New Borylation Strategy for the Construction of Borylated Heterocycles and Bodipy Derivatives

Abstract

This thesis mainly concerns: (1) developing new borylation strategies for heterocycles through addition of a B-E σ bond (E = H, N) across internal alkynes; (2) synthesizing BODIPY derivatives with interesting properties via “pre-functionalization” strategy; (3) preparing boron-based stimuli-responsive compounds for fluoride sensing. The hydroboration of alkynes is known to proceed in a cis-fashion. Catalytic trans-hydroboration has been established in the past decade while catalyst-free trans-hydroboration was unknown. In the study of hydroboration reactions, an unexpected pyridine-directed trans-hydroboration of internal alkynes was disclosed and presented. This transformation could serve as a facile method to construct novel luminescent N,C-chelate organoboron compounds. In the study of the amine directed hydroboration reaction, a facile dehydrogenative coupling between alkynyl aryl amines and 9-BBN was discovered. Based on the dehydrogenated products, a catalytic trans-aminoboration of internal alkynes was designed to give borylated indole derivatives. The reaction scope to different boranes and anilines was surveyed to generate a variety of borylated indoles. The dehydrogenative coupling of arylamines with 9-BBN described above represents a convenient method to construct a B-N covalent bond. By switching 9-BBN to monosubstituted boranes, a family of novel azaborabenzofulvenes was obtained by exploiting sequential dehydrogenative hydroboration of alkynyl arylamines. Both the reaction scope and the properties of the products were studied. Boron-dipyrromethene (BODIPY) and derivatives are excellent luminophores with bright emission in dilute solutions. However, they are rarely emissive in the solid state. In chapter 5, several spiro-BODIPYs with the “pre-functionalization” strategy were prepared. The spiro-structure successfully precludes the formation of π-stacking and leads to efficient solid state emissions. More interestingly, the spiro-BODIPYs formed J-aggregates in methanol-water mixed solvents. With the same strategy, a ferrocene functionalized BODIPY featuring direct Fc-B bond was prepared. Detailed studies on the novel compound and its analogues indicate the importance of ferrocenyl substituents and the bridge on their excited-state properties of BODIPY molecules. Trivalent organoboranes have been explored as fluoride sensors by monitoring fluorescence change, which is inconvenient in practical applications. In chapter 7, a family of boron functionalized poly-ferrocene compounds were prepared and some of these molecules can be used for colorimetric fluoride sensing.