Iodination of Anilines with Sodium Dichloroiodate

Full text of DOI:10.1080/00304948.2016.1206425

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Iodination of Anilines with Sodium Dichloroiodate
Shire Elmi, Per Heggen, Bjarte Holmelid, Didrik Malthe-Sørensen & Leiv K.
Sydnes
To cite this article: Shire Elmi, Per Heggen, Bjarte Holmelid, Didrik Malthe-Sørensen & Leiv K.
Sydnes (2016) Iodination of Anilines with Sodium Dichloroiodate, Organic Preparations and
Procedures International, 48:5, 385-392, DOI: 10.1080/00304948.2016.1206425
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Date: 21 September 2016, At: 01:11

Organic Preparations and Procedures International, 48:385–392, 2016
Copyright Ó Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2016.1206425
Iodination of Anilines with Sodium Dichloroiodate
Shire Elmi,1 Per Heggen,1 Bjarte Holmelid,1
Didrik Malthe-Sørensen,2 and Leiv K. Sydnes1
1
ꢀ
Department of Chemistry, University of Bergen, Allegt. 41, NO-5007 Bergen,
Norway
2GE Healthcare, Lindesnes Site, NO-4510 Spangereid, Norway
Iodinated aromatic compounds have been synthetic targets for decades,1–6 and a number
of methods have been developed for iodination of arenes.2–25 The simplest and mildest
are those relying on the electrophilicity of iodine in dichloroiodate salts.15–25 At least one
such reagent, sodium dichloroiodate, is utilized on an industrial scale to obtain triply
iodinated benzenes,26,27 which constitute the key structural motif in several X-ray con-
trast agents.1–6 Since this process has to be very carefully controlled to minimize the for-
mation of by-products (in particular azoarenes26,27), this inspired us to study the
iodination of some anilines with sodium dichloroiodate in greater detail.
In order to evaluate the sensitivity of the industrial process to pH, exploratory experi-
ments were performed using 2,4-dimethylaniline (1a). Under acidic conditions (pH 1–2)
iodination of 1a with sodium dichloroiodate proceeded smoothly to give a clean product
mixture from which pure 2-iodo-4,6-dimethylaniline28 (2a) was isolated in 88% yield.
However, under basic conditions 2a was not formed at all and a complex product mixture
was obtained from which the only reasonably pure product isolated in low yield (4%) was
2,20,4,40-tetramethylazobenzene (3a).29
The iodination of benzene derivatives is influenced by the nature of the substitu-
ents.6–25 It was therefore of interest to investigate the reaction of some activated and deac-
tivated aniline derivatives under the conditions applied to 1a (room temperature; acidic
and basic conditions; variable amounts of sodium dichloroiodate).
Dimethylanilines 1b–1d (Scheme 1 and Table 1) gave relatively simple reaction
mixtures under acidic conditions; in order for all the starting materials to be consumed,
an excess of sodium dichloroiodate had to be utilized. Under these conditions 2,3-dime-
thylaniline (1b) gave a 44% yield of 2b but also afforded 4,6-diiodo-2,3-dimethylaniline
(19%) with the use of a 60% molar excess of the reagent. Under the same conditions but
with a 3.8 molar excess of the reagent, 3,5-dimethylaniline (1c) gave a complex mixture;
use of a much larger excess of sodium dichloroiodate (4.8 molar) led to 2,4,6-triiodo-3,5-
dimethylaniline (78%) as the sole isolatable product (Table 1). 2,6-Dimethylaniline (1d)
exhibited a third type of reactivity; while iodination still occurred to afford 4-iodo-2,6-
dimethylaniline (2d) in 21% yield, the main product was 2,6-dimethyl-p-benzoquinone-
4-(20,60-dimethyl)anil in 28% yield.32 Under basic conditions (KOH in methanol/H2O),
Received December 12, 2015; in final form June 3, 2016.
Address correspondence to Leiv K. Sydnes, Department of Chemistry, University of Bergen,
ꢀ
Allegt. 41, NO-5007 Bergen, Norway. E-mail: leiv.sydnes@uib.no
385

386
Sydnes et al.
1a–1d behaved similarly to the anisidines (vide infra); only the corresponding azo
compounds were generated in low yields. The reactivity of 2,6-dimethylaniline (1d) is
also different from that of 1a–1c: nearly no azo compound was formed and only the anil
was isolated in 34% yield.
Scheme 1. Iodination of Dimethylanilines with NaICl2.
Table 1
Iodination of Anilines (1) Using Sodium Dichloroiodate under Acidic Conditionsa
Ratio of Reagent
to Aniline
Product
(% Yieldb)
Other productsc
(% Yieldb)
Aniline
2,4-Dimethyl (1a)
2,3-Dimethyl (1b)
3,5-Dimethyl (1c)
2,6-Dimethyl (1d)
2-Chloro (1e)
1.1
1.6
4.8
3.8
2.5
3.6
2a (88)
2b (44)
—
2d (21)
2e (61)
—
—
4,6-Diiodo-1b (19)
2,4,6-Triiodo-1c (78)
Anild (28)
4,6-Diiodo-1e (27)
2,4-Diiodo-1f (12)
4,6-Diiodo-1f (61)
2,4,6-Triiodo-1f (18)
2,6-Diiodo-1g (24)
4,6-Diiodo-1h (41)
2,4-Diiodo-1i (38)
4,6-Diiodo-1i (47)
2,6-Diiodo-1j (23)
3-Chloro (1f)
4-Chloro (1g)
2-COOMe (1h)
3-COOMe (1i)
2.5
2.5
2.5
2g (75)
2h (57)
—
4-COOMe (1j)
2.5
2j (75)
aTreatment for 3 hrs in methanol/H2O/HCl at room temperature.
bIsolated yield.
cWith the exception of the anil formed from 1d, all compounds are regarded as aniline deriva-
tives when abbreviated in this table,and the positions of the iodo substituents are given the numbers
that follow from the numbering of 1. For instance, 6-chloro-2,4-diiodoaniline is called 4,6-diiodo-
1e because positions 4 and 6 in 1e have been iodinated.
d2,6-Dimethyl-p-benzoquinone-4-(20,60-dimethyl)anil.

Iodination of Anilines with Sodium Dichloroiodate
387
All three anisidines gave black, tarry mixtures (possibly aniline oligomers, polymers
as judged from the spectroscopic data.30,31) under acidic conditions. The use of basic con-
ditions also led to poor results, the corresponding azobenzenes being the only compounds
isolated from m- and p-anisidines (1% and 5%).
Both o- and p-chloroanilines (1e and 1g) and the methyl esters of o- and p-aminoben-
zoic acids (1h and 1j) underwent iodination with NaICl2 under acidic conditions to give
reasonable yields (57–75%) of the monoiodo compounds (Scheme 2). However, since a
2.5–3.6 molar excess of dichloroiodate had to be used to achieve complete conversion,
significant amounts (>20%) of polyiodinated anilines were generated as well (Table 1).
Under these conditions, the meta isomers (1f and 1i) gave no monoiodinated products.
Under basic conditions, very poor results were obtained: the chloroanilines gave small
amounts (<10%) of the corresponding azobenzenes29 and the aminobenzoates did not
react at all.
Scheme 2. Iodination of Chloroanilines and Methyl Aminobenzoate.
In conclusion, the article describes a simple method for the iodination of some aniline
derivatives using sodium dichloroiodate. Although the reaction gives some mono-iodin-
ated anilines in 75–88% yields under acidic conditions, the results are quite sensitive to
the nature of the substituents and their position relative to the amino group. Under basic
conditions iodination does not occur.
Experimental Section
All melting points were determined on a Gallenkamp apparatus and are uncorrected. IR
spectra were recorded on a Perkin-Elmer 1310 spectrophotometer or Spectrum One FT-
IR ATR (Attenuated Total Reflection) spectrophotometer. The IR spectra were obtained
as liquid films by the attenuated total reflection technique (ATR) or as KBr discs. The
molecular absorptions frequencies are given in wave numbers (cm¡1) and indicated as
strong (s), medium (m), weak (w) or broad (b). NMR spectra were acquired on a Bruker
Spectrospin AC 200F or a Bruker Spectrospin DMX 400 with the field strengths of
200 MHz and 400 MHz, respectively. Chemical shifts are reported in d (ppm) using

388
Sydnes et al.
1
either TMS (for H) or CDCl3 (for 13C) as the internal standard for shift calibration
1
(7.28 ppm for H and 77.8 ppm for 13C). All coupling constants (J) are given in Hertz
(Hz), and the multiplicity are reported as (s) for singlet, (d) for doublet, and (bs) for broad
singlet. Mass spectra and high resolution mass spectra were determined on a JEOL Accu-
TOF T100GC which was operated in the DART mode. All new compounds were also
analysed in the CHN-mode on a VarioEL III Element Analyser using helium a carrier
gas. Due to the low contents of hydrogen in all the samples, the results of the analyses for
this element are close to the detection limit for the instrument and within a range where it
operates with poor accuracy. Consequently, the hydrogen values were calibrated against
the response of several blank combustions and given as the mean of two replicates.
Iodination of Anilines under Acidic Conditions
To a solution of the aniline (1, 20.0 mmol) in a mixture of H2O (100 mL) and MeOH
(300 mL), was added conc. hydrochloric acid was added until pH was 1–2 (pH strip).
Then an aqueous solution of NaICl2 (50% w/w 1.1–4;.8 molar equiv; for amount, see
Table) was added dropwise and the resulting mixture was stirred with a stirring bar at
room temperature for 3–4 h. Saturated aqueous NaHSO3 was added until negative starch
test was obtained and the product was worked up.
When no precipitate was clearly visible in the product mixture, the mixture was made
slightly basic (pH strip) by dropwise addition of an aqueous solution of NaOH (1 M) and
extracted with Et2O. The combined organic phases were dried (MgSO4), filtered, and con-
centrated under vacuum to give a solid.
When a visible precipitate was obtained, the solid was isolated by gravity filtration
(each case specified below) and the filtrate was then made slightly basic (pH strip) by
addition of an aqueous solution of NaOH (1 M) and extracted with Et2O. The combined
organic extracts were dried (MgSO4), filtered, and concentrated under vacuum to afford a
solid.
All the solids were analysed by TLC (silica gel; varying mixtures of hexanes/ethyl
acetate). When a solid consisted of several products, the compounds were separated and
purified by flash chromatography (silica gel; hexanes/ethyl acetate 9:1 when not other-
wise stated).
The products obtained were characterized by mps (and comparison with lit. mps), IR,
MS, 1H- and 13C-NMR spectra as well as elemental analysis for all new compounds. The
anisidines gave no products that could be characterized. Anilines 1a–1j gave products as
summarized below; most of them are known from the literature.
2,4-Dimethylaniline (1a) afforded 2-iodo-4,6-dimethylaniline (2a), mp 65–66ꢀC
(lit.28 65ꢀC).
2,3-Dimethylaniline (1b) gave 4-iodo-2,3-dimethylaniline (2b), mp 55–56ꢀC (lit.18
56ꢀC), and 4,6-diiodo-2,3-dimethylaniline (4,6-diiodo-1b), mp 91–92ꢀC (lit.18 89–90ꢀC).
3,5-Dimethylaniline (1c) gave 2,4,6-triiodo-3,5-dimethylaniline (2,4,6-triiodo-1c),
mp. 188–190ꢀC (lit.15 192ꢀC).
2,6-Dimethylaniline (1d) furnished 4-iodo-2,6-dimethylaniline (2d), mp 49–50ꢀC
(lit.28 52–5;3 ꢀC), and 2,6-dimethyl-p-benzoquinone-4-(2’,6’-dimethyl)anil, mp 74–76ꢀC
(lit.32 76–77ꢀC).
2-Chloroaniline (1e) gave 2-chloro-4-iodoaniline (2e), mp 67–69ꢀC (lit.21 68–
70ꢀC), and 6-chloro-2,4-diiodoaniline (4,6-diiodo-1e) (isolated by filtration), mp 125–
126ꢀC (lit.12 126–127ꢀC).

Iodination of Anilines with Sodium Dichloroiodate
389
3-Chloroaniline (1f) gave a mixture of two diiodoanilines, 3-chloro-2,4-diiodoani-
line (2,4-diiodo-1f) and 5-chloro-2,4-diiodoaniline (4,6-diiodo-1f), as well as one triio-
doaniline, 3-chloro-2,4,6-triiodoaniline (2,4,6-triiodo-1f), which were separated by flash
chromatography (silica gel; hexanes/ethyl acetate 85:15). 2,4-Diiodo-1f, gray powder,
mp 116–118ꢀC; IR (KBr): 3405s, 3305s, 3054w, 2922w, 1601s, 1587s, 1569s, 1447m,
1
1373m, 1297w, 1269w, 1111m, 1080s, 940w, 878m, 839m, 808s, 715s, 697s cm¡ ; 1H-
NMR (200 MHz, CDCl3): d 7.56 (d, J D 8.4, 1 H), 6.36 (d, J D 8.4, 1 H), 4.38 (bs, 2 H);
13C-NMR (50 MHz, CDCl3): d 149.1 (C), 149.0 (C), 139.4 (CH), 113.6 (CH), 86.9 (C),
80.4 (C); DART-MS: 381 (30), 379 (100, [M C H]C), 251 (20); HR-MS: 379.8208 ([M
C H]C, C6H535ClI2NC; calc. 379.8200).
Anal. Calcd for C6H435ClI2N: C, 19.00; H, 1.06; N, 3.69. Found: C, 18.94; H, 1.10;
N, 3.67.
4,6-Diiodo-1f, off-white powder, mp 117.5–119.5ꢀC; IR (KBr): 3378w, 3303m,
3171m, 2923w, 2918w, 1612s, 1567s, 1441m, 1358m, 1288m, 1256w, 1112m, 1020m,
1
908w, 877s, 841m, 802s, 715s, 704m, 697s, 655m cm¡1; H-NMR (200 MHz, CDCl3):
d 8.00 (s, 1 H), 6.87 (s, 1 H), 4.19 (bs, 2 H); 13C-NMR (50 MHz, CDCl3): d 147.8 (C),
139.4 (CH), 139.1 (C), 114.2 (CH), 82.8 (C), 81.9 (C); DART-MS: 381 (30), 379 (100,
[M C H]C), 251 (8); HR-MS: 379.8213 ([M C H]C, C6H535ClI2NC; calcd. 379.8200).
Anal. Calcd for C6H435ClI2N: C, 19.00; H, 1.06; N, 3.69. Found: C, 18.91; H, 1.09;
N, 3.67.
2,4,6-Triiodo-1f, gray powder, mp 141–143ꢀC. IR (KBr): 3398m, 3308s, 2924w,
1597s, 1569m, 1512w, 1414s, 1397w, 1333w, 1293w, 1277w, 1114m, 1073m, 936w,
1
871m, 842m, 812s, 712m, 704m cm¡1; H-NMR (200 MHz, CDCl3): d 8.08 (s, 1 H),
4.94 (bs, 2 H); 13C-NMR (50 MHz, CDCl3): 147.9 (C), 147.4 (CH), 142.3 (C), 84.4 (C),
80.3 (C), 78.7 (C); DART-MS: 507 (30), 505 (100, [M C H]C), 379 (5), 377 (15); HR-
MS: 505.7142 ([M C H]C, C6H435ClI3NC; calcd. 505.7166).
Anal. Calcd for C6H335ClI3N: C, 14.26; H, 0.60; N, 2.77; found: C, 14.35; H, 0.88; N,
2.69.
4-Chloroaniline (1g) gave 4-chloro-2-iodoaniline (2g), mp 43–44ꢀC (lit.21 42–
43ꢀC), and 4-chloro-2,6-diiodoaniline (2,6-diiodo-1g) (isolated by filtration), mp 127–
128ꢀC (lit.20 128–130ꢀC).
Methyl 2-aminobenzoate (1h) gave methyl 2-amino-5-iodobenzoate (2h), mp 78–
80ꢀC (lit.35 83–85ꢀC), and methyl 2-amino-3,5-diiodobenzoate (4,6-diiodo-1h) (isolated
by filtration), mp 106–109ꢀC (lit.36 110 ꢀC).
Methyl 3-aminobenzoate (1i) afforded two diiodoanilines, methyl 3-amino-2,6-
diiodobenzoate (2,6-diiodo-1i) and methyl 5-amino-2,4-diiodobenzoate (4,6-diiodo-1i),
which were separated by flash chromatography (silica gel; hexanes/ethyl acetate 85:15).
(2,6-Diiodo-1i): off-white powder, mp 140–144ꢀC; IR (KBr): 3405m, 3318s, 3148w,
2948w, 1709s, 1618s, 1579m, 1489w, 1431m, 1291s, 1255s, 1188w, 1119m, 1031w,
1
1
963m, 906w, 825w, 761s, 706w, 674m cm¡ ; H-NMR (200 MHz, CDCl3): d 7.47
(d, J D 8.4, 1 H), 6.48 (d, J D 8.4, 1 H), 4.08 (bs, 2 H), 3.88 (s, 3 H); 13C-NMR (50 MHz,
CDCl3): d 168.5 (CHO), 147.1 (C), 138.9 (CH), 116.2 (CH), 114.9 (C), 80.9 (C), 74.8
(C), 52.9 (OC3); DART-MS: 403 (100, [M C H]C), 343 (30); HR-MS: 403.8657 ([M C
H]C, C8H8I2NO2C; calcd. 403.8639).
Anal. Calcd for C8H7I2NO2: C, 23.85; H, 1.75; N, 3.48; found: C, 23.94; H, 1.89; N,
3.51.
(4,6-Diiodo-1i): off-white powder, mp 142–145ꢀC; IR (KBr): 3407s, 3314s,
3063w, 2948w, 1718s, 1605s, 1530w, 1429m, 1392w, 1297s, 1261s, 1126m, 1052w,
1
1
971m, 906w, 760m, 706w cm¡ ; H-NMR (200 MHz, CDCl3): d 8.19 (s, 1 H), 7.20

390
Sydnes et al.
(s, 1 H), 4.22 (bs, 2 H), 3.90 (s, 3 H); 13C-NMR (50 MHz, CDCl3): d 166.2 (C D O),
149.4 (CH), 146.8 (C), 120.2 (C), 116.1 (CH), 88.7 (C), 78.2 (C), 52.3 (OCH3);
DART-MS: 403 ([M C H]C, 100), 343 (20); HR-MS: 403.8666 ([M C H]C,
C8H8I2NO2C; 403.8639).
Anal. Calcd for C8H7I2NO2: C, 23.85; H, 1.75; N, 3.48; found: C, 23.89; H, 1.94; N, 3.44.
Methyl 4-aminobenzoate (1j) afforded methyl 4-amino-2-iodobenzoate (2j), mp
84–86ꢀC (lit.19 84–87ꢀC), and methyl 4-amino-2,6-diiodobenzoate (2,6-diiodo-1j) (iso-
lated by filtration), mp 158–160ꢀC (lit.19 163–164ꢀC).
Attempted Iodination of Anilines under Basic Conditions
Aniline (1) (20.0 mmol) and then KOH (10 g, 0.18 mol) were dissolved in a mixture of
H2O (100 mL) and MeOH (300 mL) under stirring. An aqueous solution of NaICl2 (50%
w/w of ICl) (1.1–3.5 molar eq) was added dropwise, and the mixture was stirred at room
temperature (stir bar) for 18–24 h. Saturated aqueous NaHSO3 was then added until nega-
tive starch test was obtained. The resulting mixture was extracted with Et2O and the com-
bined organic phases were dried (MgSO4), filtered, and concentrated under vacuum,
leaving a semi-solid residue behind. TLC analysis (silica gel; varying mixtures of hex-
anes/ethyl acetate) gave in most cases one spot. Purification was performed by flash chro-
matography using a silica-gel column and hexanes/ethyl acetate 97:3 as eluent.
All the anilines were reacted, and 1a–1g, 3-anisidine, and 4-anisidine furnished the
corresponding azobenzene (3). 2,6-Dimethylaniline (1d) gave in addition the same anil
that was obtained under acidic conditions. The products were characterized by their IR,
MS, 1H- and 13C-NMR spectra as well as melting points.
2,4-Dimethylaniline (1a) gave 2,20,4,40-tetramethylazobenzene (3a), mp 128–129ꢀC
(lit.37 129ꢀC) in 4% yield.
2,3-Dimethylaniline (1b) gave 2,20,3,30-tetramethylazobenzene (3b), mp 109–110ꢀC
(lit.37 110 ꢀC) in 8% yield.
3,5-Dimethylaniline (1c) gave 3,30,5,50-tetramethylazobenzene (3c), mp 136–137ꢀC
(lit.37 136–137 ꢀC) in 25% yield.
2,6-Dimethylaniline (1d) gave 2,20,6,60-tetramethylazobenzene (3d), mp 49–50ꢀC
(lit.38 48ꢀC) in 2% yield, and 2,6-dimethyl-p-benzoquinone-4-(20,60-dimethyl)anil, mp
75–76ꢀC (lit.32 76–77ꢀC) in 34% yield.
2-Chloroaniline (1e) gave 2,20-dichloroazobenzene (3e), mp 138–139ꢀC (lit.39
138ꢀC), in 3% yield.
3-Chloroaniline (1f) gave 3,30-dichloroazobenzene (3f), mp 103–104 ꢀC (lit.40
110ꢀC), in 8% yield.
4-Chloroaniline (1g) gave 4,40-dichloroazobenzene (3g), mp 189–190ꢀC (lit.40
189ꢀC), in 10% yield.
m-Anisidine afforded 3,30-dimethoxyazobenzene, mp 75–76ꢀC (lit.41, 42 73–75ꢀC),
in 1% yield.
p-Anisidine gave 4,40-dimethoxyazobenzene, mp 162–163ꢀC (lit.43 160.5–161.9ꢀC),
in 5% yield
Acknowledgments
Financial and in-kind support from Nycomed (now GE Healthcare) and the University of
Bergen is acknowledged with gratitude. Furthermore, unfailing support from the Munin
Foundation is highly appreciated. Finally, the authors would like to thank the Executive
Editor of this Journal for constructive criticism.

Iodination of Anilines with Sodium Dichloroiodate
391
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