Photoredox-Catalysed Cyclopropanation And 1,2-Elimination Reactions Via "Radical-Anion" Crossover Process

"Radical-Polar" crossover reaction has the advantages of radical reaction and ionic reaction,allowing radical to combine with carbocation or carbanion.Because of its high reactivity and good functional group compatibility,it has attracted the attention of chemists.In this paper,the cyclopropanation of alkenyl silanes and the preparation of alkenyl ethers from unsaturated acetals were investigated through photocatalytic reductive "radical-polar" crossover reactions.It mainly introduces the following two parts:The first part investigates the cyclopropanation and Giese addition of alkenylsilanes under photocatalysis.In organic synthesis,alkenyl silane is an important compounds,and the cyclopropane reaction of alkenyl silane has not been involved yet.Using alkenyl silane as radical acceptor,alkyl silicates or 4-alkyl-1,4-dihydropyridines as radical precursor,a variety of cyclopropane and Giese-type reactions products were synthesized via photoredox-catalysed "radical-polar" crossover reaction.By examining the substrates with steric hindrance,functional group compatibility and various electrical substituents,the target products were obtained in good yields,reflecting the good universality of the reaction substrates.By deuterium studies,it is confirmed that there is a reaction process of radical addition and ion termination in the reaction.In terms of applications,the derivatization experiments of desiliconization and cyclopropane organosilanes provide synthetic methods for natural products and drugs containing cyclopropyl groups.The second part is the research on the preparation of alkenyl ethers using α,β-unsaturated ketals and acetals as radical acceptors via the photocatalytic "radical-polar" crossover reaction.Functionalized vinyl ethers are a class of important molecules possessing significant biological activities and serve as important synthons in numerous reactions such as polymerization,cycloaddtion,and cross aldol reactions.Surprisingly,up to now,the investigation of radical reactions using α,β-unsaturated ketals and acetals as the radical acceptors is still untouched.A variety of alkenyl ethers were synthesized from α-substituted alkenyl acetals using alkyl silanes as radical precursors via photoredox-catalysed radical reactions.The substrate has a good scope,and the target enol ether can be obtained in good yield for substrates such as steric hindrance,electron-donating,electron-withdrawing,and heterocycle.The reaction mechanism was verified by the synthesis of cyclopropane with chloromethyl silicate as the methylene source and cyclic acetal.This protocol involving radical addition,SET reduction,1,2-elimination process.This mild reaction condition provides a new method for the synthesis of alkenyl ethers.