Chelation-assisted palladium-catalyzed activation of C-H bonds: A new avenue for carbon-heteroatom and carbon-carbon bond formation.

摘要

Carbon-hydrogen (C-H) bonds are ubiquitous in organic molecules. Utilization of such abundant chemical moieties as functional group equivalents could shorten route to synthetic targets and provide chemists with new disconnections in retrosynthesis. As such regio- and stereoselective functionalization of unactivated C-H bonds has remained one of the major challenges in organic chemistry. The majority of the transition metals have been rigorously examined for their efficacy in transforming unactivated C-H bonds (pK a >35) into useful functional groups or into C-C bonds. Among those metals, palladium is particularly effective in activating both aromatic (sp2) and aliphatic (sp3) C-H bonds. This thesis explores the reactivity of palladium catalysts in both of these areas. The research herein was conducted using directing groups for C-H cleavage with special focus on utilizing simple functionality such as carboxylic acids. Chapter one details different types of directing groups and their utility in a variety of reactions. Chapters two and three contain details of research on C-heteroatom (C-I and C-O) and C-C bond formation, respectively, with palladium acetate as a catalyst. The iodination and acetoxylation reactions proceed under mild conditions and moderate to excellent levels of diastereoselectivity (up to 99.9%) have been observed with both sp 2 and sp3 C-H bonds using oxazoline as the directing group. Mechanistic investigations have been carried out in order to understand the high level of stereoselection and, in this process, a number of palladacycle intermediates have been characterized by X-ray crystallography which led us to assign the absolute stereochemistry of C-H activation. Moreover, the iodination protocol could also be extended to prepare diiodides as intermediates for cyclopropanation which provides a new disconnection approach to construct cyclopropanes. Chapter two discusses C-C bond formation via cross-coupling reactions with organoboron reagents and carbon monoxide using the carboxylic acids as the directing group. Detailed mechanistic investigation along with characterization of intermediate palladacycle formed from sodium toluate have revealed an unprecendented directing ability of carboxylate groups in which the carbonyl oxygen, rather than the O-anion, directs palladium for C-H cleavage.