Direct C3-(sp2)-H Alkenylation and C(γ-sp3)-H Arylation of Five Membered Heteroaryl Compounds via Palladium Catalysis: An Access to Novel Compounds with Photophysical Properties

Abstract

The transition metal-catalyzed C-H bond activation has emerged as a unique strategy over the years for the synthesis of various natural products, materials, and medicines. It is also being investigated for late-stage modification. The direct activation of the unreactive C–H bonds to construct carbon-carbon and carbon-heteroatom bonds is a highlight unlike the conventional and traditional methods. In addition to other transition metals, the use of palladium catalyst for directed C-H bond activation has a great area of interest because of their high turnover numbers, reactivity for a larger range of substrates, and greater efficiency. In this thesis, various advantages of C-H bond activation over traditional methods, different types of reaction mechanisms, and the types of C-H activation reactions, along with the various applications of C-H activation for the development of organic fluorescent materials and their photophysical studies, have been explored. In order to reduce the number of stages, selective C-H bond functionalization without pre-functionalization is becoming increasingly important. The most frequently used method is directing group-mediated C-H bond activation, which involves placing the metal catalyst in close proximity to the specific C-H bond to facilitate the C-H functionalization process. As a potential strategy, we intended to achieve the direct site- and regio-selective C-H bond functionalization of several five-membered heterocyclic compounds using both this directing group and transient directing group (TDG) strategies via palladium catalysts. Through the utilization of transient directing group (TDG) and inherent directing group (DG) approaches, we have developed palladium-catalyzed C3-(sp2)-H alkenylation, C5-(sp2)-H alkenylation, and γ-C-(sp3)-H arylation of various five-membered heterocyclic scaffolds. Furthermore, as an application of this concept, we have successfully synthesized a few unique organic functional materials using this method. They exhibit novel thermally activated delayed fluorescence (TADF) properties, and a few synthesized compounds show novel mechanochromic properties.