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N,E heteroleptic ligands in the Heck reaction and other carbon - carbon bond formation processes


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Jones, Roderick C 2009 , 'N,E heteroleptic ligands in the Heck reaction and other carbon - carbon bond formation processes', PhD thesis, University of Tasmania.

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The development of a simple 'one-pot' synthesis for the preparation of substituted 2- organothiomethyl- and 2-organoselenomethylpyridines has allowed the synthesis of six new ligands (4-MepyCH₂SMe, 6-PhpyCH₂SMe, 4-MepyCH₂SPh, 6-PhpyCH₂SPh, 4-MepyCH₂SePh, and 6-PhpyCH₂SePh), together with an improved and more direct route to known ligands (pyCH₂SMe, 6-MepyCH₂SMe, pyCH₂SPh, 6-MepyCH₂SPh, pyCH₂SeMe, pyCH₂SePh, and 6-MepyCH₂SePh). These bidentate heteroleptic ligands were successfully used in the synthesis of dihalogenopalladium(II) and platinum(II) complexes, which were isolated in high yield and were characterised as predominantly square planar complexes of the form \(cis\)-MX₂(N,E) (X = halogen, E = S, Se).
X-ray diffraction techniques allowed a systematic structural study of the dihalogenopalladium(II) and platinum(II) complexes formed, producing several sets of isomorphous complexes giving a detailed insight into the coordination chemistry of palladium(II) and platinum(II). The structural studies confirm the formulation of new ligands and complexes, and identified bond length trends Pd-N > Pt-N and Pd-E > Pt-E matching earlier reports of isomorphous organometallic complexes of palladium and platinum, but the observed trend Pd-Cl\(_N\) < Pt-Cl\(_N\) is opposite to a literature report for an organometallic complex.
The influence of a 6-substituent in the pyridine ring on structural chemistry is evident. Replacement of hydrogen by methyl results in a significant Me\(^{...}\)X steric interaction which can be relieved by formation of a larger dihedral angle between the pyridine ring and the coordination plane for \(cis\)-PtX₂(N,S), but formation of an array of different structures for palladium involving \(trans\)-PdX₂(N,E) motifs in the dimer [PdI₂(μN,Se)]₂ (N~Se = 6-methyl-2-phenylselenomethylpyridine), the trimer [PdCl₂(N,S)]₃, and \(trans\)-PdI₂N₂ (monodentate ligand) in PdI₂(N,S) (N,S = 6-methyl-2-phenylthiomethylpyridine). If the substituent on the 6-position of the pyridyl ring is a phenyl group, \(ortho\)-metallation of the ligand occurs producing complexes in which a tridentate heteroleptic ligand [C,N,S] is present.
The catalytic performance of the dichloropalladium(II) complexes synthesised in this study were tested in the Heck and Suzuki reactions. The structurally simple and chemically stable pre-catalyst PdCl₂(pyCH₂SMe) gave the highest yields achieving TONs up to 10⁶ for aryl iodides and bromides in the Heck reaction. Closely related selenium containing ligands, or ligands with a 6-methyl substituent on the pyridyl ring, or cyclopalladated species, are much less active. Activation of aryl chlorides using PdCl₂(pyCH₂SMe) was also achieved using Bu\(^n\)₄NCl as a solvent, giving higher yields for both 4-chloroacetophenone and chlorobenzene than that obtained under the same conditions using N,N-dimethylacetamide as a solvent.
Ligand synthesis for the N,S-system was developed to include a ligand able to be readily attached to organic polymers via reaction of the hydroxyl group in 4-(4-hydroxypheny1)- 2-methylthiomethylpyridine with benzyl halide groups of organic polymers. This ligand was anchored to Merrifield and Wang beads, and a porous monolith of similar formulation, poly(chloromethyl-co-divinylbenzene), followed by coordination to dichloropalladium(II). These three heterogeneous catalysts were successfully tested in Heck and Suzuki reactions and were shown to be highly active for aryl iodides and bromides, including double Heck products with a >99% stereoselectivity in product formation. Under similar conditions, the heterogeneous catalyst displayed higher activity than the model complex PdCl₂(4-(4-BnOC₆H₄)pyCH₂SMe). A flow-through microreactor was developed, based upon a 250 μm internal diameter capillary containing monolith synthesized in situ and funtionalised with precatalyst in the same manner as beads and bulk monolith, and successfully trialled in the Heck and Suzuki reactions.
"Tetraaryl" products are synthesised in the catalytic reaction of aryl iodides and terminal aryl acetylenes at 120 °C using PdCl₂(pyCH₂SMe) as a palladium(II) pre-catalyst, apparently the first reported case of such products synthesised using Sonogashira conditions. Although a full and extensive study of the identity of the formed products, and possible mechanism for the reaction, are yet to be undertaken, it is of interest that 4-iodonitrobenzene and phenylacetylene gave 1,3,5-tripheny1-2-(4-nitrophenyl)benzene as the sole product in appreciable yield, with X-ray structure analysis confirming the structure and identity of the compound.

Item Type: Thesis - PhD
Authors/Creators:Jones, Roderick C
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Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references

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