Palladium zero-valent complexes, addition

Further unequivocal examples for oxidative addition of organometallic M-C bond to a zero-valent complex were obtained by the reactions of Me-MoCp(CO)3 and AcCo(CO)4 with palladium(O) and platinum(O) complexes giving (dppe)RM-M L (M = Pt, Pd. M = Mo, Co) in high yields (Eq. 3.50) [177]. Such oxidative addition reaction is considered to be reversible, since the organic group in L2RM-M Cp(CO)3 transfers to M to give RM Cp(CO)3 by reductive elimination at M [178]. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Harrison, K.N., Hoye, P.A.T., Orpen, A.G., Pringle, P.G., and Smith, M.B., Water-soluble, zero-valent, platinum-, palladium,- and nickel-P(CH2OH)3 complexes catlaysts for the addition of phosphine to formaldehyde, J. Chem. Soc., Chem. Commun., 1096, 1989. 

Terminal allenes.1 A synthesis of 1,2-dienes (3) from an aldehyde or a ketone involves addition of ethynylmagnesium bromide followed by reaction of the adduct with methyl chloroformate. The product, a 3-methoxycarbonyloxy-l-alkyne (2), can be reduced to an allene by transfer hydrogenolysis with ammonium formate catalyzed by a zero-valent palladium complex of 1 and a trialkylphosphine. The choice of solvent is also important. Best results are obtained with THF at 20-30° or with DMF at 70°. 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

Ethylene (tert-phosphine) complexes of zero-valent nickeP and platinum have been known for years. Analogous palladium complexes can be synthesized along the same lines as those reported for the nickel compounds, using ethoxy-diethylaluminum(III) as the reducing agent in the presence of ethylene. These palladium-ethylene complexes may serve as starting materials for oxidative addition reactions, since the ethylene ligand is loosely bonded. ... 

The chemistry of palladium is dominated by two stable oxidation states the zero-valent state [Pd(0), d ] and the +2 state [(Pd(II), d ]. Each oxidation state has its own characteristic reaction pattern. Thus, Pd(0) complexes are electron-rich nucleophilic species, and are prone to oxidation, ligand dissociation, insertion, and oxidative-coupling reactions. Pd(II) complexes are electrophilic and undergo ligand association and reductive-coupling reactions. A large amount of literature deals with these reactions. However, a few fundamental principles, such as oxidative addition, transmetalation, and reductive elimination, provide a basis for applying the chemistry of palladium in research. 

The carbonylation of chloroarenes has been described by Alper and Grushin [27] and Jenner and Bentaleb [28], While the former showed that square-planar complexes of divalent palladium, [ L2PdCl2], where L = tertiary phosphine, are active catalysts for the biphasic carbonylation of aromatic halides, including chloroarenes (when L = tricyclohexylphosphine), to the corresponding carboxylic acids, the latter demonstrated that chloroarenes can be converted into aromatic acids via catalytic reaction with aqueous methyl formate under biphasic conditions. [ PdCl2(PCy3)2] was the most efficient catalyst. The addition of [ Ru3(CO)12] and ammonium formate improved yield and selectivity of the carbonylation reaction. The mechanism should involve oxidative addition of the C—Cl bond to a zero-valent Pd species followed by CO insertion. However, the palladium catalyst may also directly activate methyl formate. Compared to other carbonylations of aryl-Hal compounds the procedure is quite convenient (no solvent, no initial pressurization) [27]. 

In the C-0 bond cleavage reaction of vinyl epoxides by a palladium complex, formic acid acts as a good proton and hydride donor evolving CO2 (Eqs. 3.24 and 3.25). Zero-valent palladium complex favors the attack at an allylic carbon in an Sn2 manner to give i/ -allylpalladium(II) complex with inversion of configiuation, and formate anion coordinates to the palladium center (Scheme 3.50). Then, decarboxylation of the formate affords palladium hydride, which attacks the ry -allyl moiety from the endo side. Thus, 1,2-addition of hydrogen atoms takes place regioselectively with inversion at the allylic carbon [94]. 

Oxidative addition of Ph2P(0)H to M(PEt3)3 (M = Pd, Pt) takes place under ambient conditions (Eq. 3.44) [169]. In this reaction, the P-H bond initially oxidatively adds to the zero-valent group 10 metal complex to give a (hy-drido)metallacycle complex stabilized by an internal hydrogen bond. As shown in Scheme 3.85, the reaction is applied to catalytic hydrophosphinylation of terminal alkynes promoted by zero-valent palladium complexes. 

P-Se bond oxidatively adds to zero-valent palladium and platinum complexes [170], Treatment of M(PEt3)3 (M = Pd, Pt) with PhSeP(0)(0R)2 results in oxidative addition to give tran5-M(SePh)[P(0)(0R)2](PEt3)2 (Eq. 3.45). Catalytic addition of selenophosphates PhSeP(0)(0Ph)2 to terminal alkynes RC CH giving R(SePh)C=CHP(0)(0Ph)2 is also performed by Pd(PPh3)4 via similar mechanism shown in Scheme 3.85. 

The palladium complex Pd(MeCN)2Cl2 is actually decomposed under the reaction conditions to form catalytically active nano-sized palladium clusters. However, in the case of less reactive aryl halides bearing electron-donating groups, e.g. 4-bromo toluene, addition of 10 mol% of P(0/-Pr)3 as stabilizing ligand for zero-valent palladium was necessary to rich high conversions. 

Another route to p.-Carbido complexes which we have found is oxidative addition of Lalor s halocarbyne complexes [21] to zero valent triphenylphosphine compounds of nickel, palladium and platinum. In these reactions the heterodimetalla cyclopropenes could be isolated which isomerize at higher temperatures to the dimetalla allene complexes. The first adducts of arylcarbyne complexes to zerovalent platinum complexes (arylcarbyne as analogue of ace-... 

Aryl iodides add cleanly to zero-valent PI13P (orL) complexes of palladium. The kinetics and activation parameters support a simple concerted oxidative addition process as in (126), preceded by (125). A... 

With iodo compounds yields are particularly high as they carbonylate more rapidly compared to the chloride and bromide derivatives. This can be ascribed to the greater ease of oxidative addition of the C-l bond to the zero-valent palladium complex. Activation to oxidative addition of the C-CI bond in chloroarene may be achieved by the formation of the corresponding arenetricarbonyl derivative, after which palladium-catalyzed formylation of the complex proceeds in fair yield at 30 bar CO/H2 pressure at 130°C (Eq 1.33). ° ... 

The two common catalytic intermediates 7.48 and 7.49 play critical roles in determining the overall efficiency of the coupling reactions. Complex 7.48 has palladium in the zero-valent oxidation state. Oxidative addition of ArX to 7.48, followed by isomerization of the cis complex to the trans isomer, leads to the formation of 7.49. 

In the absence of ethanol, the phosphine in combination with trace quantities of water can also act as the reducing agent. This is shown by reaction 7.4.1.2. The important point is that the precatalyst PdX must undergo reduction to give a zero-valent palladium complex that can undergo facile oxidative addition. 

Harrison KN, Hoye PAT, Orpen AG, Pringle PG, Smith MB (1989) Water soluble, zero-valent, platinum- palladium-, and nickel-P(CH20H)3 complexes eatalysts Pot the addition of PH3 to CH2O. J Chem Soc Chem Commun 1096-1097... 

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