| Dirhodium complexes have a long-standing importance in C-H functionalization chemistry, but little is known about the reactive intermediates on which this chemistry relies. The first part of this dissertation focuses on intermediates in catalytic C-H amination chemistry: In Chapter 3, the catalyst Rh 2(esp)2 (esp = &agr;,&agr;,&agr;',&agr;'-tetramethyl- 1,3-benzenedipropanoate) is studied spectroscopically and electrochemically to gain insight into its robust nature. These studies implicate the stability of a one-electron oxidized Rh2II,III state in the high level of activity reported for this catalyst. In Chapter 4, two complexes that are structurally analogous to Rh2(esp)2 with chelating dicarboxylate ligands based on a resorcinol backbone are synthesized. These new complexes are electronically dissimilar to Rh2(esp)2, since the ligands are more readily oxidized than the Rh2 core, as observed by cyclic voltammetry. In catalytic studies, it is shown that these new catalysts perform equally well in intramolecular amination as parent catalyst Rh2(esp) 2, but fail to achieve the same level of efficiency in intermolecular amination. The results of this study further implicate the importance of mixed-valency in intermolecular amination. In Chapter 5, the synthesis and catalytic activity of a new purposefully mixed-valent Rh2-amidate catalyst is reported. The new catalyst, Rh2(espn)2Cl (espn = &agr;,&agr;,&agr;',&agr;'-tetramethyl-1,3-benzenedipropanamidate), achieves higher turnover numbers in intramolecular amination than parent catalyst Rh2(esp)2. Chapter 6 is focused on the exploration of intermediates in Rh2-mediated carbenoid chemistry. Methods for preparing a metastable solution of a donor/acceptor carbene complex of Rh 2(TPA)4 (TPA = triphenylacetate) are described; the carbenoid intermediate is characterized by NMR, resonance Raman and X-ray absorption spectroscopies, all corroborated by DFT analysis.;In summary, the work presented in this dissertation establishes the importance of mixed-valency for increased turnover in C-H amination catalysts. Isolation of the first Rh2-carbenoid provides experimental support for the previously theoretically-based catalytic reactivity of Rh2-carbenoids. |
