Studies On The Synthesis Of Nido-carborane Fused Oxazoles And Regioselective Methylation Of O-Carborane

Carboranes are a class of carbon-boron molecular clusters with three dimensional aromaticity,which application prospects in functional materials,coordination and pharmaceutical chemistry based on its unique photoelectric and biochemical activity.Therefore,exploring methodologies for the selective functionalization of o-carborane is of great significance to promote the development of carborane chemistry.According to weak acidity of the C-H bonds of o-carborane,the transition metal mediated/catalyzed methods have been well developed to realize the carbon functionalization of o-carboranes.In contrast,the selective boron functionalization of o-carboranes is still a challenging subject because of the slight difference in reactivity among the 10 B-H bonds.In recent years,the transition-metal catalyzed B-H activation has emerged as a powerful stragegy for selective boron functionalization of o-carboranes,which displays much more superiority than pioneering works based on the pre-functionalized B-I bonds.By utilizing the palladium catalyzed B-H activation strategy,this thesis described the studies on“the synthesis of nido-carborane fused oxazoles and regioselective methylation of o-carboranes via B-H activation”,the main contents include the following two parts.1.Through introducing amide group to the site of B（9）in o-carborane,with the cooperation of Pd（OAc）₂,AgOAc and K₂CO₃,a one pot deboronation/cyclization of9-benzamide-o-carborane for the synthesis of nido-7,8-carborane fused oxazole has been developed.A plausible mechanism involving an amide directed electrophilic palladation of the B-H bond and deboronation/cyclization process was proposed,which based on the successful isolation and structural characterization of the key deboronated intermediate by X-ray crystallographic analysis.The work provides a platform for developing metallacarborane and BNCT agents,and has important value in design and synthesis of carborane fused heterocycles.2.Based on the changed charge density of o-carborane induced by the electron withdrawing effect of amide,we have developed an efficient method for selective methylation of B（4）-H bond directed by amide,Meanwhile,the selective methylation of B（12）-H bond was further achieved tuned by counter ion.A plausible mechanism was also proposed based on the experimental results.This work provides an alternative method for selective methylation of o-carborane via B-H activation,and has important value for selective functionalization of o-carboranes tuned by counter ions.