Effects of bentonite on p-methoxybenzyl acetate: A theoretical model for oligomerization via an electrophilic-substitution mechanism

Article abstract of DOI:10.3390/molecules16021761

Tonsil Actisil FF, a commercial bentonitic clay, promotes the formation of a series of electrophilic-aromatic-substitution products from para-methoxybenzyl acetate in carbon disulfide. The molecules obtained correspond to linear isomeric dimers, trimers, tetramers and a pentamer, according to their spectroscopic data. A clear indication of the title mechanistic pathway for the oligomerization growth was obtained from the analysis of a set of computational-chemistry calculations using the density-functional-theory level B3LYP/6-311++G(d,p). The corresponding conclusions were based on the computed dipole moments, the HOMO/LUMO distributions, and a natural-populations analysis of the studied molecules.

Full text of DOI:10.3390/molecules16021761

Molecules 2011, 16, 1761-1775; doi:10.3390/molecules16021761
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Effects of Bentonite on p-Methoxybenzyl Acetate: A Theoretical
Model for Oligomerization via an Electrophilic-Substitution
Mechanism
Manuel Salmón 1,*, Rene Miranda 2, Ines Nicolás-Vázquez 2,*, Yolanda Marina Vargas-Rodriguez 2,
Julian Cruz-Borbolla 3, María Isabel Medrano 1 and José Antonio Morales-Serna 1
1
Instituto de Química de la Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad
Universitaria Coyoacán 04510, México, D.F., Mexico;
E-Mail: morser@correo.unam.mx (J.A.M.-S)
2
Sección de Química Inorgánica, Fisicoquímica y Química Orgánica, Departamento de Ciencias
Químicas, Facultad de Estudios Superiores Cuautitlán, Universidad Nacional Autónoma de México,
Cuatitlán Izcalli, 54740, Estado de México, Mexico; E-Mails: mirruv@yahoo.com.mx (R.M.);
ym_vargas@yahoo.com.mx (Y.M.V.-R.)
3
Área Académica de Química, Universidad Autónoma del Estado de Hidalgo, Unidad Universitaria,
Km 4.5 Carretera Pachuca-Tulancingo, Mineral de Reforma, 42184, Hidalgo, Mexico;
E-Mail: jcruzborbolla@yahoo.com.mx (J.C.-B.)
* Authors to whom correspondence should be addressed; E-Mails: salmon@unam.mx (M.S.);
nicovain@yahoo.com.mx (I.N.-V.).
Received: 29 November 2010; in revised form: 14 February 2011 / Accepted: 15 February 2011 /
Published: 21 February 2011
Abstract: Tonsil Actisil FF, a commercial bentonitic clay, promotes the formation of a
series of electrophilic-aromatic-substitution products from para-methoxybenzyl acetate in
carbon disulfide. The molecules obtained correspond to linear isomeric dimers, trimers,
tetramers and a pentamer, according to their spectroscopic data. A clear indication of the
title mechanistic pathway for the oligomerization growth was obtained from the analysis of
a set of computational-chemistry calculations using the density-functional-theory level
B3LYP/6-311++G(d,p). The corresponding conclusions were based on the computed
dipole moments, the HOMO/LUMO distributions, and a natural-populations analysis of the
studied molecules.

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Keywords: bentonitic clay; para-methoxybenzyl acetate; oligomerization; electrophilic
aromatic substitution; DFT calculations
1. Introduction
Benzyltoluenes are of industrial interest due to their application as insulating oils in high-voltage
electrical devices [1], in the production of termiticide emulsions that demonstrate good penetration and
emulsion stability [2], and in the preparation of corrosion-protection products [3]. Our research group
previously developed and reported a strategy to obtain oligomeric toluene compounds with solid acid
catalysts using thermal energy and ultrasound [4]. Thus, when ortho- and para-benzyltoluenes were
studied, the outcome of the oligomerization reactions was found to be dependent of the amount of
catalyst used, reaction time, temperature and the presence of pyridine as a competitive inhibitor. The
study was carried out using Tonsil Actisil FF (TAFF), a commercial bentonite clay that has long been
employed by our research group as a Brønsted-Lowry and Lewis acid catalyst in different chemical
reactions [5-14]. An electrophilic aromatic substitution pathway has been suggested for the
oligomerization reaction [15], while a low toluene/BzCl ratio is necessary to obtain the
oligomers [16,17].
In this context, we considered the possibility of carrying out the reaction of para-methoxybenzyl
acetate in the presence of Tonsil Actisil FF to obtain oligomeric compounds, followed by a theoretical
analysis that rationalizes the observed products and some of the key reaction steps. Previous studies
carried out by our research group have established the importance of an electrophilic attack in similar
reactions [18]. Despite the complete absence of experimental data on the structures of the transition
states, calculated transition-state geometries are now commonplace thanks to the development of
software that provides tools to propose transition states. Thus, the aim of this paper is to report on the
conversion of para-methoxybenzyl acetate with TAFF, producing isomeric diphenylmethane and a
series of linear oligomers with a progressive number of units. This is accompanied by computational-
chemistry calculations performed to establish a validated mechanistic pathway to explain the growth of
the obtained linear oligomers.
2. Results and Discussion
To investigate the novel catalytic capabilities of TAFF, this work studied the transformation of
para-methoxybenzyl acetate (1). Despite the Lewis and Brønsted-Lowry acid character of the studied
clay, its acidity is very low in comparison with sulfuric or triflic acids [19]. Thus, the phenylmethanes
2-4 and linear oligomers 5-8 were obtained when the reaction was carried out in carbon disulfide at
room temperature. However, when the temperature was increased to 46 °C, the linear oligomers 9 and
10 were obtained. The structure of each isolated compound is displayed in Figure 1, where it can be
seen that the phenylmethanes 2 and 3 are ortho-substituted with respect to MeO, an electron-donating
group, whereas 4 is formed by an ipso-substitution. All other molecules are symmetrically substituted
linear benzylic oligomers formed through an electrophilic aromatic substitution (EAS) with three, four
or five linked aromatic units. In the oligomers 2, 5 and 8, the acetyl unit remains present, but it is

Molecules 2011, 16
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absent in 3 and 7, yielding a methyl group, as previously seen in pentamethylbenzyl cation reduction
[20] by the addition of a hydride provided by the clay [21]. However, this process could results from a
catalytic transfer hydrogenation from an organic donor-molecule species to any of the variety of
organic acceptors formed in the reaction medium [22]. Trimer 6 is the only isolated compound
possessing a benzylic OH group, which is probably formed after the acyl hydrolysis of 5. Finally,
when the temperature is increased, the promotion of the longer linear oligomers 9 and 10 is obtained
by further condensation with other para-methoxybenzylic carbenium ions or by an ipso substitution.
Scheme 1. Oligomerization reaction products.
O
O
1
O
Clay/CS2
O
O
O
O
O
O
O
3
4
2
O
O
OH
O
O
O
O
O
O
O
6
5
O
O
O
O
O
O
O
O
O
8
7
O
O
O
O
O
O
O
O
O
9
10
Reagents and conditions: 1.0 g of clay, 5 g of p-methoxybenzyl acetate, 40 mL of CS2, r.t., 76 h.
Figure 1. B3LYP/6-311++G(d,p) energy profile for the carbenium ion formation.

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Two possibilities were considered to explain the EAS mechanism of the para-methoxybenzyl
acetate reactions yielding 2-10 (Scheme 1) via the para-methoxybenzyl carbenium ion. The clay may
provide protons or electrons to the neutral molecule to assist the carbocation induction at the beginning
of the process. To analyze this possibility, the benzylic CH2-OAc bond distance in the monomer was
computed with a charge of –1 and a length of 1.680 Å (adding one electron) and with a charge of +1
and a length of 1.418 Å (removing an electron) (Figure 1). It is important to note that, in the neutral
molecule, the CH2-OAc bond distance is 1.463 Å. With these bond lengths, the clay may provide an
electron to enlarge the CH2-OAc bond, easing the corresponding rupture to generate an electrophilic
species. In this sense, 2 must be formed when this carbenium ion, with a dipole moment (DM) of
10.51 D and located in the plane of the benzene ring, interacts with the 3.17 D DM of a neutral species.
In this way, the interaction between both species with an ortho position to the MeO group on the
benzene ring will be favored (Figure 2). Additionally, this mechanism may suggest that the formation
of 3 is obtained from the dimeric carbenium ion (Figure 3), which is reduced by a hydride provided by
the clay, yielding the corresponding methyl group [20].
Figure 2. a) Neutral species of para-methoxybenzyl acetate. b) The carbenium ion of
para-methoxybenzylic acetate. c) The HOMO of para-methoxybenzyl acetate and the
LUMO of the para-methoxybenzyl carbenium ion (ΔEHOMO-LUMO = 1.69 eV). d) The
optimized B3LYP/6-311++G(d,p) geometries for 2. The calculated NPA charges are
reported in e-.
a)
b)
c)
d)
Figure 3. a) The LUMO of the dimeric carbenium ion that yields 3. b) The optimized
B3LYP/6-311++G(d,p) geometries for 3. The calculated NPA charges are reported in e-. c)
The carbenium dimer. d) The interaction between the LUMO of the charged carbenium
dimer with the HOMO of 3 to yield the corresponding trimer 7.
a)
b)
c)
d)

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Consequently, all compounds possessing a benzylic-ester substituent group, after dimerization, are
able to continue the chain enlongation assisted by the corresponding benzylic carbenium ion. Thus, the
oligomers 5 and 8 may be formed by two possible routes. The first route implicates an interaction
between the charged carbenium dimer 2 or a charged carbenium trimer 5 with the original substrate
(basic unit) to yield the corresponding trimer 6 and tetramer 8. This model resembles the process of
formation of 2 (Figure 4).
Figure 4. a) The interaction between the LUMO of the charged carbenium dimer with the
HOMO of the original substrate to yield the corresponding oligomer 5. b) The interaction
between the LUMO of the charged carbenium trimer with the HOMO of the original
substrate to yield the corresponding oligomer 8. c) Two possible routes to obtain 5. The
energy difference between the HOMO of the neutral species and the LUMO of the dimer
carbenium is ΔEHOMO-LUMO = 1.28 eV; therefore, this interaction is more feasible. d) The
optimized B3LYP/6-311++G(d,p) geometries for 5. The calculated NPA charges are
reported in e-.
a)
b)
c)
d)
Figure 5. a) The oligomer 5 is obtained from the benzylic carbenium monomer and the
dimer 2. b) The HOMO of 5 and the LUMO of methoxybenzyl carbenium ion that yield
tetramer 8. c) The HOMO of 2 and the LUMO of the charged carbenium dimer that yield 8.
d) According to the graph, three possible routes could lead to 8. The energy difference
between the HOMO of the neutral species and the LUMO of the trimer carbenium is
ΔEHOMO-LUMO = 1.06 eV; therefore, this interaction is more feasible.
a)
b)
c)
d)

Molecules 2011, 16
Figure 6. a) The HOMO of para-methoxybenzyl acetate and the LUMO of the
1766
methoxybenzyl carbenium ion that yield oligomer 4. b) The DM of the carbenium ion must
be aligned towards the methylene group of 4. This centered orientation prevents any further
substitution under the reported experimental conditions. c) The HOMO and LUMO of 4. d)
The optimized B3LYP/6-311++G(d,p) geometries for 2. The calculated NPA charges are
reported in e-.
a)
b)
c)
d)
The oligomer 5 may be obtained from the benzylic carbenium monomer and the dimer 2 instead of
via the second pathway (Figure 5). The same condensation is suggested to explain the formation of
tetramer 8; it is the primary interaction that occurs between the two DMs and the carbenium ion
oriented towards the benzene ring with the major HOMO distributions. Thus, this interaction is
towards any of the C atoms located ortho to the MeO group. It is also worth mentioning that these
atoms have a natural atomic charge of -0.290 e-, the largest negative value in this ring. The HOMO
distributions can be observed in the B ring. The C atoms located in ortho to the MeO group have
natural atomic charges of -0.293 and -0.294; therefore, these sites could interact with the LUMO of the
carbenium ion.
Moreover, the inhibition of the EAS was clear in 4. Its computed DM is 0.74 D and points outwards
from the methylene group. The computed HOMO distribution is widespread in an equivalent form

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over both benzene rings (Figure 6). It is also important to mention that the DM of the carbenium ion
must be aligned towards the methylene group. This centered orientation and the even distribution of
the HOMO and LUMO will prevent any further substitution under the reported experimental
conditions. The ipso substitution plays an interesting role in the oligomerization control by blocking
one side of the molecule, such as in 9 and 10 (Figures 7, 8 and 9). Thus, 9 and 10, which have DMs of
1.99 D and 2.25 D, respectively, and their HOMOs and LUMOs (Figure 10) located on opposite ends
of the chain, favor substitution, and the chain elongation proceeds step-by-step by condensing a
carbenium ion with the benzene ring that holds the HOMO orbitals. The reaction will be directed to the
ortho-methoxy carbons with the largest negative charge in the ring (Figure 7).
Figure 7. a) Interaction between the LUMO of the charged carbenium dimer and the
HOMO of 4 to yield the corresponding trimer 9. b) The interaction between the LUMO of
methoxybenzyl carbenium ion and the HOMO of the trimer of 4 to yield the corresponding
tetramer 9. c) The interaction between the LUMO of the charged carbenium trimer and the
HOMO of the monomer. d) Three possible routes to obtain 9. The energy difference
between the HOMO of the neutral species and the LUMO of the trimer carbenium is
ΔEHOMO-LUMO = 1.76 eV; therefore, this interaction is more feasible. e) Optimized
B3LYP/6-311++G(d,p) geometries for 9. The calculated NPA charges are reported in e-.
a)
b)
c)
d)
e)

Molecules 2011, 16
Figure 8. a) The interaction between the LUMO of the methoxybenzyl carbenium ion with
1768
the HOMO of 4 to yield the corresponding trimer, 4a, as an intermediary of 9. b)
Optimized B3LYP/6-311++G(d,p) geometries for 4a. The calculated NPA charges are
reported in e-. c) Carbenium trimer of 4a.
a)
b)
c)
Figure 9. a) The HOMO and LUMO of 9. b) The HOMO, HOMO-1 and LUMO of 10.
a) HOMO
b) LUMO
c) HOMO
HOMO-1
LUMO
It is convenient to note that, to classify the catalytic hydrolysis of ester molecules, organic-
chemistry textbooks state that the reaction precedes via AAc2 under acidic conditions and BAc2 under
basic conditions [23]. The above notation indicates that the O-R bond is not commonly broken [24-29].
Under acid catalysis, an alternative but less common pathway involves the alkyl-oxygen cleavage in
which an acyloxy group or its conjugated acid is the leaving group; thus, an AAl1 mechanism occurs if
the alkyl substituent comes off as a stable carbenium ion. In this particular sense, it is already known
that benzylic acetates are hydrolyzed by the AAc2 mechanism in dilute acid, whereas the mechanism
changes to AAl1 in concentrated acid [30-34]. Additionally, little is known about acetylated benzylic
alcohols as electrophiles in the EAS reaction. However, it is worth mentioning that the driving force
for this process must be the catalytic acidic promotion of an AAl1 mechanism, yielding such
interesting intermediates as the para-methoxybenzylic carbenium ion. Finally, it is important to note
that, to the best of our knowledge, this work is the first example of a proposed AAl1 pathway when
bentonitic clay is used as a heterogeneous catalyst in an EAS reaction, as supported by theoretical
calculations.

Molecules 2011, 16
Figure 10. a) Interaction between the LUMO of the methoxybenzyl carbenium ion and the
1769
HOMO of 9 to yield the corresponding pentamer 10. b) The interaction between the
LUMO of the charged carbenium dimer and the HOMO of the trimer of 4 to yield the
corresponding pentamer 10. c) and d) Two possible routes to obtain 10. The energy
difference between the HOMO of the trimer neutral species and the LUMO of the dimer
carbenium is ΔEHOMO-LUMO = 1.82 eV; therefore, this interaction is more feasible. e) The
optimized B3LYP/6-311++G(d,p) geometries for 10. The calculated NPA charges are
reported in e-.
a)
b)
c)
d)
Finally, from the mechanistic point of view, the catalytic action of TAFF should enhance the
electrophilic character of the carbonylic substrates, facilitating the formation of para-methoxybenzyl
carbonium ion. The ion is the key intermediate to understand the formation of the products. The
interaction between TAFF and 1 might be due to the protonated and unprotonated active sites
corresponding to the acidic Brönsted-Löwry and Lewis character of the clay (Scheme 2).
Scheme 2. Proposed reaction mechanism.
TAFF
TAFF
Lewis acid
Brönsted-Löwry acid
Al
Si
O
H
O
O
O
O
O
O
O
O
1
O
Clay/CS2
O
O
O
O
O
O
O
O
O
O
Ortho substitution
Ipso substitution
O
2
O
O
4
3 and 5-10

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3. Experimental
3.1. General
Tonsil Actisil FF, a cheap (US $1.30/kg) commercial Mexican bentonitic clay, is readily available
from Tonsil Mexicana S. A. de C. V. (México City, México). Examined with X-ray fluorescence, this
clay was shown to have the following composition (in %): SiO2, 74.5; Al2O3, 9.3; MgO, 0.4; Fe2O3,
1.3; CaO, 4.0; K2O, 0.4; TiO2, 0.4; H2O, 9.7. X-ray thermodiffractograms show that the laminar
structure is unstable above 150 °C. Quartz and cristobalite are also important components in the clay
composition, as observed by X-ray diffraction. The corresponding BET surface area is 198.718 m2 g−1,
and the pore volume and average pore diameter are 32.04 × 10−2 cm3 g−1 and 77.8 Å, respectively. It is
27
worth mentioning that a detailed characterization of the clay (29Si and Al MAS-NMR, SEM, IR-Py,
DTA, TG, and Ho) has already been performed and reported by our research group [12]. The particle
size is 325 mesh. The organic reagent 4-methoxybenzylacetate was prepared by the acetylation of 4-
methoxy-benzyl alcohol (Merck) with pyridine and acetyl chloride and was further purified by SiO2
column chromatography using n-C6H14/EtOAc as the eluent. IR spectra were recorded on a Nicolet
1
FTIR-5SX spectrometer. The H- and 13C-NMR spectra were measured in CDCl3 solution with a
Varian Unity 300 (operating at 300 MHz and 75 MHz, respectively) using tetramethylsilane as an
internal standard. Mass spectra were obtained with a JEOL JMS AX505HA mass spectrometer. For
thin-layer chromatographic (TLC) analyses, Merck precoated TLC plates (silica gel 60 F254, 0.25 mm,
Art 5715) were used.
3.2. Oligomerization of p-methoxybenzyl acetate with Tonsil
The typical procedure at room temperature is as follows: a suspension of p-methoxybenzyl acetate
(1, 5.0 g), carbon disulfide (40 mL) and the bentonitic earth (1.0 g) was vigorously stirred at room
temperature until disappearance of the starting material, which occurred after 76 h. The reaction was
conveniently monitored by TLC. The clay was eliminated by filtration through Celite and washed with
ethyl acetate (3 × 10 mL). The combined filtrates were dried on anhydrous Na2SO4, and the solvent
was eliminated under reduced pressure. The residue was subjected to chromatography on a silica-gel
column using n-C6H14/EtOAc as the eluent, affording phenylmethanes 2-4 and linear oligomers 5-8.
The typical procedure at reflux is identical to the above procedure, but the chromatography afforded in
that case the linear oligomers 9 and 10.
3-(4-Methoxybenzyl)-4-methoxybenzyl acetate (2). Syrupy yellow oil, isolated in 3.8% yield,
1
(C18H20O4). IR (film, cm−1): 3,007, 2,953, 2,836, 1,735 (C=O), 1,464, 1,263, and 1,032 (C-O). H-
NMR δ: 2.05 (3H, s, CH3), 3.77 (3H, s, CH3-O), 3.78 (3H, s, CH3-O), 3.90 (2H, s, Ar-CH2-Ar), 4.98
13
(2H, s, CH2O), 6.80–6.85 (3H, m, Ar), 7.05–7.21 (4H, m, Ar), C-NMR δ: 21 (CH3), 35 (CH2), 56
(2C, CH3O), 66 (CH2O), 110,114, 128, 134, 156 and 158 (C, Ar). 171 (C=O). MS m/z: 300 [M+,
100%], 241 [M-AcO]+ (54), 225 (17), 209 (16), 137 (18), 121(50), 91(10).
1-(2-Methoxy-5-methylbenzyl)-4-methoxybenzene (3). White needles m.p. 64–66 °C, 0.42%
1
(C16H18O2). IR (CDCl3 cm−1): 3,032, 2,919, 2,835, 1,466, 1,442, (CH2, CH3) 1,250, 1,032 (C-O). H-

Molecules 2011, 16
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NMR δ: 2.23 (3H, s, CH3), 3.77 (3H, s, CH3-O), 3.78 (3H,s, CH3O), 3.88 (2H, s, CH2) 6.7–6.9 (3H, m,
13
Ar), 6.7–7.11, (4H, d, Ar, J = 8.7, 8.4 Hz A2B2); C-NMR δ: 22 (CH3), 35(CH2), 56 (CH3O), 111,
114, 127, 130, 131.5 (C, Ar), 171 (C=O). MS m/z: 242 [M+, 100%], 227 [M-Me]+ (35), 211 [M-OMe]+
(43), 195 (13), 174 (17), 121 (46), 91 (15)
Bis(4-methoxyphenyl)methane (4). Syrupy yellow oil 0.3% (C15H16O2). IR (CHCl3, film, cm−1): 3,005,
2,920, 2,839, 1,511, 1,457, 1,435, 1,252, 1,027. 1H-NMR δ: 3.78 (6H, s, CH3O), 3.86 (2H, s, CH2), 6.8
13
(4H, d, J = 9 Hz, Ar), 7.05 (4H, d, J = 8.4 Hz, Ar) A2B2. C-NMR δ: 40 (CH2), 56 (2C, CH3O), 114
(4C, Ar o-H3CO-C-CH), 130 (4C, Ar, m-H3CO-C-CH-CH) 134 (2C Ar, ipso), 158 (2C, Ar, H3CO-C).
MS m/z: 228 [M+, 100%], 227 [M-H]+ (55), 197 [M-OMe]+ (85), 121 (28), 114 (12), 91 (7), 77 (4).
3-(3-(4-Methoxybenzyl)-4-methoxybenzyl)-4-methoxybenzyl acetate (5). Syrupy yellow oil 1.3%
(C26H28O5). IR (CHCl3, film, cm−1): 2,835, 1,736, (C=O) 1,463, 1,441, 1,248, 1,033. 1H-NMR δ: 2.05
(3H, s, CH3), 3.77 (9H, m, CH3O), 3.85, 3.88 (4H, s, 2CH2, 4.98 (2H, s, CH2O), 6.75–7.20 (10 H, m,
13
Ar). C-NMR δ: 21 (CH3), 35 (CH2), 56 (CH3O) 66 (CH2O) 110, 114, 128–134, 156–158 (Ar), 171
(C=O): MS m/z: 420 [M+, 37%], 360 [M-AcOH]+ (100), 345 [M-AcOH-Me]+ (56), 299 (10), 239 (12),
227 (10), 165 (6), 121 (90), 91 (6).
(3-(3-(4-Methoxybenzyl)-4-methoxybenzyl)-4-methoxyphenyl)methanol (6). Syrupy yellow oil 0.0075%
(C24H26O4). IR (CHCl3, film, cm−1): 3,425 (OH), 2,850, 2,928, 1,502, 1,247, 1,033, 1,463, 835.
1H-NMR δ: 3.77 (9H, s, CH3O), 3.84 (2H, s, CH2), 3.86 (2H, s, CH2), 4.54 (2H, s, CH2), 6.7–7.18
(10H, m, Ar); 13C-NMR δ: 30 (CH2), 35 (CH3O) 65 (CH2O) 110, 114, 126–132 (Ar); MS m/z 378 [M+,
75%], 360 [M-H2O]+ (16), 345 [M- H2O -Me]+ (15), 257 (46), 227 (42), 195 (12), 149 (15), 121 (100),
97 (11), 81 (16), 69 (34), 41 (25).
4-(2-Methoxy-5-methylbenzyl)-2-(4-methoxybenzyl)-1-methoxybenzene (7). Syrupy yellow oil 0.58%
(C24H26O4). IR (CHCl3, film, cm−1): 3,032, 2,929, 2,833, 1,520, 1,463, 1,461, 1,260, 1,035. 1H-NMR δ:
2.21 (3H, s, CH3), 3.73 (3H, s, CH3O), 3.77 (3H, s, CH3O), 3.81 (2H, s, CH2), 3.86 (2H, s, CH2),
13
6.71–7.12 (10H, m, Ar); C-NMR δ: 21 (CH3), 35 (CH2) 56 (CH3O), 110, 114, 127, 130, 131, (Ar);
MS m/z 362 [M+, 100%], 347 [M-Me]+ (8), 331 [M-OMe]+(10), 241 (77), 227 (41), 195 (6), 181 (7),
121 (95), 105 (10), 91 (6).
3-(3-(3-(4-Methoxybenzyl)-4-methoxybenzyl)-4-methoxybenzyl)-4-methoxybenzyl acetate (8). Syrupy
yellow oil 0.037% (C34H36O6); IR (CHCl3, film, cm−1): 2,925, 2,835, 1,736, (C=O), 1,619, 1,254,
1
1,033 cm−1. H-NMR δ: 2.04 (3H, s, CH3), 3.73–3.77 (12H, s, CH3O), 3.80–3.85 (6H, s, CH2), 4.95
(2H, s, CH2), 6.7–7.25 (12H, m, Ar); 13C-NMR δ: 21 (CH3), 35, 30, (CH2) 56 (CH3O), 67 (CH2O), 171
(C=O), 110, 114, 127-130, (Ar); MS m/z 480 [M+, 100%], 465 [M-Me]+ (23), 449 [M-OMe]+(11), 345
(6), 240 (22), 121 (83), 91 (4), 57 (6).
1-(5-(5-(4-Methoxybenzyl)-2-methoxybenzyl)-2-methoxybenzyl)-4-methoxybenzene (9). Syrupy yellow
oil 0.39% (C34H36O6); IR (CHCl3, film, cm−1): 2,928, 2,837, 1,611, 1,246, 1,043. 1H-NMR δ: 3.73 (3H,
s, CH2O), 3.77 (6H, s, CH3O), 3.79 (4H, s, CH2), 3.86 (2H, s, CH2), 6.7–7.12 (14H, m, Ar); 13C-NMR
δ: 35, 41 (CH2), 55 (CH2O), 110, 114, 126–134, (Ar); MS m/z 480 [M+, 100 %], 437 [M-OMe]+ (7),
347(32), 241(10), 227(31), 121(50).

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2-(3-(4-Methoxybenzyl)-4-methoxybenzyl)-4-(5-(4-methoxybenzyl)-2-methoxybenzyl)-1-ethoxybenzene
(10). Syrupy yellow oil 0.94% (C42H44O7); IR (CHCl3, film, cm−1): 2,936, 2,834, 1,736, (C=O), 1,258,
1,033. 1H-NMR δ: 2.10 (3H, s, CH3), 371 (9H, s, CH3O), 3.75 (6H, s, CH3O), 3.79 (4H, s, CH2), 3.82
13
(2H, s, CH2), 3.84 (2H, s, CH2), 4.90 2H, s, CH2O), 6.7–7.25(16H, Ar); C-NMR δ: 22 (CH3), 35
(CH2), 55 (CH3O), 66 (CH2O), 110, 113, 127–134, 155, 158, (16H, Ar); MS m/z 660 [M+, 5%], 600
[M-AcOH] (100), 479 (55), 467 (11), 368 (22), 300 (11), 241 (23), 135 (22), 121 (70), 69 (26), 57 (39).
3.3. Theoretical calculations
The geometries and electronic structures of all compounds were determined by calculations carried
out in the spin-restricted and spin-unrestricted formalisms for closed-shell and open-shell systems,
respectively. In addition, the corresponding geometry optimizations were performed by electronic-
structure approaches using density-functional theory (DFT) and specifically the functional B3LYP
[35,36]. This functional is Becke´s three-parameter functional in which the correlation functional is
provided by the Lee, Yang, and Parr expression (LYP) functional. B3LYP proved to be a suitable
method because it includes the electron-correlation effects to some extent. In other words, this scheme
was used in conjunction with the split-valence polarized basis set and diffuses, 6-311++G(d,p) [37-40],
as implemented in Gaussian 03 [41].
The located transition state (TS) was confirmed through the analysis of vibration frequencies, which
are single imaginary frequencies, defined along the reaction coordinate presented by such states. The
optimized structures were confirmed as local minima by estimating their normal vibrations and by the
absence of imaginary frequencies. The natural atomic charges, dipole moment and HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) were used to analyze
the electrophilic or nucleophilic character involved in the reactivity of the para-methoxybenzyl acetate
molecule. In addition, these properties were determined for the equilibrium geometries. The structures
and the properties were visualized with the GaussView package coupled to Gaussian 03. The natural-
population analysis (NPA) was developed to calculate the atomic charges of molecular wave functions
in general atomic-orbital basis sets. NPA is an alternative to conventional Mulliken population analysis
and exhibits improved numerical stability [42,43]. This method was used to determine the charge of
the studied molecules. The NBO 3.1 program was used as implemented in Gaussian 09 at the B3LYP
theoretical level. The NBO analysis transforms the canonical delocalized Hartree–Fock (HF) MOs into
localized orbitals that are closely tied to chemical-bonding concepts. This process involves the
sequential transformation of non-orthogonal atomic orbitals (AOs) into sets of NAOs, NHOs, and
NBOs. Each of these localized basis sets is complete and describes the wave functions in the most
economical method because electron density and other properties are described by the minimal amount
of filled orbitals in the most rapidly convergent way.
4. Conclusions
The oligomerization of para-methoxybenzyl acetate with Tonsil Actisil FF (TAFF), a commercial
bentonite clay, provided isomeric diphenylmethane and linear oligomers with a progressive number of
units. This work is the first example in which an AAl1 pathway is proposed when bentonitic clay is

Molecules 2011, 16
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used as a heterogeneous catalyst in an EAS reaction. The corresponding conclusion was based on the
computed dipole moments, the HOMO/LUMO distributions and a natural-populations analysis of the
studied molecules.
Acknowledgements
Financial support from DGAPA-UNAM under Projects PAPIIT: IN-500597, PAPIME: PE201905
and CONACyT-México. We also want to thank DGSCA-UNAM for the generous time in their
KanBalam Supercomputer.
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Sample Availability: Samples of the compounds 2-10 are available from the authors.
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