Concerted oxygen insertion

Despite the fact that a concerted oxygen insertion reaction appeared to be in complete agreement with existing experimental studies, Minisci and his group strongly advocated a... 

Oxygen atom from Cpd I is inserted into the C-H bond of saturated hydrocarbons (Scheme la) by means of hydrogen atom abstraction followed by recombination of the transient hydroxyl with the carbon radical [the so-called oxygen rebound mechanism proposed by Groves in 1976 (8, 10)]. Another possibility can be the concerted oxygen insertion into the C-H bond. Both pathways are rationalized by the two-state mechanism developed by Shaik et al. (6, 9), which describes different reactivities... 

Insertion of oxygen atom from Cpd I into the carbon-carbon double bond with formation of epoxide (Scheme Ic) reveals features characteristic for a concerted process, although formation of radical intermediates is possible in many cases. A unified description of this alternative is also provided by the two-state mechanism of catalysis by Cpd I (see the section on Hydroxylation of hydrocarbons). Essentially, the concerted oxygen insertion represents a low-spin reaction surface, whereas the distinct radical intermediate is formed on the high-spin reaction pathway. In the latter case, the carbon radical may attack the nearby heme nitrogen and modify the heme covalently. This reaction is also an important inactivation pathway of cytochromes P450 during oxidative transformations of terminal double and triple bonds. 

It has been shown by ab initio calculations that the alkane oxidation proceeds via a highly polar asynchronous transition state, which is common for either concerted oxygen insertion into the C-H bond or formation of a radical pair [48c,d]. The transition state having considerable diradical character (and also is polarized) is shown below ... 

A theoretical study on the oxidation of methane, propane and isobutane with dioxirane, dimethyldioxirane, difluorodioxirane and methyl(trifluoromethyl)di-oxirane has provided a rational for the formation of radical intermediales when dioxygen is rigorously excluded and supported the generally accepted, highly exothermic, concerted oxygen insertion mechanism for the oxidation under typical preparative conditions. The activation barriers for the oxidation of methane (44.2), propane (30.3) and isohutane (22.4 kcal mol" ) with dimethyldioxirane have been evaluated [48e]. Perfluorodialkyloxaziridines are also mild and selective reagents for the hydroxylation of alkanes [49] ... 

Such a way of activation can occur though a step-wise radical process (1) or a concerted oxygen insertion process (2). In both cases, the C-H bond breaking is initiated by the participation of the oxo Ugand. The third type is an indirect mechanism in which metal complexes promote the formation of a reactive species (usually oxyradical), which then reacts with the hydrocarbon molecule without involving the metal complex [9]. 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

On the contrary, Ostovic and Bruice have proposed rate-limiting formation of a charge transfer (CT) complex followed by rapid concerted oxygen insertion into the alkene double bond on the basis of the oxidation of olefins by 0=Fe (TDBPP) [214, 217]. The p" " value of -0.93 described above is pointed out to be similar to those reported for known concerted process such as carbene insertions into the double bond (p" " = -0.62 to -1.61) [224]. 

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