Activation of Propargylic Alcohols by Piano-Stool Ruthenium Complexes Bearing Phosphoramidite Ligands
Doctor of Philosophy
Date of Defense
Eike Bauer, PhD
Various piano-stool ruthenium complexes bearing phosphoramidite ligands have been synthesized and characterized spectroscopically and in some cases structurally. Reaction of phosphoramidite ligands with an appropriate metal precursor gives new piano-stool ruthenium complexes [RuCl(L)(arene)(phosphoramidite)], where L = Cl, PPh3, or others. The novel complexes are tested for their ability to activate propargylic alcohols catalytically as well as stoichiometrically. Specifically, catalytic substitution of propargylic alcohols via allenylidene intermediates is envisioned. Stoichiometric reactions designed to form stable, isolable allenylidenes are sought as well. h6-p-cymene complexes of the type [RuCl2(h6-p-cymene)(phosphoramidite)] activate propargylic alcohols in the reaction with carboxylic acids to form b-oxo esters. The catalytic activity of the complexes is clearly related, in part, to the steric effects of the ligands with the more hindered complexes outperforming their less sterically crowded counterparts. In these complexes the arene ligand has been shown to be labile, dissociating at elevated temperatures or after prolonged times in solution (CH2Cl2, cyclohexane) or in the solid state. The complexes overall were shown to be inactive in reactions involving allenylidene intermediates. h5-arene complexes of the type [CpRuCl(PPh3)(phosphoramidite)] and [(Ind)RuCl(PPh3)(phosphoramidite)] are viable complexes for the activation of propargylic alcohols as well. Upon coordination of a chiral phosphoramidite ligand a new stereocenter is formed at the metal. The diastereoselectivity of complex formation is highly dependent on the steric effects of the incoming phosphoramidite ligand. One complex is formed in diastereomeric purity and forms stable allenylidenes [(Ind)Ru(PPh3)(L)(=C=C=CR1R2)]PF6 (L is a phosphoramidite) in reaction with propargylic alcohols after chloride abstraction using (Et3O)PF6 in CH2Cl2. Electronic catalyst tuning can be achieved via bidentate phosphoramidite ligands utilizing a pyridyl moiety can coordinate in a chelating fashion, favoring the double substitution due to entropic reasons. A potentially general synthetic route to this new class of ligands has been developed. The effectiveness of this method of electronic tuning is still uncertain, as the coordination chemistry of the analogous ligands is dissimilar due to steric reasons. Synthesis of a small library of tuned, bidentate phosphoramidite ligands will give greater insight into the usefulness of this ligand class and will allow further tuning of the catalytic activity of the respective complexes.
Costin, Stephen Andrew, "Activation of Propargylic Alcohols by Piano-Stool Ruthenium Complexes Bearing Phosphoramidite Ligands" (2010). Dissertations. 449.