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Acylzirconocene chlorides reacted with α,β-unsaturated ketones (α,β-enone or -ynone) to give regioselectively 1,2- or 1,4-products under Pd-catalyzed conditions. In the reactions of α,β-enones, excellent regioselectivity was attained by the choice of the Pd(II) catalyst system. Thus, the 1,2- or 1,4-products were selectively prepared by choosing PdCl2(PPh3)2 or Pd(OAc)2–BF3·OEt2, respectively. The presence of a monodentate phosphine ligand brought about a 1,2-addition product, selectively. In these Pd-catalyzed reactions, the Pd(0) species was considered to be an active catalyst, which was generated in situ through transmetalation of acyl groups from zirconium to Pd(II) followed by reductive elimination of Pd(0). α,β-Ynones also reacted with acylzirconocene chloride to give regioselectively 1,4-products under the Pd-catalyzed conditions. In contrast to the reactions of α,β-enones, the presence of a triphenylphosphine ligand brought about the selective formation of 1,4-adduct. By virtue of the phosphine ligand-effect in the addition of an acyl anion to α,β-enones, enantioselective 1,2-addition of acylzirconocene chloride to cyclic α,β-enones was carried out in modest optical purity (∼66% ee) by the use of 5 mol% Pd(OAc)2-(R)-MOP (Pd/P=1:2) catalyst. The origin of the enantioselectivity was deduced by considering an (R)-MOP coordinated π-allylic acyl Pd complex. |
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