title: Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols.
abstract: In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 degrees C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields.
authors.affiliation: Catalysis Research Center and Division of Chemistry, Graduate School of Science, Hokkaido University, CREST, Japan Science and Technology Corporation (JST), Sapporo 001-0021, Japan.
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Iwasawa
authors.forename: Makoto
authors.initials: M
authors.lastname: Tokunaga
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Obora
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Tsuji