Interrogation of a dynamic multi-catalyst ensemble in asymmetric catalysisElectronic supplementary information (ESI) available: Experimental details. CCDC reference number 733391. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b910022a

• Main Authors: Eastoe, Julian, Fairlamb, Ian J. S, Fernández-Hernández, Jesús M, Filali, Emane, Jeffery, John C, Lloyd-Jones, Guy C, Martorell, Aina, Meadowcroft, Antony, Norrby, Per-Ola, Riis-Johannessen, Thomas, Sale, David A, Tomlin, Paula M
• Format: Journal Article
• Language: English
• Published: 01.01.2010
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/b910022a

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.