Pictet‐Spengler Synthesis of Fused‐Azepine‐ and 1 H ‐Azocine‐Bridged Ni II Porphyrin Dimers

Overview

This study describes the synthesis of fused-azepine- and 1H-azocine-bridged nickel(II) porphyrin dimers via Pictet-Spengler cyclization. Starting materials comprise meso-amino and meso,meso'-diamino β-β'-linked NiII porphyrin dimers, which undergo cyclization with paraformaldehyde to generate intermediate azepine-bridged dimers. Subsequent oxidative transformations produce meso-tert-butylphenyl-fused derivatives with varying regioselectivity depending on the precursor structure. The synthetic route establishes access to extended conjugated porphyrin architectures with potential for spectroscopic characterization in the near-infrared region.

Methods and approach

Pictet-Spengler cyclization was performed on meso-amino and meso,meso'-diamino β-β'-linked NiII porphyrin dimers using paraformaldehyde as the one-carbon source. Oxidative transformations employed FeCl3 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to generate meso-tert-butylphenyl-fused species. Demetalation and zinc(II) complexation were performed on selected azepine-bridged intermediates. Spectroscopic analysis included absorption and fluorescence measurements to characterize optical properties, particularly in the near-infrared region.

Key Findings

Pictet-Spengler cyclization of the meso-amino and meso,meso'-diamino β-β'-linked dimers 1Ni and 2Ni with paraformaldehyde yielded azepine-bridged NiII porphyrin dimers 3Ni and 4Ni respectively. Oxidative treatment of 3Ni with FeCl3 and DDQ provided meso-tert-butylphenyl-fused dimer 5Ni in 43% yield, whereas oxidation of 4Ni furnished the singly meso-tert-butylphenyl-fused dimer 6Ni in 94% yield, demonstrating differential regioselectivity. Pictet-Spengler cyclization of meso-amino β-meso' linked dimer 7Ni produced fused 1H-azocine-bridged dimer 8Ni. Conversion of 3Ni and 4Ni to free bases and ZnII complexes revealed red-shifted absorption bands and fluorescence emission in the near-infrared spectral region.

Implications

The development of these fused-azepine- and azocine-bridged porphyrin dimers extends the scope of accessible porphyrin architectures through sequential cyclization and oxidative strategies. The variable regioselectivity observed during oxidation of the two isomeric precursors (94% yield versus 43% yield) indicates that the positional relationship of the amino substituents on the porphyrin scaffold governs the efficiency of meso-aryl group incorporation, with potential applications in controlling synthetic selectivity for similar transformations.