Complexation reactions regeneration

Diels-Alder reactions in the presence of Lewis acids represent a case in which the Lewis acid is often used in catalytic quantities. The complexed ester (ethyl acrylate in the example given below) is substantially more reactive than the uncomplexed molecule, and the reaction proceeds through the complex. The reactive complex is regenerated by exchange of the Lewis acid from the adduct. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

If a chemical reaction regenerates the initial substance completely or partially from the products of the electrode reaction, such case is termed a chemical reaction parallel to the electrode reaction (see Eq. 5.6.1, case c). An example of this process is the catalytic reduction of hydroxylamine in the presence of the oxalate complex of TiIV, found by A. Blazek and J. Koryta. At the electrode, the complex of tetravalent titanium is reduced to the complex of trivalent titanium, which is oxidized by the hydroxylamine during diffusion from the electrode, regenerating tetravalent titanium, which is again reduced. The electrode process obeys the equations... 

Really, as illustrated in Scheme 5, the irreversible oxidation of these oxygenated complexes can involve either a one-electron or a two-electron process. This depends upon the fact that the reaction of the oxygenated complex to regenerate the non-oxygenated complex can proceed through the release of a superoxide ion or an oxygen molecule. 

Before addition of the benzyl halide, the only carbonyl adsorption peak is found at 1900 cm, indicative of the cobalt tetracarbonyl anion. After addition, this band immediately disappears and peaks at 2000 cm l are observed. These most likely represent the corresponding acyl complex. Reaction with methoxide yields the product and regenerates the cobalt anion. In the absence of sufficient methoxide, the reaction requires attack by the much... 

To simplify the complex reaction pathways of PET reactions and to make them more transferable to multiple substrates it is sometimes advisable to carry them out in a sensitized way. The sensitizer has three characteristics the substrate is excited for the primary PET process, its resulting radical ion or radical is so inert that it does not react with the substrate, and, in most cases, the sensitizer is regenerated by back-electron transfer. A simplified mechanism of a sensitized PET reaction is shown in Scheme 5. 

From all these observations and relative behaviors it can be concluded that formation of a precursor complex or regeneration of reactive sites is important in determining the overall rate of NAC reduction by surface-bound iron(II) species. Therefore, in this reaction scenario, a much weaker correlation between log kre] and h(AtN02)/0.059 V can be expected and is actually obtained (e.g., Fig. 14.10apparent correlation of the 3- and 4-substituted nitrobenzenes (slope = 0.5) may be due to a co-correlation between (r(ArN02) and the tendency of the... 

Reactions involving intermediates are classified as non-chain or chain. A chain reaction is a special type of complex reaction where the distinguishing feature is the presence of propagation steps. Here one step removes an intermediate or chain carrier to form a second intermediate, also a chain carrier. This second chain carrier reacts to regenerate the first chain carrier and the characteristic cycle of a chain is set up, and continues until all the reactant is used up (see Section 6.9). 

The olefin-laden silver solution is then pumped to a flash tank, where the pressure is lowered and the temperature raised sufficiently to reverse the complexation reaction and liberate pure ethylene. The regenerated silver nitrate solution is returned to the contactor. In this process, the high cost of the silver nitrate carrier... 

Coulometric titration procedures have been developed for a great number of oxidation-reduction, acid-base, precipitation, and complexation reactions. The sample systems as well as the electrochemical intemediates used for them are summarized in Table 4.1, and indicate the diversity and range of application for the method. An additional specialized form of coulometric titration involves the use of a spent Karl Fischer solution as the electrochemical intermediate for the determination of water at extremely low levels. For such a system the anode reaction regenerates iodine, which is the crucial component of the Karl Fischer titrant. This then reacts with the water in the sample system according to the... 

Many of the coupling reactions require a base such as OAc" or NEt3 in addition to the palladium. In the Heck reaction, for example, the base is used to effect elimination of hydrogen halide from an intermediate palladium complex, thus regenerating the palladium for use in further catalytic cycles. In the Suzuki reaction, on the other hand, the base binds to the boron atom of the boronic acid which activates the carbon-carbon bond for further reaction. 

Mo(CO)6] complex. The molybdenum intermediates were studied using in situ IR as part of an extensive mechanistic investigation. The following intermediates were identified in the reaction mixture [Mo(CO)6], [Mo(CO)5l] and [Mo(CO)4l3]. Hydrogen, which at low concentrations increases the rate of the reaction, was found to reduce [Mo(CO)4l3] to the zero-valent complex, thus regenerating the catalyst. ... 

Termination reactions regenerate the catalyst complex, therefore, the complex is a true catalyst, unlike free-radical initiators. 

The former reaction is a chain terminating reaction and cannot be important in our system since the rate coefficient is unaffected even when 2.1 Torr of O2 was added. The latter reaction regenerates the chain and produces HNO and thus is similar to Reaction 7 however, it consumes O2. If it were as important as Reaction 6, [02]/[03]o should drop below unity. Either its rate constant is measurably smaller than that of Reaction 6, or the reaction proceeds through an intermediate complex which lives long enough to react with O3, viz... 

In the absence of protons coulometry shows that the first wave changes from a four-electron to a one-electron wave (per dication of substrate) and only half a mole of PhjSb per mole of substrate dication was found. This is due to the fact that the PhjSbO formed in equation 48 cannot regenerate the substrate dication in the absence of protons but coordinates to the starting dication with formation of a new complex (reaction 51) which is electrochemically inactive at the potential of the first wave. The new complex can be isolated after exhaustive electrolysis at the potential of the first wave . 

Enzymes are proteins employed by Mother Nature to catalyze the chemical reactions necessary to sustain life in plants and animals. As catalysts, enzymes may influence the rates and/or the directions of chemical reactions involving an enormous range of substrates (reactants). Enzymes function by combining with substrates to form enzyme-substrate complexes (reaction intermediates) that subsequently react further to yield products while regenerating the free enzyme. 

A catalyst, in its simpler definition, is a substance that enables a chemical reaction to proceed at an usually faster rate or under different conditions (such as at a lower temperature) than otherwise possible. The catalyst interacts with the reagents and intermediates of reaction but is regenerated to the initial state during the reaction cycle. The turnover number (TON) indicates how many cycles a single catalytic center could perform the reaction cycle without being deactivated. In complex reactions with multiple possible products (the usual case in chemistry), the catalyst enables us to maximize a specific reaction pathway and thus to provide a selective synthesis. 

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