The effects of triplet sensitizers' energies on the photoreactivity of β,γ-unsaturated methyl ketones

Article abstract of DOI:10.1002/anie.200502619

(Chemical Equation Presented) Triplet determines reactivity: β,γ-Unsaturated ketones 1 do not undergo the oxa-di-π-methane (ODPM) rearrangement in the presence of acetophenone or 3-methoxyacetophenone as a triplet sensitizer. However, they afford the ODPM

Full text of DOI:10.1002/anie.200502619

Angewandte
Chemie
Rearrangement Reactions
DOI: 10.1002/anie.200502619
The Effects of Triplet Sensitizersꢀ Energies on the
Photoreactivity of b,g-Unsaturated Methyl
Ketones**
Diego Armesto,* Maria J. Ortiz,*
Antonia R. Agarrabeitia, and Noureddin El-Boulifi
The photochemistry of b,g-unsaturated ketones has been a
topic of great interest to organic photochemists for many
years.[1] The results of studies in this area show that these
substances undergo a 1,3-acyl migration on direct irradiation
and an oxa-di-p-methane (ODPM) rearrangement in the
presence of triplet sensitizers. Both processes usually occur
with high chemical and quantum efficiencies and high degrees
of stereoselectivity.[1] As a result, the photochemical 1,3-acyl
migration and ODPM rearrangement reactions have found
important synthetic applications.[1] Despite the fact that a
large majority of b,g-unsaturated ketones undergo triplet-
state ODPM reactions, some acyclic compounds in this series,
even those having substitution patterns that are known to
favor this reaction, are unreactive.[1a] For example, it has been
reported that the b,g-unsaturated ketones 1a,[2] 1b[3] 1c,[4] and
1d[2] do not undergo the ODPM rearrangement in the
presence of acetophenone or 3-methoxyacetophenone.
Instead, under these conditions 1a and 1b undergo alkene
E/Z isomerization.
To gain a better understanding of this unusual behavior,
we have carried out a more detailed study in which we have
probed the photochemistry of enones 1a–e by using a variety
of other triplet sensitizers. The results arising from this effort
reveal that these acyclic b,g-unsaturated ketones do indeed
undergo the ODPM process when the triplet energies of the
sensitizers used to promote the reaction are close to those of
the triplet states of the alkene moieties, and that the ODPM
reactivity of these substrates is intimately related to the
features of the triplet formed in the energy-transfer processes.
In this study, solutions of b,g-unsaturated ketones 1a–e
containing sensitizers of varying triplet energies[5] (acetophe-
none: ET = 74 kcalmolÀ1
; 3-methoxyacetophenone: ET =
71 kcalmolÀ1, depending on the absorption properties of the
enone; thioxanthone: ET = 63 kcalmolÀ1; 4-phenylbenzophe-
[*] Prof. Dr. D. Armesto, Prof. Dr. M. J. Ortiz, Prof. Dr. A. R. Agarrabeitia,
N. El-Boulifi
Departamento de Quimica Organica I
Facultad de Ciencias Quimicas
Universidad Complutense
28040 Madrid (Spain)
Fax: (+34)913-944-103
E-mail: darmesto@quim.ucm.es
mjortiz@quim.ucm.es
[**] The authors thank the Universidad Complutense (PR1/05-13271)
for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
none: ET = 61 kcalmolÀ1; chrysene: ET = 57 kcalmolÀ1) were
from the triplet excited state of 1a, a phenomenon rarely
observed in photochemistry.[6] Finally, when the photoreac-
tion of 1a was carried out with chrysene (ET = 57 kcalmolÀ1)
as the triplet sensitizer, 3a (6%) and recovered starting
material 1a (56%, 1:6 mixture of E and Z isomers) were
isolated (Scheme 1).
irradiated in an immersion apparatus through Pyrex using a
400-W medium-pressure mercury arc lamp. The results of an
earlier study showed that irradiation of the b,g-unsaturated
ketone 1a[2] (the ET of the styrene chromophore is about
62 kcalmolÀ1)[5] with acetophenone triplet sensitization (ET =
74 kcalmolÀ1) for 5 h led to 91% recovery of the enone as a
1:1 mixture of E and Z isomers. In contrast, irradiation of 1a
(1:2 mixture of E and Z isomers) in CH2Cl2 with thioxanthone
(ET = 63 kcalmolÀ1) as the sensitizer for 8.5 h resulted in
formation of the ODPM product (E)-2a (50%) and recov-
ered starting material 1a (20%), as a 2:3 mixture of E- and Z-
isomers (Scheme 1). Likewise, irradiation of 1a (1:2, E/Z)
To the best of our knowledge, these unusual observations,
which suggest that the excited-state reaction pathways
followed by enone 1a depends on the triplet energy of the
sensitizer, are unprecedented. In order to determine if this
behavior is observed with other b,g-unsaturated ketones, the
photochemistry of ketones 1b–e was also explored. We found
that (E)-1b behaves in a similar manner to 1a in photo-
reactions promoted by the triplet sensitizers listed above. The
only difference observed in this case is that the ODPM
photoproduct 2b is also formed in low yield, along with 3b as
the major product, when chrysene is used as triplet sensitizer
(Scheme 2 and Table 1). The diphenylvinyl chromophore in
1c has a triplet energy of about 53–62 kcalmolÀ1,[7] and
irradiation of 1c with 3-methoxyacetophenone (ET = 71 kcal
molÀ1) as the sensitizer leads to recovered starting material
Scheme 1. Photoreactivity of enone 1a using different triplet sensitiz-
ers. Sens.=sensitizer.
with 4-phenylbenzophenone (ET = 61 kcalmolÀ1) as sensitizer
for 8.5 h promoted formation of the ODPM product (E)-2a
(10%), the 1,3-acyl migration product 3a (28%), and
recovered starting material 1a (35%), as a 2:5 mixture of E
and Z isomers (Scheme 1). In this case, three different types
of photochemical reactions, namely E/Z isomerization,
ODPM rearrangement, and 1,3-acyl migration, take place
Scheme 2. Starting enones and photoproducts.
Table 1: Reaction conditions and yields of isolated products in the irradiation of b,g-unsaturated ketones 1 in the presence of triplet sensitizers.
Substrate
Irrad. time [h]
Triplet sensitizer
Estimated DED–A
ODPM
(yield [%])
1,3-Acyl migration
(yield, [%])
Starting material
(yield [%])
[kcalmolÀ1
]
1a
1a
1a
1a
1b
1b
1b
1b
1c
1c
1c
1c
1d
1d
1d
1d
1e
1e
1e
1e
5
8.5
8.5
8.5
11
11
11
11
11
10
10
10
14
25
25
25
5.5
5.5
5.5
5.5
acetophenone
thioxanthone
4-phenylbenzophenone
chrysene
acetophenone
thioxanthone
4-phenylbenzophenone
chrysene
3-methoxyacetophenone
thioxanthone
4-phenylbenzophenone
chrysene
acetophenone
thioxanthone
4-phenylbenzophenone
chrysene
3-methoxyacetophenone
thioxanthone
12
1
–
–
–
91[2]
20
35
2a (50)
2a (10)
–
À1
À5
12
1.7
À1
À5
13[b]
5[b]
3[b]
À1[b]
14
3
3a (28)
3a (6)
–
–
3b (15)
3b (22)
–
56
–
83[a]
30
2b (68)
2b (51)
2b (8)
–
2c (39)
2c (20)
–
15
65
80[4]
45
–
3c (10)
3c (23)
–
56
40
–
60[2]
50
2d (39)
2d (16)
2d (10)
–
2e (20)
2e (53)
2e (23)
–
1
3d (6)
3d (8)
–
63
40
72
32
20
27
À3
12
4
2
À2
–
4-phenylbenzophenone
chrysene
3e (7)
3e (30)
[a] Reinvestigated by us. Compound 1b[3] was previously reported without experimental details. [b] An average energy value of 58 kcalmolÀ1 [7] was used
for the triplet energy of the diphenylvinyl unit present in 1c.
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(80%) exclusively.[4] When thioxanthone is used as sensitizer,
1c reacts to form the ODPM photoproduct 2c (39%)
exclusively, and both 2c (20%) and 3c (10%) are obtained
when the process is sensitized by 4-phenylbenzophenone.
Finally, the chrysene-sensitized photoreaction of 1c gives only
3c (23%) (Scheme 2 and Table 1).
used deactivate by E/Z isomerization exclusively. “Warm”
triplet excited states of the alkene chromophores in the b,g-
unsaturated ketones are formed upon lowering the triplet
energy of the sensitizer, and these undergo ODPM rearrange-
ments in addition to the E/Z isomerization, and sensitizers
that have a triplet energy close to that of the alkene groups
give “cold” triplet excited states that undergo E/Z isomer-
ization, 1,3-acyl migration, and ODPM rearrangement.
Finally, when the energy of the sensitizer is slightly lower
than that of the alkene, the efficiency of the ODPM
rearrangement reaction path falls or is totally suppressed
and E/Z isomerization and 1,3-acyl migration become the
main routes followed.
It is interesting to view the conclusions presented above in
the context of basic photochemical principles. Triplet sensi-
tization is a standard method to promote triplet reactivity in
molecules that do not undergo spontaneous intersystem
crossing from their singlet excited states. This topic is
discussed in great detail in all standard photochemical
textbooks and monographs.[8] However, no reports exist, to
the best of our knowledge, in the massive amount of literature
on this topic that show that triplet excited states generated by
intermolecular energy transfer can undergo different reac-
tions depending on the energy of the triplet sensitizer used.
Consequently, the generality of these novel findings and the
accuracy of our mechanistic postulates will be tested in
further studies we plan to carry out in this area.
The photochemical behavior of dicyano-substituted
enone 1d, which has a triplet energy of about 60 kcalmolÀ1 [7]
,
also follows the general pattern described above. Thus, the
acetophenone-sensitized reaction of this substrate leads to
recovered starting material (60%) only.[2] With thioxanthone,
the photoreaction of 1d gives the ODPM product 2d (39%)
exclusively, and with 4-phenylbenzophenone 2d (16%) and
3d (6%) are obtained. Finally, chrysene sensitization also
transforms 1d into 2d (10%) and 3d (8%) (Scheme 2 and
Table 1).
The photochemistry of b,g-unsaturated ketone 1e, which
contains a piperylene group (ET = 59 kcalmolÀ1),[5] was stud-
ied to test the generality of this novel reactivity profile.
Irradiation of 1e (1:6 mixture of E and Z isomers) with 3-
methoxyacetophenone as the sensitizer gives rise to E/Z
isomerization (3:2, E/Z) only. However, with thioxanthone
the ODPM product 2e (20%) is formed in addition to
recovered starting material 1e (3:2 mixture of E- and Z-
isomers). With 4-phenylbenzophenone as the triplet sensi-
tizer, 2e (53%) and 3e (7%) are generated along with
recovered 1e (1:1, E/Z). Chrysene sensitization (DED–A
ꢀ
À2 kcalmolÀ1) also affords 2e (23%), 3e (30%), and 1e
Received: July 26, 2005
Revised: September 13, 2005
Published online: November 3, 2005
(3:1, E/Z) (Scheme 2 and Table 1).
In summary, we have explored triplet-sensitized reactions
of the acyclic b,g-unsaturated methyl ketones 1a–e by using
sensitizers with different triplet energies. It has been reported
that 1a–d[2–4] do not undergo an ODPM rearrangement when
acetophenone and 3-methoxyacetophenone are used as
triplet sensitizers: under these conditions, 1a and 1b instead
undergo alkene E/Z isomerization. Enone 1e behaves
similarly in that it undergoes E/Z isomerization exclusively
when irradiated in the presence of 3-methoxyacetophenone
as triplet sensitizer. Both of the sensitizers used in these
processes have triplet energies that are about 12 kcalmolÀ1
higher than those of the alkene moieties present in the b,g-
unsaturated enones. In contrast, photoreactions of enones
1a–e with triplet sensitizers having triplet energies that are
about 5 and 1 kcalmolÀ1 higher than those of the alkene
triplets follow ODPM rearrangement pathways to afford
cyclopropyl ketones 2a–e (Scheme 2). An ODPM rearrange-
ment still takes place (albeit inefficiently) even when the
triplet energies of the sensitizer are slightly lower (ca. À1 to
À5 kcalmolÀ1) than those of the alkene groups in 1a–e, and a
new process involving 1,3-acyl migration to form enones 3a–e
(Scheme 2) becomes competitive.
Keywords: isomerization · ketones · photochemistry ·
.
rearrangement · sensitizers
[1] For recent reviews, see: a) H. E. Zimmerman, D. Armesto, Chem.
Rev. 1996, 96, 3065 – 3112; b) S. Vishwakarma, CRC Handbook of
Organic Photochemistry and Photobiolology, 2nd ed. (Eds.: W.
Horspool, F. Lenci), CRC, New York, 2004, chap. 78 and 79;
c) V. J. Rao, A. G. Griesbeck, Molecular and Supramolecular
Photochemistry, Vol. 12, Synthetic Organic Photochemistry (Eds.:
A. G. Griesbeck, J. Mattay), Marcel Dekker, New York, 2005,
pp. 189 – 210.
[2] D. Armesto, M. J. Ortiz, A. R. Agarrabeitia, M. Martin-Fontecha,
Org. Lett. 2005, 7, 2687 – 2690.
[3] A. J. A. van der Weerdt, H. Cerfontain, Recl. Trav. Chim. Pays-
Bas 1977, 96, 247 – 248.
[4] D. Armesto, M. J. Ortiz, S. Romano, A. R. Agarrabeitia, M. G.
Gallego, A. Ramos, J. Org. Chem. 1996, 61, 1459 – 1466.
[5] S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photo-
chemistry, 2nd ed., Marcel Dekker, New York, 1993.
[6] D. Armesto, M. J. Ortiz, A. R. Agarrabeitia, S. Aparicio-Lara,
Synthesis 2001, 1149 – 1158.
These results clearly show that the energy of the triplet
sensitizer plays an important role in determining the outcome
of triplet-sensitized reactions of b,g-unsaturated ketones 1a–
e. The reactivity patterns suggest that, regardless of their
triplet energies, all sensitizers promote photo-E/Z isomer-
ization of the alkene moiety. However, the “hot” triplet
excited states, with a large excess of vibrational energy, that
are formed when sensitizers with a high triplet energy are
[7] H. E. Zimmerman, D. Armesto, M. G. Amezua, T. P. Gannett,
R. P. Johnson, J. Am. Chem. Soc. 1979, 101, 6367 – 6383.
[8] See, for example: a) G. J. Calvert, J. N. Pitts, Photochemistry,
Wiley, New York, 1966; b) N. J. Turro, Modern Molecular Photo-
chemistry, Benjamin Cummings, Menlo Park, 1978; c) A. Gilbert,
J. Baggott, Essentials of Molecular Photochemistry, Blackwell,
Oxford, 1991.
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