Electrophilic palladium center

The phosphine moiety is required to function as an anchor for efficient binding of P-NH to the electrophilic palladium center. 

The polymerization of /o-functionalized norbornenes with groups containing hetcroatoms is slower than for exo-functionalized ones or for nb with the conventional cationic catalysts. This is thought to be due to the stabilization of intermediates by coordination of the heteroatom, E, which is only feasible for the < t>-functionalized molecule (Scheme 17). The insertion into the Pd-Me bond of PdMeCl(COD) of several nb derivatives functionalized either in the exo- or in the endo-t2ice has been studied in order to check whether this neutral less-electrophilic palladium center... 

This implies that the other elementary steps in cycle B (Scheme 1), i.e., Pd-carbomethoxy formation and protonolysis of the palladium-alkenyl species, must even be considerably faster than the observed overall high reaction rate. A high rate of Pd-carbomethoxy formation (at equilibrium) could be expected for the strongly electrophilic metal center. However, the latter step, protonolysis of the Pd-alkenyl bond in l-palladium-2-carbomethoxypropene and 2-palladium-1-car-bomethoxypropene, respectively, is expected to be a slow reaction, because the proton has to overcome a relatively high barrier of (electrostatic) repulsion by the cationic palladium center on its way to the palladium-carbon bond. 

Coordination of the anions to the cationic palladium center may strongly depend on the polarity of the reaction medium. Solvation of the ion-pair by protic solvent molecules, such as methanol, is expected to facilitate cation-anion dissociation and therefore render the metal center more electrophilic and more easily accessible for substrate molecules. In relatively apolar solvents, close-contact ion-pairs are generally expected to exist. Anion displacement by substrate molecules may then require the use of noncoordinating anions, such as certain tetraaryl borates [19], with a relatively strong affinity for interaction with the solvent molecules. This will lead to a reduced barrier for displacement of these anions by monomer molecules. 

Electron withdrawal from the coordinated alkene to an electrophilic metal center makes the coordinated alkene susceptible to attack by an external nucleophilic agent or by a ligand coordinated to the metal. A classic example using modification of the chemical nature of ethylene coordinated to a cationic metal center can be seen in palladium-catalyzed Hoechst-Wacker process [111]. The catalytic cycle can be represented by Scheme 1.37, which is comprised of the main cycle to convert the ethylene coordinated to Pd(II) into acetaldehyde and auxiliary cycles to re-oxidize the Pd(0) species to Pd(II) with Cu(I). The Cu(I) produced in the process is oxidized in turn to Cu(II) with oxygen. 

This model is based on the exploration of halogenation reactions studied by Henry in the context of Wacker-type reactions [77]. As for palladium an oxidation state higher than - -II can in principle not be induced with an oxidant such as divalent copper, interaction between copper salts and a palladium center should induce electron transfer, which, consequently, should weaken the palladium-carbon bond. The resulting electrophilic character at carbon should make this center susceptible to nucleophilic attack. 

The latter step of intermolecular C-N bond formation was independently investigated within the development of a palladium(II)/palladium(IV) catalysis for C-H bond activation and amidation (Figure 16.8) [127]. Theoretical support from calculations at the LANL2DZ level show that a palladium(lV) species is generated from the oxidation of palladium(II) chelate 188 with NFSI, which does not form a neutral palladium(IV) intermediate, but rather engages in direct nucleophilic attack of the bissulfonimide anion at the methylene position next to the electrophilic palladium(I V) center. This step proceeds with an activation barrier... 

In the case of palladium(ll), the stabilization of the filled 4r/-orbitals is high, and the importance of backdonation to the olefin is very much diminished. It is the donation of the Tt-orbitals of the olefin to the metal, the chelate effect (for chelating ligands), and the strain release for strained alkenes, that are the factors mainly responsible for the stabilization of the palladium-olefin bond. In other words, the dominant character of the metallic center, when coordinated to olefins, changes from Tt-nucleophilic palladium(0) to electrophilic palladium(ll). This does not mean, however, that backdonation to the tt" -orbital of the olefin does not play any role. On the contrary, as discussed in Section 8.06.2.3, it is considered an important factor for the stabilization of five-coordinated palladium (and platinum) complexes. 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po-... 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

Another approach for the chemoselective and asymmetric iodination of unactivated C H bonds was reported with a palladium catalyst using a chiral auxiliary (Scheme 5.19). Excellent diastereoselectivities were induced by chelating the auxiliary to the palladium catalyst center followed by an electrophilic C—H activation and iodination. Studies showed that I2 acts as both the reactant and the activator to form the reactive catalyst precursor, Pd3(OAc)3. After the reaction was completed, the formed Pdl2 was precipitated from the solution and could be reused several times without losing reactivity and selectivity. 

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... 

Attack by nucleophiles on the electrophilic ir-allylpalladium complex can take place by two distinct mechanisms, which have opposite stereochemical consequences. Stereochemical inversion is achieved by attack of the nucleophile directly on the allyl ligand on the face opposite the palladium (equation 148). Retention is achieved by attack of the nucleophile at the metal center, followed by reductive elimination (equation 149). The reductive elimination step could proceed through an V-allylpalladium-Nuc species or directly from an i)3-allylpalladium-Nuc complex. Recent evidence strongly suggests the latter pathway.384... 

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