Catalytic scheme

The hydrogen set free can add to unsaturated compounds these reactions occur in the lower reaches of the Titanian atmosphere. Hydrogen cannot escape from the upper atmosphere before it reacts. The authors suggest a catalytic scheme in which reactive hydrogen atoms are converted into molecular hydrogen (H2) without a net loss of unsaturated compound (here C4H2) ... 

The trinuclear cluster [(/i-H)2Ru3(/i3-0)(C0)5(DPPM)2] is also an efficient catalyst for alkene hydrogenation reaction, for which Bergounhou proposed the catalytic Scheme 73.38... 

In the various homogeneous catalytic schemes, the solvent may be coordinated to the metal or may simply be present as bulk solvent. When a ligand leaves the coordination sphere of a metal, it may be replaced by a molecule of solvent in a process that is either associative or dissociative. There is no general way to predict which type of mechanism is operative, so in some cases the substitution reactions will be described as they relate to specific processes. Because substitution reactions have been described in Chapter 20, several other types of reactions that constitute the steps in catalytic processes will be described in greater detail. 

In the catalytic schemes to be described later, it will be seen that reactants become ligands and are... 

IFIGU RE 22.11 The catalytic scheme for polymerization by the Ziegler-Natta process. 

Cyclic voltammetric responses corresponding to the simple catalytic scheme in Figure 4.1 and to more complex schemes were discussed in detail in Section 2.2.6. The parameters that control the catalytic current have been identified and their effects quantified. Applications of homogeneous redox catalysis to the characterization of short-lived intermediates and the determination of their redox properties have been discussed in Sections 2.3 and 2.6.4. 

Regarding the participation of intermediate in the steps of detailed mechanism, Temkin (1963) classified catalytic reaction mechanisms as linear and non-linear ones. For linear mechanisms, every reaction involves the participation of only one molecule of the intermediate substance. The typical linear mechanism is the two-step catalytic scheme (Temkin-Boudart mechanism), e.g. water-gas shift... 

Combination in a catalytic scheme of this reaction, which generates [HRu(CO)J, with reaction (61), which generates Ru(CO)4l2, gives the cycle shown in Fig. 23. Transformation of Ru3(CO),2 according to (59) provides [HRu3(CO)u] and [Ru(CO)3I3], the stable species during catalysis. The outer circle represents the transformation of the reduced... 

Similarly the mechanism of the Cu(II) catalyzed oxidation of malic acid by S2Og (173) was reinterpreted on the basis of the properties of CuII-CR1R2R3 and CuIII-CR1R2R3 complexes. The following catalytic Scheme 6 was proposed (174). 

It can be seen that a key step in this process is the oxidative addition of the amine to the metal centre. The above catalytic scheme was shown to be feasible for the addition of anihne to norbomene with use of the lr(I) complex Ir(PEt3)2(C2l l4)CI as the catalyst. ... 

Of course, main-group metal hydrides such as these cannot be incorporated into a useful catalytic scheme since they do not form readily from H2. The following, apparent criteria for a suitable catalyst are based on the above discussion. 

The stoichiometric carbonylation observed using [HRu(CO)3] and the proposed catalytic schemes all involve tricarbonyl species as the active catalyst the relatively high activity of Ru3(CO)i2 is consistent with this. The relative activity of the complexes for piperidine carbonylation is [HRu(CO)3L Ru3(CO)12 > [Ru(CO)2(OCOMe)]n. The major cause of the decrease in carbonylation rates is the accumulation of formyl product although the decrease in amine concentration is also a contributing factor. This catalyst poisoning is likely attributable to com-plexation to the ruthenium, presumably via the carbonyl grouping as commonly found for formamide ligands (26). The product could compete with either amine or CO for a metal coordination site. 

Reaction of 230 with ethyl chloroformate gave a mixture of ring-opened compounds that could be reduced with lithium aluminum hydride or catalytically (Scheme 30) the latter method provides a secondary amine (231) as the major product. 

Two different variants of the electrocatalytic process are analyzed here. The first one corresponds to first-order conditions and in this case one-electron and two-electron charge transfers coupled to the chemical reaction are discussed under SWV and Voltcoulometry conditions [19, 83, 95-97], After that, a second-order catalytic scheme is presented in which the mass transport of the substrate of the chemical reaction is considered [98, 99]. 

In Figure 1, the observed reactions are indicated in the context of a proposed catalytic scheme for terminal hydroformy-lation. The reaction pathway involving the alkyl phosphine intermediate (VIII) is the most likely. Overall, the results of the NMR studies provide consistent explanations for the process parameters of selective hydroformylation, particularly of the low pressure continuous product flashoff process (.5,15). it was shown that, in contrast to prior indications (3), the tris-phosphine complex (I) is a remarkably stable and favored species in the presence of excess phosphine and H2. This complex (I) is postulated to have a key role in the reversible generation and... 

Figure 1. Catalytic scheme for alkane oxy-functionalization based on the C-H activation reaction.

FIG. 2. Some catalytic schemes proposed for Wilkinson s catalyst. 

Figure 1.6 Minimal catalytic scheme of VCPO based on crystal structures of the native enzyme and the peroxo-intermediate (Messerschmidt ef a/. 1997). Lys353 is considered to be crucial in assisting heterolytic cleavage of the side-on bound peroxide. EPR and V-EXAFS data suggest that the enzyme remains in the oxidation state throughout the cycle. Source Hasan, Z., Renirie, R., Kerkman, R., Ruijssenaars, H.J., Hartog, A.F. and Wever, R. (2006). Journal of Biological Chemistry, 281, 9738-9744. Reprinted with permission from Renirie, R., Hemrika, W., Piersma, S.R. and Wever, R. (2000). Biochemistry, 39, 11 33-1141. Copyright 2000 American Chemical Society.

Prerequisite to any catalytic activity is the ability of the metal center to interact effectively with alcohols or alcohol-derived precursors. There are several ways in which this can occur, and most of these have been observed or postulated in at least one catalytic scheme. In order to understand the specific reactivities, though, the reader should be familiar with some fundamental aspects of organo-transition-metal chemistry. These will be discussed only very briefly. The reader should also recognize that in order for catalysis to occur, a balance of reactivities is required. 

A Fenton-like mechanism was proposed to explain this product distribution. Hydroxyl radicals, formed by a reaction between the iron complex and H202, abstract protons from the substrate to form carbon radicals R- (equations 2-4) (38, 39). These are subsequently trapped by the diphenylselenide to give a phenylselenyl derivative (equation 4). Increasing the ratio of H202 to 11 switches the reactivity from stoichiometric to catalytic (Scheme 1). 

The diimide (37) reacts with CO to yield an isocyanate complex (38) (86), having a characteristic infrared absorption at 1280 cm-1 (88). This remarkable reaction suggests several catalytic schemes which might be useful for the synthesis of organic compounds containing the —N—C(O)— unit. This latter complex (38) could be viewed as the key intermediate in any catalytic cycle for the synthesis of isocyanates or urea from CO and NH3 (or N2 and H2).2 However, in our work we could not... 

Figure 4.10 Generalized catalytic scheme for palladium-catalysed aryl halide (Ar-X) functionalization...

A specific feature of catalytic schemes that imply the type (4.1) balance is the potential involvement of independent internal variables Hke Ki/K and 0Ki/6K instead of, for example, thermodynamic rushes of inter mediates K,. This is due to the existent balance relations. 

Second, if m is related to the rate constants of the individual steps in the catalytic scheme given in equation 9. In equation 15, if m is defined as (A . j + A 2)lk j. Consider a limiting case in which k is much greater than k 2- Under such circumstances, the ES complex dissociates to E and S much more rapidly than product is formed. Under these conditions (Ar.i k2). 

Photolysis would completely destroy all CO2 above the clouds in — 1.4X 10" yr and all CO2 in Venus atmosphere in —5 Myr. In addition, CO2 photolysis would produce observable amounts of O2 in —5 yr unless CO2 is reformed by another route. However, O2 is not seen in Venus atmosphere and the spectroscopic upper limit is <0.3 ppmv. Gas-phase catalytic reformation of CO2 by hydrogen, chlorine, or nitrogen gases has been proposed to solve this problem. The relative importance of the catalytic schemes depends on the H2 abundance in Venus ... 

Figure 21. RNR class I catalytic scheme as proposed by Stubbe (exp.) [1].

Catalytic scheme Detection Limit of detection Stability... 

An alternate radical-induced addition-fragmentation sequence relies on the 1,4-elimination of a leaving group to drive the reaction. In this sequence, because the fragmentation is accompanied by the expulsion of a tiibutylstannyl radical, the requirement for the initial tributyltin hydride is only catalytic (Scheme 79). The stereospecificity of the reaction course has been rationalize in terms of a conformational preference for the elimination step, as depicted in Scheme 79 where the rran.r-substituted cyclohexanone forms only one double bond geometry in the product. As shown in equations (40) and (41), the ci.r-substituted cyclohexanones undergo the four- and three-carbon expansions stereospecifically to give the (Z)-alkenes. 

Under the conditions of stoichiometric (eq. (4)) or catalytic (Scheme 2) reactions, propylene is oxidized to isopropenyl acetate as the main reaction product, along with allyl and cis- and trans -n-propenyl acetates. Higher acyclic alkenes C4-C10 are converted to mixtures of allyl and vinyl esters [5]. Cyclic alkenes also produce homoallylic esters [6, 7]. 

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