Post‐Synthetic Modification of Hangman Porphyrins Synthesized on the Gram Scale

Abstract

We report a multi-gram scale synthesis of methyl 6-formyl-4-dibenzofurancarboxylate and its subsequent use in the gram scale synthesis of a dibenzofuran-functionalized hangman porphyrin containing a pendant carboxylic acid (HPD-CO2H). HPD-CO2H can be isolated as a free carboxylic acid in high purity with minimal purification. Post-synthetic modification of HPD-CO2H allows for the introduction of any desired pendant group in good yields, resulting in a practical amount of hangman porphyrin ligand with an easily customizable second coordination sphere. The cobalt complexes of these hangman porphyrins are shown to be active proton reduction electrocatalysts.

Carbon Dioxide Reduction by Iron Hangman Porphyrins

“…As the Fe(I/0) standard potentials of all catalysts are within 100 mV of one another, a comparative analysis of the effects of the hanging groups on CO 2 reduction catalysis is impartial to the electron-donating or -withdrawing effects of the different dibenzofuran groups, where changes in turnover frequency resulting from shifts of the catalyst standard couple are minimized. 33,39 Figure 2 shows the fits to eq 1 for each of the Fe hangman catalysts. The maximum turnover frequencies, TOF max , for each catalyst at each concentration of PhOH is provided from Computational Results.…”

“…The rudimentary porphyrin synthon was 5-(4-(d ibenzo furan-6-carboxylic acid))-10,15,20-tris(3,4,5trimethoxyphenyl)porphyrin (HPD−COOH). 33 The trimethoxy-phenyl substituent was chosen for the 10,15,20-positions of the porphyrin macrocycle for ease of synthesis. Subsequently, HPD− COOH was activated via tosylation followed by treatment with an appropriate arylamine nucleophile to produce free base porphyrins bearing arenesulfonic acid (HPD−3SA), phenolic (HPD−PhOH), and diboc-guanidyl (HPD−Gnd(Boc) 2 ) hanging groups which was deprotected using neat trifluoroacetic acid to yield the hanging guanidyl group (HPD−Gnd).…”

“…Fe metalation of hangman porphyrins was accomplished in moderate yields using microwave-assisted synthesis in acetonitrile with FeCl 2 and added triethylamine (NEt 3 ). 27,33 The isolation of all known compounds was confirmed by NMR spectroscopy and mass spectrometry and compared to published values. In the cases of newly synthesized HPD−PhOH and HPD−Gnd (Scheme 1) platforms, original 1 H and 13 C NMR spectra (Figures S1−S4 Electrochemistry.…”

“…The rudimentary porphyrin synthon was 5-(4-(dibenzofuran-6-carboxylic acid))-10,15,20-tris­(3,4,5-trimethoxyphenyl)­porphyrin (HPD–COOH) . The trimethoxyphenyl substituent was chosen for the 10,15,20-positions of the porphyrin macrocycle for ease of synthesis.…”

“…Subsequently, HPD–COOH was activated via tosylation followed by treatment with an appropriate arylamine nucleophile to produce free base porphyrins bearing arenesulfonic acid (HPD–3SA), phenolic (HPD–PhOH), and diboc-guanidyl (HPD–Gnd­(Boc) 2 ) hanging groups which was deprotected using neat trifluoroacetic acid to yield the hanging guanidyl group (HPD–Gnd). Fe metalation of hangman porphyrins was accomplished in moderate yields using microwave-assisted synthesis in acetonitrile with FeCl 2 and added triethylamine (NEt 3 ). , The isolation of all known compounds was confirmed by NMR spectroscopy and mass spectrometry and compared to published values. In the cases of newly synthesized HPD–PhOH and HPD–Gnd (Scheme ) platforms, original 1 H and 13 C NMR spectra (Figures S1–S4) were recorded for free base porphyrins and mass spectra (Figures S5–S8) and UV–vis spectra (Figures S9 and S10) were recorded for free base and metalated analogues.…”