Chemical reactions of singlet oxygen

To date, several characteristic photo-oxidation reactions, in which singlet oxygen is involved, have been discovered. 

This reaction indicates a distinct stereospecificity due to the structure of singlet oxygen and its reactivity. It was found, for example, that the quantitative distribution of oxidation products from 1-methyl-2-iso-propylethylene and l,l-dimethyl-2-isopropylethylene differs when molecular or singlet oxygen is used [249]. 

This is a particularly interesting reaction, since in the first stage the anthracene molecule is excited to the triplet state and produces singlet oxygen from ground-state molecular oxygen, and then finally forms an endoperoxide. 

---

For the sensitized oxidation, the rate of production of the oxidized substrate, that is, the rate of the chemical reaction of singlet oxygen, depends on the efficiencies of singlet oxygen production and singlet oxygen reaction (Eqs. 49 and 50). 

The efficiency of the chemical reaction of singlet oxygen depending primarily on kd (Eq. 53), the choice of a solvent exhibiting weak interaction with the excited oxygen species, is advantageous. However, most sensitized oxidation reactions terminate by a sequence of thermal reactions of the intermediate product AOz in which the solvent may play a decisive role as far as the specificity and thus the chemical yield of the sensitized oxidation are concerned. The intermediate endoperoxide of furfuryl aldehyde (Eq. 48) reacts preferentially by homolysis of the O—O bond in apolar solvents, yielding a complex mixture of products due to subsequent radical reactions (Eq. 54). 

There are two main chemical reactions of singlet oxygen that are of interest here. The first is cycloaddition in a process that resembles a Diels-Alder reaction. 

Nevertheless, in the sensor study of heterogeneous reactions of singlet oxygen, it is better to employ selective sources that do not generate other active particles and produce though low, but stable concentrations of 02( ). These sources can be photochemical and chemical... 

Meyer, M.P., Leach, A.G., Kuwata, K.T., Chen, J.S., Greer, A., Foote, C.S. and Houk, K.N. (2003) Mechanism of ene reactions of singlet oxygen. A Two-Step No-intermediate mechanism. Journal of the American Chemical Society, 125, 1319-1328. 

Recently it has been reported,that singlet oxygen oeui be produced by the dismutation reaction of superoxide cmionie radicals (see chemical generation of singlet oxygen,page of this paper)(31 33) Superoxide anionic radicals take part in several polymer reactions,and are also important species in biopolymer chemistry. 

Gorman, A. A., Hamblett, I., Lambert, C., Spencer, B., Standen, M. C., Identification of Both Preequilibrium and Diffusion Limits for Reaction of Singlet Oxygen, Oji Ag), with Both Physical and Chemical Quenchers Variable temperature, Time resolved Infrared Luminescence Studies, J. Am. Chem. Soc. 1988, 110, 8053 8059. 

Figure 2.9 Chemical quenching of singlet oxygen, (a) The ene reaction — addition of singlet oxygen to an olefin with allylic hydrogen (b) the ene reaction of cholesterol (c) endoperoxide formation by singlet oxygen to imidazole residue as in histidine.

The emphasis in this work is on the scope and preparative synthetic utility of the hetero Diels-Alder reaction. No attempt has been made to carefully define or delineate the important mechanistic questions, many of which are as yet unanswered, of the various [4 -I- 2] cycloaddition reactions other than to try to provide a rationale for the facility with which the cycloadditions proceed and to provide a basis for the stereo- and regio-chemical observations. We have purposely excluded reactions of singlet oxygen as a dienophile, since extensive surveys are available elsewhere. Many miscellaneous heterodienophiles and heterodienes have not been covered if in our opinion they are not of general synthetic value. A comprehensive treatment of all recorded hetero [4 + 2] cycloadditions is beyond the scope of this monograph. However, we do hope to provide a broad survey of this reaction type as it exists today in order to furnish a foundation for continued development. 

Production of 0( P) atoms by the reaction N -1- NO is thus a preferred method. The physical and chemical behaviour of singlet oxygen can be readily studied in a discharge-flow system a considerable amount of information is known about these processes which is summarized later in this article. 

FIGURE 10 Rotating disk design for the chemical generation of singlet oxygen, 02(a A), via the two-phase reaction of chlorine gas with liquid hydrogen peroxide. 

Bortolus, R, Monti, S., Albini, A, Fasani, E., and Pietra, S., Physical quenching and chemical reaction of singlet molecular oxygen with azo dyes, /. Org. Chem., 54, 534, 1989. 

It has been suggested but now seems rather improbable, that singlet oxygen may be involved in this phagocytosis chemiluminescence [118]. This conclusion is supported experimentally by means of the most reliable method available at present for the chemical detection of singlet oxygen its unambiguous reaction with cholesterol (p. 13). 

---