Di- and Trifluorinated 2-Azidobenzimidazole Derivatives: Synthesis, Photooxygenation, and 19F NMR Prediction

Abstract

The photoreactivity of a series of hitherto unknown, multiply fluorinated 2-azidobenzimidazole derivatives was investigated. The synthesis of the starting material includes regioselective p-defluorination of nitrobenzene derivatives employing Ogoshi's conditions. If the 6-position was unsubstituted, irradiation in the presence of N-protected amino acids at 300 nm (Rayonet) led to the formation of arylesters by oxygenation of the 6-position in good to excellent yields and perfect regioselectivity. We did not observe any displacement of fluoride. If the 6-position itself was fluorinated, alternative positions of the benzene portion were attacked. Mechanistically, the reaction proceeds through ring opening of the singlet nitrene to the cyanodiimine or via the iminobenzimidazolium ion. The availability of a set of fluorinated photo-adducts prompted the quantum chemical calculation of their 19F NMR chemical shifts. Even with the most suitable method investigated (ωB97XD/TApr-cc-pVDZ), deviations of up to 5 ppm from the experimental values were observed, underlining the importance of experimental measurements.