Giant Cluster Catalyzed Reaction

When Pd compounds (PdfOAc) , Pd2(OAc)i , or Pd3(OAc)e) are used as starting material, even small additions of water (1-3%) to the NaOAc/AcOH solvent give rise to a great deal of acetaldehyde instead of vinyl acetate [11-13]. In contrast to this, the Pd metal catalysts (e. g., supported Pd or Pd black, prepared by H2 reduction of Pd complexes in combination with NaOAc) provide vinyl ester from alkene and AcOH with high selectivity, regardless of the water content up to 10% [11, 14, 15]. Further differences in the selectivity of reaction (1) with Pd and Pd° catalysts were found for the oxidative acetoxylation of higher alkenes, viz., propylene, 1-hexene, and cyclohexene [7]. All these facts apparently implied that the alkene activation came from two different origins one from Pd and another from Pd metal or, more exactly, low-valent Pd clusters formed upon Pd reduction with H2. 

The outer-sphere OAc anions can be replaced by other anions. For instance, the and PF anions readily substitute for OAc anions in an aqueous solution containing KPFft, affording the giant cluster with the idealized formula [Pdsei LeoOeoKPFeleo [Ik 16, 17]. The Pd-561 clusters exhibit a high catalytic activity in alkene acetoxylation in an AcOH solution under mild conditions (20-60 °C at 0.1 MPa). Besides reaction (1), the clusters provide the oxidative acetoxylation of propylene to allyl acetate (eq. (6)) or of toluene to benzyl acetate (eq. (7)). 

The selectivity of these reactions with respect to the products of oxidative acetoxylation is at least 95-98 %. No decrease in the selectivity was found even in 

About 50 poisonous ligand molecules per Pd cluster are necessary to stop the oxidation of C2H4, whereas for the C3H6 oxidation only about 15 poison molecules are sufficient for complete inhibition of the reaction. This is in line with 

Meanwhile, acetaldehyde is scarcely oxidized by Pd to acetic acid under these conditions. A more pronounced difference is observed for propylene in acidic aqueous solution of the Pd-561 cluster propylene is converted successively to allyl alcohol, acrolein, and acrylic acid. In contrast to the reaction mediated by Pd only traces of acetone are found. 

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Palladium(II) is also capable of mediating the oxidation of alcohols via the hydridometal pathway shown in Figure 1. Blackburn and Schwarz first reported the Pd(OAc)2-NaOAc-catalyzed aerobic oxidation of alcohols in 1977. However, activities were very low, with turnover frequencies of the order of 1 h . Subsequently, much effort has been devoted to finding synthetically useful methods for the palladium-catalyzed aerobic oxidation of alcohols. For example, the giant palladium cluster, Pds6iphen6o(OAc)tgo , was shown to catalyze the aerobic oxidation of primary allylic alcohols to the corresponding a,0-unsaturated aldehydes (Reaction 14) . 

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