Homolytic mechanism

Liquid Phase Autoxidations in the Absence of Accelerators or Inhibitors 

Many liquid phase oxidations, hereafter known as autoxidations, occur virtually spontaneously under relatively mild conditions of temperature and oxygen pressure. They are frequently subject to autocatalysis by products (i.e., hydroperoxides, peracids, etc.). The liquid phase autoxidation of hydrocarbons has been studied extensively and is the subject of several monographs and reviews.17 29 With few exceptions, the majority of liquid phase autoxidations 

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R02H, are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from studies of the reactions of alkylperoxy radicals30-33 and the parent hydroperoxides,348-d independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-characterized.35 -39 

At partial pressures of oxygen greater than approximately 100 Torr, chain termination occurs exclusively via the mutual destruction of two alkylperoxy radicals [reaction (6)]. The cross-termination reaction (5) may be neglected. The predicted rate expression, under steady-state conditions, is then given by 

Chain initiation is readily accomplished by deliberately adding initiators, that is, compounds yielding free radicals on thermal decomposition. In practice, initiators should have substantial rates of decomposition in the temperature range 50°-150°C. The rate of chain initiation, Rt, is given by 

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Dediazoniations that follow a homolytic mechanism are, however, always (as far as they are known today) faster than heterolytic dediazoniations. A good example is afforded by the rates in methanol. In a careful study, Bunnett and Yijima (1977) have shown that the homolytic rate is 4-32 times greater than the heterolytic rate, the latter being essentially independent of additives and the atmosphere (N2, 02, or argon). In water the rate of heterolytic dediazoniation, measured at pH <3, is lower than that of the homolytic reactions that take place in the range pH 8-11 (Matrka et al., 1967 Schwarz and Zollinger, 1981 Besse and Zollinger, 1981). 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

The only author to postulate a homolytic mechanism in the last few decades was Deng (1989). His arguments are based on the formation of small amounts of fluorinated bi- and polyphenyls in thermal fluoro-de-diazoniations and in mass-spec-trometric degradations of benzenediazonium tetrafluoroborate and its substituted derivatives. However, he does not include a critical discussion of his work. 

The reaction is catalyzed by light, suggesting a homolytic mechanism. 

The allylic, allenic, propargylic, 2,4-dienylic, cyclopentadienylic, and related tin compounds present special, structural features and show special reactivity by both heterolytic and homolytic mechanisms. 

The reaction of benzyl bromide with vinyltrimethylsilane was used for studying a general kinetics of addition under conditions of metal complex initiation (ref. 26). One of the crucial questions in this case is how the chain transfer step proceeds by "purely" homolytic mechanism (via benzyl bromide) ... 

A homolytic mechanism can also be considered. This could occur in two possible ways. In mechanism 2, Ni(I) would capture the product... 

The migratory aptitude for p-nitrophenyl relative to phenyl is 4.4+0.3 which was interpreted as indicating an exclusively homolytic mechanism. The following chain scheme was proposed... 

Violent ignition occurs on mixing [1], Interaction is explosive, and the products have been identified and a homolytic mechanism proposed for the reaction [2],... 

Aryl a-disulfones do undergo thermal decomposition by a homolytic mechanism but about 101 times slower than the corresponding aryl sulfinyl sulfones (Kice and Favstritsky, 1970). The reason is that AHX for homolytic dissociation of the S—S bond in the tr-disulfone (210) is about 13 kcal mol-1... 

The results for sulphonyl endoperoxide 65a are shown in Scheme 21 with the products acylated for characterization purposes. A full rationalization via an all-homolytic mechanism is depicted in Scheme 22. 

Table 1, a kinetic study suggested both polar and homolytic mechanisms for conjugate addition, depending on the substrate and the substrate conformation . ... 

Thiepane (35) has been converted to 2-acetoxythiepane (137) by a homolytic mechanism using f-butyl peracetate in the presence of a copper(I) ion catalyst (67JCS(C)1130). Similarly, a-chlorination of thiepane (35) by N- chlorosuccinimide (NCS) to yield 2-chlorothiepane (132) probably occurred by a free radical pathway (Scheme 27) (69JHCU5). 

The radical mechanism has been proposed to explain the oxidation of saturated hydrocarbons. In the previous mechanisms, the electron density of the double bond or the aromatic ring is considered essential for the attack on the peroxidic oxygen. This condition is absent in saturated hydrocarbons, and considering their inertness, their oxidation probably requires a homolytic mechanism, proceeding through radical intermediates. By analogy with vanadium... 

The use of iodoacetic acid as an aryl radical trapping agent has confirmed the intermediacy of aryl radicals in some hydrodediazoniation reactions, whether these are initiated or not.4 Spontaneous hydrodediazoniation of aryldiazonium fluoroborates occurs in warm dimethylformamide (DMF). Detailed study5 of the conversion of the 4-nitro derivative into nitrobenzene indicates a homolytic mechanism in which H-atom abstraction occurs from both sites in DMF with a formyl methyl preference of 3.5 1.0. High yields of mixed perfluorinated biaryls may be obtained by the catalytic dediazoniation of pentafluorobenzenediazonium ions in acetonitrile containing aromatic substrates and small amounts of iodide salts. The catalytic role of iodide and the isomeric product distributions indicate that arylation proceeds through the pentafluorophenyl radical in an efficient homolytic chain process.6... 

Homolytic catalysis is observed with both organometallic and coordination complexes. It is involved in a wide variety of metal-mediated transformations, often in competition with electrophilic or nucleophilic catalysis [11], For example, many metal-catalyzed oxidations involve substrate activation by homolytic catalysis (Eq. 5) [12], Similarly, oxidative additions (Eq. 6) and dioxygen activation (Eq. 7) can proceed via two-step homolytic mechanisms. 

In the first half of the 20th century it was shown that the C—Sn bond in organotin compounds, especially in tetraorganylstannanes, was easily cleaved by both heterolytic and homolytic mechanisms. This fact makes the C—Sn bond quite different (regarding its thermal and chemical stability) from the C—Si and C—Ge bonds and brought it close to the C—Pb bond. In 1945, Waring and Horton835 studied the kinetics of the thermal decomposition of tetramethylstannane at 440-493 °C, or at 185 °C at a low pressure... 

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