Studies On Transition-Metal-Catalyzed Silylation And Cascade [3+2] Annulation Of C-H Bonds

Over the past decades,the extensive research on transition-metal-catalyzed C-H activations gives us a better understanding of how C-H bond functionalizations proceed effectively.Compared with the classic palladium-catalyzed coupling reactions(such as,Heck coupling and Suzuki copuling),C-H bond activations can shorten the synthesis steps and reduce the preparation of the pre-functionalized starting materials in a more efficient and economic manner.In this dissertation,palladium-catalyzed direct intermolecular silylation of unactivaed C-H bonds and rhodium-catalyzed cascade[3+2]annulation of C-H bonds have been successfully achieved via a transition-metal-catalyzed and directing-group strategy.This dissertation mainly includes the following two parts:Part Ⅰ:A palladium(Ⅱ)-catalyzed direct intramolecular silylation of remote unactivatedβ-methyl andβ-methylene C(sp~3)–H bonds with hexamethyldisilane as the silicon source has been achieved by using a readily removable N,N-bidentate 8-aminoquinoline as the directing group.This catalytic system featured broad substrate scope and good functional group tolerance.This protocol provided an alternative convenient and simple strategy for efficient access to structural C-Si bonds.The silylation reactions can be performed in gram-scale,and the directing group can be easily removed under simple conditions.This method provides an efficient and straightforward strategy for the synthesis ofβ-silylatedα-amino acid derivatives.Moreover,this method can be applied to the silylation of remoteγ-methyl C(sp~3)-H bonds.Next,in order to expand the application scope of this silylation reaction,a palladium(Ⅱ)-catalyzed direct intramolecular silylation ofβ-C(sp~2)–H bonds with hexamethyldisilane as the silicon source has been achieved by using a readily removable N,N-bidentate 8-aminoquinoline as the directing group.This strategy provided a regio-and stereo-selective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility.The auxiliary could be readily removed via a two-step sequence to afford the corresponding ester.The practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.A stable five-membered palladacycle intermediate was isolated,which provided convictive evidence for the plausible reaction mechanism.Part Ⅱ:A rohdium(Ⅲ)-catalyzed cascade[3+2]annulation of N-aryloxyacetamides with propiolates under mild conditions using the internal oxidative O-NHAc as the directing group has been achieved.This catalytic system provides a regio-and stere-oselective access to benzofuran-2(3H)-ones bearing exocyclic enamino motifs with exclusive Z configuration selectivity,acceptable to good yields and good functional group compatibility.Mechanistic investigations by experimental and density functional theory studies suggested that a consecutive process of C-H functionalization/isomerization/lactonization is likely to be involved in the reaction.Then,a cascade[3+2]annulation and ring opening of N-aryloxyacetamides with 1-alkynylcyclobutanols via rhodium(Ⅲ)-catalyzed redox-neutral C–H/C–C activations using internal oxidative O–NHAc and-OH as the dual directing groups has been achieved.This reaction provided an efficient and regioselective approach to benzofuran derivatives with good functional group compatibility and high yields.Moreover,O–NHAc and-OH,which were traceless in the products,avoided extra steps for the removal of the undesired directing groups.In summary,a palladium-catalyzed direct intermolecular silylation of unactivaed C-H bonds and a rhodium-catalyzed cascade[3+2]annulation of C-H bonds have been successfully achieved via a transition-metal-catalyzed and directing-group strategy.