Palladium 1,3 hydrogen shift

A a-5-bonded r-alkene (r] ) intermediate (325) has been invoked to account for the hydrogenation of the thiaplatinacycle (324) to the complex (326) in which two hydrogens have been added and a hydrogen shift has occurred." When coordinated to neutral and cationic palladium(II) and platinum(II) centres, the diphosphine 2,3-bis(diphenylphosphino)propene, on treatment with benzylamine, was found to undergo isomerization to coordinated c/i-l,2-bis(diphenylphosphino)propene rather than the expected nucleophilic addition to the double bond. 

The oxidation of olefins to carbonyl compounds by palladium (II) ion can be regarded as an addition of a palladium hydroxide group to the olefin followed by a hydrogen shift. Kinetic evidence suggests the following mechanism for the oxidation of ethylene by palladium chloride in aqueous solution containing excess chloride ion 21, 49, 99). 

A mixture of palladium chloride and triphenylphosphine effectively catalyzes carboxylation of linoleic and linolenic acids and their methyl esters with water at 110°-140°C and carbon monoxide at 4000 psig. The main products are 1,3-and 1,4-dicarboxy acids from dienes and tricarboxy acids from trienes. Other products include unsaturated monocar-boxy and dicarboxy acids, carbomethoxy esters, and substituted a,J3-unsaturated cyclic ketones. The mechanism postulated for dicarboxylation involves cyclic unsaturated acylr-PdCl-PhsP complexes. These intermediates control double bond isomerization and the position of the second carboxyl group. This mechanism is consistent with our finding of double bond isomerization in polyenes and not in monoenes. A 1,3-hydrogen shift process for double bond isomerization in polyenes is also consistent with the data. 

Another attempt to account for the differences between the behavior of platinum and palladium in bond shift isomerization has been presented in a recent review by Clarke and Rooney (2). This new mechanism, which is also based on the ability of platinum and not of palladium to promote the formation of metallocyclobutanes, derives from Rooney s earlier mechanism but replaces the cr-alky 1 precursor by a metallocyclobutane, and, in the transition state, the n-olefinic bonding by a zr-allylic bonding. As in the previous mechanism, it is assumed that on platinum the metallocyclobutane is formed directly, while in the case of palladium, it would result from a 12 hydrogen shift via a transient species of zi-allylic character (Scheme 28). 

An impressive example of the possibilities of metal complex photochemistry is the substantiation of the critical step in corrin synthesis reported by Eschenmoser 145> Ring closure is brought about by antara-facial cycloisomerization of a secocorrinoidic palladium complex by photochemical 1.16-hydrogen shift. 

The impurities usually found in raw hydrogen are CO2, CO, N2, H2O, CH, and higher hydrocarbons. Removal of these impurities by shift catalysis, H2S and CO2 removal, and the pressure-swing adsorption (PSA) process have been described (vide supra). Traces of oxygen in electrolytic hydrogen are usually removed on a palladium or platinum catalyst at room temperature. 

Klette, H., T. Peters, A. Mejdell, and R. Bredesen, Development of Palladium-Based Hydrogen Membranes for Water Gas Shift Conditions, Proceedings of Eight International Conference on Greenhouse Gas Control Technologies (GHGT-8), Trondheim, June 2006. 

R. Bredesen, Development of palladium-based hydrogen membranes for water gas shift conditions, Proceedings of the 8th International Conference on Greenhouse Gas Technologies (www.GHGT8.no), 20-23 June 2006, Trondheim, Norway. 

It is worth mentioning that some precursors easily catalyze the reductive carbonylation of alkynes from the C0/H20 couple. Here, the main role of water is to furnish hydrogen through the water-gas-shift reaction, as evidenced by the co-production of CO2. In the presence of Pd /KI terminal alkynes have been selectively converted into furan-2-(5H)-ones or anhydrides when a high concentration in CO2 is maintained. Two CO building blocks are incorporated and the cascade reactions that occur on palladium result in a cyclization together with the formation of an oxygen-carbon bond [37,38]. Two examples are shown in Scheme 4. 

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