Model intermolecular asymmetric Heck reactions catalyzed by chiral pyridyloxazoline palladium(II) complexes

Abstract

The synthesis and characterization of a series of chiral pyridyloxazoline Pd(II) halide complexes, including structural determinations, are described. The use of these compounds, as well as those generated in situ from Pd(OAc) 2 and 2 equiv. of a pyridyloxazoline ligand, in the intermolecular asymmetric Heck arylation of 2,3-dihydrofuran is reported. In general, total yields after 24 h at 40 °C of the kinetic and thermodynamic products, 2-phenyl-2,5-dihydrofuran and 2-phenyl-2,3-dihydrofuran, respectively, were low (12–28%), while e.e.s (when PhOTf was used as the aryl source) were low (4–29%) for the kinetic and moderate (23–60%) for the thermodynamic product. Both yield and e.e. were compromised by facile catalyst decomposition. The higher e.e.s found for the thermodynamic than for the kinetic product imply a kinetic resolution. When PhI was used as the aryl source, racemic product was invariably generated, which strongly indicated that asymmetric induction was effected through cationic species in the reaction cycle.