Reactivity of N-Heterocyclic Carbenes for Radical Stabilization and Small Molecule Activation

DoctorSince their first isolation in 1990s, N-heterocyclic carbenes (NHCs) have been widely used not only to support transition metal complexes for useful applications, but also to activate small molecules or stabilize reactive species. Although general carbene species are unstable due to the incomplete octet structures, the NHCs can be easily prepared, isolated, and utilized for many applications in catalysis. NHCs has been also investigated as platforms to stabilize various main group elements and to activate small molecules like a transition metal center. The unique electronic property of some NHCs enables the formation of unprecedented complexes with small molecules such as CO, H2, NH3, or NO, as well as the cleavage of various inert bonds such as H–H, N–H or C–H. On the other hand, a variety of previously inaccessible organic radicals have been successfully prepared and characterized with an aid of NHCs' steric bulk and π-accepting character. In this thesis, the transition metal-like reactivity of NHCs will be discussed mainly focusing on the radical stabilization and small molecule activation. In the first chapter, general history toward the early development of NHC will be reviewed. Various stable organic radicals derived from NHC structures will be also discussed in the first chapter, including the synthesis and properties, as well as some applications of the NHC-derived organic radicals. The second chapter is about the steric modification of NHC center using bulky functional groups. NHCs with high steric bulk were designed using imidazo[1,5-a]pyridine scaffold and synthesized in a facile and efficient way, and their Cu(I), Ag(I), and Au(I) chloride complexes were also synthesized. The details about the synthesis and steric properties of the compounds will be discussed. The third chapter is about the C–F bond activation using NHCs which resembles the oxidative addition reactivity of metal complexes. An abnormal carbene precursor and its Ag(I) and Au(I) chloride complexes were synthesized through unexpected C–F bond activation of octafluorotoluene using an NHC. The synthetic method and the details about the C–F bond activation of fluoroarenes using NHCs will be discussed. In the fourth chapter, the reactivity of NHC derived iminoxyl radicals will be discussed. While nitric oxide is a well-known ligand to react with transition metal center, the reaction between NHC and nitric oxide generates NHCNO adduct, which can be described as an iminoxyl radical. This radical was found to react with silicon, carbon, and boron electrophiles to generate the corresponding adducts. The disproportionation or oxygen atom transfer reactivity of the iminoxyl radical with proton sources or oxygen acceptors, respectively, will be also discussed. In the fifth chapter, 3-azabutadiene compounds synthesized from NHCs and isocyanides will be studied as a π-basic component in classic and frustrated Lewis pair chemistry. The 3-azabutadiene compound reacts with aluminum or gallium chlorides to generate N-adduct or C-adduct, respectively, and showed one electron oxidation upon treatment with tris(pentafluorophenyl)borane or trityl cation. Also, frustrated Lewis pair composed of the 3-azabutadiene compound and triphenylborane shows a fundamental reactivity such as O–H and N–H bond cleavage