Symmetry-enforced conformational control of photochemical reactivity in 2-vinyl-1,3-terphenyl

Article abstract of DOI:10.1021/ja028251q

The photocyclization of o-vinylbiphenyl to 9,10-dihydrophenanthrene occurs with very low quantum yield because of the low ground-state population of the reactive syn rotamer. 2-Vinyl-1,3-terphenyl exists as a single rotamer which undergoes highly efficient photocyclization. Irradiation at 77 K in a rigid glass permits observation of the 8a,9-dihydrophenanthrene which rearranges to the 9,10-dihydrophenanthrene upon warming of the glass. The barriers for photocyclization and the thermal sigmatropic hydrogen shift are both remarkably small. Copyright

Full text of DOI:10.1021/ja028251q

Published on Web 10/24/2002
Symmetry-Enforced Conformational Control of Photochemical Reactivity in
2-Vinyl-1,3-terphenyl
Frederick D. Lewis,*,† Xiaobing Zuo,† Vladimir Gevorgyan,‡ and Michael Rubin‡
Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113, and Department of Chemistry,
UniVersity of Illinois at Chicago, Chicago, Illinois 60607-7061
Received August 22, 2002
The photochemical conversion of 2-vinylbiphenyl (1) to 9,10-
dihydrophenanthrene (2) was reported by Horgan et al. three
decades ago.1 They proposed a two-step mechanism: photocy-
clization to yield the unstable 8a,9-dihydrophenanthrene intermedi-
ate 1a, which then undergoes a thermal [1,n]-sigmatropic hydrogen
shift [n ) 5, 9, or 13] to yield the observed product (Scheme 1).2
Attempts to detect 8a,9-dihydrophenanthrene intermediates from
1 and other vinylbiphenyls under steady-state irradiation conditions
have been unsuccessful;1,3 however, a short-lived transient observed
by laser flash photolysis was assigned to the 10-phenyl derivative
of 1a.4 In the ground state, 1 is expected to exist as an equilibrium
mixture of syn and anti rotamers. Assuming that the NEER
(nonequilibration of excited rotamers) principle applies, only the
syn rotamer should undergo cyclization.5 Unfortunately, the low
population of syn-1 (ca. 1%) renders investigation of its photocy-
clization and thermal hydrogen shift problematic. This led us to
Figure 1. Photoreaction of 2-vinyl-1,3-terphenyl (3) monitored by UV
investigate the photochemical behavior of 2-vinyl-1,3-terphenyl (3),6
(solid lines) and fluorescence (dashed lines) changes. λex ) 260 nm,
irradiation time in minutes.
in which molecular symmetry enforces a syn relationship between
the vinyl and ortho phenyl groups. We report here that the 8a,9-
dihydrophenanthrene 3a can be observed upon irradiation of 3 in
a rigid methylcyclohexane (MC) glass at 77 K and that the
photocyclization of 3, the photochemical ring opening of 3a to yield
3, and the thermal hydrogen shift of 3a to yield 4 all have
remarkably low activation energies.
spectrum of 3 (Figure 1) is similar to that of 1. However, 3 is
essentially nonfluorescent both at room temperature and at 77 K.
Irradiation of 1 at room-temperature results in bleaching of the
absorption bands and fluorescence of 1 as well as the appearance
of new bands assigned to 2 (Scheme 1). Irradiation of 3 results in
similar changes in the absorption spectrum and the growth of a
weakly structured fluorescence band with a maximum at 330 nm
assigned to 4 (Figure 1). The quantum yields for the formation of
2 and 4 are <0.01 and 0.64, respectively
Scheme 1. Photochemistry of 2-Vinylbiphenyl
Irradiation of 1 in a MC glass at 77 K results in no change in
either its absorption or fluorescence spectrum. In contrast, irradiation
of 3 at 77 K results in the appearance of new absorption bands at
325 and 500 nm (Figure 2). The long-wavelength absorbing species
is stable in the dark at 77 K. However, 500-nm irradiation at 77 K
results in its complete reversion to 3. Upon warming the glass to
near its melting temperature (120 K), the 500-nm absorption band
rapidly disappears, accompanied by the appearance of the fluores-
cence assigned to 4. At 100 K the half-life of the 500-nm absorption
band is ca. 2 min, and its decay results in essentially quantitative
conversion to 4. At higher temperatures (>110 K), the fluorescence
of 4 appears without detectable formation of the 500-nm absorption
band. The long-wavelength absorption band observed upon low-
temperature irradiation of 3 is assigned to the intermediate 3a, in
accord with the mechanism proposed by Horgan et al.1 (Scheme
1). Support for this assignment is provided by a TD-B3LYP/3-
21G* calculation9 of the absorption spectrum of 3a. The calculated
and observed maxima and intensities are in excellent agreement
(Figure 2). In addition, the weak vibronic structure observed in the
500-nm band is consistent with a polyene structure.
Minimized geometries and potential energy surfaces for rotation
about the phenyl-vinyl bonds of 1 and 3 were obtained using the
Jaguar package.7 The phenyl-phenyl dihedral angles are ca. 60°
for both molecules. The phenyl-vinyl dihedral angles are ca. 40°
for 1 and ca. 55° for 3. The anti rotamer of 1 is more stable than
the syn rotamer by 2.6 kcal/mol, corresponding to an equilibrium
population of ca. 1% of the syn rotamer. The absorption and
fluorescence spectra of 1 in MC solution resemble those of biphenyl
and styrene,8 displaying an allowed transition at 235 nm and a weak
shoulder at 290 nm. The fluorescence quantum yield and singlet
decay time for 1 are Φf ) 0.36 and τs ) 15.5 ns. The absorption
* To whom correspondence should be addressed. E-mail: lewis@chem.nwu.edu.
† Northwestern University.
‡ University of Illinois at Chicago.
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10.1021/ja028251q CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
indicates that electrocyclic ring closure of singlet 3 and ring opening
of singlet 3a occur via a pericyclic funnel with small or nonexistent
barriers.13 Entry to this funnel from singlet 3 results in formation
of 4 with a quantum yield of 0.64, providing an upper bound of
0.36 for the quantum yield of return to the ground state of 3 via
the pericyclic funnel.
The thermal sigmatropic hydrogen shift of 3a also has a low
inherent barrier. In fact, 3a can be detected by means of steady-
state irradiation only at low temperatures and high solvent viscosi-
ties. A barrier of e2 kcal/mol for the hydrogen-shift reaction
appears to be consistent both with our data and with the laser flash
photolysis data of Lapouyade et al.4 for the R-phenyl analogue of
1. This barrier is substantially lower than the value of 41 kcal/mol
reported for the sigmatropic hydrogen shift of 1,3-cyclohexadiene.14
It also lower than the lowest barrier reported to date for a
sigmatropic hydrogen shift, the conversion of isoindene to indene
(ca. 14 kcal/mol).15 The low barrier for the rearrangement of 3a to
4 is, no doubt, a consequence of the highly exergonic nature of
this process.
Figure 2. UV spectra of irradiated 2 × 10-5 M 2-vinyl-1,3-terphenyl (3)
in methylcyclohexane at 77 K (time in minutes) and the calculated UV
spectrum of intermediate 3a.
In summary, molecular symmetry confines 3 to a single ground-
state conformation in which the vinyl and an o-phenyl group have
a syn relationship favorable for photocyclization. Cyclization yields
the intermediate 3a, the first 8a,9-dihydrophenanthrene to be directly
observed under steady-state irradiation conditions. Irradiation of
3a at 500 nm results in reversion to 3, whereas warming of the
glass or irradiation of 3 in fluid solution results in formation of 4.
Both the photochemical and thermal rearrangements are remarkable
for their low activation energies. Further details of this and related
reactions are under investigation.
The photochemical behavior of 1 can be readily explained with
reference to Scheme 1. The long-lived fluorescence is assigned to
the major (99%) rotamer, anti-1, which has a florescence quantum
yield and decay time similar to those of 1-phenylpropene.10 The
photocyclization of 1 to yield 2 is attributed to the minor (1%)
rotamer, syn-1. The absence of spectral changes upon irradiation
of 1 at 77 K is attributed to the very low population of syn-1 at
low temperature. The photochemical behavior of 3 can be explained
with reference to Scheme 2. The ground-state energies in Scheme
Scheme 2. Potential Energy Surfaces for the Photoreaction of
2-Vinyl-1,3-terphenyl (3) (Energies in eV)
Acknowledgment. Funding for this project was provided by
NSF Grants CHE-0100596 (F.D.L.) and CHE-0096889 (V.G.). We
thank Professor I. Alabugin, Florida State University, for calculating
the spectrum of 3a and for helpful suggestions.
References
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2 have been estimated using MM/PM3 calculations, and the excited-
singlet-state energies, using ZINDO calculations for 3 and 4 and a
TD-DFT calculation for 3a.9,11 The absence of fluorescence from
3 both at room temperature and 77 K, the high quantum yield for
formation of 4 at room temperature, and the formation of 3a in a
rigid MC glass at 77 K are all indicative of a low barrier for
photocyclization. In this respect, the photocyclization behavior of
singlet 3 is similar to that of cis-stilbene, for which a barrier of e1
kcal/mol has been estimated for formation of 4a,4b-dihydrophenan-
threne.12 The singlet energy of 3 lies well above that of 3a, making
adiabatic conversion of 13* to 13a* energetically feasible. However,
the observation that irradiation of 3a yields 3 even in a 77 K glass
(9) TD-DFT calculations provided by I. Alabugin.
(10) Lewis, F. D.; Bassani, D. M.; Caldwell, R. A.; Unett, D. J. J. Am. Chem.
Soc. 1994, 116, 10477-10485.
(11) ZINDO calculations: CAChe, version 4.4; Oxford Molecular Group,
Inc.: Campbell, CA, 2000. Similar values of the singlet state energies of
3a (4.25 eV) and 4 (4.08 eV) can be estimated from their absorption and
fluorescence spectra, respectively.
(12) (a) Saltiel, J.; Waller, A. S.; Sears, D. F. J. J. Am. Chem. Soc. 1993, 115,
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