On the influence of the bite angle of bidentate phosphane ligands on the regioselectivity in allylic alkylation

The natural bite angle of bidentate phosphane ligands influences the isomer distribution (syn and anti) in (1-methylallyl)(bisphosphane)Pd OTf complex es. It was found (P-31- and H-1-NMR studies) that the syn/anti ratio change s from 12 (dppp) to 1.3 (sixantphos). Molecular orbital calculations [PM3(t m) level] indicate that for ligands inducing a large bite angle, the phenyl rings of the ligand embrace the allyl moiety thus influencing the syn/anti ratio. This bite-angle effect on the syn/anti ratio is transferred to the regioselectivity in stoichiometric allylic alkylation. Ligands inducing lar ge bite angles direct the regioselectivity towards the formation of the bra nched product 2. Catalytic alkylation of (E)-2-butenyl acetate showed that for ligands with a small bite angle the regioselectivity of the catalytic a nd stoichiometric alkylation are in good agreement. This correspondence is worse for ligands with a larger bite angle, which is rationalised in terms of the relative rates of syn/anti isomerisation and alkylation. The ligand with the largest bite angle (sixantphos) gives the most active catalytic sp ecies.