Kinetically enhanced metalation model

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. 

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. 

The catalytic activities of Cu(II), Co(II) and Mn(II) are considerably enhanced by sodium dodecyl sulfate (SDS) in the autoxidation of H2DTBC (51). The maximum catalytic activity was found in the CMC region. It was assumed that the micelles incorporate the catalysts and the short metal-metal distances increase the activity in accordance with the kinetic model discussed above. The concentration of the micelles increases at higher SDS concentrations. Thus, the concentrations of the catalyst and the substrate decrease in the micellar region and, as a consequence, the catalytic reaction becomes slower again. 

The dinuclear Sr complex of the ditopic ligand 17 increases the rate of basic ethanolysis ofthe malonate derivative 19 by a surprising 5700-fold, but increases by only 9.5-fold the rate of cleavage of 14 [28]. It is remarkable that such a huge rate-enhancement occurs under extremely dilute conditions, namely 15 pM 19 and 30 pM 17-Sr2. A slightly lower rate enhancement is observed in the presence of 17-Ba2. It seems likely that under the dilute conditions of the catalytic experiments several crown-complexed metal species occur simultaneously (Scheme 5.4). Given the plethora of species involved in such a complicated system of multiple equilibria, quantitative kinetic treatment is out of reach. Nevertheless, a comparison with the reactivity of model compounds, particularly that of the malonate derivative 20, provides insight into the composition of the reactive intermediate (Table 5.8),... 

The thermalization stage of this dissociation reaction is not amenable to modelling at the molecular d5mamics level because of the long timescales required. For some systems, such as O2 /Pt(l 11), a kinetic treatment is very successful [77]. However, in others, thermalization is not complete, and the internal energy of the molecule can still enhance reaction, as observed for N2 /Fe(l 11) [78, 79] and in the dissociation of some small hydrocarbons on metal surfaces [80]- A detailed explanation of these systems is presently not available. 

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