Chelation control in the intramolecular addition of allylsilanes to carbonyl electrophiles

Palladium-catalyzed cyclization of allylsilanes with nucleophilic displacement of the silyl group

Allylsilanes containing hydroxy or tosylamide groups undergo palladium(II)catalyzed cyclization to afford derivatives of tetrahydrofuran, piperidine, and pyrrolidine. This catalytic reaction proceeds through an (eta3-allyl)palladium intermediate that is generated by allylic displacement of the silyl group of the allylsilane precursors. The internal nucleophilic attack on the (eta3-allyl)palladium intermediates proceeds with high chemo- and regioselectivity. Benzoquinone and copper(II) chloride can be used for regeneration of the palladium(II) catalyst precursor. Mechanistic studies revealed that the copper(II) chloride reoxidant also activates the (eta3-allyl)palladium intermediate towards nucleophilic attack. Kinetic studies on the formation of the (eta3-allyl)palladium intermediates showed that the reaction rate is highly dependent on the concentration of chloride ligand and the solvent. The structure and reactivity of the key intermediates of the palladadesilylation process were studied by density functional theory (DFT) calculations, which showed that coordination of the electrophilic palladium(II) catalyst precursor to allylsilanes leads to a relatively weak beta-silicon effect. The DFT studies also indicate that the cleavage of the carbon-silicon bond takes place by coordination of a chloride ion to the silicon atom.