Molecule induced homolysis

In some systems it appears that the initiation step differs from the usual two-step sequence of Eqs. 3-13 and 3-14. Thus in the t-butyl hydroperoxide-styrene system only a minor part of the initiation occurs by the first-order homolysis reaction (Eq. 3-26f), which accounts for the complete decomposition of t-butyl hydroperoxide in the absence of styrene. Homolysis of the hydroperoxide occurs at a much faster rate in the presence of styrene than in its absence. The increased decomposition rate in the t-butyl hydroperoxide-styrene system occurs by a molecule-induced, homolysis reaction which is first-order in both styrene and hydroperoxide [Walling and Heaton, 1965]. The initiation reaction may be written as... 

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. 

The direct reaction of free radical initiators with non-radical substrates resulting in the formation of free radicalshas been termed molecule-induced homolysis. An example is the reaction of a diacyl peroxide with an alkene (equation 52). ... 

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. 

Photochemical generation of free radicals from ketones 20 Ester photolysis and the meta effect 21 Induced decomposition of radical initiators 21 Molecule-induced homolysis 22... 

The second step of this chain is conventional, but the first step, which involves the Sh2 reaction of a tin radical at a tin centre to cleave an Sn-H bond is less readily accepted. The initiation step of the reaction, which with some compounds must be rapid as an inhibitor is quickly consumed, could occur via an electron-transfer process, or by molecule-induced homolysis. 

We suggested the occurrence of a "molecule-induced homolysis," process for the formation of radicals pairs in which transition states involving hydrogen abstractions were formed [Eqs. (6.3) and (6.4)]. 

The low conversion obtained in acetic acid is probably due to the strong hydrogen bond, which occurs between the hydroxyl group of the protic solvent and the O-H group present in NHPI, thus inhibiting the molecule-induced homolysis effect. However, a polar solvent is required to make NHPI soluble in the reaction medium that is why the conversion decreases in dichloroethane, whereas no product is observed in Ph-CFs, where the organocatalyst remains in suspension. 

F. Minisci, C. Gambarotti, M. Pierini, O. Porta, C. Punta, F. Recupero, M. Lucarini, V. Mugnaini, Molecule-induced homolysis of N-hydroxyphthalimide (NHPI) by peracids and dioxirane. A new, simple, selective aerobic radical epoxidation of alkanes. Tetrahedron Lett. (2006) 1421. 

A. Bravo, F. Fontana, G. Fronza, F. Minisci, L. Zhao, Molecule-induced homolysis versus "concerted oxenoid oxygen insertion" in the oxidation of organic compounds by dimethyldioxirane, /. Org. Chem. 63 (1998) 254. 

Some reactions occur thermally at temperatures too low for homolysis of any of the covalent bonds present to provide enough radicals to start the reaction. For example, mixtures of fluorine and methane explode at room temperature, and many hydrocarbons are oxidized slowly by molecular oxygen (see below). It has been postulated that the bimolecular reactions (6.10) and (6.11) are responsible. In (6.10), the formation of the very strong H-F bond makes this reaction much less endothermic than the simple homolysis of the fluorine molecule. In (6.11), a strong O-H bond is formed in the hydroperoxyl radical 18, whereas two relatively weak bonds are broken, the 0=0 n bond and the C-H bond in 16 which is weakened by the stabilization of the product benzylic radical 17. The occurrence of these molecule-induced homolysis reactions is difficult to prove because the compounds formed tend to be swamped by those from the subsequent radical reactions. 

Following the catalytic mechanism reported in Scheme 16.8, we demonstrated the role of peracetic acid, generated in situ, in promoting the formation of PINO by undergoing molecule-induced homolysis with NHPI (path a), while the acyl peroxyl radical resulted to be the effective epoxidizing agent (path b). 

The oxidation of organic compounds by dioxiranes has attracted considerable interest in recent years. Bravo et al. have reported that the oxidation of a variety of organic compounds (alkanes, alcohols, ethers, aldehydes, and alkenes) by dimethyl-dioxirane may be explained via a radical mechanism. The proposed molecule-induced homolysis of dimethyldioxirane is supported by radical trapping with CBrCls or protonated quinolines. The presence of oxygen has also been shown to have a significant effect on these reactions and supports a radical mechanism. The... 

In the textbook Free Radicals", ref. 1, I used the expression "molecule-induced homolysis" and the abbreviation MIH, a phraseology retained by Harmony (9). More recently, I have used "molecule-assisted homolysis" to avoid confusion of MAH processes with the induced decomposition of initiators caused by radicals, a propagation step, rather than an initiation process ... 

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