Generation, structure, and reactivity of o-iodobenzoyloxyl radicals. Pulsed laser photolysis of 1-(o-halobenzoyloxy)-2-pyridones

Article abstract of DOI:10.1246/cl.2000.362

The transient absorption spectrum of o-iodobenzoyloxyl radicals was observed for the first time in acetonitrile using 1-(o-iodobenzoyloxy)-2-pyridone as a precursor. On the basis of the comparison of the spectra of o-halobenzoyloxyl radicals, it is proposed that the o-iodobenzoyloxyl radicals take a planar structure with a strong interaction between the radical center oxygen atom and the neighboring iodine atom.

Full text of DOI:10.1246/cl.2000.362

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62
Chemistry Letters 2000
Generation, Structure, and Reactivity of o-Iodobenzoyloxyl Radicals.
Pulsed Laser Photolysis of 1-(o-Halobenzoyloxy)-2-pyridones
†
Ji-ichiro Hashimoto, Katsunori Segawa, Hiroki Itoh, and Hirochika Sakuragi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571
Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, Yamagata 990-8560
†
(Received December 24, 1999; CL-991091)
3
The transient absorption spectrum of o-iodobenzoyloxyl
radicals was observed for the first time in acetonitrile using 1-
o-iodobenzoyloxy)-2-pyridone as a precursor. On the basis of
the comparison of the spectra of o-halobenzoyloxyl radicals, it
is proposed that the o-iodobenzoyloxyl radicals take a planar
structure with a strong interaction between the radical center
oxygen atom and the neighboring iodine atom.
this band, the o-chloro and o-bromo radicals exhibit another
band in the shorter wavelength regions (λmax 320 nm for X = Cl
and 390 nm for X = Br). These bands are specific for the o-X
radicals but not for p-X radicals, and their appearance can be
(
3
assigned to the o-X-O interaction.
ortho-Substituted benzoyloxyl radicals show intriguing
features in structure and reactivity depending on the sub-
stituents. o-Methylbenzoyloxyl radicals abstract a hydrogen
1
atom from the o-methyl group, whereas o-methylthiobenzoyl-
2
oxyl radicals form cyclic sulfur-centered radicals. These radi-
cals were generated from photochemical and thermal decompo-
sition of peroxides. We previously reported that o-bromoben-
zoyloxyl radicals have some interaction between the oxygen
3
radical center and the bromine atom, though o-iodobenzoyl-
oxyl radicals were proposed to form cyclic iodine-centered rad-
icals.4 In the present work we generated the o-iodobenzoyloxyl
radicals by laser photolysis of a pyridone derivative, which we
found to be a useful precursor, instead of the thermally unstable
peroxide, and observed their absorption spectrum indicating a
much stronger but nonbonded interaction between the oxygen
radical center and the iodine atom.
Pyridone derivatives were reported by Redmond et al. to be
5
precursors of hydroxyl and acyloxyl radicals. We employed
these derivatives in successful observations of various aroyl-
•
6
oxyl radicals such as p-MeSC H CO . Here, o-halobenzoyl-
6
4
2
•
oxyl radicals (o-XC H CO , X = F, Cl, Br, and I) were gener-
6
4
2
7
ated from pulsed laser photolysis of the corresponding 1-aroyl-
oxy-2-pyridones (o-XBPy, X = F, Cl, Br, and I) and diaroyl
peroxides (o-XBPO, X = F, Cl, and Br) in acetonitrile, and their
behavior was examined by means of transient absorption spec-
troscopy (Figures 1 and 2).
The spectra from o-XBPy also show longer wavelength
•
bands due to o-XC H CO , and in the shorter wavelength re-
6
4
2
gion around 400 nm the band due to 2-pyridyloxyl radicals
•
overlaps those of o-XC H CO ; in particular, for X = Br
6
4
2
(Figure 2) the spectrum obtained by subtracting the band of the
2
-pyridyloxyl radicals (Figure 3) was in good agreement with
•
the spectrum of o-BrC H CO in Figure 1. The decay time
6
4
2
In Figure 1, the transient absorption spectra of o-XC H -
constants of the spectra observed for o-XBPO and o-XBPy (X
= F, Cl, and Br) were essentially identical with each other as
determined by monitoring the time profiles at 760 nm (2.4 and
2.2 µs for X = F, 0.83 and 0.78 µs for X = Cl, and 0.91 and
6
4
CO2• generated from o-XBPO are compared; all three kinds of
the radicals exhibit an absorption band characteristic of the
8
,9
aroyloxyl radicals in the 500–800-nm region.
In addition to
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
363
tion increases in the order of X = Cl < Br < I, since the longer
•
wavelength band is attributed to the free CO2 moiety. On the
•
basis of these observations, we propose that the o-IC H CO
6
4
2
radicals take a planar structure and that the interaction between
the radical center and the neighboring iodine atom is much
stronger than those observed for other o-halobenzoyloxyl radi-
•
cals and, as a result, the decarboxylation of o-IC H CO is
6
4
2
completely suppressed in acetonitrile. The strong O-I interac-
•
tion in o-IC H CO with a planar geometry was supported by
6
4
2
1
5
MO calculations. Thus, in the optimized geometry calculated
at the UHF/3-21G(*) level, the CO moiety was coplanar to the
.
2
aromatic plane and the spin density was mainly located on the
O atom directed to the I atom. Moreover, the Mulliken charge
was positive on the I atom (0.174) but negative on the O atom
(
–0.346). In contrast, for the o-Cl and o-Br radicals the
Mulliken charges on the halogen atoms were very low (0.094
on Cl and 0.022 on Br), although the negative charges on the O
atom were nearly the same as that of the o-I radicals. A much
larger difference in the Mulliken charges between the I and O
atoms was predicted at the UB3LYP/ LanL2DZ level. These
results indicate that the electrostatic interaction between the two
atoms plays an important role in the structure and reactivity of
0
.91 µs for X = Br, when generated from o-XBPO and o-XBPy,
respectively).
Previously, Leffler et al. proposed that the o-IC H CO
4
•
6
4
2
radicals formed trivalent-iodine-centered radicals, based on
isolation of a cyclic rearrangement product with a trivalent
iodine atom in the thermolysis of o-IBPO in solution. In the
present work we used a pyridone derivative, o-IBPy, to gener-
ate these radicals and examine their structure and reactivity,
since o-IBPO was too unstable to be employed as a radical pre-
cursor in photolysis in solution. Pulsed laser photolysis of o-
IBPy in acetonitrile exhibited an absorption spectrum similar to
that from o-BrBPy (Figure 2). When the band due to the 2-
pyridyloxyl radicals was subtracted, there remained an absorp-
tion band at 350–450 nm and a band with low but definite
absorbances in the 500–800-nm region (Figure 3); the peak
•
o-IC H CO .
6
4
2
References and Notes
1
J. Wang, M. Tsuchiya, T. Tateno, H. Sakuragi, and K.
Tokumaru, Chem. Lett., 1992, 563; J. Wang, M. Tsuchiya, K.
Tokumaru, and H. Sakuragi, Bull. Chem. Soc. Jpn., 68, 1213
(1995).
2
3
C. W. Perkins, J. C. Martin, A. J. Arduengo, W. Lau, A.
Alegria, and J. K. Kochi, J. Am. Chem. Soc., 102, 7753 (1980).
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Iwaizumi, and S. Yamauchi, Chem. Phys. Lett., 232, 278
(1995).
1
0
wavelength may be longer than 800 nm. These bands can be
attributed to o-IC H CO radicals. On the contrary, the radi-
cals generated from the photolysis of o-MeSBPy in acetonitrile
exhibit no absorbances in the longer wavelength region
•
6
4
2
4
5
6
7
J. E. Leffler, R. D. Faulkner, and C. C. Petropoulis, J. Am.
Chem. Soc., 80, 5435 (1958).
B. M. Aveline, I. E. Kochevar, and R. W. Redmond, J. Am.
Chem. Soc., 117, 9699 (1995); 118, 10113 and 10124 (1996).
J. Hashimoto, K. Segawa, and H. Sakuragi, Chem. Phys. Lett.,
(Figures 2 and 3), because of their cyclic S-O bonded structure
•
2
(
absence of the ArCO2 structure).
The main products in photolysis of o-IBPy were o-
iodobenzoic acid (0.86 mol/mol o-IBPy) and 2-hydroxypyri-
3
14, 261 (1999).
Laser-flash photolyses were performed using an excimer laser
Lambda Physik LPX-100, XeCl, 308 nm, 10-ns fwhm, 70
1
1
dine (0.91), but iodobenzene was not detected at all. The
product distribution in the photolysis of o-IBPy was apparently
different from those of other o-XBPy’s which gave significant
amounts of the decarboxylation products (for example, for X =
Br and Cl, o-XC H CO H/C H X = 0.41/0.43 and 0.29/0.67,
(
mJ/pulse) and a pulsed xenon arc (Wacom KXL-151, 150 W) as
a monitoring light source.
8
9
1
H. Misawa, K. Sawabe, S. Takahara, H. Sakuragi, and K.
Tokumaru, Chem. Lett., 1988, 357.
J. Chateauneuf, J. Lusztyk, and K. U. Ingold, J. Am. Chem.
6
4
2
6
5
respectively).11 In addition, the activation energy for disap-
pearance of o-IC H CO (11.0 kJ mol ; log(A/s ) = 8.5) is
in the range of those for hydrogen-atom abstraction by benzoyl-
oxyl radicals: 13 kJ mol (log(A/s ) = 7.5) for hydrogen
abstraction from solvent acetonitrile by p-MeSC H CO and
1
abstraction of o-MeC H CO , compared with 36–38 kJ mol
for disappearance of o-ClC H CO
.
–1
–1
12
6
4
2
Soc., 110, 2877 (1988).
0
S. Oishi, H. Tsubaki, and H. Matsuzawa, Chem. Lett., 1999,
805.
–
1
–1
•
6
–3
–3
11 o-XBPy’s (3–4×10 mol dm ) were irradiated with 300-nm
continuous light in acetonitrile under argon at ambient tempera-
ture.
6
4
2
7 kJ mol–1 (log(A/ s ) = 10.5) for intramolecular hydrogen
–1
•
1
–1
6
4
2
•
•13
•
12 The decay profiles of transient absorption due to o-IC H CO
and o-BrC H CO .
6
4
2
6
4
2
6
4
2
were monitored at 700 nm in the range of 7.5–34.2 °C under
argon in acetonitrile.
13 J. Wang, M. Tsuchiya, H. Sakuragi, K. Tokumaru, and H. Itoh,
Tetrahedron Lett., 35, 6321 (1994).
We previously observed that the benzoyloxyl radicals with
•
a twisted CO2 moiety from the phenyl ring exhibit the longer
wavelength band maximum in the region shorter than 800 nm.
However, the present o-XC H CO radicals show no band
maximum in such a region, indicating the planar structure of
the radicals. The intensity of the longer wavelength band rela-
tive to the shorter wavelength band decreases in the order of X
1
4
•
6
4
2
14
J. Wang, H. Itoh, M. Tsuchiya, K. Tokumaru, and H. Sakuragi,
Tetrahedron, 51, 11967 (1995).
1
5
All calculations were performed with Gaussian 98, Revision
A.6, Gaussian, Inc., Pittsburgh, PA, 1998.
=
Cl > Br > I. This observation indicates that the O-X interac-