Selective production of diiodobenzene and iodobenzene from benzene

Full text of DOI:10.1021/jo961493m

J . Org. Chem. 1996, 61, 9621-9622
9621
Other methods for benzene iodination reported in the
literature result in the formation of iodobenzene7-19 or
mixtures of polyiodobenzenes.3,20 A few reaction systems
are reported3,21 which can selectively regulate the number
of iodine atoms substituted into the aromatic ring.
However, these systems involve the use of fluorinated
solvents3 or thallium salts,21 both of which pose environ-
mental problems. Many reactions that produce 1,4-
diiodobenzene also produce a substantial amount of tri-
or tetraiodobenzene. Only iodobenzene and diiodoben-
zene are observed in the currently reported reaction.
Selective P r od u ction of Diiod oben zen e a n d
Iod oben zen e fr om Ben zen e
Linda C. Brazdil* and Carlo J . Cutler
Department of Chemistry, J ohn Carroll University,
University Heights, Ohio 44118-4581
Received August 2, 1996
Iodination of aromatic substances is an important step
in the synthesis of many organic molecules including
many pharmaceuticals and biochemicals. It is important
to establish direct, simple, and selective methods to
produce iodinated aromatics without the production of
heavy metal iodides and other wastes often associated
with these processes. Methods, such as the one described
here, do not produce heavy metal waste nor use fluori-
nated solvents for environmental reasons. When iodine
itself is used as a reagent, an additional benefit is derived
if both iodine atoms from the molecular iodine are
incorporated into the products rather than having one
of the iodine atoms become waste as an iodide salt.
Recently, it was reported1 that reaction mixtures of
iodine, diiodine pentoxide (I2O5), activated or deactivated
aromatic, and concd H2SO4 in acetic acid solvent selec-
tively produce monoiodinated products in good yield.
Trace amounts of diiodo products were seen in some
cases, but usually no diiodo product was observed.
However, both iodine atoms from molecular iodine ap-
peared to be incorporated into the iodoaromatic product.
While a large number of activated and deactivated
aromatics were tested, iodination of benzene itself by this
method was not attempted by these researchers.
For example, when 0.055 mol (4.3 g) of benzene, 0.022
mol (5.5 g) of iodine, 0.022 mol (7.3 g) of I2O5, and 60 µL
of concd H2SO4 were reacted in 25 mL of glacial acetic
acid at 60 °C for 40 h, the yield, based on benzene, of
1,4-diiodobenzene after purification was 8.7%; of 1,2-
diiodobenzene, 1.5%; and of iodobenzene, 56%. If the
amount of iodine was raised to 0.055 mol (14.0 g) while
other conditions were kept constant, the yield of 1,4-
diiodobenzene increased to 46%, that of 1,2-diiodobenzene
increased to 8.0%, and the yield of iodobenzene decreased
to 45%. Similar results were obtained with lower amounts
of I2O5 and at temperatures between 50 and 70 °C,
although no reaction was observed if I2O5 was absent.
In conclusion, a simple, safe, and effective method for
the production of diiodobenzenes and iodobenzene in
which the amount of each product can be regulated easily
is presented. No heavy metals are used, thus eliminating
the heavy metal salt waste normally associated with
aromatic iodination reactions.
In the current study, it has been found that when this
method is used to iodinate benzene, diiodobenzene is
selectively produced along with iodobenzene. Benzene
is quantitatively reacted into these products when suf-
ficient iodine is present. No other iodinated aromatic
compounds are observed, even in the presence of high
levels of iodine. These products are easily separated
since 1,4-diiodobenzene is a crystalline solid at room
temperature and 1,2-diiodobenzene and iodobenzene are
liquids. None of the products was soluble in the aqueous
acetic acid medium when water was added to stop the
reaction. Additionally, I2O5, which oxidizes the molecular
iodine, is not consumed during the reaction. The exact
nature of the activated iodine is currently under inves-
tigation, but the ability of I2O5 to activate iodine without
being consumed is a large advantage over other benzene
iodination reactions which form diiodobenzene, since the
oxidizing agents in these reactions are stoichiometrically
consumed.2-6 Also, many of these reactions involve
corrosive reagents or the use of heavy metal oxidants or
catalysts. The hazards associated with these reagents
and their waste are eliminated in the current reaction
system.
Exp er im en ta l Section
Gen er a l Com m en ts. Iodine was obtained from Mallinck-
rodt, sulfuric acid and glacial acetic acid from Fisher Scientific,
and I2O5 and benzene from Aldrich Chemical Co. All chemicals
were used without further purification. NMR spectra were
recorded at 300 MHz in acetic acid-d4. FT-Raman spectra were
obtained in glass cuvettes at 60 °C, using a NdYAG laser with
a wavelength of 1064 nm. A wattage of only 500 mW was used
because of the highly colored nature of the samples. Gas
chromatography was obtained using a 5% phenylmethylsilicone
column that is 30 m × 25 mm with a 0.25 µm film thickness.
The injector temperature was 275 °C; the helium carrier gas
held at 8 psi (1.03 mL/min initially through the column) with a
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P. J .; Asenio, G. J . Chem. Soc., Chem. Commun. 1992, 1016-1017.
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(11) Shono, R.; Matsummuar, Y.; Katoh, S.; Ikeda, K.; Kamada, T.
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* Author to whom correspondence should be addressed. Phone: 216-
397-4791. FAX: 216-397-3033. E-mail: lbrazdil@jcvaxa.jcu.edu.
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S0022-3263(96)01493-4 CCC: $12.00 © 1996 American Chemical Society

9622 J . Org. Chem., Vol. 61, No. 26, 1996
Notes
100 mL/min split flow. The temperature program starts at 50
°C for 3 min and then heats at 30 °C/min to 260 °C where it is
held for 5 min. The transfer line to the mass selective detector
is held at 280 °C.
Products were identified by NMR, GC-MS, and FT-Raman
of their acetic acid solutions. The melting point of 1,4-diiodo-
benzene was also obtained. In all cases, data matched those
for authentic samples obtained from Aldrich Chemical Co. FT-
Raman (CH3COOH): 1,4-diiodobenzene 1051 (m), 684 (m); 1,2-
diiodobenzene 1045 (m), 317 (m); iodobenzene 1017 (m), 996 (s),
657 (m), 266 (s).
Gen er a l P r oced u r e for Syn th esis of Diiod oben zen e a n d
Iod oben zen e. In a representative experimental procedure,
iodine, I2O5, and concd H2SO4 in glacial acetic acid were heated
under nitrogen in a three-necked flask with constant stirring.
When the desired temperature was reached, benzene was added
to the mixture, and the reaction mixture was heated and stirred.
The red iodine color became fainter or disappeared after time.
The reaction mixture was cooled, and distilled water was added
to dissolve the I2O5. At this point, the 1,2-diiodobenzene and
iodobenzene also separated from the aqueous layer. Filtration
separated the 1,4-diiodobenzene from a biphasic filtrate contain-
ing 1,2-diiodobenzene and iodobenzene in an oily layer. These
compounds were purified as described previously.22 The 1,4-
diiodobenzene was recrystallized from ethanol.
Ack n ow led gm en t. This material is based upon
work supported by the National Science Foundation
under Grant Number CHE-9520439. The NMR was
purchased with funds from a Kresge Foundation Science
Initiative Grant. The authors are grateful for the
support provided by these organizations. One of the
authors (C.J .C.) is also grateful for a BP Summer
Fellowship.
(22) Ogata, Y.; Nakajima, K. Tetrahedron 1964, 20, 2751-2754.
J O961493M