The Application Of Salen Catalyst In The Oxidative Self-coupling Of Primary Amines To Imines And Its Immobilization Research

Imines are widely used in the synthesis of varied pharmaceuticals,biological active compounds and fine chemicals as versatile intermediates.Recently,many protocols have been developed for the synthesis of imines.Among them,the catalytic oxidative self-coupling of primary amines has attracted more and more attentions.Most of the reported catalysts for this reaction have some drawbacks,such as complicated preparations,harsh reaction conditions,so exploring more efficient catalytic system which could make the reaction proceed under mild conditions continues to be a hot research area.Salen catalyst has been proved to be efficient in many kinds of reactions,especially in oxidation reactions.It also has advantages of low cost,facile synthesis and easy tuning of the structural properties so more attentions have been paid to it.Unfortunately,the applications of Salen catalyst in the oxidative self-coupling of primary amines to imines are rarely reported before.So,extending the application of Salen catalyst in this reaction possesses significant value in academic research and industrial application.The research was based on two main aspects.First of all,Salophen ligand was prepared according to the reaction between the salicylaldehyde and o-phenylenediamine.A series of metal Salophen complexes were synthesized subsequently.Nuclear magnetic resonances(1H NMR),FTIR and elementary analysis were used to characterize the ligand and complexes.The oxidative self-coupling of benzylamine was taken as probe reaction to evaluate the catalytic efficiency of Salophen catalyst.The optimized reaction conditions were identified and the yield was improved up to 96.7%with a conversion of>99%after the investigation of several reaction parameters.8 different amines were introduced to test the scope of the catalyst,inferring that the catalytic efficiency has a good tolerance of electronic perturbation.Relatively,the efficiency of the reaction is more sensitive to the steric hindrance.The catalyst is also suitable for the oxidation of secondary amines.Finally,a plausible mechanism was proposed based on the experimental results.The advantages of this catalyst lie in simple preparation and mild reaction conditions.On the other hand,in order to realize the recycle of the catalyst attempts were also made to immobilize Salen catalyst on support.Magnetic particles were selected to be the support and their surfaces were modified by amino groups.Sequently,carboxyl groups were introduced on the surfaces of the particles by the reaction between the amino modified particles and anhydride.During this process,various reaction conditions were investigated and the feasible condition was identified according to the characterization of FTIR.The surfaces of the particles were modified by SOCl2 further to immobilize the ligand by Friedel-Crafts acylation.Unexpectedly,the loss of magnetic response was observed on the particles so we tried to realize the immobilization by esterification alternatively.For this purpose,substitutional Salen and Salophen ligands with hydroxymethyl were prepared and characterized by 1H NMR firstly.Furthermore,the substitutional ligand was immobilized to the carboxyl modified magnetic particles by esterification in the conditions of DCC as dehydrating agent and DMAP as catalyst.Additionally,we tried the reaction betweeen the amino modified magnetic particles and CDI.