Step-wise mechanism

Three new criteria were proposed [Collman et al., 2003a] to establish that the four-electron reduction of O2 by a catalytic film represents the selectivity of an individual molecule of the catalyst (i) independence of the i /i ratio on the catalyst surface coverage (ii) much higher stabUity of the catalyst in the ORR compared with reduction of H2O2 under conditions that reproduce the concentration of H2O2 in the film that would be generated if ORR proceeded by a step-wise mechanism and (iii) the nature of the turnover-determining step. 

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

Dewar s diagram eqn. (23) (see p. 60) already gives this idea, the binding of the proton by the acceptor B being the second step reaction. Titov [39] suggested recently a scheme which would clearly show the step-wise mechanism ... 

The initial metal-catalyzed transformation of 1,4-BD (as reported for other diols [8]) on the reduced Co-Zn/porcelain catalyst leads to 4-HB formation. As it has been found by quantvun-chemical calculation the further 4-HB conversion occurs on the SiOj active sites via step-wise mechanism similar to 1,4-BD (Scheme... 

Such metal complex-catalyzed reactions, notably those involving the dissociation or formation of H—C—C, and C—bonds, proceed through step-wise mechanisms involving discrete organometallic intermediates [i.e., through pathways analogous to Eq. 

Monoamine oxidase B (MAO-B) is a membrane-bound flavoprotein that catalyzes the two-electron oxidation of primary, secondary, and tertiary amines with a preference for primary amines. Extensive studies have addressed the issue of whether the hydrogen transfer occurs via a step-wise mechanism (i.e. electron transfer followed by H+) or a concerted process (H ) (e.g. Refs. 103, 104). 

Though for s)mthetic purposes the reaction can be considered a concerted process, Zipse and co-workers (05HCA1491) have carried out studies elucidating the step-wise nature of the mechanism proceeding via the intermediary zwitterion 69 (Scheme 24). A similar study has also been carried out for 1,3,5-triazines (05JOC998), once again highhghting the step-wise mechanism. 

Hence, our calculations emphasized a step-wise mechanism over most oxides H-abstraction leads to a surface hydroxyl, in conjunction with the alkyl radical formation, which rapidly rebounds to a nearby oxygen to form a surface alkoxy. This step-wise one-electron oxidation mechanism not only offers an energetically more favorable route than the one-step two-electron oxidation mechanism such as (5+2), but also provides a plausible solution to the puzzle as contrasting the EPR results [50] to the IR results [49] (cf. Fig. 6). We propose that EPR with higher time resolution (10 -10 s) detected the radical formation, whose stability was enhanced by the higher acidity of the tungstated zirconia catalyst which retarded the rebound process. On the contrary, IR with lower time resolution (10 -10 s) was unable to capture the active alkyl intermediates, showing the peaks related to the existence of both surface hydroxy and alkoxy after rebound. 

Mutorotation continues to receive attention. Studies of the mutarotation of 2,3,4,6-tetra-O-methyl-a-D-glucopyranose in aqueous dioxan and aqueous DMSO by optical rotation over a wide range of water concentration, with and without catalysts, have shown that the uncatalysed reaction transition-state contains one mole of sugar and two moles of water in an acyclic bonded structure (6). In the pyridine-catalysed reaction one mole of water may be replaced by one mole of pyridine (Scheme 3). The evidence suggests that the reaction takes place by an intimate step-wise mechanism rather than synchronously. 

From chemical reaction evidence (color development with glycine), we first proposed that 2-carboxybenzaldehyde, 18, and 2-hydroxymethylbenzaldehyde, 21, were formed from an intermolecular Cannizzaro reaction (Route 3) as shown in Fig. 6., since the following step-wise mechanism seemed reasonable (Fig. 7). 

Scheme 11.16 Step-wise mechanism of vinyl azide photolysis and thermolysis ° ...

The products of the reaction of sihcon atoms with dimethyl ether and with methanol in an argon matrix have been assigned with the aid of B3LYP/6-311G(d,p) studies. A step-wise mechanism for the formation of Si-OMc2, Si 0(H)Me, MeOSiMe, Mc2Si = 0, l-methyl-2-oxa-l-silirane, MeOSiH and oxasilirane has been identified. 

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