New aspects of diphenylbutadiene photochemistry. Regiospecific Hula-twist photoisomerization

Article abstract of DOI:10.1021/ja043193p

In EPA glass at liquid nitrogen temperature, the E,E isomer of diphenylbutadiene (DPB) was photostable, while both the Z,E and Z,Z isomers underwent selective HT isomerization at center 1 giving the stable conformer of the double-bond isomerized trans product. That HT-1 was involved rather than the OBF process was shown by results of o,o-dimethyl-DPB. Formation of unstable trans product corresponded to simultaneous configurational and conformational isomerization. The regioselectivity was found not sensitive to a substituent effect, as shown by the similar reactivity in p,p- or o,o-bistrifluoromethyl-DPB. Copyright

Full text of DOI:10.1021/ja043193p

Published on Web 02/05/2005
New Aspects of Diphenylbutadiene Photochemistry. Regiospecific Hula-Twist
Photoisomerization
Lan-ying Yang, Robert S. H. Liu,* Kelly J. Boarman,† Natalie L. Wendt,† and Jin Liu*,†
Department of Chemistry, UniVersity of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822, and Department of
Chemistry, Murray State UniVersity, Murray, Kentucky 42071
Received November 11, 2004; E-mail: rliu@gold.chem.hawaii.edu; jin.liu@murraystate.edu
Photoisomerization of 1,4-diphenyl-1,3-butadiene (DPB, 1) was
first studied by Zechmeister.1 In 1988, Yee et al. reported a detailed
study of all three isomers under direct and triplet-sensitized
irradiation with the Z,Z isomer being absent in all product mixtures.2
Recently, photoisomerization by the volume-conserving Hula-twist
(HT) mechanism3 was shown to be involved in styrenes,4 dienes,5
and stilbenes6 under confined conditions. The same mechanism, if
involved in DPB, will not only lead to products different from that
of the conventional torsional relaxation mechanism, or one-bond-
flip (OBF) process, but also could reveal itself in two different
ways (HT-1 and HT-2) as shown with the Z,Z isomer.
Figure 1. (a) Absorption spectra of Z,E-1 recorded during its irradiation
(>310 nm, Corning 0-54 filter) in EPA glass at liquid nitrogen temperature.
Insert: log of absorbance at 337 nm versus time. (b) Difference spectra (t
- t0) from absorption spectra. Insert: partial absorption spectrum at t )
790 s minus that of residual amount of Z,E-1 (dotted line) superimposed
The E,E isomer of DPB was found to be stable under direct
with the absorption spectrum of E,E-1 (solid green line).
irradiation in EPA (ether/isopentane/ethanol ) 5:5:2) glass at liquid
nitrogen temperature. The Z,E and Z,Z isomers under the same
conditions were found to be photolabile. The results are described
below.
The progress of the reaction of Z,E-1 is shown in Figure 1a and
the difference absorption spectra (t spectrum minus t0 spectrum)
in Figure 1b. The semilog plot (insert) and isosbestic points showed
a simple one-to-one conversion. Also, positions of the peaks of
the main absorption band in the difference spectra were found to
be identical to that of an authentic sample of E,E-1. Furthermore,
we found that the relative peak heights of the spectrum of the
irradiated sample minus that of the unreacted Z,E-1 matched those
of E,E (insert in Figure 1b). Therefore, it was concluded that the
only photoproduct from Z,E in EPA glass was E,E. In agreement,
the absorption spectra of the irradiated sample before and after
warming to 180 K and recooling to 77 K were the same. Clearly,
only the stable conformer was formed in the photoreaction, which
is not in agreement with HT-2. The latter should give an unstable
E,E conformer (E,E′-1). The isomerization, therefore, is regio-
specific: HT-1. However, formally the OBF process could also
yield the same stable E,E product. While such a volume-demanding
OBF process is highly unlikely in a frozen glass medium as shown
in other studies,4-6 we decided to carry out the distinguishing
experiment depicted in Figure 2.
Figure 2. (a) Absorption spectra of Z,E-2 during its irradiation (>310
nm) in EPA glass at 77 K. Insert: a semilog plot of absorbance at 350 nm
versus time. (b) Difference spectra. Insert: the absorption spectrum at t )
1520 s (green) superimposed with that of the same sample (red) after
warming to 180 K and recooling to 77 K.
The Z,E isomer of o,o′-dimethyl-DPB (2)7 was irradiated under
the same conditions. The absorption and difference spectra are
shown in Figure 2a along with a semilog plot. The plot showed a
single-exponential decay; however, the difference spectra showed
that the photoproduct was not that of the stable conformation of
the E,E isomer. The spectrum changed when the sample was
† Murray State University.
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10.1021/ja043193p CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
warmed to 180 K and recooled to 77 K. The new spectrum was
identical to that of the stable E,E (see insert in Figure 2b). Thus,
the primary photoproduct must be a high-energy conformer of E,E-2
(E,E′′-2). The presence of the o,o′-substituents that removed
symmetry in the phenyl ring made possible detection of the
associated single-bond isomerization along with the double-bond
isomerization as expected of a HT process.
Another minor difference between results of Z,E-2 and Z,E-1 is
the slightly diffused isosbestic point for the methylated case, which
appeared to be independent of the wavelength of excitation (>300
nm, Pyrex filter, or >325 nm, naphthalene solution filter). Analysis
of the absorption spectrum showed the presence of the stable E,E-2
as a minor product at all stages of irradiation (increasing slightly
with time). Its formation is not consistent with HT-2 of Z,E-2
(yielding instead E,E′-2) but is consistent with HT-1 of the unstable
conformer Z,E-2 present in equilibrium concentration at the setting
temperature of the organic glass (∼90 K).
Figure 3. Difference spectra obtained from direct irradiation of Z,Z-1 in
EPA glass at 77 K. (a) Early period of irradiation (2-50 s). Insert:
absorption spectrum of authentic Z,E-1 (solid line) superimposed with that
of the absorption spectrum at 50 s of irradiation minus the residual amount
of Z,Z-1 (42%) and 1.8% E,E-1 (dotted line). (b) Later period of irradiation
(110-1270 s). Insert: absorption of authentic E,E-1 (solid line) superim-
posed with the absorption spectrum at t ) 1270 s of irradiation minus 62%
Z,E-1 (dotted line).
While it is premature to suggest a primary cause for the observed
regioselectivity in HT of DPB, we suspect there are two contributing
factors favoring HT-1 over HT-2. The first is the possible favorable
energy of the allyl-phenyl transition state structure for the HT-1
process (versus the vinyl-phenyl structure for HT-2). The second
is the smaller phenyl end group in its sideways sliding movement
as part of the HT-1 process (relative to the larger benzyl group for
HT-2).
We have also examined the low-temperature photochemical
behavior of Z,Z-1. The difference spectra corresponding to those
of the early (Figure 3a) and later (Figure 3b) periods of irradiation
are shown separately. The semilog plot (not shown) clearly showed
two distinct steps of reaction giving an early product without any
fine structures and a later product with fine structures in the
absorption spectra. After subtracting contributions of reactants to
the spectra of the irradiated sample, we readily concluded that the
initial product was that of the stable Z,E isomer and the later product
was that of the stable E,E isomer (see inserts), which is identical
to the insert in Figure 1b. Therefore, the photoreaction must be
consecutive HT-1 and HT-1′ processes (see scheme for Z,Z above).
Again, HT-2 or HT-2′ would lead to new unstable conformers.
Additionally, we have examined the low-temperature photo-
chemical behavior of isomers of p,p′- and o,o′-bistrifluoromethyl-
DPB (3 and 4).7 In the case of 3, photoreactions were similar to
those of the parent isomers; in the case of 4, photoreactions were
similar to those of 2. Figures corresponding to those of Figures
1-3 are available in Supporting Information. Hence, the observed
high preference for isomerization at CH-1 was unaffected by
variation of electronic properties of the substituents on the phenyl
rings.
Acknowledgment. The work was supported by grants from
Hawaii NSF-EPSCoR (CHE-01-32250) and Kentucky NSF-EPS-
CoR (4-65752-03-397).
Supporting Information Available: NMR data of isomers of 1-4
and difference absorption spectra of 3 and 4 from direct irradiation
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) Zechmeister, L. Cis-trans Isomeric Carotenoids, Vitamin A, and Arylpoly-
enes; Academic Press: New York, 1962; pp 163-168.
(2) Yee, W. A.; Hug, S. J.; Kliger, D. S. J. Am. Chem. Soc. 1988, 110, 2164-
2169.
(3) Liu, R. S. H. Acc. Chem. Res. 2001, 34, 555-562.
(4) Krishnamoorthy, G.; Asato, A. E.; Liu, R. S. H. Chem. Commun. 2003,
2170-2171.
(5) Krishnamoorthy, G.; Schieffer, S.; Shevyakov, S.; Asato, A. E.; Wong,
K.; Head, J.; Liu, R. S. H. Res. Chem. Interm. 2004, 30, 397-405.
(6) Imamoto, Y.; Kuroda, T.; Kataoka, M.; Shevyakov, S.; Krishnamoorthy,
G.; Liu, R. S. H. Angew. Chem., Int. Ed. 2003, 42, 3630-3633.
(7) Isomers of 1 were prepared according to published procedures.1,2 Similar
procedures were followed for preparation of isomers of 2-4. Characteriza-
tion data (NMR and UV) of the latter compounds are available in
Supporting Information.
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