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, and T. Guerziz
The Bristol Centre for Organometallic Catalysis, School
of Chemistry, The University of Bristol, Cantock ’s Close, Bristol,
BS8 1TS, UK
Abstract: Reaction of the C2-symmetric "Trost
modular ligand" with cationic Pd(II) allyl fragments allows isolation
of air- and bench-stable pro-catalysts 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, P,P-coordinated pro-catalyst
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 C2-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.
*Plenary and invited lectures presented at the 12th
International Symposium on Organometallic Chemistry Directed Towards
Organic Synthesis (OMCOS-12), Toronto, Ontario, Canada, 6-10 July 2003.
Other lectures are published in this issue, pp.
453-695.
