| The monocarba-closo-decaborate cluster[1-CB9H10]-and monocarba-closo-dodecaborate cluster[CB11H12]-belong to the class of 3D-delocalized closed-cage boron clusters with one BH vertex replaced by CH.They possess unique steric and electronic properties that set them apart from traditional organic or inorganic building blocks.Their sphere-like distribution of electron density gives rise to remarkable chemical and thermal stability,as well as low toxicity.The chemistry of closo-borane has fascinated chemists ever since their discovery several decades ago including understanding their fundamental properties and the applications of functionalized clusters.The modification of boron vertices traditionally relies on direct electrophilic substitution or halogenation followed by transformation to a B-X(X=C,N,O,S,P)bond.Drawbacks of these approaches are harsh conditions,limited control over selectivity of the substituted positions.Therefore,the functionalization of boron vertices by directing group-mediated,metal-catalyzed B-H activation is a highly attractive strategy.In this thesis,we have developed three methodologies for the functionalization of[1-CB9H10]-and[CB11H12]-by B-H activation as follows:1.Rhodium(?)-catalyzed dehydrogenative dialkenylation of the monocarba-closo-decaborate cluster[1-CB9H10]-by regioselective B-H activationMonocarboranes based on the[1-CB9H10]-anion are applied in areas such as medicinal chemistry,fluorescence and material science.However,the selective functionalization of the B vertex is still limited especially for the B2-5 positions which are least reactive among the B vertices.Two maj or synthetic challenges have remained:the construction of B-C bonds and access to upper belt-substituted derivatives.Here we have developed a regioselective dehydrogenative 2,3-dialkenylation of monocarbo-closo-decaborate using Rh catalysis to form novel derivatives of the[1-CB9H10]-cluster by B-H activation with amide as the directing group.The unprecedented double B-H activation occurs under mild conditions and tolerates substrates such as acrylates,sulfones and styrenes.We also tested the optical properties which suggested a certain degree of conjugation between the two alkenyl systems.These findings are in agreement with electronic effects transmitted through the 3D-delocalized carborane cage,ie.,the formal B2-B3 bond does not act as an insulator.In addition,we found using the same catalytic system with DCE instead of DMA as the reaction solvent.2,3,4,5,10-penta-alkenylated products were obtained.The structures of the products were confirmed by NMR and X-ray analysis.This methodology appears as a powerful tool for the functionalization of the[1-CB9H10]-anion,for which the limitation of existing methods has hampered widespread applications for a long time.2.Pd-catalyzed highly selective four-fold arylation of the monocarba-closo-decaborate cluster by B-H activationWe have reported Rh-catalyzed dialkenylation of monocarba-closo-decaborate using amide moiety as the directing group.In this methodology,it requires the installation of the directing group and the use of expensive Rh catalyst.Furthermore.the amide directing group can not be removed after the functionalization on B vertices.Here we developed a Pd-catalyzed tetra-arylation of the monocarba-closo-decaborate cluster by B-H activation using carboxylic acid as the directing group.This method avoids the installation of the directing group because the carboxylic acid group was introduced to the cluster since the first step of the synthesis of the cluster using glyoxylic acid.In the literature,one example of the tetra-arylation is accessed by Kumada coupling reaction between B6-10 iodinated carborane and Grignard regent.In our methodology,we can easily synthesized 23 products.Furthermore,the carboxylic acid group can be removed under microwave heating conditions3.Pd-catalyzed dialkenylation of the monocarba-closo-dodecaborate cluster[CB11H12]-by regioselective B-H activation with diarylacetylenesIn our group,we have reported Ir or Rh-catalyzed B-H activation of monocarba-closo-dodecaborate cluster with diarylacetylenes.In these methods,expensive catalysts,high temperature,high boiling-point solvents and additives are required.Here we developed a convenient method for the regioselective 2,4-dialkenylation of {CB11}cluster by Pd-catalyzed B-H activation using pyridine moiety as the directing group and diarylacetylenes as the coupling partners.The transformation proceeded at room temperature and an ambient condition without oxidants or other additives.The reaction exhibits high selectivity and the products were easily separated by filtration in good yields.A possible mech anism was proposed on the basis of the Pd complex dimer with a direct B-Pd interaction which was confirmed by X-ray crystallography. |
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