Improvement Of The Synthesis Of The Carborane Anion[CB₁₁H₁₂]^-and Transition Metal-Catalyzed B-H Bond Activation

The mono-closo-carborate(-)anion,[CB11H12]-,is a closo cage compound comprising one carbon and eleven boron atoms.It is characterized by its delocalized electron deficient bonding,which makes the carbon and boron atoms aggregate and form 3-center-2-electron bonds,resulting in the formation of icosahedral structure and hypercoordination.The anion has been widely used in fields like weakly coordinating chemistry,supramolecular chemistry,materials sciences and phamaceutical chemistry because of its unique three-dimensional structure,thermal and chemical stability,low biotoxicity and enriched boron content.In this thesis,a brief introduction of the chemical properties,synthetic methods to make[CB11H12]-and its applications is reviewed.One of the commonly used methods is the one developed by Reed's group starting from commercially available decaborane,which contains three steps.While it provides a high-yielding method,the process is time consuming and requires toxic reagents.In 2004,Kennedy reported a two-step synthesis of[CB11H12]-also starting from decaborane.However,we found that the method is difficult to reproduce,and it also uses toxic reagents.The goal of one of my projects is to optimize the preparation of[CB11H12]-in two steps based on the Kennedy route.The procedures were optimized in terms of toxicity of reagents and mildness of reaction conditions.To enable the diverse applications of carboranes in different fields,many studies nowadays focus on the synthesis of carborane derivatives.The past decades have witnessed the rapid development of parent-carborane functionalization.Substitution in the carbon vertex has been realized by deprotonation of the C-H bond.For B-H bonds however,their modification has remained less explored;deprotonation has not been accomplished,and reaction with electrophiles has met with several challenges,which limits further applications of carborane derivatives.The current boron functionalization is still mainly via electrophilic substitution,such as boron-halogenation,boron-alkylation,and the reaction sites usually lie in the relatively more reactive meta and para positions of the carbon vertex.Substitution in the ortho positions remains difficult.A major synthetic challenge therefore is how to achieve regioselective B-H bond functionalization.To solve the problems proposed above,we have developed a novel method for the regioselective B-H bond functionalization of[CB11H12]-.Inspired by the current transition metal-catalyzed C-H activation methodology and the similarity in the electrophilic substitution reaction between arenes and carboranes,we installed effective directing groups on the carbon vertex and tested the effects of coupling reagent,catalyst,additive,solvent,temperature on reaction.B-C,B-N,B-X bond formations have been realized by transition metal-catalyzed B-H activation.