Palladium complexes as catalysts

PAM sec polyacrylamide PAM see polyacrylonitrile palladium complexes, as catalysts for ATRP 492 Patterns of Reactivity scheme 11,26, 31 for prediction of reactivity ratios 365-6 lor prediction of iransfer constants 287 PB see polybutadieue PE see polyethylene... 

Octatriene reacts further with butadiene in acetic acid by using 7r-allylic palladium complex as catalyst to give a mixture of acyioxydo-decatrienes (54). 

De Groot et al. (18) prepared phosphine-functionalized carbosilane dendrimers of different generations (4, 8, 24, and 36 phosphine end groups) and used their palladium complexes as catalysts for the allylic alkylation of allyl trifluoroacetate with diethyl sodio-2-methylmalonate. 

The Heck-type reaction. The Heck reaction135 (or some modified procedure of it) is certainly one of the most powerful tools used in the preparation of precursors with acetylenic and vinylic subunits. For instance, in the case of precursors 46-49 the synthesis is conveniently achieved by a cross-coupling reaction in the presence of palladium complexes as catalyst. Two pathways are possible, as represented by equations 9 and 10108. 

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. 

To date, several homogeneously catalyzed processes involving palladium complexes as catalysts have been studied in considerable detail by computational methods. Examples comprise illustrative work on the industrially important oxida-... 

In several cases, the in situ formation of hydrogen peroxide is the first step of the process. Thus, phenol can be obtained from benzene, carbon monoxide (5 atm) and oxygen (65 atm) at 70 °C in a benzene-water-methyl isobutyl ketone mixture, with TS-1 and a palladium complex as catalysts [26]. Despite a 91% selectivity to phenol, benzene conversion (3.2%) and productivity are still too low for industrial application. The palladium complex is required to promote hydrogen peroxide formation upon reaction of oxygen, carbon monoxide and water [27[. 

The reaction between aryl or alkenyl halides or arenediazonium salts and al-kenes catalyzed by palladium complexes, the so-called Heck reaction, has been performed in aqueous media. Arylation of styrene or acryHc acid derivatives occurred in high yields in the presence of a free-ligand palladium complex as catalyst and a base (Na2C03 or K2CO3) [96-98] and, eventually, a quaternary ammonium salt [Eq. (14)] [99,100]. 

The increase of the catalyst turnover numbers is indeed one other major area where further improvements could be expected. Such improvements have recently been achieved for the standard Heck reaction by the use of high pressure conditions [86], the use of preformed palladacycles as catalysts [87], or by using a macrocyclic tetraphole as hgand [88].Dendritic diphosphine-palladium complexes as catalysts for Heck reactions have also been reported to possess superior stabihty compared to the monomeric parent compounds [89]. Transferring such iimovations to the AHR remains an important goal. 

The arylation reaction of primary and secondary amines has been investigated using nickel or palladium complexes as catalysts, and bromo- or chloroarenes as arylating agents. Among the complexes tested, the more efficient catalyst is the bis (bipyridyl) nickel (II) bromide, bipy2NiBr2, which affords for example high yields in the arylation of allylamine with /n-bromotrifluoromethylbenzene. The reduction of the haloarene, sometimes observed with the nickel complexes, becomes predominant with palladium catalysts whatever the complex used. 

Aldehydes. Aryl, heterocyclic, and vinylic halides react at 80-150° under about 1000 psi of carbon monoxide and hydrogen (1 1) in the presence of these palladium complexes as catalysts and a tertiary amine (1 eq.) to give aldehydes, usually in good yields. The reaction is actually believed to involve a series of... 

Ketone synthesis. Ketones can be prepared in 65-85% yield by the reaction of aromatic or aliphatic acyl halides with diaryl- or dialkylmercury(II) compounds in HMPT with this palladium complex as catalyst. ... 

Another interesting hydroarylation reaction of alkynes with arenes using a dinuclear palladium complex as catalyst has been reported (Eq. 71) [161], although the precise mechanism is unclear. Trialkylboranes act as effective promoters for this reaction. 

The participation of cationic diphosphine/diphosphane palladium complexes as catalysts for the co-polymerization of CO with alkenes and in the alkoxycarbonylation of alkenes has been studied extensively. Quite recently, delineation of the initiation, propagation, and termination steps of the cycle has been reported. The systems are very complex to permit readily detailed kinetics experiments. However, the mechanistic steps and reactivity of those steps have been established, by principally, NMR spectroscopic methods. 

The vast majority of methods are based on the use of palladium complexes as catalysts, although copper, ruthenium, rhodium and iridium catalysts have also been used. Progress in the understanding of the mechanisms of these reactions has only been made during the past few years. As comprehensive reviews have been recently published on aryl-aryl bond-formation reactions, covering both mechanistic and synthetic aspects of these reactions [3-7], in this chapter we wiU summarize only those mechanishc studies on metal-catalyzed arylation reactions that have been carried out in detail. 

Much effort has been devoted to developing catalysts that control the enantioselectiv-ity of these substitution reactions, as well as the regioselectivity of reactions that proceed through unsymmetrical allylic intermediates. A majority of this effort has been spent on developing palladium complexes as catalysts. Increasingly, however, complexes of molybdenum, tungsten, ruthenium, rhodium, and iridium have been studied as catalysts for enantioselective and regioselective processes. In parallel with these studies of allylic substitution catalyzed by complexes of transition metals, studies on allylic substitution catalyzed by complexes of copper have been conducted. These reactions often occur to form products of Sj 2 substitution. As catalylic allylic substitution has been developed, this process has been applied in many different ways to the synthesis of natural products. ... 

Five more papers reported on various modifications of PES involving introduction and substitution of chloro or bromoatoms. For instance, bromination of the bisphenol-A unit in a commercial PES yielded the dibromoproduct (23), which was treated with butyllithium. The lithiated PES was then reacted with methyliodide [39] or with tosylazide [40]. The resulting azide groups were finally reduced to amino groups (24). Another modification of brominated PESs utilized palladium complexes as catalysts for the... 

A new type of telomerization was found recently by Tkatchenko et aL, when they used cationic palladium complexes as catalysts [11]. Not only methoxyoctadienes were formed by butadiene and methanol, but also Cj2-, Cj6-, C20- and C24-telomers. They presume, that the reaction preferentially occurs via the condensation of Cg-units. 

Mkoyi HD, et al. (Pyrazol-l-yl)carbonyl palladium complexes as catalysts for ethylene polymerization reaction. J Organomet Chem 2013 724 95-101. 

A new procedure, for the electrochemical synthesis of methyl acetylenecarboxylates starting from alkynes, imder mild conditions, is reported using a palladium complex as catalyst, carbon monoxide (p CO=l atm) and methanol at room temperature. The process is outlined in the following reaction. 

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