Photochemical reactivity of 1-substituted-1-aza-1,4-dienes promoted by electron-acceptor sensitizers. Di-π-methane rearrangements and alternative reactions via radical-cation intermediates

Article abstract of DOI:10.1021/jo026440l

Irradiation of a series of β,γ-unsaturated imines, oxime acetates, and oxime methyl ethers, using 9,10-dicyanoanthrathene (DCA) or dicyanodurene (DCD) as electron acceptor sensitizers, affords the corresponding cyclopropanes resulting from 1-aza-di-π-methane rearrangements via radical cations. In some cases, alternative reactions of these intermediates occur to yield nitriles, dihydroquinolines, dihydronaphthalene derivatives, and cycloaddition products. Some of these products result from reactions via alkene radical-cation intermediates while others arise by pathways involving imine radical-cation intermediates. The yields of products formed in these processes were significantly higher when DCD was used as electron-acceptor sensitizer instead of DCA.

Full text of DOI:10.1021/jo026440l

P h otoch em ica l Rea ctivity of 1-Su bstitu ted -1-a za -1,4-d ien es
P r om oted by Electr on -Accep tor Sen sitizer s. Di-π-m eth a n e
Rea r r a n gem en ts a n d Alter n a tive Rea ction s via Ra d ica l-Ca tion
In ter m ed ia tes
Diego Armesto,* Maria J . Ortiz,* Antonia R. Agarrabeitia, Santiago Aparicio-Lara,
Mar Martin-Fontecha, Marta Liras,† and M. Paz Martinez-Alcazar‡
Departamento de Quimica Organica I, Facultad de Ciencias Quimicas, Universidad Complutense,
28040-Madrid, Spain
darmesto@quim.ucm.es; mjortiz@quim.ucm.es
Received September 12, 2002
Irradiation of a series of â,γ-unsaturated imines, oxime acetates, and oxime methyl ethers, using
9,10-dicyanoanthrathene (DCA) or dicyanodurene (DCD) as electron acceptor sensitizers, affords
the corresponding cyclopropanes resulting from 1-aza-di-π-methane rearrangements via radical
cations. In some cases, alternative reactions of these intermediates occur to yield nitriles,
dihydroquinolines, dihydronaphthalene derivatives, and cycloaddition products. Some of these
products result from reactions via alkene radical-cation intermediates while others arise by
pathways involving imine radical-cation intermediates. The yields of products formed in these
processes were significantly higher when DCD was used as electron-acceptor sensitizer instead of
DCA.
In tr od u ction
regioisomeric products, oxiranes and aziridines, that
would be formed in the respective ODPM and 1-ADPM
reactions have not been observed.
Since their discovery in 1967 by Zimmerman and co-
workers,1 the photoinduced di-π-methane rearrangement
has been a major topic of research in the area of organic
photochemistry. Efforts by numerous research groups
have focused on different versions of this rearrangement.
The work has led to the discovery of the di-π-methane
(DPM) rearrangement of 1,4-dienes, the oxa-di-π-meth-
ane (ODPM) rearrangement of â,γ-unsaturated ketones
and aldehydes, and the 1-aza-di-π-methane (1-ADPM)
rearrangement of 1-aza-1,4-dienes.2 These reactions are
general and typically take place with high chemical and
quantum efficiencies to afford cyclopropanes that, in
many instances, are difficult or impossible to obtain by
alternative preparative routes. In addition, the high
degree of stereoselectivity, reported for some cases,
makes these processes useful tools in synthetic organic
chemistry.2d,h
Recent results obtained in our laboratory have led to
a drastic modification of these ideas. Thus, in a report
on the photoreactivity of 2-aza-1,4-dienes, we described
the first example of a 2-aza-di-π-methane (2-ADPM)
rearrangement, promoted by triplet-sensitized irradiation
of compound 1, that yields the vinylaziridine 2 and the
cyclopropylimine 3.3 The importance of this result lies
in the fact that it is the first example of a di-π-methane
reaction, which occurs via a three-membered ring het-
erocyclic biradical, yielding a heterocyclic product. How-
ever, the most interesting observation made in this study
was that irradiation of compound 1, using 9,10-dicyano-
anthrathene (DCA) as an electron-acceptor sensitizer,
leads to formation of the vinylaziridine 2, resulting from
a 2-ADPM reaction, and the cyclopropylimine 4, arising
(2) For specific and general reviews on the different versions of di-
π-methane rearrangements see: (a) Hixson, S. S.; Mariano, P. S.;
Zimmerman, H. E. Chem. Rev. 1973, 73, 531-551. (b) Zimmerman,
H. E. In Rearrangements in Ground and Excited States; DeMayo, P.,
Ed.; Academic Press: New York, 1980; Vol. 3, pp 131-166. (c)
Zimmerman, H. E. In Organic Photochemistry; Padwa, A. Ed.; Marcel
Dekker: New York, 1991; Vol. 11, pp 1-36. (d) Zimmerman, H. E.;
Armesto, D. Chem. Rev. 1996, 96, 3065-3112. (e) Dauben, G. W.;
Lodder, G.; Ipaktschi, J . Top. Curr. Chem. 1975, 54, 73-114. (f) Houk,
K. N. Chem. Rev. 1976, 76, 1-74. (g) Schuster D. I. In Rearrangements
in Ground and Excited States; DeMayo, P., Ed.; Academic Press: New
York, 1980; Vol. 3, pp 167-279. (h) Demuth, M. In Organic Photo-
chemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1991; Vol. 11,
pp 37-109. (i) Armesto, D. In CRC Handbook of Organic Photochem-
istry and Photobiology; Horspool, W. M., Soon, P.-S., Eds.; CRC Press:
New York, 1995; pp 915-930. (j) Armesto, D. EPA Newsl. 1995, 53,
6-21.
For many years, the di-π-methane and closely related
rearrangements have been the paradigm of processes
that take place in the excited state only.2 Biradical or
concerted mechanisms have been proposed to account for
the reactions.2 The ODPM and the 1-ADPM rearrange-
ments are regioselective, yielding the corresponding
cyclopropane derivatives exclusively. The alternative
† Current address: Instituto de Quimica Organica “Lora Tamayo”,
C.S.I.C., C/ J uan de la Cierva, 3, 28006-Madrid, Spain.
‡ To whom inquires regarding the X-ray data should be addressed.
Permanent address: Universidad San Pablo-CEU, Departamento de
Quimica, Facultad de Ciencias Experimentales y de la Salud, Urban-
izacion Monte Principe, Boadilla del Monte, 28668-Madrid, Spain.
(1) Zimmerman, H. E.; Binkley, R. W.; Givens, R. S.; Sherwin, M.
A. J . Am. Chem. Soc. 1967, 89, 3932-3933.
(3) Armesto, D.; Caballero, O.; Amador, U. J . Am. Chem. Soc. 1997,
119, 12659-12660.
10.1021/jo026440l CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/23/2002
J . Org. Chem. 2002, 67, 9397-9405
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Armesto et al.
TABLE 1. Rea ction Con d ition s a n d Yield s of P r od u cts
in th e Ir r a d ia tion of 1-Aza -1,4-d ien es Usin g DCA a n d
DCD a s Electr on -a ccep tor Sen sitizer s
by an aryl-di-π-methane process. DCA-sensitized irradia-
tion of azadiene 5 also results in production of a vinyl-
aziridine 6 and the dihydroisoquinoline 7. The formation
of 2, 4, and 6 represents the first examples of di-π-
methane rearrangements that take place via radical-
cation intermediates.3,4
The results, summarized above, suggest that other â,γ-
unsaturated compounds might also be capable of par-
ticipating in di-π-methane type rearrangements via
radical-cation intermediates. To test this proposal, we
have conducted photochemical studies with a series of
1-substituted-1-aza-1,4-dienes. The results demonstrate
that these substances undergo novel 1-ADPM rearrange-
ments via radical cations promoted by single electron
transfer (SET) sensitization. In some instances alterna-
tive cyclizations have been observed. Some of the results
described here have been the subject of a preliminary
communication.5
irrad
SET- 1-ADPM other products
(yield, %)
S. M.
yield (%)
substr. time (h) sens. (yield, %)
8a
8a
8b
8b
7
2
3
0.5
0.5
2.5
1
3
1.5
1
0.75
0.25
2
0.4
13.5
3
4
1
2
DCA 9a (13)a
DCD 9a (46)a
DCA 9b (2)a
DCD 9b (11)a
DCA 9c (4)a
DCA 25a (15) 26 (2)
DCD 25a (60)
DCA 25b (28) 27 (8)
DCD 25b (55)
DCA 25c (16) 28 (9), 31 (18)
DCA 25d (4)
DCD 25d (33)
DCA 25e (2)
DCD 25e (44)
DCA 35a (10)
DCD 35a (7)
DCA
65a
30a
81a
63a
63a
66
33
30
15
26
71
34
68
41
40
37
19
61
30
16 (10)
16 (13)
21 (13)
8c
24a
24a
24b
24b
24c
24d
24d
24e
24e
34a
34a
34b
34b
34c
29 (14)
39 (23)
36 (7)
DCD
DCA 35c (3)
a
Isolated as the corresponding aldehyde.
SCHEME 1
Resu lts a n d Discu ssion
and Z-diastereoisomers), trans-2,2-dimethyl-3-phenylcy-
clopropanecarbaldehyde8 (13%), and unidentified high
polar materials (Table 1). The aldehydes are formed by
hydrolysis of the corresponding imines 8a and 9a during
chromatography.
This result demonstrates that, like the 2-aza-1,4-dienes
analogues, 1 and 5, 1-aza-1,4-diene 8a also undergoes a
di-π-methane type radical-cation rearrangement reaction.
The mechanism shown in Scheme 1 accounts for the
formation of 9a . It involves initial generation of the
radical-cation intermediate 10 that undergoes bonding
to give the cyclopropane ring containing radical-cation
11. Ring opening by bond a cleavage in 11 generates 12,
which, by back electron transfer and biradical cyclization,
yields 9a . The alternative route, involving cleavage of
bond b and forming the vinylaziridine 13, does not par-
DCA-Sen sitized Ir r a d ia tion of Im in es. The com-
pounds selected for our initial studies are the imines 8a -
c, the syntheses of which have been described previ-
ously.4b,6 Irradiation of a 1:1 mixture of (1E,4E)/(1E,4Z)
diastereoisomers of 8a ,4b in acetonitrile for 7 h, using
DCA as an electron-acceptor sensitizer and biphenyl as
a cosensitizer, leads to formation of the cyclopropylimine
9a and recovered starting material as a 2:1 mixture of
(1E,4E)/(1E,4Z) diastereoisomers (1H NMR analysis of
the crude photolyzate). Silica gel chromatography yields
2,2-dimethyl-4-phenyl-3-butenal7 (65%, 2:1 mixture of E-
(4) Within this area of SET-promoted di-π-methane rearrangements,
we have recently described the first examples of 1-ADPM and DPM
rearrangements that take place via radical-anion intermediates (ref
4b). (b) Armesto, D.; Ortiz, M. J .; Agarrabeitia, A. R.; Martin-Fontecha,
M. J . Am. Chem. Soc. 2001, 123, 9920-9921.
(5) Ortiz, M. J .; Agarrabeitia, A. R.; Aparicio-Lara, S.; Armesto, D.
Tetrahedron Lett. 1999, 40, 1759-1762.
(6) Armesto, D.; Martin, J . F.; Perez-Ossorio R.; Horspool, W. M.
Tetrahedron Lett. 1982, 23, 2149-2152. (b) Armesto, D.; Horspool, W.
M.; Martin, J .-A. F.; Perez-Ossorio R. J . Chem. Res. 1986, (S), 46-47.
(M), 0631-0648.
(7) van der Weerdt, A. J . A.; Cerfontain, H. Tetrahedron, 1981, 37,
2121-2130.
(8) Armesto, D.; Ortiz, M. J .; Romano, S. Tetrahedron Lett. 1995,
36, 965-968. (b) Armesto, D.; Ortiz, M. J .; Romano, S.; Agarrabeitia,
A. R.; Gallego, M. G.; Ramos, A. J . Org. Chem. 1996, 61, 1459-1466.
9398 J . Org. Chem., Vol. 67, No. 26, 2002

Reactivity of 1-Substituted-1-aza-1,4-dienes
SCHEME 2
SCHEME 4
SCHEME 5
SCHEME 3
of the electron-acceptor sensitized photoreactions of 1,4-
dienes 19 which yield benzhydryldihydronaphthalenes
20.12 However, it is interesting to note that dienes 19 do
not undergo radical-cation rearrangements of the di-π-
methane type.
When imine (1E,4E)-8c4b is irradiated for 30 min under
the same conditions used for 8a and 8b, it reacts to form
the (E)-cyclopropylimine 9c and a new compound (1H
NMR analysis of the crude photolyzate). Separation of
the photoproduct mixture by column chromatography on
silica gel yields 3-(1-indenylidene)-2,2-dimethylpropanal13
(63%, 2:1 mixture of E/Z diastereoisomers), dihydro-
quinoline 21 (13%), and the spirocarbaldehyde 2213 (4%).
The isolated carbonyl compounds are formed by hydroly-
sis of the corresponding imines 8c and 9c during isolation
(Scheme 4).
The identity of 21 was established based on the
comparison of its spectroscopic properties with those of
closely related compounds.3,12 The mechanism shown in
Scheme 5 accounts for the formation of dihydroquinoline
21. In the route, an olefin centered radical cation
undergoes intramolecular electrophilic addition to the
phenyl ring attached to nitrogen, generating the inter-
mediate 23, which, by aromatization and hydrogen
abstraction, yields 21.
The results obtained from studies of the DCA-sensi-
tized photoreactions of imines 8a -c demonstrate that
these compounds react to afford the corresponding cy-
clopropylimines 9a -c in low yields. These processes
represent second examples of di-π-methane type rear-
rangements that take place via radical-cation intermedi-
ticipate, because the resulting radical-cation 14 would
be considerably less stable than intermediate 11 (Scheme
1).
Irradiation of imine (E)-8b6 for 3 h, under the above
conditions, affords the cyclopropylimine 9b and a new
substance 15 (1H NMR analysis of the crude photolyzate).
Separation by silica gel column chromatography gives
2,2-dimethyl-4,4-diphenyl-3-butenal9 (81%), 2,2-dimethyl-
4-phenyl-2H-naphthalen-1-one 16 (10%), and the cyclo-
propanecarbaldehyde 1710 (2%). The isolated carbonyl
compounds are formed by hydrolysis of the corresponding
imines 8b, 9b, and 15 during chromatography (Scheme
2).
The identity of 16 was assigned based on its spectro-
scopic properties and by independent synthesis from
methyl 2-methyl-4,4-diphenyl-3-butenoate11 (Experimen-
tal Section). This result shows that two different reaction
paths are open to the radical-cation intermediate derived
from 8b. The first involves 1-ADPM rearrangement
leading to 9b, and the other is promoted by cyclization
affording 15, which is the more efficient reaction in this
instance. A possible mechanism to justify the formation
of 15 could involve intramolecular electrophilic attack of
a radical cation centered in the C-N double bond to one
of the phenyl rings at C5 to afford the intermediate 18,
which losses a proton and a hydrogen radical yielding
15 (Scheme 3). A similar cyclization process has been
observed by Zimmerman and co-workers in their study
(9) Zimmerman, H. E.; Pratt, A. C. J . Am. Chem. Soc. 1970, 92,
6259-6267.
(10) Zimmerman, H. E.; Mariano, P. S. J . Am. Chem. Soc. 1969,
91, 1718-1727.
(12) Zimmerman, H. E.; Hoffacker, K. D. J . Org. Chem. 1996, 61,
6526-6534.
(13) Armesto, D.; Gallego, M. G.; Horspool, W. M.; Agarrabeitia, A.
R. Tetrahedron 1995, 51, 9223-9240.
(11) Armesto, D.; Ortiz, M. J .; Ramos, A.; Horspool, W. M.; Mayoral,
E. P. J . Org. Chem. 1994, 59, 8115-8124.
J . Org. Chem, Vol. 67, No. 26, 2002 9399

Armesto et al.
ates. However, the main photoproducts obtained in the
photoreactions of 8b and 8c are the dihydronaphthale-
none derivative 16 and the dihydroquinoline 21, respec-
tively. With regard to the formation of these two prod-
ucts, it is worth noting that while compound 15 results
from a radical cation centered in the imine moiety, the
quinoline 21 is formed from a radical cation centered in
the alkene unit. This surprising result suggests that,
under these reaction conditions, two different and com-
petitively reactive radical-cation intermediates are gen-
erated from two donor centers with different ionization
potentials (the alkene and the imine).
DCA-Sen sitized Ir r a d ia tion of Oxim e Aceta tes.
Previous studies have shown that â,γ-unsaturated oxime
acetates undergo the 1-ADPM rearrangement more ef-
ficiently that do the corresponding imines in the triplet
excited state.14 In addition, the former compounds have
the advantage over the imines that they do not hydrolyze
readily and, therefore, are easier to handle. Conse-
quently, our studies of SET-sensitized photoreactions
were extended to the oxime acetates (1E,4E)-24a ,15 (E)-
24b,14 and (1E,4E)-24c,13 which are structurally related
to the imines 8a -c. DCA-sensitized irradiation of com-
pounds 24a -c, using biphenyl as cosensitizer, for vari-
able times, afford the corresponding cyclopropanes (E,E)-
25a 15 (15%), (E)-25b14 (28%), and (E,E)-25c13 (16%),
respectively, resulting from 1-ADPM rearrangements
(Table 1).
The results show that the 1-ADPM rearrangement via
radical-cation intermediates can be extended to â,γ-
unsaturated oxime acetates. The qualitative efficiencies
of the reactions of oxime acetates 24a -c and the yields
of products are clearly higher than those for the imines
8a -c (Table 1). Additional investigations showed that
irradiation of oxime acetates 24d -e,13,14b under the above
conditions, also leads to formation of the corresponding
1-ADPM photoproducts (E)-25d 14b (4%) and (1E,4E)-25e13
(2%), respectively (Table 1). For unknown reasons, the
yield of isolated products in these two processes is
considerably lower than those for reaction of 24a -c, even
though azadienes 24d and 24e give the corresponding
cyclopropanes in high yield in the triplet-sensitized
photoreactions. In addition to the cyclopropane (E,E)-25a ,
SET-sensitized photoreaction of (1E,4E)-24a also gener-
ates recovered starting material (66%), as a mixture 3:1
of (1E,4E)/(1E,4Z) diastereoisomers, and the nitrile (E)-
26 (2%), resulting from elimination of acetic acid.
(Scheme 6) involving generation of a C-N bond centered
radical-cation 30 that eliminates acetic acid to yield the
nitrile radical cation. Back electron transfer then yields
the nitrile.
Irradiation of oxime acetate 24c gives, in addition to
the cyclopropane 25c and the nitrile 28, unchanged
recovered started material (26%) and a new product that
was identified as the dihydroisoxazole 31 (18%), by
comparing its spectroscopic properties to those of related
substances.16,17 A possible mechanism to justify formation
of 31 involves cyclization within the radical-cation 32 to
generate the intermediate 33, which by elimination of
acetic acid yields 31 (Scheme 7). In this instance, it is
difficult to know whether the reaction is promoted by a
radical cation centered in either the alkene or the oxime
acetate moieties.
The results obtained in the study of the DCA electron-
acceptor sensitized photoreactivity of â,γ-unsaturated
oxime acetates 24 show that these compounds undergo
1-ADPM rearrangement via radical-cation intermediates
qualitatively more efficiently than the corresponding
imines 8. In some cases, radical-cation intermediates,
generated from these substances, also undergo elimina-
tion of acetic acid to form the corresponding nitriles.
Obviously, the alternative cyclization to dihydroquinoline
21 observed in the irradiation of imine 8c, which involves
the phenyl ring attached to nitrogen, cannot take place
in oxime acetates. Finally, in one instance (24c), an
alternative cyclization to dihydroisoxazole 31 was ob-
served.
The same type of elimination reaction is observed in
the irradiations of 24b, 24c, and 24e that give the
corresponding nitriles 27 (8%), 28 (9%), and 29 (14%),
respectively (Table 1). The identity of these nitriles was
easily established by use of conventional spectroscopic
techniques. The possibility that this elimination process
is thermally activated seems unlikely, since nitrile
products were not observed to form in the triplet-
sensitized irradiations of oxime acetates 24. The forma-
tion of these nitriles could by explained by a pathway
DCA-Sen sitized Ir r a d ia tion of Oxim e Eth er s. â,γ-
Unsaturated oxime ethers are hydrolytically stable C-N
double bond derivatives that are easier to handle than
the corresponding imines. However, previous photochemi-
cal studies showed that oxime ethers do not undergo
1-ADPM rearrangement.18 Therefore, it was interesting
to probe the DCA-SET-sensitized photochemical behavior
of oxime ethers 34 in order to determine if the lack of
1-ADPM reactivity of these compounds in the triplet
(14) Armesto, D.; Horspool, W. M.; Langa, F. J . Chem. Soc., Chem.
Commun. 1987, 1874-1875. (b) Armesto, D.; Horspool, W. M.; Langa,
F.; Ramos, A. J . Chem. Soc., Perkin Trans. 1 1991, 223-228.
(15) Armesto, D.; Agarrabeitia, A. R.; Horspool, W. M.; Gallego, M.
G. J . Chem. Soc., Chem. Commun. 1990, 934-936. (b) Armesto, D.;
Horspool, W. M.; Gallego, M. G.; Agarrabeitia, A. R. J . Chem. Soc.,
Perkin Trans. 1 1992, 163-169.
(16) Armesto, D.; Ramos, A.; Ortiz, M. J .; Manchen˜o, M. J .; Mayoral,
E. P. Recl. Trav. Chim. Pays-Bas 1995, 114, 514-516.
(17) Armesto, D.; Ramos, A.; Ortiz, M. J.; Horspool, W. M.; Manchen˜o,
M. J .; Caballero, O.; Mayoral, E. P. J . Chem. Soc., Perkin Trans. 1
1997, 1535-1541.
(18) Pratt, A. C.; Abdul-Majid, Q. J . Chem. Soc., Perkin Trans. 1
1987, 359-364.
9400 J . Org. Chem., Vol. 67, No. 26, 2002

Reactivity of 1-Substituted-1-aza-1,4-dienes
SCHEME 6
SCHEME 8
SCHEME 7
A possible mechanism for formation of 36 involves the
generation of radical-cation/radical-anion pair 37. In-
tramolecular cyclization of the radical cation within the
solvent cage generates the radical-cation/radical-anion
pair 38, which by intermolecular cycloaddition yields 36.
The results obtained in studies with â,γ-unsaturated
oxime ethers show that these compounds are unreactive
in the 1-ADPM mode.18 However, oxime ethers 34a and
34c undergo the 1-ADPM rearrangement via radical-
cation intermediates in low yields. The formation of
cycloadduct 36 from 34c is surprising, and it demon-
strates that unexpected alternative reaction routes are
opened to the intermediate radical cations in some
instances.
In flu en ce of th e Electr on -Accep tor Sen sitizer s on
th e P h otop r od u ct Yield s. The results summarized
thus far indicate that 1-substituted-1-aza-1,4-dienes
undergo 1-ADPM rearrangements upon irradiation using
DCA as an electron-acceptor sensitizer. In addition, â,γ-
unsaturated oxime acetates 24 were found to react
qualitatively more efficiently than the corresponding â,γ-
unsaturated imines 8 while the â,γ-unsaturated oxime
ethers 34 are less reactive. In some instances, alternative
reaction modes have been observed, yielding naphthalene
derivatives, dihydroquinolines, dihydroisoxazoles, [4 +
4] cycloadducts, and products resulting from elimination
reactions. However, from a synthetic point of view, these
reactions have little interest, since the product yields are
usually low (Table 1). Previous studies have shown that
SET-photoreaction sensitized by dicyanodurene (DCD)
often occurs with higher yields.19 However, this sensitizer
has the disadvantage that it absorbs light at shorter
wavelengths than does DCA and, therefore, it cannot be
used when the electron-donor molecules absorb strongly
above 300 nm. In an attempt to increase the yield of
products, we have explored the photoreactivity of 1-aza-
1,4-dienes 8a , 8b, 24a , 24b, 24d , 24e, 34a , and 34b, all
of which do not absorb strongly above 300 nm, using DCD
as the electron-acceptor sensitizer and biphenyl as the
cosensitizer. The results obtained, summarized in Table
1, show that reactions under these conditions require
shorter irradiation times, in most cases, and result in
increased yields of the 1-ADPM photoproducts. For
instance, DCA-sensitized irradiation of imine 8a , for 7
h, affords cyclopropane 9a in 13% yield, while DCD-
sensitized reaction of this substance for 2 h gives 9a in
excited state could be circumvented by generating the
corresponding radical cations.
Oxime ether 34b has been described previously,18 and
the related substances, 34a and 34c, were synthesized
from the corresponding aldehydes by use of conventional
procedures. DCA-sensitized irradiation of 34a for 13.5 h
promotes formation of the 1-ADPM rearrangement de-
rived cyclopropane 35a (10%), recovered starting material
(40%), and a complex mixture of products. Irradiation of
34b, under the same conditions used for 34a , for 4 h gives
recovered starting material (19%) and a complex mixture
1
of products. H NMR analysis of the crude photolyzate
showed that it does not contain the corresponding
1-ADPM product.
Oxime ether (1E,4E)-34c, when irradiated under the
above conditions for 2 h, yields the spirocyclopropane 35c
(3%), recovered starting material (30%) as a 3:4 mixture
of (1E,4Z)/(1E,4E) diastereoisomers, and a new compound
(7%) that was identified as the [4 + 4]-cycloadduct 36 by
using X-ray diffraction analysis. The identities of 35a and
35c were established based on their spectroscopic proper-
ties and by independent synthesis from the corresponding
cyclopropyl aldehydes (Experimental Section).
(19) Gassman, P. G.; De Silva, S. A. J . Am. Chem. Soc. 1991, 113,
9870-9872.
J . Org. Chem, Vol. 67, No. 26, 2002 9401

Armesto et al.
SCHEME 9
efficiently than the corresponding imines, while the
oxime ethers are the least reactive. Alternative reaction
modes are followed, in some cases yielding nitriles,
dihydronaphthalenes, dihydroquinolines, and cycloaddi-
tion products. Some of these products arise from radical
cations centered in the alkene unit, while others are
formed from radical cations located in the C-N double
bond moiety. The results show that, under SET-sensiti-
zation conditions, the substrates can yield two different
radical cations by initial SET from two functional groups
with different ionization potentials. Changing the SET-
sensitizer from DCA to DCD results in considerably
increased reaction efficiencies in all cases studied. The
yields of isolated products are also enhanced, making
these reactions useful from a synthetic perspective. In
addition, nitriles obtained in the DCA-sensitized irradia-
tion of oxime acetates are not obtained when DCD is used
as the SET sensitizer, suggesting that radical cations
centered in the C-N double bond are not formed under
the latter conditions. These results open new lines of
research in the area of di-π-methane photochemistry.
Further studies are needed to determine the factors that
control the different reaction paths open to the radical-
cation intermediates and the role of the electron-acceptor
sensitizer on the outcomes of these photoreactions.
46% yield. Similar enhancements in the yields of 1-ADPM
products occur with the other substances. One exception
to this trend is seen with oxime ether 34a , which gives
35a in a 10% yield upon 13.5 h irradiation using DCA.
In contrast, 3 h irradiation of 34a with DCD affords 35a
in lower yield of 7% along with a new product (23%),
identified based on spectroscopic evidence as the [4 + 2]
cycloadduct 39. This result is intriguing, because this is
the only case in which we have observed preferential
formation of a [4 + 2] cycloaddition product. A possible
mechanism for the formation of 39 involves initial SET
from the alkene group in 34a to excited DCD, yielding
the radical-cation/radical-anion pair 40. Intermolecular
[4 + 2] cycloaddition of this intermediate affords 39
(Scheme 9).
Finally, irradiation of oxime ether 34b, under both
DCA- and DCD-sensitization conditions, results in con-
sumption of the starting material and formation of a
complex mixture of products. This result shows that this
substance does not undergo either DCA- or DCD-SET-
sensitized 1-ADPM rearrangement. An interesting dif-
ference in reactivity upon DCA versus DCD sensitized
irradiation is observed in the cases of the oxime acetates.
Thus, while DCA-sensitized irradiation of compounds
24a , 24b, and 24e affords the corresponding nitriles 26,
27, and 29, respectively, DCD-sensitized irradiation
yields the corresponding cyclopropanes 25a , 25b, and
25e, exclusively. These observations suggest the possibil-
ity that the radical-cation centered in the C-N double
bond, postulated as being responsible for elimination of
acetic acid (Scheme 6), might not be formed under DCD-
sensitization conditions. Therefore, the 1-ADPM product
yield enhancements seen with the DCD-sensitized reac-
tions might be due to the selective formation of a radical
cation centered in the C-C double bond. In summary,
the yields of 1-ADPM products and the efficiencies of the
reaction can be increased considerably by using DCD as
an electron-acceptor sensitizer instead of DCA. Further-
more, the use of DCD suppresses alternative reaction
modes, such as the elimination of acetic acid.
Exp er im en ta l Section
Gen er a l P r oced u r es. Starting materials and reagents are
commercially available unless synthesis is decribed. Spectral
data of the known compounds were in accordance with the
literature data. Flash chromatography was performed using
silica gel 60 (40-63 µm). NMR spectra were recorded in CDCl3
solution. Chemical shifts, δ, are expressed in parts per million
(ppm), and coupling constants J are given in hertz (Hz). UV-
vis spectra were recorded in CH2Cl2 solution. Melting points
were determined in open capillaries and are uncorrected.
Imines 8a ,4b 8b,6 8c,4b oxime acetates 24a ,15 24b,14b 24c,13
24d ,14b 24e,13 and oxime ether 34b18 were synthesized by the
methods previously described.
Gen er a l P r oced u r e for th e Syn th esis of â,γ-Un sa tu r -
a ted Oxim e Eth er s. The corresponding aldehyde, O-meth-
ylhydroxylamine hydrochloride, and pyridine were refluxed in
EtOH (50 mL) for 1 h. The aldehyde/O-methylhydroxylamine/
pyridine ratio was 1:1.2:1.2 for all the experiments. The solvent
was evaporated, the crude product was dissolved in Et2O, and
the solution was washed with 10% aqueous HCl, water, and
brine. The extract was dried (MgSO4), filtered, and evaporated
to dryness. The oxime ethers were isolated and purified by
flash chromatography using hexane/Et2O (9:1) as eluent.
O-Meth yl Eth er of 2,2-Dim eth yl-4-p h en yl-3-bu ten a l
Oxim e (34a ). From 2,2-dimethyl-4-phenyl-3-butenal7 (230 mg,
1.3 mmol) as a 1:1 mixture of Z/E isomers, O-methylhydrox-
ylamine hydrochloride (201 mg, 1.4 mmol) and pyridine (0.2
mL, 1.4 mmol) yielded the oxime ether 34a (208 mg, 77%) as
an oil and as a 1:1 mixture of (1E,4Z)/(1E,4E) isomers: 1H
NMR (300 MHz) δ 7.40-7.12 (m, 5.5H), 7.05 (s, 0.5H, Z-
isomer), 6.53 (d, J ) 12.3 Hz, 0.5H, Z-isomer), 6.29 (AB system,
J
) 16.3 Hz, 1H, E-isomer), 5.59 (d, J ) 12.3 Hz, 0.5H,
Z-isomer), 3.84 (s, 1.5H, E-isomer), 3.65 (s, 1.5H, Z-isomer),
1.30 (s, 3H, E-isomer), 1.22 (s, 3H, Z-isomer); 13C NMR (75
MHz) δ 156.5, 155.8, 137.9-126.2, 61.3, 60.9, 39.1, 39.0, 27.4,
25.4; IR (neat) ν 1623, 1616 cm-1; UV (CH2Cl2) λmax 249 (ꢀ 13
116); MS m/e (%) 145 (M+ - 58, 100), 134 (22), 129 (15), 115
(23), 91 (44), 77 (19).
O-Meth yl Eth er of (1E,4E)-3-(1-In d en ylid en e)-2,2-d i-
m eth ylp r op a n a l Oxim e (34c). From (E)-3-(1-indenylidene)-
2,2-dimethylpropanal13 (1.10 g, 5.1 mmol), O-methylhydrox-
ylamine hydrochloride (0.50 g, 6.1 mmol), and pyridine (0.5
Con clu sion s
The results of this investigation show that â,γ-
unsaturated imines, oxime acetates, and oxime ethers
undergo 1-ADPM rearrangements, using DCA or DCD
as electron-acceptor sensitizers. These processes repre-
sent the second example of di-π-methane rearrangements
occurring via radical cations. Oxime acetates react more
9402 J . Org. Chem., Vol. 67, No. 26, 2002

Reactivity of 1-Substituted-1-aza-1,4-dienes
mL, 6.1 mmol) yielded the oxime ether (1E,4E)-34c (881 mg,
76%) as a yellow oil: 1H NMR (200 MHz) δ 7.53 (s, 1H), 7.32-
7.11 (m, 4H), 6.82 (dd, J ) 5.7, 1.0 Hz, 1H), 6.76 (d, J ) 5.7
Hz, 1H), 6.57 (br s, 1H), 3.88 (s, 3H), 1.45 (s, 6H); 13C NMR
(50 MHz) δ 156.0, 136.5-118.8, 61.5, 39.9, 27.5; IR (neat) ν
1606 cm-1; UV (CH2Cl2) λmax 256 (ꢀ 42 102); MS m/e (%) 227
(M+, 33), 196 (100), 181 (96), 167 (18), 115 (60), 57 (38).
Gen er a l P r oced u r e for P r ep a r a tive P h otolyses. The
photolyses were carried out in a quartz immersion well
apparatus with a Pyrex filter and a 400 W medium-pressure
Hg arc lamp. In the irradiations with DCA, a filter solution of
sodium m-vanadate 0.07M in 5% sodium hydroxide was used.
Solutions of the compounds, the sensitizer (DCA or DCD), and
the cosensitizer (biphenyl) in dry CH3CN (450 mL), were
purged for 1 h with argon and irradiated under a positive
pressure of argon. After completion of the irradiation, the
solvent was removed under reduced pressure. In the DCA
runs, the sensitizer was removed by precipitation with Et2O
and filtered, and the biphenyl and the products were separated
by flash chromatography on silica gel. When DCD was used,
the photomixture was separated by flash chromatography on
silica gel.
DCA-Sen sit ized Ir r a d ia t ion of 8a . A 1:1 mixture of
(1E,4E)/(1E,4Z)-imine 8a (497 mg, 2.0 mmol), biphenyl (306
mg, 2.0 mmol), and DCA (30 mg, 0.13 mmol) was irradiated
for 7 h. Chromatography using hexane/Et2O (97:3) as eluent
gave 2,2-dimethyl-4-phenyl-3-butenal7 (220 mg, 65%) as a 2:1
mixture of E/Z isomers and trans-2,2-dimethyl-3-phenylcyclo-
propanecarbaldehyde8 (45 mg, 13%), resulting from hydrolysis
of the corresponding imines. Further elution with Et2O af-
forded 209 mg of highly polar material.
DCD-Sen sitized Ir r a d ia tion of 8a . Imine (1E,4E)-8a
(254 mg, 1.0 mmol), biphenyl (1.52 g, 9.9 mmol), and DCD (1.4
g, 7.6 mmol) were irradiated for 2 h yielding 2,2-dimethyl-4-
phenyl-3-butenal7 (54 mg, 30%) as a 1:1 mixture of Z/E isomers
and trans-2,2-dimethyl-3-phenylcyclopropanecarbaldehyde8 (83
mg, 46%), resulting from hydrolysis of the corresponding
imines. Further elution with Et2O afforded 56 mg of highly
polar material.
DCA-Sen sitized Ir r a d ia tion of 8b. Imine (E)-8b (391 mg,
1.2 mmol), biphenyl (185 mg, 1.2 mmol), and DCA (30 mg, 0.13
mmol) were irradiated for 3 h. Chromatography using hexane/
Et2O (95:5) as eluent gave ketone 16 (30 mg, 10%) as a white
solid (mp 116-117 °C (EtOH)), 2,2-dimethyl-4,4-diphenyl-3-
butenal9 (243 mg, 81%), and cyclopropanecarbaldealdehyde
1710 (6 mg, 2%), resulting from hydrolysis of the corresponding
imines. Further elution with Et2O afforded 23 mg of highly
polar material. Ketone 16: 1H NMR (200 MHz) δ 8.06 (d, J )
7.5 Hz, 1H), 7.47-7.06 (m, 7H), 6.72 (d, J ) 7.5 Hz, 1H), 5.96
(s, 1H), 1.27 (s, 6H); 13C NMR (50 MHz) δ 203.7, 140.2-126.5,
45.3, 26.1; IR (neat) ν 1678 cm-1; MS m/e (%) 248 (M+, 100),
233 (59), 218 (9), 205 (29), 189 (8), 203 (19), 127 (4), 101 (20),
77 (7). This compound was further characterized by indepen-
dent synthesis.
In d ep en d en t Syn th esis of 2,2-Dip h en yl-4-p h en yl-1-
(2H)n a p h th a len on e (16). To a solution of i-Pr2NH (0.7 mL,
5.2 mmol) in 50 mL of dry THF were added successively, at
-78 °C under argon, BuLi (2.1 mmol, 1.6 M in hexane) and
HMPA (0.9 mL, 5.2 mmol). The resulting mixture was stirred
for 1 h, and then a solution of methyl 2-methyl-4,4-diphenyl-
3-butenoate11 (1.2 g, 4.7 mmol) in 25 mL of dry THF was added
slowly over 10 min. After 1 h at -78 °C, the solution was
allowed to warm to -40 °C, and MeI (0.3 mL, 5.6 mmol) was
added in one portion. The reaction mixture was stirred at -40
°C for 1 h, the temperature was then raised to -10 °C, and
the solution was quenched with 10% aqueous NH4Cl. The
mixture was extracted with Et2O, and the organic layer was
washed with 5% aqueous HCl, water, and brine. The extract
was dried (MgSO4), filtered, and evaporated to dryness.
Chromatography of the residue using hexane/Et2O (9:1) as
eluent yielded m eth yl 2,2-d im eth yl-4,4-d ip h en yl-3-bu ten o-
a te (1 g, 79%) as a yellow solid: mp 66-67 °C (EtOH); 1H
NMR (300 MHz) δ 7.37-7.11 (m, 10H), 6.07 (s, 1H), 3.29 (s,
3H), 1.32 (s, 6H); 13C NMR (75 MHz) δ 176.7, 143.2-127.1,
51.5, 44.0, 27.9; IR (neat) ν 1740 cm-1; MS m/e (%) 280 (M+,
19), 221 (100), 203 (10), 143 (64), 91 (42), 77 (13). Anal. Calcd
for C19H21O2: C, 81.39; H, 7.19. Found: C, 81.69; H, 7.17.
A solution of methyl 2,2-dimethyl-4,4-diphenyl-3-butenoate
(1.0 g, 3.7 mmol) in dry EtOH (20 mL) was added to a solution
of KOH (621 mg, 11 mmol) in EtOH (20 mL). After the mixture
was allowed to reflux for 24 h, the solvent was removed and
the residue was dissolved in water. The aqueous solution was
extracted with Et2O to remove any unreacted ester and then
acidified. The acid was extracted with Et2O, and the organic
layer was dried (MgSO4), filtered, and concentrated to dryness,
yielding 2,2-d im eth yl-4,4-d ip h en yl-3-bu ten oic a cid (840
1
mg, 89%) as a white solid: mp 94-95 °C (hexane); H NMR
(300 MHz) δ 7.33-7.16 (m, 10H), 6.14 (s, 1H), 1.29 (s, 6H);
13C NMR (75 MHz) δ 182.9, 142.9-127.1, 43.9, 27.2; IR (neat)
ν 3100-2650, 1740 cm-1; MS m/e (%) 266 (M+, 38), 221 (100),
178 (17), 143 (77), 128 (31), 91 (70), 51 (22). Anal. Calcd for
C
18H18O2: C, 81.16; H, 6.82. Found: C, 81.25; H, 7.06.
2,2-Dimethyl-4,4-diphenyl-3-butenoic acid (840 mg, 3.3 mmol)
was converted into the corresponding acid chloride by refluxing
in SOCl2 (0.44 mL, 4.9 mmol) for 1 h. Removal of excess SOCl2
by rotary evaporation yielded an oil which was used im-
mediately in the next step.
To a solution of anhyd AlCl3 (0.44 g, 3.3 mmol) in dry CH2-
Cl2 (10 mL) at 0 °C and under an atmosphere of argon, a
solution of the acid chloride in CH2Cl2 (15 mL) was added
dropwise. The solution was stirred for 15 min and then poured
into ice water/HCl (50%) and stirred for a further 15 min. The
mixture was extracted with Et2O, and the organic layer was
washed with saturated aqueous NaHCO3, dried (MgSO4),
filtered, and concentrated to dryness. Chromatography of the
residue using hexane/Et2O (95:5) yielded 16 (350 mg, 45%) as
a white solid (mp 116-117 °C (EtOH)). This compound proved
to be identical to the photoproduct obtained in the irradiation
of 8b.
DCD-Sen sitized Ir r a d ia tion of 8b. Imine (E)-8b (238 mg,
0.7 mmol), biphenyl (1.28 g, 8.3 mmol), and DCD (1.18 g, 6.4
mmol) were irradiated for 30 min. Chromatography using
hexane/Et2O (95:5) as eluent gave the ketone 16 (24 mg, 13%),
2,2-dimethyl-4,4-diphenyl-3-butenal9 (115 mg, 63%), and cy-
clopropanecarbaldealdehyde 1710 (20 mg, 11%), resulting from
hydrolysis of the corresponding imines. Further elution with
Et2O afforded 28 mg of highly polar material.
DCA-Sen sitized Ir r a d ia tion of 8c. Imine (1E,4E)-8c (270
mg, 0.98 mmol), biphenyl (152 mg, 0.98 mmol), and DCA (30
mg, 0.13 mmol) were irradiated for 30 min. Chromatography
using hexane/Et2O (95:5) as eluent gave 3-(1-indenylidene)-
2,2-dimethylpropanal13 (123 mg, 63%) as a 2:1 mixture of E/ Z
isomers, cyclopropanecarbaldealdehyde (E)-2213 (8 mg, 4%),
resulting from hydrolysis of the corresponding imines, and
4-(3H-inden-1-yl)-3,3-dimethyl-3,4-dihydroquinoline 21 (35 mg,
13%) as a brown oil. Further elution with Et2O afforded 43
mg of highly polar material. Dihydroquinoline 21: 1H NMR
(200 MHz) δ 8.01-6.54 (m, 9H), 5.25 (br s, 1H), 4.80 (br s,
1H), 4.49 (br s, 2H), 1.57 (s, 3H), 1.02 (s, 3H); 13C NMR (50
MHz) δ 146.5, 139.0-111.9, 58.3, 51.8, 31.7, 29.7, 25.8; IR
(neat) ν 1660 cm-1; MS m/e (%) 327 (M+, 11), 286 (4), 273 (100),
258 (93), 228 (38), 191 (12), 170 (29), 155 (29), 115 (13), 104
(74), 77 (65), 57 (39). Anal. Calcd for C20H19N: C, 87.91; H,
6.96; N, 5.13. Found: C, 87.81; H, 6.76; N, 5.25.
DCA-Sen sit ized Ir r a d ia t ion of 24a . Oxime acetate
(1E,4E)-24a (400 mg, 1.7 mmol), biphenyl (297 mg, 1.7 mmol),
and DCA (30 mg, 0.13 mmol) were irradiated for 2.5 h.
Chromatography using hexane/Et2O (95:5) as eluent gave
nitrile (E)-26 (6 mg, 2%) as an oil, starting oxime acetate (262
mg, 66%) as a 3:1 mixture of (1E,4E):(1E,4Z) isomers, and
cyclopropane (Ecyclo,EC-N)-25a 15 (45 mg, 15%). Further elution
with Et2O afforded 65 mg of highly polar material. Nitrile 26:
1H NMR (300 MHz) δ 7.40-7.26 (m, 5H), 6.76 (d, J ) 15.9
Hz, 1H), 6.03 (d, J ) 15.9 Hz, 1H), 1.55 (s, 6H); 13C NMR (75
J . Org. Chem, Vol. 67, No. 26, 2002 9403

Armesto et al.
MHz) δ 136.9, 130.4-126.5, 120.8, 35.0, 27.7; IR (neat) ν 2250
cm-1; MS m/e (%) 171 (M+, 1), 145 (11), 85 (23), 57 (100). Anal.
Calcd for C12H13N: C, 84.21; H, 7.60; N, 8.19. Found: C, 84.20;
H, 7.75; N, 8.01.
MS m/e (%) 149 (M+, 94), 123 (17), 113 (18), 97 (26), 83 (34),
71 (65), 57 (100).
DCD-Sen sit ized Ir r a d ia t ion of 24e. Oxime acetate
(1E,4E)-24e (300 mg, 1.43 mmol), biphenyl (430 mg, 2.79
mmol), and DCD (396 mg, 2.15 mmol) were irradiated for 25
min, yielding recovered starting material 24e (122 mg, 41%)
and cyclopropane (Ecyclo,EC-N)-25e13 (132 mg, 44%). Further
elution with Et2O afforded 38 mg of highly polar material.
DCA-Sen sitized Ir r a d ia tion of 34a . A 1:1 mixture of
(1E,4Z)/(1E,4E) oxime ether 34a , biphenyl (233 mg, 1.5 mmol),
and DCA (30 mg, 0.13 mmol) was irradiated for 13.5 h.
Chromatography using hexane/Et2O (95:5) as eluent gave
recovered starting material 34a (122 mg, 40%) as a 1.3:1
mixture of (1E,4Z)/(1E,4E) isomers, cyclopropane (Ecyclo,EC-N)-
35a (25 mg, 8%) as an oil, and cyclopropane (Ecyclo,ZC-N)-35a
(6 mg, 2%) as an oil. Further elution with Et2O afforded 123
mg of highly polar material. (Ecyclo,EC-N)-35a : 1H NMR (200
MHz) δ 7.37-7.09 (m, 6H), 3.79 (s, 3H), 2.18 (d, J ) 5.7 Hz,
1H), 1.87 (dd, J ) 8.5, 5.7 Hz, 1H), 1.23 (s, 3H), 0.83 (s, 3H);
13C NMR (50 MHz) δ 151.5, 129.8-126.2, 61.5, 38.2, 36.4, 28.8,
25.7, 22.6; IR (neat) ν 1624 cm-1; MS m/e (%) 203 (M+, 6), 175
(19), 171 (19), 157 (21), 145 (100), 129 (32), 105 (27), 91 (57),
71 (43), 57 (97), 55 (69). (Ecyclo,ZC-N)-35a : 1H NMR (200 MHz)
δ 7.31-7.12 (m, 5H), 6.41 (d, J ) 8.3 Hz, 1H), 3.86 (s, 3H),
2.47 (dd, J ) 8.3, 5.7 Hz, 1H), 2.15 (d, J ) 5.7 Hz, 1H), 1.30
(s, 3H), 0.85 (s, 3H); 13C NMR (50 MHz) δ 151.3, 129.0-126.4,
61.9, 38.5, 36.4, 25.7, 23.0, 21.8.
The oxime ether 35a was further characterized by indepen-
dent synthesis. From trans-2,2-dimethyl-3-phenylcyclopro-
panecarbaldehyde8 (25 mg, 0.14 mmol), O-methylhydroxyl-
amine hydrochloride (18 mg, 0.22 mmol), and pyridine (0.02
mL, 0.22 mmol) yielded (Ecyclo,EC-N)-35a (11 mg, 38%) and
(Ecyclo,ZC-N)-35a (8 mg, 28%). The spectral characteristics of
35a were identical to those observed for the photoproduct
obtained in the DCA irradiation of 34a .
DCD-Sen sitized Ir r a d ia tion of 34a . A 1:1 mixture of
(1E,4Z)/(1E,4E) oxime ether 34a , biphenyl (362 mg, 2.3 mmol),
and DCD (333 mg, 1.8 mmol) were irradiated for 3 h, yielding
recovered starting material 34a (131 mg, 37%) as a 1.3:1
mixture of (1E,4Z)/(1E,4E) isomers, cyclopropane (Ecyclo,EC-N)-
35a (23 mg, 7%) as an oil, and 39 (137 mg, 23%) as an oil.
Further elution with Et2O afforded 100 mg of highly polar
material. Compound 39: 1H NMR (200 MHz) δ 7.49 (s, 1H),
7.30-6.98 (m, 5H), 4.54 (d, J ) 1.4 Hz, 1H), 4.13 (d, J ) 1.4
Hz, 1H), 3.83 (s, 3H), 2.43 (s, 6H), 2.24 (s, 6H), 1.42 (s, 3H),
1.30 (s, 3H); 13C NMR (50 MHz) δ 191.5, 153.8, 138.2, 137.4,
135.7, 133.3, 128.9, 127.2, 118.0, 115.4, 77.3, 61.5, 56.6, 39.3,
22.9, 21.9, 18.7, 17.3; IR (neat) ν 2221, 1676, 1597 cm-1; MS
m/e (%) 387 (M+, 26), 356 (11), 287 (25), 260 (21), 105 (26),
101 (100), 91 (27), 77 (22).
DCD-Sen sit ized Ir r a d ia t ion of 24a . Oxime acetate
(1E,4E)-24a (263 mg, 1.14 mmol), biphenyl (20 mg, 0.13 mmol),
and DCD (25 mg, 0.11 mmol) were irradiated for 1 h yielding
recovered starting material 24a (87 mg, 33%) as a 3:1 mixture
of (1E,4E):(1E,4Z) isomers and cyclopropane (Ecyclo,EC-N)-25a 15
(131 mg 60%). Further elution with Et2O afforded 10 mg of
highly polar material.
DCA-Sen sitized Ir r a d ia tion of 24b. Oxime acetate (E)-
24b (325 mg, 1.05 mmol), biphenyl (163 mg, 1.05 mmol), and
DCA (30 mg, 0.13 mmol) were irradiated for 3 h. Chromatog-
raphy using hexane/Et2O (95:5) as eluent gave nitrile 276b (20
mg, 8%), starting oxime acetate 24b (97 mg, 30%), and
cyclopropane (E)-25b14b (81 mg, 28%). Further elution with
Et2O afforded 97 mg of highly polar material.
DCD-Sen sitized Ir r a d ia tion of 24b. Oxime acetate (E)-
24b (206 mg, 0.67 mmol), biphenyl (216 mg, 1.4 mmol), and
DCD (200 mg, 1.08 mmol) were irradiated for 90 min, yielding
recovered starting material 24b (30 mg, 15%) and cyclopropane
25b14b (112 mg, 55%) as a 3:1 mixture of Z/ E isomers. Further
elution with Et2O afforded 60 mg of highly polar material.
DCA-Sen sitized Ir r adiation of 24c. Oxime acetate (1E,4E)-
24c (290 mg, 1.1 mmol), biphenyl (175 mg, 1.1 mmol), and DCA
(30 mg, 0.13 mmol) were irradiated for 1 h. Chromatography
using hexane/Et2O (95:5) as eluent gave nitrile 28 (23 mg, 9%)
as an oil and as a 1:3 mixture of Z:E isomers, 5-(3-indenyl)-
4,4-dimethyl-4,5-dihydroisoxazole (31) (44 mg, 18%) as a yellow
oil, starting oxime acetate 24c (76 mg, 26%), and cyclopropane
(Ecyclo,EC-N)-25c13 (46 mg, 16%). Further elution with Et2O
afforded 72 mg of highly polar material. Nitrile 28: 1H NMR
(200 MHz) δ 7.49 (d, J ) 5.7 Hz, 0.75H, E-isomer), 7.35-7.20
(m, 4H), 6.99 (d, J ) 5.7 Hz, 0.75H, E-isomer), 6.79 (d, J )
5.4 Hz, 0.25H, Z-isomer), 6.31 (d, J ) 5.4 Hz, 0.25H, Z-isomer),
6.28 (s, 0.75H, E-isomer), 6.02 (s, 0.25H, Z-isomer), 1.76 (s,
1.5H, Z-isomer), 1.69 (s, 4.5H, E-isomer); 13C NMR (50 MHz)
δ 141.9-118.9, 35.8, 32.8, 29.5, 28.4; IR (neat) ν 2237 cm-1
;
MS m/e (%) 195 (M+, 96), 180 (70), 167 (47), 149 (100), 131
(27), 103 (37), 69 (46), 57 (52). Compound 31: 1H NMR (300
MHz) δ 7.53-7.19 (m, 4H), 7.03 (s, 1H), 6.58 (br s, 1H), 5.22
(br s, 1H), 3.43 (br s, 2H), 1.48 (s, 3H), 0.93 (s, 3H); 13C NMR
(75 MHz) δ 155.7, 146.0-119.8, 86.2, 52.0, 37.8, 25.4, 20.7; IR
(neat) ν 1635 cm-1; MS m/e (%) 171 (M+ - 42, 100), 155 (37),
149 (29), 141 (25), 129 (40), 115 (20), 106 (40), 91 (20).
DCA-Sen sitized Ir r a d ia tion of 24d . Oxime acetate (E)-
24d (300 mg, 0.69 mmol), biphenyl (107 mg, 0.69 mmol), and
DCA (30 mg, 0.13 mmol) were irradiated for 45 min. Chro-
matography using hexane/Et2O (95:5) as eluent gave recovered
starting material 24d (214 mg, 71%) and cyclopropane (E)-
25d 14b (7 mg, 4%). Further elution with Et2O afforded 52 mg
of highly polar material.
DCA-Sen sitized Ir r a d ia tion of 34b. Oxime ether 34b
(240 mg, 0.9 mmol), biphenyl (132 mg, 0.9 mmol), and DCA
(30 mg, 0.13 mmol) were irradiated for 4 h. Chromatography
using hexane/Et2O (95:5) as eluent gave recovered starting
material 34b (45 mg, 19%). Further elution with Et2O afforded
150 mg of highly polar material.
DCD-Sen sitized Ir r a d ia tion of 34b. Oxime ether 34b
(223 mg, 0.8 mmol), biphenyl (85 mg, 0.55 mmol), and DCD
(119 mg, 0.43 mmol) were irradiated for 1 h, yielding recovered
starting material 34b (136 mg, 61%). Further elution with
Et2O afforded 80 mg of highly polar material.
DCD-Sen sitized Ir r a d ia tion of 24d . Oxime acetate (E)-
24d (300 mg, 0.69 mmol), biphenyl (255 mg, 1.66 mmol), and
DCD (235 mg, 1.27 mmol) were irradiated for 15 min, yielding
recovered starting material 24d (101 mg, 34%) and cyclopro-
pane 25d 14b (99 mg, 33%) as a 1:2 mixture of Z/E isomers.
Further elution with Et2O afforded 70 mg of highly polar
material.
DCA-Sen sitized Ir r adiation of 24e. Oxime acetate (1E,4E)-
24e13 (290 mg, 1.38 mmol), biphenyl (213 mg, 1.38 mmol), and
DCA (30 mg, 0.13 mmol) were irradiated for 2 h. Chromatog-
raphy using hexane/Et2O (95:5) as eluent gave nitrile (E)-29
(30 mg, 14%) as an oil, starting oxime acetate 24e (196 mg,
68%), and cyclopropane (Ecyclo,EC-N)-25e13 (5 mg, 2%). Further
elution with Et2O afforded 30 mg of highly polar material.
Nitrile 29: 1H NMR (200 MHz) δ 6.51 (dd, J ) 15.1, 10.8 Hz,
1H), 5.72 (d, J ) 10.8 Hz, 1H), 5.30 (d, J ) 15.1 Hz, 1H), 1.72
(s, 6H), 1.38 (s, 6H); 13C NMR (50 MHz) δ 137.3, 130.7, 126.2,
DCA-Sen sitized Ir r a d ia tion of 34c. Oxime ether (1E,4E)-
34c (404 mg, 1.8 mmol), biphenyl (274 mg, 1.8 mmol), and DCA
(30 mg, 0.13 mmol) were irradiated for 2 h. Chromatography
using hexane/Et2O (95:5) as eluent gave recovered starting
material 34c (120 mg, 30%) as a 4:3 mixture of (1E,4E)/(1E,4Z)
isomers, cyclopropane (Ecyclo,EC-N)-35c (10 mg, 3%) as an oil,
and 36 (52 mg, 7%) as a white solid (mp 200-202 °C (EtOH)).
Further elution with Et2O afforded 194 mg of highly polar
material. Cyclopropane 35c: 1H NMR (200 MHz) δ 7.48 (d,
J ) 9.0 Hz, 1H), 7.33 (d, J ) 7.2 Hz, 1H), 7.22-7.04 (m, 3H),
6.88 (d, J ) 5.7 Hz, 1H), 6.40 (d, J ) 5.7 Hz, 1H), 3.74 (s, 3H),
123.5, 123.2, 34.4, 27.6, 25.8, 18.2; IR (neat) ν 2237, 1658 cm-1
;
9404 J . Org. Chem., Vol. 67, No. 26, 2002

Reactivity of 1-Substituted-1-aza-1,4-dienes
2.65 (d, J ) 9.0 Hz, 1H), 1.46 (s, 3H), 1.42 (s, 3H); 13C NMR
(50 MHz) δ 148.8, 144.4-121.4, 61.6, 48.7, 40.0, 33.5, 23.3,
21.5; IR (neat) ν 1625 cm-1; MS m/e (%) 227 (M+, 8), 212 (8),
197 (16), 182 (14), 166 (13), 152 (27), 139 (12), 115 (35), 87 (7),
91 (7). Compound 36: 1H NMR (200 MHz) δ 8.05-7.20 (m,
9H), 7.01 (d, J ) 7.5 Hz, 1H), 6.85 (t, J ) 6.7 Hz, 1H), 6.48
(dd, J ) 7.8, 1.2 Hz, 1H), 5.20 (d, J ) 3.1 Hz, 1H), 4.50 (dd,
J ) 7.2, 3.1 Hz, 1H), 3.80 (d, J ) 7.2 Hz, 1H), 3.76 (s, 1H),
2.70 (s, 3H), 1.60 (s, 3H), 1.17 (s, 3H); 13C NMR (50 MHz) δ
139.7-120.0, 81.3, 68.8, 59.9, 56.4, 52.8, 36.5, 32.0, 26.5; IR
(neat) ν 2399 cm-1; MS m/e (%) 429 (M+- 2, 2), 398 (3), 383
(1), 355 (4), 228 (100), 207 (14), 196 (76), 181 (30), 152 (11),
127 (5), 101 (13), 87 (18). The structure assignment was
established by X-ray crystallography.
Cyclopropane 35c was further characterized by independent
synthesis. From cyclopropane 2213 (49 mg, 0.24 mmol), O-
methylhydroxylamine hydrochloride (25 mg, 0.29 mmol), and
pyridine (0.02 mL, 0.29 mmol) yielded (Ecyclo,EC-N)-35c (32 mg,
31%) and (Ecyclo,ZC-N)-35c (32 mg, 31%). The spectral charac-
teristics of (Ecyclo,EC-N)-35c were identical to those observed
for the photoproduct obtained in the DCA irradiation of 34c.
Cyclopropane (Ecyclo,ZC-N)-35c: 1H NMR (200 MHz) δ 7.42-
7.13 (m, 4H), 6.95 (d, J ) 5.7 Hz, 1H), 6.78 (d, J ) 8.2 Hz,
1H), 6.50 (d, J ) 5.7 Hz, 1H), 3.84 (s, 3H), 3.18 (d, J ) 8.2 Hz,
1H), 1.54 (s, 3H), 1.48 (s, 3H); 13C NMR (50 MHz) δ 148.0,
143.9-121.5, 61.8, 49.1, 41.3, 36.1, 23.3, 21.4.
Ack n ow led gm en t. S.A.-L. and M.M.-F. thank the
Ministerio de Educacion, Cultura y Deporte for post-
graduate fellowships. We thank the Subdireccion Gen-
eral de Proyectos de Investigacion del Ministerio de
Ciencia y Tecnologia (Grant no. BQU2001-1282) for
financial support. We acknowledge the support provided
by the Centro de Resonancia Magnetica, the Centro de
Microanalisis, and the Servicio de Espectrometria de
Masas of the Universidad Complutense. Dedicated to
the memory of the late Professor Dr. Manuel Lora
Tamayo.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
all compounds lacking analyses and X-ray crystallographic
data for 36. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O026440L
J . Org. Chem, Vol. 67, No. 26, 2002 9405