Rebound mechanism, oxygen

To explore the mechanism of allylic hydroxylation, three probe substrates, 3,3,6,6-tetradeuterocyclohexene, methylene cyclohexane, and /l-pinenc, were studied (113). Each substrate yielded a mixture of two allylic alcohols formed as a consequence of either retention or rearrangement of the double bond. The observation of a significant deuterium isotope effect (4-5) in the oxidation of 3,3,6,6-tetradeuterocyclohexene together with the formation of a mixture of un-rearranged and rearranged allylic alcohols from all three substrates is most consistent with a hydrogen abstraction-oxygen rebound mechanism (Fig. 4.48). 

Figure 7.19 The Groves oxygen rebound mechanism for alkane hydroxylation.

The oxygen rebound mechanism was supported by experimental evidence including (1) high kinetic isotope effects, (2) partial positional or stereochemical scrambling, and (3) allylic rearrangements. For instance, in the presence of [Fe(TPP)Cl] and PhIO, dx-stilbene was stereospecihcally epoxidized. In addition, it was found that cis-stilbene was 15 times more reactive than trans-stilbene in competitive epoxidations. (see Figure 7.20). " ... 

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

Fig. 10 Oxygen-rebound mechanism for P-450-catalyzed hydroxylation (reproduced from reference 393 with the permission of the American Chemical Society).

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

Scheme 5. The concerted oxenoid versus the stepwise oxygen-rebound mechanism for the C-H oxidation by dioxi ranes.

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

Zeolite encapsulated complexes catalyse the oxygenation of alkanes with peroxides according to oxo chemistry, following a mechanism very similar to the oxygen-rebound mechanism encountered in monooxygenase enzymes. 

Figure 3.13. The proposed mechanism for Q to T conversion during the substrate oxidation process catalazed by MMO. The mechanism shown is analogous to the conventional oxygen rebound mechanism proposed for P450 (Jin and Lipcomb, et al., 2000). Reproduced with permission.

Figure 18 The concerted, nonsynchronous concerted, and radical (oxygen) rebound mechanisms of C-H bond oxidation by the P450 Compound I intermediate...

Scheme 18.3b illustrates a recent mechanistic proposal. This bears similarities to the oxygen-rebound mechanism, proposed for the hydroxylation of alkanes by high-valent transition metal species in biomimetic and enzymatic systems [31, 33, 34]. The generation of the active species and other implications of the mechanism are discussed in detail in Section 18.11.3. 

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... 

This is called an oxygen-rebound mechanism. Large intramolecular isotope effects (Ath/Zto = 11) for substrates with CHD groups [120, 121], C-H bond selectivi-... 

Finally, Fig. 4.88 presents two examples of such radical-type mechanisms catalyzed by cytochromes P450 and leading to aliphatic hydroxylation, through the so-called oxygen-rebound mechanism, and epoxidation, respectively. 

Key Words Ethylene oxide, Propylene oxide. Epoxybutene, Market, Isoamylene oxide. Cyclohexene oxide. Styrene oxide, Norbornene oxide. Epichlorohydrin, Epoxy resins, Carbamazepine, Terpenes, Limonene, a-Pinene, Fatty acid epoxides, Allyl epoxides, Sharpless epoxidation. Turnover frequency, Space time yield. Hydrogen peroxide, Polyoxometallates, Phase-transfer reagents, Methyltrioxorhenium (MTO), Fluorinated acetone, Alkylmetaborate esters. Alumina, Iminium salts, Porphyrins, Jacobsen-Katsuki oxidation, Salen, Peroxoacetic acid, P450 BM-3, Escherichia coli, lodosylbenzene, Oxometallacycle, DFT, Lewis acid mechanism, Metalladioxolane, Mimoun complex, Sheldon complex, Michaelis-Menten, Schiff bases. Redox mechanism. Oxygen-rebound mechanism, Spiro structure. 2008 Elsevier B.V. 

---