Ligand-modified Catalysis

A major advance in homogeneous catalysis was the introduction of a trialkyl-phosphine to supplement the role of carbon monoxide in catalyst stabilization. [3] A ligand modifier such as trialkylphosphine serves three principal roles in a homogeneous catalytic process. It stabilizes the metal, it influences the reaction rate, and it influences process selectivity. 

Hermanns E, Hasenjager J, Driefien-Holscher B (2008) PEG-Modified Ligands for Catalysis and Catalyst Recycling in Thermoregulated Systems. 23 53-66 Hermanns J, see Schmidt B (2004) 13 223-267 Herrmann W, see Freimd C (2007) 22 39-77... 

In principle, with cobalt catalysts similar pathways for deactivation exist. Because of the low price of cobalt compared to rhodium, this is less important in unmodified cobalt catalysis, but the deposition of cobalt clusters and metallic cobalt can cause nozzles and valves to plug up, resulting in the shutdown of the plant. In case of a ligand-modified cobalt catalysis, the same problems of ligand deterioration e.g., by oxygen and peroxides, arise, necessitating a meticulous purification of the starting materials. 

In addition to the enantioselective effect, cinchona alkaloids also produce a rate acceleration, i.e. this is an example of ligand accelerated catalysis [14]. The model of a non-closepacked ordered array of cinchonidine molecules adsorbed on platinum, proposed by Wells and co-workers, was abandoned in their later study [15]. Augustine [16] deduced from the behaviour of this system at low modifier concentrations that the chiral sites are formed at the edge and comer platinum atoms, which involve the adsorbed cinchonidine and a metal adatom. The different authors agreed that the quinoline ring of the modifier is responsible for the adsorption on platinum, the quinuclidine part, through the nitrogen atom, interacts with... 

Apart from phosphanes, typical systems used as chiral ligands in catalysis are alcohols, amines, amino alcohols and other N-functionalities including imines, amides and N-modificd carbonyl groups. Dicarbonyl compounds and their chirally modified equivalents such as dioximes or crown ethers, as well as sulfoxides, arsines, and cyclopentadienyl ligands with chiral side chains have been used. 

While the detailed mode of action of the modified catalyst is not understood, our results are consistent with a "ligand accelerated" catalysis. In this model, the modifier is reversibly adsorbed on surface Pt atoms, thereby leading to a chiral active site that on the one hand is more active than the unmodified one and on the other hand is able to control the stereochemistry of the hydrogen addition. 

In the case of ethylbenzene oxidation in the beginning of reaction of oxidation in the presence of complexes Ni(II)(acac)2 (3.0-10 mol/1) with monodentate ligand-modifier MP, by-products AP and MPC (P=AP, or MPC) are formed consistently, at decomposition of PEH, as well as at catalysis Ni(ll)(acac)2 without additives MP 0 at i— 0. However... 

The interesting result was received in the case of research of cumene oxidation. At introduction of catalyst (with and without of ligand-modifier) in reaction of oxidation of cumene changes in the mechanism of formation of products it was not observed, as opposed to oxidation of ethylbenzene and toluene. So as well as in uncatalyzed oxidation or at catalysis by Ni(ll)(acac)2, and by system Ni(II)(acac)2 + MP (complexes Ni(II) (acac)2 2MP and Ni2(OAc)3(acac) MP), ketone AP and hydroperoxide CH are formed parallel w 0 at t- 0), but DMPC is product of CH decomposition t—>0) during all reaction of oxidation... 

The favorable effects of phosphine ligands in catalysis have been known for more than half a century. One of the first reports involves the use of triphenylphosphine in the Reppe chemistry, the reactions of alkynes, alcohols and carbon monoxide to make acryhc esters [2]. An early example of a phosphine-modified catalytic process is the Shell process for alkene hydroformylation nsing a cobalt catalyst containing an alkylphoshine [3]. 

Homogeneous unmodified or ligand-modified rhodium catalysts are predominantly utilized for the transformation of olefins with a chain length major advantages of rhodium catalysis are the reduced syngas pressure and lower reaction temperatures. These features have also been recognized by the chemical industry. Thus, in 1980 less than 10% of hydroformylation was conducted with rhodium, and by 1995 this had been increased to about 80% [3]. In some cases, a combination of Co and Rh can be advantageous [4]. 

Obviously, in Rh-catalyzed hydroformylation extreme care has to be taken to avoid all loss of the precious metal compound from the process. This requires keeping the metal in its most stable active form in the process and avoiding any process conditions that would harm the catalyst s life-time. This is the reason why no industrial process is known that uses non-ligand-modified Rh-catalysis even though pure Rh-carbonyl is an active hydroformylation catalyst. The breakthrough in industrial Rh-catalyzed hydroformylation came with the discovery of triarylphosphine-... 

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