Ketone–Olefin Coupling of Aliphatic and Aromatic Carbonyls Catalyzed by Excited-State Acridine Radicals

Abstract

Ketone–olefin coupling reactions are common methods for the formation of carbon–carbon bonds. This reaction class typically requires stoichiometric or super stoichiometric quantities of metal reductants, and catalytic variations are limited in application. Photoredox catalysis has offered an alternative method toward ketone–olefin coupling reactions, although most methods are limited in scope to easily reducible aromatic carbonyl compounds. Herein, we describe a mild, metal-free ketone–olefin coupling reaction using an excited-state acridine radical super reductant as a photoredox catalyst. We demonstrate both intramolecular and intermolecular ketone–olefin couplings of aliphatic and aromatic ketones and aldehydes. Mechanistic evidence is also presented supporting an “olefin first” ketone–olefin coupling mechanism.

Chromium/Photoredox Dual Catalyzed Synthesis of α‐Benzylic Alcohols, Isochromanones, 1,2‐Oxy Alcohols and 1,2‐Thio Alcohols

“…The ESI‐HRMS detected TEMPO adducts with both 2 aa and 2 ba corroborate with the former statement (Figure 7D). Entry 4 g (Figure 4) provides an authentication for a radical clock experiment, with no open chain products obtained [7, 40] …”

“…Entry 4 g (Figure 4) provides an authentication for a radical clock experiment, with no open chain products obtained. [7,40] With this information in hand, a plausible reaction mechanism was outlined for the benzylation process (Figure 8). Once 2 a gets oxidized by the excited photocatalyst PC1, benzylic radical 2 a' is formed which is stabilized by the delocalization into the phenyl ring.…”

Enantioselective diversification of alkene radical anions

“…For instance, the Yoon group revealed that Ru(bpy)3Cl2 serves as an efficient photocatalyst for generating radical anions from activated alkene-aryl enones under visible light irradiation, and developed efficient intra-and intermolecular [2+2] enone cycloadditions (25,26). Quite recently, Polyzos (27) and Nicewicz (28) reported breakthrough discoveries involving the application of photoexcited reduced species of [Ir(ppy)2(dtb-bpy)]PF6 and acridinum salt as super SET donors towards the reduction of styrenes and conjugated alkenes to the corresponding radical anions upon multiphoton excitation using stoichiometric N,N-diisopropylethylamine (DIPEA) as a sacrificial electron donor. The resultant nucleophilic radical anions can then be added to ketones in an intra-or intermolecular manner, followed by further SET-reduction and protonation to yield reductive hydrofunctionalization products.…”

Chromium-Catalyzed Defluorinative Reductive Coupling of Aldehydes with gem-Difluoroalkenes

“…9 In 2019, the Jamison group reported intermolecular ketyl−olefin coupling by using p-terphenyl as the photoredox catalyst. 10 Related photocatalytic ketyl−olefin coupling processes have also been reported by Rueping, 11 Glorius, 12 Doyle, 13 Nicewicz, 14 Yoon, 15 and others. 16 The electrochemical generation of ketyl radicals has also been demonstrated as an alternative to stochiometric reductants for the generation of ketyl radicals.…”