Light-triggered elimination of CO2 and absorption of O2 (artificial breathing reaction) in photolysis of 2-(4-nitrophenyl)-1H-indole derivatives

A new chromophore, 2-(4-nitrophenyl)-1H-indole (NPI), was synthesized as a potential photolabile protecting group. Caged benzoic acids featuring the NPI chromophore were synthesized as model compounds. Benzoic acid was released in moderate yields (similar to 40-60%) upon photolysis of the caged benzoic acids without any additional chemical reagents. Interestingly, an aldehyde, 1-(5-(1-formyl-1H-indo1-2-yl)-2-nitrophenyl)ethyl benzoate, was isolated in approximate to 20% together with benzoic acid 40%) in photolysis of a caged benzoic acid, 2-(2-(3-(1-(benzoyloxy)ethyl)-4-nitrophenyl)-1H-indol-1-yl)acetic acid. The functional group, CH2COOH, at the indole nitrogen was transformed into the aldehyde group, CHO, under photolysis conditions in air. The similar photochemical transformation was observed in the photolysis of 2-(2-(4-nitrophenyl)-1H-indol-1-yl)acetic acid, in which the benzoate group is not attached at the nitrophenyl ring. Products analysis, transient absorption spectroscopy, and computational study suggested that intramolecular electron transfer is key for the elimination of CO2 and absorption of O-2 for the formation of the aldehyde. The artificial breathing-type reaction can apply to transition metal-free oxidation of amino acids under mild conditions. [GRAPHICS] .

Automated summary

A new chromophore, NPI, was synthesized as a potential photolabile protecting group, and caged benzoic acids featuring this chromophore were found to release benzoic acid in moderate yields upon photolysis, along with the formation of aldehyde products. Under photolysis conditions, the functional group at the indole nitrogen was transformed into an aldehyde group. Intramolecular electron transfer was determined to be crucial for the formation of the aldehyde, demonstrating potential applications in transition metal-free oxidation of amino acids.