Singlet oxygen , reactant

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

Kummler68 reports a private communication from Mahan which suggests that the rate constant for the reaction between 02(1A9) and ethylene is slower, but not more than ten times slower, than the reaction between atomic oxygen and ethylene. Thus the rate constant should be between about 2 x 107and2 x 108 liter mole-1 sec-1. Absolute measurements of the reactivity of singlet oxygen with olefins have recently been made by Broadbent et al.,51 using both emission intensity measurements and the rate of reactant removal, to follow the reaction. A preliminary rate constant of 2.3 x 105 liter mole-1 sec-1 has been obtained for the reaction... 

The excited 02 states are singlets (xAg, g), the first of which is relatively low lying (94 kJ mol-1, equivalent to near-infrared radiation at 1270 nm) and thus can be easily populated by the energy transfer from the triplet states of most dyes the second 02 singlet 0Xg), is more difficult to reach (157 kJ mol-1), but is much more reactive. The first step of this photo-oxidation pathway is usually the singlet oxygen attack on the reactant molecule in its ground state ... 

Zweig, A., Henderson, W.A. 1975. Singlet oxygen and polymer photooxidations. I. Sensitizers, quenchers and reactants. J. Polym. Sci. Part A 1, 717-736. 

Table 12.2 gives a qualitative survey of photolysis products that are important reactants in surface and atmospheric waters. Some of the important physicochemical properties of these photoreactants have already been discussed in Chapter 11. As the table illustrates, the formation of singlet oxygen, of OH radicals, of O /HO, and of organic peroxides is especially important. Some reaction schemes are given in Figure 12.3. 

Termination. Just as peroxy radicals are key to the propagation sequence, so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (25). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. As a standard, we have assumed a value close to that determined from oxygen absorption (26) and by ESR spectra (27) for oxidized polypropylene films. 

The photoperoxidation of high oleic sxmflower oil with singlet oxygen is an efficient reaction for the synthesis of functionalized triglyceride derivatives. In this reaction, high oleic sunflower oil and oxygen are the only reactants, which makes this transformation very attractive from an environmental point of view (74). 

In the autoxidation of CLA, which yields identified products likely produced by a singlet oxygen pathway, i.e., furan fatty acids, the identified products are not exceptionally stable relative to the reactants (12). The reason that this reaction proceeds is then more likely due to the presence of initiators. Few reports that identify the products of CLA autoxidation are available in the literature (8-13). The results of an experiment performed in our laboratory are presented below to illustrate the effect of initiators. 

The dye Rose-Bengal, in solution or as a suspension, is a very suitable sensitizer for generating singlet oxygen (Foote, 1%8 Kearns, 1971). There are several limitations to the dyes as photosensitizers (a) When the sensitizer is used in a solution, solvents which can be used for the reaction are limited, (b) The dye bleaches if longer reaction times are used, (c) The dye can sometimes react with the reactants or with the products, (d) Separation of the products from the dye in the reaction mixture is often difficult. 

For a reaction leading to hydroperoxides, a useful criterion of mechanism is the product distribution when 1,2-dimethylcyclohexene is oxidized by that method. Autoxidation under initiation by free radicals leads to introduction of the hydro-peroxy group at the 3-position to the extent of 54% (10), this mode of reaction being undetectably small when either thermally- or photo-generated singlet oxygen is the reactant. 

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