Coordination of the Trost modular ligand to palladium allyl fragments: Oligomers, monomers, and memory effects in catalysis

G C Lloyd-Jones, S C Stephen, I J S Fairlamb, A Martorell, B Dominguez, P M Tomlin, M Murray, J M Fernandez, J C Jeffery, T Riis-Johannessen, T Guerziz

Research output: Contribution to journalArticle (Academic Journal)peer-review

44 Citations (Scopus)

Abstract

Reaction of the C-2-symmetric "Trost modular ligand" with cationic Pd(II) allyl fragments allows isolation of air- and bench-stable procatalysts for the asymmetric allylic alkylation of racemic cycloalkenyl esters. In solution, three distinct complexation modes are observed. When mixed in a ligand/Pd ratio of 1/2, a binuclear bis-P,O-chelate complex is generated. This species does not induce enantioselectivity in the reaction. In contrast, with a ligand/Pd ratio of 1/1, a highly enantioselective, PP-coordinated procatalyst system is generated in which there are two basic coordination modes: monomeric and oligomeric. The monomeric form is mononuclear and exists as two 13-membered chelates, isomeric through loss of C-2-symmetry in the ligand. The oligomeric form is polynuclear and forms chains and rings of alternating ligand and cationic Pd(allyl) units, one of which was identified by single-crystal X-ray diffraction. In solution, the monomeric and oligomeric species are in dynamic equilibrium with populations and interconversion rates controlled by concentration, temperature, and counterion. Isotopic desymmetrization analysis Suggests that the monomer-oligomer equilibrium plays a Crucial role in both the selectivity and efficiency of the asymmetric allylic alkylation reaction.

Original languageEnglish
Pages (from-to)589-601
Number of pages13
JournalPure and Applied Chemistry
Volume76
Issue number3
Publication statusPublished - Mar 2004

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