1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

Discussion and Results 3
At first sight, the spectrum displayed in Figure 30 seems to be comparable to the one of 53
as it shows the expected features like e.g. the appearance of seven β-pyrrolic resonances
most downfield and three singlets around 1.58 ppm representing the three t-butyl groups in
the periphery. Both facts nicely reflect the C1 symmetry pattern.
While, analog to 53, the signal for the inner ring protons is strongly shifted to lower field, the
shift appears extremely huge for 58. The signal, being observed at -0.56 ppm, is shifted over
2.1 ppm compared to the precursor 60 and also more than 1 ppm in comparison to 53. Thus,
the electronic structure of the macrocycle appears strongly influenced by changing the size
of the exocycle.
Furthermore, a closer look reveals that the aromatic region is completely contrary to the
situation observed for 53 as the protons on the non-annulated phenyl rings give solely rise to
well resolved signals (o-/m-ArH) while surprisingly the resonances of the tethered phenyl
ring appear significantly broadened (5 3-6 ). Additionally, the splitting pattern of the signals
assigned to the freely rotatable phenyl rings looks like three sets of two doublets overlaying
at about 8.0 and 7.7 ppm, respectively. This almost seems to equal the situation being
observed in non-annulated symmetric porphyrin systems. Thus, it can be concluded, that in
58 the two half-spaces above and below the porphyrin plane are not distinguishable so that
three pseudo-AB systems appear in the spectrum. These observations imply that six-
membered ring analog 58 is not stable in configuration as the exocycle seems to be
invertible. This is also supported by the line-broadening of the signal for β-pyrrolic proton on
position 7 at the neighboring pyrrolic unit.
Further discussion on structural details is provided together with those on the second model
compound in paragraph 3.2.6.5.
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