Oxygen Palladium complexes

Relative differences between S 2p3/2 and O 1 s ionization potentials show a characteristic separation for oxygen-bound and sulphur-bound sulphoxides. It is clearly shown in Table 20 that sulphur-bound complexes have (O 1 s-S 2p3/2) relative shifts of 365.0 eV, while oxygen-bound complexes have relative shifts of 365.8 eV. Infrared and X-ray crystallographic results also show that most neutral platinum and palladium dialkyl sulphoxide complexes contain metal-sulphur rather than metal-oxygen bonds, while first-row transition metals favour oxygen-bonded sulphoxide. 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

Although there are indeed only few reported methods of direct activation of molecular oxygen via transition metals, there are many reports of indirect oxidation. The majority of this research is based on palladium-based oxidation as summarized in equation 32. The palladium complex catalyzed oxidation reactions have been reviewed previously186 and also only very recently187 and in this book the palladium catalyzed oxidation of dienes and polyenes will be discussed separately and therefore will not be discussed... 

Some synthetically useful isomerization reactions of alkenes, other than nitrogen- or oxygen-substituted allylic compounds, were reported by the use of a catalytic amount of transition metal complexes. The palladium complex, /ra r-Pd(C6HsCN)2Gl2, effectively catalyzed the stereoselective isomerization of /3,7-unsaturated esters to a,/3-unsaturated esters (Equation (26)). 

Oxidative addition involving carbon-to-oxygen bonds is of relevance to the catalysis with palladium complexes. The most reactive carbon-oxygen bond is that between allylic fragments and carboxylates. The reaction starts with a palladium zero complex and the product is a ir-allylic palladium(II) carboxylate Figure 2.16. 

The nucleophilic attack of the water or hydroxide species takes place in an anti fashion i.e. the oxygen attacks from outside the palladium complex and the reaction is not an insertion of ethene into the palladium oxygen bond. This has been demonstrated in a model reaction by Backvall [4], The reaction studied was the Wacker reaction of dideuterio-ethene (cis and trans) in the presence of excess of LiCl, which is needed to form 2-chloroethanol as the product instead of ethanal. The latter product would not reveal the stereochemistry of the attack Note that all of the mechanistic work has been carried out, necessarily, on systems deviating in one aspect or another from the real catalytic one. The outcome depends strongly on the concentration of chloride ions [5],... 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

Nitrogen nucleophiles, in a similar manner to oxygen- and sulfur-based functionality, undergo transition metal-catalyzed cross-coupling with halopyridines. The use of palladium(O) catalysts is most effective in combination with chelating bis-(phosphine) ligands such as BINAP that prevent the formation of pyridine-palladium complexes that... 

On the other hand, Wacker-type oxidative cyclization is a versatile approach for the construction of oxygenated stereocenters [213,214]. The synthesis of a number of dihydrobenzofurans catalyzed by an in situ-formed carbene-palladium complex has been reported by Muniz [215]. When Pd(TFA)2 in combination with IMes were employed, high yields in pure cyclized products were obtained after simple work-up (Table 9). However, palladium salts containing chlorine or acetate groups led to the formation of mixtures containing the desired product and its six-membered ring isomer. 

Many of the reactions outlined in Section 4 for Pd-C bonds occur in cyclometalated complexes. However, the existence of the Pd-C bond in a chelate ring imposes some kinetic stability on the bond. In general, mild acids and bases and oxygen are unreactive with cyclometalated palladium complexes. More vigorous reagents will lead to useful reactions, however. 

In this context, it is worthwhile to note that the use of alcohols in catalytic hydrogenations may lead to related aldehydes or ketones which in turn are capable of producing stable imidazolid-4-one derivatives with the N-terminal amine group.Moreover, when carrying out hydrogenations in an alcohol, oxygen has to be removed meticulously from the system to avoid as a serious side reaction N-alkylation via aldehyde and related imine derivatives which has been observed to occur readily, particularly with methanol as the solvent.f l An additional inconvenience observed when air is not rigorously excluded, is the formation of palladium complexes with the peptides. 

Cyclopalladated sulfur-containing < 1995JOC1005> and oxygen-containing complexes <2003OM3967,2005CEJ3268> have also been synthesized. The insertion of phenylacetylene 122 into the palladium-carbon bond of complex 121 yielded the palladacycle 123 (Equation 43). 

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