DIPHENYLCARBENE PHOTOCHEMISTRY
and prolonged irradiation with the 254-nm light of a low
pressure mercury lamp did not result in any detectable
photochemistry.
To detect possible paramagnetic intermediates involved in the
rearrangement of 5, we investigated its matrix photochemistry
also using the very sensitive EPR spectroscopy (Figure 3). As
expected, all transitions assigned to T-5[38,41] slowly disappear
during 445-nm irradiation. A radical signal centered at 340 mT
of an unknown species appears, but no signals are observed that
could be attributed to T-9 or other paramagnetic intermediates.
In conclusion, diphenylcarbene 5 rearranges to cyclohep-
tatetraene 8 upon direct irradiation with an intense laser into
its weak transitions in the 430–450-nm range. In contrast to the
highly photolabile phenylcarbene 1, diphenylcarbene 5 is stable
toward UV photolysis, and the 445-nm photochemistry is
extremely slow and inefficient. Nevertheless, the photochemical
formation of 8 supports the proposed carbene–carbene rear-
rangement as the preferred mechanism for the formation of 7.
Under the conditions of our experiments, 8 is rather photostable,
and upon UV irradiation neither rearranges back to carbene 5
nor produces fluorene 7 or carbene 9.
Figure 2. IR spectra showing the rearrangement of 5 to 8. (a) IR
spectrum of 5 in argon at 5 K generated by 530-nm photolysis of 6. (b)
Difference IR spectrum showing the 5 → 8 rearrangement. Bands
pointing downward are disappearing during overnight irradiation with
λ = 445 nm, bands pointing upwards are appearing. (c) IR spectrum of 8
obtained upon overnight irradiation of 5 with λ = 445 nm. (d) IR spectrum
of 8, calculated at the B3LYP/6-311++G(d,p) level of theory
This photochemical transformation could also be followed by
IR spectroscopy (Figure 2). Again, 445-nm photolysis results in
the slow bleaching of all bands assigned to 5. A new compound
Acknowledgement
This work was supported by the Cluster of Excellence RESOLV
(EXC 1069) funded by the Deutsche Forschungsgemeinschaft.
is formed with the strongest absorptions at 725 and 790 cmÀ1
.
The new bands were compared with the calculated IR spectra
of a number of isomers of 5, but the only convincing match
was found with 1-phenyl-1,2,4,6-cycloheptatetraene 8 (Table
S1). To confirm this assignment, we also generated the per-
deuterated carbene d10-5, which was transformed to the corre-
sponding allene d10-8. Again, the experimental band positions
and band intensities are very nicely reproduced by the DFT
calculations (Figure S1 and Table S2). As with other
cycloheptatetraenes described in the literature,[1] the C–C–C
stretching vibration of the allene moiety in 8 is very weak and
difficult to observe in the spectrum. For 8, the C–C–C stretching
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Figure 3. X-band EPR spectra showing the 445-nm photochemistry of 5,
matrix-isolated in argon at 5 K (a) EPR spectrum of 5 with the zfs param-
eters D = 0.417 cmÀ1, E = 0.019 cmÀ1. (b) Same matrix after overnight
irradiation with λ = 445 nm
J. Phys. Org. Chem. 2015, 28 71–74
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