1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
of rotational or mirror symmetry thereby also gives rise to symmetrical distortions of the
non-annulated pyrrolic units hence providing NMR spectra only differentiable by knowledge
of the precursor.
With that in mind, however, the classification was achieved by taking into account the
spectral data displayed in Figure 38 and computational models of those systems.
For both 67 and 69, four doublets are observed with characteristic three-bond couplings of
8.1-8.6 Hz resembling the appearance of two different pseudo-AB spin systems – one for
each “free” phenyl ring. This indicates that the two present half-spaces in the systems are
identical. While this is straightforward as 69 is concerned, the situation for 67 needs further
explanation. On the one side, based on computed structural models, the ketone
functionality is nearly perfectly coplanar with the porphyrin so that a pseudo-AB spin system
for the 20-phenyl substituent becomes feasible. On the other side, only the ortho methyl
groups are able to deliver a differentiation between the half-spaces. But, based on our
experience (see discussion on p. 38-39), those are too small to be capable of that which
explains the second pseudo-AB spin system. Together with the 13 C NMR spectra, where the
t-butyl groups appear as two sets of three signals in a 1:1:2 ratio – two differentiable signals
for the “free” phenyl rings and one combined for the tethered ones – the assignment was
accomplished. Thus, 67 is to be of Cs symmetry while 69 appears C2 symmetric.
In 68 and 70, the situation is different as the corresponding resonances are observed as four
doublets of doublet of corresponding couplings of 3 J ∼ 8.1 Hz and 4 J ∼ 1.7 Hz, respectively,
being in compliance with the appearance of ABCD spin systems. That means, upper and
lower half-space represent different surroundings close to the regarded phenyl rings causing
different chemical shifts. But as additionally each signal stands for 2 H, both ABCD systems
appear identical, telling that the situation is exactly the same for both “free” phenyl rings.
That is corroborated by the 13 C NMR spectra, where only one signal for both rings is
observed instead of the two signals in a 1:1 ratio for 67 and 69. Thus, taking also the
precursor’s symmetry into account, 68 is to be C2 symmetric while 70 appears to be of Ci
symmetry.
Compound 71, in contrast, represents a far easier example as only a C2 symmetric
conformation appears reasonable. That assumption can be confirmed by the corresponding
NMR data as for the two cis-standing phenyl substituents (positions 10 and 20) signals are
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