Visible-light-induced Asymmetric Catalysis with Chiral-at-rhodium Lewis Acids

Author:Yuan Wei

Supervisor:eric meggersgong lei


Degree Year:2017





Interfacing visible light activated photochemistry with asymmetric catalysis therefore provides new opportunities for the efficient and green synthesis of non-racemic chiral molecules.However,the high reactivity of photoexcited substrates,intermediate radical ions or radicals,and the low activation barriers for follow-up reactions provide significant hurdles for the development of efficient transformations that under sterochemical control.Here we show that a chiral-at-rhodium complex can serve as a sensitizer for photoredox catalysis and at the same time provide very effective asymmetric induction for the enantioselective catalysis.The metal centre of this novel asymmetric photoredox catalyst could simultaneously serves as the exclusive source of chirality,catalytically active Lewis acid centre,and photoredox centre.In Chapter 1,progress in visible-light-induced asymmetric catalysis over the past decade is briefly summarized.In Chapter 2,synthetic methods of the chiral-at-rhodium/irdium Lewis acid are discribed first.Then,we demonstrate how the chiral-at-metal rhodium complex was applied to the aerobic catalytic asymmetric dehydrogenative cross-coupling between two C(sp3)-H groups.Mechanistic studies confirm that the chiral-at-metal rhodium complex is capable of forming the visible-light-active rhodium-ketone intermneidate in addition to control the stereochemistry during the catalytic process.The light activated rhodium-ketone intermeidate could serve as a week photo-oxidant and induce a single electron oxidation of tertiary amine.This work represents the first example of photoredox reaction catalyzed by rhodium complex.In Chapter 3,we demonstrate that the chiral-at-rhodium Lewis acid could efficiently catalyze the enantioselective cross-coupling of ketones with aryl silyl enolates under photo-induced aerobic oxidation conditions.A variety of 1,4-dicarbonyl compounds with potential biological activities could be obtained.In Chapter 4,the visible-light-induced stereo-selective alkylation of remote,unactivated C(sp3)-H bonds is developed by using the chiral-at-rhodium Lewis acid and photosentisizer as the dual catalysis system.The process involves visible-light-induced proton-coupled electron transfer(PCET),1,5-hydrogen atom transfer(1,5-HAT),and chiral Lewis acid catalysis.A carbon radical-radical coupling mechanism has been proposed.As a potentially useful approach for selective function-alization of inert C(sp3)-H bonds,the method further demonstrates that the chiral-at-rhodium Lewis acid is well compatible with other catalysts and additives in the complicated system.The works in this thesis is supposed to provide a new insight in design of novel visible-light-induced asymmetric catalysts and development of highly efficient photoactivated asymmetric catalysis.