Palladium complexes displacement reactions

Palladium-catalyzed displacement reactions with carbon nucleophiles are not only regioselective but also highly stereoselective. In the first step, displacement of the leaving group by palladium to form the 7i-allylpalladium complex occurs from the less hindered face with inversion. Subsequent nucleophilic substitution of the intermediate 7t-allylpalladium complex with soft nucleophiles such as amines, phenols, or mal-onate-type anions also proceeds with inversion of the stereochemistry. The overall process is a retention of configuration as a result of the double inversion. 

Further studies revealed that electron-deficient alkenes are capable of displacing O2 from a peroxopalladium(II) complex. The reaction of nitrostyrene derivatives with (bc)Pd(02) results in quantitative displacement of dioxygen and formation of the (bc)Pd(ns ) complex (i.e., the reverse reaction in Eq. 21) [138]. Moreover, preliminary results reveal that dioxygen and BQ undergo reversible exchange at a bathocuproine-coordinated Pd center (Eq. 22) (Popp BV, Stahl SS, unpublished results). This observation is the most direct experimental result to date that establishes the similar reactivity of dioxygen and BQ with palladium. 

If the reaction mixture also contains a nucleophile, then the acyl-palladium complex might undergo displacement of the metal, which usually leads to the formation of a carboxylic acid derivative. The side product in this process is a palladium(II) complex that undergoes reductive elimination to regenerate the catalytically active palladium(O) complex. 

It has been suggested (162) that there exists only negligible 7r-backbonding in [AuCl(CO>], and a number of displacement reactions have been described (162, 163). Vibrational and NMR spectroscopic studies have been made of this complex (164), and the results have been compared with those for carbonyl complexes of palladium, platinum, rhodium, and iridium. 

A similar borderline system, fran -bis[2-(2-chloroethyl)pyridine]palladium chloride (1), has been prepared and structurally characterized by X-ray spectroscopy and computational study.2 A study on the elimination reaction of (1) induced by quinuclidine in acetonitrile has been performed (Scheme 1). The results suggest that the initial product of elimination is a palladium complex of vinylpyridine and that displacement from this complex is partially rate determining in the formation of the uncoordinated product. Despite experimental efforts, it was not possible to distinguish between two possible mechanisms, E2 concerted or ElcB. 

Both ring-expansion reactions proceed via a putative pallada(sila)cyclopentane intermediate. In the course of the mechanistic elucidation studies, Tanaka succeeded in preparing a l-pallada-2-silacyclopentane complex with quantitative conversion (Scheme 51) <19970M3246>. Formation of the complex is reversible, and the starting SCB can be released by addition of an acetylene, which acts as a ligand for palladium and displaces the SCB. 

Sn2 reaction. This problem together with the associated stereochemical ambiguity was described in Chapter 23. In contrast, jt-aUyl cation complexes of palladium allow both the stereochemistry and regiochemistry of nucleophilic displacement reactions to be controlled. 

A variety of other nucleophiles have been used for the metal-catalyzed displacement of chlorine in nonactivated chloroarenes, including arylthiolate [191-193] and iodide [194, 195] anions, primary and secondary amines [196, 197], tertiary phosphines [198,199], and aminophosphines [200]. All these reactions are catalyzed by either preformed or generated in situ Ni(0) complexes. Very recently, however, Reddy and Tanaka [201] and Koie et al. [202] reported the arylation of secondary amines with chlorobenzene and other chloroarenes, catalyzed by palladium complexes containing bulky, electron-rich phosphines,... 

Malonate anion, with a palladium catalyst, displaces the acetoxy of 3- and 4-(acetoxymethyl)qninolines, probably via a three-centered benzylic equivalent of a 7t-allyl complex. The reaction fails with the 2-isomer, bnt also works with the 3- and 4-(l-acetoxyethyl) compounds and 4-(l-acetoxyethyl)isoqninoline. ... 

The palladium complex Pd(PhCN)2Cl2, which is readily prepared by reaction of PdClj in benzonitrile, has proved a valuable source of olefin complexes through displacement of benzontrile ... 

Trost proposed the following mechanism to account for these catalytic transformations. Reaction of the palladium catalyst with 377 generates jt-alkene palladium complex 378. Palladium removes the allylic hydrogen, with expulsion of the acetate moiety to generate the Jt-allyl palladium complex (379). Attack of a nucleophile at Ca leads to 380, with expulsion of the PdL2 species, whereas attack at Cb leads to 381. Palladium coordinates on the face of the alkene distal to the acetate (distant from the acetate Ca rather than Cb). Palladium displaces acetate with inversion (378 - 379). When the nucleophile displaces the palladium, a second inversion occurs at Ca or Cb, whichever is less sterically hindered, to give a net retention of configuration for the conversion 377 - 380 and/or 381. 

Carbopalladation. Allylic amines and sulfides form palladium complexes in high yield with dilithium tetrachloropalladate (THF, 20°, 6-8 hours). These complexes react with certain carbanions at the jS-position, a site that is not susceptible to nucleophilic displacement in allylic amines and sulfides. The complexes need not be isolated these reactions are conducted in THF at room temperature by reaction of the allylic amine or sulfide with the carbanion in the presence of 1 equiv. of LizPdCU (equations I and II). The products are the palladium complexes 1 and 3. These are reduced by sodium borohydride or sodium cyanoborohydride... 

Solution-phase synthetic procedures leading to (NHC)2Pd complexes revolve around ligand displacement reactions involving palladium centers bearing weakly coordinating ligands such as phosphines or olefins (see Scheme 1). For instance, a 14-electron species such as Pd(P(o-tolyl)3)2 reacts with free NHC... 

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