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
2
O
2
as oxidant agent. Iodine was introduced
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.
Keywords Phenol Á Iodination Á Solid acids
1 Introduction
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
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.
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
3
/H
2
SO
4
, HIO
3
;
H
2
SO
4
/HIO
4
;CF
3
SO
3
H/NIS) was reported [2–9].
Since molecular iodine has weaker electrophilicity than
molecular bromine and chlorine, formation of aryl iodides
by using I
2
reagent is difficult and direct iodination of
arenes are not still well developed [10]. Among the various
reported methods we recall the use of molecular iodine
with heavy metal salts (I
2
/HgX
2
,I
2
/HgO, I
2
/Ag
2
SO
4
,
I
2
/Pb(OAc)
4
/HOAc) [11–16], molecular iodine with various
oxidants (I
2
/KMnO
4
,I
2
/NaIO
3
,I
2
/NaIO
4
)[17–19], or other
systems employing less common reagents (I
2
/F-TEDA-BF
4
,
ICl, I
2
/PhI(OAc)
2
, NIS, NaOCl-NaI) [20–26] in organic
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
[29–31].
Following the growing concern over environmental
pollution and sustainable development, ‘‘greener’’ iodin-
ation synthetic methods were recently developed, in
P. Carniti (&) Á 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
S. Colonna
Dipartimento di Scienze Molecolari Applicate ai Biosistemi,
Universita
`
degli Studi di Milano, Via Venezian, 21,
20133 Milan, Italy
123
Catal Lett (2010) 137:55–62
DOI 10.1007/s10562-010-0283-6