Reaction palladium exchange

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

Diimines such as bpy and phen replace 2-(arylazo)pyridine ligands (aap) in [Pd(aap)Cl2] by a simple second-order process, whose detailed mechanism may depend on the nature of the incoming ligand (254). Three phthalocyanine units, each containing Zn2+, can be bonded to tetrahedral phosphorus, to give [PPh(pc-Zn)3]+. Mechanistic proposals are advanced for this novel exchange reaction in which palladium-bound phthalocyanine replaces phenyl on phosphorus (255). 

The a- and y-position activation has a remarkable impact on the regiochemical outcome for the Pd-catalyzed reaction of heterocycles. For example, the palladium-catalyzed reaction of 2,5-dibromopyridine takes place regioselectively at the C(2) position, although the halogen-metal exchange reaction takes place predominantly at C(5). Pd-catalyzed reactions of chloropyrimidines take place at C(4) and C(6) more readily than at C(2). 

Mixed trialkylstannyl and silyl derivatives have also been used in coupling reactions, with subsequent replacement of the silyl substituent by bromine, leading to species that are capable of undergoing further coupling reactions. This process was amply demonstrated by the recent synthesis of micrococcinic acid 203, which involved four palladium-catalyzed crosscoupling reactions on stannylated substrates, two palladium-catalyzed trimethylstannane replacements of bromine, two trimethylsilyl displacements by bromine, and a total of four bromine-lithium exchange reactions, on three different thiazole derivatives and one pyridine derivative (91-TL4263). 

The two most used reversible covalent reactions are disulfide exchange and palladium-catalyzed olefin metathesis. We first probed the incorporation of olefin units into the H bonded duplexes by subjecting the modified duplexes to a Pd (Gmbb s) catalyst. Based on a duplex template with the same unsymmetrical H bonding sequence used for directing the formation of the /3-sheet structures, we prepared two groups (strands 17 and 18) of five olefins covalently linked to the two template strands (Fig. 9.13). Mixing each one of components 17 with each one of components 18 in a 1 1 fashion results in a small library of 25 (5 x 5) members. 

Nickel has been reported to show behaviour similar to platinum [236], Further, in the reactions of cycloalkenes with deuterium, the product cycloalkanes are much more extensively exchanged over palladium than over nickel or platinum [236]. Such behaviour is not unexpected by comparison with the results obtained in the hydrogenation of linear alkenes (Sect. 3, p. 25). 

These results show that the palladium exchange reaction takes place predominately with retention of configuration at the carbon bearing the mercury. 

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