Enantioselective Radical Reactions Using Chiral Catalysts

Abstract

Scheme 24 Enantioselective Radical Hydroacylation of Enals with α-Ketoacids byMerging Photoredox Catalysis with Amine Catalysis.Jang et al. exploited enantioselective tandem Michael addition/oxyamination of α,β-unsaturated aldehydes by combining asymmetric iminium catalysis using organocatalyst with photoinduced SOMO catalysis using N719/TiO2. 60 Here, TiO2 acts as a second cooperative photocatalyst along with Ru photocatalyst N719 which bound to the latter, increases the enantio-and diastereoselectivity of the process. Adamantane carboxylic acid (30 mol%) was used as an additive which promotes formation of iminium ion 2.1.48A from aldehyde 2.1.48 and chiral amine catalyst. This iminium intermediate then reacts with diethyl malonate to provide intermediate 2.1.48B which either undergoes hydrolysis to form β-substituted aldehyde 2.1.50 or photo-oxidation by the photoexcited Ru(II) dye to form enamine radical 2.1.48C. Subsequent addition of TEMPO to radical intermediate 2.1.48BC followed by hydrolysis afforded the desired product 2.1.49 (Scheme 25). This mechanism is obeying to a polar/radical crossover sequence where the first stereocontrol arises from the addition of the malonate.Recently, Bach group reported promising preliminary results of enantioselective [2+2] photocycloaddition of cinnamaldehyde in the presence of ruthenium photocatalyst and proline derived organocatalyst. 61

Visible-light mediated catalytic asymmetric radical deuteration at non-benzylic positions

“…Motivated by the growing interest in merging asymmetric organocatalysis with photocatalysis [25][26][27][28][29][30][31][32] and in view of the paucity of practical methods for asymmetric radical deuterations, we recently questioned whether a photocatalytic radical deuteration could be achieved in a highly enantioselective and cost-effective fashion. In particular, we hypothesized that a combination of chiral thiols with deuterium oxide (D 2 O) might be a potential solution, given the widespread use of achiral thiols as a catalyst for non-asymmetric radical deuteration 4,[33][34][35][36] and the encouraging stereocontrol that chiral thiol catalysts exerted in a handful of prior work [37][38][39][40] .…”

Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

“…Then, product 4 is formed via an intramolecular radical–radical coupling reaction, which is compatible with the experimental conditions described in this work. 11 …”

A Guide to Low‐Valent Titanocene Complexes as Tunable Single‐Electron Transfer Catalysts for Applications in Organic Chemistry

“…The use of SET catalysts that get and remain actively coordinated by a substrate is particularly intriguing. This approach opens the door to catalyst‐controlled SET and radical reactions, even enantioselective reactions, overcoming the substrate‐induced selectivity of traditional diffusion‐controlled processes [10–14] . In the design of such reactions, the adjustment of the electron‐transfer properties of the catalyst is of crucial importance for achieving a successful and selective transformation.…”