Nancy W. Eilerts

Bisphenol ligands in the stereocontrol of transformations catalyzed by titanium and tungsten

Abstract Bisphenol ligands have a rich coordination chemistry in the early transition elements. 1,1′-Bi-2-naphthols are of particular interest because of their continued utility in controlling the stereochemistry of metal catalyzed processes. In several projects in our laboratory, we have endeavored to examine the potential scope and mechanism by which modifications of bisphenolate ligands can influence the stereoselectivity of electrocyclic reactions. In this paper we discuss two of these studies: an examination of stereochemically modified ROMP precatalysts and a study of well characterized binaphtholate titanium chloride Lewis acid catalysts.

The role of two-component catalysts containing chelating bisaryloxide ligands in controlling the stereochemistry of the metathesis polymerization of norbornene

Abstract The ring-opening metathesis polymerization of norbornene and 5,5-dimethylnorbornene has been studied with a range of tungsten(VI) ring-opening metathesis polymerization procatalysts containing chelating diolate ligands, including catechols, 1,8-dihydroxynaphthalene, 1,1′-bi-2-naphthol, biphenanthrol and bis(2-hydroxyphenyl) methane. One class of these catalysts, W(X)(OArO)Cl 2 (THF) (X = O or NArMe 2 ) was produced via alcoholysis reactions between W(X)Cl 4 and the respective diol. Polymers of norbornene produced with these catalysts showed a reasonable correlation between ring size and cis -selectivity. This effect is most likely steric in origin. Analyses of the tacticity of poly-5,5-dimethylnorbornene showed a correlation betwen high cis -olefin content and syndiotacticity, even when produced with catalysts containing asymmetric chelating diolate ligands.

4,4′-Biphenolate complexes of titanium and zirconium

Abstract 4,4′-Biphenolate ligands (R4BIPOH2) bearing methyl or tert-butyl substituents in the 3,3′,5,5′-positions participate in alcoholysis reactions with metal precursors, including TiCl4, Ti(OiPr)4 and Cp2MCl2. The tetra-tert-butyl derivative shows evidence of an intermediate monotitanium species (Bu4BIPOH)Ti(OiPr)3 during titrations of the ligand with Ti(OiPr)4, while (Bu4BIPO)(TiOiPr)3)2 exists as a simple monomeric molecule in solution. (Me4BIPO)(Ti(OiPr)3)2 shows evidence of aggregation into oligomeric species in solution, although its insolubility hampered the complete characterization of these species. A reaction between the d1 complex TiCl3(THF)3 and 3,3′,5,5′-tBu4-4,4′-biphenoquinone produced (Bu4BIPO)[TiCl3(THF)]2, while dirct reactions between TiCl4 and Bu4BIPOH2 in toluene resulted in the isolation of the dealkylated product, (Bu2BIPO)[TiCl3(Et2O)]2 (where the tBu groups occur in the 3,3′-positions), after treatment with ether. Refluxing Me4BIPOH2 and Cp2MCl2 (M = Ti or Zr) in toluene produced bismetallocene chloride complexes.

Dimetallacyclobutadiene derivatives: Catalysts or catalyst precursors for the ROMP of norbornene?

Abstract Metallacyclobutadiene complexes with the formula (Me3SiCH2)4M2(μ-CSiMe3)2 (M = Ta or W) induce the ring-opening metathesis polymerization (ROMP) of norbornene. The tungsten compound acts as a stand-alone catalyst, polymerizing norbornene with a first-order dependence on both W2 and monomer. In contrast, the tantalum complex requires oxygen as an initiator, and generates an as yet undefined catalyst that shows a zero-order dependence on the monomer concentration.