Alkoxy radicals system

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

In cyclic systems, the fragmentation of alkoxy radicals can be a reversible process. The 10-decaIyloxy radical can undergo fragmentation of either the C(l)—C(9) or the C(9)-C(10) bond ... 

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

The Mn + complexed ions appear to be the most selective and efficient initiator system for grafting to polysaccharides so far described. The mechanism of the initiation reaction has been studied in our laboratories by model experiments and ESR spectroscopy. There are two possible reactions indicated. Bond cleavage of a vicinal diol according to reaction (22) is one possibility. Another and faster reaction with Mn3+ giving radicals appears to be oxidation of aldehyde groups (24) to alkoxy radicals ... 

The simplest hydrocarbon, methane, has posed a wealth of challenges to experimentalists and theoreticians seeking to discern its combustion mechanism. Methane s reactions have been explored in a wide variety of contexts over the past several decades. We have discussed these briefly the interested reader is referred to the reviews cited in our previous discussion for further details. Due to the scope of this review, we are primarily interested in these reactions insofar as they provide useful benchmarks for the reactions of larger alkylperoxy (RO2 ) and alkoxy (RO ) systems. With respect to the reactive intermediates present in methane combustion and their implications for larger systems, Lightfoot has published a review on the atmospheric role of these species, while Wallington et al. have provided multiple overviews of gas-phase peroxy radical chemistry. Lesclaux has provided multiple reviews of developments in peroxy radical chemistry. Batt published a review of the gas-phase decomposition reactions available to the alkoxy radicals. ... 

This conversion has been rationalized33 as depicted in (a) to Id) the alkoxy radical (a) formed during the photolysis of the corresponding nitrite is favorably placed with respect to the 7r-system of the double bond to undergo an intramolecular addition, the resulting intermediate alkyl radical (b) then combines with NO to furnish the oxime (d) via the nitroso compound (c). 

Hypohalites IRO-Hal) are similar to nitrates (see p. 155 in their photochemical behaviour. Ultraviolet irradiation gives an (n,Ji > excited state that cleaves to form an alkoxy radical and a halogen atom. The radical may undergo alpha-cleavage before recombination with the halogen atom occurs, and this accounts for the formation of 5-iodopentanal (5.69) from the hypoiodite of cydopentanol such hypoiodites are generated in situ from the alcohol. Iodine and merturyfll oxide. In open-chain systems the alkoxy radical can... 

The mechanisms for ambient temperature oxidations become more complex as the alkyl radical R becomes more complex and as the reactions proceed. Thus, with azo-2-methylpropane (52) the pyrolysis of the tert-butoxy radical must be included, and in all but the initial stages, reactions of alkoxy radicals with products containing weak C—H bonds must be included. Numerous tertiary reactions can then occur. As with most free radical systems, useful information can be obtained only if the degree of conversion of the starting material is kept low. 

Garner, Long, and Temple (19), using a flow system so that large amounts of reaction products could be collected, observed a ratio of acetaldehyde to formaldehyde of about 4.5 to 1 in the oxidation of hexane at 310° C. in a stoichiometric mixture. Bailey and Norrish (2) studied the oxidation of hexane in the cool-flame region. The considerable quantities of formaldehyde in the products were presumed to arise from alkoxy-radical decomposition and a methyl radical-oxygen reaction. 

On the basis of analogous reaction systems, it would be expected a priori that decomposition of this alkoxy radical to methylglyoxal CH3C(0)CHO would predominate [27],... 

Cyclization via epoxide fragmentation. Treatment of the epoxythionoimidaz-olide 1 with Bu3SnH and AIBN generates an alkoxy radical by fragmentation of the epoxy group with cyclization to a ds-fused bicyclic system (2). 

Consequently some of the predicted compounds could not be detected while others were formed which were not expected from direct alkoxy radical cleavage. Furthermore, the amounts of volatiles present varied markedly with heating time. It is not surprising, therefore, that at high temperatures, and particularly with polyunsaturated systems, it has been extremely difficult to establish the nature and extent of preferential hydroperoxide scission. 

Similar to alkoxy radicals, few organic thiyl radicals have been observed in photochemical liquid systems. However, asymmetric spectra attributed to RS radicals with anisotropic g-tensors, have been observed in UV-irradiated thiols, sulphides, and disulphides in a solid matrix at 77°K (103,242-245). A typical "sulfur pattern" is shown in Figure 10, and the typical values... 

Mascarenas appUed a strategy related to type c with an alkoxy radical to the chemistry of oxabicyclic systems (see 89 90). He uncovered a very... 

Similarly, the alkoxy radical intermediates generated by reacting carbi-nolimides 124 with the selenurane I2I/I2 system under irradiation with visible light gave, after fragmentation, 3,5-substituted cyclic imides 125 in good yields [63] (Eq. 29). 

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