Exploiting New Chiral Cobalt Complexes For Asymmetric Photoreactions And Beyond

Free radicals are an important class of active intermediates in organic chemistry and are widely used in organic synthesis.Compared with the ionic reaction,the stereochemistry of the reactions is more difficult to control due to the higher reactivity of free radicals.Traditional methods require the use of chiral auxiliary groups and stoichiometric chiral catalysts to achieve asymmetric transformation of free radical intermediates,which atomic economy and stereoselectivity are not ideal.Last decade,important progress has been made in photoredox catalysis due to it's facile,efficient,green and good functional group compatibility.However,chiral catalysts and photosensitizers of precious metals such as ruthenium,rhodium and iridium are required for many types of conversion.In order to improve the economy and stereoselectivity of the asymmetric transformation of free radicals,a new type of octahedral chiral cobalt complex was designed and synthesized,which was successfully applied to the visible-light-induced free radical asymmetric transformations.Firstly,inspired by a class of chiral octahedral iron or manganese complexes widely used in asymmetric oxidation reactions.Starting from cheap chiral diamines,simple synthetic coupling and alkylation reactions were used to synthesize a series of new chiral N4 ligands then coordinated with cobalt salts to synthesize a series of chiral cobalt complexes.The three-dimensional cis-? octahedral coordination configurations of these complexes were confirmed by single crystal ray diffraction.Subsequently,as an efficient chiral Lewis acid catalyst this kind of novel chiral octahedral cobalt complex was successfully applied to the visible-light-induced asymmetric alkyl/aryl radical conjugate addition reactions.A series of chiral carbonyl and 1,4-dicarbonyl compounds were synthesized with 4-alkyl/acyl substituted Hantzsch ester derivatives as free radical precursors and ?,?-unsaturatedacyl-imidazole as Michael receptors under visible-light-induced chiral cobalt catalysis(up to>99%and 97:3 e.r.).This chiral cobalt catalysis system can be successfully extented to the visible-light-induced HAT(hydrogen atom transfer)reaction,realizing the C-H bond directly asymmetric transformations,which has good functional groups compatibility with substituted toluene derivatives,cycloalkanes and aldehydes.Simultaneously,in order to further explore the application of the new chiral cobalt catalyst,we successfully used this chiral cobalt catalyst to realize the visible-light-induced synthesis of chiral y-amino acid derivatives and the construction of chiral quaternary carbon center using easily available?-silylamines as ?-amino radical precursor and ?-substituted-?,?-unsaturated acyl-imidazole.Mechanism investigations reveal that it is not a conventional conjugate addition pathway but a free radical recombination process.Notably,this facile protocol tolerates a broad substrates scope affording the corresponding chiral products in generally good yields and high enantioselectivities.Efficient construction of chiral olefin compounds by Wittig reaction has always been a difficult problem in organic synthesis.The key point is how to efficiently construct chiral phosphorus ylide reagents.Here,we show that highly functionalized chiral P-ylides can be easily synthesized through a copper-catalyzed asymmetric propargylic alkylation reaction from phosphonium salts and racemic propargylic esters.The subsequent Wittig reactions enable the synthesis of versatile alkene building blocks,chiral ?-propargylic acrylates,and ?-propargylic allenoates,with a wide substrate scope and satisfactory functional group compatibility(up to 98%yield and>99%ee).