1.1 Porphyrins - Friedrich-Alexander-Universität Erlangen-Nürnberg

O
N
NH HN
N
O
O
N
H
NH N
N
N
NH
H
N
N
Discussion and Results 3
becomes more pronounced when the temperature is further reduced. At -90 °C, two sets of
two singlets each are detected at -1.68 and -1.94 ppm and at -1.94 and -2.07 ppm,
respectively, where the signal at -1.94 ppm represents an overlap of two signals as
integration proved. Thereby, the β-pyrrolic signal (position 2) and the NH-signals become
shifted upfield as Figure 19 shows.
This whole behavior is considered to be due to the presence of different tautomeric
structures at lowered temperature as tautomerism is then slowed down and thus
distinguishable structures are to be observed. That assumption is in good agreement with
findings of M. J. CROSSLEY and co-workers who described an analog splitting of NH signals in
other 2-substituted 5,10,15,20-tetraphenyl-porphyrins. 101 The detected shift of signals to
higher field might be explained by either conformational changes going in hand with
stabilization of distinct tautomeric structures or by changes in the π-electron delocalization
pathway (see Scheme 36, p. 71) or both. 102 The fact that the signal for the β-pyrrolic proton
on position 2 also experiences an upfield shift corroborates the latter explanation in analogy
to literature data obtained for naturally occurring chlorins like e.g. rhodin g7 trimethyl ester
or chlorin e6 trimethyl ester 102 .
For our explicit case, the tautomers to be taken into consideration are shown in Scheme 33.
O
N
T1 T3 T5
N
H
N
H
N
T2 T4 T6
O
O
N
N
N
HN
H
N
N
H HN
Scheme 33. Stationery tautomeric forms of 53 (T1 - T6) and the way the formally form.
N
51