Oxidation by Pd

This is a genuine organometalUc reaction in which ethene is oxidized by Pd(n) to yield acetaldehyde (eq. 8.1) [3] ... 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

Benzohydrofuranes. 2-Allylphenols are cyclized oxidatively by Pd(OAc)2 (1 equiv.) to isomeric benzofuranes. In the presence of oxygen, the Pd(II) salt can be used in catalytic amounts. An example is formulated in equation (I). ... 

In a 1 1 ethylene glycol-tetrahydrofuran solvent, ethylene ketals would be expected, since these would be the products if the olefins were directly oxidized by Pd(II) salts in ethylene glycol (139) ... 

Evidence that deoxymercuration does not occur to give free olefin, followed by oxidation by Pd(II) to give product, is provided by the fact that undec-l-ene was not oxidized by PdCl2 under the reaction conditions in 2 hours, but a 63% yield was obtained in 30 minutes when the mercurial was employed. A very significant deuterium-labeling experiment was also carried out. 2-d -3,3-Dimethylbut-l-ene was prepared. 

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. 

Alcohols are oxidized by Pd(II) species. In this case, carbonyl compounds are formed by the S-H elimination from the Pd alkoxides 41, and the reactions may be expressed by either 41 or 41a. 

Patterns 2.1 throngh 2.4 for oxidation by Pd (P-elimination + reductive elimination)... 

Several catalytic oxidative reactions have been developed using O2 as the terminal oxidant, most notably using Pd. In these reactions, the substrate is typically oxidized by Pd, which is in turn oxidized by O2, without transfer of oxygen to the substrate. The mechanistic details of the oxidation of Pd complexes by O2 are therefore of great interest. At the centre of this debate is the question whether a Pd° hydride (formed upon product release in most oxidative Pd-catalyzed reactions) reacts directly with O2 (Scheme 12.1, 1 2), or the Pd hydride undergoes reductive elimination to form Pd followed by reoxidation (Scheme 12.1, 3-+4->2). 

Bifiinctional bimetallic alloys in which both catalytic and magnetic functions are simultaneously provided can be applied. For example, Co-Pd bimetallic alloy NP catalysts were prepared and employed for the aerobic oxidation of a variety of alcohols in water. The catalysts then were magnetically recovered and reused for further oxidation. Leaching of Co and Pd was in orders of only 10 mol% and 10 mol%, respectively. The proposed explanation for the low leaching involves an electrolytic protection of the more expensive Pd component E° of Pd(0)/Pd(II) (0.95 V) is higher than E° of Co(0)/Co(II) (0.28 V). Hence, Co(0) is oxidized by Pd(II) to give Co(II), while Pd(0) aggregates with initial NPs and remains within the catalyst. 

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