The absence of E/Z isomerization of the 4-cyanophenylvi-
nyl unit in the triplet-sensitized reactions of diene 4e is
surprising. However, this observation is consistent with
literature precedents that show that the E/Z isomerization of
4-cyanostilbenes occurs only from their singlet excited
states.12
In order to determine whether 1,4-dienes with electron-
donor groups at C-1 would also undergo DPM rearrangement
in the triplet manifold, the photochemical reactivity of
compound 4f with 4-methoxyphenyl substitution at C-1 was
investigated. Direct irradiation of 4f (E/Z, 3:5) generates 5f
(22%, E/Z 1:1) and recovered starting material 4f (54%, E/Z
3:2). In contrast, acetophenone-sensitized irradiation of 4f
leads to E/Z isomerization of the alkene unit, exclusively.
However, when 3-methoxyacetophenone, thioxanthone, 4-phe-
nylbenzophenone, or chrysene are used as sensitizers, 4f
undergoes DPM reaction to generate photoproduct 5f with
the respective efficiencies displayed in Table 1.
The formation of 5f in the sensitized irradiation of 4f
represents the first example of a triplet DPM rearrangement
in an acyclic 1,4-diene that contains an electron-donating
group at C-1. In this instance, the regiochemistry of the
reaction does not depend on the multiplicity of the excited
state, and cyclopropane 5f, with the 4-methoxyphenyl group
attached to the vinyl unit, is obtained from both direct and
triplet-sensitized irradiation promoted reactions.
The study was extended to include 1,4-diene 4g that has
two 4-methoxyphenyl groups at C-1 and two 4-cyanophenyl
substituents at C-5. The photochemistry of 4g was previously
described by Zimmerman and Welter.6 These authors
observed that DPM rearrangement of 4g takes place on direct
irradiation to yield 5g (30%) and that this substance is
unreactive under 3-methoxyacetophenone-sensitized irradia-
tion conditions.
of high triplet energy (acetophenone and 3-methoxyac-
etophenone) and low triplet energy (4-phenylbenzophe-
none and chrysene) but that the reaction does not occur
when thioxanthone is used (Table 1).
In summary, previous studies of the photochemistry of
acyclic 1,4-dienes led to the establishment of the general
rule of thumb that these substances undergo DPM rearrange-
ments on direct irradiation via the singlet excited states,
whereas their triplet excited states lead to Z/E isomerization,
exclusively.3 However, the results obtained in the current
effort show that these substrates do indeed undergo DPM
reactions on triplet-sensitized irradiation via the triplet excited
state and that the chemical efficiencies of the triplet processes
depend on the nature of the sensitizer used. Although our
previous studies with related unsaturated ketones suggested
that a possible correlation exists between the triplet energy
of the sensitizer and the photoreactivity observed, the
observations made in this study demonstrate that this
correlation does not apply to dienes 4. Therefore, other
factors (still unknown) must play an important role in the
outcome of triplet-sensitized reactions of acyclic 1,4-dienes.
We have tried to correlate the reactivity observed with
different physical constants of the sensitizers and the dienes
without success. In an attempt to obtain information about
this issue, diene 4a was irradiated in t-BuOH. The results
indicate that significant differences in the reactivity profile
of 4a do not exists for reactions in CH2Cl2 or the t-BuOH.
It is clear that additional studies are needed to clarify the
influence of sensitizer type and triplet energy on the triplet
reactivity of 1,4-dienes. Nevertheless, the obvious conse-
quence of the investigation described above is that other
compounds that have been previously reported to be unre-
active in their triplet excited states might actually be reactive
when suitable triplet sensitizers are used.
In contrast with these previous observations, the results
of the current studies show that acetophenone-sensitized
irradiation of 4g affords the DPM photoproduct 5g (14%).
Likewise, when 3-methoxyacetophenone and 4-phenylben-
zophenone are used as sensitizers, 5g is obtained in 30%
and 36% yield, respectively. However, irradiation with
thioxanthone as a triplet sensitizer leads to recovery of
starting material (90%). Finally, chrysene-sensitized irradia-
tion of 4g efficiently yields the DPM product 5g (73%)
(Figure 2 and Table 1). The regiochemistry of the triplet-
DPM reaction in this instance is the same as that
previously reported for the direct irradiation reaction of
4g.6 These results are surprising in light of the fact that
4g undergoes efficient DPM rearrangement with sensitizers
Acknowledgment. M.M.-F. and N.E.-B. thank the MEC
and MAEC, respectively, for postgraduate fellowships. We
thank the Ministerio de Educacio´n y Ciencia (Grant No.
CTQ2005-01400) and Universidad Complutense of Madrid
(Grant No. CCG06-UCM/PPQ-1326) for financial support.
We acknowledge the support provided by the Centro de
Resonancia Magnetica of the Universidad Complutense de
Madrid.
Supporting Information Available: Experimental pro-
cedures for the synthesis of dienes 4e and 4f and photolysis
of 4a-g. Analytical and spectroscopic data for all photo-
products. This material is available free of charge via the
(12) King, N. R.; Whale, E. A.; Davis, F. J.; Gilbert, A.; Mitchell, G. R.
J. Mater. Chem. 1997, 7, 625–630, and references cited therein.
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