CNHCNCNHC-Pincer Complexes of Group 9 and Their Catalytic Activities in Homogeneous Catalysis

Abstract

With the interest in N-heterocyclic carbenes (NHCs), this thesis deals with the design and synthesis of CNHCNCNHC-pincer complexes with transition-metals of group 9 and their applications as catalysts in small molecule isomerization and semihydrogenation of alkynes. The details are as follows. In the first chapter, the nucleophilic Meinwald rear range ment of functionalized terminal epoxides into methyl ketones under mild condi tions was realized using [Rh(bimcaHomo)]. An excellent regio- and chemoselectivity is obtained for the first time for aryl oxiranes. In the second chapter, further development of the Meinwald rearrangement was made with the more reactive [Rh(bimcaMe,Homo)] and the substrate scope was extended to internal epoxides. In addition, mechanistic investigations showed that the catalytic cycle follows a β-hydride elimination–reductive elimination pathway after the nucleophilic ring opening of the epoxide. In the third chapter, two methods for the synthesis of the bis(imidazolin-2-ylidene)carbazolide cobalt(I) complex [Co(bimcaHomo)] have been developed. The first route relies on the direct transmetallation of the in situ generated lithium complex [Li(bimcaHomo)] with Co(PPh3)3Cl. The second one is a two-step synthesis that consists of the transmetallation of [Li(bimcaHomo)] with CoCl2 followed by reduction of the Co(II) complex to yield the desired Co(I) complex. The analogous rhodium and iridium complexes were prepared by transmetallation of [Li(bimcaHomo)] or [K(bimcaHomo)] with [M(COD)(μ-Cl)]2. The catalytic activity of [Co(bimcaHomo)] and [Ir(bimcaHomo)] in the epoxide isomerization was tested with and without the presence of H2. When [M(bimcaHomo)] (M = Rh or Ir) were exposed to 1 bar H2 at 80 °C single crystals formed whose X-ray structure analyses revealed the hydrogenation of the N-homoallyl moieties and formation the dimeric hydrido complexes [Ir(bimcan-Bu) (H)2]2 and [Rh(bimcan-Bu) (H)2]2. In the fourth chapter, the selective isomerization of various N-Boc protected terminal aziridines to enamides is presented using the highly reactive nucleophilic [Rh(bimcaMe,Homo)] with the Lewis acid LiNTf2 as co-catalyst under moderate conditions. The reaction proceeds smoothly with only 1 mol% catalyst loading and excellent yields were achieved. An intermediate containing an enamide with a non-conjugated terminal C=C double bond was detected during the course of the reaction, which isomerizes to form the thermodynamically favored 2-amido styrene. Mechanistic insight is gained based on these observations. In the last chapter, the semihydrogenation of both terminal and internal alkynes was developed with [Co(bimcaHomo)]. Catalytic reactions were performed under ambient conditions. Several terminal and internal alkynes were semihydrogenated to yield olefins without over-reduction. In the case of internal alkynes, cis-olefins were achieved exclusively after semihydrogenation. Full reduction of alkynes was realized by tuning the reaction conditions.