Dirhodium Carbenes - The Structure and Stability of Chiral Dirhodium Carbenes & α-Silyl Diazo Compounds as Sequential Cylopropanation/Cross-Coupling Reagents

This thesis is concerned with the examination of the broadly applied and highly effective dirhodium paddlewheel catalysts and their carbene complexes formed via decomposition of diazo compounds. The first project aims at a better structural understanding of Rh2(DOSP)4-, Rh2(BTPCP)4- and Rh2(BNP)4- carbene complexes. NMR analysis of the carbene complexes provided data on the purity of the carbene formation and the stability of the resulting carbenes. The examined donor-donor substituted carbenes of Rh2(DOSP)4 and its derivatives were stable at room temperature for more than 12 h. A comparison of the examined Rh2(DOSP)4 derivatives showed that the purity of the carbene is dependent on the structure of the ligand. The examined carbenes of Rh2(BTPCP)4 and Rh2(BNP)4 were less stable than the Rh2(DOSP)4 carbenes. Further on the ligand geometry of the Rh2(DOSP)4 derivatives Rh2(naphtyl-2-sulfonylprolinate)4 (α,α,β,β) and Rh2((pentafluorophenyl)sulfonylprolinate)4 (α,α,α,α) were identified via X-ray crystal structure analysis. Despite copious experimentation, no single crystal of a dirhodium carbene complex was obtained. The examined donor-donor carbene complexes with DOSP type ligands are stable enough to be isolated as powders, but they lack crystallinity. The second part of this thesis aims at the development of a new synthetic methodology using α-silylated diazo compounds for a sequential dirhodium catalyzed cyclopropanation and palladium catalyzed cross-coupling reaction to access a broad range of substituted cyclopropane rings with predictable and reliable stereochemistry. Besides α-silylated diazo esters, also so far unknown α-silylated diazoacetonitrile was obtained. Both types of diazo compounds undergo dirhodium catalyzed cyclopropanation, in which α-silylated diazoacetonitrile exhibits a high diastereoselectivity (dr = 8:1). The resulting α-silylated cyclopropylnitriles undergo fluoride activated palladium catalyzed cross-coupling with retention of the stereochemistry, giving the first example of a sequential dirhodium catalyzed cyclopropanation, cross coupling with α-silylated diazo compounds. As a side reaction protodesilylation of the starting material occurs. The examined α-silylated cyclopropanylester do not undergo cross-coupling, but rather quantitative protodesilylation. This study shows that α-metallated diazoacetonitriles hold promise as multifunctional building blocks and that further research is warranted to explore the development of a powerful synthetic methodology