Tetraalkylgermanes:Synthesis And Applications In Radical Reactions

Compounds of Group 14 elements have been playing important roles in the development of radical chemistry.As visible-light mediated photocatalysis becomes the main protocol for radical generation,tin and lead compounds have been abandoned due to their toxicity.Meanwhile,silicon compounds and radicals are still widely utilized in multiple conditions.Located between silicon and tin in the periodic table,organogermanium compounds are usually non-toxic and more reactive than the silicon analogues.However,the combination of organogermanium compounds with visible-light photocatalytic reactions remains unexplored to date.In order to enrich photocatalysis through the introduction of germanium compounds,the radical reactions of several typical compounds are summarized in Chapter One.From these,alkyltrimethylgermanes are selected for the study of their potential application as radical precursors.Both the synthesis and photocatalytic reactions are discussed in the following three chapters.In Chapter Two,sporadic reports on the synthesis of tetraalkylgermanes are summarized and developed into three generic methods:The Grignard reagent method,the GeCl2 insertion method,and the HGeCl3 addition method.Specifically,in the GeCl3 insertion method,unexpected functional group tolerance in the reaction between MeMgBr and alkyltrichlorogermane provides a convenient way to produce various substituted alkyltrimethylgermanes.However,functionalized 2°alkyltrimethylgermanes are still hard to generate using the above methods.Therefore,a copper-promoted reaction using alkyl bromide and germyl electrophiles has been developed to provide moderate compatibility.This protocol can also be used to synthesize several organogermanium compounds with other structures.Finally,the stability of the trimethylgermyl group towards other functional group transformations has been checked during the derivation of several functionalized trimethylgermyl compounds.In Chapter Three,alkyltrimethylgermanes are applied to visible-light photocatalytic reactions as neutral and stable nucleophilic radical precursors.By using[Acr-Mes]type photocatalyst,alkyltrimethylgermanes with various structures can be oxidized to corresponding alkyl radicals,followed by addition reactions with different electron-deficient alkenes.Under alternative photocatalytic conditions,trimethylgermane and carboxylic acid can be orthogonally oxidized to alkyl radicals as well.Additionally,cross-coupling reactions are possible between alkyltrimethylgermanes and aryl bromides with the introduction of Ni catalysts.Active functional groups such as aldehyde and sulfonyl amide can be tolerated.Intramolecular competitive reactions suggest that the assistance of a nucleophile is necessary for the cleavage of radical cation after the single electron transfer step.In Chapter Four,alkyltrimethylgermanes are used in an innovative hydrogen atom transfer(HAT)process initiated by α-fluoroalkyl radicals.Unactivated 1° and 2° alkyl C-H bonds can be cleaved to give the corresponding alkyl radicals,which are then introduced into Ni-catalyzed cross-coupling reactions to generate remote acylation or arylation products.This process provides a novel method for functionalizing unactivated alkyl C-H bonds initiated from sp3 C-radicals,greatly expanding the scope of remote C-H functionalization reactions.