Zero valent palladium

A low temperature catalytic process has been reported (64). The process involves the divalent nickel- or zero-valent palladium-catalyzed self-condensation of halothiophenols in an alcohol solvent. The preferred halothiophenol is -bromothiophenol. The relatively poor solubiHty of PPS under the mild reaction conditions results in the synthesis of only low molecular weight PPS. An advantage afforded by the mild reaction conditions is that of making telecheHc PPS with functional groups that may not survive typical PPS polymerization conditions. 

It is apparent from mechanistic considerations that an active species in the palladium-catalyzed dimerization of butadiene is a zero-valent palladium complex, which forms bis-ir-allylic complex 20. 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

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°. 

The coupling product occurred in high yield with iodobenzene (80% vs 3-bromothio-phene). With aryl bromides, an electron-withdrawing group was required to achieve the coupling reaction (with FG = p-CN and o-MeOCO, yields are 47% and 40%, respectively, vs 3-bromothiophene). Conversely, no coupling product was observed in the presence of electron-donor substituents. In this last case, the deactivated aryl bromide is certainly unreactive towards zero-valent palladium. 

Shi el al. reported one of the first examples of bimetallic catalytic systems that allowed the insertion of C02 into the rather unreactive fin-carbon bond [45], The concept behind this system was to exploit, in the same system, the ability of a transition metal to catalyze crosscoupling reactions and C02 activation. For instance, tributyl(allyll)tin does not react with C02 in solution even under high-pressure. To run the same reaction in the presence of zero-valent palladium species (Pd(PPh3)4 or Pd(PBu3)4) will quantitatively afford carboxylates 2 (90%) and 3 (10%) (Scheme 5.10), although the reactivity of the system is limited to allylstannanes. 

One of the starting materials, the bromoindolinemesylate 183 was obtained from the commercially available 5-hydroxyindole by mesylation followed by successive treatment of the resulting indole derivative with sodium cyanoborohydride and bromine. Coupling of 183 with the known boronic acid 184 in the presence of zero valent palladium complex led directly to the lactam 185, the intermediate carbinolamine 186 formed initially in the reaction suffering facile aerial oxidation during work-up. On reduction with sodium (2-methoxyethoxy)aluminium-hydride, the amide 185 yielded the aminophenol 187 which on chromatography underwent oxidative aromatisation to 182 in 54% yield. 

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... 

Some new and attractive methods, for example eliminations under phase-transfer conditions and couplings under the influence of zero-valent palladium compounds, have been included. 

For the crucial stage of the oxidative addition of an electrophile to the zero-valent palladium, double synergistic interaction takes place ... 

The basic assumptions are as follows The precatalyst is first reduced to the zero-valent palladium species 4.42. Oxidative addition of Ar-Cl to 4.42 gives... 

There are two main uncertainties associated with this general mechanism. First, there are a number of C-C coupling reactions where there is no direct evidence for the reduction of the Pd(II) precatalyst into a zero-valent palladium species. Second, like the hydrosilylation system, a number of these reactions may involve colloidal palladium. Also, the general catalytic cycle needs to be substantially modified to rationalize the successful use of 7.63 as a precatalyst. 

The sum of all these steps is the net reaction 8.1. To make the reaction catalytic, 8.8 must be converted back to 8.1. This is achieved either in the same reactor or in another one by oxidizing zero-valent palladium with copper(II) chloride (Eq. 8.2) and the resultant cuprous chloride by dioxygen (Eq. 8.3). 

This phosphine complex, however, is not reduced by alcohol to zero-valent palladium and oxalate ester, nor is it formed by insertion of carbon monoxide into the palladium-carbon bond of the related aikuxycarbonyl species. 

Complexes of zero-valent palladium (d ) are tetrahedral if the coordination number is four, trigonal if the coordination number is three, and hnear if the coordination number is two. For Pd (d ), the geometry is square planar, unlike nF, which is normally octahedral, but like PF. With a very bulky ligand, 14-electron three-coordinate T-shaped species can be stabilized and structurally characterized. For example, the complex Pd(Ar)l[P(/-Bu)3], where Ar = 2,4-xylyl, can be isolated and the aryl group is opposite the open coordination site. For the few Pd (d ) complexes that have been structurally characterized, octahedral geometry is found. 

The zero-valent palladium complex Pd(nbd)(ma), nbd = norbomadiene, ma = maleic anhydride, is a useful precursor to novel Pd° complexes with nitrogen a-donor ligands. In THF, pyridine, diethyl amine, aniline, and even NH3 displaced the nbd ligand to form E2Pd(ma) complexes. ... 

Zero-valent palladium carbonyls without phosphine ligands have been prepared by two groups of workers by the technique of matrix isolation at low temperatures. Carbonyls of the type Pd(CO) (m = 1 to 4) have been characterized by IR spectra. Diffusion studies indicate that the lower carbonyls react readily with CO to give Pd(CO)4 (46, 167). 

The following mechanism was proposed for the carbonylation of olefin-palladium chloride complex (10). The first step is coordination of carbon monoxide to the complex. Insertion of the coordinated olefin into the palladium-chlorine bond then forms a -chloroalkylpalladium complex (IV). This complex undergoes carbon monoxide insertion to form an acylpalladium complex (V), as has been assumed for many metal carbonyl-catalyzed carbonylation reactions. The final step is formation of a )8-chloroacyl chloride and zero-valent palladium by combination of the acyl group with the coordinated chlorine. 

During the following 15 years, only small advances were achieved in increasing catalyst efficiencies. Independently, Fenton [9a] at Union Oil and Nozaki [9b] at Shell Development Company (USA) discovered several related palladium chlorides, palladium cyanides, and zero-valent palladium complexes as catalysts. Sen and co-workers [10] reported that cationic bis(triphenyl-phosphine)-palladium tetrafluoroborate complexes in aprotic solvents such as dichloromethane, produced ethylene/carbon monoxide copolymers under very mild conditions. The reaction rates were, however, very low, as were the molecular weights. 

Carbonylphosphine complexes of zero-valent palladium are considerably less stable and more reactive than their Ni counterparts. Most common triphenyl-phosphine complexes of Pd are excellent catalysts for various carbonylation reactions of aryl iodides and bromides [15,129-131 ]. It is conceivable that the palladium-catalyzed alkoxycarbonylation of ArCl proceeds via a mechanism similar to that proposed for the analogous reactions of bromo- and iodoarenes (Scheme 5) [45,159,160,161]. 

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