Reaction intermediate rebound” mechanism

These results are inconsistent with a radical rebound mechanism because this mechanism is a two-step process that requires the involvement of intermediates. Instead the results suggest that the hydroxylation is a concerted process, much like a singlet carbene reaction, which does not involve intermediates. However, this conclusion is in conflict with the properties of singlet carbene reactions discussed above. Subsequent studies on a number of substituted methylcyclopropanes and other stained hydrocarbon systems established that these findings were not anomalous. 

Whereas several transient species have been observed for dioxygen activation by MMOH, no intermediates were found by rapid-mixing spectroscopic methods for the actual methane hydroxylation step. Mechanistic probes, i.e. certain non-natural substrates that are transformed into rearranged products only if the reaction proceeds via a specific intermediate such as a radical or a cation, give ambivalent results Some studies show that products according to a pathway via cationic intermediates are obtained in sMMO hydroxylations and at least one study suggests the presence of a radical intermediate [40]. Computational analyses of the reaction of MMOHq with methane suggest a so-called radical recoil/rebound mechanism in which MMOHq... 

The rebound mechanism, though in a modified version, has been recently supported by theoretical calculations of KIF using the density functional theory (Yoshizawa et al., 2000). The calculations demonstrate that the transition state for the H-atom abstraction from ethane involves a linear [FeO.H...C] array a resultant radical species with a spin density of nearly one is bound to an iron-hydroxy complex, followed by recombination and release of product ethanol. According to the calculation of the reaction energy profile, the carbon radical species is not a stable reaction intermediate with a finite lifetime. The calculated KIF at 300 K is in the range of 7-13 in accord with experimental data and is predicted to be significantly dependent on temperature and substituents. It was also shown from femtosecond dynamic calculations in the FeOVCH4 system that the direct abstraction mechanism can occur in 100-200 fs. 

Velocity and angular analysis of the product CsCl from Cs + CCI4 was obtained by Wilson [142] and Herschbach [8]. This system is intermediate in mechanism between the stripping and rebound mechanisms (see Section 3.3.1), and it was confirmed, as had often been inferred from kinematic arguments, that most of the heat of reaction is liberated as internal excitation, rather than as kinetic energy, of the products. 

Radical rebound mechanisms are frequently invoked for reactions of metal-oxo catalysts," " bnt also for some functionalizations proceeding throngh metal nitrenoids." Since the formation of radical intermediates (11) is involved, selectivity for the functionalization of weak C—H bonds is typical in such protocols. Stereoselective C—H functionalizations are often difficult to achieve with this mechanistic manifold due to the stereochemical liability of 11," but can be overcome if radical recombination is fast (e.g., in enzymes)." ... 

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