Iodine(I) reagents in hydrochloric acid-catalyzed oxidative iodination of aromatic compounds by hydrogen peroxide and iodine

Article abstract of DOI:10.1002/adsc.201300127

Hydrochloric acid activates the oxidative iodination of aromatic compounds with the iodine- hydrogen peroxide system through the formation of an iodine(I) compound as the iodinating reagent. Activation with hydrochloric acid is more powerful than that with sulfuric acid. The formation of dichloroiodic(I) acid (HICl₂) with various forms of hydrogen peroxide was followed using UV spectroscopy. The HICl₂ was used as the iodinating reagent. In the preparative oxidative iodinaton of various aromatic compounds, hydrochloric acid was used in a catalytic amount and the iodine(I) reagent was formed in situ with 0.5 equiv. hydrogen peroxide and 0.5 equiv. molecular iodine. Two types of reactivity were observed in oxidative iodination with iodine(I) species catalyzed by hydrochloric acid: in the iodination of anisole 1a better yields of iodination were observed with a smaller amount of hydrochloric acid, while on the contrary 4-tert-butyltoluene 1b gave better yields of iodination upon increasing the amount of hydrochloric acid. Reactivity was further manipulated by the choice of the solvent (MeCN, trifluoroethanol, hexafluoro-2-propanol). Copyright

Full text of DOI:10.1002/adsc.201300127

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DOI: 10.1002/adsc.201300127
Iodine(I) Reagents in Hydrochloric Acid-Catalyzed Oxidative
Iodination of Aromatic Compounds by Hydrogen Peroxide and
Iodine
a,b,
ˇa
Leon Bedrac and Jernej Iskra *
a
ˇ
Department of Physical and Organic Chemistry, Laboratory for Organic and Bioorganic Chemistry, “Jozef Stefan”
Institute, Jamova 39, 1000 Ljubljana, Slovenia
Fax : (+386)-1251-9385; phone: (+386)-1477-3631; e-mail: jernej.iskra@ijs.si
The Centre of Excellence for Integrated Approaches in Chemistry and Biology of Proteins (CIPKeBiP), Jamova 39, 1000
Ljubljana, Slovenia
b
Received: February 7, 2013; Revised: April 12, 2013; Published online: April 30, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300127.
Abstract: Hydrochloric acid activates the oxidative
iodination of aromatic compounds with the iodine-
hydrogen peroxide system through the formation of
an iodine(I) compound as the iodinating reagent.
Activation with hydrochloric acid is more powerful
than that with sulfuric acid. The formation of di-
chloroiodic(I) acid (HICl2) with various forms of
hydrogen peroxide was followed using UV spectros-
copy. The HICl2 was used as the iodinating reagent.
In the preparative oxidative iodinaton of various ar-
omatic compounds, hydrochloric acid was used in
a catalytic amount and the iodine(I) reagent was
formed in situ with 0.5 equiv. hydrogen peroxide
and 0.5 equiv. molecular iodine. Two types of reac-
tivity were observed in oxidative iodination with io-
dine(I) species catalyzed by hydrochloric acid: in
the iodination of anisole 1a better yields of iodina-
tion were observed with a smaller amount of hydro-
chloric acid, while on the contrary 4-tert-butylto-
luene 1b gave better yields of iodination upon in-
creasing the amount of hydrochloric acid. Reactivi-
ty was further manipulated by the choice of the sol-
vent (MeCN, trifluoroethanol, hexafluoro-2-
propanol).
preparation of organometallic reagents and metal-cat-
alyzed cross-coupling reactions, while they are also
important per se.[2] In medicine, iodoaromatics are ex-
ploited as contrast agents or radioactively labelled
markers.[3] Furthermore, they are able to form poly-
valent iodine compounds such as PIDA and IBX,
which are of ever-increasing value in modern organic
synthesis.[4]
Due to the increasing importance of the iodinated
aromatic compounds, synthetic pathways for their
preparation have been intensively studied but the low
electrophilic character of molecular iodine makes its
direct use difficult. To overcome this problem, iodina-
tion is often performed under oxidative conditions in
strong acidic media (i.e., KI/KIO3/H2SO4).[5] Howev-
er, strong acids and oxidizing reagents can lead to un-
desirable side reactions. Iodine(I) reagents have been
devised for iodination, such as iodosuccinimide,
IPy2BF4 and ICl, and their reactivity has been en-
hanced by the addition of a strong acid, i.e., NIS/
TfOH.[6] Although reactivity is improved, drawbacks
including higher cost and a high environmental
burden resulting from low atom economy, the use of
toxic and hazardous reagents and the large amount of
waste produced. As a consequence, an oxidative iodi-
nation method was developed, where both iodine
atoms in the molecule of I2 react as electrophiles due
to the regeneration of HI by its re-oxidation in situ.[7]
By using either H2O2 or O2, this method has great
“green” potential since the only side product is
water.[7a] Reactivity is limited by that of I2 and it can
Keywords: halogenation; hydrogen peroxide; iodi-
nation; oxidation; oxidative iodination
Iodine-containing aromatic compounds have been be further modulated by addition of an acid.[8]
known for over a century and play an indispensable An interesting approach in the oxidative iodination
role in chemical synthesis, the pharmaceutical indus- would be to activate iodine by converting it to the
try, agricultural chemistry and medicine.[1] Iodoaro- more electrophilic iodonium(I) reagent. Tetraalkylam-
matics are valuable starting compounds for carbon- monium dichloroiodates(I) are interesting iodinating
carbon and carbon-heteroatom bond formation, the reagents, although their high reactivity can only be
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Leon Bedrac and Jernej Iskra
COMMUNICATIONS
achieved in super acidic media.[9] We have recently ditions. This was checked by examining a MeCN solu-
shown that these salts can be prepared using a simple tion of H2O2, I2 and acid using UV spectroscopy.
procedure from I2 and H2O2.[10] This finding indicates Figure 1 shows how iodine is completely converted
that iodine(I) species could be formed under the reac- into iodine(I) species in the presence of HCl and HBr
tion conditions of oxidative iodination using H2O2 as with the formation of strong new signals with lmax at
the oxidant.
343 nm and 373 nm, respectively, whereas in the pres-
In this study we show that HCl is a catalyst for ence of sulfuric acid only a small signal with lmax at
H2O2-mediated oxidative iodination of aromatic mol- 332 nm is apparent. With all the other acids the UV
ecules with I2 by enabling in situ formation of iodi- spectrum remained the same as that of I2 (Figure 1).
À [10]
ne(I) species. The nature of active iodinating species The signal at 343 nm corresponds to R4N+ICl2 ,
in I2/H2O2/HCl system was also studied.
while the signal at 332 nm corresponds to HOSO3I
We initially investigated if HCl differs from other (vide infra).
acids in the ability to activate oxidative iodination.
This formation of iodine(I) species was further in-
Anisole 1a is used as a model substrate because it vestigated in different solvents. First, iodine, an equiv-
does not react with 0.5 equiv. of I2 and 0.5 equiv. of alent of 50% aqueous H2O2 and 4 equivalents of 37%
50% aqueous H2O2 in MeCN at room temperature aqueous HCl, as it is needed for the formation of
under neutral conditions. An equivalent of acid was HICl2 [Eq. (1)], were placed in a mortar and ground
used to activate the iodination and it was found that
the highest yields of iodinated product 2a were
formed in the presence of HCl and H2SO4, while 4-
bromoanisole 3a was the main product in the pres-
ence of HBr (Scheme 1). There is no clear correlation
between the strength of the acid and activation of io- until homogenous without addition of organic solvent.
dination. A possible reason for this is the formation We observed the disappearance of iodine and forma-
of an iodine(I) species under the acidic reaction con- tion of a yellow solution. The reaction was monitored
using UV spectroscopy by dissolving an aliquot of the
reaction mixture in ethanol. As the reaction pro-
gressed, the peak at 446 nm decreased, while a new
peak at 343 nm appears (Figure 1).[11] After 3 h the
iodine signal is no longer detectable and dichloroio-
AHCTUNGTRENNUNG
dic(I) acid[12] (HICl2) was obtained quantitatively as
a yellow solution. The oxidation of iodine was also
quantitative when MeCN was used as the solvent (in
24 h) but this was not the case when using either etha-
nol, dichloromethane (DCM) or hexane. 2,2,2-Tri-
fluoroethanol (TFE) activated the reaction pronoun-
cedly and the conversion of iodine into HICl2 was
completed within an hour.
Scheme 1. Iodination of anisole (1a) with the reaction
system I2/H2O2/HA (0.5:0.5:1).
Figure 1. UV spectra of MeCN solution of I2, H2O2 in the presence of acids (1:1:4) (left). UV spectra of the reaction between
I2 and 50% H2O2 (1 equiv.) in the presence of HCl (4 equiv.) by grinding in a mortar (right).
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Iodine(I) Reagents in Hydrochloric Acid-Catalyzed Oxidative Iodination of Aromatic Compounds
Table 1. Oxidation of I2 with H2O2 in the presence of HCl.[a]
confirmed by mass spectrometry and iodometric titra-
tion [Eq. (2)].
Entry
Peroxide source
I2:H2O2
Time[b] [h]
1
2
3
4
5
6
7
50% aq. H2O2
30% aq. H2O2
30% aq. H2O2
UHP
1:1
1:1
1:2
1:1
1:2
1:1
1:2
1
24[c]
1
4
1
UHP
Next, we tested the iodinating activity of HICl2 and
its in situ formation in the reaction of oxidative iodi-
nation with a catalytic amount of HCl. The aim was
SPC[d]
24[c]
3
SPC
[a]
I2:H2O2:HCl (1:1:4) were dissolved in 1 mL of TFE and to explore the effect that the amount of acid has on
0.2 mL of PhCF3 and stirred in a 25-mL flask at room
temperature.
Time needed for complete conversions monitored by UV
spectroscopy at 343 nm.
Conversion was not complete.
SPC=sodium percarbonate.
the iodination of anisole 1a using two different acids:
H2SO4 as classic activator and HCl as promoter of the
iodine(I) reagent. We varied the amount of acid in
the iodination of 1a with I2/H2O2 in MeCN at room
temperature (as in Scheme 1). We observed two types
of correlation between iodination and the amount of
added HCl. Contrary to expectations, the yield of io-
dination of 1a increases with decreasing amount of
[b]
[c]
[d]
We next used different forms of hydrogen peroxide HCl, being 100% with 0.1 equiv. of HCl after 24 h at
in TFE to check its effect on the oxidation of I2. Due room temperature and 65% with 2 equiv., respectively
to low solubility of iodine in TFE, we added 17 vol% (Figure 2a). A different effect of the amount of acid
of PhCF3 to the reaction mixture. Besides a 50% was observed in the H2SO4 activated reaction, where
aqueous solution of H2O2, a complex of urea-hydro- iodination was more efficient when using higher
gen peroxide (UHP) was also able to completely oxi- amounts of acid, albeit the difference was not pro-
dize iodine using an equimolar amount of peroxide nounced (59% with 0.1 equiv. and 65% with 2 equiv.).
and iodine, although the reactivity was lower This effect was further explored using less reactive p-
(Table 1). A 30% aqueous H2O2 solution, in an equi- tert-butyltoluene 1b (Figure 2b). Both acids had a simi-
molar amount to I2, was inefficient and did not oxi- lar effect on the iodination of 1b, and the effect of the
dize iodine quantitatively after 24 h. However, when acid was again more pronounced and the reactivity
a two-fold excess of H2O2 was used, the reaction was higher in the case of HCl.
fast and a yellow solution was obtained within one
hour. A similar reactivity was observed using sodium tivity of other active reagents including I2, HICl2, ICl
percarbonate (SPC). and HOSO3I. Iodine was immediately eliminated as it
This observation prompted us to compare the reac-
To prove that dichloroiodic(I) acid is formed during is not sufficiently reactive to iodinate 1a at room tem-
the oxidation of I2 with H2O2 in the presence of HCl, perature. When using pure HICl2, 27% of 2a was
we isolated HICl2 by extracting it out of the diluted formed after 6 h, while ICl gave a 94% yield under
aqueous reaction mixture using DCM, then washing the same reaction conditions. HOSO3I was prepared
the organic phase with water before drying it with by heating a solution of I2/H2O2/2H2SO4 in MeCN for
Na2SO4. Organic solvent was removed under reduced 6 h. Complete conversion of iodine was determined
pressure and the residue was a yellow oil which was by UV spectroscopy (product has lmax at 332 nm).
confirmed as HICl2 by using UV spectroscopy, iodo- When it was used for iodination, no reaction occurred
metric titration and HR-MS analysis. Raman spec- at room temperature and only 18% of 2a was formed
troscopy clearly showed the absence of iodine, while after 6 h at 608C. Judging by these results, it is proba-
two signals are present - 268 cmÀ1 (major) and ble that the HCl-catalyzed I2/H2O2 oxidative iodina-
309 cmÀ1 (minor). The major one is similar to the tion proceeds via the formation of ICl. When the con-
signal of ICl2À salt (261 cmÀ1). HICl2 was also generat- centration of HCl is low and the substrate is active
À
ed from the Et4N+ICl2 salt with the addition of sulfu- (i.e., 1a), iodination with ICl prevails. When the con-
ric acid as proven by Raman spectra (see the Support- centration of HCl is higher and/or the substrate is less
ing Information). Interestingly, the concentrated reactive (i.e., 1b), ICl reacts with HCl to form HICl2,
aqueous solution of HICl2 is stable for days as are its which is less reactive than ICl (Scheme 2). This was
solutions in different organic solvents (DCM, EtOH, further proved by the fact that iodination of 1b with
MeCN and TFE). At a higher pH decomposition ICl is getting slower with increasing amounts of HCl
occurs. In a biphasic system of DCM-H2O, HICl2 (Figure 2c).
slowly disproportionates. After 24 h of stirring, the
These results indicate that HCl can be used for in
DCM solution becomes violet due to the presence of situ generation of an iodine(I) reagent for oxidative
I2, while the aqueous phase contains the iodate, as iodination with the I2-H2O2 system. Furthermore, HCl
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perature and 98% of 2a was isolated after column
chromatography (Table 2). The same result was ob-
tained by using a two-fold excess of 30% H2O2 (see
Table 1), however, in a typical reaction we used 50%
H2O2 since an excess of oxidant is not required. We
applied this catalytic reaction system to the iodination
of different aromatic substrates (Table 2). 1,3-Dime-
thoxybenzene (1c) gave a similar result, while for the
iodination of less reactive substrates more activating
solvent – TFE had to be used in order to have com-
plete iodination. 1,2-Dimethoxybenzene (1d), 4-tert-
butylanisole (1e) and 4-tert-butylphenol (1f) were
quantitatively and regioselectively iodinated without
the formation of any diiodinated products (Table 2,
entries 3–5). 4-Nitrophenol 1g gave mainly a diiodinat-
ed product 4g, which was quantitatively formed when
using a 2-fold excess of iodine and H2O2 (Table 2,
entry 6).
In addition, the reaction of mesitylene 1h in TFE
resulted in complete iodination into 2h, while durene
1i gave monoiodo 2i and diiodo product 4i. Similarly,
during iodination of 4-tert-butylaniline (1j), the for-
mation of 2-iodo-4-tert-butylaniline (2j) was accompa-
nied by the formation of small amounts of 1,5-diiodo-
4-tert-butylaniline (4j). With an excess amount of I2
conversion was 100%, nevertheless 2j was still the
main reaction product.
The HCl-catalyzed oxidative iodination of 4-fluo-
roanisole (1k) gave a 20% yield of 2-iodo-4-fluoroani-
sole (2k) after 16 h at room temperature, while con-
version was moderate even at an elevated tempera-
ture (61%). Hence we chose hexafluoro-2-propanol
(HFIP) as an even better activator of electrophilic
Figure 2. The influence of the amount of the acid (HCl and processes than TFE. Surprisingly, iodination did not
H2SO4) on the iodination of 1a (a) and 1b (b) with the reac-
tion system I2/H2O2/HA (0.5/0.5/x) for 24 h at room temper-
ature. (c) Reaction of 1b with ICl and various amounts of
HCl (MeCN, room temperature, 6 h).
occur and only 2-chloro-4-fluoroanisole 5k was
formed (10% yield) after 24 h at room temperature.
Since the solubility of I2 in HFIP is very low, oxida-
tion of HCl with H2O2 prevails and chlorine reacts
with 1k in place of I2. Because a catalytic system in
HFIP leads to chlorination, we used preformed HICl2
in HFIP as the reaction system for the iodination of
deactivated substrates that do not react completely in
TFE. HICl2 was formed in only 15 min in HFIP under
typical reaction conditions and after addition of 1k io-
dination was the main reaction channel that was ac-
companied by formation of small amount of chlori-
nated product 5k (8%) (Scheme 3). 4-tert-Butylto-
Scheme 2. Iodination of arenes with in situ generated HICl2 luene (1b) and 4-nitroaniline (1l) were quantitatively
and selectively iodinated with pre-formed HICl2 at
reflux, while the catalyzed reaction did not lead to
can be used as catalyst as it is re-generated during the complete conversion (Scheme 3). Iodination of highly
reaction. 0.1 mmol of HCl per 1 mmol of substrate is deactivated 4-nitrotoluene (1m) did not occur even
equivalent to 5 mol% of its required quantity [see Eq. under these reaction conditions (Scheme 3).
(1)]. First, we made a preparative reaction with 1a
In summary, we find that oxidative iodination in
and iodination of 1 mmol of 1a with 0.5 mmol of I2, the presence of HCl proceeds differently than the
0.5 mmol of 50% aqueous H2O2 and 0.1 mmol of HCl H2SO4-activated reaction. While the latter process is
was completed quantitatively within 6 h at room tem- activated via the acidic function of sulfuric acid, oxi-
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Iodine(I) Reagents in Hydrochloric Acid-Catalyzed Oxidative Iodination of Aromatic Compounds
Table 2. Iodination of aromatic compounds using the I2:H2O2:HClACTHUNGTRENNUNG
Entry
1
Solvent
MeCN
Time [h]
6
Conversion[b]
100% (98%)
2
3
MeCN
TFE[c]
4
100% (99%)
100% (94%)
12
4
5
TFE
TFE
16
4
100% (95%)
100% (99%)
6
7
TFE[d]
TFE
16, 608C
100% (86%)
100% (97%)
4
8
TFE
12
6
2i: 77% (71%)+4i: 10% (8%)
2j: 88%+4j: 12%
9
TFE[d]
[a]
1 mmol of substrate, 0.5 mmol I2, 0.1 mmol 37% HCl, 0.5 mmol of 50% H2O2, 1 mL TFE and 0.2 mL PhCF3.
Conversion based on the starting compound determined by H NMR spectra, number in bracket presents the isolated
[b]
1
yield.
[c]
[d]
5 mol% excess of I2 was used.
1 mmol of substrate, 1.0 mmol I2, 0.1 mmol 37% HCl, 1.0 mmol of 50% H2O2, 1 mL TFE and 0.2 mL PhCF3.
dative iodination in the presence of HCl leads to the When using activated arenes, iodination is even faster
formation of the iodine(I) species. Furthermore, acti- when using HCl in catalytic amounts. Various arenes
vation via the formation of an iodine(I) reagent is were iodinated using 0.5 equiv. of I2, and H2O2 and
more effective and HCl can be used as a catalyst. 0.1 equiv. of HCl. Research into the synthesis and
properties of other dihalo halogen(I) species and
their role in catalytic halogenation of organic mole-
cules is currently underway.
Experimental Section
Typical Procedure for Catalytic Oxidative Iodination
of Aromatic Compounds with H2O2 in the Presence
of 0.1 Equivalent of HCl
Scheme 3. Iodination of aromatic compounds with HICl2 in
HFIP
Into a 25-mL flask was placed 127 mg (0.50 mmol) of finely
powdered iodine, dissolved in 1 mL of MeCN or trifluoro-
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Leon Bedrac and Jernej Iskra
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Acknowledgements
The financial support of the Slovenian Research Agency
(PR-04115) is greatly appreciated. The authors are grateful to
the Slovenian NMR Centre at the National Institute of
Chemistry, Dr. Zoran Arsov (F-5 JSI) for IR, Dr. Melita
ˇ
ˇ
ˇ
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