Selective B-H Bond Functionalization Of Closo-dodecaborate Anions

The dodecahydro-closo-dodecaborate dianion[B12H12]2-is an icosahedral cluster molecule that consists of 12 identical B-H vertices.Due to its extraordinary symmetry,selective functionalization of one B-H vertex or several specific B-H vertices faces significant challenges.To date,most of synthesized and isolated derivatives are mono-substituted or per-substituted.Di-substituted or multi-substituted derivatives,especially with different substituents have rarely been reported because of the difficulty of control over the substitution sites as well as the substitution degree.To expand the application of[B12H12]2-,new approaches to the synthesis of multi-functionalized cage derivatives are highly required.In this thesis,we have developed some novel methodologies for selective B-H bond functionalization as follows:1.Perchlorination of ammonio/hydroxo-dodecahydro-closo dodecaborates The perchlorinated close-dodecaborate dianion[B12Cl12]2-can be used as a weakly coordinating anion for stabilization of reactive cations.However,due to its double negative charge,the solubility of[B12Cl12]2-salts in nonpolar organic solvent is very low,which limits its practical applications.In order to increase the solubility,we use ammonio/hydroxo-dodecahydro-closo-dodecaborates as starting materials and sulfuryl chloride as the chlorination reagent to prepare ammonio/hydroxo-dodecachloro-closo-dodecaborates.The solubility is further increased by introduction of organic groups on the ammonio/hydroxo site without sacrificing the weakly coordinating ability.The target anions can serve as new weakly coordinating anions 2.Rh(?)-catalyzed alkenylation/annulation of closo-dodecaborate anions using ureido directing groupsThe main approaches of constructing B-C bonds of closo-dodecaborates are based on cross coupling of iodinated dodecaborates with Grignard reagents or zinc reagents.Direct transformation of B-H bonds to B-C bonds is challenging due to the low reactivity of B-H bonds.Inspired by direct C-H bond activation with the assistance of directing groups,we have developed a methodology for B-H bond alkenylation/annulation via Rh catalysis at room temperature using ureido moieties as directing groups.The reaction exhibited excellent selectivity with good functional group tolerance and medium to high yields.A possible mechanism was proposed by control experiments.This is also the first example of transition-metal catalyzed B-H bond activation/functionalization of closo-dodecabora,tes.3.Rh(III)-catalyzed alkenylation/annulation of closo-dodecaborate anions using aryl amido directing groupsThe closo-dodecaborate precursors bearing ureido moieties are not stable and slowly decompose to[B12H11NCO]2-.In order to find some more stable precursors with functional groups that can be further derived,we tested different kinds of aryl amido groups as directing groups and found the aryl amides with ortho-substituted directing group is not effective due to the steric hindrance while ortho-C-H aryl amido groups serve as effective directing groups for B-H bond alkenylation/annulation reaction without the observation of products from competitive C-H bond activation.The reaction exhibited excellent selectivity with good functional group tolerance and medium to high yields.The products showed solid state fluorescence emission.VT-NMR showed that the intermediate with B-H-Rh agnostic-like interation is stable and the coordiantion between boron cluster and Rh center is weak.The intermediate with a direct B-Rh bond is very reactive and it is able to react with the solvent acetonitrile to form a B-C bond.4.Rh(III)-catalyzed alkylation/annulation of closo-dodecaborate anions using aryl amido directing groupsThe closo-dodecaborates with direct alkyl substituents have rarely been reported due to the lack of reliable approaches.In this work,1,2,3-trisubstituted closo-dodecaborates with B-O,B-N,and B-C(alkyl)bonds as well as a fused borane oxazole ring have been synthesized by rhodium-catalyzed direct cage B-H bond alkylation and annulation of aryl amido boranes.The reaction proceeded under mild condition and exhibited good selectivity for efficient monoalkylation with excellent functional group tolerance.Typically,unactivated terminal alkenes can also be used as alkylation reagents in the reaction.All the major products were mono-alkylated derivatives.A small amount of di-alkylated species were also detected.