Regioselectivity of E/Z photoisomerization of fluorinated cisoid (1E,3E)-1,4-diphenylbutadienes via direct irradiation

Article abstract of DOI:10.1016/j.tetlet.2003.09.043

Direct irradiation of the 1E,3E isomers of six cisoid fluorinated butadienes in an organic solvent at room temperature showed a predominant formation of the 1Z,3E isomers.

Full text of DOI:10.1016/j.tetlet.2003.09.043

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8103–8107
Regioselectivity of E/Z photoisomerization of fluorinated cisoid
(1E,3E)-1,4-diphenylbutadienes via direct irradiationꢀ
Jin Liu,* Eric L. Suits and Kelly J. Boarman
Department of Chemistry, Murray State University, Murray, KY, 42071, USA
Received 24 August 2003; revised 4 September 2003; accepted 4 September 2003
Abstract—Direct irradiation of the 1E,3E isomers of six cisoid fluorinated butadienes in an organic solvent at room temperature
showed a predominant formation of the 1Z,3E isomers.
© 2003 Elsevier Ltd. All rights reserved.
The mechanisms of photo-induced cis/trans isomeriza-
tion of arylethenes such as stilbenes have been exten-
sively investigated during the past four decades.1 The
photochemical and photophysical properties of
arylethenes have been well understood.2 As compared to
the number of studies of arylethenes, only a few studies
of photoisomerization of 1,4-diphenyl-1,3-butadienes
have been reported,3 due to the complexity caused by
the existence of three E/Z isomers. In 1988, Yee and his
co-workers4 reported their findings on direct and sensi-
tized photoisomerization of 1,4-diphenyl-1,3-butadi-
enes. In 1995, Kauffman and his co-worker5
investigated solvent effects on direct photoisomerization
of 1,4-diphenyl-1,3-butadienes. Photoisomerization of
(E,E)-1,4-diphenyl-1,3-butadiene gives only one (E,Z)-
isomer, because of the equivalence of two aryl groups in
its structure. However, photoisomerization will produce
two different isomers (1Z,3E) and (1E,3Z) when two
aryl groups are not identical in the structures of phenyl-
substituted 1,4-diphenyl-1,3-butadienes. Photoisomer-
ization will be a useful synthetic method, if the
(E,E)-isomer of a substituted 1,4-diphenyl-1,3-butadi-
ene could be selectively converted to one of its E,Z
isomers. Nevertheless, our current understanding of
regioselectivity of E/Z photoisomerization of unsym-
metrically substituted 1,4-diphenyl-1,3-butadienes is
very limited.6
glet state are single bond s-cis/s-trans and double bond
cis/trans isomerizations. In order to better focus on
studies of double bond cis/trans isomerization, we chose
a cisoid 1,4-diphenyl-1,3-butadiene system,9,10 in which
a conjugated diene was locked. To differentiate between
the two phenyl rings that were attached to the conju-
gated diene, at lease one of the rings was substituted
with an electron-withdrawing or electron-donating
group.11 Herein, we report our results observed in the
E/Z photoisomerization of unsymmetrically substituted
cisoid (1E,3E)-1,4-diphenylbutadienes by direct irradia-
tion.
First, four fluorinated cisoid butadienes (1–4) were
synthesized. Symmetrical cisoid 1,4-diphenyl-1,3-buta-
dienes were prepared by Liu et al. in 1997 using the
Wittig reaction of a conjugated enone in Scheme 1 with
benzylidenetriphenylphosphorane.9 Due to the relative
low yield of the Wittig reaction, preparation of
the unsymmetrical 1,4-diphenyl-1,3-butadienes was
achieved by means of the McMurry reaction.10 The
enone prepared from the Aldol condensation of racemic
norcamphor with benzaldehyde was coupled with pen-
tafluorobenzaldehyde to produce compound 1 (in ca.
30% yield) and two dimers, which were formed from
dimerization of pentafluorobenzaldehyde and the
Previous studies7,8 have found that major products of
photoisomerization of 1,3-butadienes in the excited sin-
ꢀ Supplementary data associated with this article can be found at
doi:10.1016/j.tetlet.2003.09.043
* Corresponding
author.
Fax:
270-762-6474;
e-mail:
jin.liu@murraystate.edu
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.043

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J. Liu et al. / Tetrahedron Letters 44 (2003) 8103–8107
Scheme 1.
enone, respectively.12 Compound 1 contained two
stereoisomers in a ratio of 95:5. The two isomers as
white solids were separated by silica gel column chro-
matography. The same synthetic approach was used for
the preparation of the cisoid butadienes 2–4.
Figure 2. The X-ray structure of the (1E,3E)-isomer of 2 at
173 K; a pair of the enantiomers was found in the asymmetric
unit.
To properly assign the E/Z configuration of the iso-
mers, the major isomers of compounds 1 and 2 were
recrystallized from ethyl acetate to give crystals that
were suitable for X-ray crystallographic studies. The E
configuration of the double bonds in the two structures
was confirmed by crystal structures (Figs. 1 and 2).13
Therefore, it was deduced that the minor isomers of 1
and 2 from the synthesis were the 1Z,3E isomers. Also,
the two X-ray crystal structures indicate that the aver-
age bond length of the carbon–carbon double bonds of
(1E,3E)-isomer are shifted downfield relative to those
of the (1Z,3E)-isomer. A similar pattern was found
1
from the H NMR study of the two isomers of 2. Also,
UV-vis absorptions of the two 1E,3E isomers of com-
pounds 1 and 2 are appreciably red-shifted from those
of the 1Z,3E isomers. Because suitable single crystals of
compounds 3 and 4 were not obtained, the correct
configuration of the 1E,3E isomers of 3 and 4 was
assigned by comparing the downfield shifts of the vinyl
Hs and the red-shifted UV absorptions of the 1E,3E
isomers to those of the corresponding 1Z,3E isomers
(see the Supplementary Data).
,
the cisoid 1,3-butadienes is ca. 1.34 A. The average
bond length of the central carbon–carbon single bonds,
which attaches to the two carbon double bonds, is ca.
,
1.49 A.
Direct irradiation of the cisoid butadienes (1–4) in
chloroform-d was carried out using a 200 W Hanovia
medium pressure mercury lamp with the Corning glass
filter O-52 (>340 nm light) (Scheme 2). In Figure 3, the
progress of direct photoisomerization of the (1E,3E)-
1H NMR spectra of the (1E,3E) and (1Z,3E) isomers
of compound 1 indicates the vinyl-H resonances of the
1
isomer (1) is illustrated. During the reaction, H NMR
spectra were recorded every 15 min until no further
change was observed. The photoisomerization of the
(1E,3E)-isomer to the (1Z,3E)-isomer was indicated by
the decreasing intensity of two norbornyl bridgehead-H
signals of (1E,3E) at l 3.48 and 2.88, and the increasing
intensity of two bridgehead-H signals of (1Z,3E) at l
3.41 and 3.03. The very low intensity of two proton
resonances at l 2.93 and 2.78 indicated the barely
formed (1E,3Z)-isomer at the end of the photo-reac-
Figure 1. The X-ray structure of the (1E,3E)-isomer of 1 at
173 K.
Scheme 2.

J. Liu et al. / Tetrahedron Letters 44 (2003) 8103–8107
8105
The E/Z photoisomerization of 2–4 was studied under
the same photoirradiation conditions, and the results
also showed the preferred isomerization at the carbon–
carbon double bonds connected with the fluorinated
phenyl rings. However, the ratios of (1Z,3E)/(1E,3Z) of
2, 3, 4 were 23:1, 13:1 and 1.5:1, respectively (Table 1).
The gradually decreasing selectivity appears to be
related to the number of F-substituents on the phenyl
rings.
Some substituent effects on photochemistry of
stilbene14–16 and 1,4-diphenyl-1,3-butadiene17 were
reported in the literature. The photochemical behavior
of 1–4 by direct irradiation is clearly sensitive to the
F-substituents on the phenyl rings. Although the aver-
age bond length of the carbon–carbon double bonds
connecting with the fluorinated rings is the same as that
of the double bonds with the phenyl rings, the two
types of double bonds are not the same in the excited
singlet state.18 The strong electron-withdrawing effect
of fluorine substituents weakens the carbon–carbon
double bonds facilitating the isomerization process. The
results from 1–3 demonstrate the strong isomerization
preference for the carbon–carbon double bonds closer
to the F-substituents. The mono-fluorinated case (4)
shows a low selectivity (1.5:1), as the number of
fluorine substituents decreases.
Figure 3. 1H NMR spectra (a)–(d) were recorded at irradia-
tion time 0, 15, 30, 45 min. Symbols (*), (ꢁ) and (ꢂ) are
used for the signals from the (1E,3E)-isomer, the (1Z,3E)-iso-
mer and the (1E,3Z)-isomer, respectively.
tion. After the irradiation, the remaining (1E,3E)-iso-
mer in the mixture was isolated by silica gel column
chromatography, and the photo-conversion yield
(73.5%) of the E,E isomer (1) to the E,Z isomers was
determined. The two E,Z isomers, partially separated
by column chromatography, were further purified by
preparative HPLC. The (1Z,3E)/(1E,3Z) ratio of 1 was
determined to be 26:1. In Figure 4, the UV-vis absorp-
tion spectra of the three isomers (1) are shown. The
results from the direct irradiation of 1 indicated the
preferred E/Z photoisomerization at the carbon–car-
bon double bond on the pentafluorophenyl side. Other
organic solvents such as benzene-d6 and hexafluoroben-
zene were found to have little effect on the regioselec-
tivity of the photoisomerization. Different concentra-
tions of 1 in chloroform-d (0.002ꢀ0.015 M) had no
influence on the observed regioselectivity.
To further confirm the electron-withdrawing fluorine
effect on the regioselectivity, we prepared unsymmetri-
cal non-fluorinated compounds (5–8). The (1E,3E)-iso-
mers and the (1Z,3E)-isomers of 5–8 were prepared
using the same synthetic approach, and the structures
were fully characterized. Interestingly, the two E,Z
isomers of 5–6 in an almost equal amount were
obtained from the photoisomerization of the (1E,3E)-
isomers of 5–6 by direct irradiation (Table 1). The
results were in agreement with our earlier observation
from 4. Surprisingly, the E/Z photoisomerization of
7–8 under the same irradiation conditions was not
observed, and the starting materials were recovered.
The unexpected observation might indicate that the
strong electron-donating methoxy groups strengthened
the carbon–carbon double bonds of 7–8 in the excited
singlet states.
The different photochemical properties of 7 and 8
prompted us to prepare two unsymmetrically substi-
tuted cisoid butadienes with substituents on both of the
phenyl rings (9–10). Figure 5 shows UV-vis absorption
spectra of the 1E,3E isomers of 7–10 in hexanes, illus-
trating the similar absorption bands of 7–10. Under the
same irradiation conditions, the E/Z photoisomeriza-
tion of 9 and 10 was not only found, but the regioselec-
Figure 4. Absorption spectra of three E/Z isomers (1) in
hexanes.

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J. Liu et al. / Tetrahedron Letters 44 (2003) 8103–8107
Table 1. The percentages of three E/Z isomers at the end of photoisomerization of the (1E,3E)-isomers of 1–10
Compound
(1E,3E) (%)
(1Z,3E) (%)
(1E,3Z) (%)
(1Z,3E)/(1E,3Z) ratio
1
2
3
4
5
6
7
8
9
26.6
33.3
41.7
50.0
88.2
32.6
>99
>99
47.4
45.0
70.8
63.9
54.2
30.0
5.9
34.8
0
0
2.7
2.8
4.1
20.0
5.9
32.6
0
0
10.5
4.0
26:1
23:1
13:1
1.5:1
1:1
1.1:1
–
–
4:1
42.1
51.0
10
13:1
1,3-butadienes, and the role of substituents on pho-
toisomerization mechanisms of butadienes20 are under
investigation.
Supplementary material
Representative procedures and spectroscopic data for
new compounds are included in Supplementary Data,
which is available on the web.
Acknowledgements
This work was supported by grants from Kentucky
NSF EPSCoR and Howard Hughes Medical Institu-
tion. We thank Drs. William W. Brennessel and Victor
G. Young, Jr at Department of Chemistry, University
of Minnesota, for the X-ray crystallographic studies of
1 and 2.
Figure 5. Absorption spectra of compounds 7–10 in hexanes.
tivity of the photoisomerization was also regained
(Table 1). The photo-conversion yields of 1E,3E iso-
mers of 9–10 were about the same (53% and 55%).
However, the (1Z,3E)/(1E,3Z) ratio of 10 was higher
than that of 9. In the case of 10, the better selectivity
was due to the fact that the electron-withdrawing effect
of F-substituents was much stronger than the electron-
donating effect of the single methoxy group. Thus, we
believe that the methoxy groups strengthened the car-
bon–carbon double bonds of 7–8 in the excited singlet
states.
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