Peroxides, cyclic 1,2-Dioxetanes

See Other DIOXETANES, STRAINED-RING COMPOUNDS CYCLIC PEROXIDES... 

As mentioned earlier (see p. 122) the previously postulated dioxetane intermediate in firefly bioluminescence has been challenged as no 180 is in-corporated in the carbon dioxide released during oxidation of firefly luciferin with 18C>2. In view of the crucial significance of the 180. experiments De Luca and Dempsey 202> rigorously examined the reliability of their tracer method. They conclude from their experiments that all available evidence is in favour of a linear, not a cyclic peroxide intermediate — in contrast to Cypridina bioluminescence where at least part of the reaction proceeds via a cyclic peroxide (dioxetane) as concluded from the incorporation of 180 into the carbon dioxide evolved 202,203). However, the dioxetane intermediate is not absolutely excluded as there is the possibility of a non-chemiluminescent hydrolytic cleavage of the four-membered ring 204>. 

The brilliant emissions resulting from the oxidation of certain oxalic acid derivatives, especially in the presence of a variety of fluorophores, are the bases of the most active area of current interest in CL. This group of chemiluminescent reactions has been classified as peroxyoxalate chemistry because it derives from the excited states formed by the decomposition of cyclic peroxides of oxalic acid derivatives called dioxetanes, dioxetanones, and dioxetanediones. 

This group of highly strained cyclic peroxides, though thermodynamically unstable, contains some compounds of sufficient kinetic stability to exist as solids at ambient temperature [1], Interest in these compounds is increasing, most are very unstable, several have proved explosive when isolated [2,3], Not only are lower 1,2-dioxetanes dangerously unstable but so, above 0°C, are the precursor 1,2-bromohydroperoxides [4],... 

Section II covers the synthesis of the cyclic peroxides with medium ring size from 5 to 7. Section HI covers the synthesis of 1,2,4-trioxanes. Classification in sub-sections and sub-sub-sections is done according to the type of reaction by which the cyclic peroxide system is formed. Syntheses of dioxirans, 1,2-dioxetanes, trioxolanes (ozonides), tetrox-anes, and macrocyclic peroxides are not discussed in this review. 

In this Section, we will focus on acyclic dialkyl hydroperoxides and we will not discuss cyclic peroxides. Dioxiranes are discussed in Chapter 14 and dioxetanes in Chapter 15 of this book. 

The oxidation of organic substances by cyclic peroxides has been intensively studied over the last decades , from both the synthetic and mechanistic points of view. The earliest mechanistic studies have been carried out with cyclic peroxides such as phthaloyl peroxide , and more recently with a-methylene S-peroxy lactones and 1,2-dioxetanes . During the last 20 years, the dioxiranes (remarkable three-membered-ring cyclic peroxides) have acquired invaluable importance as powerful and mild oxidants, especially the epoxidation of electron-rich as well as electron-poor alkenes, heteroatom oxidation and CH insertions into alkanes (cf. the chapter by Adam and Zhao in this volume). The broad scope and general applicability of dioxiranes has rendered them as indispensable oxidizing agents in synthetic chemistry this is amply manifested by their intensive use, most prominently in the oxyfunctionalization of olefinic substrates. 

In the present chapter, we have considered the main recent developments in dioxetane studies. Our incentive to concentrate on the contemporary facets of dioxetane chemistry was to assure the reader that interest in these unique cyclic peroxides has not subsided since their discovery about 35 years ago. Particularly the last decade has witnessed impressive advances in this fascinating field, from both the mechanistic and applied points of view. Moreover, a novel feature of dioxetane chemistry pertains to harnessing them as building blocks in asymmetric synthesis, hardly thought feasible about a decade ago. 

For isolated HEI such as dioxetanes and other cyclic and linear peroxides that act directly as reagents in the excitation step, kinetic studies lead to rate constants and activation parameters for this excitation step and conclusions with respect to the mechanism of chemiexcitation can be obtained from the structural and conditional dependence of these parameters. For complex CL systems, in which the HEI is formed in rate-limiting steps prior to the excitation step, the kinetic parameters of this essential reaction step can only be obtained indirectly (see below). 

The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (a-peroxylac-tones) is the simplest and most exhaustively studied example of a thermal reaction that leads to the formation, in this case in a single elementary step, of the electronically excited state of one of the product molecules. The mechanism of this transformation was studied intensively in the 1970s and early 1980s and several hundreds of 1,2-dioxetane derivatives and some 1,2-dioxetanones were synthesized and their activation parameters and CL quantum yields determined. Thermal decomposition of these cyclic peroxides leads mainly to the formation of triplet-excited carbonyl products in up to 30% yields. However, formation of singlet excited products occurs in significantly lower yields (below... 

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. 

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