Catalyst combined action

In tire preceding example, tire stmcture of tire catalyst combined witli reactants and products was detennined and tire data were used to infer a cycle. Stmctures of tire highly reactive intennediates in catalysis are generally elusive and infonnation about tliem based only on inference. In prospect, tire most incisive infonnation about tire workings of a catalyst can be obtained by observations of tire catalyst in action. The following example illustrates this... 

In light of the previous discussions, it would be instructive to compare the behavior of enantiomerically pure allylic alcohol 12 in epoxidation reactions without and with the asymmetric titanium-tartrate catalyst (see Scheme 2). When 12 is exposed to the combined action of titanium tetraisopropoxide and tert-butyl hydroperoxide in the absence of the enantiomerically pure tartrate ligand, a 2.3 1 mixture of a- and /(-epoxy alcohol diastereoisomers is produced in favor of a-13. This ratio reflects the inherent diasteieo-facial preference of 12 (substrate-control) for a-attack. In a different experiment, it was found that SAE of achiral allylic alcohol 15 with the (+)-diethyl tartrate [(+)-DET] ligand produces a 99 1 mixture of /(- and a-epoxy alcohol enantiomers in favor of / -16 (98% ee). 

A very first example involving the combined action of an enzyme and a metal catalyst is the direct one-pot conversion of glucose into mannitol (3 x as expensive as... 

In particular, the combined action of a transition metal catalyst and a lipase in organic solvents for the racemization and esterification steps, respectively, has been applied for the conversion of racemic secondary alcohols into their esters... 

Christiansen shows that if a reaction takes place by a chain mechanism and if a molecule of the negative catalyst combines with or deactivates one of the atoms or molecules participating in the propagation of the chain, then not only is this one atom or molecule put out of action, but, virtually, all those which would normally have reacted in the rest of the chain. Since the chain might have been a very long one, it is evident that one molecule of the negative catalyst may thus stop the reaction of hundreds or thousands of molecules. 

Available vast information in the scientific literature on synthesis and mechanism of heterogeneous metal-complex catalysts (HMC) action points to combination of advantages in both homogeneous and heterogeneous catalysts. The catalytic activity of HMC has been investigated in a series of processes, including oligomerization of lower olefins [1]. However, in connection with limitations of physicochemical methods of research of HMC, the data on formation of catalytic active centers on the surface of the support and on mechanism of their action are not numerous and are contradictory. 

It has been earlier demonstrated that sulphonic acids increase the charcoal yield. The amount of sulphuric acid in wood soaked with the EORA tar catalyst is no more than 0.7 to 0.8% on the o.d wood basis. However the combined action of these cellulose dehydration [S omoters causes an increase in the charcoal yield... 

Nowadays interest in mankind-designed cascade reactions is rapidly growing. A recent review of Bmggink et al.[ 18] lists a total of 62 cascade reactions, of which 21 examples involve bio-bio catalysis by mixtures of enzymes, 26 examples involve bio-chcmo catalysis, combined action of an enzyme and a chemo-catalyst, and 15 examples pertain to the action of two or more chemo-catalysts. 

Hydrogenation. The polymer is degraded by the combined actions of heat, hydrogen and in many cases catalysts. 

Only under combined action of anion and cation the most essential changes of the peroxide fragment (C-O-O-C) conformation in the ROOR molecule are observed. It is the dominant factor of peroxide bond activation. Considering models V and VI it is impossible to prefer finally only one structure. But comparative analysis of all considered models allows conclude about selective association of peroxide with tetraethylammonium bromide ions i.e. catalyst attack should be stereospecific. 

The methods described were used to investigate many different kinds of adsorbents and catalysts. As a result of these studies, the universal nature of chemisorption-induced strength lowering was established. The durability of the catalyst was decreased due to the combined action of the... 

The retro-Hofmann consideration is based on the availability of pyridine-3-carboxamide on a large scale. Actually, this compound, known as nicotinamide or vitamin B3, is produced in multi-ton quantities from 3-methylpyridine [20, 21]. For this building block, the catalytic industrial process is based on the one-pot cyclization of acrolein, ammonia and propanal in the presence of oxides of Sb(II), Ti(IV) or V(V) as catalysts. 3-Methylpyridine is submitted to amoxidation, the combined action of oxygen and ammonia to obtain 3-cyanopyridine (Scheme 8.9) [22]. 

Figure 31.7 Synergetic effect obtained by entrapping two catalysts in the same silica sol-gel matrix highly efficient detoxification of chlorinated environmental pollutants by the combined action of Pd nanoparticles and [Rh(cod)CI)2.

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