A Unified Picture of Radical Anion Photoredox Chemistry

Abstract

Radical anions are competent reagents for supporting photoredox transformations of exceptionally strong chemical bonds. However, the excited states of radical anions are extremely short-lived, making them impractical for directly accomplishing photochemical transformations with meaningful quantum yields. Herein, we examine the radical anion of 9,10-dicyanoanthracene (DCA •– ), which has previously been reported to activate aryl chloride substrates. We show that 10-cyanoanthrolate (10-CA), the product of the reaction of DCA •– with oxygen, is a competent photocatalyst for reductive transformations of select aryl chlorides but not electron-rich aryl chlorides, suggesting another mode of photoreactivity. We show that DCA •– yields highly reducing solvated electrons via photodetachment when excited with blue light. Near-infrared femtosecond transient absorption spectroscopy measurements show that spectral features assigned to solvated electrons are quenched by electron-rich aryl chlorides that cannot be reduced by 10-CA. Moreover, we demonstrate the generality of solvated electron generation using other previously reported photoactive radicals, such as naphthalene monoimide radical anion and a 9-mesityl-3,6-di- tert -butyl-10-phenylacridinium radical. Taken together, we now present a unified picture of radical anion photoredox chemistry in which the radical anion is susceptible to react with electrophiles by an ECE (electron-chemical-electron) process to furnish a closed shell super-reducing photoreagent. Alternatively, radical anions are sufficiently reduced that a solvated electron may be produced by charge transfer to solvent (CTTS) under sufficiently energetic excitation. Both pathways result in super-reducing reagents that can activate exceptionally strong chemical bonds.

Topics & Concepts