Turnover catalytic

Acetoxybenzene is prepared by the reaction of benzene with Pd(OAc)2[325,342-345], This reaction is regarded as a potentially useful method for phenol production from benzene, if carried out with only a catalytic amount of Pd(OAc)2. Extensive studies have been carried out on this reaction in order to achieve a high catalytic turnover. In addition to oxygen and Cu(II) salts, other oxidants, such as HNOi, nitrate[346,347], potassium peroxodisulfate[348], and heteropoly acids[349,3S0], are used. HNO is said to... 

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. 

AbouKhair, N. K., Ziegler, M. M., and Baldwin, T. O. (1984). The catalytic turnover of bacterial luciferase produces a quasi-stable species of altered conformation. In Bray, R. C., et al. (eds.), Flavins Flavoproteins, Proc. Int. Symp., 8th, pp. 371-374. de Gruyter, Berlin. 

Complexes [Ru30(0Ac)6L3]"+ (L = H20, PPh3) have been found to be catalysts for the oxidation of primary and secondary alcohols to aldehydes and ketones under fairly mild conditions (65°C, 3 atm 02) with high catalytic turnovers [104],... 

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

For time scales shorter than that of a catalytic turnover (typically 10 2 to 102s) the three phenomena are indistinguishable. Looking at the Na-promoted Pt surface on the cover of this book and imagining that CO oxidation is taking place on that surface, there is no way to distinguish if this is a classically promoted surface where Na has been added from the gas phase,... 

Similar would be the situation on a Pt surface decorated with O2", the only difference being the experimental difficulty of introducing 02 with classical promotion and its short lifetime on the catalyst surface, only A times longer than the catalytic turnover. 

The first electrochemical H2 generation catalyzed by a hetero-nuclear Fe-Ni complex [Ni(L)Fe2(CO)g] (27) [L - = (CH3C6H3S2)2(CH2)3 ] (Fig. 9) with tri-fluoroacetic acid was reported by Schoder and coworkers in 2006 [211]. Based on their electrochemical behavior, spectroscopic data, and DFT calculations of 27, an EECC mechanism was mled out and therefore an ECCE or ECEC mechanism involving the formation of Fe°-H and Ni -H intermediates is likely. In this cycle, six catalytic turnovers were achieved. This value is comparable to those for... 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

The alternative mechanism (Fig. 18.16, mechanism B) is based on the fully reduced [(dipor)Co2] state as the redox-active form of the catalyst. The redox equilibrium between the mixed-valence and fully reduced forms is shifted toward the catalytically inactive mixed-valence state, and hence controls the amount of catalytically active species in the catalytic cycle and contributes to the — 60 mV/pH dependence. The fully reduced form is known to bind O2 (probably reversibly) in organic solvents [LeMest et al., 1997 Fukuzumi et al., 2004], and the resulting diamagnetic adducts are typically viewed as a pair of Co ions bridged by a peroxide, which are of course quite common in the O2 chemistry of nonporphyrin Co complexes. To obtain the —60 mV/pH dependence of the catalytic turnover rate, a protonation step is required either prior to the TDS or as the TDS. Mechanism B cannot be extended to monometallic cofacial porphyrins or heterometallic porphyrins with a redox-inert ion, but there is no reason to assume that the two classes of cofacial porphyrin catalysts, with rather different catalytic performance (Fig. 18.15), must follow the same mechanism. 

The equilibrium constant K for (por)Fe(OH2) (por)Fe, which determines the molar fraction of the 5-coordinate redox-active Fe catalyst. This constant was estimated from analysis of the catalytic turnover frequencies in the presence of varying concentrations of an inhibitor, CN, which competes with both O2 and H2O for the 5-coordinate Fe porphyrin. 

A variety of aldehyde/alkyne reductive couplings involving the stoichiometric use of early transition metals (Ti and Zr) have been developed (Scheme 27) [68-70]. The low cost and ease of handling of titanium alkox-ides render these stoichiometric processes very practical despite the lack of catalytic turnover. Recent variants of stoichiometric processes involving titanium alkoxides have demonstrated impressive scope in relatively complex applications [71-73]. 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

Selective cleavage of peptides and proteins is an important procedure in biochemistry and molecular biology. The half-life for the uncatalyzed hydrolysis of amide bonds is 350 500 years at room temperature and pH 4 8. Clearly, efficient methods of cleavage are needed. Despite their great catalytic power and selectivity to sequence, proteinases have some disadvantages. Peptides 420,423,424,426 an(j proteins428 429 can be hydrolytically cleaved near histidine and methionine residues with several palladium(II) aqua complexes, often with catalytic turnover. 

Ru3(CO)12(117)3] and [H4Ru4(CO)11(117)] as catalyst precursors in the hydrogenation of non-activated alkenes under biphasic conditions. Each cluster displays activity under moderate conditions, ca. 60 atm. H2 at 60 °C with catalytic turnovers up to ca. 500. The trinuclear clusters undergo transformations during reaction but can be used repeatedly without loss of activity.325... 

MeOH was studied. Using Mel as co-catalyst, catalytic turnover numbers of 1,800 were obtained within 14 h.71... 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

Komiyama at al. have prepared two oligonuclear Zn(II) complexes (22 and 23) and tested their hydrolytic activity toward different diribonucleotides [45,46] (catalytic turnover was not demonstrated). The dimer and trimer structures of the active species were confirmed by measuring the hydrolytic activity as a function of Zn/L ratio, which show sharp maxima at the expected 2/1 and 3/1 ratios, respectively. The oligomer complexes have high ribonuclease activity (e.g. the hydrolysis of UpU is accelerated more than 4 and 5 orders of magnitude by 22 and 23, respectively), whereas the effect of the monomer complex 24 was not... 

Schneider and coworkers have reported on the hydrolysis of BNPP by Pr(III) in the presence of the potentially dinucleating ligand 45 [66]. An aqueous solution of a 2 1 Pr(III)/45 complex which was prepared in organic solvent is 70 times more reactive toward BNPP than the metal salt alone at pH 7.0 and 323 K. The rate enhancement over spontaneous hydrolysis is 5 x 106-fold. The authors suggest cooperation of two metal ions, but there is no direct evidence for the presence of a dinuclear-Pr complex in aqueous solution. Catalytic turnover was not demonstrated. 

Only for a very few dinuclear phosphoesterase mimics has catalytic turnover been demonstrated. Catalysis may be prevented either by slow kinetics of substrate/product exchange, as in the case of cobalt(III)... 

The hydroarylation of olefins is also achieved by using a ruthenium catalyst, TpRu(CO)(NCMe)(Ph) (Tp = hydridotris(pyrazolyl)borate) (Equation (34)).39 The reaction of benzene with ethene is catalyzed by the ruthenium complex to give ethylbenzene (TN = 51, TOF = 3.5 x 10 3mol 1 s-1 at 90 °G for 4h). The ruthenium-catalyzed reaction of benzene with propene gives the hydroarylation products with a 1.6 1.0 ratio of -propyl to isopropylbenzene, with 14 catalytic turnovers after 19 h. 

Carreira and Kruger reported facile transmetallation of silicon enolates to other soft metal enolates including Gu derivatives.499 They reasoned that the use of soft metal fluoride complexes enabled silyl metal transmetallation with catalytic use of a soft metal source. The concept is illustrated in Scheme 103. Normal Lewis acid-catalyzed reactions of silicon enolates with aldehydes proceed via activation of aldehydes by carbonyl oxygen coordination to Lewis acids, as shown in the upper equation of Scheme 103. A key step for catalytic turnover is the desilyation of 233 by the... 

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