Zerovalent palladium complexes

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

The second mechanism involves the oxidative addition of methanol to the divalent acylpalladium complex 14 (19, Figure 12.14). This reaction has the only advantage that the new hydride initiator is formed in one step, but apart from this it is an unlikely reaction. Oxidative addition of alcohols is only known for electron-rich zerovalent palladium complexes [46],... 

The reaction starts with an oxidative addition of an allylic compound to palladium(O) and a Tt-allyl-palladium complex forms. Carboxylates, allyl halides, etc. can be used. In practice one often starts with divalent palladium sources, which require in situ reduction. This reduction can take place in several ways, it may involve the alkene, the nucleophile, or the phosphine ligand added. One can start from zerovalent palladium complexes, but very stable palladium(O) complexes may also require an incubation period. Good starting materials are the 7t-allyl-palladium intermediates ... 

Bromophenyl)ethanol undergoes a palladium-catalyzed carbonylation which results in the formation of isochroman-l-one (79H(12)92l). It is considered that the reaction involves formation of an aryl-palladium complex (624) through insertion of the zerovalent palladium complex (623) into the aryl halide. Insertion of carbon monoxide followed by reductive elimination of the metal as a complex species leads to the isochromanone (Scheme 242). 

Mechanistic studies (ref. 6, 7) suggest the intermediacy of a zerovalent palladium complex produced by reductive elimination of an acid chloride, and the need for hydrogen chloride in order to cleave the C-OMe bond by preliminary protonation of the methoxy group leading to an allylpalla-dium complex which undergoes carbonylation (Scheme 1). 

In the case of experiments performed under the conditions of run 6, but in the presence of 1 ml of methanol, 1.6 equivalent of dimethyl carbonate was obtained according to GC analysis. No dimethyl carbonate was observed in the absence of hydrogen chloride. Therefore, in the early stage of the carbonylation of 3, Pd/C is partly oxidized to palladium chloride (eqn. 2). This compound reacts in turn with CO and MeOH to give, according to one of the routes described in Scheme 2, dimethyl carbonate and a zerovalent palladium complex (noted [Pd]). 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

Amatore, C., Azzabi, M., Jutand, A. Role and effects of halide ions on the rates and mechanisms of oxidative addition of iodobenzene to low-ligated zerovalent palladium complexes PdO(PPh3)2. J. Am. Chem. Soc. 1991,113, 8375-8384. 

This reaction is catalyzed by nickel or palladium complexes. However, the selectivity and activity of nickel catalysts are lower than those of palladium catalysts. Palladium-catalyzed reactions give linear dimers selectively and no cydization takes place. Not only the zerovalent palladium complexes but also certain bivalent ones can be used as active catalysts in combination with excess PPh3. However, the zerovalent palladium complexes are somewhat tedious to prepare and unstable in oxygen, so easily available and stable bivalent palladium compounds such as Pd(OAc)2 are generally used with PPh3. A proposed mechanism is given in Figure 1 [1 f]. 

Amatore, C., Carr6, E., Jutand, A. et al. (1995) Rate and mechanism of the formation of zerovalent palladium complexes from mixtures of Pd(OAc)2 and tertiary phosphines and their reactivity in oxidative additions. Organometallics, 14, 1818-26. 

Jutand, A. and Mosleh, A. (1995) Rate and mechanism of oxidative addition of aryl triflates to zerovalent palladium complexes. Evidence for the formation of cationic (aryl)palladium complsx s. Organometallics, 14, 1810-7. 

Alkyl halides are usually considered to be less suitable for double carbonylation because of the possibility of the direct reaction of alkyl halides with nucleophiles and of instability of alkyl-transition metal complexes involved in the catalytic process. However, allylic halides were found amenable to double carbonylation promoted by zerovalent palladium complex. It is well known that allylic halides undergo ready oxidative addition with a Pd(0) species to produce Tj -allylpalladium halide complexes. Thus, it was reasoned that the double carbonylation process might be realized if CO insertion into the aUyl-palladium bond proceeds before attack of amine on the 17 -allylpaUadium halide takes place. On the basis of fundamental studies on the behavior of i7 -allylpalladium halide complexes with CO and secondary amines, double carbonylation processes of substituted aUyl halides to give a-keto amides in high yields have recently been achieved (Eqs. 15 and... 

Zerovalent palladium complexes with the unsymmetric iVyV-ligands 8-(2-pyridyl)quinoline, 8-(6-methyl-2-pyridyl)-quinoline, or /ra j -4,5-bis(2-pyridyl)norbornane and ma or fn have been obtained from Pd2(dba)3, the chelating ligand. 

The reaction of zerovalent palladium complexes with a,/ -unsaturated aldehydes or ketones, -coordinated through the C-C double bond, with Lewis acids (LAs) produce O-substituted T/ -allylic complexes. Protonation of these complexes or reaction with methyl triflate lead to the coordination of the carbonyl carbon to palladium and formation of cationic Pd(ii) 77 -allyl complexes (Equation (36)). ... 

Amatore C, Jutand A, Suarez A (1993) Intimate mechanism of oxidative addition to zerovalent palladium complexes in the presence of halide-ions and its relevance to the mechanism of palladium-catalyzed nucleophilic substitutions. J Am Chem Soc 115 9531... 

Kakino R, Nagayama K, Kayaki I, Shimizu I, Yamamoto A. Formation of a paUadalactone complex by C—O bond cleavage of diketene promoted by a zerovalent palladium complex. Chem Lett. 1999 28 685-686. 

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