Theoretical Study On The Activation Mechanisms Of C-H Bonds In Unsaturated Molecules

Activation of C-H bonds is an important means to effectively construct C-C,C-N and C-O bonds and plays an important role in the organic synthesis.Because of the atomic economy and replacement of a C-H bond with C-C or C-X bonds(X = O,N or S)in one step,great progress has been made to transition metal catalyzed C-H bond activation.Theoretical studies of the C-H activation mechanism at molecular level help understand the reaction process and rationalize the experimental phenomena.They will also provide useful information to guide effectively construct C-C or C-X bonds.In this thesis,we used quantum chemistry methods to investigate the two transition metal catalyzed C-H activation in unsaturated molecules.We first performed the mechanistic study of Cu-catalyzed cross-dehydrogenative coupling reaction of thiazoles with THF using density functional theory.Previously it was believed that the reaction followed the so-called concerted metalation-deprotonation(CMD)mechanism.But our calculations show that this reaction most likely follows a seven-membered ring mechanism.In this mechanism,a seven-membered ring is formed first,and then a reduction-elimination step happends,leading to the final product.This mechanism has a very low reaction barrier,especially for the breaking of the C-H of thiazoles.Our calculations also reveal the importance of potassium sulfate which first dissociates into sulfate anion free radicals,consistent with the experimental results.The kinetic Carlo Monte simulation identified the dominant pathway leading to the final product:In the pathway,Cu catalyst first acts with thiazoles,resulting in a reaction complex.The formed reaction complex then reacts with tetrahydrofuran.On the other hand,the pathway in which tetrahydrofuran first reacts with thiazoles,then the formed reaction complex reacts with Cu catalyst hardly contributes to the formation of the final product.In chapter 4 we investigated the mechanism of Hg(II)-catalyzed cyclopropanation of diazooxindoles.We found that the structure affects slightly on the calculated chemical shift of the 31P.On the other hand,the basis sets used to calculate the NMR has a great influence on the calculation results.Based on the determined structure of the active species,we found that diazooxindoles first loses a N2 under the attack of the catalyst,forming a metal carbene complex.This step is the rate-determining step.Then,two different reaction intermediates are formed by the combination of the metal carbene complex with styrene.The final product is produced from the intermediates.The above studies provide meaningful information for the activation of C-H bonds in unsaturated molecules.They also deepen the understanding of the Cu-catalyzed cross-dehydrogenative coupling reaction and the cyclopropane process by Hg catalysts.