Aerobic oxidation catalysis is a green approach to organic substrate transformations. Molecular oxygen has high relative abundance, low cost and negligible environmental toxicity making it an ideal catalyst co-oxidant. We report successful aerobic oxidation of various organic substrates using a visible-light active tellurorhodamine chromophore 1 (9-mesityl-3,6-bis(dimethylamino)telluroxanthylium hexafluorophosphate) as both the sensitizer and oxidation catalyst. Catalytic oxidation of thiols to disulfide, silanes to silanols and phosphine to phosphine oxide was achieved with 5% 1 under ambient temperature and pressure. The proposed mechanism, supported experimentally and computationally, involves the formation of a tellurorhodamine telluroxide 2 from reaction with water and singlet oxygen generated by irradiation of 1. The oxidation to the 2 is accompanied by the formation of hydrogen peroxide. 2 reduces back to 1 upon oxidation of thiols, silanes or phosphines. Mechanistically, DFT suggests a substitution-like mechanism for the reaction between 1 and substrate to give a trigonal-bipyramidal Te(IV). This intermediate in each case undergoes concerted loss of the corresponding oxidized substrate to regenerate 1. Derivatives of 1 that contain electron withdrawing and electron donating groups on the mesityl ring have been synthesized and characterized to tune the electronic properties of the photocatalyst. Photophysical properties of each derivative are defined, including triplet quantum yields, and compared to 1. The reactivity of these derivatives towards oxidizing organic substrates are compared to 1 using the well characterized reaction with aromatic silanes.

Photocatalytic reactivity of tellurorhodamine chromophore derivatives towards aerobic oxidation of organic substrates

2019 Fall National Meeting

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