Efficiency as catalysts

Note 2. Prepared by azeotropic removal of water from the commercial, water-containing product by means of benzene. This solvent was removed in a water--pump vacuum and the remaining mass was powdered. The water-containing acid appeared to work much less efficiently as catalyst. 

Ni-cyclam, Ni(CR), or Ni(tet a) can be used efficiently as catalyst in DMF, and in the presence of NH4CIO4 as proton source [71-74]. Ni species generated electrochemically react rapidly with organic halides to generate alkyl, alkenyl, or aryl radicals which add intramolecularly to a double or triple bond, then leading to cyclopentanoids (Table 7, entries 3-7a). 

Also, linear or cross-linked polyphenylene of type 599 worked efficiently as catalyst in lithiation processes from functionalized chlorinated materials, dichlorinated compounds and benzofused cyclic ethers, with lithium powder in THF at —78 °C to room temperature. Yields are similar to those obtained in solution . 

Enzymes possess high specificity and efficiency as catalysts. However, their rates of reactions can be influenced by certain factors, which include the following ... 

More recently, a very efficient yttrium-based catalyst, yttrium 2-methoxyeth-oxide, has been applied successfully for /f-butyrolactone polymerisation which proceeded easily at room temperature [99], It is worth mentioning that rare-earth metal alkoxides (derived from yttrium and lanthanum) exhibit outstanding efficiency as catalysts for the polymerisation of cyclic esters such as e-caprolactone [132] and lactide [133]. 

An important characteristic of zeolites, determining to a large extent their efficiency as catalysts, adsorbents, and ion exchangers, is their structural resistance to various factors, including thermal, chemical, and mechanical ones. Different theoretical aproaches are now being applied to this problem. Although quantum-chemical methods are only now coming into use in this field, certain results have already been obtained and some of them are summarized below. 

Metal nanoparticles embedded in thermosensitive core-shell microgel particles can also work efficiently as catalyst for this reaction. Figure 13 shows the oxidation reaction of benzyl alcohol to benzaldehyde in aqueous media by using microgel-metal nanocomposite particles as catalyst. All reactions were carried out at room temperature using aerobic conditions. It is worth noting that the reaction conditions are very mild and no phase transfer catalyst is needed. It has been found that microgel-metal nanocomposites efficiently catalyze the aerobic oxidation of benzyl alcohol at room temperature. No byproducts have been detected by GC after the reaction, and water is the only product formed besides the aldehyde. 

The preparation of metal colloids and their use in catalysis is almost as old as the study of catalysis itself and their efficiency as catalysts is well established. In fact the high activity of colloidal metals in a number of catalytic processes has sometimes been a complicating factor in the investigation of... 

Even though proline displays a very poor efficiency as catalyst (TONs generally range in the interval 2-5), its performance is partially mitigated by its low cost and low molecular weight. 

Inspired by the work of Inoue, Lipton and coworkers replaced the imidazole side chain of t icZo-peptide 50, which was efficient as catalyst in cyanohydrin synthesis from aldehydes (Scheme 13.29), with a more basic guanidine moiety in order to afford a catalyst capable of accelerating proton transfer in the Strecker reaction. The modified catalyst 52 was found to be effective in the synthesis of a-amino nitriles with very high yields from aromatic and aliphatic M-benzhydiyl imines, giving enantioselectivities of up to 99% (Scheme 13.30). ... 

Metal NPs embedded in thermosensitive core-shell miao-gel particles can also work efficiently as catalyst for oxidation... 

Cobalt complexes with pyridine ligands, for example, catalyzed the oxidation of neat ethylbenzene to acetophenone in 70% conversion and 90% selectivity [35]. Mn porphyrin complex catalyzes the ethylbenzene oxidation with dioxygen to 3 14 mixture of methylphenylcarbinole and acetophenone in the presence of acetaldehyde [36]. The system CUCI2-crown ether in the presence of acetaldehyde is efficient as catalyst of oxidation of ethylbenzene, indane, and tetralin by dioxygen (70°C) into the corresponding alcohols and ketones with high TON [37]. The oxidations were established to occur via a radical pathway and not by a metal-oxo... 

The catalyst for the oxidative carbonylation to give PhNHCOOEt was metallic palladium with iodide compounds as promoters [11]. The selectivity was superior to 97 %, with a turnover of 300 g/g Pd h. Other platinum group metals, particularly rhodium, were active, although less efficient as catalysts [12], The reaction is conducted under 80 atm of CO and 6 atm of O2 at 160-170 °C and can also be applied to aliphatic amines. Diphenylurea is intermediately formed. 

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