Aromatic iodination with iodine monochloride by using a catalytic amount of ferrocenium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

Article abstract of DOI:10.1016/S0040-4039(00)01454-4

Direct iodination reaction of several aromatic compounds with 1.1-2.0 molar amounts of iodine monochloride (IC1) proceeded smoothly to afford the corresponding aromatic iodides in good to excellent yields by using 5 mol% of Cp₂FeB[3,5-(CF₃)₂C₆H₃]₄ (1) in the coexistence of DDQ or ZnO. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01454-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 9383–9386
Aromatic iodination with iodine monochloride by using a
catalytic amount of ferrocenium
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate†
Teruaki Mukaiyama,* Hideo Kitagawa and Jun-ichi Matsuo
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku,
Tokyo 162-8601, Japan
Received 5 August 2000; revised 31 August 2000; accepted 1 September 2000
Abstract
Direct iodination reaction of several aromatic compounds with 1.1–2.0 molar amounts of iodine
monochloride (ICl) proceeded smoothly to afford the corresponding aromatic iodides in good to excellent
yields by using 5 mol% of Cp2FeB[3,5-(CF3)2C6H3]4 (1) in the coexistence of DDQ or ZnO. © 2000
Published by Elsevier Science Ltd.
Keywords: catalysis; halogenation; ferrocenes; aryl halides.
Aromatic iodides have widely been used as synthetic intermediates in the formation of new
carbonꢀcarbon or carbonꢀheteroatom bonds via replacement of their iodine atoms with
electrophiles.1 However, synthetic methods leading to aromatic iodides2 by direct iodination of
aromatic compounds are relatively limited compared with those of preparing aromatic chlorides
and bromides because of the low electrophilicity of iodine.
Iodination of aromatic compounds was carried out by using molecular iodine together with a
strong oxidizing agent such as nitric acid, sulfuric acid, iodic acid, sulfur trioxide or hydrogen
peroxide3 in order to generate a better electrophile by oxidation of molecular iodine. It was also
reported that the aromatic iodination with molecular iodine was catalyzed by stoichiometric
amounts of metal halides, such as AlCl3ꢀCuCl2 or SbCl5,5 and a direct iodination was carried
4
out by using NH4I and catalytic amounts of NOBF4 in CF3COOH/CH2Cl2 or CF3COOH/
CH3COOH with molecular oxygen.6 In those methods, large amounts of aromatic compounds
were needed and the reactions had to be conducted under strongly acidic conditions. Now, we
would like to report direct iodination of several aromatic compounds with 1.1–2.0 molar
* Corresponding author. Tel: +81-3-3260-4271 ext. 1709; fax: +81-3-3260-4185; e-mail: mukaiyam@rs.kagu.sut.ac.jp
†
Dedicated to Professor H. H. Wasserman on the occasion of his 80th birthday.
0040-4039/00/$ - see front matter © 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)01454-4

9384
amounts of iodine monochloride (ICl) by using a catalytic amount of Cp2FeB[3,5-(CF3)2C6H3]4
(1) in the coexistence of DDQ or ZnO.
In order to carry out the direct iodination reaction under milder conditions, catalytic
7
activation of an iodinating agent with mild Lewis acids having B(C6F5)4 or B[3,5-(CF3)2C6H3]4
anion was considered based on the following evidence. The above anions have been known as
low-coordinating anions and were frequently employed as counter ions of several Lewis acid
catalysts because they enhanced the Lewis acidities of the counter metals. Then, mild Lewis
acids were expected to be obtained by combining those low-coordinating anions with counter
metals, which were normally thought to have a low Lewis acidity.
First, lithium was chosen and LiB(C6F5)4 (2) was used as the mild Lewis acid catalyst in the
direct iodination of aromatic compounds. When catalytic iodination of toluene with ICl8 was
tried by using 5 mol% of 2, the effect of the catalyst was not observed and a similar result to
that of non-catalyzed iodination was obtained (Table 1, entries 1 and 3). Next, LiB[3,5-
(CF3)2C6H3]4 (3) was used and the desired products were then obtained in 68% yield (entry 5).
The yields of the desired iodinated aromatic compounds were greatly improved in both 2- and
3-catalyzed reactions (entries 4 and 6) in the coexistence of DDQ, which might have worked as
a radical trapping agent to suppress benzylic iodination.
Table 1
Effect of catalysts and DDQa
Entry
Catalyst
Additive (0.5 equiv.)
Time/h
o/pb
Yield%b
1
2
3
4
5
6
7
8
9c
None
None
LiB(C6F5)4 (2)
2
LiB(3,5-C6H3(CF3)2)4 (3)
3
Cp2FeB(3,5-C6H3(CF3)2)4 (1)
1
1
None
DDQ
None
DDQ
None
DDQ
None
DDQ
DDQ
8
8
8
49/51
49/51
44/56
48/52
46/54
48/52
46/54
51/49
53/47
48
47
50
91
68
92
61
8
8
10
10
10
0.5
8
Quant.
a Diiodo and chlorinated products were not detected.
b Determined by GC analyses.
c The yields of o- and p-iodotoluene greatly decreased to 28% in a prolonged reaction time (24 h).
A ferrocenium ion was considered to have interacted with an electron at the outer sphere.9 It
was then figured that the ferrocenium ion would exhibit low Lewis acidity and that 1 would
work as a mild Lewis acid in the iodination of aromatic compounds. Recently, Molins and
co-workers reported on the preparation of 110 and described that this ferrocenium salt could be
purified easily by recrystallization and stored in air for at least one month. The iodination
reaction using 1 alone did not give as good a result as 2- and 3-catalyzed reactions did, while
the addition of DDQ was effective and the iodide was obtained in good yield.

9385
Then, direct iodination of several more reactive aromatic compounds with ICl was tried by
using 5 mol% of 1 (Table 2). It was found that 1-catalyzed iodination effectively took place in
the coexistence of zinc oxide (ZnO).11 It was assumed that ZnO captured hydrogen chloride
formed during the iodination reaction and it helped the smooth turnover of the catalyst 1.
Table 2
Synthesis of aromatic iodidesa
A typical experimental procedure for the iodination of toluene with ICl is as follows: to a
mixture of 1 (0.05×10−1 mmol) and DDQ (0.55 mmol) in dichloromethane (2.20 mL), a solution
of ICl in dichloromethane (1.0 M, 2.21 mL) was added at room temperature. After stirring for
30 min, toluene (117.6 mL, 1.11 mmol) was added and the reaction mixture was stirred for 8 h
at room temperature.
The reaction was quenched by adding saturated aqueous sodium sulfite, and the mixture was
extracted with ethyl acetate. The combined organic layer was washed with H2O and brine, dried
over MgSO4, filtered and concentrated in vacuo. The yield and the isomeric ratio of aromatic

9386
iodides were determined by GC analysis (SHIMADZU CAPILLARY COLUMN HiCap Series
CBP10-M25-0.25) of the resulting residue (quantitative, o/p=53/47) by comparison with those
of authentic samples. Neither diiodo nor chlorinated products were detected.
It is noted that the iodination of aromatic compounds with 1.1–2.0 molar amounts of ICl was
effectively promoted by a catalytic amount of 1 in the coexistence of DDQ or ZnO at room
temperature. The results shown in Table 2 demonstrate that 1 could serve as a mild Lewis acid
catalyst in the present iodination reaction. Further investigation on applications of this novel
catalyst system to other halogenation reactions is now in progress.
Acknowledgements
This work was financially supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture of Japan.
References
1. (a) Fanta, P. E. Synlett 1974, 9. (b) Nilsson, M. Tetrahedron Lett. 1966, 679. (c) House, H. O.; Respess, W. L.;
Whitesides, G. M. J. Org. Chem. 1966, 31, 3128. (d) Matsui, K.; Tobita, E.; Ando, M.; Kondo, K. Chem. Lett.
1981, 1719. (e) Burdon, J.; Coc, P. L.; Marsh, C. R. J. Chem. Soc., Chem. Commun. 1967. 1259. (f) Setsune, J.;
Matsukawa, K.; Wakemoto, H.; Kitano, T. Chem. Lett. 1981, 367. (g) Suzuki, H.; Kobayashi, T.; Yoshida, Y.;
Osuka, A. Chem. Lett. 1983, 193. (h) Suzuki, H.; Abe, H.; Osuka, A. Chem. Lett. 1980, 1363.
2. Merkushev, E. B. Synthesis 1988, 923.
3. (a) Pizey, J. S. Synthetic Reagents; Wiley: New York, 1977; Vol. 3, pp. 227. (b) Boothe, R.; Dial, C.; Conaway,
R.; Pagni, R. M.; Kabalka, G. W. Tetrahedron Lett. 1986, 22, 2207. (c) Sugita, T.; Idei, M.; Takegami, Y. Chem.
Lett. 1982, 1481. (d) Suzuki, H.; Haruta, Y. Bull. Chem. Soc. Jpn. 1973, 46, 589. (e) Suzuki, H. Organic Syntheses
1988; Coll. Vol. 4, pp. 700.
4. Sugita, T.; Idei, M.; Ishibashi, Y.; Takegami, Y. Chem. Lett. 1982, 1481.
5. Umemura, S.; Onoe, A.; Okano, M. Bull. Chem. Soc. Jpn. 1974, 47, 147.
6. Radner, F. J. Org. Chem. 1988, 53, 3548.
7. (a) Mori, A.; Nie, J.; Sonoda, T.; Yagupolskii, Y. Chem. Lett. 2000, 62. (b) Nishida, H.; Takada, N.; Yoshimura,
M. Bull. Chem. Soc. Jpn. 1984, 57, 2600. (c) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345.
8. (a) Olah, G.; Pavlath, A.; Varsanyl, G. J. Chem. Soc. 1957, 1823. (b) Keefer, R. M.; Andrews, L. A. J. Am.
Chem. Soc. 1956, 78, 5623. (c) Miller, L.; Kayawa, E. P.; Campbell, C. B. J. Am. Chem. Soc. 1970, 92, 2821.
9. Carney, M. J.; Lesniak, J. S.; Likar, M. D.; Pladziewwicz, J. R. J. J. Am. Chem. Soc. 1984, 106, 2565.
10. Molins, E.; Roig, A.; Maniukiewicz, W.; Manriquez, J. M. J. Organomet. Chem. 2000, 601, 126.
11. Commercial ZnO (Kanto Chemical Co., Inc.) was used without further purification.