Behavior of naphthoyloxyl and methoxynaphthoyloxyl radicals generated from the photocleavage of dinaphthoyl peroxides and 1-(naphthoyloxy)-2-pyridones

Article abstract of DOI:10.1246/bcsj.76.575

1-Naphthoyloxyl and 2-naphthoyloxyl radicals were generated from photocleavage of dinaphthoyl peroxides and 1-(naphthoyloxy)-2-pyridones in acetonitrile. The difference in product distribution between the precursors is ascribed to the contribution of the two-bond cleavage in the peroxide decomposition in the singlet state. A series of methoxynaphthoyloxyl radicals were also generated from the corresponding (methoxynaphthoyloxy)pyridones and their behavior was compared with that of unsubstituted naphthoyloxyl radicals. The introduction of a methoxy group in the naphthalene ring stabilizes the naphthoyloxyl radicals to prevent their decarboxylation completely and reduces remarkably their reactivities in the addition to olefins and hydrogen-atom abstraction. The structure of the naphthoyloxyl radicals was discussed on the basis of their absorption spectra and MO calculations.

Full text of DOI:10.1246/bcsj.76.575

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 575–585 (2003)
575
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Behavior of Naphthoyloxyl and Methoxynaphthoyloxyl Radicals
Generated from the Photocleavage of Dinaphthoyl Peroxides and
1-(Naphthoyloxy)-2-pyridones
Behavior of Naphthoyloxyl Radicals
T. Najiwara et al.
03
5
Toshihiro Najiwara, Ji-ichiro Hashimoto, Katsunori Segawa, and Hirochika Sakuragi*
5
Department of Chemistry, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571
(Received September 4, 2002)
SJA8
09-2673
254
1-Naphthoyloxyl and 2-naphthoyloxyl radicals were generated from photocleavage of dinaphthoyl peroxides and
1-(naphthoyloxy)-2-pyridones in acetonitrile. The difference in product distribution between the precursors is ascribed
to the contribution of the two-bond cleavage in the peroxide decomposition in the singlet state. A series of methoxy-
naphthoyloxyl radicals were also generated from the corresponding (methoxynaphthoyloxy)pyridones and their behavior
was compared with that of unsubstituted naphthoyloxyl radicals. The introduction of a methoxy group in the naphtha-
lene ring stabilizes the naphthoyloxyl radicals to prevent their decarboxylation completely and reduces remarkably their
reactivities in the addition to olefins and hydrogen-atom abstraction. The structure of the naphthoyloxyl radicals was
discussed on the basis of their absorption spectra and MO calculations.
02
02
Diaroyl peroxides have been useful precursors of aroyloxyl
radicals, for which still newer aspects have been presented.1–3
Radical precursors other than peroxides provide some merits
such as thermal stability, availability, and low reactivity of pair
radicals.4,5 We previously reported behavior of 1- and 2-naph-
·
thoyloxyl radicals (NpCO2 ), which are more stabilized than
·
benzoyloxyl radicals (PhCO2 ) by conjugate electron delocal-
ization.6 Very recently we found a similar but more marked
conjugation effect in the lower reactivity of anthroyloxyl (an-
thracenecarbonyloxyl) radicals, which were generated photo-
chemically from oxime esters.5,7,8 Decarboxylation of benzo-
yloxyl radicals is accelerated by introduction of methyl groups
or chlorine atoms in both of the ortho-positions, resulting in
twisting of the carbonyloxyl group from the molecular plane
because of steric hindrance.9 On the contrary, the decarboxyl-
ation rate of p-methoxybenzoyloxyl radicals is significantly re-
duced, compared with unsubstituted benzoyloxyl radicals, in
particular, in polar solvents.10,11 The decarboxylation was in-
hibited in o-iodobenzoyloxyl radicals by the interaction be-
tween the carbonyloxyl moiety and the iodine atom, as studied
by employing 1-aroyloxy-2-pyridones as radical precursors.4
However, effects of substituents on the reactivity of naphthoyl-
oxyl and anthroyloxyl radicals have not been studied. In this
work, we studied the reactivity of unsubstituted and methoxy-
substituted naphthoyloxyl radicals generated from dinaphthoyl
peroxides and 1-(naphthoyloxy)-2-pyridones by means of sta-
tionary photolysis and transient absorption spectroscopy.12
Results
Fig. 1. Absorption spectra of dinaphthoyl peroxides (a) and
naphthoyloxy- and benzoyloxypyridones (b) in acetoni-
trile.
1. Absorption and Fluorescence Spectra.
Absorption
spectra of di(1-naphthoyl) and di(2-naphthoyl) peroxides (1-
NPO and 2-NPO, respectively, Scheme 1) in acetonitrile, as
depicted in Fig. 1a, are very similar to those of the correspond-
ing methyl naphthoates. The absorption coefficients at the ab-
sorption maxima of 1-NPO around 300 and of 2-NPO around

576 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Behavior of Naphthoyloxyl Radicals
Table 1. Product Yields in 300-nm Stationary Photolyses of Dinaphthoyl Peroxides, Naphthoyloxy-,
and Methoxynaphthoyloxypyridones in Acetonitrile
Product yield (mol/mol precursor)
Precursor
1-NPO
2-NPO
1-NPy
Acid (A)
0.63
0.19
0.49
0.19
0.93
0.95
0.70
0.59
Naphthalene (N)
0.95
Ester
0.08
0.07
—
Pyridone
—
N/(A + N)
0.60
0.89
0.23
0.71
0
0
0
0
0
1.59
0.15
—
0.72
0.54
0.62
0.85
0.84
0.87
0.85
2-NPy
0.47
—
2-MeO-1-NPy
4-MeO-1-NPy
1-MeO-2-NPy
3-MeO-2-NPy
6-MeO-2-NPy
none
—
none
—
none
—
none
—
0.70
none
—
in Table 1, are larger in the photolyses of 1-NPO and 2-NPO
than in the cases of 1-NPy and 2-NPy, respectively, though the
2-naphthoyloxyl precursors give the larger values than the 1-
naphthoyloxyl precursors, respectively.
Product studies were also performed under similar condi-
tions for (methoxynaphthoyloxy)pyridones (MeONPys, 4 ×
10−4 mol dm−3), such as 1-(2-methoxy-1-naphthoyloxy)- (2-
MeO-1-NPy),
1-(4-methoxy-1-naphthoyloxy)- (4-MeO-1-
NPy), 1-(1-methoxy-2-naphthoyloxy)- (1-MeO-2-NPy), 1-(3-
methoxy-2-naphthoyloxy)- (3-MeO-2-NPy), and 1-(6-meth-
oxy-2-naphthoyloxy)-2-pyridones (6-MeO-2-NPy) (Scheme
1). As listed in Table 1, the corresponding acids and 2-pyri-
done were obtained as products, but no decarboxylation prod-
ucts (i.e., methoxynaphthalenes) were detected for any MeO-
NPys employed. These observations indicate that the intro-
duction of a methoxy group displays a more remarkable effect
on radical stabilization in naphthoyloxyl radicals than in ben-
zoyloxyl radicals,10,11 as discussed below.
3. Transient Absorption Spectra. Pulsed laser photolysis
of 1-NPy (8 × 10−4 mol dm−3) and of 2-NPy (4 × 10−4
mol dm−3) at 308 nm under argon in acetonitrile at ambient
temperature exhibited essentially identical absorption spectra
(600–800 nm) with those observed for 1-NPO and 2-NPO (4
× 10−4 mol dm−3), respectively, under similar conditions, ex-
cept for overlapping of the band due to 2-pyridyloxyl radicals
around 390 nm,4,14 as shown in Figs. 2a and 3a, respectively.
Lifetimes of the transients were independent of the precursors,
as monitored at 780 nm for both 1- and 2-naphthoyloxyl radi-
cals (Figs. 2b, 2c, 3b, and 4b): 2.4 and 2.5 µs for 1-NPy and 1-
NPO, respectively, and 0.47 and 0.45 µs for 2-NPy and 2-
NPO, respectively. These results indicate that the dynamic be-
havior of the naphthoyloxyl radicals is not affected by the pair
radicals. Similar phenomena were observed in the laser pho-
tolysis of unsubstituted and substituted dibenzoyl peroxides
and benzoyloxypyridones.3,4
Scheme 1.
330 nm are nearly 1.5 and 2 times larger than those of the cor-
responding esters. In the absorption spectra of 1-(1-naphthoyl-
oxy)- and 1-(2-naphthoyloxy)-2-pyridones (1-NPy and 2-NPy,
respectively) in acetonitrile (Fig. 1b), a band due to 2-pyridone
moiety is located around 300 nm, in addition to those of the
naphthoyloxyl moieties, as indicated from the spectrum of 1-
benzoyloxy-2-pyridone (BPy).
Methyl 1- and 2-naphthoates exhibit a broad and a struc-
tured fluorescence spectrum, respectively, with an emission
maximum around 360 nm in acetonitrile. The fluorescence
quantum yields were determined to be 0.15 and 0.21 for the 1-
and 2-naphthoate, respectively, by comparison with naphtha-
lene as a standard (0.21).13 On the contrary, 1-NPO and 2-
NPO exhibit only a very weak fluorescence spectrum with an
emission maximum around 350 nm. Actually, the fluorescence
quantum yields of the peroxides are 5 to 10 times lower (0.03
for 1-NPO and 0.02 for 2-NPO) than those of the correspond-
ing methyl esters.
2. Product Studies. 1-NPO and 2-NPO (1 × 10−4 mol
dm−3) were photolyzed almost completely with 300-nm sta-
tionary light in acetonitrile at ambient temperature to give
naphthalene, the corresponding naphthoic acid, and naphthyl
naphthoate as a geminate product. The products and their
yields, as determined by HPLC analyses, are listed in Table 1.
Similar irradiation of 1-NPy and 2-NPy (2 × 10−4 mol dm−3)
gave naphthalene, naphthoic acid, and 2-pyridone (Table 1).
The relative yields of naphthalene, as evaluated from the ratio
of naphthalene (N) to the sum of naphthalene (N) and acid (A)
In the transient absorption spectrum from 1-NPO, the decay
time constant at 440 nm was identical to that measured at 780
nm; however, the decay curve monitored at 380 nm was not
able to be analyzed by a single exponential, indicating that the
440 nm band can be ascribed to 1-naphthoyloxyl radicals, but
that there is an overlapping of other transient species in the 380
nm region. No rise components were detected under argon or
oxygen in the wavelength region employed.
In the laser photolysis of 2-NPO, a component was observed

T. Najiwara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 577
Fig. 3. Transient absorption spectra observed 117 and 140 ns
after pulsed laser excitation of 2-NPO (ꢂ) and 2-NPy (ꢃ),
respectively, at 308 nm in acetonitrile under argon (a), and
a decay profile of the spectrum from 2-NPy monitored at
780 nm (b).
shown in Fig. 4a, as obtained just by subtracting the spectrum
under argon (A) from that under oxygen (B). The spectrum in
the 400-800 nm region is in good agreement with that reported
for 2-naphthyldioxyl radicals in the pulse radiolysis of 2-bro-
monaphthalene in methanol under air.15
Pulsed laser photolyses of the peroxides and the pyridones
were also carried out in the presence of triplet quenchers. The
initial spectral intensities observed on laser photolyses of 1-
NPO and 2-NPO (4 × 10−4 mol dm−3) in acetonitrile were not
affected by 1,3-pentadiene (0.005–0.26 mol dm−3), as moni-
tored at 780 nm, though the decay rates of the spectra were ac-
celerated by the diene (vide infra). Molecular oxygen (0.002–
0.009 mol dm−3) also did not affect the spectral intensity at
780 nm. Similar phenomena were observed in laser photoly-
ses of 1-NPy and 2-NPy (4 × 10−4 and 8 × 10−4 mol dm−3,
respectively) in the presence of 1,3-pentadiene (0.012–0.75
mol dm−3) or oxygen (0.002–0.009 mol dm−3) under similar
conditions. These results indicate that the present precursors
are decomposed from the excited singlet state on direct irradia-
tion.
Fig. 2. Transient absorption spectra observed 234 ns after
pulsed laser excitation of 1-NPO (ꢀ) and 1-NPy (ꢁ) at
308 nm in acetonitrile under argon (a), and time profiles of
the spectra from 1-NPO (b) and 1-NPy (c) monitored at
780 nm.
to grow at the 360 nm region concomitantly with the decay of
the band due to 2-naphthoyloxyl radicals (Figs. 4a and 4b).
This component may be assigned to 2-naphthyl radicals result-
ing from decarboxylation of the 2-naphthoyloxyl radicals. The
lifetime of this component was ca. 2 µs. The laser photolysis
of 2-NPO under air or oxygen exhibited another absorption
band growing in a broad wavelength region of 350-650 nm
with a time constant similar to the decay time constant at 780
nm (Figs. 4a and 4c); the decay profile at this wavelength was
not affected by oxygen. A difference spectrum (B − A) is
MeONPys (4 × 10−4 mol dm−3) were photolyzed with 308-
nm laser pulses under argon in acetonitrile at ambient tempera-
ture. In Figs. 5 and 6 are shown the transient absorption spec-
tra observed for MeO-1-NPys and MeO-2-NPys, respectively.

578 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Behavior of Naphthoyloxyl Radicals
Fig. 5. Transient absorption spectra observed on 308-nm
pulsed laser excitation of 2-MeO-1-NPy and 4-MeO-1-
NPy in acetonitrile (a), and their time profiles monitored at
700 nm (b).
same lifetime as that of the 600–800-nm band (Fig. 5). Among
the three MeO-2-NPys, 6-MeO-2-NPy exhibits a band similar
to that of 2-NPy in the 500–800 nm region; however, the band
maxima in 1-MeO-2-NPy and 3-MeO-2-NPy appear at the
wavelength regions (around 700 and 600 nm, respectively)
shorter than those of 2-NPy and 6-MeO-2-NPy (Fig. 6). When
bis(o-methoxybenzoyl) peroxide was photolyzed under similar
conditions, a band assignable to o-methoxybenzoyloxyl radi-
cals appeared around 650 nm. In a nonpolar solvent such as
carbon tetrachloride, o-methoxybenzoyloxyl radicals exhibit
absorption bands in a broader wavelength region of 500–800
nm with intensities increasing towards the longer wave-
lengths.16
Fig. 4. Transient absorption spectra observed on laser pho-
tolysis of 2-NPO in acetonitrile; 312 (ꢃ) and 664 ns (ꢂ)
after laser pulse under argon, 351 (ꢁ) and 937 ns (ꢀ) after
laser pulse under oxygen, and the difference spectrum (ꢄ)
between those under oxygen and argon (a), and time pro-
files of the spectrum from 2-NPO under argon (b) and un-
der oxygen (c).
The lifetimes of MeOC10H6CO2· were determined by moni-
toring the absorption bands at 700, 620 (for 6-MeO-2-
·
·
C10H6CO2 ), or 590 nm (for 3-MeO-2-C10H6CO2 ), and are
listed in Table 2. The lifetimes obtained are much longer than
those of the unsubstituted parent radicals. In the presence of
1,3-pentadiene, the lifetimes were reduced, but the initial spec-
tral intensities were not affected at all.
All five pyridones employed exhibited absorption spectra as-
signable to the corresponding methoxynaphthoyloxyl radicals,
·
MeOC10H6CO2 . The sharp peaks at 390 nm observed in some
4. Kinetic Measurements. Activation parameters for dis-
appearance of the naphthoyloxyl radicals were obtained from
the first-order decay rates, which were determined in acetoni-
trile at 18–50 °C by monitoring the transient absorptions from
NPys and MeONPys at 700 or 590 nm (for 3-MeO-2-NPy).
MeONPys are assigned to the pair radicals, 2-pyridylox-
yls.4,14 For 2-MeO-1-NPy and 4-MeO-1-NPy, the spectra are
·
very similar to that of 1-NpCO2 in the 600-800 nm region, and
4-MeO-1-NPy exhibited another band around 560 nm with the

T. Najiwara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 579
Fig. 6. Transient absorption spectra observed on 308-nm
pulsed laser excitation of 1-MeO-2-NPy, 3-MeO-2-NPy,
and 6-MeO-2-NPy in acetonitrile (a), and their time pro-
files monitored at 700, 600, and 620 nm, respectively (b).
Fig. 7. Eyring plots for disappearance of unsubstituted and
methoxy-substituted 1-naphthoyloxyl (a) and 2-naphthoyl-
oxyl radicals (b).
absorptions of the radicals at 700 or 590 nm; kexptl = k0 +
k2[substrate], where k0 refers to first-order reactions by which
the radical decays at zero substrate concentration. In Table 3
the k2 values are listed together with those of benzoyloxyl3 and
anthroyloxyl radicals.5,8
Table 2. Lifetimes (at 23 °C) and Activation Parameters for
Disappearance of Unsubstituted and Methoxy-Substituted
Naphthoyloxyl Radicals in Acetonitrile
∆H‡
∆S‡
Radical
τ/µs
/kJ mol−1 /J mol−1 K−1
5. Molecular Orbital Calculations.
MO calculations
·
were performed to examine the geometries of the naphthoylox-
yl and some benzoyloxyl radicals by the UHF/PM3 method.
The energies (heats of formation) of the radicals are changed
when the carbonyloxyl moiety is rotated around the Cipso–Cα
bond; the energy minima appear at a planar and/or a twisted
geometry. Among the three kinds of 1-naphthoyloxyl radicals
that have a double-minimum energy surface,17 the parent 1-
naphthoyloxyl and 4-methoxy-1-naphthoyloxyl radicals are
most stabilized at the planar geometry; however, in 2-meth-
oxy-1-naphthoyloxyl radicals an almost perpendicularly twist-
ed geometry is ca. 9 kJ mol−1 stabler than the planar geometry.
On the contrary, the 2-naphthoyloxyl and benzoyloxyl radicals
employed in this work present a single-minimum energy sur-
face; the radicals having a methoxy group at the position adja-
cent to the ipso-carbon, such as 1-methoxy-2-naphthoyloxyl,
3-methoxy-2-naphthoyloxyl, and o-methoxybenzoyloxyl radi-
cals, show the energy minimum at the twisted geometry, and
the others that have no steric constraint in the CO2 moiety take
the planar geometry. For those methoxylated radicals, the en-
ergy barriers to the bond rotation are smaller than 4 kJ mol−1.
1-NpCO2
2.4
30
14
9.4
29
10
15
8.7
29
−38
−110
−120
−30
−120
−88
·
·
2-MeO-1-C10H6CO2
4-MeO-1-C10H6CO2
18
16
0.47
15
5.5
·
2-NpCO2
·
·
·
1-MeO-2-C10H6CO2
3-MeO-2-C10H6CO2
6-MeO-2-C10H6CO2
9.0
0.22
−120
−21
·a)
PhCO2
a) Ref. 11.
Typical Eyring plots are depicted in Fig. 7. The obtained acti-
vation parameters are listed in Table 2.
Bimolecular rate constants, k2, for the reactions of the naph-
thoyloxyl radicals with different substrates were evaluated
from their pseudo-first-order decay rates, kexptl, obtained at am-
bient temperature in the presence of benzene (0.56–4.2
mol dm−3), toluene (0.47–3.5), cyclohexane (0.46–2.3), cyclo-
hexene (0.0024–0.15), 1,3-pentadiene (0.012–0.75), and 1,3-
cyclohexadiene (0.0026–0.066) by monitoring the transient

580 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Behavior of Naphthoyloxyl Radicals
Table 3. Rate Constants (k2/105 mol−1 dm3 s−1) for Reactions of Aroyloxyl Radicals with Some Substrates in
Acetonitrile
Substrate
Radical
a)
b)
c)
d)
c-C6H12
3.5
< 0.1
< 0.1
13
0.1
1.3
0.1
370
3.9
C6H5CH3
7.2
0.1
< 0.1
48
< 0.1
2.3
0.3
470
22
C6H6
1.5
< 0.1
< 0.1
12
< 0.1
1.0
0.1
140
6.4
< 0.1
< 0.1
< 0.1
c-C6H10
290
61
1,3-C5H8
610
c-C6H8
15000
37
18
26000
54
15000
1800
13000
12000
·
1-NpCO2
·
·
2-MeO-1-C10H6CO2
4-MeO-1-C10H6CO2
3.1
0.7
1100
3.5
140
6.9
4600
350
11
590
63
280
41
520
290
43
·
2-NpCO2
·
·
·
1-MeO-2-C10H6CO2
3-MeO-2-C10H6CO2
6-MeO-2-C10H6CO2
· e)
PhCO2
· e)
p-MeOC6H4CO2
· f)
1-C14H9CO2
2-C14H9CO2
9-C14H9CO2
3.3
1.4
< 0.1
· f)
9.1
5.5
· f)
2.6
11
a) Cyclohexane. b) Cyclohexene. c) 1,3-Pentadiene. d) 1,3-Cyclohexadiene. e) Ref. 4. f) Ref. 8.
Table 4. Selected Structural Parameters and Electron Densities for the Optimized Structures of the Naphthoyloxyl and Benzoyloxyl
Radicalsa)
·
·
·
X in X-1-C10H6CO2
X in X-2-C10H6CO2
1-MeO 3-MeO
X in XC6H4CO2
H
2-MeO
4-MeO
H
6-MeO
H
o-MeO
p-MeO
Bond length/pm
O1–C2
C2–O3
132.1
122.3
148.4
131.9
121.9
149.0
132.1
122.4
148.2
131.7
122.4
148.2
131.6
122.0
149.0
131.6
122.0
149.0
131.7
122.5
148.1
131.8
122.4
148.1
131.7
122.1
148.7
131.8
122.5
147.9
C2–C4
Bond angle/degree
O1–C2–O3
C2–C4–C5
110.3
121.2
118.7
112.3
119.5
120.5
110.0
121.5
118.3
110.1
119.0
120.6
112.9
121.4
118.0
112.5
118.6
121.4
109.9
119.0
120.6
109.6
118.9
111.9
118.1
109.6
118.9
121.2b)
C2–C4–C13
121.1b)
122.4b)
Dihedral angle/degree
O1–C2–C4–C13
−178.8
81.0
−179.9
0.0
88.2
71.6
−0.3
−176.9
117.3c) −179.7c)
Electron density
O1
C2
O3
C4
C5
C7
C9
C11
C13
6.103
3.602
6.360
4.117
4.007
4.079
4.082
4.058
4.031
6.079
3.583
6.353
4.143
3.981
4.082
4.081
4.043
3.888
6.105
3.597
6.366
4.150
3.988
4.074
4.065
3.880
4.000
6.083
3.605
6.361
4.128
4.020
4.079
4.087
4.021
4.074
6.072
3.589
6.355
4.135
3.889
4.091
4.090
4.021
4.065
6.072
3.590
6.354
4.135
4.022
4.076
4.086
3.999
3.899
6.084
3.604
6.363
4.137
4.012
4.061
3.914
3.991
4.068
6.085
3.603
6.360
4.141
4.035
4.063
4.053
6.074
3.586
6.358
4.172
4.018
4.036
3.865
6.089
3.597
6.365
4.185
4.000
3.871
4.010
a) Numbering of the atoms is as follows:
·
·
b) Bond angle C2-C4-C9 for XC6H4CO2 . c) Dihedral angle O1-C2-C4-C9 for XC6H4CO2 .
Selected values of the structural parameters for the optimized
geometries are listed in Table 4, where selected values for the
calculated electron densities are also given. The benzoyloxyl
radicals with a sterically hindered CO2 group such as 2,6-di-
methyl- and 2,6-dichlorobenzoyloxyl radicals are predicted to
take a twisted geometry; their energy barriers to the bond rota-
tion are ca. 14 and 2 kJ mol−1, respectively.
The heats of formation and the geometries of the naphtho-

T. Najiwara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 581
Table 5. Heats of Formation for the Optimized Structures of the Naphthoyloxyl and Benzoyloxyl Radicals and Their
Decarboxylation Transition States
∆Hfa)
Selected structural parameter at TS
Eac)
·
ArCO2
TSb)
87
Ar· + CO2
5
C2–C4d)
181.5
181.1
183.1
181.6
182.3
182.4
181.3
182.0
183.3
182.2
O1–C2–C4–C13e)
91.2
·
1-C10H7CO2
2-C10H7CO2
14
7
74
75
75
75
77
76
76
79
81
81
·
82
3
−94.0
94.5
−92.3
−99.6
−92.2
92.1
−87.0
92.2
93.3
·
·
·
·
·
2-MeO-1-C10H6CO2
4-MeO-1-C10H6CO2
1-MeO-2-C10H6CO2
3-MeO-2-C10H6CO2
6-MeO-2-C10H6CO2
−137
−141
−136
−144
−154
−60
−62
−66
−60
−68
−79
19
−141
−146
−139
−147
−159
−58
·
C6H5CO2
·
o-MeOC6H4CO2
p-MeOC6H4CO2
−212
−222
−131
−141
−208
−218
·
a) ∆Hf: heat of formation/kJ mol−1. b) TS: transition state for decarboxylation. c) Ea: activation energy for decarbox-
ylation/kJ mol−1. d) Interatomic distance/pm. Numbering of the atoms is shown in Table 4. e) Dihedral angle/
degree; O1–C2–C4–C9 for the bottom three kinds of radicals.
yloxyl and benzoyloxyl radicals in the decarboxylation transi-
tion states were also calculated by the UHF/PM3 method, and
are summarized in Table 5 together with selected structural pa-
rameters of the transition states. The activation energies (Ea)
for decarboxylation were estimated as the differences in heat
of formation between the optimized structure and the transition
state, as summarized in Table 5. The Ea values (74–77
kJ mol−1) of the naphthoyloxyl radicals are not affected by in-
troduction of a methoxy group, even when it induced the rota-
tion of the CO2 moiety. This is the case for the benzoyloxyl
radicals. As for the geometry of the transition state, the CO2
moiety is almost perpendicularly twisted regardless of the ge-
ometry of the initial optimized structure, and the interatomic
distance between the Cipso and Cα is shorter for the initially pla-
nar radicals.
such as 1,3-pentadiene and molecular oxygen did not affect the
radical yields in the laser photolyses of NPOs. Each peroxide
exhibits a weak fluorescence spectrum with a band maximum
around 350 nm, and the fluorescence quantum yields are 5 to
10 times lower than those of the corresponding methyl naph-
thoates. This may be due to a high efficiency of the bond
cleavage from the excited singlet state.
For the naphthoyloxypyridones, the homolysis of the N–O
bond is assumed to take place from the excited singlet state,
since the efficiency of photocleavage was not affected by trip-
let quenchers (1,3-pentadiene and molecular oxygen), as esti-
mated from the radical yield in the laser photolyses. The 2-pyr-
idone moiety has an absorption band in the 300 nm region,
where 1- and 2-naphthoyl moieties also have an absorption
band (Fig. 1). It is not clarified which moiety is responsible for
the photocleavage on 300 or 308 nm excitation of 1-NPy and
2-NPy; however, the bond cleavage of BPy is clearly attributed
to the pyridone moiety since the benzoyl moiety has an absorp-
tion band in the shorter wavelength region and only the pyri-
done moiety is excited with the light of these wavelengths.
The efficiency of photocleavage of aroyloxypyridones seems
not to depend much on the structure or the singlet energy of the
aroyl moiety, such as benzoyl, naphthoyl, and anthroyl, since
the absorption spectra of the aroyloxyl radicals can be clearly
observed on laser photolyses of the pyridones with 308 nm
pulses under the same conditions. Thus, the photocleavage of
the present pyridones may take place from the excited singlet
state of the pyridone moiety.
Discussion
1. Decomposition Mechanisms of the 1- and 2-Naphtho-
yloxyl Radical Precursors. On direct irradiation, the usual
dibenzoyl peroxides (BPOs) are cleaved in the excited singlet
state,18,19 except for methylthio-substituted BPOs, which are
cleaved in the triplet state.3 The formation of a significant
amount of the geminate product is a characteristic of the sin-
glet mechanism, which is composed of two modes of bond
cleavage: a simple O–O bond cleavage giving two benzoyloxyl
radicals (Eq. 1) and a simultaneous or successive O–O plus
C
ipso–Cα bond cleavage producing a benzoyloxyl and a phenyl
radical in the solvent cage (Eq. 2).18
The efficiency of decarboxylation of 1- and 2-naphthoyloxyl
radicals, as reflected in the relative yield of naphthalene (the
ratio of naphthalene to the sum of naphthalene and acid in
Table 1), is highly dependent on the precursors, though the rel-
ative yields are larger for the 2-naphthoyloxyl precursors.
Since the decay profiles of the naphthoyloxyl radicals are inde-
pendent of the precursors as observed in the laser photolyses, it
can be assumed that the free naphthoyloxyl radicals should
give naphthoic acid and naphthalene with the same ratios as
observed in the photolyses of the pyridones. Consequently, the
higher efficiencies of decarboxylation in the peroxide photoly-
ses could be ascribed to the contribution of the aforementioned
h
ν
ν
·
(1)
(2)
(3)
ArCO2OCOAr
ArCO2OCOAr
2ArCO2
h
ArCO2 + CO2 + Ar·
·
ArCO2 + Ar·
ArCO2Ar (geminate product)
·
The dinaphthoyl peroxides, 1-NPO and 2-NPO, are also
supposed to be photochemically decomposed in the excited
singlet state, since each peroxide gives the geminate product,
naphthyl naphthoate, in nearly 10% yield (Table 1). This
mechanism is supported by the fact that the triplet quenchers

582 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Behavior of Naphthoyloxyl Radicals
two-bond cleavage, in which the O–O and Cipso–Cα bonds are
cleaved simultaneously or successively to produce a naphtho-
yloxyl and a naphthyl radical in the solvent cage (Eq. 2). For
the pyridones, the two-bond cleavage at the N–O and Cipso–Cα
bonds may not be operative, since the bond dissociation energy
is expected to be much larger for the N–O bond (ca. 220
kJ mol−1)20 than for the O–O bond (ca. 130 kJ mol−1).21
2. Spectral Behavior of the 1- and 2-Naphthoyloxyl Radi-
cals. In the laser photolyses of 1-NPO and 2-NPO, the broad
bands were observed in the 500–800 nm region (Figs. 2 and 3,
respectively); the spectral intensity increases towards the visi-
ble end. These bands are assigned to the 1- and 2-naphthoyl-
oxyl radicals, respectively, as reported previously.6 In the case
of 2-NPO, another absorption band grows in the 360 nm region
concomitantly with the decay of 2-naphthoyloxyl radicals (Fig.
4). This species may be assigned to 2-naphthyl radicals result-
ing from decarboxylation of the 2-naphthoyloxyl radicals (Eq.
4) and finally giving naphthalene via hydrogen-atom abstrac-
tion. Under oxygen, another new band was observed in the
broad wavelength region of 350–650 nm instead of that due to
the 2-naphthyl radicals. The broad band is assigned to 2-naph-
thyldioxyl radicals,15 which may be formed by a reaction of the
2-naphthyl radicals with molecular oxygen (Eq. 5).
tral behavior is very similar to that of o-methoxybenzoyloxyl
radicals (λmax ca. 650 nm) as well as that of 2,6-disubstituted
benzoyloxyl radicals (λmax 700–730 nm).9
The spectral behavior of the aroyloxyl radicals may be asso-
ciated with their geometries. The long-wavelength absorption
band of the aroyloxyl radicals is most probably due to a transi-
tion in the carboxyl plane from the 2B2 ground state to the 2A1
state.10,22–24 The unpaired electron which resides in the oxygen
p-type orbitals in the ground state does not interact with the ar-
omatic π system; however, in the aroyloxyl radicals with a
twisted geometry, the SOMO can overlap with the π system to
some extent to result in a blue shift of the absorption band.
2,6-Dimethyl- and 2,6-dichlorobenzoyloxyl radicals with a
sterically hindered CO2 moiety are predicted to take a twisted
geometry by the UHF/PM3 method. o-Methoxybenzoyloxyl
radicals are also predicted to take a twisted geometry, of which
the energy is nearly 4 kJ mol−1 lower than that of the planar
geometry. Consequently, the absorption bands from these ben-
zoyloxyl radicals around 650–720 may be due to their twisted
geometries, as previously discussed.9,16 The twisted geometry
is assumed to be more stabilized in polar solvents than in non-
polar solvents on the basis of the facts that the o-methoxyben-
zoyloxyl radicals exhibit their absorption bands in the region
of wider wavelengths of 600–800 nm in carbon tetrachloride16
than that around 650 nm in acetonitrile, and that their lifetime
is longer in acetonitrile than in carbon tetrachloride.16
In the case of 1-NPO, no rise components were detected un-
der argon or oxygen, though some spectral overlapping was
observed in the shorter wavelength region. This may be as-
cribed to a smaller decarboxylation efficiency of the 1-naph-
thoyloxyl radicals compared with that of the 2-naphthoyloxyl
radicals.
Similar features are suggested by the UHF/PM3 calcula-
tions for the naphthoyloxyl radicals bearing a methoxy group
at the adjacent position of the ipso carbon. As shown in Table
·
·
4, for 2-MeO-1-C10H6CO2 , 1-MeO-2-C10H6CO2 , and 3-
·
·
·
(4)
(5)
MeO-2-C10H6CO2 the geometry has been optimized where the
2 - C10H7CO2
2 - C10H7 + CO2
CO2 moiety is twisted from the molecular plane, involving not
only an increase in the Cipso–Cα bond length and the O–C–O
bond angle but also a decrease in electron density at the radical
center oxygen, compared to the parent radicals. The spectral
·
2 - C10H7 + O2
2 - C10H7OO·
The laser photolyses of 1-NPy and 2-NPy exhibited essen-
tially identical absorption spectra with those observed for 1-
NPO and 2-NPO, respectively, at 500–800 nm (Figs. 2 and 3,
respectively), though the band due to 2-pyridyloxyl radicals
was overlapped in the shorter wavelength region around 390
nm.4,14 Since the lifetimes monitored at 780 nm were indepen-
dent of the precursors for both 1- and 2-naphthoyloxyl radi-
cals, the dynamic behavior of the naphthoyloxyl radicals is not
affected by the pair radicals. Similar phenomena were ob-
served in laser photolyses of unsubstituted and substituted
dibenzoyl peroxides and 1-(benzoyloxy)-2-pyridones.3,4
·
·
behavior of 1-MeO-2-C10H6CO2 and 3-MeO-2-C10H6CO2
can be understood in accordance with the above considerations
for the substituted benzoyloxyl radicals; however, 2-MeO-1-
·
C10H6CO2 , which have the absorption band up to 800 nm,
might take a planar geometry through some intramolecular
charge-transfer interactions associated with the expanded aro-
matic system. This may be attributed to the stabilization of 1-
NpCO2· to a higher extent than in the case of 2-NpCO2· due to
the intramolecular charge transfer in the polar solvent.6 Thus,
the cooperative effects of the expanded π system of 1-naphthyl
moiety and the electron-donating methoxy group might stabi-
lize the planar geometry in acetonitrile.
4. Reactivity of the Methoxynaphthoyloxyl Radicals.
The lifetimes of the methoxynaphthoyloxyl radicals (6–18 µs)
were much longer than those of the parent radicals (Table 2).
The product distributions giving no methoxynaphthalenes indi-
cate that the methoxylated radicals do not decarboxylate at
room temperature, but disappear through hydrogen-atom ab-
straction. The activation parameters for disappearance are
consistent with this process (Table 2). As an example, the acti-
vation enthalpies for the parent naphthoyloxyl radicals are in
accord with that of benzoyloxyl radicals, and are among the
typical values for the decarboxylation process. The smaller
values for the methoxynaphthoyloxyl radicals are similar to
3. Spectral Behavior of the Methoxynaphthoyloxyl Radi-
cals. The laser photolyses of MeONPys exhibited transient
absorption spectra assignable to the corresponding methox-
·
ynaphthoyloxyl radicals, MeOC10H6CO2 , in the 500–800 nm
region (Figs. 5 and 6). The product distributions in the station-
ary photolyses of MeONPys (Table 1) are consistent with the
intermediacy of these radicals. The spectra of 2-MeO-1-
C10H6CO2· and 4-MeO-1-C10H6CO2· are very similar to that of
·
1-NpCO2 in the 600–800 nm region. Among MeO-2-
·
·
C10H6CO2 radicals, 1-MeO-2-C10H6CO2 and 3-MeO-2-
·
C10H6CO2 , which have a methoxy group at the position adja-
cent to the ipso carbon, showed the band maximum at the
wavelengths (around 700 and 600 nm, respectively) shorter
·
than those of 2-NpCO2· and 6-MeO-2-C10H6CO2 . This spec-

T. Najiwara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 583
·
those for the intramolecular hydrogen-atom abstraction of o-
methylbenzoyloxyl radicals (15 kJ mol−1)25 and the intermo-
lecular hydrogen-atom abstraction of p-methylthiobenzoyloxyl
radicals (11 kJ mol−1) in acetonitrile.3 Thus, the effect of the
methoxy substituent on the stability of naphthoyloxyl radicals
is quite remarkable. The lifetime of p-MeOC6H4CO2· is much
longer than that of PhCO2· in acetonitrile; however, those radi-
cals still decarboxylate to some extent in the same solvent.11
The activation energies estimated for decarboxylation of the
naphthoyloxyl radicals by the UHF/PM3 method are not af-
fected significantly by the introduction of a methoxy group.
Furthermore, the estimated values are rather smaller than those
of the methoxybenzoyloxyl radicals, which decarboxylate sig-
nificantly at room temperature.11 However, the introduction of
the methoxy group induces an increase and a decrease in elec-
tron density at the Cipso and the CO2 moiety, respectively; the
effect is more remarkable in 1-naphthoyloxyl radicals than in
2-naphthoyloxyls (Table 4). Thus, the lack of decarboxylation
in the methoxynaphthoyloxyl radicals can be rationalized in
terms of a charge-transfer effect displayed by the methoxy
group more significantly in polar solvents upon the naphthoyl-
oxyl radicals with a more expanded conjugate electron system
than the benzoyloxyls.
Inspection of bimolecular rate constants of the naphthoylox-
yl radicals with typical substrates (Table 3) shows that the
methoxynaphthoyloxyl radicals are much less reactive in hy-
drogen-atom abstraction (PhMe, c-C6H12, c-C6H10, and 1,3-cy-
clohexadiene) and addition to olefins and aromatics (c-C6H10,
PhH, and PhMe) than the parent naphthoyloxyl radicals as
well as benzoyloxyl radicals, and that their reactivity is almost
comparable with that of the anthroyloxyl radicals.5,8 These
facts indicate that the methoxy group stabilizes the naphthoyl-
oxyl radicals through the charge-transfer effect (Scheme 2) to
an extent similar to that affected by conjugation of the an-
thracene moiety, and its effect as a substituent is more promi-
nent in the naphthoyloxyl radicals than in the benzoyloxyl rad-
icals.
C10H6CO2 exhibit the band maximum around 700 and 600
nm, respectively. MO calculations predict that the CO2 group
would be rotated from the molecular plane in the latter radi-
cals.
The methoxynaphthoyloxyl radicals have much longer life-
times than those of the unsubstituted parent radicals, and do
not decarboxylate at room temperature but disappear through
hydrogen-atom abstraction from the solvent. They are much
less reactive in hydrogen-atom abstraction and addition to ole-
fins and aromatics than the parent naphthoyloxyl radicals as
well as benzoyloxyl radicals; the reactivity is almost compara-
ble with that of anthroyloxyl radicals. The effect of the meth-
oxy group is more prominent in the naphthoyloxyl radicals
than in the benzoyloxyl radicals.
Experimental
General. Proton and carbon 13 NMR spectra were recorded
on a Varian Gemini-200 (200 and 50 MHz, respectively) or a
JEOL FX-270 (270 and 67.5 MHz, respectively) spectrometer.
UV absorption and fluorescence spectra were measured on a JAS-
CO Ubest 55 or a Shimadzu 1600 spectrophotometer and a
Hitachi F-4000 spectrofluorimeter, respectively. Elemental analy-
ses were performed at the Analysis Center of the University of
Tsukuba. High-resolution mass analyses were performed on a Hi-
tachi M80B Mass Spectrometer at the National Institute of Ad-
vanced Industrial Science and Technology (AIST).
Materials. Solvent acetonitrile (Dojin, spectrograde) was
used as received. Di(1-naphthoyl) (1-NPO) and di(2-naphthoyl)
peroxides (2-NPO) were prepared from the corresponding naph-
thoyl chlorides and sodium peroxide with water in toluene at −(4–
10) °C. The peroxides were crystallized from chloroform and
methanol. The attempted preparation of bis(methoxynaphthoyl)
peroxides was unsuccessful.
1-NPO: mp 97–99 °C (lit. 98.2 °C26,27); UV (MeCN) λmax 304
nm (ε 10000).
2-NPO: mp 140–142 °C (lit. 138,27 140 °C26); UV (MeCN)
λ
max 244 nm (ε 123000), 276 (15400), 336 (3500).
1-(1-Naphthoyloxy)-2-pyridone (1-NPy). To a stirred solu-
tion of 1-naphthoyl chloride (3.81 g, 0.02 mol) and 2-hydroxypyr-
idine N-oxide (2.22 g, 0.02 mol) in dichloromethane was added 4-
(dimethylamino)pyridine (DMAP; 2.08 g, 0.022 mol), and the so-
lution was stirred for 20 h at room temperature. The colorless pre-
cipitate obtained after usual work-up was crystallized by precipi-
tation from ethyl acetate solution with hexane (2.4 g, 44%); mp
1
122–124 °C; H NMR (CDCl3) δ 6.28 (td, J = 6.9, 1.2 Hz, 1H),
Scheme 2.
6.83 (dt, J = 9.3, 0.8 Hz, 1H), 7.40–7.71 (m, 5H), 7.94 (d, J = 7.6
Hz, 1H), 8.16 (d, J = 8.2 Hz, 1H), 8.54 (d, J = 7.4 Hz, 1H), 8.90
(d, J = 8.0 Hz, 1H). Found: C, 72.18; H, 4.16; N, 5.25%. Calcd
for C16H11NO3: C, 72.45; H, 4.18; N, 5.28%.
Conclusion
The dinaphthoyl peroxides and the naphthoyloxypyridones
are photochemically decomposed in the excited singlet state;
the former involve the two-bond cleavage, but the latter do not.
Pulsed laser photolyses of 2-NPO under argon and oxygen ex-
hibited the absorption bands assignable to 2-naphthyl radicals
and 2-naphthyldioxyl radicals, respectively, in addition to
those due to 2-naphthoyloxyl radicals.
1-(2-Naphthoyloxy)-2-pyridone (2-NPy).
1-(2-Naphthoyl-
oxy)-2-pyridone was prepared similarly from 2-naphthoyl chlo-
ride (3.81 g, 0.02 mol) and 2-hydroxypyridine N-oxide (2.22 g,
0.02 mol) and purified by precipitation from ethyl acetate solution
1
with hexane (1.7 g, 31%); mp 160–162 °C; H NMR (CDCl3) δ
6.27 (td, J = 6.9, 1.6 Hz, 1H), 6.81 (dd, J = 9.4, 1.2 Hz, 1H),
7.40–7.54 (m, 2H), 7.56–7.72 (m, 2H), 7.79 (quartet, J = 8.0 Hz,
3H), 8.15 (dd, J = 8.6, 1.8 Hz, 1H), 8.82 (s, 1H). Found: C,
71.83; H, 4.29; N, 5.20%. Calcd for C16H11NO3: C, 72.45; H,
4.18; N, 5.28%.
The spectral profiles of methoxynaphthoyloxyl radicals are
strongly dependent upon the position of the methoxy group; 2-
·
·
MeO-1-C10H6CO2 , 4-MeO-1-C10H6CO2 , and 6-MeO-2-
·
C10H6CO2 exhibit the band increasing in intensity towards
800 nm or more, while 1-MeO-2-C10H6CO2 and 3-MeO-2-
1-(2-Methoxy-1-naphthoyloxy)-2-pyridone (2-MeO-1-NPy).
2-Methoxy-1-naphthaldehyde (0.93 g, 0.005 mol) was treated
·

584 Bull. Chem. Soc. Jpn., 76, No. 3 (2003)
Behavior of Naphthoyloxyl Radicals
with potassium permanganate (0.01 mol) in alkaline aqueous solu-
tion for 8 h at 70–80 °C to give the corresponding acid (0.32 g,
32%). The acid (0.2 g) was converted to the acid chloride with
thionyl chloride (6 mL). The acid chloride (0.22 g, 0.001 mol)
was treated with 2-hydroxypyridine N-oxide (0.11 g, 0.001 mol)
in the presence of DMAP (0.24 g, 0.002 mol) in a manner similar
to that employed in the preparation of 2-NPy. The product was
crystallized from diethyl ether (0.18 g, 41%); mp 102–104 °C;
1H NMR (CDCl3) δ 4.05 (s, 3H), 6.27 (td, J = 7.2, 1.6 Hz, 1H),
6.84 (dt, J = 9.3, 1.4 Hz, 1H), 7.33 (d, J = 9.0, 1H), 7.40–7.49
(m, 1H), 7.56 (dd, J = 7.2, 1.6 Hz, 1H), 7.64 (td, J = 7.1, 1.4 Hz,
1H), 7.82 (d, J = 8.2 Hz, 1H), 8.02 (d, J = 9.0 Hz, 1H), 8.51 (d, J
= 7.8 Hz, 1H). Found: m/z 295.0852. Calcd for C17H13NO4: M,
295.0845.
1-(4-Methoxy-1-naphthoyloxy)-2-pyridone (4-MeO-1-NPy).
4-Methoxy-1-naphthaldehyde (5.4 g, 0.029 mol) was treated with
potassium permanganate (0.06 mol) in alkaline aqueous solution
for 8 h at 70–80 °C to give the corresponding acid (1.82 g, 31%).
The acid (1.0 g) was converted to the acid chloride with thionyl
chloride (8 mL). The acid chloride (1.1 g, 0.005 mol) was treated
with 2-hydroxypyridine N-oxide (0.55 g, 0.005 mol) in the pres-
ence of DMAP (0.73 g, 0.006 mol) in a manner similar to that em-
ployed in the preparation of 2-NPy. The product was crystallized
from dichloromethane and hexane (1.0 g, 67%); mp 155–157 °C;
1H NMR (CDCl3) δ 4.11 (s, 3H), 6.26 (td, J = 6.9, 1.6 Hz, 1H),
6.81 (dd, J = 9.4, 1.6 Hz, 1H), 6.89 (d, J = 8.4 Hz, 1H), 7.39–
7.71 (m, 4H), 8.37 (d, J = 8.0 Hz, 1H), 8.62 (d, J = 8.4 Hz, 1H),
8.96 (d, J = 8.0 Hz, 1H). Found: m/z 295.0910. Calcd for
C17H13NO4: M, 295.0845.
0.0025 mol) and 2-hydroxypyridine N-oxide (0.28 g, 0.0025 mol)
in dichloromethane was added DMAP (0.43 g, 0.0035 mol), and
the solution was stirred for 8 h at room temperature. The colorless
precipitate obtained after the usual work-up was crystallized by
precipitation from dichloromethane solution with hexane (0.33 g,
44%); mp 178–180 °C; 1H NMR (CDCl3) δ 3.97 (s, 3H), 6.26 (td,
J = 6.8, 1.8 Hz, 1H), 6.80 (dd, J = 9.3, 1.7 Hz, 1H), 7.18–7.26
(m, 2H), 7.38–7.52 (m, 2H), 7.85 (t, J = 9.6 Hz, 2H), 8.11 (dd, J
= 8.6, 1.9 Hz, 1H), 8.73 (s, 1H). Found: m/z 295.0824. Calcd for
C17H13NO4: M, 295.0845.
Photolyses of NPOs and NPys. A solution (4 mL) of a radi-
cal precursor (1 × 10−4 mol dm−3) in acetonitrile was irradiated
for 2 min in a quartz cuvette (10 mm path length) with 300-nm
light (Rayonet RPR-3000 lamps); the products were analyzed
qualitatively and quantitatively by HPLC with a multichannel UV/
vis detector.
Laser-Flash Photolyses. Laser-flash photolyses were per-
formed by using an excimer laser (Lambda Physik LPX-100, Xe-
Cl, 308 nm, 10-ns fwhm, 70 mJ/pulse) and a pulsed xenon arc
(Ushio UXL-159, 150 W) as a monitoring light source. The moni-
toring beam obtained from the xenon lamp was oriented perpen-
dicularly to the exciting laser beam, passed through a sample cell
and a grating monochromator (JASCO CT-25C), and detected
with a photomultiplier (Hamamatsu Photonix R928). The ampli-
fied signal was recorded as the time profile of a transmittance
change on a storage oscilloscope (Iwatsu TS-8123), transferred to
a personal computer (NEC PC-9801VX21), and accumulated for
3–5 times to be averaged. The system was computer-controlled
and decay curves were analyzed by this computer system. The
temperature of sample cells was controlled with a thermostated
cell holder and a circulating water bath (Haake F3-K).
1-(1-Methoxy-2-naphthoyloxy)-2-pyridone (1-MeO-2-NPy).
1-Hydroxy-2-naphthoic acid (9.4 g, 0.05 mol) was treated with
dimethyl sulfate (10 mL) in 15% sodium hydroxide aqueous solu-
tion. The obtained 1-methoxy-2-naphthoic acid was converted to
the acid chloride with thionyl chloride. The acid chloride (0.22 g,
0.001 mol) was treated with 2-hydroxypyridine N-oxide (0.11 g,
0.001 mol) in the presence of DMAP (0.18 g, 0.0015 mol) in a
manner similar to that employed in the preparation of 2-NPy. The
product was crystallized from diethyl ether (0.17 g, 56%); mp
MO Calculations. Semiempirical (UHF/PM3) calculations
were performed to determine the optimum geometries and the de-
carboxylation transition states of naphthoyloxyl and benzoyloxyl
radicals using a program of WinMOPAC V3.0.28 The vibration
analyses gave only one imaginary frequency corresponding to
CO2 elimination from the aroyloxyl radicals at the transition state.
1
131–133 °C; H NMR (CDCl3) δ 4.18 (s, 3H), 6.26 (td, J = 6.9,
References
1.6 Hz, 1H), 6.80 (dd, J = 8.8, 1.6 Hz, 1H), 7.39–7.54 (m, 2H),
7.58–7.72 (m, 3H), 7.91 (dd, J = 6.9, 2.0 Hz, 1H), 8.08 (d, J =
8.8 Hz, 1H), 8.32 (dd, J = 8.0, 1.8 Hz, 1H). Found: m/z 295.0838.
Calcd for C17H13NO4: M, 295.0845.
1
S. Oishi, H. Tsubaki, and H. Matsuzawa, Chem. Lett.,
1999, 805.
2
T. Karatsu, Y. Yoshida, K. Fukui, and A. Kitamura, Chem.
1-(3-Methoxy-2-naphthoyloxy)-2-pyridone (3-MeO-2-NPy).
3-Hydroxy-2-naphthoic acid (9.4 g, 0.05 mol) was treated with
dimethyl sulfate (10 mL) in 15% sodium hydroxide aqueous solu-
tion. The crude product was washed with ethyl acetate and the
residue was crystallized from dichloromethane and hexane (2.1 g,
21%). The obtained 3-methoxy-2-naphthoic acid (0.50 g) was
converted to the acid chloride with thionyl chloride (6 mL). The
acid chloride (0.55 g, 0.0025 mol) was treated with 2-hydroxypyr-
idine N-oxide (0.28 g, 0.0025 mol) in the presence of DMAP
(0.30 g, 0.003 mol) in a manner similar to that employed in the
preparation of 2-NPy. The product (0.46 g) was treated with sili-
ca-gel column and crystallized from diethyl ether (0.05 g, 7%);
mp 76–78 °C; 1H NMR (CDCl3) δ 4.03 (s, 3H), 6.25 (td, J = 6.8,
1.8 Hz, 1H), 6.80 (dd, J = 9.3, 1.3 Hz, 1H), 7.38–7.46 (m, 2H),
7.50–7.62 (m, 2H), 7.77 (d, J = 8.4 Hz, 1H), 7.89 (d, J = 8.4 Hz,
1H), 8.72 (s, 1H). Found: m/z 295.0867. Calcd for C17H13NO4:
M, 295.0845.
Lett., 1999, 877.
3
J. Hashimoto, K. Segawa, and H. Sakuragi, Chem. Phys.
Lett., 314, 261 (1999).
4
J. Hashimoto, K. Segawa, H. Itoh, and H. Sakuragi, Chem.
Lett., 2000, 362.
5
Y. Saitoh, K. Segawa, H. Itoh, and H. Sakuragi, Chem.
Lett., 2001, 82.
6
T. Tateno, H. Sakuragi, and K. Tokumaru, Chem. Lett.,
1992, 1883.
7
Y. Saitoh, K. Segawa, H. Itoh, and H. Sakuragi, Tetrahe-
dron Lett., 41, 8353 (2000).
8
Y. Saitoh, K. Segawa, H. Itoh, and H. Sakuragi, Bull.
Chem. Soc. Jpn., 75, 2013 (2002).
9
J. Wang, M. Tsuchiya, H. Sakuragi, K. Tokumaru, and H.
Itoh, Tetrahedron Lett., 35, 6321 (1994); J. Wang, H. Itoh, M.
Tsuchiya, K. Tokumaru, and H. Sakuragi, Tetrahedron, 51, 11967
(1995).
1-(6-Methoxy-2-naphthoyloxy)-2-pyridone (6-MeO-2-NPy).
To a stirred solution of 6-methoxy-2-naphthoyl chloride (0.55 g,
10 J. Chateauneuf, J. Lusztyk, and K. U. Ingold, J. Am. Chem.
Soc., 109, 897 (1987); J. Chateauneuf, J. Lusztyk, and K. U.

T. Najiwara et al.
Bull. Chem. Soc. Jpn., 76, No. 3 (2003) 585
Ingold, J. Am. Chem. Soc., 110, 2877 (1988); J. Chateauneuf, J.
Lusztyk, and K. U. Ingold, J. Am. Chem. Soc., 110, 2886 (1988).
11 H. Misawa, K. Sawabe, S. Takahara, H. Sakuragi, and K.
Tokumaru, Chem. Lett., 1988, 357.
19 S. Yamauchi, N. Hirota, S. Takahara, H. Sakuragi, and K.
Tokumaru, J. Am. Chem. Soc., 107, 5021 (1985); S. Yamauchi, N.
Hirota, K. Sawabe, H. Misawa, S. Takahara, H. Sakuragi, and K.
Tokumaru, J. Am. Chem. Soc., 111, 4402 (1989).
12 T. Najiwara, K. Segawa, and H. Sakuragi, Chem. Lett.,
2001, 1064.
13 S. L. Murov, I. Carmichael, and G. L. Hug, “Handbook of
Photochemistry,” 2nd ed, Revised and Expanded, Marcel Dekker
(1993), p. 31.
20 H. Sakuragi, M. Yoshida, H. Kinoshita, K. Utena, K.
Tokumaru, and M. Hoshino, Tetrahedron Lett., 1978, 1529.
21 S. L. Murov, I. Carmichael, and G. L. Hug, “Handbook of
Photochemistry,” 2nd ed., Revised and Expanded, Marcel Dekker
(1993), Table 11–3, p. 282.
14 B. M. Aveline, I. E. Kochevar, and R. W. Redmond, J. Am.
Chem. Soc., 117, 9699 (1995); B. M. Aveline, I. E. Kochevar, and
R. W. Redmond, J. Am. Chem. Soc., 118, 10113 (1996); B. M.
Aveline, I. E. Kochevar, and R. W. Redmond, J. Am. Chem. Soc.,
118, 10124 (1996).
22 H. G. Korth, W. Müller, J. Lusztyk, and K. U. Ingold,
Angew. Chem. Int. Ed. Engl., 28, 183 (1989).
23 J. Chateauneuf, J. Lusztyk, B. Maillard, and K. U. Ingold,
J. Am. Chem. Soc., 110, 6727 (1988); H. G. Korth, J. Chateauneuf,
J. Lusztyk, and K. U. Ingold, J. Org. Chem., 56, 2405 (1991).
24 Also, a transition from the second HOMO to the SOMO
was suggested by MO calculations.1
15 Z. B. Alfassi, G. I. Khaikin, and P. Neta, J. Phys. Chem.,
99, 265 (1995).
16 J. Wang, T. Tateno, H. Sakuragi, and K. Tokumaru, J.
Photochem. Photobiol. A: Chem., 92, 53 (1995).
17 Strictly speaking, these 1-naphthoyloxyl radicals have two
planar geometries, each of which is located at an energy mini-
mum, since the two C–O bonds are different in length from each
other.
18 A. Kitamura, H. Sakuragi, M. Yoshida, and K. Tokumaru,
Bull. Chem. Soc. Jpn., 53, 1393 (1980), and references cited there-
in.
25 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).
26 D. H. Hey and E. W. Walker, J. Chem. Soc., 1948, 2213.
27 M. S. Kharasch and R. C. Dannley, J. Org. Chem., 10, 406
(1945).
28 WinMOPAC V3.0, Fujitsu, Tokyo (2000).