Phosphorus-based catalysts activities

Organocatalytic asymmetric carbonyl reductions have been achieved with boranes in the presence of oxazaborolidine and phosphorus-based catalysts (Section 11.1), with borohydride reagents in the presence of phase-transfer catalysts (Section 11.2), and with hydrosilanes in the presence of chiral nucleophilic activators (Section 11.3). 

The key to the successful development of homogeneous catalysts has been the exploitation of the effects that ligands exert on the properties of metal complexes by tailoring the electronic and steric properties of a catalytically active metal complex, activities and selectivities can be altered considerably. This especially holds for phosphorus based ligands, which are the most commonly encountered ligands as.sociated with organometallic compounds. 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

The chiral bicyclic phosphines 5 (and in particular 5a [7b]) are currently the most active phosphorus-based acylation catalysts, enabling use of low reaction temperatures. Under these conditions (i.e. —40 °C) selectivity factors as high as 370-390 were achieved (Scheme 12.2). This is the best selectivity factor ever reported for metal-free, non-enzymatic kinetic resolution. As a consequence, very good enantiomeric purity of both the isobutyric esters 7 and the remaining alcohols 6 was obtained, even at substrate conversions approaching 50% (Scheme 12.2) [7, 8],... 

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

Tungsten-based hydrotreating catalysts have been studied much less than the classical molybdenum-based catalysts. It is expected, however, that phosphorus addition should lead to similar effects in both cases since tungsten is chemically similar to molybdenum. Atanasova et al. (101) reported that phosphorus increases the thiophene HDS activity, especially that of a sequentially impregnated NiW—P/Al catalyst. Halachev et al (135) found that a maximum hydrogenation activity for naphthalene conversion is attained when the catalyst contains 0.6 wt% P2O5. Cruz Reyes et al (58) reported that phosphorus on a W/Al catalyst notably enhances gas oil HDS and pyridine HDN. 

Effects of phosphorus have also been proposed from different points of view. First, phosphorus may decrease the polarization of the Mo—S bond and therefore increase its covalent character. Since molybdenum-based catalysts with highly covalent Mo —S bonds are supposed to have high HDS activities, phosphorus can thus improve HDS activity (84). Second, the presence of phosphorus increases the formation of octahedral molybdenum, cobalt, and nickel oxo-species which could be the precursors of the catalyti-cally active phase (38,88). Finally, phosphorus strongly promotes hydrogen activation in MoP/Al catalysts (59), which could be beneficial for all the hydrotreating reactions. 

Literature descriptions of active sites on oxide catalysts are often speculative and very often just generate a picture of the surface active site by extrapolation of the bulk structure. In general they envisage approach of the starting material to the active site in a preferred orientation without any indication of how the preferred orientation is established. In addition, the description of the active site is usually restricted to a small number of molecules. For example, the vanadium phosphorus oxide catalysts used for n-butane oxidation to maleic anhydride is based on the vanadyl pyrophosphate structure and an active site architecture is... 

Alkyl alkyl ketones have also been enantioselectively hydrosilylated with rhodium catalysts containing phosphorus-based ligands. The results were similar to those from the reactions of 1-phenylethanone3-5. The highest value was 72% ee for the hydrosilylation of 3,3-dimethyl-2-butanone to (S)-3,3-dimethyl-2-butanol with diphenylsilane using a rhodium/Amphos system, based on the optically active aminophosphane ligand Amphos22. 

Addition of phosphorus-based additives to polyesters synthesised with high-activity catalysts, with addition taking place immediately before melt processing [34]... 

The first step in RNA hydrolysis is the intramolecular nucleophilic attack of the phosphorus atom by the 2 -OH of ribose. This step is activated by the coordination of the phosphodiester linkage in RNA to the lanthanide(III) ion in the bimetallic cluster [R 2(OH)2] , since the electrons are withdrawn by the metal ion from the phosphorus atom. This electron withdrawal promotes the electrophilicity of the P atom, although it is not so drastic as the effect achieved by the Ce(IV) in DNA hydrolysis (cf. sect. 5). Furthermore, the hydroxide ion bound to another lanthanide(III) ion in the bimetallic cluster functions as a general base catalyst, and enhances the electrophilicity of the 2 -OH by removing its proton. Alternatively, the 2 -OH is directly coordinated to this metal ion, and its dissociation to alkoxide ion is facilitated. In this way, both the nucleophilic center (the oxygen in the 2 -OH) and the electrophilic center (the phosphorus atom) are simultaneously activated by the bimetallic cluster, and thus the intramolecular nucleophilic attack proceeds efficiently. 

It is a peculiar property of V/P/O based catalysts that deactivation occurs with an increase of the activity and a decrease in the selectivity to maleic anhydride (1839). Deactivation phenomena are not well explained in the patent and scientific literature, but very likely are due to overoxidation of the catalyst and/or migration of phosphorus. 

Among various vanadia-based catalysts, the vanadium phosphorus oxides (VPOs) have been proved to be excellent catalysts for selective O- and N-insertion reactions of aliphatic and methylaromatics, in particular for the oxidation of -butane to maleic anhydride and the ammoxidation of methylaromatics and heteroaromatics to their corresponding aldehydes and nitriles [12,44,59-65], Various VPO precursors of different structure and vanadium valence state were studied in recent years with an aim to elucidate their reaction behavior and to improve their catalytic performance in the earlier mentioned processes. Regarding the nature of active phase in these catalysts and the way its structure influences the catalytic activity and selectivity of these catalysts, comprehensive investigations were made by direct observation of the catalysts under reaction conditions with the help of various spectroscopic in situ methods. More recently, a comprehensive picture of structure-reactivity relationship for the industrially important ammoxidation of toluene to benzonitrile was demonstrated experimentally for VPO catalysts [5] ... 

With the aim of avoiding the use of phosphorus-based ligands, we had previously developed catalysts based on silica supported bidentate iminopyridine ligand systems. These coordinate Pd well, and the catalysts are active and stable in both the Heck (20,22) and Suzuki (21,22) reactions. Since the catalysts are based on the functionalisation of an aminopropyl groups attached to the silica, these are ideal candidates to translate to a chitosan-based support, and indeed this was achieved in a straightforward manner (Figure 2). 

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