879689-47-1Relevant academic research and scientific papers
Catalyst life in imidazolium-based ionic liquids for palladium-catalysed asymmetric allylic alkylation
Guerrero-Rios,Martin
, p. 7533 - 7539 (2014/05/20)
A Pd/(S)-BINAP system was successfully applied to the asymmetric allylic alkylation of rac-1,3-diphenyl-3-acetoxyprop-1-ene (I) using imidazolium-based ionic liquids (ILs) attaining up to 225 h-1 TOF and 88% ee of the (R)-product. Although the system was barely active in the recycling experiments, the catalyst life was confirmed after recharging the system with substrate/reactants resulting in an alkylated product. In the latter case, the conversion rates and enantiomeric excesses were similar or lower compared to those in the first cycle. In order to explain the observed catalyst performance in the recycling as well as in the recharging experiments, we investigated the reactivity between the catalyst precursors, substrate and reactants in ILs. We were able to identify the species involved in the catalytic reactions under various conditions by means of 31P NMR analyses. Allylpalladium intermediates (3) were found to be the active and selective species at a high substrate concentration. When the substrate was consumed, competing reactions took place leading to different palladium complexes. [PdCl(NHC Bu,Me)((S)-BINAP)]Cl (4), together with [Pd((S)-BINAP)2] (5), were recognised as the species responsible for the loss of activity, meanwhile, the decrease in enantioselectivity was accounted for by the formation of mixed (NHC)(monophosphine)-palladium species. This journal is the Partner Organisations 2014.
A highly active palladium catalyst for intermolecular hydroamination. Factors that control reactivity and additions of functionalized anilines to dienes and vinylarenes
Johns, Adam M.,Utsunomiya, Masaru,Incarvito, Christopher D.,Hartwig, John F.
, p. 1828 - 1839 (2007/10/03)
We report a catalyst for intermolecular hydroamination of vinylarenes that is substantially more active for this process than catalysts published previously. With this more reactive catalyst, we demonstrate that additions of amines to vinylarenes and dienes occur in the presence of potentially reactive functional groups, such as ketones with enolizable hydrogens, free alcohols, free carboxylic acids, free amides, nitriles, and esters. The catalyst for these reactions is generated from [Pd(η3-allyl)CI]2 (with or without added AgOTf) or [Pd(CH3CN)4](BF 4)2 and Xantphos (9,9-dimethyl-4,5-bis(diphenylphosphino) xanthene), which generates complexes with large P-Pd-P bite angles. Studies on the rate of the C-N bond-forming step that occurs by attack of amine on an η3-phenethyl and an η3-allyl complex were conducted to determine the effect of the bite angle on the rate of this nucleophilic attack. Studies on model η3-benzyl complexes containing various bisphosphines showed that the nucleophilic attack was faster for complexes containing larger P-Pd-P bite angles. Studies of substituted unsymmetrical and unsubstituted symmetrical model η3-allyl complexes showed that nucleophilic attack on complexes ligated by Xantphos was faster than on complexes bearing ligands with smaller bite angles and that nucleophilic attack on unsymmetrical allyl complexes with larger bite angle ligands was faster than on unsymmetrical allyl complexes with smaller bite angle ligands. However, monitoring of catalytic reactions of dienes by 31P NMR spectroscopy showed that the concentration of active catalyst was the major factor that controlled rates for reactions of symmetrical dienes catalyzed by complexes of phosphines with smaller bite angles. The identity of the counterion also affected the rate of attack; reactions of allylpalladium complexes with chloride counterion occurred faster than reactions of allylpalladium complexes with triflate ortetrafluoroborate counterion. As is often observed, the dynamics of the allyl and benzyl complexes also depended on the identity of the counterion.
