Cleavage, reductive mechanisms

Tetracyanoethylene oxide (TCNEO) not only oxidizes sulfides to sulfoxides but also reduces sulfoxides to sulfides with generation of two molecules of carbonyl cyanide (17). The reduction mechanism involves a zwitterion intermediate (15) that produces sulfide and two molecules of (17) by simultaneous cleavage of the C—C and O—S bonds. A mechanism (Scheme 14) that involves a zwitterion (16) as a common intermediate is proposed for the formation of ylide and sulfoxide.311... 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

When oxycelluloses are placed in alkaline solution, large reductions in molecular weight take place because of the scission of glycosidic bonds. As a consequence, the oxidation of cellulose does not, in general, lead directly to rupture of the molecules, but renders those linkages near the point of attack extremely susceptible to alkaline cleavage. < Several mechanisms have been proposed for explaining these effects. 

The protonation of an Ni-coordinated cysteine after heterolytic cleavage has also been proposed.85 From the resulting intermediate, the reduction mechanisms evolve through a structurally different Ni-C intermediate (compare Figures 7.7 and 7.8), but similarly regenerates the Ni-SI intermediate. No corresponding low-energy pathway has been found for the initial coordination of H2 to the Ni center. 

In the cathodic reduction of substrates with appropriate leaving groups X, these groups can be replaced by hydrogen in a reaction which is termed cathodic cleavage . A mechanism is assumed involving the reduction of the substrate to a radical anion that dissociates into a radical and X, sometimes both reactions occur in a single step (dissociative electron transfer) [153]. The radical is further reduced to an anion that is subsequently protonated (Eq. 19). 

Althought this decomposition mechanism is not yet well-defined (homolytic cleavage, reductive dimerization. hydride formation, or other mechanisms), it is practically meaningful. Styrene polymerization by CpTiMea/FAB carried out in the low-temperature regime (below the decomposition temperature) produces only atactic polystyrene, possibly via a car-bocationic polyaddition. However, at higher polymerization temperatures (above the decomposition temperature), highly syndiotactic polystyrene can be obtained by coordinative 2,1-polyinsertion. ... 

Hemnann et have indicated that the standard palladacycle trans-di(fju-acetato)-bis[o-(di-o-tolylphosphino)benzyl]-dipaUadium(II) (A) might be a catalyst precursor to active palladium(O) complexes (Scheme 41). In other words, the palladacycle may act as a thermally stable reservoir for the real catalytic species, which is released by heterolytic Pd—C bond cleavage and is activated by subsequent reduction. If this is the acmal case a tfaditional catalytic cycle via Pd(0)/Pd(II) has to be postulated also with palladacycles. In addition, for cross-coupling and amination reactions there is strong evidence for the reduction mechanism of phosphapaUadacycle A into a Pd(0) species.f ... 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

The cleavage of the isoxazole ring by organomagnesium compounds may proceed by either one or both of two alternative mechanisms. Magnesium subhalides produced during the associated reaction may act as reducing agents as proved in specific cases.Another possibility is that the reduction involves a six-membered cyclic complex (171). 

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