Porphycene bridged

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. 

As in the porphycene series one method to obtain an isocorroie is based on the McMurry coupling of a tetrapyrroledicarbaldehyde, e.g. 1.2 4 The reductive cyclization forms a C —C double bond bridge leading directly to the isocorroie, e.g. 2, which is at the correct oxidation level. 

Porphycenes 51 bearing substituents at ethylenic carbons have been prepared from 5,5 -acyl bipyrroles 53 (05OL1887) (Scheme 26). However, oxidation of the intermediate annulene 54 is difficult, owing to the presence of the alkyl chains on the alkene bridges. These reduce conformational flexibility, resulting in a near-nonplanar geometry, which resists oxidation and/or aromatization (08CSR215). 

The general synthetic approaches for expanded porphycenes rely upon the insertion of alkylidene or alkylidyne moieties either as the meso-bridge or between the 2 and 2 positions of the bipyrrole subunits. Thus, the vinylogous dialdehyde 72 (Scheme 35) obtained from 4,4 -dipropyl-2,2 -bipyrrole with 3-(dimethyl-l-amino)acrolein has been converted to (1993PAC143) to 73 using a standard McMurry-type coupling. Expanded porphycene 73 with TFA precipitates as dication 74. 

An interesting feature of system 3.123 is the finding that the two porphycene rings are well separated. As a result, there is no apparent barrier to rotation about the piperazine bridge. Further, the visible absorption spectrum of 3.123 ( max = 380 nm and 618 nm in CH2CI2) is rather unperturbed relative to the monomer 3.119 and displays no evidence of appreciable exitonic coupling between the rings." ... 

The A,7V -bridged porphycenes 3.119-3.121 were found to exhibit little tendency to act as viable metal-chelating agents." By contrast, the mono-A-methylpor-phycene 3.110 has been demonstrated to be capable of forming stable metal complexes." " For instance, treatment of A(l)-methylporphycene 3,110 with a metal... 

Porphyrin has fourfold symmetry, with the pyrrole rings connected by alternate meso bridges. About eight isomers can be sketched down by changing the sequence of the subunits, of which porphycene (37), hemiporphycene, corrphycene, and isoporphycene have been synthesized to date. Expanded porphyrins are, by definition, those possessing more than 16 atoms in the smallest circuit of the macrocycle. Sapphyrin (39) is the common expanded porphyrin derived by the insertion of an additional pyrrole ring into the macrocycle [40]. 

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