Di-π-methane reactions promoted by SET from electron-donor sensitizers [13]

Full text of DOI:10.1021/ja0164428

9920
J. Am. Chem. Soc. 2001, 123, 9920-9921
Scheme 1
Di-π-methane Reactions Promoted by SET from
Electron-Donor Sensitizers
Diego Armesto,* Maria J. Ortiz,*
Antonia R. Agarrabeitia, and Mar Martin-Fontecha
Departamento de Quimica Organica I
Facultad de Ciencias Quimicas
UniVersidad Complutense, 28040-Madrid, Spain
the azadiene 4a6 was irradiated for 20 min, in acetonitrile, using
N,N-dimethylaniline (DMA) as an electron-donor sensitizer.7 The
solvent and sensitizer were removed by vacuum distillation giving
1
ReceiVed June 19, 2001
an oily residue, which was analyzed by H NMR showing the
presence of the cyclopropylimine 5a. Separation of product
mixture by column chromatography on silica gel afforded the
aldehyde 6a8 (21%) resulting from hydrolysis of the imine 5a
(Scheme 1).
Studies of di-π-methane rearrangements of 1,4-unsaturated
compounds have been the focus of several research efforts in
organic photochemistry for many years. These reactions have
proved to be very general for 1,4-dienes,1,2 â,γ-unsaturated
ketones,1,3 and 1-aza-1,4-dienes.1,4 We have reported recently the
extension of the rearrangement to 2-aza-1,4-dienes.5
Investigations carried out for more than 30 years have
established these rearrangements as the paradigm of processes
that take place in the excited state, exclusively.1-4 In all cases
studied, biradical or concerted mechanisms have been proposed
to account for the rearrangement processes. However, results from
recent efforts in our laboratory have led to a drastic modification
of these ideas. For example, irradiation of the 2-aza-1,4-diene 1,
using 9,10-dicyanoanthrathene (DCA) as an electron-acceptor
sensitizer, yields the N-vinylaziridine 2 and the cyclopropylimine
3.5
The study was extended to 1-azadienes 4b,9 4c, and 4d.
Substrates 4c and 4d were synthesized by condensation of the
corresponding aldehydes with aniline by using standard proce-
dures. DMA-sensitized irradiation of these compounds under the
conditions described above yielded, after silica gel column
chromatography, the respective cyclopropane-carbaldehydes 6b10
(15%), 6c11 (15%), and 6d12 (11%). The isolated products are
formed by hydrolysis of the primary cyclopropylimines photo-
products 5b, 5c, and 5d (see Supporting Information for irradiation
conditions).
The formation of 2 and 3 represent the first examples of di-
π-methane reactions (a 2-aza-di-π-methane reaction producing 2
and an aryl-di-π-methane process generating 3) that take place
via radical-cation intermediates. Furthermore, the formation of
the heterocyclic product 2 is unprecedented in all of the studies
carried out before on di-π-methane rearrangements. In addition,
we found that 1-aza-1,4-dienes also undergo di-π-methane
rearrangement promoted by excited-state electron transfer via
radical-cation intermediates. Accordingly, 4a affords the imine
5a on irradiation using DCA as sensitizer.6
The results clearly show that 1-ADPM rearrangements of
azadienes 4 occurs in DMA, a well-known electron-donor,7 by
single electron transfer (SET)-sensitized reactions. This interpreta-
tion is tentative since we previously observed triplet 1-ADPM
reactivity of azadiene 4b by using acetophenone as sensitizer.9
Since the DMA has a triplet energy of 68.4 kcal/mol,7 in these
processes efficient energy transfer to the diphenylvinyl unit (triplet
energy of ∼53-62 kcal/mol)13 is possible. The question that arises
is whether the DMA-sensitized reactions take place via SET or
The aza-di-π-methane rearrangement (ADPM) of aza-1,4-
dienes via radical-cations suggests the possibility that other
radical-ion intermediates (e.g., radical-anions) could also be
responsible for this rearrangement reaction. To test this proposal,
(1) Zimmerman, H. E.; Armesto, D. Chem. ReV. 1996, 96, 3065-3112.
(2) (a) Zimmerman, H. E. In Rearrangements in Ground and Excited States;
DeMayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, pp 131-166.
(b) Zimmerman, H. E. In Organic Photochemistry; Padwa, A., Ed.; Marcel
Dekker: New York, 1991; Vol. 11, pp 1-36.
(3) (a) Dauben, G. W.; Lodder, G.; Ipaktschi, J. Top. Curr. Chem. 1975,
54, 73-114. (b) Houk, K. N. Chem. ReV. 1976, 76, 1-74. (c) Schuster, D. I.
In Rearrangements in Ground and Excited States; DeMayo, P., Ed.; Academic
Press: New York, 1980; Vol. 3, pp 232-279. (d) Demuth, M. In Organic
Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York, 1991; Vol. 11,
pp 37-109.
(4) (a) Armesto, D. In CRC Handbook of Organic Photochemistry and
Photobiology; Horspool, W. M., Soon, P.-S., Eds.; CRC Press: New York,
1995; pp 915-930. (b) Armesto, D. EPA Newsl. 1995, 53, 6-21.
(5) Armesto, D.; Caballero, O.; Amador, U. J. Am. Chem. Soc. 1997, 119,
12659-12660.
(6) Ortiz, M. J.; Agarrabeitia, A. R.; Aparicio-Lara, S.; Armesto, D.
Tetrahedron Lett. 1999, 40, 1759-1762.
(7) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449.
(8) Armesto, D.; Ortiz, M. J.; Romano, S. Tetrahedron Lett. 1995, 36, 965-
968.
(9) Armesto, D.; Horspool, W. M.; Fernandez-Martin, J. A.; Perez-Ossorio,
R. J. Chem. Res. (S) 1986, 46-47.
(10) Zimmerman, H. E.; Mariano, P. S. J. Am. Chem. Soc. 1969, 91, 1718-
1727.
(11) Armesto, D.; Gallego, M. G.; Horspool, W. M.; Agarrabeitia, A. R.
Tetrahedron 1995, 51, 9223-9240.
(12) Armesto, D.; Ortiz, M. J.; Romano, S.; Agarrabeitia, A. R.; Gallego,
M. G.; Ramos, A. J. Org. Chem. 1996, 61, 1459-1466.
(13) Zimmerman, H. E.; Armesto, D.; Amezua, M. G.; Gannett, T. P.;
Johnson, R. P. J. Am. Chem. Soc. 1979, 101, 6367-6383.
10.1021/ja0164428 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/18/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9921
triplet energy-transfer mechanisms. To clarify this issue, azadienes
4 were irradiated for the same period of time, by using DMA in
the nonpolar solvent hexane. Under this condition, radical-anion
formation should not be favored. We observed that DMA-
sensitized reactions of 4 occur in hexane to yield the correspond-
ing cyclopropanecarbaldehydes 6a (2%), 6b (2%), and 6c (4%).
However, the yields of these processes are much lower than in
the acetonitrile runs that afford 21% of 6a, and 15% of 6b and
6c. These results support the proposal that radical-anions are
involved as intermediates in photoreactions in acetonitrile. In the
case of compound 4d, the yields of aldehyde 6d from the
acetonitrile and hexane reactions were similar (ca. 11%), casting
doubts again on whether this process occurs via SET or triplet
energy sensitization.
Scheme 2
In an attempt to obtain a more definite answer to this question,
we searched for a substrate that would not undergo the di-π-
methane rearrangement in the triplet excited-state manifold. With
this goal in mind, the study was extended to the 1,4-diene 7.14
Numerous studies have demonstrated that triplets of acyclic 1,4-
dienes do not undergo di-π-methane rearrangements.1,2 The
photochemical reactivity of 7 was studied earlier by Zimmerman
and Pratt14 who showed that it rearranges efficiently to yield the
cyclopropane 8 on direct irradiation but that the process is very
inefficient when benzophenone triplet sensitization is used.
Therefore, compound 7 was considered to be a good candidate
to test the possibility of promoting di-π-methane reactions by
using electron-donor sensitizers. Irradiation of 7 in acetonitrile,
for 3 h, by using DMA as a sensitizer, yielded cyclopropane 8 in
34% yield. Changing the solvent from acetonitrile to hexane does
not modify significantly the efficiency (22%) of the process.
However, when diene 7 was irradiated in hexane for 3 h, with
benzophenone as sensitizer, the cyclopropane 8 was obtained in
only 5% yield, in agreement with the earlier results.14 Since the
triplet energy of benzophenone (69.2 kcal/mol)7 is comparable
to the value reported for the DMA (68.4 kcal/mol),7 both
sensitizers should be able to transfer their triplet energy to the
diphenylvinyl unit with the same efficiency. Therefore, the
decrease in the yield of product observed in the reactions run in
hexane, caused by changing the sensitizer from DMA to benzo-
phenone, cannot be explained by less efficient triplet energy
transfer from the benzophenone to the diene 7. An alternative
explanation could invoke the possible involvement of singlet
energy transfer from the DMA to the diene 7. However, this
possibility seems unlikely since the energy of the diphenylvinyl
singlet excited state (97.9 kcal/mol)7 is considerably higher than
the singlet excited state of DMA (88.8 kcal/mol).7
might be due to a very fast rearrangement of the radical-anion 9
within solvent cages (Scheme 2, path a). In cases in which this
intermediate escapes from the cage before rearrangement occurs,
a significant influence of the polarity of the solvent would have
been observed. This is the situation in DMA-sensitized reactions
of 4a, 4b, and 4c (Scheme 2, path b).
The results reported herein provide strong evidence in favor
of the involvement of radical-anions in electron donor-sensitized
1-aza-di-π-methane rearrangements of azadienes 4 and also in
the di-π-methane reaction of diene 7. These observations open
new lines of research in an area in which, due to the large number
of studies carried out for more than 30 years, apparently there
were very few things to uncover. Finally, photoreactions promoted
by photoinduced electron transfer have became one of the main
areas of research in organic photochemistry.15 However, most of
the studies in this field involve the generation of radical-cations
using electron-acceptor sensitizers, while the photochemical
production of radical-anions using electron-donor sensitizers has
received less attention.15 Our results, which demonstrate that these
intermediates can participate in di-π-methane processes, could
lead to an increased interest in these reactions. Further studies
are in progress to determine the scope and synthetic applications
of these reactions.
Acknowledgment. Dedicated to Professor Dr. Jose Luis Soto on the
occasion of his 70th birthday. M.M.-F. thanks the Ministry of Education
and Science of Spain for a F.P.I. predoctoral fellowship. We thank the
Direccion General de Investigacion Cientifica y Tecnica (Grant No. PB
97/0235) for financial support. We acknowledge the support provided
by the Centro de Resonancia Magnetica and the Centro de Masas of the
Universidad Complutense.
The above results lead to the conclusion that rearrangement of
7 to the cyclopropane 8 occurring in the reactions sensitized by
DMA must take place via radical-anion intermediates. Considering
that the C-N double bond in compounds 4 should be a better
electron acceptor than the diphenylvinyl unit in 7 it is logical to
assume that the rearrangement of 4 also occurs via similar ion-
radical intermediates. A possible mechanism for these reactions
is shown in Scheme 2 for compound 4a. The absence of a solvent
polarity effect on the efficiency of photoreactions of 4d and 7
Supporting Information Available: Experimental procedures for the
synthesis of 4a, 4c, and 4d, and for the irradiations (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0164428
(15) For a comprehensive review on electron-transfer processes, see:
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
2001; five volumes.
(14) Pratt, A. C.; Zimmerman, H. E. J. Am. Chem. Soc. 1970, 92, 6259-
6267.