First examples of superelectrophile initiated iodination of alkanes and cycloalkanes

Article abstract of DOI:10.1016/S0040-4039(01)02371-1

Direct iodination of alkanes and cycloalkanes in the presence of superelectrophiles has been accomplished for the first time. The reactions of saturated hydrocarbons with I₂ in the presence of CCl₄·2AlI₃ at -20°C afforded monoiodides in good yields and selectivities.

Full text of DOI:10.1016/S0040-4039(01)02371-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1333–1335
First examples of superelectrophile initiated iodination of alkanes
and cycloalkanes
Irena Akhrem,* Alexander Orlinkov, Sergei Vitt and Anatolii Chistyakov
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow, GSP-1,
28 Vavilova st., Russia
Received 29 October 2001; revised 7 December 2001; accepted 13 December 2001
Abstract—Direct iodination of alkanes and cycloalkanes in the presence of superelectrophiles has been accomplished for the first
time. The reactions of saturated hydrocarbons with I2 in the presence of CCl4·2AlI3 at −20°C afforded monoiodides in good yields
and selectivities. © 2002 Elsevier Science Ltd. All rights reserved.
Elaboration of one-pot selective transformations of
alkanes and cycloalkanes into their functionalized
derivatives is a topic of current interest.1 In recent
years, topics for study have become not only thermody-
namically favorable reactions of alkanes but also those
occurring with consumption of energy.2–5 Iodination of
saturated hydrocarbons is an example of this type of
reaction.
The substitution of a hydrocarbon hydrogen by iodine
in electrophilic reactions has been feasible only for
aromatics.7–13 Most typically, iodinating systems were
based on I2 combined with oxidizing agents7–12 (Ag+
− 9
salts,7 (NH4)2S2O8,8 NO+ BF4 , Ce10) and iodosuccin-
imide in CF3SO3H.13 The latter system will even iodi-
nate
deactivated
aromatics
effectively.
The
CF3SO2OI+ [CF3SO3]− complex is assumed to be
responsible for the high activity of this system.13 Elec-
trophile-initiated iodination of alkanes and cycloalka-
nes has not been previously described.
The enthalpies of halogenation of propane, Et and Et0
(zero-point energy) in kcal/mol, which we have calcu-
lated by a DFT-PBE method,6 are as follows:
This work reports the first one-pot transformations of
alkanes and cycloalkanes into the corresponding
monoiodides in the presence of superelectrophilic sys-
tems, CX4·2AlI3 (X=Cl, Br). At −20°C propane,
mono-, bi- and tricycloalkanes react with I2 in the
presence of equimolar amounts of the CX4·2AlI3 com-
plexes in CH2Br2 to give the corresponding monoio-
dides in 50–80% yields after 1.5–2 h (Table 1).
C3H8+X2ꢀꢀꢀꢀꢀꢁiC3H7X+HX
Et=−105.3 (Et0=−106.5), X=F; −28.5 (−31.0), X=Cl;
−8.9 (−12.3), X=Br; 10.2 (3.3), X=I.
(1)
These calculations show that fluorination, chlorination
and bromination are exothermic with enthalpies
decreasing by 3- and 10-fold on passing from the
reaction with F2 to those with Cl2 and Br2, respectively.
In contrast, iodination is endothermic. Nevertheless,
the free-radical iodination of alkanes and cycloalkanes
has been performed in photo-induced reactions using
CCl ·2AlI
4
3
RH+I2ꢀꢀꢀꢀꢀꢀꢀꢀꢁRI
−20°C
RH=propane, cyclopentane, cyclohexane, norbornane,
adamantane.
t
either the BuCOI/HgCl2 system as a source of tert-
butylhypoiodide3, or a mixture of I2/Cl2 serving as a
donor of the ICl species.4 Recently, a promising
method has been reported for the preparation of
monoiodides from saturated hydrocarbons by the reac-
tion with CH2I2/solid NaOH.5
All reactions (except for the case of n-pentane) give rise
to a single isomer, while diiodides and bromides are
absent or produced in trace amounts only. The iodina-
tion of n-pentane is less effective, since it affords a 2:1
mixture of two isomers in a total yield of 19%. At
−40°C, the yields of monoiodides decreases to 10% and
at 0°C these reactions become unselective. At the stoi-
chiometric I2/CX4 ratio, CX4 is quantitatively reduced
into CHX3.
* Corresponding author. Fax:
ineos.ac.ru
7 (095) 1355085; e-mail: cmoc@
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02371-1

1334
I. Akhrem et al. / Tetrahedron Letters 43 (2002) 1333–1335
Table 1. Ionic iodination of alkanes and cycloalkanes (RH) with I2 in the presence of the CCl4·2AlI3 system in CH2Br2;
molar ratio of I2/CCl4=1, −20°C, 1.5–2 h
The systems CX4·2AlI3 (X=Cl, Br) turned out to be
unique since aluminum iodide alone (in the absence of
CX4) does not initiate iodination of alkanes and
cycloalkanes with I2. At the same time, the CCl4·2AlI3
complex itself (without I2) causes iodination of
cyclopentane, although the yield of cyclopentyl iodide is
two times lower as compared with that for the run with
I2+CCl4·2AlI3. The use of the CCl4·2AlBr3 complex
instead of CCl4·2AlI3 completely suppresses the iodina-
tion by I2 with the corresponding bromides becoming
the products of the reactions.
According to the above scheme, the CX3+ Al2I6Cl−
cationic complex14 abstracts hydride to afford a carbo-
cation. The latter immediately captures an iodide anion
either from the Al2I6Cl− anion or the I2 molecule.
Therefore, this reaction can be considered as a supere-
lectrophile-initiated nucleophilic substitution of an
alkane (cycloalkane) hydrogen by iodide. In this
respect, it is noteworthy that the above calculations
seem to exclude the alternative mechanism of the direct
electrophilic iodination, and in fact, the formation of
alkyl iodides was not observed in the reaction of alka-
nes with the I2/Al2I6 system.
The following scheme can be suggested for the iodina-
tion of alkanes and cycloalkanes.
(i) Generation of a carbocation:
Earlier we have found that I2·2AlBr3 can promote an
effective cracking of n-octane at 20°C.15 Aluminum
iodide is known to be the most powerful Lewis acid
among the aluminum halides.16 Therefore, the I2·2AlI3
complex obviously belongs to the category of superelec-
trophilic systems. However, the results of the present
work indicate that the two types of superelectrophilic
systems, i.e. CCl4·2AlI3 and Hal2·2AlI3, differ consider-
ably in activities at −20°C. The lower activity of
CCl4·2AlI3 compared with that of I2/CCl4·2AlI3 is prob-
ably due to the rapid I/Cl exchange between Al2I6 and
CCl4 (or its reduced forms). As a result, more favorable
chlorination occurs rather than iodination due to the
increased chlorine content in the mixed Al2InCl6−n spe-
cies. Similarly, the I2/CCl4·2AlBr3 system acts as a
brominating agent only.
AlI
3
AlI
3
CCl4ꢀꢀꢀꢀꢁCCl3 + AlI3Cl−ꢀꢀꢀꢀꢁCCl3 + Al2I6Cl−
RH+CCl3 + Al2I6Cl−ꢀꢀꢀꢀꢁR+ Al2I6Cl−+CCl3H
(ii) Iodination in the absence of I2:
R+ Al2I6Cl−ꢀꢀꢀꢀꢁRI+Al2I5Cl
(iii) Iodination in the presence of I2:

I. Akhrem et al. / Tetrahedron Letters 43 (2002) 1333–1335
1335
The DFT-PBE quantum-chemical calculations6 revealed
that, in contrast to the endothermic nature of reaction
(1),theiodinationofpropanebytheI2/CCl4·2AlI3 system,
reaction (2), becomes exothermic by −16.5 kcal/mol:
Kotz, K. T.; Jeston, J. S.; Wilkens, M.; Frei, H.; Bergman,
R. G.; Harris, C. B. Science 1997, 278, 260–263; (d)
Crabtree, R. H. Chem. Rev. 1995, 95, 987–1007; (e)
Sommer, J.; Bukala, J. Acc. Chem. Res. 1993, 26, 370–376;
(f) Akhrem, I. S.; Orlinkov, A. V. Russ. Chem. Bull. 1998,
47, 771–795.
C3H8+CCl4+Al2I6ꢀꢀꢀꢀꢁiC3H7I+CHCl3+Al2I5Cl
Et=−16.5 kcal/mol (Et0=−17.1).
(2)
2. Akhrem, I. S.; Vitt, S. V.; Churilova, I. M.; Orlinkov, A.
V. Russ. Chem. Bull. 1999, 48, 2279–2285.
3. Liquori, L.; Bjorsvik, H.-R.; Bravo, A.; Fontana, R.;
Miniski, F. J. Chem. Soc., Chem. Commun. 1997, 1501–
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In conclusion, the one-pot selective transformations of
propane and cycloalkanes into monoiodides by superelec-
trophiles has been performed for the first time.
1. Experimental
5. Schreiner, P. R.; Lauenstein, O.; Butova, E. D.; Fokin, A.
A. Angew. Chem., Int. Ed. 1999, 38, 2786–2788.
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‘PRIRODA’ (version 1-10) reported in (a) Perdew, J. P.;
Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 11,
3865–3868; (b) Laikov, D. DFT2000—Satellite Symp. 10th
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June 2000; (c) Laikov, D. N. Chem. Phys. Lett. 1997, 281,
151–156. Calculated total energies of molecules (Et a.u.)
participating in the reactions (1) and (2) are as follows: C3H8
CCl4 (0.13 g; 0.85 mmol) was added to a pre-cooled
(−30°C) stirred solution of 0.7 g (1.7 mmol) of anhydrous
AlI3 in 2.5 mL of CH2Cl2. Then 0.22 g (0.85 mmol) of
powdered iodine was introduced and the mixture was
allowed to warm to −20°C. After 10 min the reaction flask
was connected to a source of gaseous propane (or
alternatively, 6.0 mmol of the corresponding liquid
hydrocarbon was added to the reaction mixture). The
reaction mixture was stirred at −20°C for 1.5 h and was
then treated with a cooled aqueous solution of Na2SO3.
The products were carefully extracted with ether, the ether
extracts were washed with diluted NaHCO3 solution,
dried with CaCl2 and analyzed by gas chromatography
(GC) and GC-mass spectrometry (MS) using n-amyl
iodide (or n-heptyl iodide) as internal standards. For
nuclear magnetic resonance (NMR) studies, ether and
other light products were distilled from the ether extracts.
i
(119.0595); CCl4 (1884.0924); CHCl3 (1423.1972); C3H7F
(218.3801); iC3H7Cl (579.9753); iC3H7Br (2724.9965);
iC3H7I (7235.9160); F2 (199.6069); Cl2 (922.9777); Br2
(5213.0482); I2 (14234.9154); HF (100.4843); HCI
(462.1074); HBr (2607.1254); HI (7118.04320); Al2I6
(43130.6255); I2Al(ICl)AlI2 (36534.6905); I2AlI2AlClI
(36534.6904).
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575–590.
13. Olah, G. A.; Wang, Q.; Sandford, G.; Prakash, G. K. S.
J. Org. Chem. 1993, 58, 3194–3195.
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A.; Heiliger, L.; Prakash, G. K. S. J. Am. Chem. Soc. 1989,
111, 8020–8021; (b) Vancik, H.; Perkas, C.; Sunko, D. E.
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1.1. Selected spectral data
Mass spectra; m/z (I, %): 2-iodopropane; 170 (M+, 24),
127 (4), 43 (100), 41 (28), 27 (26); iodocyclopentane; 196
(M+, 8), 127 (20), 69 (100), 67 (57), 68 (17); iodocyclohex-
ane; 210 (M+, 7), 127 (9), 83 (100), 55 (87), 67 (50), 54
(34); 2-iodonorbornane: 254 (M+, 1), 127 (41), 95 (99),
94 (26), 79 (64), 78 (26), 67 (48), 66 (100); 1-iodoadaman-
tane: 135 (100), 127 (12), 93 (24), 79 (34), 77 (16), 67 (18),
55 (12). 13C NMR; l, ppm: 2-iodonorbornane, 15.16
(CHI), 28.37, 28.61, 36.21, 37.86 (CH2), 45.06, 47.84
(CH).
The calculations of polyhalomethyl cations and related
systems: (a) Chistyakov, A. L.; Stankevich, I. V.; Akhrem,
I. S.; Gambaryan, N. P.; Vol’pin, M. E. Russ. Chem. Bull.
1996, 45, 514–521; (b) Olah, G. A.; Rasul, G.; Yudin, A.;
Burrichter, A.; Prakash, G. K. S.; Chistyakov, A. L.;
Stankevich, I. V.; Akhrem, I. S.; Gambaryan, N. P.;
Vol’pin, M. E. J. Am. Chem. Soc. 1996, 118, 1446–1451;
(c) Akhrem, I. S.; Chistyakov, A. L.; Gambaryan, N. P.;
Stankevich, I. V.; Vol’pin, M. E. J. Organomet. Chem.
1997, 536, 489–494; (d) Olah, G. A.; Rasul, G.; Heiliger,
L.; Prakash, G. K. S. J. Am. Chem. Soc. 1996, 118,
3580–3583; (e) Franking, G.; Fau, S.; Marchandm, C. M.;
Grutzmacher, H. J. Am. Chem. Soc. 1997, 119, 6648–6655.
15. Akhrem, I. S.; Afanas’eva, L. V.; Orlinkov, A. V.; Vol’pin,
M. E. Mendeleev Commun. 1994, 1, 131–134.
Acknowledgements
This work was supported by the Russian Foundation of
Basic Research (Grant Nos. 99-03-33006 and 01-03-
32105).
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