1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

3 Discussion and Results
effects 128 . But also here, in most cases, it is dealt with achiral porphyrins adopting chiral
supramolecular structures. To conclude, it can be stated, that chiral porphyrin systems
without stereogenic centers existing in stable and resolvable conformations without any
auxiliaries seem to be unknown up to now.
3.2.5.2 Inherent Chirality in Cycloketo-Porphyrin Systems
Cycloketo-Porphyrin systems like 53 can be seen as analogs to chiral porphyrin 56 as their
chirality also evolves from a distorted π-system due to reasons already discussed, but, based
on the VT NMR data, their conformations are considered to be perfectly stable at room
temperature in solution without additional stabilizers making a resolution possible. The
enantiomers already shown in Scheme 32 can be assigned using the recommendations for
axial chirality since the chirality arises from the positioning of substituents along a chiral axis
which is represented by the elongation of the C meso -C α -bond. For the assignment of
descriptors the substituent of lowest priority – in this case o-ArCH3 – is placed in front
downwards while the opposite position is denoted priority 1 (Scheme 40). The neighbored
pyrrolic positions are then to be 2 and 3 with the annulated position being higher in priority.
If the rotation arising from those priorities is clockwise, the configuration is denoted Ra or P,
else Sa or M (counter-clockwise).
72
2
N
O
1
H
3
N
H 1
O
3 2
N N
S a or M R a or P
N
NH HN
N
O
O
N
NH HN
N
Scheme 40. Assignment of
stereochemical descriptors in 53.
View along the C meso -C α -bond
from the 5-phenyl substituent
towards the porphyrin core.
The corresponding enantiomeric
structures are shown below.
As cycloketo-porphyrin 53 is lacking functionalities an enantiomerically pure chiral auxiliary
could be attached to, a chemical generation of diastereomers is not possible. For such a