| Popis: |
The bite angle of bidentate phosphane ligands has a pronounced influence on the degree of retention of the double bond geometry of the allylic substrate 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 substrates. The alkylation of substrates with an (E) conformation of the double bond results in the preferential formation of the linear (E) product. A larger bite angle of the ligand results in an increase of the regioselectivity to >98% for the Sixantphos ligand. Analogously, the alkylation of (Z) substrates results in the formation of the linear (Z) product. Remarkably, for (Z) substrates, a larger bite angle of the ligand leads to an increased regioselectivity for the formation of the branched product instead of the linear product, up to 47.5% for Sixantphos. The observed regioselectivities are rationalized in terms of: a) a competition between syn-anti isomerization and alkylation, and b) a combination of steric and electronic effects in the transition state of the reaction. For all ligands tested, the reaction is faster for the (E) than for the (Z) substrate. However, competition experiments using the Sixantphos ligand show a relatively fast reaction rate for the (Z) substrate, which indicates that the coordination of the substrate to palladium is the discriminating, but not the rate-determining, step when both substrates are present. |
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