Divalent transition metal ion-exchanged faujasites as mild, efficient, heterogeneous Friedel-Crafts benzylation catalysts

Article abstract of DOI:10.1081/SCC-120015565

Ni²⁺-, Cu²⁺-, and Zn²⁺-exchanged faujasite (MY) zeolites efficiently catalyze the Friedel-Crafts benzylation of arenes in a clean and simpler method.

Full text of DOI:10.1081/SCC-120015565

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Divalent Transition Metal Ion-Exchanged Faujasites
as Mild, Efficient, Heterogeneous Friedel–Crafts
Benzylation Catalysts
M. Kumarraja a & K. Pitchumani a b
a School of Chemistry, Madurai Kamaraj University, Madurai, India
b School of Chemistry, Madurai Kamaraj University, Madurai, 625 021, India
Version of record first published: 29 Jun 2011
To cite this article: M. Kumarraja & K. Pitchumani (2003): Divalent Transition Metal Ion-Exchanged Faujasites as Mild,
Efficient, Heterogeneous Friedel–Crafts Benzylation Catalysts, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:1, 105-111
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 1, pp. 105–111, 2003
Divalent Transition Metal Ion-Exchanged
Faujasites as Mild, Efficient, Heterogeneous
Friedel–Crafts Benzylation Catalysts
*
M. Kumarraja and K. Pitchumani
School of Chemistry, Madurai Kamaraj University,
Madurai, India
ABSTRACT
Ni2þ-, Cu2þ-, and Zn2þ-exchanged faujasite (MY) zeolites efficiently
catalyze the Friedel–Craftsbenzylation of arenesin a clean and
simpler method.
Key Words: Benzylation; Arenes; Substituted benzylchlorides;
Lewisacidity of zeolites.
*Correspondence: K. Pitchumani, School of Chemistry, Madurai Kamaraj
University, Madurai 625 021, India. Fax: 91-452-859139; E-mail: pit12399@
yahoo.com.
105
DOI: 10.1081/SCC-120015565
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106
Kumarraja and Pitchumani
Zeolitesare inorganic oslidswhich find extenisve applicationsin
organic transformations[1] and photochemical reactions.[2] Intrazeolite
radical reactions,[3] electron-, energy-, and proton-transfer reactions[4]
are also extensively reported. In the area of production of fine chemicals,
the merits of zeolites, in addition to the usual requisites of mildness,
selectivity, and cleanliness, over soluble acidic catalysts are significant
and have appeared in several papers and reviews.[5–7] Reduction of salt
and waste production, replacement of hazardous acids, prevention
of corrosion, availability of variety of structures, and pore dimensions
leading to shape selectivity, tunable acidity, and easy regeneration are
some of the advantages, which accrue upon their usage as heterogeneous
solid acid-catalysts.
Aluminosilicates-mediated Friedel–Crafts alkylations have received
particular attention as the products serve as useful starting materials for
synthesis of pharmaceuticals and polymers. Montmorillonite-
supported transition metal salts are reported[8,9] asefficient oslid
Friedel–Crafts alkylation catalysts, which reduce the problems associated
with the standard reactions using anhydrous AlCl3. Izumi et al.[10] have
shown that, smectites clays with simple exchange of metal ions, particu-
larly Zn2þ-exchanged nontronite and montmorillonite, exert much higher
catalytic activity for the alkylation than ZnCl2/K10-montmorillonite.
Preparation of diphenylmethane from benzyl chloride and benzene
using a simple mixture of ZnCl2 and K10/KSF-montmorillonite clays
(upon activation by ultrasonic waves) is reported.[11]
These observations, coupled with our interest in exploring alumino-
silicates[12,13] as versatile catalysts, have prompted us to study
the Friedel–Craftsbenzylation of areneswith divalent cation-exchanged
zeolites as solid acid catalysts. It is relevant to note here that the H-form
zeolites[5–7] and CaY[14] (with an ability to generate Bronsted acidity) are
studied in detail in many systems. Friedel–Craft’s acylation of toluene
[15]
and p-xylene with carboxylic acid isfound
to be catalyzed by CeY
zeolite and the para-isomer is the predominant product. In a recent
report,[16] benzyl chloride when included in NaY zeolite, isfound
to undergo dimerization in a meta-selective Friedel–Crafts pathway
(in contrast to its reaction in AlCl3, wherein extensive polymerization
isobesrved). However, modulation of zeolite acidity by exchange with
transition metal ions, with a subsequent increase in Lewis acidity
and much milder reaction conditionshasnot been explored so far. The
present work is directed towards this goal in mind.
In a typical procedure, to a mixture of the arene (0.01 mol) and the
substituted benzyl chloride (0.003 mol) taken in a round bottomed flask

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Ni2þ-, Cu2þ-, and Zn2þ-Exchanged Faujasite Zeolites
107
equipped with magnetic bead, 500 mg of the activated (preheated to
450ꢀC for 6 h in a muffle furnace) zeolite isadded and tsirred for 4 h
at room temperature. After the addition of 15 mL of dichloromethane,
the mixture is stirred again overnight, the solvent is removed and
evaporated. The reaction mixture isanalyzed in a Shimadzu 17A GC
(SE-30 10% capillary column, FID detector), unless reported otherwise.
In all the cases, retention times of unconsumed benzyl chloride is
taken asthe internal reference. 1H-NMR spectra of diphenylmethane,
isolated in the case of benzene, shows peaks at 6.78 ꢀ (s, 10H) and
3.6 ꢀ (s, 2H). With substituted benzenes and benzyl chlorides, isomeric
productsare ioslated (for Entries1, 2, and 4 in Table 2) by column
chromatography (silica gel; 5% ethyl acetate–95% petroleum ether
1
mixture) and are identified by their H-NMR spectra (2H singlet for
the methylene group at 3.5–4.0 ꢀ and para-isomers by their characteristic
AB pattern).
The results indicate clearly that benzylation of arenes (Sch. 1) is
catalyzed very efficiently (Tables1 and 2) by divalent cation-exchanged
zeolites(NiY, CuY, and ZnY) and there isno reaction with non-acidic
zeolites, namely NaY and KY. With CaY the yield has improved signifi-
cantly, but not asextensive asZnY and NiY. The reaction isfaster with
areneshaving electron-releaisng groupsand becomeslsower when the
arene and benzyl chloride[8] are substituted with electron-withdrawing
groups (Table 2). Compared to previous results, zeolites provide much
higher isolated yield and lower polyalkylation, which is attributed to their
well defined cages and consequent structural characterization.
The reaction isinefficient with benzyl alcohol asevident from the
following: treatment with PhCH2OH and NiY hasresulted in only 21%
conversion (with 4% of PhCHO, 10% of Ph2CH2 and others7%) and
with HY only 3% conversion is observed (1% of Ph2CH2 and others
2%). Thisalos rulesout the generation of a free benzylic carbonium
ion which isalso confirmed by carrying out the reaction with NiY/CaY
Scheme 1.

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108
Kumarraja and Pitchumani
Table 1. Formation of (C6H5)2CH2 from C6H6 and C6H5CH2Cl catalyzed by
a
variouscation-exchanged zeolitesat room temperature.
Yieldb (%) of
Conversion of
Entry
Zeolite
Time (h)
PhCH2Cl (%)
Ph2CH2
Others
1
2
3
4
5
6
7
8
9
NaY
KY
4
4
4
4
4
4
1
2
4
4
3
4
3
58
80 (75)c
99 (85)c
96 (84)c
6
—
—
10
—
—
—
—
—
15
CaY
CuY
NiY
NiYd
ZnY
ZnY
ZnY
68
80
99
96
6
22
100
22
85 (80)c
a0.01 mol of benzene and 0.003 mol of benzyl chloride in 500 mg of activated
zeolite.
bGC yield: Error limit ꢀ 5%.
cNumbers in parentheses refer to isolated yield (based on the amount of benzyl
chloride taken).
dNiY reused after three times of benzylation.
Table 2. Benzylation of substituted benzene (C6H5R0) with RC6H4CH2Cl in the
presence of NiY zeolite at room temperature.a
Yieldb (%) of
Conversion of
Time (h) RC6H4CH2Cl (%) Ortho Para Others
Entry
R
R0
1
2
3
4
5
6
7
H
H
H
H
H
CH3
–OH
–Cl
–OCH3
–NO2
H
4
4
4
4
12
6
100
100
12
100
44
47
3
43
—
—
—
44
53
6
57
—
—
—
12
0
3
—
9
—
—
9
2-NO2
4-NO2
No reaction
No reaction
H
6
a0.01 mol of substituted benzene and 0.003 mol of substituted benzyl chloride in
500 mg of activated zeolite.
bGC yield: Error limit ꢀ 5%.

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Ni2þ-, Cu2þ-, and Zn2þ-Exchanged Faujasite Zeolites
109
zeolite in the presence of styrene and benzene in hexane slurry. Only
styrene derived products are formed, and the coupled product is
absent. Similarly there is no reaction when benzylation is carried out
with unactivated NiY/ZnY zeolite, in which the cationsof interets are
solvated and hence are less readily available. It is likely that the entry into
the cage ishindered by water molecules, and the reaction thustakesplace
on the surface.
The observed results indicate clearly that the Lewis acidity of the
transition metal ions in zeolite is mainly responsible for the efficient
benzylation. Ni2þ, Zn2þ, and Cu2þ (with ionic radii of 0.72, 0.74, and
0.69 A, respectively) are all borderline acids according to the HSAB
theory of acidsand baessand by their ready binding to the covalently
linked chloride ion (a borderline base), polarize the carbon–chloride
bond in benzyl chloride more efficiently. Thisisfollowed by the facile
attack of the incipient carbonium ion on the aryl ring. It also explains
why the reaction isslow with metal ionsasNa
þ, Kþ (hard acids), and
with Ca2þ (though divalent, hasa higher ionic radiusof 0.98 A , making it
a weaker Lewisacid, compared to NiY, CuY, and ZnY). Izumi et al. have
also proposed[10] that only Lewis acidity is responsible for the
efficient benzylation by Zn2þ-exchanged smectites. It is relevant to
note here that in a recent report,[17] zinc-exchanged zeolite BEA is
found to be an excellent catalyst for the intramolecular hydroamination
of 6-aminohex-1-yne.
Thus the observed results amply demonstrate the efficiency of
divalent transition metal cation-exchanged zeolites (NiY, ZnY, and
CuY) as versatile Friedel–Crafts alkylation catalysts. Lewis acidity
of the metal cations is primarily responsible for this industrially relevant
reaction. In addition to ensuring simpler work-up, cleaner, and efficient
conversions, the other advantages of employing these solid heterogeneous
acid catalysts, are their environmentally benign nature, well defined
structural characterization, reduced polyalkylation, much higher isolated
yield, presence of basic sites in the framework to trap HCl liberated in the
reaction, and reusability (after three times, efficiency of the NiY is
still above 95%).
ACKNOWLEDGMENT
Financial support from Department of Science and Technology
(DST), New Delhi isgratefully acknowledged.

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110
Kumarraja and Pitchumani
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Ni2þ-, Cu2þ-, and Zn2þ-Exchanged Faujasite Zeolites
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Received in Japan August 23, 2001

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