Towards Polymer Supported Iridium Borylation Catalysts for Organic Synthesis

Abstract

The first chapter contains a detailed explanation of the borylation of arenes using iridium catalysts with different ligands. Phosphine, N,N-aryl and carbene ligands have been studied previously for the borylation of substituted aromatic and heteroaromatic compounds. The importance of polymer supported catalysts is shown. Examples of polymer supported iridium catalysts for the borylation of arenes are given. Chapter 2 discusses the preparation of 2-arylquinoline and quinolone derivatives. These were prepared by reaction of 3-methoxy and 3,5-dimethoxy aniline with malonic acid to generate the 2,4-dichloro quinoline derivatives. These in turn were then coupled with a range of aryl boronic acids in Suzuki-Miyaura cross-coupling reactions. A study documents the borylation selectivity of 2-(4’-methoxyphenyl)-4-chloro-7-methoxyquinoline 230. Various conditions in Stille cross-coupling reaction were used to prepare nonsymmetrical 4,4’-substituted-2,2’-bipyridine derivatives 250 and 285 in chapter 3. These were prepared through coupling of stannyl pyridine 266 with 2-chloro- and bromo-4-substituted pyridine derivatives in presence of metal salts. Ligands 250 and 285 were evaluated in the borylation of m-xylene and compared to the activity of the literature standard ligand 4,4’-di-tert-butyl-2,2’-bipyridine dtbpy 22. Chapter 4 describes the preparation of 2,4,6-substituted pyridine derivatives. These compounds were prepared by one of two methods. The borylation of 2-chloro-4-substituted pyridine derivatives afforded the corresponding boronate esters, which were then coupled with a range of aryl halides. This was followed by an aromatic nucleophilic substitution reaction with a range of amines. Alternatively, aromatic nucleophilic substitution of 2-chloro-4-substituted pyridine derivatives with amines afforded the corresponding 2-aminopyridines. Subsequent borylation of these subtrates followed by Suzuki-Miyaura cross-copuling was also an effective strategy. Chapter 5 reports the synthesis of symmetrical phenanthroline 347 using the Altman protocol. Attachment of a linker to enable coupling to a polymer support afforded modified ligand 367. Phenanthrolines 347 and 367 were evaluated in the borylation of m-xylene compared to the commercially available 3,4,7,8-tetra-methyl-1,10-phenanthroline tmphen 66. The commercially available MCM-41 was chosen as a suitable polymer for the polymer supported iridium catalyst. Different strategies were investigated to attach the phenanthroline ligand to the polymer. These strategies involved attaching an amine linker to the polymer before coupling it with lithium phenanthroline carboxylate 368. Chapter 6 provides all the experimental details.