Palladium , monomer-dimer

Sterrer, M., Risse, T., Giordano, L., Heyde, M., Nihus, N., Rust, H. R, Pacchioni, G., and Freund, H. J. 2007. Palladium monomers, dimers, and trimers on the MgO(OOl) surface viewed individually. Angewandte... 

The dimeric amido complexes underwent reductive elimination after cleavage to form two monomeric, 3-coordinate, 14-electron amido complexes. In the case of the anilido dimer 20, a half-order rate dependence in the palladium complex showed that the reductive elimination occurred after reversible cleavage of the dimer to form two monomers. In the case of the f-butylamido complex 21, rapid reductive elimination occurred after irreversible dimer cleavage. This conclusion was supported by reaction rates that were first order in palladium dimer and by the lack of crossover during the reductive elimination reactions containing two doubly-labeled dimers. 

Telomerization of Isoprene.—Reviews have appeared on isoprene and chloro-prene, and on the complex reactions of isoprene to form terpenoids (in Japanese). Isoprene reacts with magnesium, especially in the presence of Lewis acids, and the resulting complex gives adducts with aldehydes. As usual in this type of reaction, a very complex mixture is obtained. The palladium-chloride-catalysed reaction of isoprene with acetic acid gives different products in different solvents. Monomers predominate in benzene [2-methylbut-2-enyl acetate (5) and 3-methylbut-2-enyl acetate (6)] while dimers [(7), (8), neryl (9), and geranyl (10) acetates] tend to be formed in tetrahydrofuran. Further details of the synthesis of Cio alcohols from isoprene and naphthyl-lithium are available, as well as of the in situ oxidation,but there is little of novelty (see Vol. 1, p. 17). 

The vinyl ester exchange has also been studied in the chloride-free palladium(II) acetate system, using vinyl propionate as substrate. Of the three Pd(II) species present in this system (Section II, A, 2), the dimer is most reactive, with the trimer Pd3(OAc)g next, and the monomer unreac-tive. The rate expression for exchange catalyzed by the dimer is (214). 

Chain propagation of CO/ethylene copolymerization proceeds by a strictly alternating insertion of CO and olefin monomers in the growing chain. It is safe to assume that double CO insertion does not occur for thermodynamic reasons [Ic]. However, the complete absence of double ethylene insertions is remarkable because ethylene insertion in a Pd-alkyl species must be exothermic by about 20 kcal/mol (84 kJ mol). The observation of strict alternation is the more surprising since the same palladium catalysts also efficiently dimerize ethylene to butenes [25]. The perfect alternation is maintained even in the presence of very low concentrations of carbon monoxide. When starting abatch polymerization at a high ethylene/CO ratio, error-free copolymer is produced until all the CO is consumed then the system starts forming butenes (with some catalyst systems at about twice the rate of copolymerization ). 

Similar to the bipyrimidine palladium complex, Bercaw and coworkers demonstrated that hydroxy-bridged dimers [ (diimine)Pd(OH) 2]2+ 11 could also effect activation of the allylic C-H bond [25]. Mechansitic studies revealed that the solvent (trifluoroethanol or MeOH) assisted dissociation of dimer 11 to monoca-tionic monomer 12, followed by the rate-limiting -coordination of indene to palladium center, and finally by fast C-H activation (Scheme 9). 

Powell et al. (113) have utilized NMR to aid in the detection of a tri-haptoidimexy-pentahaptoijaonovaeT) equilibrium for a series of hepta-2,6-dienylpalladium acetate complexes in chloroform solution. The chemical shifts for C(l) and C(2) in IV (W = H, X = O2CCF3) are compatible with a coordinated olefin, but for IV (W = Cl, X = O2CCF3) these shifts are almost identical to those of III (W = Cl, X = hfacac) in which the olefin C(l) = C(2) is not coordinated to palladium. This evidence together with molecular weight data led the authors to conclude that IV existed in an /t (dimer)-A (monomer) equilibrium the position of equilibrium being dependent on both the olefinic substituent W and the carboxylate substituent R. 

The allyl carbonates [10a,b] and [lla,b] were reacted by palladium(0)-catalysis with different equivalents of hydroquinone to direct the product formation either to the monomer or to the dimer. This is indeed possible as indicated by Eqs. 11 and 12 and Tables 1 and 2. 

In the Mitsunobu reaction with the allylic alcohols [2] and [5a,b] as with the palladium(0)-catalyzed reaction, good yields are obtained for the monomers [23] and [27] with an excess of nucleophile [22]. The dimers [24] and [28], however, ean be prepared only in moderate yields. Beeause the reaction does not proeeed via a n-allyl palladium complex with two reaetion sites, additional isomers are not produeed in this reaction. The conversion of [2] therefore yields only [27] and [28] as product. 

Ea = 41.5 kJmol. The palladium(ii) hepta-2,6-dienyl complexes (64) exist in solution at normal temperatures as a rapidly equilibrating mixture of the A -dimer and A -monomer forms (Scheme 9). The position of equilibrium is seen to be affected... 

An extension to work on platinum(i) isocyanides discussed above has been undertaken, showing that it is possible to coordinate metals other than platinum. In the first instance, platinum and palladium compounds were added to the dinuclear Pt(i) compound 103 to give either the linear trinuclear complex 105 (when a -M(0) source was used), or a combined dimer-monomer complex 124 (when a -M(ii) source was added) (Scheme 28). An A-frame complex 125 results when the -source is added to 102. 

Maleic anhydride monomer cont.) oxime adducts, 228 palladium complexes, 213 peracid formation, 246 phenols photochemical reaction, 180 phosphine reactions, 230 phosphite reactions, 206 photo-adduct applications, 213 photo-adduct sterochemistry, 182, 183, 192-194 photochemical dimerization, 188 photochemical polymerization, 195, 239, 241-243, 247, 248... 

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