Analysis of stereochemical convergence in asymmetric pd-catalysed allylic alkylation reactions complicated by halide and memory effects.
A quantitative two-term description of memory effects arising in Pd-catalysed allylic alkylation reactions formally proceeding through 'meso'-type pi-allylpalladium intermediates is presented. The utility of this description (stereochemical convergence (sc) and global enantiomeric excess (ee(g))) is demonstrated by application to a series of Pd-catalysed allylic alkylation reactions involving racemic cyclopentenyl esters. Analysis of such reactions, by employing a range of enantiomerically pure monophosphine ligands, reinforces the conclusion that selectivities (enantiomeric excess (ee) values) obtained under standard 'benchmark' type conditions may be very misleading when powerful memory effects are operative. However, by comparison of sc and ee for a given ligand/solvent combination under a range of related conditions, one may predict the limiting ('latent') selectivity that will be obtained when the memory effect is negated. This technique is exemplified with one particular ligand (4 b, 'MAP') for which a number of strategies were employed to find conditions that negate the memory effect and reveal the limiting selectivity of the ligand. These conditions give a higher limiting global selectivity than that obtainable by using standard diastereoisomer equilibration methods such as added halide. Thus, the analysis of sc versus ee(g) also allows subtle changes in selectivity to be discerned. The difference in limiting selectivity (chloride versus non-chloride conditions) is proposed to arise through the nucleophilic attack of neutral monodentate versus cationic bidentate MAP (4 b) or MOP (4 a) complexes.