Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes

Abstract

Photoredox catalysis and organocatalysis represent two powerful fields of molecule activation that have found widespread application in the areas of inorganic and organic chemistry, respectively. We merged these two catalysis fields to solve problems in asymmetric chemical synthesis. Specifically, the enantioselective intermolecular α-alkylation of aldehydes has been accomplished using an interwoven activation pathway that combines both the photoredox catalyst Ru(bpy)3Cl2 (where bpy is 2,2′-bipyridine) and an imidazolidinone organocatalyst. This broadly applicable, yet previously elusive, alkylation reaction is now highly enantioselective and operationally trivial.

Eosin Y Catalyzed Visible-Light-Promoted Aerobic Oxidative Cyclization of 2-Aminobenzothiazole

“…In this sequence visible light photoredox catalysis has recently received much attention in organic synthesis owing to readily availability, sustainability, non-toxicity and ease of handling of visible light. [10][11][12][13][14][15] In their revolutionary work in this area, MacMillan, [16] Yoon [17] and Stephenson [18][19] have used Ruthenium and Iridium complexes as the photoredox catalyst, which has inspired the development of several powerful methods for various chemical transformations useful in organic synthesis.…”

Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

“…[1][2][3][4][5][6][7][8][9] Ru-and Ir-based coordination complexes have received enormous attention because of their excellent visible-light-harvesting properties, modest to extremely high oxidation and reduction potentials, relatively long excited-state lifetimes, and reasonably good chemical and photostabilities under synthetic oxidative and reductive conditions. [2] In addition, considerable efforts have been made to develop metal-free photoredox catalytic methods with organic dyes [8,[10][11][12][13][14] such as eosin Y [10,11] or rhodamine derivatives [13,14] and organic heterogeneous photocatalysts [15] for synthetic transformations.…”

The new breed of cutting-edge catalysts

“…When photoredox catalysts absorb light, an electron leaps from the metal ion to the ligand and becomes stuck there, leaving the molecule in an unstable state. "The catalyst becomes desperate to fill the hole in the metal and get rid of the electron in the ligand, " explains David MacMillan, a chemist at Princeton University in New Jersey who first reported 6 the idea in collaboration with chemist David Nicewicz from the University of North Carolina at Chapel Hill. But the only way the photoredox system can accomplish this is to trade electrons with the standard catalyst, supercharging it and triggering chemical transformations that were previously impossible.…”