Facile and Efficient SynthesisofFluorene and Indenoarene Carboxylates from Biaryldiazoacetates via BlueLight‐promoted Intramolecular CarbeneC−H Insertion

Abstract

AbstractInspired by the recent discovery that aryldiazoacetates could be excited with visible light to generate carbenes in a mild and catalyst‐free manner, we tested the possibility to induce the cyclization of fluorenes from biaryldiazoacetate precursors simply by visible light irradiation at ambient temperature. The experimental results showed that 455 nm blue light is most effective on the synthesis of a diversity of fluorenes and indenoarenes in excellent yields. Analysis of UV absorbance and electronic transition properties of biaryldiazoacetates revealed that their absorbance peaks in the blue light region are due to the n–π* transition of diazo group. Their small ϵ420 nm values well explains the fact that only when exposed to intense blue light could biaryldiazoaetates be excited to generate carbenes. Theoretical calculations and kinetic isotopic effect (KIE) study illustrated that the fluorene cyclization proceeds via the intramolecular carbene C−H insertion pathway rather than the cyclopropanation pathway. In addition, a furan ring‐opening mechanism for the unexpected formation of an indene‐based unsaturated aldehyde was also proposed based on the calculated potential energy surfaces of three plausible reaction pathways.