Browsing by Author "Khoza, Khethukuthula Petronella Mallecia."

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Synthesis, structural characterization, and subsequent applications of salicyldiminato Pd (II) complexes as efficient catalysts for C-C bond-forming reactions.
(2024) Khoza, Khethukuthula Petronella Mallecia.; Sithebe, Siphamandla.
Palladium-catalysed cross-coupling reactions (i.e., Heck, Stille, Liebeskind Strogl, and Suzuki-Miyaura) are perceived as one of the most prominent, successful, and widely used strategies for the construction of complex organic compounds such as biaryls from simple precursors. Amongst the known carbon-carbon bond-forming reactions, the Suzuki-Miyaura (SM) cross-coupling reaction has emerged as one of the most efficient and powerful methodologies due to its high versatility, use of non-toxic reagents, mild reaction conditions, broad substrate scope, and functional group tolerance. The SM cross-coupling has not only greatly changed the landscape of the field of organic chemistry, but also profoundly impacted pharmaceuticals, agrochemicals, natural products, material science, and various related industries. Stable supporting ligands are key in the catalytic activities of Pd complexes. The design and development of supporting ligands that will result in stable Pd complexes while maintaining high catalytic activity and catalytic turnover through stereo-electronic effect alteration are highly desirable. With the aim of developing stable highly catalytic Pd complexes, our research focus is directed towards the synthesis, characterisation, and application of novel N, O-bis Schiff base chelated Pd (II) complexes (PdC1 - PdC7) from Schiff base ligands (L1 - L7). The synthesis of (E)-2-((phenylimino)methyl) phenol (L1), (E)-2-((4 tolylimino)methyl) phenol (L2), (E)-2-(((4-ethylphenyl)imino)methyl) phenol (L3), (E)-2-(((4-methoxyphenyl) imino)methyl) phenol (L4), (E)-2-(((4-fluorophenyl)imino)methyl) phenol (L5), (E)-2-(((4-bromophenyl)imino)methyl) phenol (L6), and (E)-4-((2-hydroxybenzylidene)amino) benzonitrile (L7) was achieved in excellent yields (80% - 94%) through a condensation reaction of salicyldehyde with various electron-withdrawing and donating para-substituted anilines. The reaction between L1 - L7 and Palladium acetate Pd (OAc)2) afforded PdC1 - PdC7 with yields ranging from 78% - 95%. The successful synthesis of L1 - L7 was verified by NMR, FTIR, and MS analyses, while all complexes (PdC1 - PdC7) were characterised using NMR, FTIR, MS, and Single X-ray crystallography. However, a comprehensive X-ray data report was only obtained for PdC2, as the X-ray crystallographic structures of PdC3 - PdC7 proved difficult to solve. The crystallographic analysis of PdC2 revealed that L2 is coordinated to the Pd metal center in a bidentate mode via the N-donor of the imine bond and the O-donor of the phenolic group. The complex displayed a distorted square planar geometry and crystallised in a monoclinic crystal system with a P21/c space group. All the complexes were tested for catalytic activity in the SM cross-coupling reaction between sodium (trihydroxy)phenylborate salt and 4-bromotoluene. The reactions were catalysed with a catalyst loading of 2 mol% for 24 hours in ethanol, yielding 18% - 32% of the desired biaryl product. Motivated by the successful cross-coupling reaction between sodium (trihydroxy)phenylborate salt and 4-bromotoluene, we opted to broaden the application scope of the complexes using the established reaction conditions in SM cross-coupling acylation reaction. We used the most active catalysts, PdC2 and PdC6 for constructing biaryl ketones through the reaction of benzoyl chloride with sodium (trihydroxy)phenylborate salt in toluene for 24 hours. The reactions yielded 15% and 30% of the desired products, respectively.