Disproportionation reactions involving

The behavior of oligosiloxanediols in the presence of strong bases is different. The contribution to the overall process of the disproportionation reaction, involving a migration of the ultimate siloxane unit between siloxane molecules, is much greater and may even completely dominate the polycondensation reaction (80). The reactivity enhancement of the siloxane bond adjacent to the sHanolate anion can be understood in terms of n(0) (7 (SiO) conjugation. 

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

Note that the disproportionation reaction involves intermolecu-lar hydrogen abstraction. Due to steric factors, tertiary alkyl radicals undergo disproportionation rather than radical combination. Radicals forming addition products with compounds containing C=C bonds ... 

This should not be taken for granted. After all, the related oxygen disproportionation reaction involving tetrahydropyran and 1,4-dioxane (28, cyclo-l,4-X(CH2)4 with X = CH2 and O respectively)... 

There have been extensive investigations into the kinetics of disproportionation reactions involving chlorofluorocarbon compounds.4-6 Phase equilibrium constants in relation to contact time and temperature have been determined and reported in various publications.1-3... 

Lesser amounts of methane and ketene also are formed as the result of disproportionation reactions involving hydrogen-atom transfers of the types we have encountered previously in radical reactions (see Section 10-8C) ... 

Consider now the following disproportionation reaction involving benzophenone (52, R = Ph) ... 

Attempted insertions of fluorocarbenes into Sn—Mn and Sn—Fe bonds were unsuccessful thus, the thermal reaction of (CO)sMnSnMe3 with Me3SnCF3 (a good source of carbene CF2) led instead to a disproportionation reaction involving formation of (CO)5MnSnMe2(CF3) and SnMe4344. 

Actual cleavage of the polymer molecules, or chain scission, results from disproportionation reactions involving hydroperoxides in the polymer structure. The mechanisms are presented below. 

A3. A disproportionation reaction involves a substance that acts as both an ox idizing and a reducing agent, producing both higher and lower oxidation states of the same element in the products. Which of the following disproportionation reactions are spontaneous under standard conditions ... 

However, all chain terminations by recombination and disproportionation reactions involve radical-radical reaction... 

As soon as polymer amide groups are formed, they can take part in both types of transacylation reactions (23) and (24). The disproportionation reactions involving polymer amide groups and/or anions produce acyllactam and diacylamine structures entering into the polymerization process... 

The scenario is best visualized with reference to Figure l.a. Here we consider the activated complex for a bimolecular disproportionation reaction involving dissimilar molecular pairs. The six nearest neighbour sites can be populated by A, B molecules or they can remain vacant. 

Disproportionation reactions involve the transfer of a hydrogen radical from one growing polymer radical to another. This results in the joining of a hydrogen radical with a growing polymer radical and the formation of a carbon-carbon bond in the other growing polymer radical. This is shown as ... 

Side reactions or postcuring reactions are possible in the formation of silicone networks. In most cases, the silicon-bound hydride is in stoichiometric excess to enhance reaction rates. Disproportionation reactions involving terminal hydride groups have been reported (222). A major side reaction consumes Si—H to give redistributed siloxane in the resulting polymers and gaseous silane as a by-product. 

The third step in the vulcanization sequence is considered to be the formation of polysulphide cross-links. The most probable route for the conversion of the rubber-bound intermediate into cross-linked polysulphides is by disproportionation reactions, involving cleavage of S—S bonds these reactions may be initiated by mercaptide ions (XS ) derived from the zinc mercaptide (cf., Section 17.2) ... 

The Tishchenko reaction is more than 100 years old and, with its different variants, has grown over the decades into a niche method for the selective production of dimeric esters from two aldehyde molecules. The reaction also known as the Claisen-Tishchenko reaction, occurs via a Lewis acid-catalysed disproportionation reaction involving a hydride shift (the rate determining step) as depicted in Scheme 18.23. Catalysts with higher Lewis acidity (like pure aluminium(m) alkoxides) afford in most cases the esters. 

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