Study On The Free Radical Activation Reaction Mode Based On Trivalent Phosphin

Tertiary phosphines have been widely used as efficient catalysts in organocatalytic reactions,promoting various transformations in a racemic and enantioselective manner.However,in modern organic synthesis,tertiary phosphines are relatively rarely used in the reaction of free radicals.Phosphine-centered radicals can be generated from various tertiary phosphines through a phosphine-oxidant charge transfer process,photoredox catalysis,and electrochemical oxidation.In recent years,the importance of radical transitions associated with tertiary phosphines has been rediscovered.Based on the tertiary phosphine SET（single-electron-transfer）process,many excellent synthetic methods have been developed.The SET process associated with tertiary phosphines,which can be combined with photoredox catalysis or electrochemical synthesis from readily available starting materials by simple manipulation,provides an efficient approach on challenging structures.Reduction of nitro groups is one of the most straightforward methods to prepare aliphatic or aromatic amines.Among many traditional methods for reducing nitro groups,hydrogenation or transfer-metal catalytic hydrogenation are generally carried out under the conditions of Pd/C,Pt O₂,Raney Ni or transition metal catalysts.However,these methods lack functional group compatibility and usually require high pressure equipment and hazardous reagents（hydrazinium hydroxide solution）,which limit the application of this method.For metal dissolution reductions involving zinc,iron,indium or samarium,these reductions are less compatible with the presence of halogen atoms.In this paper,a photochemical reduction of nitroarenes mediated by p-toluenethiophenol and tricyclohexylphosphine under mild solvent-free and metal-free conditions was designed.By optimizing and improving the conditions of this reduction method,the yield of the final model reaction can reach 98%,and the amine derivatives（3a-3x）with different substituents and substitution patterns can be obtained with 17%-98%yields.All target compounds were structurally characterized by ¹H NMR,¹³C NMR,¹⁹F NMR and HRMS.For this reduction method,we also carried out amplifying reaction,applied research and preliminary discussion on the mechanism.The target product can still be obtained with a yield of 88%after the reaction is amplified ten times,and a yield of 53%can also be obtained after changing the light source to sunlight.We utilize the SET activation mode and wide range of reaction substrates provided by trivalent phosphines to achieve efficient,fast and concise construction of amine-containing and amine derivative molecules.Amine compounds with various substituents were synthesized by a simple and efficient method through a free radical reduction reaction mode under solvent-free,metal-free and mild conditions.