the bare enol and in R1b, re-
spectively; thus the electron transfer proceeds more easily
during nucleophilic attack.
À1
TS2bW’’, 15.1 kcalmol higher in energy than the reactants,
therefore, this is a kinetically less favored route than the path
that involves initial heterocyclization and a closing dehydration
step.
Likewise, we have estimated the role of the oxide in the fol-
lowing cyclization and dehydration steps. The water-assisted
heterocyclization under uncatalyzed conditions proceeds
through an earlier transition state (TS2bW’) as the developing
NÀC bond is longer than that under catalysis (1.668 vs.
Conclusion
We report for the first time the Friedlꢃnder condensation cata-
lyzed by basic carbon materials, carbon-supported MgO, which
lead to the corresponding N-containing heterocyclic com-
pounds of interest. The reaction proceeds under solvent-free
and mild conditions. The scope of the methodology has been
demonstrated by the reaction of different 2-aminoaryl alde-
hydes and 1,3-dicarbonyl compounds.
1.572 ꢆ) and the Wiberg bond order is lower (0.713 vs. 0.793).
In the absence of the MgO surface, an intramolecular H bond
1.960 ꢆ) between the alcohol group and the ester carbonyl
(
stabilizes the transition structure. Notably, the energy barrier
for this step in both cases is caused only by entropic effects as
Although a decrease of the surface area and microporosity
of the carbon materials under study was observed, our results
indicate that MgO is the main catalytically active species re-
sponsible of the formation of the corresponding heterocyclic
compounds. Remarkably, the activity of the catalysts does not
only correlate to the MgO loading but also apparently depends
on the available active catalytic surface of the MgO crystals on
the hybrid materials.
À1
the enthalpy barrier is very low (~1 kcalmol ); however, the
entropic effects for the uncatalyzed system are greater than
those of the catalyzed clamped structure TS2bW, which leads
to a higher free-energy barrier (22.4 for TS2bW’ vs. 9.1 kcal
À1
mol for TS2bW). The uncatalyzed dehydration of the amino-
alcohol moiety takes place through a transition structure
À1
(
TS3bW’) that involves a moderate barrier (19.0 kcalmol ),
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[
42]
The carbon support affects the catalytic activity of the inves-
tigated materials slightly as demonstrated if the carbon Norit
RX3 was used. The lower activity found for Norit RX3 could be
because of the changes in the reaction mechanism; p–p stack-
ing interactions between catalyst and substrate could be
behind the operative pathway of the reaction.
tubes have been identified by a genetic algorithm as the puta-
tive global minima for clusters with (MgO)3k (k=1–5). For other
cluster compositions, often more cubelike structures are predicted,
but there is no common agreement on their structures in the liter-
ature yet. A recent combined experimental (gas-phase IR spectros-
copy) and theoretical (DFT) investigation gives clear evidence that
the (MgO) (n=1–20) clusters have a hexagonal tube structure,
with the exception of (MgO)4. With these precedents in mind,
we compared the cubic and hexagonal tubular structures of
a (MgO)15 cluster. The size n=15 was selected as large enough to
contain the reactant structures. In agreement with the above con-
clusions, the hexagonal tube structure was more stable (by
7.3 kcalmol ). Hence, this structure was chosen for our DFT
[43]
n
The computational analysis of the reaction mechanism sug-
gests dual activation by the catalyst, as both carbonyl com-
pounds are activated for the condensation. The catalyst plays
a role until the last dehydration step to stabilize the species
and promote the reaction. Moreover, the regioselectivity of the
condensation for asymmetric dicarbonyl compounds has been
justified by the calculations based on energy and electronic
factors.
[44]
À1
1
calculations.
[45]
Calculations were performed by using Gaussian09.
hybrid DFT method with the 6-31G(d,p) basis set was used to
optimize the geometries. B3LYP is a reliable level of theory used
The B3LYP
[46]
Finally, the advantage of using carbon-supported MgO ma-
terials for the Friedlꢃnder reaction relies on their easy prepara-
tion from a natural MgO source and high thermal stability.
These materials constitute a highly efficient, sustainable alter-
native to amino-grafted and bifunctional mesoporous silicas.
[47]
commonly in the study of different nanostructures. Vibrational
frequencies were also calculated at the same level to confirm that
all the stationary points correspond to true minima on the poten-
tial energy surface and to extract vibrational zero-point and ther-
mal corrections from the thermodynamic results. Transition state
calculations were performed using the same level of theory. The in-
trinsic reaction coordinate (IRC) pathways have been traced to
verify two desired minima connected by the transition states. To
obtain insights into the adsorption/desorption processes, relaxed
scans of the PES were performed by changing the distance be-
tween two key atoms progressively. Electronic parameters, such as
frontier molecular orbitals and natural bond orbital (NBO) analy-
[48]
Experimental Section
Catalytic performance
The reactions were followed by TLC chromatography performed
on DC-Aulofolien/Kieselgel 60 F245 (Merck) using mixtures of
CH Cl /EtOH 98:2 as eluent.
2
2
[49]
ses, and energy calculations, were computed on the optimized
structures at the same level.
The characterization of the reaction products was performed by
1
H NMR spectroscopy. NMR spectra were recorded by using
1
1
a Bruker AVANCE DPX-300 spectrometer (300 MHz for H). H chem-
ical shifts (d) in [D ]DMSO are referenced to internal
tetramethylsilane.
6
Acknowledgements
The reactions were performed in the liquid phase under atmos-
pheric pressure by using a multiexperiment work station StarFish
This work has been supported by MICINN (projects CTQ2009–
10478 and CTQ2011–27935). We are grateful to the Centro de Su-
percomputaciꢀn de Galicia (CESGA) for generous allocation of
computing resources.
(
Radley’s Discovery Technologies UK).
Typically, 2-aminoaryl aldehyde (0.5 mmol) and the 1,3-dicarbonyl
compound (5 mmol) were added to a three-necked vessel of
1
0 mL capacity, equipped with a condenser, thermometer, and
Keywords: carbon · magnesium · nitrogen heterocycles ·
reaction mechanisms · supported catalysts
magnetic stirrer (0.8 cm). Subsequently, the catalyst (25 mg) was
added and the reaction mixture was stirred (250 rpm) at RT (293 K)
for the time indicated in the figures or tables. The samples were
withdrawn at different reaction times by diluting a small amount
of the reaction mixture in dichloromethane (0.5 mL). Subsequently,
the catalyst was collected by filtration, and the solvent evaporated
in vacuo.
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Published online on && &&, 0000
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Oxide, outside: MgO supported on
carbon is able to catalyze the synthesis
of interesting N-containing heterocyclic
compounds efficiently under mild con-
ditions. MgO on the carbon surface is
responsible for the catalytic behavior.
These carbon materials are environmen-
tally friendly catalysts for the Friedlꢃnd-
er reaction.
M. Godino-Ojer, A. J. Lꢀpez-Peinado,
R. M. Martꢁn-Aranda, J. Przepiꢀrski,
E. Pꢂrez-Mayoral,* E. Soriano*
&& – &&
Eco-Friendly Catalytic Systems Based
on Carbon-Supported Magnesium
Oxide Materials for the Friedlꢀnder
Condensation
ꢄ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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