Palladium associative ligand substitutions

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

The transmetallation of oxidative addition products of alkenyl halides and zerovalent palladium species by organotin reagents is an important step in Stille type reactions and its mechanism depends on many variables. A recent review on this topic accounts for transmetallation of alkenylpalladium(ii) species.The involvement of 14-electron T-shaped species [PdX(R)(L)] in Stille reactions involving alkenyl and other substrates has been refuted as unrealistic. Instead, associative ligand substitution at palladium is advocated, which has been corroborated by full and quantitative evaluation of kinetic data as well as MO considerations. ... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Although the results of these experiments suggest that the palladation proceeds by an electrophilic aromatic substitution, the transformations are probably more complex than the above results suggest. Indeed, the reaction of alkyl palladium complex 6a with KOPh in MeCN was almost completely inhibited by the addition of lequiv. of PPhs [29], which indicates that ligand substitution, presumably by an associative mechanism, occurs during the C—H bond-activation process. Biden-... 

Chiral pyridine-based ligands were, among various Ar,AT-coordinating ligands, more efficient associated to palladium for asymmetric nucleophilic allylic substitution. Asymmetric molybdenum-catalyzed alkylations, especially of non-symmetric allylic derivatives as substrates, have been very efficiently performed with bis(pyridylamide) ligands. 

Because substitution chemistry at square-planar palladium is dominated by associative processes [48], coordination of the alkene in 22.2 would undoubtedly initially generate penta-coordinate intermediate 22.6. Complex 22.6 could then either evolve to square-planar complex 22.5 by a series of pseudorotations and eventual expulsion of the halide ligand or undergo... 

The transmetallation step (iii) is certainly the most enigmatic part of the catalytic cycle. Generally, it is assumed to be rate limiting, and several mechanisms are proposed depending on the solvent. An open transition state with inversion of the stereochemistry would arise with polar solvents which are able to stabilize the transient partial charges , whereas a cyclic transition state with retention of the stereochemistry would arise in less polar solvents. It should be noted that the nature of the ligands on the palladium may influence dramatically the kinetics of the transmetallation step. A 1000-fold rate enhancement was observed when replacing triphenylphosphine by tri(2-furyl)phosphine . However, the dissociative or associative nature of the substitution on the palladium is stiU under discussion . ... 

Such a scheme has been proposed based on the UV spectrum and polaro-gram of (7r-allyl)(77-cyclopentadienyl)palladium (212). A complete X-ray study of this compound carried out by Struchkov et al. (213) revealed that the palladium atom has a vacant coordination site. In essence, the scheme proposed is a version of associative substitution via change in ligand bond character in the transition state. 

The normal neutral pathway (22 24 25 27) was ruled out by conducting the reaction with monodentate phosphine BINAP ligand mimics (Scheme 12.5). The products obtained were of low enantiomeric excess relative to reactions employing BINAP. The direct cationic pathway (24-> 26) was also eliminated due to the fact that the opposite stereochemistry was obtained under cationic conditions with the addition of silver salts. The switch in stereoselectivity in the presence of silver salts, moreover, indicates that oxidative insertion is not the enantioselective step. j8-Hydride elimination was also discounted as the enantioselective step due to the influence of the double-bond geometry of the starting material on the enantioselectivity of the cyclization. The proposed enantioselective step is the formation of the cationic intermediate 26 by an associative displacement (24-> 28-> 26). In the case of square planar pafladium(n) complexes, substitution chemistry can occur through associative processes. Axial coordination of the alkene would form the pentacoordinate pafladium(II) complex 28. Reports of isolated and characterized pentacoordinate palladium(II) species provide support for this proposed intermediate. 

The generally accepted mechanism for Pd-catalyzed allylic substitution involves association of the palladium(0) catalyst to the substrate, and oxidative addition to provide a ir-aUyl complex. The equilibrium between the neutral 7r-allyl complex and the more reactive cationic 7r-allyl complex depends on the nature/concentration of phosphine Ugand. Nucleophilic addition to the ligand involves direct attack on the ligand when stabilized enolates are employed. After dissociation of the product, the palladium is able to continue in the next catalytic cycle (Scheme 2). In general, the reaction proceeds via a Pd(0)/Pd(II) shuttle, although a Pd(II)/Pd(IV) pathway is also possible. 

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