On the influence of the bite angle on the allylic alkylation of (E) and (Z) substrates: Loss and retention of double bond stereochemistry

The bite angle of bidentate phosphane Ligands has a pronounced influence on the degree of retention of the double bond geometry of the allylic substra te in the allylic alkylation reaction. To study the effect of the ligand on the regioselectivity, (Z)- and (E)-pent-2-enyl acetate were used as substr ates. The alkylation of substrates with an (E) conformation of the double b ond results in the preferential formation of the linear (E) product. A larg er bite angle of the Ligand results in an increase of the regioselectivity to >98% far the Six-antphos ligand. Analogously, the alkylation of (Z) subs trates results in the formation of the linear (Z) product. Remarkably, for (Z) substrates, a larger bite angle of the ligand leads to an increased reg ioselectivity for the formation of the branched product instead of the line ar product, up to 47.5% for Six-antphos. The observed regioselectivities ar e rationalized in terms of: a) a competition between syn-anti isomerization and alkylation, and b) a combination of steric and electronic effects in t he transition state of the reaction. For all Ligands tested, the reaction i s faster for the (E) than for the (Z) substrate. However, competition exper iments using the Six-antphos Ligand show a relatively fast reaction rate fo r the (Z) substrate, which indicates that the coordination of the substrate to palladium is the discriminating, but not the rate-determining, step whe n both substrates are present.