Alpha-Methyl Functionalization of Electron-Poor Heterocycles and Synthesis of Novel Dipyridine and Phenanthroline Metallocyclic Palladium Complexes.
Date of Award
Doctor of Philosophy (PhD)
The (alpha)-methyl functionalization of 2,9-dimethyl-1,10-phenanthroline was conducted via a four step procedure to give 2,9-bis(chloromethyl)-1,10-phenanthroline in high yield. Attempts to prepare this compound via a one-step free radical halogenation using NCS were unsuccessful, however, a comparative evaluation of this approach in other heterocycles demonstrated the potential for direct mono-chlorination. Additional functionalization procedures were investigated via the olefination of 2,2'-dipyridine and 1,10-phenanthroline. Following the oxidation of 6,6'-dimethyl-2,2'-dipyridine to the dialdehyele, standard Wittig reaction conditions were employed to obtain 6,6'-divinyl-2,2'-dipyridine, however, yields were low because of aldehyde insolubility. This method was replaced by the Wittig-Horner reaction utilizing heterocyclic phosphonates as precursors to olefin formation. This approach led to the formation of hydroxy- and methoxy-substituted vinyl derivatives of dipyridine and phenanthroline. The additional degree of stability imposed by the heterocycle on the phosphorus stabilized anion is the cause of these unexpected products. A series of cyclometallated palladium(II) complexes was prepared possessing overal cis geometry in which partial coordination to the metal is achieved via an sp('3) carbon anionic bond. The cis ligands were synthesized by treatment of the appropriate chloromethyl heterocycle with alkyl malonates under basic conditions. Complexation was subsequently conducted via combination of the ligand and PdCl(,2) in acetonitrile with anhydrous K(,2)CO(,3) as base. The phenanthroline ligands potentially capable of forming the 5(.)5(.)5 fused tricyclic ring system yielded only one C-Pd bond. The X-ray structure demonstrated that the phenanthroline backbone is too rigid to permit the distortions necessary for accommodation of the second C-Pd bond. Similarly, the cis 6(.)5(.)6 dipyridine and phenanthroline ligands were capable of only one C-Pd bond, however, X-ray data showed that steric interactions precluded formation of the bis-C-Pd bonded species in this case. The 5(.)7(.)5 and 5(.)6(.)5 ligands obtained via insertion of an ethano or carbonyl bridge between the 2,2'-dipyridine linkage afforded the bis-C-Pd bonded complexes. Their unusual stability is attributed to an increased ligand flexibility which allows for a greater variation of metal interactions. The infrared spectral analyses of these and other C-Pd bonded complexes were conducted to establish the C-Pd stretching frequency.
Kiefer, Garry Edward, "Alpha-Methyl Functionalization of Electron-Poor Heterocycles and Synthesis of Novel Dipyridine and Phenanthroline Metallocyclic Palladium Complexes." (1984). LSU Historical Dissertations and Theses. 4021.