Curious oxygen effect on photosensitized electron-transfer reactions of benzophenone oxime O-methyl ethers: One-way photoisomerization of an iminic double bond

Article abstract of DOI:10.1016/S0040-4039(00)01412-X

Under an oxygen atmosphere, the E and Z isomer ratio of N-methoxy-4-methoxyphenyl-4'-methylphenylmethanimine (initially, [E]/[Z]=1/1) reached 4/96 upon irradiation (>360 nm) of 9,10-dicyanoanthracene (DCA) as a photosensitizer in acetonitrile. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01412-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8101–8106
Curious oxygen effect on photosensitized electron-transfer
reactions of benzophenone oxime O-methyl ethers: one-way
photoisomerization of an iminic double bond
Yasuhiko Kawamura,* Ryusaku Takayama, Masaki Nishiuchi and Masao Tsukayama
Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima,
Minamijosanjima-cho, Tokushima 770-8506, Japan
Received 12 June 2000; revised 16 August 2000; accepted 18 August 2000
Abstract
Under an oxygen atmosphere, the E and Z isomer ratio of N-methoxy-4-methoxyphenyl-4%-
methylphenylmethanimine (initially, [E]/[Z]=1/1) reached 4/96 upon irradiation (>360 nm) of 9,10-
dicyanoanthracene (DCA) as a photosensitizer in acetonitrile. © 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: photoreaction; electron transfer; isomerization; oximes; superoxide.
Photochemical cis–trans isomerizations of CꢀC1 and NꢀN2 double bonds have been investi-
gated thoroughly, yet limited information is available on photoisomerization of a CꢀN double
bond.3 There has been virtually no report on the isomerization of a compound containing the
CꢀN double bond derived especially from a diaryl ketone. During the course of our investiga-
tion on photosensitized electron-transfer (PET) reactions of compounds consisting of a CꢁNꢁO
skeleton,4 we found a novel oxygen-sensitive geometrical isomerization of N-methoxydiaryl-
methanimine (1) under the PET conditions.5
(1)
* Corresponding author. Tel: (+81)-88-656-7401; fax: (+81)-88-655-7025; e-mail: kawamura@chem.tokushima-u.ac.jp
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01412-X

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Table 1
Oxidation potentials and kinetic data of the quenching of the DCA fluorescence with methanimines 1
ox a
1/2
b
c
Compd.
E
(V)
Ksv (M−1
)
kq (1010 M−1 s−1
)
DGd (kJ mol−1
)
1a
1b
1.54
1.88
171
59.5
1.12
0.389
−11.4
−3.51
a Half peak of oxidation potentials of 1 vs. SCE measured in dry MeCN under Ar. Supporting electrolyte:
n-Bu4N+ClO4−
.
b Stern–Volmer constants for the fluorescence quenching of DCA with 1.
c Rate constants for the fluorescence quenching.
d Free energy change6 for the electron transfer from the excited singlet state of DCA to 1.
Fluorescence of DCA employed as a photosensitizer was quenched by 1 in MeCN at nearly
ox
1/2
diffusion-controlled rates. The oxidation potential (E ) of 1 was moderately to fairly low (<1.9
V vs. SCE) and the free energy changes (DG) for the electron transfer from 1 to the excited
singlet state of DCA were calculated to be negative by the Rehm–Weller equation6 employing
ox
1/2
E
values of 1. The photosensitized electron-transfer process is, therefore, the most probable
mechanism for the fluorescence quenching of DCA. These data are summarized in Table 1.
Table 2
Photoisomerizations of N-methoxy-1,1-diarylmethanimines 1a
Sens.
Atmosphere
Isomer ratio Compd. 1a
Isomer ratio Compd. 1b
E
Z
E
Z
DCAb
Ar
O2
48
52
96
56
44
4
78
22
TMKc
Ar
O2
45
44
55
56
45
51
55
49
Directd
Ar
O2
33
29
67
71
38
34
62
66
1
a Reaction time: 3 h. Isomer ratios were determined by H NMR spectra (400 MHz in MeCN-d3).
b 9,10-Dicyanoanthracene.
c Thio-Michler’s ketone.
d hw\280 nm.
Irradiation of 17 (Eq. (1)) with or without photosensitizers gave the results as summarized in
Table 2.8 Features of the photoreactions are: (1) similar E/Z selectivities of 1a and 1b were
observed upon direct irradiation of each under Ar. No different selectivities were observed in the
reactions under O2 from those under Ar. (2) When a typical triplet sensitizer, thio-Michler’s
ketone (TMK), was employed in the reactions under Ar, the ratios of E-1a and E-1b increased

8103
by ca. 10% compared to the results in the direct irradiation. In addition, the DCA-sensitized
photoreaction of 1a under Ar showed a little selectivity in geometry of 1. (3) In DCA-sensitized
photoreactions under O2, surprisingly high geometrical selectivity in 1 was observed, especially
in 1a.9 Moreover, diaryl ketone (2)10 and methyl nitrite (3)10 were detected as minor byproducts
in the photoreactions of both methanimines. Note that the Z isomer is a major component in
the PET reaction of 1a, whereas the E isomer is major in the case of 1b. By contrast, the
TMK-sensitized reaction showed no such remarkable oxygen effect.
In order to elucidate the peculiar oxygen effect on the stereoselectivity under PET conditions,
some control experiments were done and the results are summarized as follows: (1) no
displacement of isomer ratios was observed at all in a methylene blue-sensitized photoreaction.
The result rules out the participation of singlet oxygen in the photoisomerization. (2) Despite
almost the same triplet energies of DCA (175 kJ mol−1)11 and TMK (176 kJ mol−1),11 apparent
differences in selectivity were observed in the reactions under O2 (Table 2). This suggests that
excited triplet states of 13b and/or triplet molecular oxygen are not responsible for the observed
stereochemical outcome. (3) The PET reaction of 1a employing 2,6,9,10-tetracyanoanthracene
(TCA)12 as a photosensitizer was suppressed upon irradiation for the same period (3 h) as the
reaction reached the maximum isomer ratio in the DCA–PET reaction under O2. The result
suggests that a superoxide anion as an active oxygen species plays a major role in controlling the
stereochemical outcome.13 How superoxide takes part in the reaction of E-1a is considered as
follows (Scheme 1). Once electron transfer between a singlet excited DCA and N-
methoxymethanimines occurs, the resultant DCA anion radical reacts further with triplet oxygen
to generate superoxide anion.14 A mode of the interaction of these radical ion pairs is most likely
to be that of a thermally allowed [p2s+p2a] cycloaddition. Accordingly, the transition state has
a crisscrossed geometry as in the cycloaddition of neutral species. A similar explanation was
proposed previously in the cis selective formation of olefins with unstable phosphorus ylides and
aldehydes (a Wittig olefination).15 In addition, a non-triplet mechanism for geometrical isomer-
ization of olefins is already known.16
To explain the origin of the preferential Z selectivity of 1a and E selectivity of 1b in the PET
reactions under O2, we wish to propose a route to an azadioxetane via the direct interaction of
the radical ion pair, i.e. an N-methoxymethanimine radical cation and a superoxide anion.
Energy optimized structures (the semiempirical PM3 molecular orbital calculations17) of
radical cations of 1 are shown in Scheme 1. The p-methylphenyl groups in E- and Z-1a+ are
’
inclined in 89.5 and 88.4°, respectively, against the residual molecular frameworks.18 A positive
charge and a spin are delocalized over the p-anisyl-CꢀNꢁO system in both E and Z isomers.
Alternatively, both could be localized in the nitrogen lone pair, but this state is surely higher in
energy. A spin density is the highest on the nitrogen and it is essentially a p radical similar to
aminyls19 and alkoxyaminyls.20 Then if concerted [2s+2a] interaction of both ion radicals occurs
and an azadioxetane is formed at once, it would be difficult to explain the observed stereochem-
istry because it isn’t possible to expect so much energy difference in the isomers of azadioxe-
tanes. Radical cation 1a+ and superoxide initially make one new bond at the benzylic carbon
’
atom at which attack of superoxide is most likely to occur. However, the attacking oxygen of
the superoxide anion is discriminated in the crisscrossed geometry made up of both ion radicals,
due to steric hindrance imposed on the inclined p-methylphenyl group in E-1a+ and an oxygen
’
terminus of superoxide. In such a stepwise view, the interaction yields an intermediate 1,4-birad-
ical. According to the Skell–Woodworth hypothesis, there is then competition between bond
rotation that causes loss of E and Z, and the spin inversion necessitated for the second bond

8104
Scheme 1. Proposed reaction mechanism with the aid of the energy-minimized structures (PM3 semiempirical MO
calculations, the half electron method for open-shell compounds) of the intermediates. a: The values are those of the
triplet 1,4-diradicals

8105
formation. Perturbation of the terminal oxygen leads the methoxyaminyl p radical20 to spin flip
and concomitantly such loss of spin momentum is compensated by change of orbital momen-
tum. That is, a proximal oxygen affects a p aminyl radical, which then causes the spin flip and
the simultaneous change of an electronic configuration to a s aminyl radical. Considering the
values of DHf for the energy optimized triplet E- and Z-1a*ꢁO2*, the path from E-1a*ꢁO2* to
Z-1a*ꢁO2* is apparently uphill and energetically unfavorable. Nevertheless, the subsequent large
exothermic process to an azadioxetane (or a singlet biradical) would displace the equilibrium to
the right. Then the second bond formation of nitrogen and oxygen in Z-1a*ꢁO2* occurs and a
proposed azadioxetane is formed. Before complete ring closure, the intermediate releases oxygen
to give finally the observed preferential geometrical isomer of a CꢀN double bond. Byproducts
2 and 3 can be explained by decomposition of the azadioxetane. On the other hand, an
interaction of the Z-1a+ with superoxide proceeds via Z-1a*ꢁO2* without any leakage to
’
E-1a*ꢁO2*. Or if any, the following spin inversion in E-1a*ꢁO2* with a CꢁN bond rotation
again gives rise to the formation of Z-1a*ꢁO2* (singlet) or of the azadioxetane due to the
directionality of the s methoxyaminyl formed in the process. The root cause of the E selectivity
of 1b can be considered similarly.21
In summary, the PET reactions of the title N-methoxymethanimines (substituted benzophe-
none oxime O-methyl ethers) under O2 gave rise to the unprecedented geometrical isomerization.
The feature substantiates the unique PET reactivity of the oxime ethers. Further work including
the isomerization of the compounds containing the CꢀC and CꢀN double bonds is in progress.
References
1. Tokumaru, K.; Arai, T. Bull. Chem. Soc. Jpn. 1995, 68, 1065–1087 and references therein.
2. Drewer, R. J. In The Chemistry of the Hydrazo, Azo and Azoxy Groups; Patai, S. Ed.; Wiley: New York, 1975;
part 2.
3. (a) Padwa, A. Chem. Rev. 1977, 77, 37–68; (b) Takeda, Y.; Misawa, H.; Sakuragi, H.; Tokumaru, K. Bull. Chem.
Soc. Jpn. 1989, 62, 2213–2218.
4. (a) Kawamura, Y.; Iwano, Y.; Shimizu, Y.; Tokai, Y.; Horie, T. Chem. Lett. 1994, 707–710; (b) Iwano, Y.;
Kawamura, Y.; Miyoshi, H.; Yoshinari, T.; Horie, H. Bull. Chem. Soc. Jpn. 1994, 67, 2348–2350; (c) Iwano, Y.;
Kawamura, Y.; Horie, T. Chem. Lett. 1995, 67–68.
5. Lewis, F. D. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M. Eds.; Elsevier: Amsterdam, 1988; part
C, chapter 4.1, pp. 8–14.
6. Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271.
7. N-Methoxymethanimines were prepared generally by a Friedel–Crafts acylation of benzoyl chloride derivatives,
followed by the condensation of the ketones obtained with NH2OH·HCl, and then O-methylation of the oximes
with MeI. A formyl group on one of the aryl groups in 1b was introduced by a controlled chromium oxidation
of 4,4%-dimethylbenzophenone. Geometrical notation of E about an iminic double bond of 1b shows the opposite
arrangement of N-methoxy and p-formylphenyl groups across the bond. Similarly, the notation E of 1a shows
the opposite relationships of N-methoxy and p-methoxyphenyl groups. The E/Z ratio of 1 was 1/1 before
photoirradiation.
8. Geometrical isomers of 1a were not separated even by high pressure liquid chromatography (HPLC) using a silica
gel column eluted with 10% EtOAc in hexane. But those of 1b were separated by HPLC with difficulty. Isomer
1
ratios were, therefore, determined by H NMR (400 MHz) in all experiments. Geometry of 1 was determined by
NOE experiments on degree of enhancement of the peak areas of the aromatic protons upon irradiation of
N-methoxyl signals. Experimental conditions: [1]=10−3 M. [DCA]=10−5 M. Dry MeCN solutions of samples
were irradiated with a high-pressure mercury lamp [Riko® 400 W (Toshiba® glass filter L-39, >360 nm for the
PET reactions; Pyrex® glass filter, >280 nm for the direct irradiation)] at <10°C in a merry-go-round apparatus
(Riko® RH400-10W) after purging Ar or O2 for 30 min in Pyrex® test tubes.

8106
9. The value of the isomer ratio shown in Table 1 is not that in the photostationary state. We believe that it is still
meaningful relevant to the term, one-way photoisomerization of the CꢀN double bond, according to the
following experimental result and the argument below.21 A PET reaction becomes complicated generally, when
it is performed under O2. In our preparative run of 1a ([1a]=1.2×10−2 M), the mixture of isomer (E-1a/Z-1a=35/
65) and benzophenone derivative were obtained in 73 and 2.6% yields, respectively, based on the starting 1a after
4 h photoirradiation under O2. It is considered that the E/Z PET isomerization is the major one.
10. Both compounds were identified by comparison with the authentic samples.
11. Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry; New York: Marcel Dekker, 1993; 2nd ed.
pp. 59–60.
12. The compound does not produce the superoxide anion as judged by the redox potential of it and that of
molecular oxygen.
13. Song, K.; Wu, L.-Z.; Yang, C.-H.; Tung, C.-H. Tetrahedron Lett. 2000, 41, 1951–1954. In this paper, Song et al.
partly studied the DCA-sensitized photooxidation of 3-styrylthiophene and they concluded that the oxidation
proceeded via superoxide.
14. Eriksen, J.; Foote, C. S.; Parker, T. L. J. Am. Chem. Soc. 1977, 99, 6455–6456.
15. Yamataka, H.; Nagareda, K.; Takatsuka, T.; Ando, K.; Hanafusa, T.; Nagase, S. J. Am. Chem. Soc. 1993, 115,
8570–8576.
16. (a) Yang, N. C.; Cohen, J. I.; Shani, A. J. Am. Chem. Soc. 1968, 90, 3264–3266; (b) Saltiel, J.; Neuberger, K. R.;
Wrighton, M. J. Am. Chem. Soc. 1969, 91, 3658–3659; (c) Caldwell, R.; James, S. P. J. Am. Chem. Soc. 1969, 91,
5184–5186; (d) Caldwell, R. A. J. Am. Chem. Soc. 1970, 92, 1439–1441.
17. Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221–264.
18. The values of DHf for the neutral E- and Z-1a are 16.9 and 19.6 kcal/mol, respectively. These values for the
radical cations of E- and Z-1a are 206.4 and 210.8 kcal/mol, respectively. By contrast, the corresponding values
for the optimized structures of 1a having the p-methoxyphenyl group arranged perpendicularly to the rest of the
molecular framework were 218.4 and 212.2 kcal/mol for the E- and Z-1a radical cations, respectively. In order
to avoid assigning erroneous optimized structures as the most energetically favorable ones due to the presence of
multiple energy minima, the calculations were performed with starting several initial structures of 1.
19. (a) Danen, W. C.; Neugebauer, F. A. Angew. Chem., Int. Ed. Engl. 1975, 14, 783–789; (b) Norman, R. O. C.
Chemistry in Britain 1970, 6, 66–70 and references cited therein.
20. Danen, W. C.; West, T. C.; Kensler, T. J. Am. Chem. Soc. 1973, 95, 5716–5724.
21. About this mechanism, one of the referees pointed out that compound 1a gave solely the Z isomer and 1b gave
the E isomer, otherwise, this reaction cannot be one-way isomerization. Actually, decomposition products were
formed and coloration of the reaction mixture developed during the progress of the reaction. The analysis of the
product ratio in the limit was difficult for these reasons. Concerning this reaction, we found recently a similar
photoreaction of a CꢀC double bond. It was confirmed that one isomer isomerized perfectly to another isomer
upon irradiation of DCA. Considering the importance of the proper usage of the term, one-way isomerization,
in the present reaction, we believe that the reaction is where one isomer of 1 isomerizes selectively into another,
similar to the reaction of the CꢀC double bond.
.