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Ferrocene ligands; Chiral PN Ligands; Pd-catalysed asymmetric allylic alkylation; Mechanistic studies
Abstract:
The preparation of four novel ferrocenylphosphinamine ligands, possessing both planar and central chirality, is described. The absolute configuration of two of the ligands was confirmed by X-ray crystallographic analysis. The palladium complexes of these new ligands were applied in the allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate to study the effect of planar chirality and/or the trimethylsilyl group. Quantitative yields with ees of up to 77% were obtained with the ligand that bears both (R)-planar and (R)-central chirality. Application of its trimethylsilyl analogue gave significantly lower ees, whereas the presence of the trimethylsilyl group in the ligand that bears (S)-planar and (R)-central chirality had a beneficial effect on enantioselectivity. Solution 1H NMR spectroscopic studies of the 1,3-diphenylallyl palladium complexes of two of the ligands revealed the major diastereomer in solution does not give rise to the major product observed in allylic alkylation, assuming nucleophilic attack trans to phosphorus.
