Mechanism of the 10-methylacridinium ion-sensitized photooxidation of N,N-dibenzylhydroxylamine and its derivatives in acetonitrile

Article abstract of DOI:10.1039/b009206l

The 10-methylacridinium ion (MA⁺)-sensitized photooxidation of substituted N,N-dibenzylhydroxylamines (1) in acetonitrile occurred mainly by a superoxide ion mechanism to give N-benzylidenebenzylamine N-oxides (2) and hydrogen peroxide quantitatively. Analysis of substituent effects on the limiting quantum yield for formation of 2 showed that back electron transfer (ET) from the 10-methylacridinyl radical (MA^.) to the radical cation 1^+. proceeds in the Marcus 'normal region'. In addition, this back ET was found to take place in preference to one-electron reduction of O₂ by MA^..

Full text of DOI:10.1039/b009206l

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Mechanism of the 10-methylacridinium ion-sensitized
photooxidation of N,N-dibenzylhydroxylamine and its derivatives
in acetonitrile
2
Yasuhiro Ohba,a Kanji Kubo,b Tetsutaro Igarashia and Tadamitsu Sakurai*a
a Department of Applied Chemistry, Faculty of Engineering, Kanagawa University,
Kanagawa-ku, Yokohama 221-8686, Japan
b Institute of Advanced Material Study, 86, Kyushu University, Kasuga-koen, Kasuga,
Fukuoka 816-0811, Japan
Received (in Cambridge, UK) 16th November 2000, Accepted 27th February 2001
First published as an Advance Article on the web 20th March 2001
The 10-methylacridinium ion (MA+)-sensitized photooxidation of substituted N,N-dibenzylhydroxylamines (1) in
acetonitrile occurred mainly by a superoxide ion mechanism to give N-benzylidenebenzylamine N-oxides (2) and
hydrogen peroxide quantitatively. Analysis of substituent effects on the limiting quantum yield for formation of 2
+
ؒ
ؒ
showed that back electron transfer (ET) from the 10-methylacridinyl radical (MA ) to the radical cation 1 proceeds
in the Marcus ‘normal region’. In addition, this back ET was found to take place in preference to one-electron
ؒ
reduction of O2 by MA .
of the same reaction mixture. In addition, the absorption spec-
Introduction
trum of each product has a maximum around 300 nm, where
Much attention has been devoted to the electron transfer-
MAP exhibits only weak absorption. This is in agreement with
initiated photooxygenation reactions of various electron-rich
the absorption spectrum of the corresponding authentic sample
alkenes, including aromatic compounds, from synthetic and
2 (Scheme 1). Control experiments showed that the oxidation of
1 occurs to a negligible extent without either MAP or O2.
mechanistic points of view,1 whereas there are only a few
studies regarding the photosensitized oxidations of hydroxyl-
amines.2 In previous studies we demonstrated that the
cyano-substituted anthracene-sensitized photooxidation of
substituted N,N-dibenzylhydroxylamines (1) is a good system
for exploring the mechanism of the oxidation reaction of
hydroxylamines initiated by electron transfer (ET).3 An inter-
esting finding in our study is that the limiting quantum yield
for the photooxidation is determined by the relative rate of
back ET within the initially formed geminate radical ion pair.
Since ET from 1 to a salt as a sensitizer induces no net charge
separation, we might be able to enhance the formation of free
hydroxylamine radical cations and, hence, to lower the relative
rate for back ET, compared with ET to cyano-substituted
anthracenes.4 In order to find out the factors that control the
sensitized photooxidation efficiency of hydroxylamines, we
chose 10-methylacridinium perchlorate (MAP) as an ET-type
photosensitizer and 1a–f as substrates and investigated
substituent effects on the MAP-sensitized photooxidation of 1
in acetonitrile (MeCN).
Scheme 1
A typical plot given in Fig. 1 demonstrates that the fluor-
Results and discussion
escence of MAP is quenched by 1 according to the Stern–
An O2-purged MeCN solution of 1b (1.0 × 10Ϫ3 mol dmϪ3) con-
taining MAP (5.0 × 10Ϫ4 mol dmϪ3) was irradiated for 3.5 h
with light of wavelength longer than 340 nm at room tem-
perature. 1H NMR analysis of the product mixture using
N-(1-naphthylidene)-1-naphthylmethylamine N-oxide as an
internal standard revealed that N-benzylidenebenzylamine
N-oxide (2b) is formed in 97% yield when 62% of the starting 1a
is consumed and MAP remains unchanged. No change in the
MAP concentration is further substantiated by the finding that
the UV absorption of MAP undergoes negligible change during
irradiation in the wavelength range 330–460 nm. Hydrogen per-
oxide was also found to be obtained in 100% yield by iodometry
Volmer equation I0/I = 1 + ketτS[1], where I and I0 are the
fluorescence intensities of MAP with and without 1, respect-
ively, ket is the bimolecular quenching rate constant, and τS is
the fluorescence lifetime of MAP in the absence of 1 (36.2 ns).
From the slopes (quenching constants) obtained from the
linear plots, the ket values for 1a–f as quenchers were estimated
(Table 1). To confirm the occurrence of the MAP emission
quenching by ET, the free energy changes for ET (∆etG) from
1a–f to the excited singlet-state MAP were calculated on the
basis of the simplified Weller equation5 ∆etG/kJ molϪ1
=
96.5(Eox Ϫ Ered) Ϫ ES, where Eox is the oxidation potential of 1
(Table 1), Ered is the reduction potential of MAP (Ϫ0.43 V vs.
DOI: 10.1039/b009206l
J. Chem. Soc., Perkin Trans. 2, 2001, 491–493
This journal is © The Royal Society of Chemistry 2001
491

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Table 1 Free energy changes for ET (∆etG and ∆etЈG), fluorescence quenching constants (ketτS), limiting quantum yield for formation of 2 (Φ2,lim
and related parameters (Eox, ket and ketЈRel) in the MAP-sensitized photooxidation of 1 in MeCN
)
Compound
Eox a/V vs. SCE
ket/1010
M
Ϫ1 sϪ1
∆etG/kJ molϪ1
Φ2,lim
ketτS b/MϪ1
ketτS c/MϪ1
ketЈRel
∆etЈG/kJ molϪ1
1a
1b
1c
1d
1e
1f
0.67
0.76
0.76
0.85
0.88
1.00
1.3
1.2
1.2
1.1
1.1
0.80
Ϫ159
Ϫ151
Ϫ151
Ϫ142
Ϫ139
Ϫ127
0.049 0.003
0.019 0.001
0.0073 0.0003
0.0083 0.0005
0.012 0.001
0.0046 0.0003
470
450
420
400
400
290
120
210
270
120
220
160
0.37
1.0
2.6
2.3
1.6
4.2
Ϫ106
Ϫ115
Ϫ115
Ϫ124
Ϫ126
Ϫ138
a Previous data (see ref. 3b). b Determined from the fluorescence quenching of MAP by 1. c Estimated from the dependence of Φ2 on the concentration
of 1. 1 M = 1 mol dmϪ3
.
Fig. 2 Plot of Φ2b versus [1b]Ϫ1 for the photooxidation of 1b in the
Ϫ1
Fig. 1 Stern–Volmer plot for the fluorescence quenching of MAP
(1.0 × 10Ϫ4 mol dmϪ3
temperature. Excitation wavelength is 366 nm.
presence of MAP (1.0 × 10Ϫ4 mol dmϪ3) in O2-purged MeCN at room
temperature. Irradiation wavelength is 366 nm.
)
by 1b in N2-saturated MeCN at room
SCE in MeCN)6, and ES is the first singlet excitation energy of
MAP (265 kJ molϪ1).6 These are collected in Table 1. The
magnitude of the ∆etG values evaluated, therefore, leads
us to conclude that the fluorescence quenching occurs by
an ET mechanism.
Since no induction of net charge separation by ET to the
excited-state MAP strongly suggests that solvent cage effects
play a minor role, we are able to propose Scheme 2 (in which
absorption around 300 nm (where only the weak absorption of
MAP is detected) made it possible to determine Φ2 spectro-
photometrically at very low conversions of 1 (<1%). As shown
in Fig. 2, we obtained linear plots of 1/Φ2 against the reciprocal
of the concentration of 1 (1/[1]) and then estimated the limiting
quantum yields for the appearance of 2 (Φ2,lim = α) and the
quenching constants (ketτS) from the intercept and the ratio of
the intercept to the slope of these linear plots, respectively
(Table 1). Comparison of the ketτS values determined according
to the two different procedures confirms that the quenching
constant estimated from the concentration dependence of Φ2
is comparable to that obtained by the MAP fluorescence
quenching. This finding also substantiates the ET mechanism
that is shown in Scheme 2. On the other hand, an inspection of
substituent effects on the limiting quantum yield reveals that
Φ2,lim is smaller than that of the cyano-substituted anthracene-
sensitized photooxidation by about an order of magnitude, but
has a clear tendency to decrease as the electron-withdrawing
ability of the substituent R is increased.3 Because Φ2,lim is equal
to α, the observed poor oxidation efficiency is due to the much
slower rate of ET [ketЈ(O2)] from the 10-methylacridinyl radical
+
ؒ
+
ؒ
ؒ
(MA ) to O2 than that of back ET [ketЈ(1 )] to 1 . The ability
ؒ
of MA to reduce O2 is considered to be much lower, compared
with the reducing ability of cyano-substituted anthracene-
derived radical anions.
Scheme 2
It has been shown previously that back ET from the 9,10-
dicyanoanthracene (DCA)-derived radical anion to 1+ takes
ؒ
superoxide ion is involved as the major oxidizing species in the
process) on the basis of the results described above. Application
of the steady-state approximation to Scheme 2 gives eqn. (1),
place in the Marcus ‘normal region’ and its rate is not very
sensitive to the electronic effects of the substituent R.3b Since we
may assume that the ET rate ketЈ(O2) undergoes no substituent
+
ؒ
effects, the equation: Φ2,lim = α = ketЈ(O2)/[ketЈ(O2) + ketЈ(1 )]
allows us to derive eqn. (2). The ketЈRel values calculated by
1/Φ2 = (1/α){1 + 1/(ketτS[1])}
(1)
where Φ2 refers to the quantum yield for the formation of 2 and
α is the ratio of ET to O2 to the overall ET (to 1+ and O2). The
(Φ2a–f,limϪ1 Ϫ 1)/(Φ2b,limϪ1 Ϫ 1) =
ؒ
+
Rel
fortunate observation that the product 2 exhibits strong UV
ketЈ(1a–f+ )/ketЈ(1b ) = ketЈ
(2)
ؒ
ؒ
492
J. Chem. Soc., Perkin Trans. 2, 2001, 491–493

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Experimental
UV absorption and fluorescence spectra were recorded on a
Shimadzu UV-2200 spectrophotometer and a Shimadzu RF-
1
5000 spectrofluorimeter, respectively. H NMR spectra were
taken with a JEOL JNM-A500 spectrometer. Chemical shifts
were determined using tetramethylsilane as an internal
standard. IR spectra were taken with a Hitachi Model 270-30
infrared spectrometer. The fluorescence lifetime of MAP was
measured under nitrogen with a time-correlated single-photon
counting apparatus (Horiba NAES-700; excitation wave-
length = 366 nm; cut-off wavelength = 410 nm), which was
equipped with a flash lamp filled with hydrogen. Ten thousand
counts were sampled in the peak channel.
Substituted N,N-dibenzylhydroxylamines (1a–f) and N-
benzylidenebenzylamine N-oxides (2a–f) were prepared and
purified according to the previously described procedures.3
Physical and spectroscopic properties of 1 and 2 were consistent
with those of previously prepared samples. MAP was prepared
according to the method of Roberts et al. and purified by
recrystallization from methanol.8 The structure of MAP was
established by IR and 1H NMR spectroscopy. Acetonitrile
was purified according to the standard method.9 N-(1-
Naphthylidene)-1-naphthylmethylamine N-oxide, used as an
internal standard, was the same as that employed in a previous
study.10 CD3CN and CDCl3 (Aldrich) were used as received.
Fig. 3 Relative rate of back electron transfer from the 10-
methylacridinyl radical to the radical cation of 1 as a function of the
free energy change in MeCN.
using this equation are collected in Table 1. In addition, we
+
ؒ
ؒ
evaluated the free energy change for back ET from MA to 1
based on the relation, ∆etЈG/kJ molϪ1 = Ϫ96.5(Eox Ϫ Ered
)
(Table 1). In Fig. 3 is shown the Marcus plot of log ketЈRel
versus Ϫ∆etЈG. The plot clearly indicates that the rate of
back ET increases with increasing exothermicity for this
process and, hence, leads us to conclude that the ET occurs in
the Marcus ‘normal region’.7 This conclusion is consistent with
the finding that Φ2,lim decreases as the electron-withdrawing
A
potassium tris(oxalato)ferrate() actinometer was
employed to determine the quantum yields for the appearance
of 2 at low conversions of the starting hydroxylamine 1
(<1%).11 A 1% conversion of 1 ([1] = 2.5–20 × 10Ϫ3 mol dmϪ3)
into 2 corresponds to a change in the absorbance of 0.45–4.6
around 300 nm, because 2 in MeCN has a molar absorption
coefficient of 1.8–2.3 × 104 dm3 molϪ1 cmϪ1 around this wave-
length.3b A 450 W high-pressure Hg lamp was used as the light
source from which 366 nm light was selected with Corning
0-52, Corning 7-60, and Toshiba IRA-25S glass filters. The
previously determined molar absorption coefficients of 2a–f
were utilized to quantify the formation of 2.3b All of the
quantum yields are an average of more than five determin-
ations. Quantitative iodometric analysis of H2O2 produced was
carried out by using the linear calibration curve for this per-
oxide, obtained under the same analytical conditions.
ability of the substituent R is increased. The much smaller
+
ؒ
ؒ
exothermicity of back ET for the MA –1 system than that
+
for the DCAϪ –1 system may be responsible for a more
sensitive dependence of the back ET rate for the former system
on Ϫ∆etЈG.
ؒ
ؒ
Since the possibility of a singlet oxygen mechanism in MeCN
still remains, we investigated the solvent deuterium isotope
effects on Φ2b using CD3CN as a solvent in order to estimate the
extent to which this mechanism contributes to the overall
MAP-sensitized photooxidation of 1. From eqn. (3)3 (where D
1
1
Φ2b(D)/Φ2b(H) = {kd (H) ϩ kr1[1b]}/{kd (D) ϩ kr1[1b]} (3)
References
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1
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dmϪ3, respectively. A comparison of these calculated isotope
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Conclusion
Through the analysis of substituent effects on the limiting
quantum yield for the appearance of 2, we found that both the
stability of the hydroxylamine radical cation 1+ and the ability
ؒ
of the 10-methylacridinyl radical to reduce O2 play key roles in
determining the efficiency of the photooxidation reaction of 1,
initiated by ET.
J. Chem. Soc., Perkin Trans. 2, 2001, 491–493
493