Palladium cations

A new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

Shimizu et al. (2000) investigated the state of palladium cations in H-mordenite at 623 K in various atmospheres (after oxidation, in 1000 ppm of NO, and in a mixture of NO, CH4, and O2). An integrating sphere was combined with fiber optics. First, dehydration of Pd(H20),Y 1 complexes was observed then, interaction of the resultant Pd(Ozeo) 2+ complexes with NO was indicated by a slight broadening and a shift of the band at 480 nm to 450-470 nm. Only a slight further shift to lower wavelengths occurred in the reaction mixture, and it was concluded that Pd2+ ions were the principal species present during SCR of NO with CH4. 

As shown in the case of cinnamaldehyde, addition to the carbonyl functionality occurs in preference to attack at the electron-poor double bond. The extent to which stereogenic aldehydes can control the stereochemical course of these cycloadditions is discussed in Section 1.6.1.2.3.2. The success of the tin-based cocatalyst to induce reaction is explained in terms of the formation of an intermediate stannyl ether which exhibits greater reactivity towards 7t-allyl palladium cations than free alkoxide ions. The addition to ketones is less general and one of the more successful examples is given. 

In addition, Brookhart has reported low-temperature NMR spectroscopic studies of alkyl palladium cations that have shown that the secondary alkyl is more stable than the primary when the metal is electrophilic and an open coordination site is present cis to the alkyl group (Scheme 3.1). This combination of factors gives rise to a 3-agostic interaction (see the discussion of a-complexes in Chapter 2) that is stronger to the primary C-H bond than to a secondary C-H bond and stabilizes the secondary alkyl complex. Coordination of a fourth ligand reverses the order of stability because of the loss of the agostic interaction, and the primary alkyl complex becomes more stable than the secondary alkyl isomer. 

An efficient palladium cationic catalyst was used for the selective production of methyl methacrylate from propyne. The active catalytic system consists of a ligand containing a 2-pyridylphosphine moiety, a palladium(II) species, and a proton source containing weakly... 

Jt-Allyl palladium cations can be regarded as soft electrophiles, and react smoothly with soft nucleophiles. For carbon-based nucleophiles, soft methylene compounds (conjugate acids with a < 25) with two electron-... 

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