Laying the Groundwork for a Tantalum Dearomatization Agent

Abstract

Chapter 1 provides a brief description of the synthesis of the !-basic dearomatization agents {Os(NH3)5}, {TpRe(CO)L} (L=tBuNC, PMe3, py, MeIm, and NH3) and {TpW(NO)(PMe3)}. It also submits a theoretical explanation for the phenomenon of dearomatization via ! back bonding. The basic principles for the synthesis of a dearomatization agent that have been derived from these past syntheses are discussed in the context of the characteristics that we hoped to design into a new tantalum based dearomatizing metal fragment.Chapter 2 provides a literature review of low valent tantalum chemistry in order to elucidate the synthetic tools that were available for the synthesis of new low valent tantalum species. It also describes multiple pathways that were envisioned for the synthesis of a tantalum dearomatization agent based on these tools. Pathways initially based on high valent tantalum chemistry are also discussed. Chapter 2 also reports the attempts that were made to use the high valent TaCl5 as starting materials for the synthesis of low valent tantalum species. The difficulties and eventual abandonment of this pathway are noted. The synthesis of [Et4N][Ta(CO)6] is discussed and the failure of [Ta(CO)6]to have its ligand set modified by thermal substitution is reported. Chapter 3 focuses on the modification of [Ta(CO)6] -. Chapter 3a describes the photolytic substitution of carbonyl ligands on [Ta(CO)6] -. [Et4N][Ta(CO)5(PPh3)], [Et4N][Ta(CO)5(PMe3)], and [Et4N][Ta(CO)5(4-DMAP)] were synthesized. Electrochemical and infrared data is reported for these species. The substitutional lability of the 4-DMAP ligand of [Et4N][Ta(CO)5(4-DMAP)] was explored. Efforts to synthesize iv 2.8 Experimental 2.9 References Chapter 3 Part A: Ligand Substitution Via Photolysis 3a.1 Introduction 3a.2 Direct Substitution 3a.3 Substitution of Carbonyls for Monodentate ! donors 3a.4 Substitution of 4-DMAP 3a.5 Substitution of Carbonyls of Polydentate ! donors 64 3a.6 Conclusion 72 Part B: Ligand Substitution Via Oxidation vi Figure 3ab.3: Depiction of compound 6. Figure 3ab.4: Depiction of compound 7. List of Schemes Scheme 1.1: Aromatic compounds successfully bound by the {Os(NH3)5} 2+ metal fragment. 3 Scheme 1.2: Ratio of phenol tautomers for the free and {Os(NH3)5} 2+ bound phenol Scheme 1.3: [Os] 2+ bound phenol species undergoes electrophilic attack in the presence of maleic anhydride. Scheme 1.4: The reduction of aromatic and non-aromatic species. 9 Scheme 1.5: Synthesis of a tungsten based dearomatization agent.Scheme 2.1: Attempted Tp-Ta(V) syntheses. 23 Scheme 2.2: Attempted reductions and substitutions with TaCl5. 25 Scheme 2.3: Low pressure and high pressure synthesis of [Ta(CO)6] -28 Scheme 2.4: Mono-and Disubstituted [Ta(CO)6]derivatives. Scheme 2.5: Compounds accessible from oxidation of [Ta(CO)6] -. 38 Scheme 2.6: Plan for the synthesis of a tantalum dearomatization agent. 40 Scheme 2.7: Substitutions attempted with [Ta(CO)6] -. 42 Scheme 3a.1: Photochemical reactions of 1 and C-C double bond containing ! acids. 56 Scheme 3a....