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.

Solvent-free aerobic oxidation of hydrocarbons and alcohols with Pd@N-doped carbon from glucose

“…From the late 1990s, a number of homogeneous Pd-catalytic systems, such as Pd/coordinating solvent 9,10 , Pd/ligand [11][12][13][14][15] , have been investigated for aerobic oxidation. Compared with homogeneous catalysts, the heterogeneous catalysts, which are convenient for recycling from the reaction environment and readily used in flow reactors, are more popular, in particular in large-scale production 16 .…”

Transition-Metal Complexes with Nano-Sized Phosphine and Pyridine Ligands-Catalysis, Fluxional Behavior and Molecular Recognition

“…tetraphenylphenyl moiety 4 Nano-sized pyridine ligands having tetraphenylphenyl moieties (1a-e) were synthesized by utilizing Diels-Alder reaction of ethynylpyridine with corresponding 2,3,4,5-tetraarylcyclopentadienone (Figure 1). 12 Complexation of 1 with Pd(OAc) 2 afforded Pd(OAc) 2 (1) 2 quantitatively.…”

“…There have been several reports in which nano-sized ligands were used in transition-metal catalyzed reactions, but so far their success was rather limited. 3 In this review, we summarize recent progress from our laboratoty in the field of nano-sized dendrimer pyridine and phosphine ligands having tetraphenylphenyl-, 4 m-terphenyl-, 5 and poly(benzylether) moieties. 6 These ligands show remarkable catalytic performance in Pd-catalyzed alcohol oxidation 4 and Rh-catalyzed ketone hydrosilylation.…”

“…3 In this review, we summarize recent progress from our laboratoty in the field of nano-sized dendrimer pyridine and phosphine ligands having tetraphenylphenyl-, 4 m-terphenyl-, 5 and poly(benzylether) moieties. 6 These ligands show remarkable catalytic performance in Pd-catalyzed alcohol oxidation 4 and Rh-catalyzed ketone hydrosilylation. 5 In addition, we synthesized novel phosphine ligands having calixarene moieties (phosphinocalixarenes).…”

Transition‐Metal Complexes with Nanosized Phosphine and Pyridine Ligands — Catalysis, Fluxional Behavior and Molecular Recognition

“…The diphosphinated calix [4] With regard to the phosphinocalixa p n oieties (6) X-ray crystal analysis showed 7a has a capsule-shaped structure having three iridium metals and two calix [6]arene moieties ( Figure 6). The complex possessed an inner nano-sized cavity and one CH 2 Cl 2 molecule was encapsulated in the cavity.…”