Hydroboration Of Cyclopropanes

The hydroboration of unsaturated organic compounds is a well-established method for the synthesis of alkyl boranes,which can be converted to the stable boronic esters in a straightforward manner.The boron atom in these boronic esters can be readily converted into a wide variety of functional groups under mild conditions,providing simple versatile synthesis of organic compounds.Olefins and cyclopropane derivatives—the representative unsaturated organic compounds are related in many ways.While distinct differences in reactivity do exist—the double bond usually being more reactive than the three-membered ring—there are no fundamental differences in behavior.Our group has been working on the development of C-C bond activation and functionalization,this thesis mainly studies the C-C bond cleavage of unactivated cyclopropanes to build a reactive C-B bond.Herein,we have developed a method for cyclopropane ring-opening reaction that boron tribromide effectively promotes the C-C bond cleavage of unactivated cyclopropanes to provide anti-Markovnikov boronic esters.One-carbon homologation of the corresponding olefin hydroborating product is readily realized by utilizing this strategy.This transformation exhibits excellent yield and good functional group compatibility and can serve as powerful synthetic tools for late-stage functionalization of complex compounds.In particular,it is the first time to realize the hydroboration of cyclopropane gas.Preliminary mechanism experiments show that boron tribromide mediate C-C bond cleavage of unactivated cyclopropanes to generate a carbocation intermediate,which subsequently undergoes 1,2-hydride migration and nucleophilic attack to give the hydroborating product.The chiral control experiments at different temperatures can explain the racemization of the substrates containing chiral carbon atoms,and verify this reaction involves a mechanism of carbocation intermediate.Nevertheless,the Brown hydroboration reaction proceeds via a four-membered transition state and undergoes a concerted mechanism.The above is the innovation of this thesis,and also the significance of this research.