Ligand-free catalysts

The same group found that heteroaromatic halides can be smoothly phenylated using NaBPh4 as the transmetallating agent and ligand-free Pd(OAc)2 as a catalyst with water or AT-methylformamide (NMF) as the solvent (Scheme 21) [39], The experiments were all performed using power control. 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Simple Pd salts and complexes which contain neither phosphines nor any other deliberately added ligands are well known to provide catalytic activity in cross-coupling reactions. Such catalytic systems (often referred to as ligand-free catalysts ) often require the use of water as a component of the reaction medium.17 In the majority of cases such systems are applicable to electrophiles easily undergoing the oxidative addition (aryl iodides and activated bromides), although there are examples of effective reactions with unactivated substrates (electron-rich aiyl bromides, and some aryl chlorides).18,470... 

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29],... 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

Ligand-free catalysts have been prepared from the following types of nickel(II) compounds nickel salts of long-chain aliphatic or aromatic carboxylic acids (10, 11) or of sulfonic acids (11), nickel enolates of /3-diketones (11) [e.g., nickel acetylacetonate (4, 12)] or their imino derivatives (11, 13), nickel phenolates (11), dithiocarbamates (14), and mer-captides (15). 

Using a chiral diphosphine Rh(I) complex as the catalyst, high catalytic activity and a good branched/linear ratio can be achieved, but in most cases the ee values remain moderate.122 The enantioselectivity of Rh(I)-catalyzed reactions depends on the amount of added chiral ligand because of the much higher catalytic activity of ligand-free rhodium species. Usually 4 6 equivalents... 

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

IR studies on Ligand-free Rhodium Carbonyl Catalysts 237... 

In this chapter, we will focus only on the latest reports, first dealing with a ligand-free system and later with multidentate N,N- and N,0-ligand systems. As a general rule of thumb, non-heme iron catalysts work best if they have exchangeable ligands [28]. 

Because aryl phosphines are not only costly but can also act as aryl sources themselves, giving rise to unwanted byproducts, there has been steady interest in extending ligand-free Heck reactions to aryl bromides and aryl chlorides. Reetz and de Vries recently found that these can be performed with high efficiency using stabilized colloidal Pd catalysts [21]. If the palladium is kept at a low concentration between 0.01 and 0.1 mol%, precipitation of the Pd(0) is avoided and the colloids serve as a reservoir for the catalytically active species (Scheme 5). This economically attractive method has been successfully applied on an industrial scale by DSM [22]. 

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