Pd cationic complexes

The reaction usually proceeds with retention of configuration at the reacting centre. As in SN2 reactions going with retention (Chapter 37), this can mean only a double inversion. Coordination of Pd to the double bond of the allylic acetate occurs on the less hindered face opposite the leaving group and the nucleophile adds to the face of the 7t-allyl Pd cation complex opposite the Pd. The net result is displacement of the leaving group by the nucleophile with retention. Thereafter, the... 

A more recent experimental and computational study of rf-allyl-Pd cationic complexes confirmed the tendency of nucleophiles to attack the terminal positions of the allyl ligand as long as jx-accepting ligands are present.54 If a-donating ligands are attached to palladium, which would pump electron density into the allyl ligand, attack occurs mainly at the C-2 position. Equations 8.37 and 8.38 summarize these results. 

Since the 1950s, other catalysts have been used to polymerize substituted acetylenes [25]. Free radical initiators (e.g., benzoyl peroxide, di-f-butylperoxide) are active catalysts, as are complexes of other transition metals like Ni and Co. Recently, both Rh and Pd cationic complexes have been shown to polymerize monosubstituted acetylenes [119-121]. Thermal polymerizations and y-... 

Keto esters are obtained by the carbonylation of alkadienes via insertion of the aikene into an acylpalladium intermediate. The five-membered ring keto ester 22 is formed from l,5-hexadiene[24]. Carbonylation of 1,5-COD in alcohols affords the mono- and diesters 23 and 24[25], On the other hand, bicy-clo[3.3.1]-2-nonen-9-one (25) is formed in 40% yield in THF[26], 1,5-Diphenyl-3-oxopentane (26) and 1,5-diphenylpent-l-en-3-one (27) are obtained by the carbonylation of styrene. A cationic Pd-diphosphine complex is used as the catalyst[27]. 

Carbon monoxide also reacts with olefins such as ethylene to produce high molecular weight polymers. The reaction of CO with ethylene can be initiated by an x-ray irradiator (62) or transition-metal cataly2ed reactions (63). The copolymeri2ation of ethylene with carbon monoxide is cataly2ed by cationic Pd (II) complexes such as Pd[P(CgH )2] (CH CN) (BF 2 where n = 1-3. With this catalyst, copolymeri2ation can be carried out at 25°C and pressures as low as 2.1 MPa. 

Interaction of the latter with triphenylphosphine in the presence of sodium perchlorate leads to the cationic complex [(dppm)Au2(li-L)Pd(C Fj)(PPh3)]C10. Reactions of [L Au(p-L)AuL ] (L = bibenzimidazolate L = PPh, dppm)] with [Pd(0C103)(C Fj)(PPh3)2] do not lead to the tetranuclear products. Only the bibenzimidazolate (L) dinuclear product [(PPh3)(C F3)Pd(p-L)Pd(C, F3)(PPh3)] could be isolated in both cases. 

Scheme 5.2-3 Formation of a Pd-carbene complex by deprotonation of the imidazolium cation.

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The bis-phosphonium salt 56 in presence of Pd(OAc)2 leads to the formation of the neutral bis-ylide 57 which reacts with TICIO4 to give the dinuclear cationic complex 58 (Scheme 22) [89,90]. The bis-ylide part, which has potentially two carbons and one oxygen donor atoms, acts as a C,C-chelating ligand through its two soft ylidic carbons. 

Allyl-Pd(ll) complexes featuring a single NHC ligand have been explored for norbomene polymerisation. The first such disclosure appears in a patent of 2006 [39] in which complexes of the form 34 (Fig. 4.12) are activated with AgSbF and LiB(CgFj). The resulting cationic complexes catalyse the polymerisation of norbomene and polar-substituted norbomenes with very high activity. The best result... 

So far, there is only one report describing the use of chiral NHC-metal complexes in catalytic asymmetric arylation of imines. This was achieved by using C -symmetric cationic NHC-Pd diaquo complex 20 (Scheme 7.6) [38]. The arylation of a variety of A-tosylimines with different arylboronic acids was carried out under mild conditions. The presence of electron-withdrawing or electron-donating substituents on both partners did not seem to affect the reaction and the corresponding chiral diarylamines were obtained in good to excellent yields and high enantiomeric excess. 

Early work by Strassner and co-workers showed that the chelating bis-NHC Pd complexes 32a and 32c were capable of promoting the oxidation of methane, whilst the iodo-analogues 32b and 32d were inactive under the same reaction condition [45], Indeed, in a mixture of TEA and TFAA, in the presence of potassium peroxodisulfate under 20-30 bar of methane, trifluoroactic acid methyl ester is produced, using 32a or 32c as catalyst (Scheme 10.14). hi a more recent work, the authors disclosed the use of pyrimidine-NHC Pd complexes for the same reaction. A shghtly better catalytic activity was obtained with the unexpected cationic complex 34 [46],... 

Uptake measurements were made [16] at several oxide/solution ratios, reported as surface loading (SL) or m2 oxide surface/liter of solution, as PdCl, 2 concentration was increased and pH was held constant at the optimal value (Figure 6.10a). Each SL indeed indicated a plateau near the steric value [16], For Pt and Pd ammine cations, the maximum surface density over many oxides appears to be a close-packed layer, which retains two hydration sheaths representative results for PTA uptake over silica from a recent paper [19] are shown in Figure 6.10b. The physical limit of cationic ammine surface density thus appears to be 0.84 pmol/m2, or about 1 cationic complex/2 nm2. Cationic uptake, therefore, is inherently half of anion uptake in many cases. 

In the same research group the cationic hydridopalladium complex [Pd(H)(H20)(PCy3)2] [BF4] has been shown to catalyze the hydroxycarbony-lation of triple bonds. As a representative example the dehydration occurring to give the dienoic acid is displayed in Scheme 3 [35]. The same cationic complex is able to activate a carbon oxygen bond in a-allenic alcohols to provide dienoic acids but with the COOH group in the branched position (Scheme 3) [36]. 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

The most active and selective catalysts for both the copolymerisation process and for the apparently simpler ethene carbonylation to monocarbonylated products MP or DEK are cationic square planar Pd(II) complexes in which the metal centre is czs-coordinated by a bidentate P - P ligand, by a Ugand involved in the initial step of the catalysis or in the process of forming the product and with the fourth vacant site coordinated by CO or ethene or a keto group of the growing chain or MeOH (or H2O, always present in the solvent even when not added on purpose) or even by a weakly coordinating anion. 

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