Palladium systems

One of the first efficient catalytic palladium systems for asymmetric allyhc alkylation that does not involve phosphines has been reported by Pfaltz [37], who used bisoxazohnes. 

Cazin and co-workers recently reported on the use of the well-defined dimer complexes [Pd( a-C1)(C1)(NHC)]2 that are commercially available, and perform exceedingly well in the Suzuki-Miyaura reaction involving aryl chlorides [108]. The Cazin group has also recently disclosed well-defined mixed NHC/phosphite palladium systems of the type [PdCl2(NHC) P(OR)3j], enabling the Suzuki-Miyaura of aryl chlorides at 0.1 mol% Pd loading [109]. 

Hydroxycarbonylation and alkoxycarbonylation of alkenes catalyzed by metal catalyst have been studied for the synthesis of acids, esters, and related derivatives. Palladium systems in particular have been popular and their use in hydroxycarbonylation and alkoxycarbonylation reactions has been reviewed.625,626 The catalysts were mainly designed for the carbonylation of alkenes in the presence of alcohols in order to prepare carboxylic esters, but they also work well for synthesizing carboxylic acids or anhydrides.137 627 They have also been used as catalysts in many other carbonyl-based processes that are of interest to industry. The hydroxycarbonylation of butadiene, the dicarboxylation of alkenes, the carbonylation of alkenes, the carbonylation of benzyl- and aryl-halide compounds, and oxidative carbonylations have been reviewed.6 8 The Pd-catalyzed hydroxycarbonylation of alkenes has attracted considerable interest in recent years as a way of obtaining carboxylic acids. In general, in acidic media, palladium salts in the presence of mono- or bidentate phosphines afford a mixture of linear and branched acids (see Scheme 9). 

Several reports covering carbonylation and other aspects of homogeneous catalysis include the hydroxycarbonylation reaction with palladium systems.310,311,629 A review summarizing the results and findings about the mechanism has been published.630 The carbonylation of vinyl... 

The critical role of the counter anion in the hydroxycarbonylation of styrene for different phosphorus-modified palladium systems has been reported. The regioselectivity can be controlled... 

Most researchers currently agree that the hydrido mechanism is more common than the alkoxycarbonyl path in the alkoxycarbonylation of alkenes with palladium systems. However, carbalkoxy complexes are putative intermediates in carbonylation reactions giving succinates and polyketone diesters, with metals like Co, Rh, or Pd.137... 

In addition to the foregoing late transition metal catalyst developments, which have led to the discovery of sophisticated palladium systems capable of the catalytic (coordination) copolymerization of ethylene with acrylates, there have also been some interesting studies into systems where it eventually transpired that the mechanism was actually free radical rather than catalytic. 

Palladium(O) forms a complex with quinone that is now electron rich and can be protonated to give hydroquinone and palladium(II). The latter can start a new cycle via a carbomethoxy species after reaction with methanol and CO (c.f. reaction (6), Figure 12.4). Thus we have formally switched from a hydride initiator to a carbomethoxy initiator species. Addition of quinone to a nonactive or moderately active palladium system is a diagnostic tool that tells us whether zerovalent palladium is involved as an inactive state. Likewise, one might add dihydrogen to a system to see whether palladium(II) salts need to be converted to a hydride to reactivate our dormant catalyst. 

The aldol condensation/hydrogenation reaction was carried out in a continuous flow microreactor. The catalysts (0.5 g) were reduced in situ in a flow of H2 at atmospheric pressure at 723 K for 1 h for the palladium systems and 2 h for the nickel systems. The liquid reactant, acetone (Fisher Scientific HPLC grade >99.99%), was pumped via a Gilson HPLC 307 pump at 5 mL hr into the carrier gas stream of H2 (50 cm min ) (BOC high purity) where it entered a heated chamber and was volatilised. The carrier gas and reactant then entered the reactor containing the catalyst. The reactor was run at 6 bar pressure and at reaction temperatures between 373 and 673 K. Samples were collected in a cooled drop out tank and analyzed by a Thermoquest GC-MS fitted with a CP-Sil 5CB column... 

Prior to 1991, no high-yield double silylation had been reported using simple peralkyldisilanes such as hexamethyldisilane. In that year, Ito and Tanaka independently reported that palladium systems, with isocyanate and P(OCH2)3CEt ligands, respectively, promote insertions into the Si-Si bond of unactivated disilanes. 

In contrast to the corresponding palladium system, water is not involved as the oxygen source. In fact it is an inhibitor, and the presence of a dehydrating agent such as 2,2-dimethoxypropane speeds up the reaction. 

Several groups have been successful at the catalytic conversion of carbon dioxide, hydrogen, and alcohols into alkyl formate esters using neutral metal - phosphine complexes in conjunction with a Lewis acid or base (109). Denise and Sneeden (110) have recently investigated various copper and palladium systems for the product of ethyl formate and ethyl formamide. Their results are summarized in Table II. Of the mononuclear palladium complexes, the most active system for ethyl formate production was found to be the Pd(0) complex, Pd(dpm)2, which generated 10/imol HCOOEt per /rniol metal complex per day. It was anticipated that complexes containing more than one metal center might aid in the formation of C2 products however, none of the multinuclear complexes produced substantial quantities of diethyl oxalate. 

As important as it is, the hydrogen-palladium system is rather limited in its application. For this reason, other electronic materials that could operate on the same principle have been investigated. Organic semiconductors proved to be most useful in that respect. 

Section 6 describes the use of the kinetic theory for studying physicochemical mechanics problems. An example of the theory is used to explain the observed properties for hydrogen-palladium systems. It is also used to self-consistently connect the local adsorption and diffusion characteristics of hydrogen atoms with the deformations within the metal membrane. 

Aakermark B, Hansson S, Krakenberger B, Vitagliano A, Zetterberg K (1984) Alkylation of (7t-allyl)palladium systems. Mechanism and regiocontrol. Organometallics 3 679-682... 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

Ipaktschi and Sharifl reported the palladium-catalyzed synthesis of 2,7-diamino fluorenones by two indirect routes due to the base sensitivity of fluorenones [182]. First, 2,7-dibromofluorene was reacted with secondary amines, and subsequent oxidation of the product formed the diaminofluorenone. Alternatively, reaction of ami-nostannanes derived from secondary amines with 2,7-dibromofluorenone gave yields of the fluorenone ranging from 42 to 58%. Beletskaya has used the DPPF-ligated palladium system to conduct selective monoarylation of ethylene diamine, diethylene triamine, triethylene tetra-amines, and 2,2-dimethyl butane-1,3-diamine [183]. 

Abstract N-heterocyclic carbenes (NHCs) have attracted increasing attention since their discovery. Notably, they have allowed for major advances in palladium-catalyzed reactions. Mainly known for their application in cross-coupling reactions, this review intends to provide a broader overview of (NHC)-palladium systems in organic transformations. 

The Heck reaction was the first catalytic application reported with palladium-NHC complexes [49,50] and since then it has been considered a standard reaction for testing new palladium systems. Not only for their reactivity, but also stability as in most of the cases long reaction times under harsh conditions are required to obtain good yields [51-66]. 

Despite the many phosphine/palladium systems developed for the coupling of organohalides with organotin compounds [117-119], this reaction re-... 

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