Volume 145, 2010

Interrogation of a dynamic multi-catalyst ensemble in asymmetric catalysis

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 Pdallyl 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 Pdallyl 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.

Supplementary files

Article information

Article type
Paper
Submitted
20 May 2009
Accepted
17 Jun 2009
First published
17 Sep 2009

Faraday Discuss., 2010,145, 27-47

Interrogation of a dynamic multi-catalyst ensemble in asymmetric catalysis

J. Eastoe, I. J. S. Fairlamb, J. M. Fernández-Hernández, E. Filali, J. C. Jeffery, G. C. Lloyd-Jones, A. Martorell, A. Meadowcroft, P. Norrby, T. Riis-Johannessen, D. A. Sale and P. M. Tomlin, Faraday Discuss., 2010, 145, 27 DOI: 10.1039/B910022A

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements