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

Pyridinyl azole ligands, such as pyridinylpyrazoles, pyridinylimidazoles, a nd 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-allylpall adium complexes worked as effective catalysts for cyclopropanation of keten e silyl acetals with allylic acetates whereas the cationic pi-allylpalladiu m complex having bipyridyl ligand was almost unreactive. Reaction of cinnam yl acetate with ketene silyl acetal of ethyl 2-methylpropionate in the pres ence of the palladium-pyridinylimidazole catalyst and sodium acetate in DMS O gave a cyclopropane derivative selectively in a 83% yield. This reaction was also applicable for asymmetric cyclopropanation. When chiral pyrazole-p alladium catalysts were used, chiral cyclopropane derivatives were obtained up to 54%ee. To examine the difference between cationic and neutral pallad ium complexes, X-ray diffraction analysis and temperature variable NMR stud ies were performed. Bond lengths between palladium and nitrogens of the neu tral pyridinylimidazole complex were shorter than those of the cationic com plex in the crystal structures. Further, kinetic constants of syn-syn and a nti-anti exchange of the allyl moiety in the neutral complex was smaller th an that in the case of cationic complex in DMF-d(7) and DMSO-d(6). These re sults shows that palladium-nitrogen bonds of the neutral complex are strong er than that of the cationic complex, and these difference affects the reac tivity for catalytic cyclopropanation.