Porphycene dihydro

The reduction of porphycenes, e.g. 1, can be achieved by catalytic hydrogenation22-25 or chemical reduction with alkali metals.22 With alkali metals one of the C —C double bond bridges in 1 is reduced to yield a 19,20-dihydroporphycene2. On catalytic hydrogenation of porphycene (1), 2,3-dihydro porphycene (3) is formed which can be compared to chlorin, the dihydrogenated form of porphyrin. 

Porphycene (2).2 This isomer (2) of porphin is obtained in 2-3% yield by McMurry coupling of the diformylbipyrrole 1 in THF/Py with TiCl4-Zn. The reaction presumably involves a dihydro derivative (a). 

The reduced porphycene 3.90 may be considered as an analog of chlorin 3.91 (Figure 3.1.18), a species which, in turn, may be considered as a dihydro-analog of porphyrin. In the specific case of 3.90, confirmation of the chlorin-like nature of the system in terms of formal 7i-electron bookkeeping, was obtained from H NMR experiments. These same experiments provided insights into the specific site of reduction. For instance, it was found that the P-pyrrolic CHCH protons resonated at lower field than analogous protons in pyrrole (i.e., at 5 = 8.2-9.3 ppm). On the other hand, the CH2CH2 protons of the reduced pyrrolic subunit were found at a relatively upheld location (i.e., 8 = 4.6-4.9 ppm) this is as one would expect for aliphatic protons of this type. 

Reducing porphycene in the presence of alcoholic sodium metal gives rise to a different reduction product. Under these conditions, it was found, for instance, that the unsubstituted porphycene 3.2 affords the 9,10-dihydroporphycene 3.93 (Scheme 3.1.17) instead of an jV,iV-dihydro- or 2,3-dihydroporphycenes, such as 3.36 and 3.90 (afforded by the H2/Pd-C reduction process discussed above). A species such as 3.93 thus bears a structural analogy to the phlorins of the porphyrin series. Unfortunately, structural information for this class of molecules, including 3.93, is currently lacking. 

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