Palladium complexes electron-transfer reactions

Nitroso compounds as a reaction intermediate for nitro compound reductions were investigated as substrates for mechanism understanding by Cenini and his colleagues [18]. The isolation of a paramagnetic complex suggested that the first step of the interaction between Pd(0) and the nitroso species is an electron transfer reaction. The palladium carbomethoxy complex from the reaction of a palla-dium(O) complex with PhNO under CO pressure in methanol was also isolated, which fits with the published reaction mechanism [19]. 

Reaction of this lO-S-3 [279] tetraazapentalene derivative with [Pd(PPhj) ] or [Rh(PPh3)3)Cl] results in the formal substitution of sulfur by the transition metal accompanied by a redox reaction (see Figure 4.93) [280], The endocyclic sulfur atom is transferred to a PPhj ligand (oxidation of phosphorus to PhjP=S). At the same time the transition metal is oxidis (palladium from 0 to +11 rhodium from +1 to +III), which leaves sulfur to be reduced by four electrons (it is -II in Ph I S and thus must have been +II in the tr-sulfurane starting material). It follows from this electron transfer analysis that the rt-sulfurane is indeed better desaibed as the sulfur complex of a doubly amide functionahsed NHC ligand. 

These reactions turned out to be preparatively very useful since they allow the performance of radical cyclization reactions but produce an organozinc halide as a final product (Scheme 9-33) [65-70]. The treatment of an unsaturated alkyl halide 36 (X = Br, I) with a palladium(O) or nickel(O) complex produces, via a one-electron transfer [72], a paramagnetic nickel(I) or palladium(I) salt MLj,(X) (M = Ni, Pd) and a radical 37 which undergoes a smooth cyclization reaction and produces, after recombination with the transition-metal moiety, the nickel(ll) or palladium(II) species 38. After transmetallation with a zinc(II) salt, a stable organozinc cyclopentylmethyl derivative of type 39 is produced. 

The mode of interaction of the oxidant with the acetoxypalladation adduct is not certain. The oxidant could be removing electrons from Pd as the Pd(II)—C bond is broken and Pd(0) is never formed, or the Pd(II) could be oxidized to Pd(IV) which would leave much more easily than Pd(II). Another possibility is that the organic radical is transferred to the oxidant followed by decomposition. It would be difficult to distinguish between the various possibilities. Related reactions are the cleavage of a-bonded palladium complexes with Collins reagent (280), decomposition of rr-allyls with oxidants (164), and the decomposition of oxypalladation adducts of diolefins with oxidants (Section IV, B). 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system pd(II)-BQ-ML where ML" is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPp)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

Direct arylations of arenes are, however, not restricted to palladium-catalyzed transformations, but were also accomplished with, inter alia, iridium complexes. Thus, the direct coupling of various aryl iodides with an excess of benzene in the presence of [Cp IrHCl]2 afforded the corresponding biaryl products, but usually in moderate yields only (Scheme 9.30) [69]. The reaction is believed to proceed via a radical-based mechanism with initial base-mediated reduction of iridium(III) followed by electron transfer from iridium(II) to the aryl iodide. Rather high catalyst loadings were required and the phenylation of toluene (90) under these reaction conditions provided a mixture of regioisomers 91, 92, and 93 in an overall low yield (Scheme 9.30) [69]. 

Modifications of the palladium-phenothiazine derivative complex procedure of Ryan ° have been applied to the quantitative analysis of propiomazine hydrochloride successfully. j The colorimetric procedure is based on the reaction of palladium with propiomazine in an aqueous solution buffered at about pH 3 to form a colored complex which is spectrophotometrically measured at U65 m/u. Since this complex formation is based on an electron transfer from the sulfur moiety to the palladium ions, the procedure provides a method to assay propiomazine in the presence of its corresponding sulfoxide oxidative decomposition product. 

Pti-x ZXjc supported on carbon or alumina, Kt/Kb is proportional to x, suggesting electron transfer from platinum to zirconium, as predicted by the Engel-Brewer theory, and (2) chemisorption of sulfur on platinum has been shown to decrease electron density of the surface, while carbon has the opposite effect. The ratio Kt/Kb was very large for ruthenium, about 10 for rhodium and about unity for palladium, which may help to explain their different activities in these and other reactions. An extensive kinetic study of the hydrogenation of mixtures of benzene and toluene on NiA zeolite has however revealed a situation of some complexity, and it is not certain that the original simple concept is totally valid. 

The success of this triple catalytic system relies on a highly selective kinetic control. From a thermodynamic point of view, there are 10 possible redox reactions that could occur in this system. However, the energy barrier for six of these (O2 + diene, O2 + Pd(0), etc.) are too high, and only the kinetically favored redox reactions shown in Scheme 11.14 occur. A likely explanation for this kinetic control is that the barrier is significantly lowered by coordination. Thus, the diene coordinates to Pd(II), BQ coordinates to Pd(0), HQ coordinates to (ML,), and Oj coordinates to ML ,. In a related system for aerobic oxidation, a heteropolyacid was employed in place of the metal macrocyclic complex (ML ,) as oxygen activator and electron transfer mediator [72]. Recent immobilization of the macrocyclic complex in ZeoHte-Y, led to eflBcient reoxidation of the HQ in the palladium-catalyzed 1,4-diacetoxylation [73]. 

In contrast with the above r rted mechanism of the NiCl2(dppe) catalysis, the PdCl2(PPh3)2 catalysis is found to proceed with a succession of elementary steps involving only paramagnetic palladium complexes. The catalytic cycle is a succession of two-electron transfer steps (which have been characterized by their potential) and chemical steps. The oxidative addition is the rate determining step of the catalytic cycle while the nucleophilic attack of CO2 by the anion Ar is a fast reaction. 

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