Phototransposition of Indazoles to Benzimidazoles: Tautomer-Dependent Reactivity, Wavelength Dependence, and Continuous Flow Studies

A detailed investigation of the photomediated transformation of indazoles to benzimidazoles through a nitrogen–carbon transposition is herein reported. This phototransposition has been observed to occur in low yield when 1H-indazoles are subjected to high-energy UVC irradiation. It was found that the 2H-tautomer of indazole absorbs light more strongly than the 1H-tautomer at longer wavelengths. This improved absorbance profile was leveraged to develop a general system for the high-yielding conversion of N₂-derivatized indazoles (prepared from the corresponding 1H-indazoles) to the corresponding benzimidazoles under UVB or UVA irradiation, with yields of up to 98% being achieved. The substrate scope was investigated, revealing a strong correlation between reaction yield and electron density at N₂ of the indazole substrate, suggesting that the availability of the lone pair at this position plays an important role in reaction efficiency. In addition, wavelength-dependent reactivity was evaluated through the generation of a photochemical action plot, and it was revealed that the highest conversion is achieved not only at the substrate’s maximum absorbance wavelength but also on its red-side, thereby enabling high yields under longer wavelength irradiation. Building on these insights, a continuous flow protocol was established, enabling the phototransposition to be performed on a preparative scale.

• Bartholomew, GL
• Kim, SF
• Oyamada, Y
• Sbordone, F
• Carroll, JA
• Jurczyk, JE
• Yeung, CS
• Barner-Kowollik, C
• Sarpong, R

• UC Berkeley: University of California Berkeley, Chemistry, UNITED STATES OF AMERICA

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