Role of Steric Hindrance in the Newman-Kwart Rearrangement and in the Synthesis and Photophysical Properties of Arylsulfanyl Tetrapyrazinoporphyrazines

Conditions for the Newman-Kwart rearrangement
of phenols into thiophenols
were investigated in relation to the bulkiness of substituents at
the 2 and 6 positions of the starting phenol derivative with an emphasis
on eliminating side reactions. Thiophenols with different 2,6-disubstitution
patterns (including hydrogen, methyl, isopropyl or tert-butyl groups) were used for the synthesis of 5,6-bis­(arylsulfanyl)­pyrazine-2,3-dicarbonitriles
that underwent cyclotetramerization leading to the corresponding zinc
tetrapyrazinoporphyrazines (TPyzPz), aza-analogues of phthalocyanines.
Several methods for the cyclotetramerization were attempted to eliminate
problematic side reactions. Magnesium butoxide was found as the most
suitable cyclotetramerization agent and afforded TPyzPzs in reasonable
yields of approximately 30% under mild conditions. The varying arrangements
of the peripheral substitutions resulting from the different bulkiness
of the substituents were demonstrated by the X-ray structures of the
pyrazine-2,3-dicarbonitriles. The prepared zinc arylsulfanyl TPyzPzs
showed an absorption maximum at a Q-band over 650 nm, fluorescence
quantum yields between 0.078 and 0.20, and singlet oxygen quantum
yields ranging 0.58–0.69. TPyzPzs with isopropyl groups were
found to be the best derivatives in this series as they combined facile
cyclotetramerization, no aggregation, and good photophysical properties,
which makes them potentially suitable for photodynamic therapy.