Activation of CH Bonds through Oxidant‐Free Photoredox Catalysis: Cross‐Coupling Hydrogen‐Evolution Transformation of Isochromans and β‐Keto Esters

Abstract

The direct and controlled activation of a C(sp(3) )H bond adjacent to an O atom is of particular synthetic value for the conventional derivatization of ethers or alcohols. In general, stoichiometric amounts of an oxidant are required to remove an electron and a hydrogen atom of the ether for subsequent transformations. Herein, we demonstrate that the activation of a CH bond next to an O atom could be achieved under oxidant-free conditions through photoredox-neutral catalysis. By using a commercial dyad photosensitizer (Acr(+) -Mes ClO4 (-) , 9-mesityl-10-methylacridinium perchlorate) and an easily available cobaloxime complex (Co(dmgBF2 )2 ⋅2 MeCN, dmg=dimethylglyoxime), the nucleophilic addition of β-keto esters to oxonium species, which is rarely observed in photocatalysis, leads to the corresponding coupling products and H2 in moderate to good yields under visible-light irradiation. Mechanistic studies suggest that both isochroman and the cobaloxime complex quench the electron-transfer state of this dyad photosensitizer and that benzylic CH bond cleavage is probably the rate-determining step of this cross-coupling hydrogen-evolution transformation.

Photoredox Catalysis in Organic Chemistry

“…This strategy has also been applied to the C–H functionalization of N -arylamines and isochromans with β-keto esters. 139 While this catalysis mode is currently limited to substrates which can be oxidized by the photocatalyst, the extremely mild conditions under which acceptorless dehydrogenation can be achieved is notable.…”

Dual Catalysis Strategies in Photochemical Synthesis

“…Wu has exploited a similar design strategy for the addition of β-keto esters to photocatalytically generated oxocarbenium ions. 310 …”

Progress on the Photocatalytic Organic Hydrogen-Evolution Coupling/Aromatization Reaction

“…化剂的双催化体系, 实现了含富电子芳环结构的醚的 α-sp 3 -碳官能化, 偶联产物最高收率达到 89% (Scheme 9) [38] . 机理研究表明, 该反应中醚底物中的芳烃被激发 的光催化剂氧化后, 可以形成关键的苄基阳离子自由 基.…”