Activation of Propargylic Alcohols by Piano-Stool Ruthenium Complexes Bearing Phosphoramidite Ligands

Various piano-stool ruthenium complexes bearing phosphoramidite ligands have been synthesized and characterized spectroscopically and in some cases structurally. Reaction of phosphoramidite ligands 41 with an appropriate metal precursor gave new piano-stool ruthenium complexes [RuCl(L)(arene)(phosphoramidite)], where L = Cl, PPh3, or others. The novel complexes were tested for their ability to activate propargylic alcohols catalytically as well as stoichiometrically. Specifically, catalytic substitution of propargylic alcohols via allenylidene intermediates was envisioned. Stoichiometric reactions designed to form stable, isolable allenylidenes were sought as well.;eta6-p-cymene complexes of the type [RuCl2(eta6-p-cymene)(phosphoramidite)] (43, 45) activate propargylic alcohols in the reaction with carboxylic acids to form beta-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 such as 43b 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.;eta5-arene complexes of the type [CpRuCl(PPh3)(phosphoramidite)] (43) and [(Ind)RuCl(PPh3)(phosphoramidite)] ( 111) 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. The best results were obtained for the complexes bearing the ligand 41b (69b, 111b), as they can be isolated in diastereomeric purity (111b forms as a single diastereomer). 111b forms stable allenylidenes [(Ind)Ru(PPh3)(41b)(=C=C=CR 1R2)]PF6 (116) in reaction with propargylic alcohols (5) after chloride abstraction using (Et3O)PF6 in CH2Cl2.;Bidentate phosphoramidite ligands utilizing a pyridyl moiety ( 135) 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 for 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.