Study On The Mechanism Of Rhodium-Catalyzed Cyclization Reaction

In organic synthesis,transition metal catalyzed cyclization reactions provide the most powerful methods for the construction of carbon rings and heterocyclic product.In the past decades,due to the unique reactivity of accumulated double bonds,allenes have been considered as important reaction units in transition metal catalyzed cyclization reactions.Their cyclic compounds widely exist in the molecular structures of medicine,pesticides,functional materials and natural products.In this paper,the detailed reaction mechanism of transition metal rhodium catalyzed cyclization is systematically studied by M06 method of density functional theory.The main contents are as follows:1.The experimental progress and related theoretical calculations of the preparation of cyclic compounds are briefly described.2.The ab initio self-consistent method,density functional theory,transition state theory and the application of calculation software involved in the study of reaction mechanism are introduced.3.The reaction mechanism of[3+2+1]cycloaddition reaction of vinylidene cyclopropane and carbon monoxide catalyzed by rhodium is studied in detail.The active structures of two catalysts are studied:monocarbonyl[Rh（CO）Cl]and dicarbonyl[Rh（CO）₂Cl].When the catalyst is monocarbonyl[Rh（CO）Cl],two reaction mechanisms are proposed.Mechanism A is divided into oxidative addition,carbonyl insertion,olefin insertion and reductive elimination.Mechanism B is divided into oxidative addition,olefin insertion,carbonyl insertion and reductive elimination.When the catalyst is dicarbonyl[Rh（CO）₂Cl],the mechanism is divided into oxidative addition,carbonyl insertion,olefin insertion and reductive elimination.Through the calculation of density functional method,it is concluded that the reaction is the most feasible when the catalyst is dicarbonyl[Rh（CO）₂Cl].The reaction starting complex 1 opens the ring of cyclopropane through oxidative addition,then the carbonyl group is inserted into the adjacent Rh–C4 bond to form intermediate4c,then the olefin is inserted to form bicyclic intermediate 7,and finally the final product is obtained through reductive elimination.The rate-determining step of the whole catalytic cycle is the reductive elimination step,which needs to overcome the energy barrier of 29.3 kcal/mol.4.The reaction mechanism of rhodium catalyzed cycloisomerization of allene–allenylcyclopropanes is studied in detail.The mechanism of the formation of monocyclic product product 1 in this system is mainly divided into two steps:cyclopropane ring opening and reductive elimination.In the ring opening step of cyclopropane,the rhodium catalyst is coordinated with the diene substrate,and then oxidative addition occurs to open the ring of cyclopropane,and then the reduction is eliminated,and the C–C coupling is constructed to form a monocyclic product.The reductive elimination step is the rate-determining step,and the energy barrier to be overcome is 33.2 kcal/mol.The mechanism of formation of bicyclic product product 2 is mainly divided into four steps:oxidative coupling,cyclopropane ring opening,deenerylation and reductive elimination.In the oxidative coupling step,the allene in Rh（Ⅲ）complex B1 is oxidative coupled with the allene fragment to form a 7/5 rhodium bicyclic intermediate.In the ring opening reaction of cyclopropane,the C1-C3 bond is broken,and the Rh-C3 bond forms a bond to form a 7/8bicyclic intermediate.Then,the 7/6 bicyclic intermediate is formed through the de Olefination process,and finally the final 7/5 bicyclic product is obtained through reductive elimination.The calculation results show that oxidative coupling is the rate-determining step of the reaction,which needs to overcome the energy barrier of 36.2 kcal/mol.5.On the whole,the energy barrier of the rate-determining step of the formation of single ring product product 1 is similar to that of the formation of double ring product product 2（33.2 kcal/mol vs 36.2 kcal/mol）,which is consistent with the experimental results,including the formation of both single ring product and double ring product.