Generation and behavior of Naphthoyloxyl radicals in photocleavage of 1-(Naphthoyloxy)-2-pyridones

Article abstract of DOI:10.1246/cl.2001.1064

A series of 1- and 2-naphthoyloxyl radicals are generated from photocleavage of 1-(naphthoyloxy)-2-pyridones in acetonitrile. Introduction of a methoxy group in the naphthalene ring stabilized the naphthoyloxyl radicals to prevent decarboxylation completely and reduced remarkably their reactivities of addition to olefins and hydrogen-atom abstraction.

Full text of DOI:10.1246/cl.2001.1064

1
064
Chemistry Letters 2001
Generation and Behavior of Naphthoyloxyl Radicals in Photocleavage
of 1-(Naphthoyloxy)-2-pyridones
Toshihiro Najiwara, Katsunori Segawa, and Hirochika Sakuragi*
Department of Chemistry, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571
(Received August 6, 2001; CL-010754)
A series of 1- and 2-naphthoyloxyl radicals are generated
from photocleavage of 1-(naphthoyloxy)-2-pyridones in aceto-
nitrile. Introduction of a methoxy group in the naphthalene ring
stabilized the naphthoyloxyl radicals to prevent decarboxylation
completely and reduced remarkably their reactivities of addition
to olefins and hydrogen-atom abstraction.
Diaroyl peroxides have been prevailing precursors of
aroyloxyl radicals, for which still new aspects have been pre-
sented.1 We previously reported that 1- and 2-naphthoyloxyl
–3
radicals (NpCO •) are more stabilized by conjugate electron
2
4
delocalization than benzoyloxyl radicals (PhCO •). Very
2
recently we found a similar but more marked conjugation effect
in the lower reactivity of anthroyloxyl radicals generated photo-
5
,6
chemically from oxime esters.
However, effects of sub-
stituents on the reactivity of naphthoyloxyl and anthroyloxyl
radicals have not been studied. In this work, we studied the
reactivity of unsubstituted and methoxy-substituted naphthoyl-
oxyl radicals generated from 1-(naphthoyloxy)-2-pyridones by
means of stationary photolysis and transient absorption spec-
troscopy.
for some methoxylated naphthoyloxypyridones, such as 1-(2-
methoxy-1-naphthoyloxy)- and 1-(6-methoxy-2-naphthoyloxy)-
2-pyridones (2MeO-1NPy and 6MeO-2NPy, respectively). As
listed in Table 1, the corresponding acids and 2-pyridone were
obtained as products; no decarboxylation products were detect-
ed for any (methoxynaphthoyloxy)pyridones employed. These
observations indicate that the introduction of a methoxy group
exerts a much more remarkable effect on radical stabilization in
Stationary irradiation of 1-(1-naphthoyloxy)- and 1-(2-
naphthoyloxy)-2-pyridones (1NPy and 2NPy, respectively; 2 ×
–
4
–3
1
0
mol dm ) with 300-nm light (Layonet RPR-3000 lamps)
in acetonitrile at ambient temperature gave naphthalene, naph-
thoic acids, and 2-pyridone. The products are listed in Table 1,
together with those from di(1-naphthoyl) and di(2-naphthoyl)
peroxides (1NPO and 2NPO, respectively) on being photolyzed
under similar conditions. In these cases small amounts of naph-
thyl naphthoates (esters) were also obtained as geminate prod-
ucts. The efficiency of decarboxylation of the naphthoyloxyl
radicals, as reflected in the ratio of naphthalene to acid, seems
to be highly different between the precursors though the ratios
are larger in the 2-naphthoyloxyl precursors. The difference in
product distribution may be ascribed to contribution of two-
bond cleavage in the peroxide decomposition in the singlet
state.2 For the pyridones, the homolysis of the N–O bond
takes place from the excited singlet state attributed to the pyri-
done moiety to produce a naphthoyloxyl radical together with a
8
,9
naphthoyloxyl radicals than in benzoyloxyl radicals.
–
4
–3
Pulsed laser photolysis of 1NPy (8 × 10 mol dm ) and
–
4
–3
2NPy (4 × 10 mol dm ) at 308 nm under argon in acetonitrile
at ambient temperature exhibited essentially identical absorp-
tion spectra (600–800 nm) with those observed for 1NPO and
4
2NPO, respectively, except for superposition of that of 2-
1
0,11
pyridyloxyl radicals around 390 nm.
Lifetimes of the tran-
sients were independent of the precursors as monitored at 780
nm for both 1- and 2-naphthoyloxyl radicals; 2.4 µs for 1NPy,
2.5 µs for 1NPO, 0.47 µs for 2NPy, and 0.45 µs for 2NPO.
These results indicate that behavior of the naphthoyloxyl radi-
cals is not affected by the pair radicals at all.
The (methoxynaphthoyloxy)pyridones were photolyzed
under similar conditions. Figures 1 and 2 show absorption
spectra observed for 2MeO-1NPy and 6MeO-2NPy, respective-
ly. All the five pyridones employed exhibited transient absorp-
tion spectra assignable to the corresponding naphthoyloxyl radi-
cals. Their lifetimes (6–18 µs) were much longer than those of
the unsubstituted parent radicals (Table 2). The activation
parameters for disappearance of the naphthoyloxyl radicals in
acetonitrile were obtained from the first-order decay rates deter-
mined at 18–50 °C by monitoring the transient absorptions at
780 nm (Table 2); the activation enthalpies for 1-NpCO2• and
2-NpCO2• are among the typical values for decarboxylation, as
,7
2-pyridyloxyl radical; however, for the peroxides there are two
modes of bond cleavage, a simple O–O bond cleavage giving
two naphthoyloxyl radicals and a simultaneous or successive
O–O plus C(ipso)–C(α) bond cleavage producing a naphthoyl-
oxyl and a naphthyl radical in the solvent cage. When we
assume that the free naphthoyloxyl radicals should give naph-
thoic acid and naphthalene with the same ratios as observed for
the pyridones, the higher ratios of the decarboxylation product
in the peroxide photolysis could be ascribed to the contribution
of the two-bond cleavage.
Product studies were also made under similar conditions
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1065
NpCO • with different substrates were evaluated from pseudo-
2
first-order decay rates: kexptl = k + k [substrate], where k refers
0
2
0
to first-order reactions by which the radical decays at zero sub-
strate concentration. In Table 3 the k values are compared with
2
those of benzoyloxyl and anthroyloxyl radicals. Table 3 shows
that the methoxynaphthoyloxyl radicals are much less reactive in
hydrogen-atom abstraction (PhMe, c-C H , and c-C H ) and
6
12
6
10
addition to olefins and aromatics (c-C H , PhH, and PhMe) than
6
10
the parent naphthoyloxyl radicals as well as benzoyloxyl radicals,
and almost comparable with the anthroyloxyl radicals. These
facts indicate that the methoxy group stabilizes the naphthoyloxyl
radicals through the electron donating effect (Scheme 1) to an
extent similar to that affected by conjugation of the anthracene
moiety, and the effect as a substituent is more prominent in the
naphthoyloxyl radicals than in the benzoyloxyl radicals.
References and Notes
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1
compared to that of PhCO •. However, the smaller values for
the methoxynaphthoyloxyl radicals can be ascribed to the
2
process of hydrogen-atom abstraction in accord with the prod-
uct distributions.11,12
Bimolecular rate constants, k , for the reactions of the
2
methoxynaphthoyloxyl radicals as well as 1-NpCO • and 2-
2