Direct green iodination of phenol over solid acids

Article abstract of DOI:10.1007/s10562-010-0283-6

Examination of several solid acid catalysts of different nature (acid resins, zeolites, mixed oxides, Nb-oxide, and Nb-phosphate) was performed for the direct iodination reaction of phenol by using molecular iodine. The experiments were carried out in mil

Full text of DOI:10.1007/s10562-010-0283-6

Catal Lett (2010) 137:55–62
DOI 10.1007/s10562-010-0283-6
Direct Green Iodination of Phenol over Solid Acids
Paolo Carniti
•
Stefano Colonna Antonella Gervasini
•
Received: 25 November 2009 / Accepted: 21 January 2010 / Published online: 16 April 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Examination of several solid acid catalysts of
different nature (acid resins, zeolites, mixed oxides, Nb-
oxide, and Nb-phosphate) was performed for the direct
iodination reaction of phenol by using molecular iodine.
The experiments were carried out in mild and green con-
ditions (50 °C at ambient pressure) in methanol in the
presence of H O as oxidant agent. Iodine was introduced
iodoarenes compounds are biologically active molecules
used in medicine as drugs and diagnostic aids as radioac-
tively labeled markers or contrastors, thus justifying the
high interest that they attract. Thus, intensive research over
the years resulted in numerous different experimental
procedures for the effective synthesis of a variety of
iodoarenes.
2
2
in reduced amount, stoichiometric for the formation of di-
iodinated phenol, to obtain information on the regio-
selectivity of the iodination reaction. The catalysts proved
to be all efficient for the introduction of iodine into the
aromatic substrate, yielding mono-, di-, and tri-iodo deri-
vates. Different selectivity distributions to the iodo-com-
pound formed were observed over the different catalysts.
Catalysts could be grouped into distinct families on the
basis of their ortho/para orientation.
Electrophilic reaction process for the direct introduction
of iodine into organic molecules necessitates of severe
conditions. Extensive use of strong acids in combination
with various iodinating agents (HNO /H SO , HIO ;
3
2
4
3
H SO /HIO ; CF SO H/NIS) was reported [2–9].
2
4
4
3
3
Since molecular iodine has weaker electrophilicity than
molecular bromine and chlorine, formation of aryl iodides
by using I reagent is difficult and direct iodination of
2
arenes are not still well developed [10]. Among the various
reported methods we recall the use of molecular iodine
with heavy metal salts (I /HgX , I /HgO, I /Ag SO ,
Keywords Phenol ꢀ Iodination ꢀ Solid acids
2
2
2
2
2
4
I /Pb(OAc) /HOAc) [11–16], molecular iodine with various
2
4
oxidants (I /KMnO , I /NaIO , I /NaIO ) [17–19], or other
2
4
2
3
2
4
1
Introduction
systems employing less common reagents (I /F-TEDA-BF ,
2 4
ICl, I /PhI(OAc) , NIS, NaOCl-NaI) [20–26] in organic
2
2
Aromatic iodinated compounds have been used for over a
century as valuable chemicals in pharmaceutical and
agricultural chemistry or as intermediates in organic syn-
thesis, especially for C–C coupling reactions [1]. Many
solvents. When the use of molecular iodine is concerned,
formation of hydrogen iodide occurs thus causing proteo-
lytic cleavage of particular sensitive compounds. This is
the reason why oxidant agents were employed to reoxidize
the formed iodide ions to molecular iodine [1–28]. The
methodologies for the electrophilic introduction of an
iodine atom into organic compounds using molecular
iodine with other reagent systems have been revised
recently and the results are reported in excellent papers
P. Carniti (&) ꢀ A. Gervasini
Dipartimento di Chimica Fisica ed Elettrochimica, Universita`
degli Studi di Milano, Via Camillo Golgi, 19, 20133 Milan, Italy
e-mail: paolo.carniti@unimi.it
[
29–31].
Following the growing concern over environmental
S. Colonna
Dipartimento di Scienze Molecolari Applicate ai Biosistemi,
Universit a` degli Studi di Milano, Via Venezian, 21,
pollution and sustainable development, ‘‘greener’’ iodin-
ation synthetic methods were recently developed, in
2
0133 Milan, Italy
123

5
6
P. Carniti et al.
particular aerobic oxidative procedures [32–37] and the use
of system I /H O deserves interest [38–40]. Hydrogen
both the resins were dried at 100 °C under vacuum
(*20 Torr) in a rotative oven (BUCHI mod. GKR-50).
Particles between 25 and 35 mesh, obtained as reported in
¨
2
2 2
peroxide is accepted as a green oxidant and it has several
advantages over other oxidants: it is relatively cheap, non-
toxic, and it breaks into benign byproducts. Some hazard-
ous with its use is connected with highly concentrated
solution of H O , higher than 70–80% aqueous H O .
Ref. [49], were used for the catalytic tests.
H-ZSM5 and LZ-Y82 zeolites, SiAl and SiZr oxides,
NBO and NBP were first crushed in a mortar and then
sieved, collecting the 25–35 mesh fraction. The amount of
water in the catalysts was determined by drying at 100 °C
under vacuum (*20 Torr) in a rotative oven up to constant
weight. The knowledge of the water content in each cata-
lyst permitted evaluating the exact quantity of dry sample
employed in the catalytic test. Before use, they were dried
at 120 °C in an oven overnight (about 16 h).
Specific surface area and pore volume were determined
by conventional N2 adsorption/desorption at -196 °C
using an automatic analyzer of surface area (Carlo Erba
Sorptomatic). Prior to the analysis, the samples (H-ZSM5,
LZ-Y82, SiAl. SiZr, NBO, and NBP) were outgassed at
350 °C overnight (16 h) to eliminate moisture and volatile
compounds from the surface and pores. BET equation was
used for the calculation of the surface area and pore volume
values.
2
2
2 2
Moreover, the use of solid acids [41] instead of concen-
trated liquid acids, accounting for an heterogeneous syn-
thetic process for the direct introduction of iodine into
organics, is in the direction of the development of greener
reactive processes. The enormous advantages of the het-
erogeneous catalysis in comparison with the homogeneous
one have been well assessed in very numerous processes of
the fine, primary, and secondary chemistry [42–45]. In our
knowledge, experimental data concerning the direct
iodination of aromatics over solid acids have not been
appreciably revealed in the literature [46, 47].
In this work, we present the direct iodination of phenol,
chosen as representative aromatic substrate. The electro-
?
philic I species was generated from diodine by the pres-
ence of the catalytic action of some different solid acids;
the formed iodide ions were reoxidized by the oxidant
agent H O . The protocol employed to perform the reac-
2-Phenylethylamine (PEA, from Fluka, pure reagent,
99%), was utilized as basic probe for the acid site titrations
of the samples. The experiments were performed in a liquid
chromatographic line (HPLC), consisting of a pump
(Waters model 510) and an UV detector (Waters model
2487, k = 254 nm). The catalyst sample was placed in a
small stainless steel tube (length 12 cm, i.d. 2 mm)
mounted in place of the chromatographic column. The tube
was surrounded by a glass jacket thermostated by circu-
lating water at 17 °C.
2
2
tions is mild and very simple from the operative point of
view, as compared with the most reported methods using
homogeneous catalysts. The present approach could open a
new interesting perspective in the functionalization of
activated and deactivated arenes by iodine atom introduc-
tion assisted by an heterogeneous catalytic action.
2
Experimental
The apparatus can work in dynamic mode by successive
injections of pulses of defined and constant PEA concen-
tration (20 lL, 0.15 M) up to surface saturation or in re-
circulating mode. In the recirculating mode, solution of
PEA recirculated onto the sample until adsorption equi-
librium was achieved. The PEA concentration was varied
adding small quantities of PEA (50 lL, 0.15 M) to the
recirculating solution. Details on the method and operative
conditions have been given elsewhere [48–50]. The col-
lected data allowed to draw adsorption isotherms. 1:1
Stoichiometry for the PEA adsorption on the acid site was
assumed, the total number of acid sites per catalyst mass
2
.1 Materials and Characterization
The catalysts studied were three resins in acidic form
Amberlite IR-120 (H) and Amberlite 200, named A120 and
(
A200, respectively, and SAC-13, Nafion supported on sil-
ica), two acidic zeolites (MFI and faujasite types, named H-
ZSM5 and LZ-Y82, respectively), two amorphous mixed
oxides (silica-alumina and silica-zirconia, named SiAl and
SiZr, respectively), and two Nb-containing materials (nio-
bium oxide and niobium phosphate, named NBO and NBP,
respectively). They all are commercial materials. The cat-
alysts and their characteristics are listed in Table 1.
-
1
were expressed as mequiv g
.
A120 Amberlite was supplied in acidic form, it needed
only to be rinsed, under stirring, with distilled water to
eliminate any mineral acidic residue. The procedure was
repeated up to complete absence of both acidity and colour
in the wash waters. A200 Amberlite was supplied in
Na-form, it was activated by ionic exchange with HCl
2.2 Catalytic Reaction
The tests of catalytic iodination of phenol were performed
in a recirculation reaction line schematically shown in
Fig. 1. The line comprised: a peristaltic pump (Watson-
Marlow), a glass pre-heater and fixed-bed flow reactor
(20 cm long, 2 cm internal diameter) thermostated by
1
0%, then rinsed with distilled water up to neutrality. Then,
1
23

Direct Green Iodination of Phenol over Solid Acids
57
123

5
8
P. Carniti et al.
The reaction was followed for only a limited extent in
order to study the ratio of the ortho-, para-substitution in
the first part of the reaction, on the basis of the selectivity
to mono-, di-, and tri-iodinated compounds. The employed
ratio of substrate/iodine (1:2) was stoichiometric for the
formation of di-iodo-phenol, thus limiting the extent degree
of reaction.
3
Results and Discussion
The chosen catalysts belong to different catalyst families:
unsupported (A120 and A200) and silica supported (SAC-
Fig. 1 Scheme of the reaction line working in complete recirculation
mode used for the catalytic experiments. M = reservoir containing
the reaction mixture; P = recirculation pump; FB = catalytic packed
bed; B = thermostatted fluid bath; T = precision thermometer
13) acidic resins of organic nature, crystalline silico-alu-
mina catalysts (H-ZSM5 and Y zeolites), amorphous mixed
oxides (SiO –Al O and SiO –ZrO ), Nb-oxide, and Nb-
2
2
3
2
2
circulating water, and a reservoir containing the reaction
mixture. The catalyst sample (1 g, 25–35 mesh particles)
was held in the middle of the reactor, between two sand
beds. The initial reaction solution contained phenol (Carlo
Erba, 99.5% purity), molecular iodine (Carlo Erba, 99.9%
purity), and H O (Sigma–Aldrich, 50% water solution) at
phosphate. They all possess high acid properties due to
various surface groups, as detailed in Table 1. The amount
of the surface acid sites was determined by basic titration
employing PEA as molecular probe. The high basicity of
PEA (pKa = 9.84) guaranteed the titration of all the acid
sites of the different solids [51]. When titrations were
performed by two different methodologies, pulse method
[50] or recirculation-equilibrium [52], both gave similar
final results. Expressing the acidity per mass of sample, the
two non-supported acid resins and zeolite Y type were
found to have the highest amount of acid sites (more than 2
2
2
0
.1 M, 0.1 M, and 0.2 M concentration, respectively, in
methanol. The solution continuously circulated
-
1.40 mL min ) through the reaction line and on the solid
1
(
catalyst maintained at constant temperature of 50 °C.
At defined time intervals, samples of solution (0.1 mL)
were withdrawn from the reservoir for the analysis. For
each sample, after iodine elimination by Na S O and
-
1
mequiv g ), the two mixed oxides have an amount of acid
-
1
sites around 1 mequiv g , and the Nb-samples, H-ZSM5,
and SAC-13 have an amount of acid sites in the range from
2
2 3
extraction by CH Cl , gas-chromatographic analysis was
2
2
-
0.2 to 0.6 mequiv g .
1
performed (GC from HP 5890 series II, mounting a cap-
illary Supelco column, SPB5, 32 m long, and operating at
isothermal temperature of 120 °C for 50 min, then under
linearly increasing temperature at controlled rate,
The results obtained from the iodination of phenol over
the tested solid catalysts are summarized in the Tables 2, 3,
4, 5. At given reaction times, the percent extent of phenol
conversion (XPhOH/%) and selectivity distribution to the
two mono-iodinated compounds (2-IPhOH and 4-IPhOH),
two di-iodinated compounds (2,4-di-IPhOH and 2,6-di-
IPhOH), and tri-iodinated compound (2,4,6-tri-IPhOH)
were calculated. From an examination of the results, it can
-
1
4
°C min , up to 270 °C and maintaining the final tem-
perature for 30 min). Standard solutions of the pure sub-
strate (phenol) and iodinated-products in methanol at
various defined concentrations were used for the qualitative
and quantitative calibration of the GC analysis.
Table 2 Results obtained in the
iodination of phenol over the
acid resin catalysts
a
PhOH /% Selectivity/%
Sample code Time/h
X
2-IPhOH 4-IPhOH 2,4-di-IPhOH 2,6-di-IPhOH 2,4,6-tri-IPhOH
A120
2
4
7
2
4
7
2
4
7
23.3
40.0
54.7
20.4
34.3
50.0
16.9
28.1
40
55.2
46.4
40.7
58.4
50.9
43.8
67.2
57.3
49.7
35.8
35.5
35.5
33.2
32.7
34.0
26.3
27.6
26.1
1.7
3.6
5.7
1.5
3.5
5.5
0
7.3
10.9
13.3
6.9
0
3.5
4.8
0
A200
10.8
14.0
6.5
2.1
2.7
0
SAC-13
2.2
3.8
11.5
15.7
1.3
4.6
a
Phenol conversion
1
23

Direct Green Iodination of Phenol over Solid Acids
59
Table 3 Results obtained in the
iodination of phenol over the
zeolite catalysts
a
PhOH /% Selectivity/%
Sample code Time/h
X
2-IPhOH 4-IPhOH 2,4-di-IPhOH 2,6-di-IPhOH 2,4,6-tri-IPhOH
H-ZSM5
LZ-Y82
2
4
7
2
4
7
19.1
31.7
43.9
34.0
49.9
59.3
65.7
57.6
52.5
53.3
47.3
40.2
22.3
20.1
19.7
20.2
20.8
21.2
2.1
3.4
9.9
14.4
15.8
12.3
12.8
11.7
0
4.5
6.9
6.8
9.2
13.4
5.1
7.4
9.9
13.5
a
Phenol conversion
Table 4 Results obtained in the
iodination of phenol over the
mixed oxide catalysts
a
PhOH /% Selectivity/%
Sample code Time/h
X
2-IPhOH 4-IPhOH 2,4-di-IPhOH 2,6-di-IPhOH 2,4,6-tri-IPhOH
SiAl
SiZr
2
4
7
2
4
7
21.1
36.8
45.5
27.5
40.7
49.6
65.4
56.3
53.8
45.3
41.4
38.5
21.2
20.4
17.7
20.8
21.2
22.1
1.9
3.7
11.5
16.3
17.0
11.9
11.9
11.4
0
3.3
5.1
6.4
8.1
13.9
15.7
14.9
9.7
13.1
a
Phenol conversion
Table 5 Results obtained in the
iodination of phenol over the
Nb-catalysts
a
PhOH /% Selectivity/%
Sample code Time/h
X
2-IPhOH 4-IPhOH 2,4-di-IPhOH 2,6-di-IPhOH 2,4,6-tri-IPhOH
NBO
NBP
2
4
7
2
4
7
27.1
44.7
66.4
22.8
43.4
61.7
48.3
39.5
31.0
61.2
51.6
40.7
36.0
38.7
40.9
26.2
24.4
24.7
7.8
12.0
16.3
2.6
7.8
8.1
0
1.8
3.3
0
8.5
9.9
5.5
13.4
15.7
5.1
9.8
9.1
a
Phenol conversion
be inferred that a common trend could be observed over all
the catalysts. Phenol was principally ortho-iodinated with
higher formation of 2-IPhOH than of 4-IPhOH. The two
mono-iodinated compounds could act as further substrates
to iodinate, accordingly, the selectivity to 2-IPhOH
decreased with reaction time, while the selectivity to
with a consecutive mechanism from which it derives. Its
selectivity was increasing with time in a more or less
marked way depending on the different catalysts.
The most interesting feature of the obtained results relies
on the ortho- and para-orientation with which the catalytic
iodination occurred. The higher formation of 2-IPhOH over
4-IPhOH was an unexpected result on the basis of several
reports of the literature [11, 20, 26, 47] dealing with
iodination of phenol by different homogeneous catalysts
and/or iodinated agents. Generally, the literature results
indicate that phenol was principally para-iodinated over
the ortho-compound, beside the formation of ortho-and
para-diiodinated compound. On the opposite, the higher
formation of 2-IPhOH than 4-IPhOH here observed cannot
be only justified by the statistical consideration that there
are two attack points to the phenol substrate leading to
2-IPhOH compared with the unique position available that
leads to the 4-IPhOH compound. To investigate on the
reactivity of the ortho- vs. para-position of iodine
4
-IPhOH was maintained on a quite fixed value.
Iodination of both the two mono-iodinated compounds
gave rise to 2,4-di-IPhOH, while 2,6-di-IPhOH was formed
only from 2-IPhOH. We observed that the selectivity to
2
,6-di-IPhOH was always higher than that of 2,4-di-
IPhOH, so confirming the already observed preferred
ortho-orientation of the iodine in the aromatic substrate on
all the tested solid catalysts.
Both the di-iodinated compounds could act as further
substrate to iodinate, giving rise to the full-iodinated
compound (2,4,6-tri-IPhOH). The tri-iodinated compound
was observed among the products for long time of reaction
(
in general at reaction times higher than 2 h), in agreement
123

6
0
P. Carniti et al.
introduction into phenol, the selectivity ratio of 2-IPhOH
taken as one half) to 4-IPhOH, Sortho/para, was calculated
complex, involving the two reagents (iodine and phenol)
and catalytic site on the different catalyst groups.
(
for all the catalysts and at all the reaction time studied.
When the Sortho/para values are plotted as a function of the
selectivity to the two iodinated compounds (2-IPhOH ? 4-
IPhOH), three distinct lines decreasing with phenol con-
version (i.e., reaction time) could be identified (Fig. 2). On
each line, the points representing Sortho/para for several
catalysts lay together along the same line. The catalyst
groups comprised: (1) HZSM-5, LZ-Y82, SiAl, and SiZr
The identification of the three catalyst groups on the
basis of the trend of Sortho/para against selectivity to mono-
iodinated compounds is corroborated by another interpre-
tation of the collected data. Reporting the selectivity to all
the formed iodinated products as a function of phenol
conversion, increasing or decreasing linear trends could be
found for the different products formed. Once again, the
same catalyst groups already identified on the basis of
Fig. 2, are found grouped together, as depicted in Fig. 3, 4,
and 5. To enlighten the peculiar situation of the SiZr cat-
alyst, that showed the highest ability in the formation of di-
and tri-iodinated compounds compared with the ability of
all the other catalysts, it has been reported alone in Fig. 6.
The results shown in the Figs. 3, 4, 5, 6 clearly show the
(
even if SiZr could be considered apart from the other three
samples), (2) SAC-13 and NBP, (3) A120, A20, and NBO.
By extrapolating the lines to 100% selectivity to the di-
iodinated compounds (i.e., initial reaction time), it is possible
to find the Sortho/para value around 1.8 for the (1) group of
catalysts, the Sortho/para value around 1.3 for the (2) group
of catalysts, and the Sortho/para value of 1 for the (3) group
of catalysts. A value higher than 1 for Sortho/para signifies a
higher reactivity of the ortho-position over the para posi-
tion. The results of Fig. 2 show that the (1) group, in
particular, and (2) group of catalysts promote most the
reactivity of the ortho-position of phenol despite this
position is more sterically hindered than the para-position.
The (3) group of catalysts with the Sortho/para value of 1
does not cause significant difference of reactivity between
the ortho- and para-position of the phenol substrate.
However also in this case, the absence of the expected
decreasing of ortho-position reactivity, due to sterical
effects, is in line with a little higher activity of the ortho-
position. The observed behavior could be rationalized
invoking different molecular structures of the activated
80
6
4
2
0
0
0
0
0
20
40
60
80
PhOH conversion / %
Fig. 3 Selectivity to the products obtained from the iodination of
phenol as a function of phenol conversion over the H-ZSM5 (black
symbols), LZ-Y82 (grey symbols), and SiAl (open symbols) catalysts.
The different products are associated with different symbols: circle, 2-
IPhOH; square, 4-IPhOH; triangle, sum of the two di-IPhOHs; and
diamond, 2,4,6-tri-IPhOH
2
1
1
0
0
.0
.5
.0
.5
.0
8
6
4
2
0
0
0
0
5
0
60
70
80
90
100
Selectivity to iodinated compounds (2-IPhOH + 4-IPhOH) / %
H-ZSM5
SiO2-ZrO2
SAC13
NBP
A200
0
0
20
40
60
80
LZ-Y82
IR-120(H)
NBO
PhOH conversion / %
SiO2-Al2O3
Fig. 4 Selectivity to the products obtained from the iodination of
phenol as a function of phenol conversion over the SAC-13 (black
symbols) and NBP (open symbols) catalysts. The different products
are associated with different symbols: circle, 2-IPhOH; square, 4-
IPhOH; triangle, sum of the two di-IPhOHs; and diamond, 2,4,6-tri-
IPhOH
Fig. 2 Selectivity ratio of 2-IPhOH (taken as one half) to 4-IPhOH as
a function of the sum of the two mono-iodinated compounds. Distinct
lines identify families of catalysts operating the phenol iodination
with the same selective ortho-, para-orientation. Arrows indicate the
course of reaction
1
23

Direct Green Iodination of Phenol over Solid Acids
61
8
6
4
2
0
0
0
0
30
H-ZSM5
LZ-Y82
SiAl
SiZr
2
0
0
0
SAC-13
NBP
A200
A120
NBO
1
0
0
20
40
60
80
PhOH conversion / %
10
20
30
40
PhOH conversion / %
Fig. 5 Selectivity to the products obtained from the iodination of
phenol as a function of phenol conversion over the A200 (black
symbols), A120 (grey symbols), and NBO (open symbols) catalysts.
The different products are associated with different symbols: circle, 2-
IPhOH; square, 4-IPhOH; triangle, sum of the two di-IPhOHs; and
diamond, 2,4,6-tri-IPhOH
Fig. 7 Iodine conversion against phenol conversion determined at
fixed reaction time (2 h) over all the catalysts. Dotted line represents
the I conversion value to obtain the 2,-IPhOH and 4-IPhOH mono-
2
iodinated products
in a wide range of both I (7 to 27%) and phenol (17 to
2
3
5%) conversion. This indicates wide differences among
8
6
4
2
0
0
0
0
the catalysts in the rate of the iodination reaction. SAC-13,
A200, A120, ZSM-5, and SiAl can be considered to be
comprised in a quite narrow conversion range, but NBO,
SiZr, and LZ-Y82 are well apart from the others; they are
shifted towards higher phenol conversion, and in particular,
iodine conversion values. This indicates their superior rate
of direct introduction of one or more I-atom into phenol.
Moreover all the catalysts, and in particular NBO, SiZr,
and LZ-Y82, appear over the line that indicates the iodine
conversion to mono-iodinated compounds (2-IPhOH and 4-
IPhOH). As higher was the catalyst ability to introduce
more than one iodine atom into phenol, with the formation
0
0
20
40
60
80
PhOH conversion / %
Fig. 6 Selectivity to the products obtained from the iodination of
phenol as a function of phenol conversion over the SiZr catalyst. The
different products are associated with different symbols: circle,
2-IPhOH; square, 4-IPhOH; triangle, sum of the two di-IPhOHs; and
diamond, 2,4,6-tri-IPhOH
of di- and tri-iodinated compounds, as higher the I than
2
PhOH conversions were observed.
As concerning the relations between catalyst activity
and acidity properties, it seems that the lower or higher
catalytic ability to iodinate phenol was not simply con-
nected with the amount of acid sites of each surface. It
should be more valid to consider that beside the acid site
amount, also the site acid strength govern the catalyst
activity. Concerning the influence of surface area, any clear
relation between activity and catalyst surface area could
not be identified. For example, NBO catalyst with
marked decreasing trend of the selectivity to 2-IPhOH
compound with the extent of reaction over all the catalysts
and the increasing trends to the di-iodinated and tri-iodin-
ated compounds. The highly negative slopes of the lines
relevant to 2-IPhOH formation indicate the higher reac-
tivity of this compound compared with the other mono-
iodinated compound (4-IPhOH).
2
-1
108 m g achieved 66% phenol conversion by 2 h, while
2
-1
The iodination of phenol was studied for a limited extent
in order to study the reaction mechanism and regio-selec-
tivity of the reaction. Some kinetic considerations on the
reaction rate could de done from the collected data, as
reported in Fig. 7. Comparing the catalyst activity in terms
of both molecular iodine conversion and phenol conversion
at fixed reaction rate, it is possible to classify the catalysts
on the basis of their kinetic ability to iodinate the aromatic
SiZr catalyst with 293 m g did not exceed 50% con-
version. The specific effect of the catalyst structure and/or
active sites on the reaction mechanism seems to hold, so
outlining several catalyst groups.
The distinguishing action of the active acid site is
associated with the molecular iodine activation, causing the
-
?
formation of the (I ꢀꢀꢀꢀI ) adsorbed species. The presence
of an active vicinal site involved in the phenol adsorption
?
-
substrate. Figure 7, which reports the I against PhOH
2
to the activated (I ꢀꢀꢀꢀI ) species could justify the prevalent
?
attack of the electrophilic I species to the ortho position
conversion, shows that the employed catalysts are effective
123

6
2
P. Carniti et al.
8
9
. Patil BR, Bhusare SR, Pawar RP, Vibhute YB (2005) Tetrahe-
dron Lett 46:7179
. Castanet AS, Colobert F, Broutin PE (2002) Tetrahedron Lett
4
3:5047
1
0. March J (2000) Advanced organic chemistry, 4th edn. Wiley-
Interscience, New York
1. Baird WC Jr, Surridge JH (1970) J Org Chem 35:3436
2. Firouzabadi H, Iranpoor N, Shiri M (2003) Tetrahedron Lett
δ+
δ-
I
I
1
1
4
4:8781
13. Bachki A, Foabelo F, Yus M (1994) Tetrahedron 50:5139
1
O
4. Krassowska-Swiebocka B, Lulinski P, Skulski L (1995) Synthesis
26
5. Sy WW (1993) Tetrahedron Lett 34:6223
9
H
1
1
6. Orito K, Hatakeyama T, Takeo M, Suginome H (1995) Synthesis
1
273
1
1
1
7. Ogata Y, Aoki K (1968) J Am Chem Soc 90:6187
8. Lulinski P, Skulski L (2000) Bull Chem Soc Jpn 72:115
9. Lulinski P, Skulski L (2000) Bull Chem Soc Jpn 73:951
Fig. 8 Proposed molecular scheme of I
leading to the preferential formation of 2-IPhOH
2
and phenol interaction
20. Johnsson R, Meijer A, Ellervik U (2005) Tetrahedron 61:11657
21. Kajigaeshi S, Kakinami T, Watanabe F, Okamoto T (1989) Bull
Chem Jpn 62:1349
2
2
2
2. Zupan M, Iskra J, Stavber S (1997) Tetrahedron Lett 38:6305
3. Jereb M, Stavber S, Zupan M (2003) Synthesis 853
4. Hubig SM, Jung W, Kochi JK (1994) J Org Chem 59:6233
of phenol, following the scheme depicted in Fig. 8. Kiran
et al. [31] already observed the predominant formation of
25. Carreno MC, Ruano JLG, Sanz G, Toledo MA, Urbano A (1996)
Tetrahedron Lett 37:4081
2
2
2
-iodophenol during iodination of phenol with I /NaNO in
2 3
6. Edgar KJ, Falling SN (1990) J Org Chem 55:5287
7. Espuna G, Arsequell G, Valencia G, Barluenga G, P e´ rez M,
Gonz a´ lez JM (2000) Chem Commun 1307
water–methanol solution. This behavior could be influ-
enced not only by the catalyst action but also by the solvent
nature. This point will merit more attention in our future
works.
28. Adimurthy S, Ramachandraiah G, Ghosh PK, Bedekar AV (2003)
Tetrahedron Lett 44:5099
2
3
3
9. Stavber S, Jereb M, Zupan M (2008) Synthesis 1487
0. Pavlinac J, Zupan M, Stavber S (2007) Org Biomol Chem 5:699
1. Kiran YB, Konakahara T, Sakai N (2008) Synthesis 2327
4
Conclusions
32. Branytska OL, Neumann R (2003) J Org Chem 68:9510
3
3
3
3. Wan S, Wang SR, Lu W (2006) J Org Chem 71:4349
4. Iskra J, Stavber S, Zupan M (2008) Tetrahedron Lett 49:893
5. Das B, Krishnaiah K, Venkateswarlu K, Reddy VS (2007) Tet-
rahedron Lett 48:81
Mild and effective reagent combination, I /H O /solid
2 2
2
acid, for aromatic iodination has been presented. We
proved that solid acids are suitable for this transformation
and that they can offer the possibility to operate according
to an eco-friendly reactive process. The choice of the cat-
alyst nature affords the possibility to tune the reaction rate
and selectivity distribution of mono-, di-, and tri-iodo-
compounds.
36. Stavber G, Iskra J, Zupan M, Stavber S (2008) Adv Synth Catal
50:2921
7. Stavber G, Iskra J, Zupan M, Stavber S (2009) Green Chem
1:1262
8. Pavlinac J, Zupan M, Stavber S (2006) Synthesis 2603
3
3
3
1
39. Pavlinac J, Zupan M, Stavber S (2006) J Org Chem 71:1027
4
4
4
4
4
4
4
0. Jereb M, Zupan M, Stavber S (2004) Chem Commun 2614
1. Misono M, Okuhara T (1993) CHEMTECH November 23
2. Sartori G, Maggi R (2006) Chem Rev 106:1077
3. Corma A (1995) Chem Rev 95:559
4. Corma A, Garc ´ı a h (2003) Chem Rev 103:4307
5. Sheldon RA (2008) Chem Commun 3325
6. Khadilkar BM, Tilve RD, Varughese MA (2002) Tetrahedron
Lett 43:9457
Acknowledgment BRACCO Imaging S.p.A. (Italy) is gratefully
acknowledged for the financial contribution.
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