Kinetically enhanced metalation

Mechanisms Complex-Induced Proximity Effect Process, Kinetically Enhanced Metalation, and Overriding Base Mechanism... 

An alternative scenario in which complexation of the base and proton abstraction occur simultaneously in a kinetically controlled one-step reaction was postulated by Schleyer (Fig. 26.4a) [13, 14]. In this kinetically enhanced metalation (KEM) model, this is not precomplexation that is important but the existence of a stabilizing metal-substituent interaction at the rate-limiting transition structures [13]. The fact that KIEs of inter- and intramolecular lithiation of anisole by -BuLi in Etp are identical (kjkj =2.5 0.2) [63] supports this one-step mechanism. 

The effects estimated for the aggregation based on Lewis acid function of Li are not realized clearly in the present stage. The importance of precomplexation of substrate ( complex-induced proximity effects ) has been proposed by Beak [63]. On the other hand, Schleyer advocates the term kinetically enhanced metalation, proposing that directing and activating effects are transition-state phenomena [64]. In either case, the Lewis acid function of Li plays critical roles. 

However, this simple model does not explain the (kinetically) enhanced reactivity of arenes relative to alkanes, considering the higher (Dc-h) bond energy of arenes, i.e. Dc-h, benzene — 111 lccal mol-1 vs. Dc-h,alkane 95 lccal mol-1. Many more examples of metal-activated cleavage of C-H bonds are known for aromatic compounds than for alkanes [37]. To account for this difference, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking. 

Comparative studies of other metal halides as dopant precursors for treating NaAlIij have shown that similar levels of kinetic enhancement of the reversible dehydrogenation can be achieved upon doping with chlorides of zirconium, vanadium, and several lanthanides. Lower levels of catalytic activity have been reported to occur in hydride that was charged with FeCl2 and... 

Bogdanovic and Schwickardi found that the kinetic enhancement upon Ti-doping can be extended to the reversible dehydrogenation of LiNa2AlH to LiH, 2 NaH, and Al. They found the mixed alkali metal alanate to have significantly lower plateau pressure at 211 °C and thus a higher than... 

NickeI(Il).— The rate constants have been measured for complex formation and dissociation with neutral and anionic ligands in the presence of polyelectrolytes [sodium polystyrene sulphonate, sodium poly(ethylene sulphonate), and poly-(ethylenimine) hydrochloride]. In the reaction with the murexide anion, the formation rate constant kt was decreased by the presence of either anionic or cationic polyelectrolyte, whereas the dissociation rate constant remained unchanged. With the neutral ligands (phen, bipy, and pada), however, the polyelectrolytes with hydrophobic residues enhanced kt (the value of k again being unchan ). It appears that the increase in kt is associated with a reduction in the activation enthalpy e.g. 13.5 kcal mol for bipy with no added polyelectrolyte but 11.2 kcal mol with 1.5x 10 equivl of sodium polystyrene sulphonate). The authors discuss their results in terms of the formation of polyelectrolyte-metal complexes but it is quite clear that the presence of innocent species can have large effects on the kinetics of metal complex formation - even with neutral ligands which apparently follow the normal mechanism. 

Noncnzymc-Catalyzcd Reactions The variable-time method has also been used to determine the concentration of nonenzymatic catalysts. Because a trace amount of catalyst can substantially enhance a reaction s rate, a kinetic determination of a catalyst s concentration is capable of providing an excellent detection limit. One of the most commonly used reactions is the reduction of H2O2 by reducing agents, such as thiosulfate, iodide, and hydroquinone. These reactions are catalyzed by trace levels of selected metal ions. Eor example the reduction of H2O2 by U... 

Studies of the influence of irradiation on the kinetics of oxidation have been confined to post-irradiation work. In general, prior irradiation increases reactivity, although there are considerable inconsistencies in the enhancements obtained The effects can be derived from an increased surface area associated with the swelling voids produced in the metal by the irradiation, and can also probably arise to a lesser extent from chemical effects of the fission products. 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

Kinetic data for the decompositions of several metal hydrides are summarized in Table 12 to which the following information can be added. The acceleratory period in the decomposition of BeH2 (a < 0.35) is ascribed [673] to the random formation of metal nuclei followed by linear growth. The increase in rate consequent upon exposure to X-irradia-tion is attributed to enhanced nucleation. Grinding similarly increased the... 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

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