DFT calculations on the Friedel-Crafts benzylation of 1,4-dimethoxybenzene using ZnCl2 impregnated montmorillonite K10 - Inversion of relative selectivities and reactivities of aryl halides

Article abstract of DOI:10.2478/s11696-011-0073-7

Zinc was bound on montmorillonite K10 by cation exchange to obtain a catalyst named clayzic. In the Friedel-Crafts benzylation of 1,4-dimethoxybenzene, this catalyst was used for the synthesis of substituted diphenylmethanes using 4-chlorobenzyl chloride and 4-bromobenzyl bromide. During the reaction, sub-products from a second benzylation reaction process were observed. For a better understanding of their formation, reactions were carried out at different times to obtain data on the progress of benzylation and the relative ratio of each product was calculated using two different analytical methods. It was shown that the selectivity and reactivity of both aryl halides were reversed under these experimental conditions contrary to those obtained using the more conventional catalyst, zinc dichloride. These results were explained by geometrical and electronic considerations. It was found that the formation of transition states and Wheland intermediates from aryl bromide and chloride in the presence of clayzic can be explained in terms of preferential absorption. Moreover, the high percentage of 4-chlorobenzyl chloride conversion was attributed to its covalent radius, which is smaller than that of Br. At the same time it was shown that the presence of a Bronsted acid, due to the liberation of HCl during the benzylation, is responsible for the poisoning of the clayzic catalyst. Moreover, poisoning effect of the bromine anion could not be excluded.

Full text of DOI:10.2478/s11696-011-0073-7

Chemical Papers 65 (6) 873–882 (2011)
DOI: 10.2478/s11696-011-0073-7
ORIGINAL PAPER
DFT calculations on the Friedel–Crafts benzylation
of 1,4-dimethoxybenzene using ZnCl impregnated montmorillonite
2
K10 – inversion of relative selectivities and reactivities
of aryl halides
a,b
a,c
a,d
a,d
Christophe Waterlot*, Daniel Couturier, Beno ˆı t Rigo, Alina Ghinet,
a,e
Marc De Backer
aUniversité Lille Nord de France, 1bis rue Georges Lef `e vre, F-59044 Lille, France
bLaboratoire Génie Civil et géoEnvironnement (LGCgE) Lille Nord de France (EA 4515), Equipe Sols et Environnement,
Groupe ISA, UCLille, 48 Boulevard Vauban, F-59046 Lille, France
cLaboratoire d’Ingénierie Moléculaire, USTL, 59655 Villeneuve d’Ascq, France
dLaboratoire de Pharmacochimie, HEI, EA 4481 (GRIIOT), UCLille, 13 rue de Toul, F-59046 Lille, France
eUMR CNRS 8516 (LASIR), USTL, 59655 Villeneuve d’Ascq, France
Received 12 January 2011; Revised 12 May 2011; Accepted 14 June 2011
Zinc was bound on montmorillonite K10 by cation exchange to obtain a catalyst named clayzic.
In the Friedel–Crafts benzylation of 1,4-dimethoxybenzene, this catalyst was used for the synthesis
of substituted diphenylmethanes using 4-chlorobenzyl chloride and 4-bromobenzyl bromide. During
the reaction, sub-products from a second benzylation reaction process were observed. For a better
understanding of their formation, reactions were carried out at different times to obtain data on
the progress of benzylation and the relative ratio of each product was calculated using two different
analytical methods. It was shown that the selectivity and reactivity of both aryl halides were
reversed under these experimental conditions contrary to those obtained using the more conventional
catalyst, zinc dichloride. These results were explained by geometrical and electronic considerations.
It was found that the formation of transition states and Wheland intermediates from aryl bromide
and chloride in the presence of clayzic can be explained in terms of preferential absorption. Moreover,
the high percentage of 4-chlorobenzyl chloride conversion was attributed to its covalent radius, which
is smaller than that of Br. At the same time it was shown that the presence of a Br o¨ nsted acid,
due to the liberation of HCl during the benzylation, is responsible for the poisoning of the clayzic
catalyst. Moreover, poisoning effect of the bromine anion could not be excluded.
ꢀ
c 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: zinc, montmorillonite, diarylmethane, alkylation, orbital, DFT calculations
Introduction
or trifluoromethanesulfonic acid (Carey and Trem-
per, 1968; Gribble et al., 1977; Olah et al., 1986;
Waterlot et al., 2001b). Diarylmethanes have been
recently synthesized using other acids and hybrids,
e.g., AlCl3 and polymethylhydrosiloxane in various
solvents such as dichloromethane, carbon tetrachlo-
The synthesis of substituted diarylmethanes is of-
ten carried out via reductive alkylation of benzophe-
nones or benzhydrols using triethylsilane or sodium
tetrahydroborate in combination with trifluoroacetic
*Corresponding author, e-mail: c.waterlot@isa-lille.fr

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74
C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Clayzic catalyst
CH Cl ; 25 °C
X
X
X
2
2
+
+
X
X
X
X
OMe
IIa: X = Br
IIb: X = Cl
IIIa: X = Br
IIbI: X = Cl
IVa: X = Br
IVb: X = Cl
Va: X = Br
Vb: X = Cl
Fig. 1. Reactions of 1,4-dimethoxybenzene, I, with aryl halides, IIa and IIb, in the presence of clayzic.
ride and nitromethane (Waterlot et al., 2000a, 2001b)
The authors highlighted the formation of several by-
products and the mechanisms of their formation were
discussed (Waterlot et al., 2000b).
Since the late 1980’s, the use of clean catalysts such
as metal halides dispersed in inorganic substrates in
chemical industry has grown considering environmen-
tal problems. As a consequence, superacidic sulfated
zirconia (S-ZrO2) (Yadav et al., 1993; Yadav & Tho-
rat, 1996), zeolite (Koltunov et al., 2004; Bidart et al.,
dimethoxybenzene, I, was conducted using unsup-
ported ZnCl2 as the Lewis acid catalyst and ZnCl2 ex-
changed montmorillonite K10 clay following the pro-
cedure described in a previous study (Waterlot et al.,
2000c). For both these catalysts, reactions were per-
formed in triplicates, at different times, in order to
obtain data on the alkylation progress. In this contri-
bution, results of these experiments are reported and
the reactivity of aryl halides in the presence of clayzic
using the density functional theory (DFT) calculations
is explained.
2
2
006), silica (Miller et al., 1997, 1998; Orlovic et al.,
001), and clay (Cseri et al., 1995) catalysts have been
used in the Friedel–Crafts alkylation reactions in or-
der to substitute powerful Lewis acids such as ZnCl2
or AlCl3. Among these heterogeneous catalysts, zinc-
exchanged montmorillonite K10, named clayzic (Clark
et al., 1989; Cornélis et al., 1991a, 1991b), has been
reported as a highly active and selective reagent for
the benzylation of aromatic hydrocarbons compared
with unsupported ZnCl2.
Experimental
For each data point, Friedel–Crafts reactions were
carried out in triplicates. Analyses were performed
on a Varian 5000 HPLC chromatograph with a
multi-wavelength UV-VIS detector (Varian, Essonne,
France) under reversed-phase conditions (octadecyl-
t
bonded silica column, Supelco France, S -Germain-en-
From an industrial point of view, inexpensive and
environmentally friendly clayzic is the right choice
especially in the preparation of a variety of diaryl-
methanes which have found many applications in the
fields of biologically active molecules, polymers, food
additives and agrochemicals. Indeed, the achieved con-
version of benzyl halides was about 100 % in a short
time whereas both clay and ZnCl2 require several
hours to catalyze the benzylation reactions (Laszlo,
Laye, France) using acetonitrile–water (ϕr = 6 : 4,
Acr ¯o s Organics, Noisy-le-Grand, France) as the mo-
bile phase. NMR spectra were run on a Bruker AC300
spectrometer (Marne-la-Vallée, France) at 300 MHz
for 1H and 75 MHz for C, using CDCl3 (Acr o¯ s
Organics, Noisy-le-Grand, France) as a solvent and
tetramethylsilane (Acr ¯o s Organics, ) as an internal
standard. Chemical shifts (δ) are reported in parts
per million. Spectroscopic data and elemental analy-
ses were described in detail in previous studies (Wa-
terlot et al., 2000c, 2001b). The treatment of each
mixture was carried out following the procedure de-
scribed below. The relative ratio of compounds I, III,
IV, and V was determined using HPLC after the de-
termination of the retention time related to each com-
pound and establishment of the calibration curves.
These compounds were characterized by the wave-
length (λ/nm) at which absorption occurs (Table 1).
13
1
987; Laszlo & Mathy, 1987; Clark et al., 1989); more-
over, diarylmethanes were isolated with good yields
and without by-products (Vanden Eynde et al., 1995).
For our research activities and in order to provide
environmentally friendly chemical reactions, clean
synthesis of diarylmethanes, III, was essential. The
Friedel–Crafts reaction of 1,4-dimethoxybenzene, I,
with halides, II, was also used as an alternative pro-
cedure (Fig. 1). Compared with all sub-products ob-
tained during the reduction of benzophenones or ben-
zhydrols (Waterlot et al., 2000a, 2001b), it was ob-
served that only by-products IV and V were also
formed in low yields when using a heterogeneous cata-
lyst (Waterlot et al., 2000c). The reported chemistry of
1
In addition, H NMR spectra were performed for each
mixture and the relative ratio was determined on ba-
sis of the 1H NMR signals of methoxy groups (Ta-
ble 1).
1,4-Dimethoxybenzene (1 g, 7.24 mmol, Sigma–
Aldrich, Saint–Quentin–Fallavier, France) in dichloro-
methane (5 mL, Acr ¯o s Organics, Noisy-le-Grand,
France) was added to a stirred solution of 4-bromoben-
zyl bromide, IIa, or 4-chlorobenzyl chloride, IIb
(7.24 mmol, Sigma–Aldrich), and ZnCl2 (1.46 g,
the Friedel–Crafts reaction using ZnCl (Olah, 1963,
2
1
973) and our results were compared.
To explain apparent discrepancies between the
results obtained with the two catalysts and for
the purpose of this study, the alkylation of 1,4-

C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
875
Table 1. Chemical parameters used in HPLC (λ/nm) and 1H NMR (δ) for the determination of the relative ratio of compounds
I, III, IV, and V
Parameter
I
IIIa
IIIb
IVa
IVb
Va
Vb
λ/nm
δ
196, 228, 290
3.80
198, 224, 292
3.75, 3.78
198, 224, 292
3.74, 3.76
198, 218, 296
3.69
198, 218, 296
3.70
198, 224, 286
3.56, 3.66
198, 224, 286
3.57, 3.67
Table 2. Benzylation of 1,4-dimethoxybenzene using ZnCl2 as the catalyst
t
III
IV
V
I
Conversion of II
Entry
Aryl halides
h
%a
%
1
2
3
4
5
6
7
8
IIa
IIb
IIa
IIb
IIa
IIb
IIa
IIb
118
118
142
142
166
166
312
312
29
28
33
36
37
37
5
2
6
3
7
5
0
0
1
0
2
66
70
60
62
54
56
39
32
47
42
55
51
96
92
2
b
b
b
54
60
11
9b
10
25
24
b
b
7
a) Yields were determined by 1H NMR and HPLC; b) isolated yields.
Table 3. Benzylation of 1,4-dimethoxybenzene using clayzic as the catalyst
t
III
III
V
I
Conversion of II
Entry
Aryl halides
h
%a
%
c
3c
16
8c
c
1
2
3
4
5
6
7
8
9
IIa
IIb
IIa
IIb
IIa
IIb
IIa
IIb
IIa
19b
19
54
49
52
49
52
49
42
48
48
2
41
31
35
30
32
29
24
26
24
64
89
78
91
84
93
100
96
b
c
c
c
c
c
c
4
c
24
24
30
30
5
c
c
15
6
c
c
10
15
17
17
18
6
7
12
7c
8
b,d
48
48
96
c
c
b,d
100
a) Isolated yields; b) the same results as in Waterlot et al. (2000c); c) yields determined by 1H NMR and HPLC; d) quantitative
conversion of aryl halides.
1
.08 mmol, Sigma–Aldrich) in dichloromethane (5 mL).
Results and discussion
The mixture was stirred at room temperature for the
required time. Then, the reaction was quenched with
water (50 mL). After stirring, the organic layer was
extracted with dichloromethane (2 × 10 mL), dried
over CaCl2 and concentrated under vacuum. Benzy-
lation of 1,4-dimethoxybenzene with clayzic was con-
ducted following the procedure described in detail by
Waterlot et al. (2000c). This catalyst was prepared
following the general procedure described by Clark
et al. (1992) and Clark (1994) with slight modifica-
tions. ZnCl2 (10 g) and montmorillonite K10 (60 g,
Sigma–Aldrich) were added to dry methanol (100 mL,
Acr ¯o s Organics). The resulting slurry was stirred at
room temperature for 5 hours forming a clay which
was then filtered and washed with dichloromethane
In course of the process requiring the use of di-
arylmethanes, IIIa and IIIb, to produce a redox poly-
mer (Waterlot et al., 2001a, Waterlot & Couturier,
2010), differences between our results using unsup-
ported ZnCl2 and ZnCl2 exchanged montmorillonite
K10 clay were examined. A systematic study was car-
ried out with the two catalysts in order to understand
the preliminary results.
Benzylation of 1,4-dimethoxybenzene, I, with aryl
halides IIa and IIb was first conducted with ZnCl2. In
ꢀ
this reaction, 2,5-dimethoxy-l-(4 -halobenzyl)benzene,
III, was produced and dibenzylated derivatives from
a second benzylation reaction appeared (Fig. 1). The
p-isomer, IV, was formed selectively after 144 hours.
At a longer reaction time, m-isomer, V, was detected,
and at the end of the reaction, m- and p-isomers were
isolated in similar yields (Table 2).
(
30 mL, Acr ¯o s Organics). The resulting powder was
◦
thermally activated overnight at 120 C (Cornélis et
al., 1991b; Phukan et al., 2003).

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76
C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
7
5
3
1
5
A
B
4
3
m
m
p
p
2
1
1
00
150
200
250
300
350
0
20
40
60
80
100
Time/h
Time/h
Fig. 2. p-Dibenzylated derivatives/m-dibenzylated derivatives molar ratio (– – IVa/Va; — IVb/Vb) after various reaction times:
A) using ZnCl2 as catalyst; B) using clayzic.
1
00
100
75
50
25
0
A
B
7
5
2
5
0
5
0
0
50
100 150 200 250 300 350
0
25
50
75
100
Time/h
Time/h
Fig. 3. Conversion of aryl bromide, IIa ( ), and aryl chloride, IIb (
values (n = 3) and standard deviation are given.
◦
): A) using ZnCl2 as the catalyst; B) using clayzic. Average
Regarding the results in Tables 2 and 3, the selec-
tivity of clayzic used as the catalyst was different than
that of ZnCl2 at the beginning, during and at the end
of the reaction. For instance, it is undeniable that the
time required for total conversion of aryl halides us-
ing clayzic (Table 3, entries 7 and 9) is shorter than
that obtained using ZnCl2 (Table 2, entries 7 and 8).
Moreover, second benzylation occurred in the pres-
ence of clayzic earlier than with ZnCl2. These results
can be explained by the high local concentration of
zlo, 1987; Laszlo & Mathy, 1987). In contrast, differ-
ences in terms of activity occurred when clayzic was
used. Reactivity of aryl chloride was higher than that
of bromide after the reaction beginning. Twenty hours
after the beginning of the reaction, 90 % of the ini-
tial amount of aryl chloride were consumed whereas
63 % of the amount of aryl bromide were used for the
benzylation of 1,4-dimethoxybenzene. After this time,
conversion of aryl chloride, IIb, was almost stopped.
In contrast, total conversion of aryl bromide, IIa, was
achieved after 40 hours of the reaction.
2+
Zn within the pores of montmorillonite creating a
high activated region towards incoming alkyl halide
derivatives.
It is well-known that major structural changes
occur in montmorillonite K10 during its activation
at higher temperatures. In our experiments, thermal
treatment of the ZnCl2/montmorillonite K10 catalyst
As shown in Fig. 2, selectivity of the second benzy-
lation resulting in m- and p-isomers, IV and V, relies
on the nature of the catalyst and the benzyl halide, IIa
or IIb. As a consequence, the conversion rate of ben-
zyl halides depends on the Lewis catalyst (Fig. 3). In
our experiments, reaction rates of 4-bromobenzyl bro-
mide, IIa, and 4-chlorobenzyl chloride, IIb, are very
similar when using ZnCl2 (Fig. 3a). Therefore, there
is no significant difference between the activities of
the aryl halides studied up to 118 hours. After this re-
action time, the conversion rate of aryl bromide, IIa,
became significantly faster than that of aryl chloride,
IIb, as reported in previous studies (Olah, 1973; Las-
◦
was carried out at 120 C overnight. Under these con-
ditions it was shown that the basal spacing of our
catalyst is high (Choudhari & Mantri, 2002; Phukan
et al., 2003). In contrast, activity of the acid sites ac-
◦
tivated at 120 C was lower than of those activated
at higher temperatures. With regard to the activation
procedure of our heterogeneous catalyst, the exchange
of Zn2 ions for the exchangeable cations of montmo-
rillonite K10 can occur. During the activation of the
+
◦
catalyst at 120 C, loss of water from the interstitial
spaces of the clay was limited, reflecting a little de-

C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
877
X
OMe
H
OMe
OMe
Si
CH2
Clayzic
catalyst
O
X
X
Zn
O
X
X
X
X
OMe
Si
Wheland intermediate
VIa; X = Br
IIa; X = Br
IIb; X = Cl
IIIa; X = Br
IIIb; X = Cl
IIc
VIb; X = Cl
Fig. 4. Mechanism of 1,4-dimethoxybenzene (I) benzylation.
X
X
OCH3
δ+
OCH3
Si
CH2
δ+ CH2
X
Si
X
H
O
-
δ
O
-
1
δ
O
H
1
Zn
Zn
δ'+
δ''+
O
O
O
O
CH3
CH3
C2
Si
Si
C1
Fig. 5. Schematic forms of two possible transition states with hexacoordinated zinc.
crease of its Br o¨ nsted acidity. Furthermore, formation
of bonds between physically absorbed ZnCl2 and clay
can take place creating new active sites, which is fol-
lowed by the realease of the HCl gas. However, this
mechanism for the activation of benzyl chloride to
produce the carbocationic intermediate IIc in the pres-
ence of clayzic was proposed. Similar mechanisms were
proposed for other heterogeneous catalysis and other
metal salts (Cseri et al., 1995; Choudhary & Jana,
2001; Singh et al., 2004; Bachari & Cherifi, 2007;
Choudhary & Jha, 2008). Formation of a chlorine rad-
ical, much more reactive than the bromine one, can
also explain the high conversion rate of 4-chlorobenzyl
chloride after the beginning of the reaction up to 19
hours of its duration.
◦
phenomenon seems to be limited at 120 C (Choud-
hary & Mantri, 2002). High reactivity of aryl bromide
in the presence of clayzic can be also explained by the
highly polar clayzic pores into which the more polar
or polarizable substrate diffuses (Clark et al., 1994).
Obviously, this phenomenon does not explain the ob-
tained results.
Benzylation scheme of 1,4-dimethoxybenzene, I, is
shown in Fig. 4 and it follows the general mechanism
of the Friedel–Crafts alkylation.
Even if the formation of a hexacoordinated sphere
is less evident with the Zn2 heterogeneous catalyst
than with other metal-composites (Ahmed & Dutta,
2005), the high conversion of 4-chlorobenzyl chloride
up to the reaction time of 19 hours can be explained
by a possible chelation of this compound and 1,4-
dimethoxybenzene with the presence of two catalytic
acid sites, Br ¨o nsted and Lewis, of clayzic (Fig. 5). In
schematic form, the transition state C2 seems to be
more suitable due to the Br ¨o nsted activity of our cat-
alyst. As reported by Cornélis et al. (1993), forma-
tion of this transition state can occur in two steps:
i) attack by the aryl halides, bromine or chlorine,
on the zinc Lewis acid already coordinated to 1,4-
dimethoxybenzene due to the low temperature of the
reaction (Barlow et al., 1993); ii) Br o¨ nsted acid cataly-
sis of the leaving halide due to the superacidic clay sur-
face, with the acidity of the remaining water molecules
enhanced through their coordination to zinc.
+
If the reaction proceeds through a carbocationic in-
termediate, theory predicts that relative reactivity of
alkyl halides is dictated by the catalytic activity, the
Lewis acid (Clark & Macquarrie, 1996), the structure
of the halide reagent and the energy of the halogen—
carbon bonds (Price, 1947; Olah, 1973). In the ex-
periments conducted with clayzic, aryl halides can be
absorbed on the catalyst to produce carbenium ions,
involving the steric properties of aryl halides IIa and
IIb. Therefore, at the beginning of the reaction, high
conversion rate of 4-chlorobenzyl chloride, IIb, can be
explained by the low covalent radius of Cl compared
to that of Br of 4-bromobenzyl bromide, IIa.
Based on the results related to the paradoxical se-
lectivity of the Friedel–Crafts benzylation using com-
mercial clays (Choudhary & Jana, 2002), a redox

8
78
C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
X
X
δ+
δ+
OCH3
OCH3
CH2
CH2
Si
O
Si
X
X
X
H
H
-
O
-
δ
O
H
δ
O
H
1
1
Zn
X
Zn
δ'+
δ'+
O
O
O
O
CH3
CH3
C3
C4
Si
Si
Fig. 6. Schematic forms of possible transition states with hexacoordinated zinc.
III -a Br
III b- C
ꢀ
Fig. 7. Representation of the HOMO (in grey) and the LUMO (gridded) orbitals of 2,5-dimethoxy-1-(4 -bromobenzyl)benzene,
ꢀ
IIIa, and 2,5-dimethoxy-1-(4 -chloro-benzyl)benzene, IIIb.
In the second alkylation, possible simultaneous
chelation of diarylmethanes IIIb or IIIa, and aryl
halides IIb or IIa with already coordinated zinc can
be considered (Fig. 6). Regarding the properties of
the chlorine atom (i.e., electronegativity and covalent
radius), the formation of transition states C3 and C4
is easier when X = Cl than when X = Br. This might
explain the consumption of 4-chlorobenzyl chloride up
to the reaction time of 19 hours and the higher amount
of the (p- and m-) isomers IVb and Vb than that of
the (p- and m-) isomers IVa and Va.
Table 4. Energies of HOMO and LUMO orbitals obtained by
DFT calculations
Compound
I
IIa
IIb
IIIa
IIIb
kJ mol−1
Orbital energy
HOMO
LUMO
–825
37
–925
–41
–927
–25
–836
11
–833
14
To understand the formation of by-products IV
and V, geometries and electronic parameters of com-
pounds I, II, and III were optimized using DFT cal-
culations (the Gaussian base used was 6.31 G** with
s, p orbital for hydrogen atoms and s, p, and d or-
bitals for the others atoms 6.31 G**), by means of the
Jaguar programs (Schr o¨ dinger, Inc. 1991–2000; Fries-
ner, 1991) and SPARTAN optimized using the semi-
empirical AM1 method (Dewar et al., 1985; Wavefunc-
tion Inc., 1999).
tude (Table 4). As a consequence, LUMO orbital of
the aryl halides approached the HOMO orbital of 1,4-
dimethoxybenzene, I. Competition between the first
and the second alkylation can be explained by the
nearly equal HOMO energies of diarylmethanes IIIa
and IIIb and 1,4-dimethoxybenzene, I.
Graphical representations of the HOMO and
LUMO orbitals of IIIa and IIIb are shown in Fig. 7.
HOMO positions of these compounds indicate that the
second alkylation takes place on the 1,4-dimethoxy-
benzene group.
Energy of the lowest unoccupied molecular orbital
(
LUMO) of each aryl halide, IIa and IIb, is notice-
The distribution of isomers varied significantly
with the reaction time reaching the thermodynamic
equilibrium after 48 hours (Table 3, Fig. 2b). On the
other hand, selectivity of the second alkylation of IIIb
ably lower than that of compounds I and III for which
the highest occupied molecular orbital (HOMO) ener-
gies are the highest and in the same order of magni-

C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
879
d1
d
1
d
2
d3
d
2
α1
d
d
4
d
3
d
4
α1
α
1
d1
α 2
d2
α
2
III
3
I
4
V
Fig. 8. Structure of compounds III–V determined by calculations of the C—C lengths and angles.
OMe
OMe
H
OMe
X
X
X
+
X
X
H
OMe
OMe
OMe
VIIa: X = Br
VIIb: X = Cl
VIIIa: X = Br
VIIIb: X = Cl
IIIa; X = Br
IIIb; X = Cl
OMe
OMe
X
X
X
+
X
OMe
OMe
IVa: X = Br
IVb: X = Cl
Va: X = Br
Vb: X = Cl
Fig. 9. Mechanism of diarylmethanes (III) benzylation with clayzic as the catalyst.
seemed to be higher than that of IIIa in particular
in the first 19 hours of the reaction time. Interest-
ingly, stabilization of the aryl chloride, IIb, conver-
sion occurred at the same time (Table 3, Fig. 2b).
These results suggest that the release of HCl during
the Friedel–Crafts alkylation contributes to the signifi-
cant decrease of the clayzic activity. As a consequence,
HCl can be considered as a poison to the clayzic cat-
alyst.
for geometry optimized structures energy of each com-
pound and their Wheland intermediates were then cal-
culated taking into account the partial positive charge
on the aromatic ring.
A reaction coordinate diagram for each elec-
trophilic aromatic substitution was thus obtained
(Figs. 10 and 11). The first diagram indicates that
the stability of products IIIa and IIIb is lower than
that of 1,4-dimethoxybenzene, I, (Fig. 10) implying
◦
that changes of the Gibbs free energy (∆G ) related
Theoretical investigations
to the monobenzylation is endergonic. According to
the Hammond postulate, Wheland complexes derived
from the methoxylated intermediate are formed via
late transition states. For this monobenzylation re-
action, the energy of the Wheland intermediate was
106.7 kJ mol for IIIa and 84.9 kJ mol for IIIb,
showing high stability of intermediate VIIb in com-
parison with VIIa. As a consequence, the formation of
intermediate compound IIIb was more favorable than
that of IIIc and so, obviously, monobenzylation of 1,4-
dimethoxybenzene conducted with aryl bromide, IIa,
was slower than that with aryl chloride, IIIb (Table 3,
entries 1, 2 and 3, 4).
on the formation of di-substituded
1
,4-dimethoxybenzene in presence
of clayzic
−
1
−1
Friedel–Crafts reactions of diarylmethanes IIIa and
IIIb with aryl halides IIa and IIb, respectively, were in-
vestigated from a theoretical point of view. The mech-
anism described in Fig. 6 was considered and the C—
C lengths (d) and angles (α) (Fig. 8) of compounds
III–V, and of the Wheland intermediates (Wheland,
1
9
942; Tyagi & Yadav, 1990) VIa and VIb (Figs. 8 and
) were determined.
As reported in Table 5, the comparison of lengths
The energy coordinate diagram for the benzyla-
tion of IIIa and IIIb with aryl halides IIa and IIb
was then considered. As reported in Fig. 11, the end-
products have higher energies than the starting prod-
ucts, reflecting that these reactions are endergonic too.
and angles of bromide and chloride compounds with
those of their Wheland intermediates did not show
large differences. Hence, these considerations did not
explain the previous results. The total energy values

8
80
C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
Table 5. Structural characteristics, energy of compounds III–V, and of their corresponding Wheland transition
d1
d2
d3
D4
α1
α2
E
Compound
˚
◦
kJ mol−1
IIIa
IIIb
VIa
VIb
IVa
IVb
VIIa
VIIb
Va
1.491
1.492
1.486
1.486
1.492
1.491
1.493
1.493
1.515
1.513
1.488
1.488
1.492
1.491
1.542
1.542
1.491
1.491
1.489
1.489
1.516
1.513
1.494
1.495
–
–
–
–
–
–
–
–
112.65
112.56
111.21
111.19
112.56
112.67
110.94
110.92
116.20
113.56
114.12
114.44
–
–
–
–
51.94
51.32
106.71
84.86
80.55
79.29
133.13
108.43
85.12
83.40
137.93
114.88
1.491
1.491
1.542
1.542
1.516
1.513
1.542
1.543
1.492
1.491
1.486
1.486
1.515
1.513
1.486
1.485
112.56
112.67
111.19
111.22
116.20
113.56
111.20
114.44
Vb
VIIIa
VIIIb
1
1
60
20
Transition-state
Transition-state
VIa
VIb
8
0
IIIa
IIIb
∆
E
40
0
G°
I
Reaction coordinate
Fig. 10. Energy coordinate diagram for the monobenzylation of 1,4-dimethoxybenzene, I.
Transition-state
Transition-state
1
1
60
20
VIIIa
VIIa
Va
Vb
IIIa
VIIIb
E
VIIb
8
4
0
0
IVb
IVa
IIIb
Reaction coordinate
Fig. 11. Energy coordinate diagram for the benzylation of diarylmethanes IIIa and IIIb with aryl halides IIa and IIb, respectively.
Furthermore, p-isomers IV are more stable than m-
isomers V due to some steric effects which is in ac-
cordance with the theory. Interestingly, there are no
important differences between the energy of the Whe-
land intermediates associated to the chloride (∆ = 6.4
kJ mol ) and bromide isomers (∆ = 4.8 kJ mol ).
In contrast, significant differences in the Wheland in-
termediates energy between arenium chloride ions and
arenium bromide ions were observed. Finally, it is in-
teresting to note that the transition-state energies nec-
essary to obtain Wheland intermediates VIIb and VI-
IIb are lower than those for VIIa and VIIIa. These
results explain why the combined yield of the diben-
zylated products IVb and Vb, obtained by a reaction
of IIIb with aryl chloride IIb, is higher than that of
brominated compounds IVa and Va (Table 3, entries
1 and 2, 3 and 4, 5 and 6).
From our results on the first and second benzyla-
tion of 1,4-dimethoxybenzene, I, using clayzic as the
catalyst it seems that 4-chlorobenzyl chloride, IIb, re-
acts faster than 4-bromobenzyl bromide, IIa. This is in
accordance with the conversion rate of halides at the
beginning of the reaction and up to 19 hours (Fig. 3b).
After this time, the release of HCl, related to the mech-
anism of the Friedel–Crafts benzylation, was more sig-
nificant than that of HBr at the same reaction time
(Fig. 3b). As the acid treatment of montmorillonite
K10 increases its reactivity (Olah, 1963; Tyagi & Ya-
−
1
−1

C. Waterlot et al./Chemical Papers 65 (6) 873–882 (2011)
881
dav, 1990; Pai et al., 1997; Rhodes et al., 1991), re-
duction of the catalyst activity can be attributed to
the counter ion of the acid (Chouday at al., 1991; Van-
den Eynde at al., 1995). Poisoning mechanism of the
clayzic catalyst was also shown to be due to HCl for-
mation during the Friedel–Crafts benzylation. How-
ever, from this comparative study of the benzylation
using chloride or bromide derivatives, poisoning effect
of the bromine anion cannot be excluded.
Choudhary, V. R., & Jha, R. (2008). GaAlClx-grafted Mont.K10
clay: Highly active and stable solid catalyst for the Friedel-
Crafts type benzylation and acylation reactions. Catalysis
Communications, 9, 1101–1105. DOI: 10.1016/j.catcom.2007.
10.014.
Choudhary, V. R., & Mantri, K. (2002). Thermal activation
of a clayzic catalyst useful for Friedel–Crafts reactions: HCl
evolved with creation of active sites in different thermal treat-
ments to ZnCl2/Mont-K10. Catalysis Letters, 81, 163–168.
DOI: 10.1023/A:1016564603798.
Clark, J. H. (1994). Catalysis of organic reactions by supported
inorganic reagents (pp. 126). New York, NY, USA: Wiley.
Clark, J. H., Cullen, S. R., Barlow, S. J., & Bastok, T.
W. (1994). Environmentally friendly chemistry using sup-
ported reagent catalysts: structure–property relationships for
clayzic. Journal of the Chemical Society, Perkin Transac-
tions, 2, 1117–1130. DOI: 10.1039/P29940001117.
Conclusions
This study allows a better understanding of the
role of clayzic as the catalyst in comparison with
ZnCl2 in the course of the 1,4-dimethoxybenzene ben-
zylation with aryl halides. Using density functional
theory calculations it was shown that the higher per-
centage of 4-chlorobenzyl chloride conversion, com-
pared to that of 4-bromobenzyl bromine, was due to
the high stability of the transition states of chloride,
and smaller covalent radius of Cl compared to that of
Br. Thus, the mechanism related to the poisoning of
clayzic due to the release of HCl led to total conversion
of aryl bromide prior to aryl chloride.
Clark, J. H., Kybett, A. P., & Macquarrie, D. J. (1992). Sup-
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1
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&
metal salts as Friedel–Crafts alkylation catalysts. Journal of
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