Design and Synthesis of New N-heterocyclic Carbenes: Coumaraz-2-on-4-ylidene (CONY) and Indol-2-ylidene (INDY)

다국어 초록 (Multilingual Abstract)

Carbene is an organic compound with only a divalent bond that does not satisfy the octet rules. Their unstable structure has been widely applied as an intermediate in various organic reactions or as a ligand for transition metal and main-group complexes. From about 40 years ago, various strategies have been studied to effectively stabilize the inherent instability of carbenes by introducing various functional groups. In particular, N-heterocyclic carbene (NHC) is one of the effectively stabilized carbene structures, and they have been applied as ligands and organic catalysts to date. In this thesis, I summarized a general overview of various stable carbenes and their reactivities studied so far. Furthermore, I also presented how to design and synthesize new stable carbenes.
The first chapter mainly deals with the overall history of the carbene’s discovery and its applications. In particular, focusing on persistent carbenes, I briefly reviewed their various applications and further summarized the several reactivities of various types of persistent carbenes with small molecules.
In the second chapter, I discussed in detail the representative persistent carbenes stabilized by the introduction of various functional groups. In particular, focusing on their frontier orbital energy levels that represent their chemical properties, I performed the computational studies on their electronic properties. Various factors influencing their electrical properties were also considered, such as the ring size effect and the functional group effect.
The third chapter is mainly about the reactivities of NHCs with various oxidants, such as nitric oxide (NO), tetracyanoethylene (TCNE) and ferrocenium hexafluorophosphate (FcPF6). Through the reactions between the NHC and these oxidants, various adduct structures, such as the NHCNO radical (Part 3.2~3.4), the zwitterionic NHC-TCNE adduct (Part 3.5~3.7) and the NHC-Fc adduct salt (Part 3.8~3.10), were successfully obtained and characterized respectively. Their experimental details will be discussed in Chapter 3.
In the fourth chapter, the development of electronically tunable NHCs will be discussed. I focused on some advantages for newly designed NHCs, such as easy synthetic methods, easy structural modification and broad substrate scope. As a result, I could successfully design and synthesize the precursors of coumaraz-2-on-4-ylidene (CONY) and indol-2-ylidene (INDY), and obtain the CONY- and INDY-based transition metal complexes.