Influence of electron-donor sensitizers on SET-promoted photochemical reactions of β,γ-unsaturated aldehydes

Article abstract of DOI:10.1021/ol049263e

β,γ-Unsaturated aldehydes undergo photochemical oxa-di-π-methane and decarbonylation reactions using DMN as an electron-donor sensitizer, whereas, using DMA, other photoproducts resulting from proton abstraction by ketyl- and alkene-centered radical-anion

Full text of DOI:10.1021/ol049263e

ORGANIC
LETTERS
2004
Vol. 6, No. 13
2261-2264
Influence of Electron-Donor Sensitizers
on SET-Promoted Photochemical
Reactions of â,γ-Unsaturated Aldehydes
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
darmesto@quim.ucm.es; mjortiz@quim.ucm.es
Received April 22, 2004
ABSTRACT
â,γ-Unsaturated aldehydes undergo photochemical oxa-di-π-methane and decarbonylation reactions using DMN as an electron-donor sensitizer,
whereas, using DMA, other photoproducts resulting from proton abstraction by ketyl- and alkene-centered radical-anions are also obtained.
Radical-ions, generated from organic molecules by single-
electron transfer (SET), are important intermediates in a
variety of chemical reactions, particularly, in cyclization
processes that lead to the construction of a variety of carbo-
and heterocyclic ring systems.1,2 Radical-ion intermediates
can be generated by ground-state single-electron transfer
oxidations and reductions and by photoinduced electron
transfer (PET) either directly or by using electron-donor and
electron-acceptor sensitizers.3 As part of our studies in this
area, we have observed that 1-aza-di-π-methane (1-ADPM)
rearrangements take place through radical-cation intermedi-
ates. For example, sensitized irradiation using the electron-
acceptor sensitizer 9,10-dicyanoanthrathene (DCA) of a series
of 1-aza-1,4-diene derivatives leads to formation of the
corresponding cyclopropanes.4 In another effort, we found
that 2-aza-1,4-diene radical-cations, generated under these
photosensitized conditions, undergo 2-aza-di-π-methane (2-
ADPM) reactions affording N-vinylaziridines.5
In contrast to the large number of studies focusing on SET
photocyclizations that take place via radical-cation intermedi-
ates, those exploring radical-anions as intermediates in these
reactions have received much less attention.3 There are only
a few examples in the literature of intramolecular reductive
cyclization reactions via radical-anions other than ketyl.3
However, we have recently reported 1-ADPM and di-π-
methane (DPM) reactions that occur via radical-anion inter-
mediates. For example, irradiation of a series of â,γ-un-
saturated imines such as 1 and the 1,4-diene 3 using N,N-
dimethylaniline (DMA) as an electron-donor sensitizer, leads
to production of cyclopropanes 2 and 4, respectively (Scheme
1).6 Because di-π-methane rearrangements have been con-
(1) For a comprehensive review on electron-transfer processes, see:
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
2001; Vols. 1-5.
(2) For other reviews see: (a) Photoinduced Electron-Transfer I, II, III,
IV and V. In Top. Curr. Chem.; Mattay, J., Ed.; Springer-Verlag: Berlin,
1993; Vols. 156, 158, 159, 163, and 168, respectively. (b) Electron-Transfer
I and II. In Top. Curr. Chem.; Mattay, J., Ed.; Springer-Verlag: Berlin,
1996; Vols. 169 and 177, respectively.
(4) Armesto, D.; Ortiz, M. J.; Agarrabeitia, A. R.; Aparicio-Lara, S.;
Martin-Fontecha, M.; Liras, M.; Martinez-Alcazar, M. P. J. Org. Chem.
2002, 67, 9397-9405.
(5) (a) Armesto, D.; Caballero, O.; Amador, U. J. Am. Chem. Soc. 1997,
119, 12659-12660. (b) Armesto, D.; Caballero, O.; Ortiz, M. J.; Agarra-
beitia, A. R.; Martin-Fontecha, M.; Torres, M. R. J. Org. Chem., 2003, 68,
6661-6671.
(3) Hintz, S.; Heidbreder, A.; Mattay J. Radical Ion Cyclizations. In Top.
Curr. Chem.; Mattay, J., Bassi, R., Eds.; Springer-Verlag: Berlin, 1996;
Vol. 177, pp 77-124 and references cited therein.
(6) Armesto, D.; Ortiz, M. J.; Agarrabeitia, A. R.; Martin-Fontecha, M.
J. Am. Chem. Soc. 2001, 123, 9920-9921.
10.1021/ol049263e CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/27/2004

1-aza-1,4-diene and 1,4-dienes analogues, â,γ-unsaturated
aldehydes 5 also undergo di-π-methane-type rearrangements
via radical-anion intermediates.
Scheme 1. Examples of 1-ADPM and DPM Rearrangements
via Radical-Anion Intermediates
Possible mechanistic pathways for formation of 6 and 7
involve the intermediacy of radical-anions 10, which undergo
bonding to give the cyclopropane ring-containing radical-
anions 11. Rearrangement of 11 affords 12, which by back-
electron transfer (BET) generates 6. The reversibility of the
step leading to 11 would account for the observed C-C
double-bond isomerization of 10a. Alternatively, radical-
anions 10 can be protonated to afford radicals 13 that cyclize
generating intermediates 14, which lose hydrogen atoms to
yield 7 (Scheme 3).
sidered for many years to be the paradigm of reactions that
take place in the excited state of organic molecules, the
observation that these processes can take place in ground-
state radical-ion intermediates has opened new lines of
research in this area.
Scheme 3
In an extension of this effort, we have investigated DMA
electron-donor-sensitized photoreactions of a series of â,γ-
unsaturated aldehydes that are structurally related to the
previously studied 1-aza-1,4-dienes and the 1,4-diene. All
of the aldehydes probed in this investigation undergo efficient
oxa-di-π-methane (ODPM) rearrangement on triplet-sensi-
tized irradiation.
Irradiation of a solution of (E)-5a7 in acetonitrile contain-
ing DMA as an electron-donor sensitizer for 10 min, using
a 125 W medium-pressure Hg arc lamp, followed by column
chromatography on silica gel, gave recovered 5a (52%) as a
1:1 mixture of E:Z isomers and two photoproducts, identified
as the cyclopropanecarbaldehyde 6a8 (12%) and dihy-
dronaphthalene derivative 7a (4%) (Scheme 2). The study
Compound 8 is formed by a conventional radical-anion
reduction of the alkene unit.3 This result is surprising since
the carbonyl group should be a better electron-acceptor than
the alkene moiety. However, the stabilization provided by
diphenyl substitution in 5b promotes the competitive genera-
tion of alkene- and ketyl-centered radical-anions. The forma-
tion of 9 in the irradiation of 5b, possibly by the path shown
in Scheme 3, is the first example of a decarbonylation
reaction in â,γ-unsaturated aldehydes via radical-anion
intermediates.
The study was extended to aldehyde 15.11 Irradiation of
15 for 30 min, under the same conditions used for 5b,
followed by chromatography, afforded recovered 15 (11%)
and five other photoproducts, identified as spirocyclopropane
derivative 1611 (1%), saturated aldehyde 17 (8%), γ-hy-
droxyaldehyde 18 (13%), spirodihydrofuran 19 (16%), and
cyclopropanol 20 (4%) (Scheme 4).
Scheme 2. DMA-Sensitized Irradiations of Aldehydes 5
Scheme 4
was extended to aldehyde 5b.9 DMA-sensitized irradiation
of 5b in acetonitrile, using a 400 W medium-pressure Hg
arc lamp, for 90 min yielded 6b10 (4%), 7b (10%), recovered
starting material (17%), and two new compounds, character-
ized as the saturated aldehyde 8 (10%) and the alkene 97
(10%) (Scheme 2). These results demonstrate that, like their
(7) van der Weerdt, A. J. A.; Cerfontain, H. Tetrahedron 1981, 37, 2121-
2130.
(8) Armesto, D.; Ortiz, M. J.; Romano, S.; Agarrabeitia, A. R.; Gallego,
M. G.; Ramos, A. J. Org. Chem. 1996, 61, 1459-1466.
(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.
Clearly, compounds 17-19 are formed by way of alkene-
centered radical-anion intermediates. Possible mechanistic
2262
Org. Lett., Vol. 6, No. 13, 2004

pathways for formation of 17-19 involve the intermediacy
of radical-anion 21, the protonation of which yields radical
22. Hydrogen abstraction by 22 then produces 17. Alterna-
tively, 22 could participate in BET to give carbocation 23,
which is then trapped by spurious water, always present in
acetonitrile, to yield 18. Finally, cyclization of 18 would form
the cyclic hemiketal 19 (Scheme 5). The reasons radical 22
Irradiation of aldehyde 2614 for 15 min, under the same
conditions used for 5b, after chromatography afforded
recovered 26 (30%), cyclopropanecarbaldehyde 27,8 (22%)
and alkene 28 (11%) (Scheme 7). In this instance the
Scheme 7
Scheme 5
formation of products resulting from proton transfer to
alkene- or ketyl-centered radical-anions is not observed.
The results outlined above show that radical-anions, gen-
erated by DMA-sensitized irradiation of â,γ-unsaturated
aldehydes in acetonitrile, undergo ODPM-type rearrangement
with variable efficiencies. In addition, a variety of other
reactions involving proton abstraction and addition of water
have been observed. This is somewhat surprising since
DMA-sensitized irradiation of â,γ-unsaturated imines in
acetonitrile afford the corresponding 1-ADPM photoproducts,
exclusively.6
In an attempt to simplify the outcome of these reactions,
we have irradiated aldehydes 5a,b, 15, and 26 under con-
ditions that minimize proton transfer from the sensitizer
radical-cation. A previous study demonstrated that DPM re-
arrangement of diene 3 via radical-anion intermediates occurs
when the reaction is carried out in hexane.6 Therefore, 5a,b,
15, and 26 were irradiated in hexane, using 1,4-dimethoxy-
naphthalene (DMN) as an electron-donor sensitizer. As
expected, under these conditions formation of products re-
sulting from proton transfer to the alkene and ketyl radical-
anions was suppressed (Table 1). Consequently, aldehyde
5a affords only the ODPM photoproduct 6a. Compounds
5b, 15, and 26 also yield the corresponding cyclopropanes
6b, 16, and 27, respectively, when subjected to these donor-
sensitized irradiation conditions. In these instances, alkenes
formed by decarbonylation of the corresponding radical-
anions were also obtained. Thus, aldehyde 5b affords alkene
9 as the major product. Compound 15 gives an inseparable
mixture of 9-isobutylidene-9H-fluorene (29) and 9-(2-
methylpropenyl)-9H-fluorene (30). Finally, aldehyde 26
decarbonylates, providing a mixture of alkenes 28 and 2-iso-
propylidene-1,2,3,4-tetrahydronaphthalene (31). The yield of
ODPM photoproducts, under these conditions, increases
considerably for compounds 15 and 26 but remains practi-
cally the same for aldehydes 5a and 5b.
has this strong tendency to undergo BET to generate
carbocation 23 are unclear at this point.
The formation of cyclopropanol 20 is interesting. Although
reductive intramolecular cyclization of ketyl radical-anions
tethered to a double bond at a suitable distance has been
used in the synthesis of different carbo- and heterocycles,12
there is only one report in the literature of cyclopropane
formation by this procedure using â,γ-unsaturated carbonyl
compounds.13 Cyclopropane 20 could come from radical-
anion 21 or from a ketyl radical-anion. To clarify this point,
15 was reacted with SmI2 in tBuOH/THF, affording 20 (61%)
exclusively. Earlier efforts have shown that SmI2 reduces
carbonyl groups, but as far as we are aware, it does not
promote the generation of radical-anions in simple alkenes.12
Therefore, the formation of 20 supports the involvement of
a carbonyl-centered radical-anion in this cyclization reaction.
The mechanism shown in Scheme 6 accounts for the for-
Scheme 6
mation of 20. Thus, the radical-anion 24, generated thermally
or photochemically, can undergo a 3-exo-trig cyclization
yielding 25, which by proton transfer and hydrogen abstrac-
tion gives 20.
In summary, the results arising in this study show that, in
a manner similar to 1-aza-1,4-dienes and 1,4-dienes, â,γ-
unsaturated aldehydes 5a,b, 15, and 26 undergo SET-pro-
moted ODPM rearrangements. These processes are the first
examples of ODPM reactions that occur via radical-anion
intermediates. However, other products arising by proton
transfer to alkene- and ketyl-centered radical-anions are also
(11) Armesto D.; Gallego M. G.; Horspool, W. M.; Agarrabeitia, A. R.
Tetrahedron 1995, 51, 9223-9240.
(12) (a) Molander, G. A. Chem. ReV. 1992, 92, 29-68. (b) Molander,
G. A.; Harris, C. R. Chem. ReV. 1996, 96, 307-338. (c) Krief, A.; Laval,
A.-M. Chem. ReV. 1999, 99, 745-777.
(13) Villar, H.; Guibe, F. Tetrahedron Lett. 2002, 43, 9517-9520.
(14) Armesto, D.; Ramos A. Tetrahedon 1993, 49, 7159-7168.
Org. Lett., Vol. 6, No. 13, 2004
2263

Table 1. Yields of Products in the Irradiation of â,γ-Unsaturated Aldehydes Using DMA and DMN as Electron-Donor Sensitizers
ODPM
decarbonylation
(yield, %)
CdC reduction
other
starting material
(yield, %)
substrate
SET-sensitizer
solvent
(yield, %)
(yield, %)
(yield, %)
5a
5a
5b
5b
15
15
26
26
DMA
DMN
DMA
DMN
DMA
DMN
DMA
DMN
MeCN
hexane
MeCN
hexane
MeCN
hexane
MeCN
hexane
6a (12)
6a (7)
6b (4)
6b (5)
16 (1)
16 (25)
27 (22)
27 (69)
7a (4)
52
77
17
47
11
39
30
20
9 (10)
9 (24)
8 (10)
17 (8)
7b (10)
18 (13), 19 (16), 20 (4)
29, 30 (16)
28 (11)
28, 31 (6)
obtained when the irradiations are promoted by using DMA
as an electron-donor sensitizer in acetonitrile. These results
contrast those obtained from studies of the reactivity of 1-aza-
1,4-diene radical-anions. The latter intermediates undergo
the 1-ADPM rearrangement exclusively under these reaction
conditions. However, the intervening proton-transfer pro-
cesses, which result in diminished yields, can be suppressed
by using DMN as an electron-donor sensitizer and hexane
as a solvent. Under these conditions, competitive decarbo-
nylation reactions are observed for aldehydes 5b, 15, and
26. These are the first examples of reactions that are similar
to the well-known Norrish type I process that take place via
radical-anion intermediates.
Another interesting observation in this study is the for-
mation of cyclopropanol 20 upon irradiation of aldehyde 15.
There is only one previous report in the literature describing
cyclopropane formation by a route involving 3-exo-trig
cyclization of radical-anions derived from â,γ-unsaturated
carbonyl compounds.13 Compound 20 is also obtained in
good yield when 15 is reacted with SmI2 in tBuOH/THF.
These results open new lines of research in the area of
the â,γ-unsaturated carbonyl compound photochemistry.
Further studies are underway to elucidate the factors that
control the photoreactivity of these compounds via radical-
anion intermediates.
Acknowledgment. Dedicated to Dr. Juan Carlos del
Amo, member of our Department, who was killed by
terrorists in Madrid on March 11, 2004. M.M.-F. thanks the
Ministerio de Educacion y Ciencia for a postgraduate
fellowship. We thank the Ministerio de Ciencia y Tec-
nologia (Grant BQU2001-1282) for financial support. We
acknowledge the support provided by the Centro de Reso-
nancia Magnetica, the Centro de Microanalisis, and the
Servicio de Espectrometria de Masas of the Universidad
Complutense.
Supporting Information Available: Experimental details
for the photolysis and the reaction of 15 with SmI2 and
analytical and spectroscopic data for all photoproducts. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL049263E
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Org. Lett., Vol. 6, No. 13, 2004