Oxygen rebound

Another method for conducting cyclizations catalytic in Cp2TiCl is shown in Scheme 14. It relies on the thermodynamically favorable ring closure of THF from 5-titanoxy radicals [81,82]. This step is mechanistically related to the oxygen rebound steps of oxidation reactions. While the scope of this transformation remains to be established, the presence of substituted THF-derivatives in many natural products renders the method potentially attractive. 

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). 

Bowry VW, Ingold KU. A radical clock investigation of microsomal cytochrome-P-450 hydroxylation of hydrocarbons—rate of oxygen rebound. J Am Chem Soc 1991 113(15) 5699-5707. 

Newcomb M, Letadic MH, Putt DA, et al. An incredibly fast apparent oxygen rebound rate-constant for hydrocarbon hydroxylation by cytochrome-P-450 enzymes. J Am Chem Soc 1995 117(11) 3312—3313. 

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). " ... 

The mechanism of cytochrome P450 catalysis is probably constant across the system. It is determined by the ability of a high valent formal (FeO) species to carry out one-electron oxidations through the abstraction of hydrogen atoms or electrons. The resultant substrate radical can then recombine with the newly created hydroxyl radical (oxygen rebound) to form the oxidized metabolite. Where a heteroatom is the (rich) source of the electron more than one product is possible. There can be direct recombination to yield the heteroatom oxide or radical relocalization within the... 

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. 

The oxidation of dioctyl Fe(III)PPIX-Cl with iodosylbenzene (9) showed that the octyl sidechains had been hydroxylated and that 60% of the hydroxylation had occurred at C( 4) and C(5) in the middle of the chain. Molecular models indicate that these two carbon centers have the most favorable access to the center of the porphyrin ring, supporting the idea that the mechanism of this hydroxylation is an intramolecular oxygen rebound (25, 26) from iodine to iron and into the C-H bond (Scheme 5). 

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.

Groves, J.T. and McGlusky, G.A. (1976) Aliphatic hydroxylation via oxygen rebound. Oxygen transfer by iron, J. Am. Chem. Soc. 98, 859-861. 

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