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

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.

Regiodivergent Addition of Phenols to Allylic Oxides: Control of 1,2‐ and 1,4‐Additions for Cyclitol Synthesis

“…The 1,4-addition is accessible using the (–)- 1 oxide ( 7 ; equation 2) via attack at the pro-( S ) carbon. The selectivity of this addition is consistent with Lloyd-Jones and co-workers’ studies 11 of the proposed transition states of the asymmetric allylic alkylation of cycloalkenyl esters.…”

Palladium-Catalyzed Stereoselective Allylic Alkylation of Lithium Enolates

“…In addition, the lithium salt might also influence the aggregation of the transition metal catalyst. 11 An additional improvement comes from a change of the leaving group in the allylic substrate (entries 7 and 8). Remarkably, no allylation occurs in the absence of lithium chloride at -78°C (entries 9 and 10).…”

New Developments in Stereoselective Palladium-Catalyzed Allylic Alkylations of Preformed Enolates

“…It turned out that the presence of lithium chloride was crucial to the stereoselectivity, an effect that can be attributed to the disaggregation of the preformed enolate 7 or to the structure of the palladium complex. 50 The application of this protocol to cyclopentanone leads to the formation of ketone 75b in 93% ee.…”