Interrogation of a dynamic multi-catalyst ensemble in asymmetric catalysis

Julian Eastoe, Ian J. S. Fairlamb, Jesus M. Fernandez-Hernandez, Emane Filali, John C. Jeffery, Guy C. Lloyd-Jones*, Aina Martorell, Antony Meadowcroft, Per-Ola Norrby, Thomas Riis-Johannessen, David A. Sale, Paula M. Tomlin

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The 'Trost Standard Ligand' (2) is a chiral diphosphine ligand that distinguishes itself by the high selectivity it induces in the Pd-catalysed reactions of allylic substrates that generate slim cyclic or small linear intermediates. However, a range of unusual features, including memory effects, inverse dependence of selectivity and rate on catalyst concentration, high sensitivity to counter-ion, particularly chloride, and decreasing enantioselectivities at lower temperatures, are often encountered, thus requiring considerable optimisation of reaction conditions to attain optimum selectivity. These features can be accounted for by a model involving a dynamic multi-catalyst ensemble. To gain evidence for this model, the manner in which diphosphine 2 interacts with Pd-allyl cations, and in particular the higher-order systems it generates, has been investigated by use of NMR, isotopic labelling, polarimetry, UV, neutron scattering, X-ray crystallography and molecular modeling. Ligand 2 coordinates to Pd-allyl cations to generate a mononuclear P,P-chelate. This is found to readily form non-chelate oligomers, present in a range of forms, including rings, for which high homochiral selectivity in oligomerisation is demonstrated by the technique of pseudoenantiomers. All of these species are in relatively rapid equilibrium, with half-lives for interconversion in the range 2-6 s. Higher-order aggregation is also detected, and thus at even moderate concentrations (>50 mM) large rod-like aggregates are formed.

Original languageEnglish
Pages (from-to)27-47
Number of pages21
JournalFaraday Discussions
Volume145
DOIs
Publication statusPublished - 2010

Keywords

  • ALLYLIC ALKYLATION REACTIONS
  • TROST MODULAR LIGAND
  • ENANTIOSELECTIVE ALLYLATION
  • KINETIC RESOLUTION
  • WORKING MODEL
  • SELECTIVITY
  • RECEPTOR
  • COMPLEX
  • ION

Cite this