A new direct homolytic iodination reaction of alkanes by perfluoroalkyl iodides

Article abstract of DOI:10.1039/a703332j

A new method for the direct free-radical chain iodination of alkanes by perfluoroalkyl iodides is described.

Full text of DOI:10.1039/a703332j

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A new direct homolytic iodination reaction of alkanes by perfluoroalkyl iodides
Lucia Liguori, Hans-Rene´ Bjørsvik, Anna Bravo, Francesca Fontana and Francesco Minisci*
Dipartimento di Chimica del Politecnico, via Mancinelli 7, I-20131 Milano, Italy
A new method for the direct free-radical chain iodination of
alkanes by perfluoroalkyl iodides is described.
from THF, whose a-position is particularly activated by both
enthalpic and polar [eqn. (3)] factors, has been evaluated1 at 6.1
3 105 m21 s21. On the other hand, alkyl radicals easily abstract
iodine atom from Rf–I, as shown by the effective free-radical
addition of Rf–I to alkenes2 [eqns. (4) and (5)], with the polar
effect playing a significant role.
The direct free-radical iodination of alkanes by iodine, unlike
the other halogens, is not feasible owing to its large positive
enthalpy of hydrogen abstraction [eqn. (1)]. The rates of
Thus, the conditions of alkane iodination according to the
radical chain of eqns. (6) and (7) appeared to be fulfilled.
Iodocyclohexane was obtained in 70% yield, based on C4F9I,
by refluxing a solution of cyclohexane and C4F9I in acetic acid
in the presence of catalytic amounts of ButOOH and Fe(OAc)3.
The reaction is explained by the radical chain of eqns. (6) and
(7), initiated by a redox system [eqns. (8)–(10)] and represents
a new simple, direct iodination of alkanes; we believe3 eqn. (8)
to be operating in this case, rather than eqn. (11).
In order to probe the reactivity–selectivity relationship of the
reaction, an investigation was carried out with heptane,
adamantane and 1-chlorohexane; heptane gave a mixture of all
the iodoheptane isomers (Table 1, including a comparison with
chlorination by N-chloroamines).
∆H ~ 20–33 kcal mol–1 (1)
+ I•
R•
+
R
H
H
+
H
I
+ Rf•
R•
+
R
(2)
(3)
Rf
H
‡
Rf
H
δ–
Rf•
+
Rf
H
δ+
O
O
H
•
O
Rf•
+
(4)
(5)
Rf
C
C
C•
‡
Rf•
+
δ+
δ–
I
Rf
Enthalpic and steric effects contribute to the regioselectivity:
the CH2 groups in positions 3 and 4 have substantially the same
reactivity, while the CH2 in position 2 is more reactive due to
steric factors and the methyl group is less reactive for enthalpic
reasons. The behaviour is qualitatively similar to that for
chlorination by protonated N-chloroamines4 (Minisci chlorina-
+
Rf
C
I
Rf
Rf
C C•
Rf
C
C
I
+ R•
H
Rf•
+
(6)
H
I
R
Rf
tion),5 in which the selectivity is determined by hydrogen
+
Rf•
+
(7)
(8)
·
R•
+
abstraction by R2NH .
Rf
R
I
The free-radical reactivity of adamantane is interesting
because the selectivity of hydrogen abstraction is little affected
by enthalpic factors, but it is greatly influenced by polar effects6
ButOOH + FeIII
ButO•
ButO• + OH – + FeIV
(high selectivity with R2NH+ 6 and ArC(O)OO 7 radicals and in
electron-transfer processes8). The yields obtained according to
eqns. (6) and (7) (84.7% 1-iodo- and 15.3% 2-iodo-ada-
·
·
•
(9)
CH3 + MeC(O)Me
•
•
(10)
(11)
+ CH3
C4F9
I
CH3
+
I
C4F9
Table 1 Iodination of heptane
Iodination per hydrogen (%)a
Iodinating
ButOO• + H+ + FeII
ButOOH + FeIII
agent
1-iodo
2-iodo
3-iodo
4-iodo
hydrogen abstraction from C–H bonds [eqn. (2)] by perfluoro-
3
·
alkyl radicals (Rf ) are > 10 times larger than those of the
C4F9I
Me2NHCl+
Bui2NCl
1 (4.1)
1
1
18.7 (51.0)
75.8
75.3
11.1 (30.3)
39.5
26.4
10.7 (14.6)
39.0
26.1
analogous hydrocarbon radicals.1 Enthalpic (the higher energy
of Rf–H compared to R–H bonds) and polar (the electrophilic
·
character of Rf ) effects influence the rates of hydrogen
·
abstraction. Thus, the rate of hydrogen abstraction by C7F15
a Figures in brackets indicate the yield of each isomer.
Table 2 Iodination of 1-chlorohexane
Iodination per hydrogen (%)a
Iodinating
agent
1-iodo
2-iodo
3-iodo
4-iodo
5-iodo
6-iodo
C4F9I
Me2NHCl+
Cl2
0.2 (2.1)
—
0.09
0.3 (3.5)
—
0.4
1 (12.0)
1
1
2.2 (26.7)
2.9
1.1
4.2 (50.0)
10.5
1.2
0.3 (5.7)
0.2
0.5
a Figures in brackets indicate the yield of each isomer.
Chem. Commun., 1997
1501

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mantane) confirm the expected influence of the polar effect, due
Footnote and References
·
to the electrophilic character of C4F9 , the higher stability of
* E-mail: fontana@dept.chem.polimi.it
1-adamantyl+ with respect to its (secondary) 2-isomer9 and the
oxidation potential of adamantane (Eox = 2.72 V vs. SCE),
which is lower than that of 2,3-dimethylbutane (Eox = 3.45 V
vs. SCE).8
1 W. R. Dolbier, Jr., Chem. Rev., 1996, 96, 1557.
2 M. Yoshida, N. Kamigata, H. Sawada and M. Nakayama, J. Fluorine
Chem., 1990, 49, 1.
3 F. Minisci, E. Vismara and F. Fontana, J. Org. Chem., 1989, 54, 5224;
F. Minisci, F. Fontana, S. Araneo, F. Recupero, S. Banfi and S. Quici,
J. Am. Chem. Soc., 1995, 117, 226; F. Minisci, F. Fontana, S. Araneo,
F. Recupero and L. Zhao, Synlett, 1996, 119.
4 R. Bernardi, R. Galli and F. Minisci, J. Chem. Soc. B, 1968, 324.
5 N. C. Deno, Methods in Free Radical Chemistry, ed. E. S. Huyser,
Marcel Dekker, New York, 1975, p. 143.
The iodination of 1-chlorohexane, reported in Table 2 and
compared with chlorination by Me2NHCl+ and Cl2, further
supports our ideas about the influence of enthalpic and polar
·
effects in hydrogen abstraction by C4F9 .
The s-inductive and field effects of the chlorine atom in
1-chlorohexane are significant in the iodination, but they
are lower than in chlorination by Me2NHCl+. This result
6 F. Minisci, F. Fontana, L. Zhao, S. Banfi and S. Quici, Tetrahedron
Lett., 1994, 35, 8033.
·
suggests that the increase in hydrogen abstraction rates for Rf
compared to the analogous hydrocarbon radicals must be
mainly ascribed to the enthalpic effect (higher energy of the Rf–
H bonds), the polar effect being a significant but minor
factor.
7 A. Bravo, H. R. Bjørsvik, F. Fontana, F. Minisci and A. Serri, J. Org.
Chem., 1996, 61, 940.
8 F. Mella, M. Freccero, T. Soldi, E. Fasani and A. Albini, J. Org. Chem.,
1996, 61, 1413.
9 G. A. Olah, G. Liang and G. D. Mateesen, J. Org. Chem., 1974, 39,
3750.
10 A. Bravo, H. R. Bjørsvik, F. Fontana, L. Liguori and F. Minisci, J. Org.
Chem., in the press.
We have recently10 reached the same conclusion concerning
·
the increase in addition rates of Rf to aromatic substrates,
compared to addition of alkyl radicals, which is due mainly to
the enthalpic effect, with a significant but minor polar
contribution.
Received in Liverpool, UK, 14th May 1997; 7/03332J
1502
Chem. Commun., 1997