Enantioselective Catalysis of Photochemical Reactions

Abstract

The nature of the excited state renders the development of chiral catalysts for enantioselective photochemical reactions a considerable challenge. The absorption of a 400 nm photon corresponds to an energy uptake of approximately 300 kJ mol(-1) . Given the large distance to the ground state, innovative concepts are required to open reaction pathways that selectively lead to a single enantiomer of the desired product. This Review outlines the two major concepts of homogenously catalyzed enantioselective processes. The first part deals with chiral photocatalysts, which intervene in the photochemical key step and induce an asymmetric induction in this step. In the second part, reactions are presented in which the photochemical excitation is mediated by an achiral photocatalyst and the transfer of chirality is ensured by a second chiral catalyst (dual catalysis).

Electron donor-free photoredox catalysis via an electron transfer cascade by cooperative organic photocatalysts

“…Recent research activities have demonstrated a vast number of structural design methods of small molecular 8,10,[16][17][18][19][20] or macromolecular [21][22][23][24][25][26][27][28][29][30] OS systems as efficient photocatalysts for visible light-promoted photoredox reactions. Nevertheless, most developed OS photocatalysts, similar to traditional transition metal complexes, [31][32][33][34][35][36][37][38] are single photocatalytic systems. As illustrated in Fig.…”

Exploiting Octahedral Stereocenters: From Enzyme Inhibition to Asymmetric Photoredox Catalysis

“…[41] We were particularly inspired by MacMillans seminal work on merging photoredox catalysis with asymmetric catalysis. [42] Specifically, in 2008, Nicewicz and MacMillan demonstrated that the enantioselective a-alkylation of aldehydes can be accomplished by employing a combination of a chiral secondary amine for enamine catalysis and a photoredox catalyst for photoinduced electron transfer. [43] We wondered whether our chiral bis-cyclometalated iridium Lewis acids could be uniquely suited to merge visible-light-induced photoredox chemistry and asymmetric catalysis with just a single catalyst.…”

Brønsted‐Säure‐Katalyse der [2+2]‐Photocycloaddition von Enondithianen bei Bestrahlung mit sichtbarem Licht

“…[5,7,8] Aufd er Suche nach Substraten, die keine nennenswerte langwellige Absorption aufweisen, die aber nach Aktivierung mit einer Säure eine starke bathochrome Verschiebung zeigen, haben wir Enone getestet, deren Carbonylgruppe in ein 1,3-Dithian überführt wurde (Enondithiane). Das mutmaßlich durch Protonierung entstehende Thioniumion sollte im sichtbaren Bereich anregbar sein (l abs > 380 nm) [9] und damit eventuell eine Brønsted-Säure-katalysierte [2+ +2]-Photocycloaddition eingehen.…”