Mechanistic Studies Of Silicon-based Cross-coupling Reactions And Cycloadditions Of Isochromenyliums

Cross-coupling reaction has been widely studied as a common way to construct carbon-carbon bonds in chemistry.It is reported that the siloxane transfer reagents can be involved in the transition metal catalyzed cross-coupling reactions of aryl halides with aryl Iithium under mild conditions at room temperature and reused repeatedly.According to these reports,the reaction mechanism of aryl lithium and aryl iodide catalyzed by palladium and copper and the reaction mechanism of palladium catalyzed aryl lithium and aryl chloride were studied.It is found that the assistance of cuprous iodide promotes the dissociation of lithium iodide from palladium silicon complex and reduces the reaction activation energy barrier in the palladium and copper co-catalyzed system.DFT calculations indicated that cuprous iodide initially accelerates the Si-Pd(?)transmetalation of the organosilicon intermediate by the formation of Cul2-.After consumption of cuprous iodide,subsequently Cul2-works as a shuttle between the Si-Cu(?)and Cu(?)-Pd(?)transmetalation processes.We are attracted by the fact that the reaction system of aryl lithium and aryl chloride under the catalysis of palladium can work,while the system of high reactive aryl lithium and aryl iodide does not work.The reason is that the difference in stability of palladium silicon complex structure caused by different halogen.At the same time,the study of the corresponding substituents on silicon transfer reagents shows that the increase in steric hindrance effect of substituents on silicon will cause difficulties in the nucleophile attack of phenyl lithium and the instability of lithium alkoxide.The two trifluoromethyl groups on the benzyl position of siloxane reagent have good electron-withdrawing effects to make siloxane reagent work perfectly.Besides,the proper reduction of the potential steric resistance on the benzyl position can effectively reduce the activation energy barrier of the transmetalation step in order to expand the range of the reaction substrates.Theoretical calculations predict a series of new siloxane reagents with one trifluoromethyl group or one phenyl group or one methyl group on the benzyl position involved in the aryl-coupling reactions.The behavior phenomenon of post-transition states bifurcation is widely discovered in organic reactions.It has been reported and researched in the transition metal catalytic systems,non-metallic systems and biomolecular synthesis systems.Ortho-alk,ynylbenzaldehydes can form intramolecular isopyran ring under the activation of alkynyl by transition metals.This structure has the structural characteristics of the pyran ring,and also has the structural characteristics of 1,3-dipole metal carbene.Because of this special structure,it can produce[4+2]or[3+2]ring cycloaddition products with olefins or alkynes.The theoretical calculation results show that it is a transition state with two modes.Therefore,this transition state is an example of the bifurcation behavior phenomenon by the transition metal catalyzed cycloaddition reactions.The combination of experiment research and calculation results show that the catalytic systems of most kinds of transition metals can be roughl,y divided into three categories:(1)tungsten metal system,which reacts as the form of isochromenylium tungsten carbene with alkenyl ethers to produce[3+2]ring intermediate,and the insertion of C-H bond to the tungsten carbene as a subsequent reaction to generate polycyclic skeleton molecules;(2)metal copper and palladium systems,which react as the form of benzo isopyran dienyl with styrenes to produce[4+2]ring intermediate,and the subsequent reaction of opening oxygen ring to obtain naphthalene molecules;(3)metal silver and gold systems,their benzo isopyran structures can occur with styrene or norbornene to carry out both[3+2]cycloaddition and[4+2]cycloaddition between the above two classes.In addition to the effect of transition metals on the abimodel transition state and the subsequent[3+2]and[4+2]intermediates,the substituents on isopyran structures also play a role.Theoretical calculation shows that the electron donor methyl substitution is beneficial to the formation of[4+2]intermediates while hydrogen substitution is beneficial to the formation of[3+2]intermediates.The trajectories of the subsequent structural changes of the transition state are simulated by the molecular dynamics calculation.According to the statistical analysis,the tungsten metal system has the characteristics of abimodel transition states.There are the whole trajectories of the[3+2]intermediates and the[4+2]intermediates from reactants and the proportion of the trajectories is definitely influenced by the substituents on the benzo isopyran skeleton in tungsten metal system.However,the trajectories of the copper system show that the trend of the[4+2]intermediate and we did not find any trajectories of the[3+2]intermediate.Further analysis shows that the platform process of post-transition states in the copper metal system is the process of continuous structure change,which is more conducive to the formation of the[4+2]intermediate.In addition,in the related study of various transition metals,it is found that not all metal systems have the abimodel transition state.For example,the cycloaddition reaction of metal platinum is carried out stepwise.The formation of[3+2]intermediates and[4+2]intermediates are controlled by their corresponding transition states.