Chiral Photocatalyst Structures in Asymmetric Photochemical Synthesis

Abstract

Asymmetric catalysis is a major theme of research in contemporary synthetic organic chemistry. The discovery of general strategies for highly enantioselective photochemical reactions, however, has been a relatively recent development, and the variety of photoreactions that can be conducted in a stereocontrolled manner is consequently somewhat limited. Asymmetric photocatalysis is complicated by the short lifetimes and high reactivities characteristic of photogenerated reactive intermediates; the design of catalyst architectures that can provide effective enantiodifferentiating environments for these intermediates while minimizing the participation of uncontrolled racemic background processes has proven to be a key challenge for progress in this field. This review provides a summary of the chiral catalyst structures that have been studied for solution-phase asymmetric photochemistry, including chiral organic sensitizers, inorganic chromophores, and soluble macromolecules. While some of these photocatalysts are derived from privileged catalyst structures that are effective for both ground-state and photochemical transformations, others are structural designs unique to photocatalysis and offer insight into the logic required for highly effective stereocontrolled photocatalysis.

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] .…”

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

“…The major isomer 28 was obtained in high enantioselectivity. It has been suggested that a chromenylium structure is formed (30). This involves excited state intramolecular proton transfer (ESIPT) [80][81][82].…”

“…In fact, the dispersion of this energy can be controlled [21] and selective, also stereoselective or stereospecific reactions are observed. In the literature numerous of such reactions are reported [22][23][24][25][26][27][28][29][30]. Often supramolecular structures in the broadest sense are frequently involved [31][32][33][34][35].…”

Controlling the optical and catalytic properties of artificial metalloenzyme photocatalysts using chemogenetic engineering

“…10 Given that many photocatalysts are only involved in the initial electron or energy transfer to/from a substrate and that the resulting high energy intermediates can undergo rapid subsequent reactions, the substrate must either be in a chiral environment during the excitation step and remain there during the selectivity-determining step or later engage with a chiral cocatalyst during the selectivity-determining step. 11 Reactions proceeding via electron transfer can also proceed via radical chain mechanisms that do not involve the catalyst and compete with the desired closed catalytic cycle to erode catalyst controlled selectivity. 12,13 Many enzyme-catalyzed reactions, 14,15 including recently reported examples of non-native photocatalytic reactions, 16 are initiated via SET, and highly efficient ET occurs in photosystems I and II.…”