Development of Imidazopyridine based N-Heterocyclic Carbene Ligands

Abstract

In the field of organotransition-metal chemistry, the development of novel and efficient catalysts for homogeneous catalysis has been extensively studied. N-Heterocyclic carbenes (NHCs) as strong σ-donor ligands have been the focus of a variety of research in homogeneous catalysis, since the isolation of the stable free carbene by Arduengo in 1991. Numerous structural variations on the prototypical imidazolylidene skeleton have been used to modulate the electronic and steric properties of NHC ligands. Imidazo[1,5-a]pyridin-3-ylidenes (ImPy) are rigid, bicyclic variant of NHCs that show unique electronic and steric properties. ImPy ligands are strong σ-donors, as their extended π-system can increase the electron density at the carbene center. The bicyclic ImPy substituents on the bicyclic ImPy can be projected deeply into the metal coordination sphere. In line with our interests in designing new, high-performance NHC-based catalysts, we suggest that ImPy ligands could serve as a versatile framework for tuning the electronic and steric properties of ligands. The present thesis describes the development of imidazopyridine-based NHC transition metal complexes and their application to chemical reactions: direct C–H carboxylation with CO2 and olefin polymerization. Carbon dioxide (CO2) serves as a useful C1 feedstock for organic synthesis owing to its abundance, cheapness and renewable source. Utilization of CO2 for value-added chemicals has attracted much attention as a promising field for green chemistry. Transition metal-catalyzed direct C–H carboxylation with CO2 is a straightforward and atom-economical approach to providing carboxylic acid derivatives used as useful building blocks in pharmaceuticals, polymer chemistry, pharmaceuticals and other science. Chapter 2 describes the synthesis of diethylene glycol-functionalized imidazopyridine ligand (DEG-ImPy) and their application in the direct C–H carboxylation of benzoxazole with CO2. Polyether units are known as a CO2-philic building block and have been utilized for CO2 capture. The molecular structure of ImPy-Cu(I) complexes were determined by X-ray crystallography and ImPy-Cu catalysts showed good catalytic activity with good yield. Importantly, the Cu catalyst generated in situ with the DEG-ImPy·HCl salts efficiently catalyzed the direct C–H carboxylation of various heterocyclic compounds with CO2, resulting in higher yield than that achieved by non-functionalized NHC-Cu catalysts. Functional polyolefins have attracted much attention due to enhance material properties such as adhesion, dyeability, printability, compatibility and toughness. Transition-metal-catalyzed coordination-insertion copolymerization of olefins with polar monomers has emerged as an elegant strategy to produce functional polyolefins in a controlled manner. A variety types of late transition metal catalysts have been developed in copolymerization of ethylene with polar monomers due to their low oxophilicity compared with early transition metal catalyst. Chapter 3 discusses palladium complexes bearing abnormal imidazopyridine-based NHC ligands (aImPy) developed for the homopolymerization of olefins and the copolymerization of olefins and polar monomers. The highly electron-donating nature of these abnormal N-heterocyclic carbenes (aNHCs) embedded in the carbene-phenolate chelating ligand scaffold resulted in good catalytic activity, generating linear polyethylene with a high molecular weight (Mn = 237, 000). aImPy-Pd complexes efficiently catalyzed propylene polymerization to afford polypropylene free of regio-defects. These catalytic systems exhibit good tolerance of various polar monomers and afford the desired copolymers. Up to 3.0% of the polar monomers were incorporated into the main chain, as determined by 1H and 13C NMR analysis. Furthermore, methyl methacrylate (MMA), as a 1,1-disubstituted ethylene derivative, was successfully incorporated into the main chain of polyethylene. Computational studies indicated that ethylene insertion into a Pd-alkyl bond by aImPy-Pd catalyst 6a is faster than that by Pd catalyst bearing normal NHC analogs, which could be attributed to high linearity and high molecular weight of polyethylene.