Palladium-Catalyzed O-Carborane Functionalization

As an icosahedral cage compound,o-carboborane has the characteristics of three-dimensional superaromatics,great thermal and chemical stability,and has a broad development prospect in the fields of military,medicine,coordination chemistry and luminescent materials.Therefore,it is of great scientific significance to develop efficient synthesis methods of o-carbonborane derivatives.In this paper,palladium-catalyzed o-carborane functionalization reaction was studied.The main contents and results are as follows:1.An asymmetric coupling of o-carborane B(3)-B(6)is carried out at room temperature through palladium-catalyzed oxidative dehydrogenation of pyridine,as the guiding group,without the protection of an inert gas.With 1-(2-pyridyl)-o-carboranes and 1-((5-(2-methyl)phenyl)-2-pyridyl)-o-carboranes as a template for the substrate,screening the B(3)-B(6ˊ)asymmetric coupling reaction,the optimal conditions.Based on the above optimal reaction conditions,the universality of asymmetric coupling reactions of pyridinyl o-carborane dehydrogenation was investigated by changing the types of two substrates.The results show that the reaction can still occur when there is alkyl on pyridine ring or alkyl on benzene ring.It is worth emphasizing that the new method can combine active functional groups such as aldehydes and ketones which are easily oxidized.The reaction can be carried out when naphthalene,pyrene and trimethylsilane are substituted on the benzene ring.A total of 22 o-carborane B(3)-B(6)compounds have been synthesized by this method.In addition,a metal intermediate was detected by HRMS experiments,and the possible mechanism of the reaction was speculated.2.Based on the pyridine-directed group strategy,the vertex amination of o-carborane B(3)-H/B(3,6)-H was achieved under palladium acetate catalysis,and a new method for direct amination of o-carborane B-H bond was established.Using1-pyridine-o-carborane and methyl carbamate as template substrates,the optimal reaction conditions for direct amination of o-carborane B(3)-H vertex were screened,and1-pyridine-o-carborane was selected as one of the substrates.By changing the type of amino source,Ten amino substituted o-carboborane derivatives including amino acid esters,alkylamides,aryl amides,heterocyclic amides and aminopyridine were synthesized.3.Using 1-((5-bromine)-2-pyridyl)-o-carborane as raw material,Suzuki coupling reaction was used to achieve the efficient synthesis of 1-pyridyl-o-carborane derivatives.Using 1-((5-bromine)-2-pyridyl)-o-carborane and thiophen-2-boric acid as template substrates,the optimum reaction conditions for o-carborane substrates were selected.The universality of the method was investigated by changing the type of arylboric acid using the optimal reaction conditions.The results show that the reaction can be applied to arylboric acid and heterocyclic substituted boric acid including phenyl,biphenyl,naphthalene,pyrene and other groups.