Palladium complexes stabilization

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

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 reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

Other works have shown that phosphonium yhdes a-stabilized by a cyano or keto group can behave as ambidentate hgands towards palladium complexes (Scheme 21) [85-89]. 

Works developed by Navarro and coworkers for the palladium and stabilized ylides complexes have been extended to the platinum chemistry (Scheme 25) [97,98]. 

In summary, these results demonstrate that air-stable POPd, POPdl and POPd2 complexes can be directly employed to mediate the rate-limiting oxidative addition of unactivated aryl chlorides in the presence of bases, and that such processes can be incorporated into efficient catalytic cycles for a variety of cross-coupling reactions. Noteworthy are the efficiency for unactivated aryl chlorides simplicity of use, low cost, air- and moisture-stability, and ready accessibility of these complexes. Additional applications of these air-stable palladium complexes for catalysis are currently under investigation. 

The use of a basic solvent (in this case diethylamine) is important to stabilize acetylenic anions.(9) The third catalyst system component, triphenyl phosphine, is presumably added to help replace lost triphenyl phosphine ligands on the palladium complex and thus prevent metal agglomeration. 

While apparently no metallasilsesquioxane complexes of nickel and palladium have yet been prepared, several literature reports have appeared on platinum compounds containing silsesquioxane ligands. Abbenhuis reported the synthesis and characterization of three platinum(II) complexes stabilized by the chelating diphosphine ligand dppe (= 1,2-bis(diphenylphosphino)ethane) as outlined in Scheme 63. ... 

The most recent development concerns the heterocyclic (amino)(ylide)carbenes AYC. Such compounds have been known for some years [203] but so far had little impact compared to their diamino stabilized relatives. Both phosphorus ylide (86) and sulfur ylide (87) stabilized AYC ligands have been generated in situ and were stabilized at suitable metal centers (Fig. 27) [204, 205]. The palladium complex 88 with an anionic (amino) [bis(ylide)]carbene is also known [206]. 

Table 6 Stability constant data for palladium complexes... 

When another palladium complex, diiodobis(l, 3-dimethylimidazolium-2-ylidene)palladium(II), was used as a catalyst (257), it resulted in a large improvement in catalyst stability in the same ionic liquid. The Heck reaction performed better in the ionic liquid than in organic solvents such as dimethylfuran (DMF). In the reaction of bromobenzene with styrene, the yield of stilbene was increased from 20% in DMF to 99% in [NBu4][Br]. The ionic liquid showed excellent solubility for all the reacting molecules. 

Detailed structural studies of bicyclic azetines have been difficult to carry out because of the instability inherent in this type of ring system. The presence of fluorine-containing substituents though can stabilize some bicyclic azetines sufficiently to obtain physical data in support of these structures. X-ray crystallographic examination of palladium complex (117) showed that each l-azabicyclo[2.2.0]hexadiene ring is planar. Selected bond angles and distances for (117) are listed in Table 9 (76CPB2219). 

All of these processes are of limited synthetic utility because of the requirement of the use of stoi chiometric amounts of palladium complexes. However, by judicious choice of reactants and condition the above-mentioned impediments to catalysis can be overcome. For example, an efficient palladium(II) catalyzed cyclization of o-allyl- and o-vinyl-anilines to indoles has been developed (equation 14).28 Be cause arylamines are -106 less basic than aliphatic amines, and because the cyclized product in thi system gave an enamine (indole) stabilized by aromatization, the problems of catalyst poisoning by sub strate or product were circumvented, and catalysis was successfully achieved. The system was quit tolerant of a variety of functional groups and was used to prepare indoloquinones in excellent yieli... 

Equation 76) <1993OM3019>, which react as ester and ketone enolate equivalents, respectively. The latter reaction requires the use of fluoride ion activation (tetrabutylammonium fluoride, TBAF) to actuate the addition. Central carbon alkylation is less common for allylpalladium reactions despite this, nucleophilic alkylation of TMEDA-stabilized 1,3-diphenylallyl palladium complexes proceeds selectively to the central carbon (Equation 77) <1995AGE100>. 

Tests made in a batch-wise system showed that Go-5 had a lower activity than the corresponding monomeric phosphino ester palladium complex. Although less active, the metallodendrimer Go-5 displayed higher stability... 

---