Palladium ligand substitutions

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

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... 

An efficient aqueous phase Suzuki-Miyaura reaction of activated aryl chlorides with aryl boronic acids has been reported. The method uses a new D-glucosamine-based dicyclohexylarylphosphine ligand for the palladium catalyst and works well with nitro-and cyano-activated chlorides.32 The aryl fluoride bond has been considered inert to palladium-catalysed substitution reactions. However, a computational study, backed up by experiment, shows that the presence of a carboxylate group ortho to fluorine will allow reaction both with phenylboronic acids in a Suzuki-type reaction and with organotin reagents in a Stille-type reaction the presence of the adjacent oxyanion stabilizes the transition state.33... 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

The generally accepted mechanism of the palladium-catalyzed substitution reaction is shown in Scheme 1. For a more detailed discussion, the reader is referred to the original chapter [1], A number of papers which probe the enanti-odetermining step for specific ligands have been published over the past four years but will not be discussed further [15-28]. The effect of catalyst loading [29], the ionization of I with different Pd° complexes [30,31], the mechanism of the q3-q -q3 isomerization [32] of intermediate II, and the behavior of the ole-fin-Pd(O) complex III have been studied [33]. 

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. 

Thus the iron atom in (Tr-cyclopentadienvl)iron dicarbonyl bromide forms additional bonds with the halide ligands on palladium. Carbonyl substitution on the generated intermediate (XV) then occurs. In this case the cyclopentadienyl group acts as a bridge. In general, such a mechanism corresponds to an A-type substitution, where the halogens in (7T-tetraphenyl-cyclobutadiene)palladium dichloride function as nucleophiles. 

Despite the richness of this field of study, however, a number of anomalies and some major uncertainties remain. One curious feature, which is probably no more than an historical accident, is that though most studies of nucleophilic ligand substitution have been carried out on complexes of platinum(II) and palladium(II) and few on complexes of rhodium(I) and iridium(I), the reverse distribution is apparent for studies of oxidative additions. The scope for rectifying this imbalance is vast. On the other hand, a fundamental and persistent uncertainty in this field of study concerns the very nature of square-planar compounds in solution. We address this problem in some detail. 

The palladium-mediated allylation proceeds via an initial oxidative addition of an allylic substrate to Pd(0). The resultant TC-allylpalladium(II) complex A is electrophilic and reacts with carbon nucleophiles generating the Pd(0) complex B, which undergoes ligand exchange to release the product and restart the cycle for palladium. With substituted allylic compounds, the palladium-catalyzed nucleophilic addition usually occurs at the less substituted side. 

This set of results is at variance with those in the nickel-palladium-platinum triad, where palladium appears to be the most reactive species [11.3.2.1.3(i)], indicating the outstanding role played by palladium in substitution reactions. On the other hand, presence of the cyclopentadienyl ligand in the systems of reaction (a) could introduce a perturbation due to the possibility of a -> rj" mechanism, with n being an odd number < 5. 

We suppose that the planar quadratic coordination of the palladium(II) chelates allow CO2 molecules or other small molecules like traces of the synthesis solvent to be coordinated to the Pd(II) centre and therefore change the polarity /solubility of the chelate. Voluminous and flexibel residual ligand substitutes protect the central palladium ion against interaction of small molecules and we can observe a drastic increase of solubility. Otherwise, if the coordination sphere of the chelate centre ion is octahedral or tetrahedral, other molecules (e.g. the solvent) cannot interact with the metal centre easily. 

In particular, the palladium complex with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, was selected as the most efficient catalyst. The presence of the methyl substitution in 2,9-positions was necessary to avoid the formation of binuclear complexes like 1 (detected using 1,10-phenanthroline as palladium ligand), which are stable in reaction conditions and are responsible of the fast irreversible deactivation of the catalyst. 

Significant advances have been made in asymmetric nucleophilic additions to -ir-allyl palladium complexes using chiral ligands. Substitution of allylic acetates in the presence of the chiral phosphine hgand 224 (or other chiral phosphine ligands) can occur with very high levels of enantioselection (1.224). The reaction works best with diaryl-substituted allylic acetates. 

Two other spectrophotometric SF studies have increased the amount of data available on Pd(II) ligand substitution processes. In one the formation of tetra-chloropalladate(II) from bis(oxalato)- and bis(malanato)palladium(II) in aqueous add chloride solution has been studied. It is found that [Pd(mal)2] is much more labile than [Pd(ox)2] , and that in both cases the formation of [PdCU] " proceeds in two consecutive steps both of which are characterized by the two-term rate law (39). The rate-determining steps characterized by Icq and k are thought to be the formation of reactive intermediate species in which the leaving ligand... 

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