Peripheral Hexabromination,Hexaphenylation, and Hexaethynylation of meso‐Aryl‐Substituted Subporphyrins

Effective peripheral fabrication methods of meso‐aryl‐substituted subporphyrins were explored for the first time. Hexabrominated subporphyrins 2 were prepared quantitatively from the bromination of subporphyrins 1 with bromine. Hexaphenylated subporphyrins 3 and hexaethynylated subporphyrins 4 and 5 were synthesized by Suzuki–Miyaura coupling and Stille coupling, respectively, in good yields. X‐ray crystal structures of 2 b , 3 b , 4 b , and 5 a revealed preservation of the bowl‐shaped bent structures with bowl depths similar to that of 1 . Hexaethynylated subporphyrins exhibit large two‐photon‐absorption cross‐sections due to effective delocalization of the conjugated network to the ethynyl substituents.     

Singly and Doubly Quinoxaline-Fused BIII Subporphyrins

B-Phenyl B^III subporphyrin-α-diones prepared in a three-step reaction sequence from the parent subporphyrin were condensed with 1,2-diaminobenzenes to give the corresponding quinoxaline-fused subporphyrins in variable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl B^III subporphyrin was transformed to the corresponding subporphyrin-α-dione in the same three-step reaction sequence, which was then condensed with 1,2-diaminobenzene to give doubly quinoxaline-fused subporphyrin. These quinoxaline-fused subporphyrins exhibit redshifted absorption and fluorescence spectra compared with the parent one. A singly quinoxaline-fused subporphyrin bearing three meso-bis(4-dimethylaminophenyl)aminophenyl substituents shows blueshifted fluorescence in less polar solvent, which has been ascribed to emission associated with charge recombination of intramolecular charge transfer (CT) state.     

Synthesis of Peripherally Nitrated,Aminated, and Arylaminated Subporphyrins

2‐Nitro‐5,10,15‐tri(4‐tert‐butylphenyl)subporphyrin 2 was prepared by the nitration of 5,10,15‐tri(4‐tert‐ butylphenyl)subporphyrin 1a with five equivalents of Cu(NO₃)₂ ? 5 H₂O in a mixed EtOAc/Ac₂O solution and was reduced into 2‐amino‐5,10,15‐tri(4‐tert‐butylphenyl)subporphyrin 3 . Bromination of 5,10,15‐triphenylsubporphyrin 1b with 1.5 equivalents of N‐bromosuccinimide (NBS) gave 2‐bromo‐5,10,15‐triphenylsubporphyrin, which was converted into various 2‐arylamino‐5,10,15‐triphenylsubporphyrins ( 4a , 4b , 4c , 4d ) and 2‐benzamido‐5,10,15‐triphenylsubporphyrin 5 through Pd‐catalyzed cross‐coupling reactions. These molecules constitute the first examples of mono‐β‐substituted subporphyrins. These subporphyrins exhibit significantly perturbed optical and electrochemical properties, which reflect a large influence of the peripherally attached substituents on the electronic networks of subporphyrins.     

Stable Subporphyrin meso-Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

Most aminyl radicals studied so far are resonance-stabilized by neighboring heteroatoms, and those without such stabilization are usually short-lived. We report herein that subporphyrin meso-2,4,6-trichlorophenylaminyl radicals and a bis(5-subporphyrinyl)aminyl radical are fairly stable under ambient conditions without such stabilization. The subporphyrin meso-2,4,6-trichlorophenylaminyl radical crystal structure displays a characteristically short C_meso−N bond and a perpendicular arrangement of the meso-arylamino group. The stabilities of these radicals have been ascribed to extensive spin delocalization over the subporphyrin π-electronic network as well as steric protection around the aminyl radical center.     

Nucleophilic Aromatic Substitution Reactions of meso‐Bromosubporphyrin: Synthesis of a Thiopyrane‐Fused Subporphyrin

meso‐Bromosubporphyrin undergoes nucleophilic aromatic substitution (S_NAr) reactions with arylamines, diarylamines, phenols, ethanol, thiophenols, and n‐butanethiol in the presence of suitable bases to provide the corresponding substitution products. The S_NAr reactions also proceed well with pyrrole, indole, and carbazole to provide substitution products in moderate to good yields. Finally, the S_NAr reaction with 2‐bromothiophenol and subsequent intramolecular peripheral arylation reaction affords a thiopyrane‐fused subporphyrin.