Generation and Characterization of Highly Strained Dibenzotetrakisdehydro[12]- and Dibenzopentakisdehydro[14]annulenes

To generate dibenzotetrakisdehydro[12]- and dibenzopentakisdehydro[14]annulenes ([12]- and [14]DBAs) having a highly deformed triyne moiety, [4.3.2]propellatriene-anneleted dehydro[12]- and
dehydro[14]annulenes were prepared as their precursors. UV irradiation of the precursors resulted
in the photochemical [2 + 2] cycloreversion to generate the strained [12]- and [14]DBAs, respectively.
The [12]DBA was not detected by 1H NMR spectroscopy, but it was intercepted as Diels−Alder
adducts in solution, suggesting its intermediacy. Its spectroscopic characterization was successfully
carried out by UV−vis spectroscopy in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K
and by FT-IR spectroscopy in an argon matrix at 20 K. On the other hand, the [14]DBA was stable
enough for observation by 1H and 13C NMR spectra in solution, though it was not isolated because
of the low efficiency of the cycloreversion. The [14]DBA was also characterized by interception as
Diels−Alder adducts in solution and by UV−vis spectroscopy in a MTHF glass matrix at 77 K.
The kinetic stabilities of the DBAs are compared with the related dehydrobenzoannulenes with
respect to the topology of the π-systems. In addition, the tropicity of the [14]DBA is discussed based
on its experimental and theoretical 1H NMR chemical shifts.