Theoretical Studies On Mechanism Of Silver(?)-mediated Synthesis Of Allene And Molybdenum(0)-catalyzed Ring-opening Reaction Of Thiophene

Quantum calculation has been an important method for chemistry studies in recent years.With this tool,the reaction mechanism can be thrown light on,and it is possible to predict the occurrence of some reactions.A combination of quantum calculation and experimental study can give a strong driving force for the development of chemistry.As a popular quantum calculation method,density functional theory(DFT)attains a well balance between the accuracy and the cost of calculation.So it is used for the silver(?)-mediated enantioselective synthesis of allenes and molybdenum(0)-catalyzed ring-opening reaction of thiophene.The mechanism of the two reactions is clearly elucidated,and some experimental facts are rationally explained by the calculation results.Firstly,the silver(?)-catalyzed synthesis picture of axially chiral allenes based on propargylamines have been outlined using DFT method.Our calculations find that,the most possible route for this reaction is that the silver(?)coordinates into the triple bond of propargylamines at anti-position of nitrogen,then the formed silver(?)complex undergoes a hydride transfer to give a vinyl-silver,finally the vinyl-silver goes through an anti-periplanar elimination to give(-)-allene.The hydride transfer with the energy barrier of 44.8 kJ·mol-1 is the rate-limiting step in whole catalytic process.The energy barrier of the anti-periplanar elimination of vinyl-silver is 26.9 kJ·mol-1 lower than that of the syn-periplanar elimination,supporting that(-)-allene is the main product of this reaction.This work provides insight into why this reaction has very high enantioselectivity.Secondly,the same method was employed to investigate the mechanism of the ring-opening reaction of thiophene catalyzed by Mo(PMes)6.Through calculations on species in all possible routes,the most practical way of this reaction was found,which is presented in the following.Firstly one PMe3 ligand dissociates from the six-coordinated catalyst precursor to give an active five-coordinated catalyst in a square-pyramid-like structure.Secondly,another PMe3 ligand in the bottom dissociates to attain a four-coordinated active catalyst species.Thirdly,thiophene coordinates with the four-coordinated active catalyst species through S atom to generate ?1-S-thiophene complex.Fourthly,?1-S-thiophene complex convert into ?2-C,S-thiophene complex.Finally the C-S bond of thiophene breaks and the ring-opened product is obtained.The conversion of the ?1-S-thiophene complex to ?2-C,S-thiophene complex is the rate-limiting step in whole reaction,with the energy barrier of 35.4 kJ·mol-1.The calculation results agree well with the experiment facts that the reaction can be carried out under mild conditions.