Synthesis of neutral π-allylpalladium complexes having bisnitrogen ligands and palladium-catalyzed cyclopropanation of ketene silyl acetals with allylic acetates

Pyridinyl azole ligands, such as pyridinylpyrazoles, pyridinylimidazoles, and pyridinylpyrrole, were used as a new type of bisnitrogen ligands for pi-allylpalladium complexes. Reaction of pi-allylpalladium chloride dimer with pyridinyl azole ligands formed cationic pi-allylpalladium complexes, which could be converted into neutral forms with bases. The neutral pi-allylpalladium complexes worked as effective catalysts for cyclopropanation of ketene silyl acetals with allylic acetates whereas the cationic pi-allylpalladium complex having bipyridyl ligand was almost unreactive. Reaction of cinnamyl acetate with ketene silyl acetal of ethyl 2-methylpropionate in the presence of the palladium-pyridinylimidazole catalyst and sodium acetate in DMSO gave a cyclopropane derivative selectively in a 83% yield. This reaction was also applicable for asymmetric cyclopropanation. When chiral pyrazole-palladium catalysts were used, chiral cyclopropane derivatives were obtained up to 54%ee. To examine the difference between cationic and neutral palladium complexes, X-ray diffraction analysis and temperature variable NMR studies were performed. Bond lengths between palladium and nitrogens of the neutral pyridinylimidazole complex were shorter than those of the cationic complex in the crystal structures. Further, kinetic constants of syn-syn and anti-anti exchange of the allyl moiety in the neutral complex was smaller than that in the case of cationic complex in DMF-d(7) and DMSO-d(6). These results shows that palladium-nitrogen bonds of the neutral complex are stronger than that of the cationic complex, and these difference affects the reactivity for catalytic cyclopropanation.