Studies On Metal-free Selective B-H Bond Functionalization Of Carboranes

Icosahedral carboranes,in analogy to 2-D benzene,have 3-D aromaticity and good thermal and chemical stability.Owing to their useful properties such as enriched boron content,low biological toxicity,good chemical modification,carboranes are finding a large variety of applications in many fields such as organic catalysis,biomedicine,optoelectronic materials,supramolecular design and nanomaterials.In order to further expand the application of carboranes in the above-mentioned fields,the modification of carboranes has become an important research area for boron chemists.Compared with cage C-H bonds functionalization of carboranes,the site-selectivity of cage B-H bonds is more challenging to be achieved.Only in recent years,significant progress has been made in regioselective B?H functionalization,they mainly focus on two strategies:transition metal catalyzed B?H functionalization assisted by guide groups and transition metal catalyzed direct B?H functionalization.It is still difficult to realize direct B?H functionalization under mild conditions without metal catalysis.Therefore,this study focused on the direct B-H functionalization of unmodified carboranes promoted by hydrogen bond donor solvents or Bronsted acids.The research contents covered the following two parts:1.Hexafluoro-2-propanol-Promoted Selective B(9)-Selenation of Carboranes.In this chapter,hexafluoroisopropanol has been used as a hydrogen-bond donor solvent to realize the selective synthesis of 9-aryselanyl-carboranes.Compared with the previous work,this method does not require transition metal catalysis.The reactions were conducted at room temperature in open air and the products were obtained in high to excellent yields with good functional group tolerance.For example,the diaryl disenlenides containing electron-rich,electron-poor and hindered substituents worked well in this system.In addition,this methodology can be scaled up with a similar yield.The same system can facilitate regioselective carborane-cage B(9)-H iodination and bromination.On the basis of the previously reported literatures,a mechanism involving an electrophilic substitution pathway was proposed.2.Friedel-Crafts Reactions of Alcohols and CarboranesIn this chapter,we studied the direct carborane-cage B-H alkylation promoted by organic superacids such as trifluoromethanesulfonic acid,using commercially available alcohol as the alkyl source.This reaction is suitable for incorporate highly electronically deactivated benzylic alcohols and a series of mono-benzylated products were achieved in good yields.In addition,this methodology can facilitate carborane-cage B(8/9)-H fluorination and sulfhydrylation On the basis of the existing literatures,a mechanism cycle similar to Friedel-Crafts alkylation reaction was proposed.