Molecules 2001, 6
870
ArICl2, yellow crystalline compounds, are light- and heat-sensitive and often unstable to storage.
They do not usually give satisfactory microanalyses and their melting/decomposition points are
uncertain, depending upon the purity of their freshly prepared batches, the time elapsed since their
preparation, and the rate of heating during their melting point estimations [1].
In 1886 Willgerodt [2] developed the most common method up to now for preparing ArICl2, by
passing the stream of Cl2 through cooled solutions of ArI in CHCl3. Recently, Japanese chemists [3]
repeatedly produced PhICl2 from PhI (in 94% crude yield) on a 20 kg scale using this classic method.
Next, they selectively monochlorinated 4-aminoacetophenone with crude PhICl2 on a 24.8 kg scale, in
87% yield.
To avoid the hazardous and inconvenient use of gaseous Cl2 to prepare ArICl2 from ArI, a
considerable number of various either two-phase (CCl4/conc. aq. HCl) or monophasic liquid-phase
methods were reported; the full account of all those methods is given in our latest review [4]. The
dichlorine was generated there in situ from conc. hydrochloric acid, used either as a separate phase or
as cosolvent, under the action of various oxidants: 2HCl + [O] → Cl2 + H2O; mostly, such procedures
are easy and relatively safe. However, all former methods [1-4] required the use of iodoarenes, ArI, as
the starting substrates which are then chlorinated at their iodine atoms to afford ArICl2. Recently, we
presented [5] a novel, one-pot (two-stage) method for preparing eleven ArICl2 from corresponding
arenes (ArH) and diiodine used as the starting substrates. ArH were first oxidatively substituted in
appropriate anhydrous I2/NaIO4 or NaIO3/AcOH/Ac2O/conc. H2SO4 mixtures with some iodine(III)
transient species, I3+, to form in situ the organoiodine(III) intermediates, ArISO4, viz. ArH + I3+
+
H2SO4 → ArISO4 + 3H+. Next, excess conc. (36%) hydrochloric acid was added to the resulting
reaction mixtures, containing the soluble ArISO4 intermediates, to precipitate out the corresponding
ArICl2, isolated in 46-88% crude yields, viz. ArISO4 (not isolated) + 2HCl → ↓ArICl2 + H2SO4. Our
environmentally benign method avoided the use of costly iodoarenes, the hazardous application of
gaseous Cl2 and chlorinated solvents and, in our opinion, would be particularly suitable for large-scale
preparations of ArICl2 from the respective arenes; cf. Ref. [3]. But there are also some inherent
limitations in the applicability of this method. Of course, only those isomeric RC6H4ICl2 may
predominantly be obtained from the monosubstituted benzenes RC6H5, which are formed in agreement
with common orientation rules in the electrophilic substitutions of the used substrates RC6H5 by said
strongly electrophilic I3+ transient species, generated in situ in the anhydrous iodinating reaction
mixtures. We have emphasized [5] that further studies are necessary to establish more precisely the
limits of the scope of this novel method. Consequently, such novel "model" studies are presented
below, but with using chromium(VI) oxide, CrO3, as the oxidant. Hence, it is necessary to recall the
results reported in our two former papers [6, 7], where CrO3 was also applied as the effective oxidant.
In 1997 we developed [6] a simple oxidative method for the iodination of various arenes in
variable anhydrous ArH/I2/CrO3/AcOH/Ac2O/conc. H2SO4 systems. For benzene, halobenzenes, and
activated arenes, the highest iodination yields were obtained with the following ratio of the reagents:
6ArH : 3I2 : 2CrO3, which favoured the generation of some iodine(I) transient species, I+, acting there
as the iodinating agents. In contrast, for deactivated aromatics, the following ratio of the reagents: