Indian Journal of Chemistry
Vol. 45B, April 2006, p p. 972-975
Natural kaolinitic clay: A remarkable catalyst for highly regioselective
chlorination of arenes with Cl2 or SO2Cl2
B Jayachandran, Prodeep Phukan,# Thomas Daniel & A Sudalai*
National Chemical Laboratory, Pune 411 008, India
E-mail: [email protected] .in
Received 30 September 2004; accepted (revised) 7 November 2005
Natural kaolinitic clay containing transitio n metals s
as Fe and Ti in its lattice has been found to exhibit unus ual
regioselectivity in the liquid-phase chlorination of arenes with either Cl2 or SO 2Cl2 as the chlorinating agent para -Chlorinated
products are predominant for most of the substrates with an exceptional case of ortho-selectivity for the chlorination of aniline.
Keywords : Chlorination, chlorine, s ulfuryl chloride, aniline, ortho-chloro aniline
IPC: Int.Cl.7 C 07 C
Introduction of chlorine into aromatic rings by
electrophilic substitution is an important synthetic
transformation because chlorinated hydrocarbons are
recognized as versatile starting materials and additives
in th e p rod uc tio n o f h igh qu ality in sec tic ide s,
fungicides, herbicides, dyes, pharmaceuticals etc. 1
Moreover, they can serve as precursors for numerous
functionalities, such as phenols, aromatic ethers and
thioethers, amines, aryl hydrazines, benzonitriles,
be nzalde hyd es , fluor o ar oma tics a nd ar om atic
hydrocarbons2. In general, chlorination of aromatic ring
is achieved either by using chlorine or SO 2Cl 2 as
chlorine source in presence of Lewis acid catalysts
such as Al3+, Fe3+, Sn 4+ or Zn 2+ chlorides, acetic acid
etc.3 But use of homogenous catalyst has the drawback
such as poor regioselectivity, difficulty in separation
of the catalyst from the reaction mixture, generation
of toxic waste etc. Secondly, use of mineral acids and
metallic halides create work-up and effluent problem
in the industrial scale production of these compounds.
Use of solid acid is an easy, safe and economical
process, which does not create any environmental
problem. Several efforts have been made for the
replacement of these homogeneous catalysts with solid
#
acids such as zeolites for the chlorination of arene4.
Although para -selectivity was achieved using zeolite4
because of it’s shape selective nature, there is not such
report for ortho -selective chlorination. Sheldon
reported a highly regioselective ortho-chlorination of
phenol with SO 2Cl2-amine systems in homogeneous
phase 5. In recent years, acid catalysis of organic
transformations by clay aluminosilicates6 is an area of
considerable potential and interest due to the ease of
handling and w ork-up, a bsence o f toxicity and
corrosion and low cost of clays. Due to their Br ¥onsted
and Lewis acidities, clays function as efficient catalysts
for a variety of impor tant transformations 7 . An
interesting feature of clay catalyst is that it gives
unexpected ortho-selective products in a few cases,
which is very specific for a particular combination of
substrates8. This peculiar property is sometimes very
much useful for the synthesis of some important ortho substituted compounds. Herein is reported the use of
natural kaolinitic clay for the chlorination of arenes
with substantial degree of regioselectivity with Cl2 or
SO2Cl2 as the chlorinating agent (Scheme I ).
T he k ao lin itic clay w a s pr o cu re d f ro m th e
Padappakara mine of Quilon District, Kerala, India and
Present address: Department o f chemistry, G auhati University, Guwahati 781 014, Assam
JAYACHANDRAN et al.: CATALYST FOR REGIOSELECTIVE CHLORINATION
973
R
R
R
Kaolinitic clay,
Cl2 or SO2 Cl2
Cl
+
CCl4,80oC
R = H, CH3, Cl, OMe, OH, NH2
Cl
Scheme I
it was subsequently purified 9 by separating the coarser
mineral impurities from clay particles. This is achieved
by suspending the crude lumps in water (500 g of the
clay lump in 5 L water), stirring vigorously and
allowing the coarser mineral to separate. Particles of
size below 45 µm are collected and washed free of
solubles. Clay particles thus separated from coarser
mineral impurities are dried and calcined at 550°C for
6 h. The calcined clay sample (1 part by wt.) is boiled
with 2M HCl (4 parts by wt) for 45 min. The leached
samples are then washed free of chloride ions and dried
at 110 ° C for 12 h. The clay has been thoroughly
characterized by FT IR, XRD, UV-Vis, EPR, SEM and
chemical analysis by atomic absorption spectroscopy.
The XRD pattern of the clay exhibits 3 major peaks
located at 2θ values of 26.8°. The FT IR spectra of
the clay showed the presence of two different
bands: one at 3710 cm -1 assigned to SiOH and the
other band centered at 3590-3490 cm -1 assigned to
hydroxyl groups in the clay lattice. The presence of
Lewis and Br ¥onsted-acid sites in the clay has been
confirmed by its FT IR study of adsorbed pyridine. The
IR bands at 1630, 1495 and 1445 cm -1 is indicative of
its Lewis acid sites while the IR bands at 1630, 1549
and 1495 cm -1 observed for pyridine adsorbed clay
samples indicate the Br ¥onsted acid sites. The SEM of
the clay shows average particle size of 2-5 µm in the
form of clusters. The UV-Vis spectrum of the clay
shows absorption maximum, λ max at 320-240 mm
indicating the presence of Ti (anatase form) as mostly
its oxide. A broad EPR spectrum of the clay recorded
at 293K (the g value of 2.13) is indicative of Fe3+ state
in the clay. Finally, the chemical component of the clay
was determined by wet chemical analysis10 (in %):
SiO2 = 67.45, Al2O3 = 22.20, TiO2 = 3.45, Fe2O3 = 6.1
and K = 0.8.
In a typical reaction procedure, a mixture of aniline
and clay catalyst (10 weight % based on aniline) in
CCl4 was heated to reflux. To this mixture, chlorine
gas was bubbled slowly for 2 h. The catalyst was
filtered off, the products analyzed by GLC capillary
column ( ortho -product 79.5% and para -product
20.5%) and purified by flash chromatography to
afford 2-chloroaniline (77%) and 4-chloroaniline
(20 %). The products were characterized by comparing
the IR and 1H NMR data with those reported in the
literature11 .
Table I lists the results for the various aromatic
s ub str ates , w hich un de rw e nt r e gios ele ctiv e
chlorinations with chlorine as well as SO 2Cl2 catalyzed
by the cla y. As c an b e s een from Ta b le I , the
conversions are quantitative in all substrates except
benzene with strong electrophile Cl2 whereas it is only
moderate in the case of weak electrophile, SO 2Cl2. A
n ov el f e atur e o f th is c a ta lyst is the un us ua l
regioselectivity exhibited in case of aniline ( ortho79.5 %; para- 20.5% with Cl2 and ortho-90%; para10% with SO 2Cl 2). For all other substrates, para selectivity is the dominant feature. It is also remarkable
that for all the cases studied, poly-chlorination did not
occur with the clay catalyst. In the case of toluene, it
is to be noted that no side-chain chlorination took
place. The observed ortho preference with aniline may
be due to H-bonding of anilinium cation with the
hydration sphere of these cations. The catalyst was
recovered and reused 5 times in case of toluene with
virtually no loss of activity and selectivity.
In conclusion, an efficient, simple, and economic
process for highly regioselective chlorination of arenes
has been demonstrated. Chlorine was found to be more
effective as compared to SO 2Cl 2 for this process.
Altho ugh par a -selectivity was the p redominant
974
INDIAN J. CHEM., SEC B, APRIL 2006
Table I—Kao linitic clay-c atalyzed chlo rinatio n of arenes
Compd
Substrates
With SO 2Cl2a
Conv.b
p /o
With Cl2
1
Conv.
b
p /o ratio
b
Iso lated Yield (%)
orth o
(%)
67
2
d
72
23
24
1.20
2.76
71
25
25
3.00
100
9.00
85
9
100
9.00
100
2.77
70
23
30
1.5
0.26
20
77
55
0.11
(%)
(GC)
a
70
c
b
100e
3.00
100
para
ratio b
Cl
c
OMe
d
OH
e
NH2
f
100
a) Reaction conditions : Sub strate (5 mmol); clay: 10% ass; SO 2Cl2 (5 mmo l); S olvent C Cl4; 360K, 2hr; b) Determined by G LC;
HP -5880 silico n gum c apillary co lumn; c) 41% chloro benzene + 56% 1,4-d ichloro benzene +3%, 1,2-dichlo robenzene; d) o nly 2%
chlo robenzene is formed; e) in the ab sence o f catalyst, the conversions were <2%.
outcome for most of the substrates, exceptional othoselectivity was achieved in case of chlorination of
aniline.
purified by flash chromatography using pet. ether –
EtOAc (15-25%) to afford 2-chloroaniline (1.9 g,77 %)
and 4-chloroaniline (0.5 g, 20%).
Experimental Section
P rocedure for chlorination with SO 2Cl2
A mixture of aniline (0.465 g, 5 mmoles), clay
catalyst (0.046 g, 10 weight %) and SO 2Cl2 (0.68 g, 5
P rocedure for chlorination with Cl2
A mixture of aniline (1.86 g, 20 mmoles) and clay
catalyst (186 mg, 10 weight %) was taken in CCl4 (30
mL) and heated to reflux. Chlorine gas was bubbled
slowly through this mixture for 2 h. After the reaction
was complete (TLC), the catalyst was filtered off and
the products analyzed by GLC. After evaporating the
solvent under reduced pressure, the crude product was
mmoles) were taken in CCl4 (10 mL) and refluxed for
2 h. The catalyst was filtered off and the products were
analyzed by GLC.
Acknowledgement
The authors (BJ and PP) thank CSIR (New Delhi)
for the award of research fellowship.
JAYACHANDRAN et al.: CATALYST FOR REGIOSELECTIVE CHLORINATION
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