Studies On Boryl And Silyl Radicals Involved Stereoselective And Diverse Cascade Reactions

Stereoselectivity control in radical reactions remains a marked challenge in synthetic chemistry.In terms of the stereoselectivity control issue of hydrogen atom transfer(HAT)to acyclic carbon radical intermediates,the author proposed a synergistical mode of NHC-BH3/thiol catalyst and NHC-BH3/Znl2-chelation systems to achieve stereoselective HAT process in this thesis,and diastereodivergent boron-tethered heterocycles were afforded in the end.Besides,the author had developed a silyl radical triggered addition-translocation-cyclization cascades,using the tris(trimethylsilyl)silane as the silyl radical precursor.Subsequently diverse silicon-incorporated indolines were delivered according to the different properties of substrates.In chapter one,the author described the research progress about diastereoselective hydrogen atom transfer to acyclic radicals,established through either a substrate-controlled approach or a Lewis-acid assisted process.In addtion,their applications in HAT process were listed out.The second chapter introduced the studies of boryl radical-involved stereoselective and diastereodivergent radical cascades.An NHC-boryl radical triggered radical cascade to assemble heterocyclic molecules anchoring a boron-substituted acyclic stereocenter was disclosed.The stereochemistry of HAT to acyclic radical intermediates was determined by NHC-BH3/thiol catalyst and NHC-BH3/ZnI2-chelation systems.Experimental and computational studies suggested that these two modes operated in complementary ways.The synthetic utility of this diastereodivergent process was demonstrated,offering great foundations for stereoselective organoboron and heterocycles synthesis.In the last chapter,silyl-substituted acyclic radical was generated via a silyl radical-triggered radical addition-translocation-cyclization(RATC)process in which tris(trimethylsilyl)silane served as the silyl radical precursor.And the property of the acyclic radical intermediate was closely related to its substituents.The experimental results disclosed that tris(trimethylsilyl)silyl-substituted product was obtained through hydrogen atom transfer to this acyclic radical intermediate when terminal alkyne was used as the starting material,while a α-silyl ester moiety was accessed through a radical Brook-type rearrangement/HAT sequence if an ester group was installed to the alkyne.The generating silyl enol ether underwent isomerization on silica column,leading to the bi(trimethylsilyl)silyl-substituted product.Further transformations to access synthetically useful building blocks were also demonstrated.