Photochemical transformation of γ,δ-unsaturated ketone O-(p-cyanophenyl)oximes to 3,4-dihydro-2H-pyrrole derivatives

Article abstract of DOI:10.1246/cl.2000.338

γ,δ-Unsaturated ketone O-(p-cyanophenyl)oximes are transformed to 3,4-dihydro-2H-pyrroles by the photosensitized electron transfer process.

Full text of DOI:10.1246/cl.2000.338

338
Chemistry Letters 2000
Photochemical Transformation of γ,δ-Unsaturated Ketone O-(p-Cyanophenyl)oximes
to 3,4-Dihydro-2H-pyrrole Derivatives
Tetsuhiro Mikami and Koichi Narasaka*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received December 27, 1999; CL-991103)
γ,δ-Unsaturated ketone O-(p-cyanophenyl)oximes are
transformed to 3,4-dihydro-2H-pyrroles by the photosensitized
electron transfer process.
Alkylideneaminyl radicals, conventionally called iminyl
radicals, are attractive intermediates for the construction of
nitrogen-containing heterocycles.1 As a leading group in this
area, Zard and co-workers have presented various systems for
the generation of alkylideneaminyl radicals.2 Their latest report
described a new system for generating these species using nick-
el powder and acetic acid in 2-propanol.2f Weinreb and col-
leagues also developed the generation of alkylideneaminyl radi-
cals from O-(2,6-dimethylbenzenesulfinyl)oximes via the modi-
fied Hudson reaction.3
Recently, we have reported that the cyclization of γ,δ-un-
saturated ketone O-(2,4-dinitrophenyl)oximes proceeds on the
oxime nitrogen atom by treatment with sodium hydride and 3,4-
methylenedioxyphenol, giving 3,4-dihydro-2H-pyrroles.4 Be-
cause this cyclization is considered to proceed via one-electron
transfer from the sodium phenolate to the dinitrophenyloxime,
it was expected that similar electron transfer would occur
between a sensitizer and electron deficient O-aryloximes by
photoirradiation (Scheme 1).
9% conversion yield and in a trace amount, respectively.
Concerning the influence of wavelength, the irradiation of
the dinitro derivative 1 with shorter wavelength >300 nm of
UV increased the yield of the ketone 3 and the azine 4 (Entry
2), and that with longer wavelength (>340 nm) proceeded more
slowly than that with >320 nm (Entry 3).
Since a fluorescence quenching of excited DMN by an
adding of the oxime 1 was observed, the electron transfer pre-
sumably proceeds in an excited singlet state, giving an anion
radical A. The anion radical A would cyclize to generate alkyl
radical intermediate B, which is trapped with 1,4-cyclohexadi-
ene to yield the cyclic imine 2a. The ketone 3 might be pro-
duced by hydrogen abstraction of the alkylideneaminyl radical
In our first experiment (Table 1, Entry 1), UV irradiation
of 1-phenylhept-6-en-3-one O-(2,4-dinitrophenyl)oxime (1)
was performed in the presence of 1,5-dimethoxynaphthalene
(DMN, a 0.5 molar amount) as a sensitizer for the electron-
transfer5 and 1,4-cyclohexadiene (7.6 molar amounts) as a
hydrogen source in acetonitrile–2,2,2-trifluoroethanol (1 : 1)6
by 250 W mercury–xenon lamp through a UV cut filter (>320
nm). The desired cyclization product, 2-methyl-5-(1-phenyl-
ethyl)-3,4-dihydro-2H-pyrrole (2a), was obtained in 69%
conversion yield, while side products, 1-phenylhept-6-en-3-one
(3) and 1-phenylhept-6-en-3-one azine (4), were generated in
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
339
While the complete conversion of 9a was hardly attained
in a mixture of acetonitrile and 2,2,2-trifluoroethanol even after
36 h irradiation (Table 1), the reaction was highly improved to
consume the oxime 9a after 24 h irradiation in acetonitrile, giv-
ing the cyclized product 2a in 77% yield (Entry 9). Under
lower concentration of 1,4-cyclohexadiene, the yield of 2a
slightly decreased (Entry 10), and, even without 1,4-cyclohexa-
diene, the cyclization proceeded to afford the cyclic imine 2a in
47% yield (Entry 11). The reaction in acetonitrile-d3 afforded
the corresponding 2-monodeuterized methyl product 2a, which
indicated the hydrogen donor to be acetonitrile.
As summarized in Table 2, the photochemical reaction of
various γ,δ-unsaturated ketone O-(p-cyanophenyl)oximes was
examined in acetonitrile with DMN as a sensitizer and 1,4-
cyclohexadiene as a hydrogen source by 500 W mercury–xenon
lamp through the UV cut filter (>320 nm). Mono cyclic imines
2a-e were obtained selectively by 5-exo cyclization of γ,δ-
unsaturated ketone oximes 9a-e in 53-78% yield. Only in the
case of oxime 9f having a styrene moiety, the desired product 2f
was obtained in only 13% yield along with many side products.
Bicyclic products, hexahydroazapentalene 2g and hexa-
hydroindole 2h were also prepared by the present photo-
chemical cyclization in 78% and 69% yield, respectively. In
addition, the cyclization of oxime 9i gave bicyclic imine 2i and
enamine 10 in 60% and 15% yield, respectively.
Thus, 5-exo cyclization proceeds successfully, while the
trial of 6-exo cyclization of δ,ε-unsaturated ketone oxime 9j
gave a complex mixture, which contained the corresponding
ketone and azine as major products.
In conclusion, γ,δ-unsaturated ketone O-(p-cyanophenyl)-
oximes are converted into dihydropyrrole derivatives by using
photosensitized electron transfer process.
References and Notes
1
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2
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Biadatti, A.-C. Callier-Dublanchet, J. H. Horner, B.
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C, which is generated by O–N bond cleavage of the anion radi-
cal A, followed by hydrolysis of the resulting imine.7 The for-
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ization of the aminyl radical C.
3
4
5
In order to screen the substituent effect of O-phenyl group,
various oximes having electron-deficient O-aryl groups were
prepared. The reaction of a mononitro derivative, O-(p-nitro-
phenyl)oximes 5, proceeded slowly as compared with the dini-
tro derivative 1 (Entry 4), and O-(p-trifluoromethyl)phenyl de-
rivative 6 cyclized scarcely (Entry 5). Though the reaction of
o-cyanophenyl derivatives 7 and 8 gave good conversion of the
starting material, a considerable amount of the side product, the
ketone 3, was increased (Entries 6 and 7). Photoreaction of O-
(p-cyanophenyl)oxime 9a proceeded most efficiently and af-
forded the cyclized product 2a in 70% conversion yield (Entry
8).
K. Uchiyama, Y. Hayashi, and K. Narasaka, Chem. Lett.,
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6
7
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