Cyclic peroxides chemiluminescence

Although reaction of the acid chlorides with hydrogen peroxide and base did not lead to the isolation of definite cyclic peroxides, chemiluminescence was observed. On reacting 9,10-diphenyl anthracene-2,3-dicarboxylicacid dichloride (30) with hydrogen peroxide/tert. amine (e. g. ethyl-dicyclohexylamine-, dicyclo-hexylamine - or urea/H202) in dimethyl phthalate light is emitted. The emission spectrum matched the fluorescence of 9,10-diphenylanthracene-2,3-dicarboxy-late (Xmax. ca. 460nm). The quantum yield is low, with ca. 10 einsteins/mol. 

Later, fireflv oxyluciferin was successfully synthesi2ed (403. 408) and has been isolated and identified in firefly lanterns (luciola cruaciata) after the lanterns were treated with pyridine and acetic anhydride to prevent decomposition (409). In 1972, Suzuki and Goto firmly established that oxyluciferin is involved in the bioluminescence of firefly lanterns and in the chemiluminescence of firefly luciferin (403. 410).. A. mechanism involving a four-membered ring cyclic peroxide has been proposed for the reaction (406. 411). However, it was not confirmed by 0 -labelinE experiments (412). 

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. 

In the presence of hydrogen peroxide and base, acridinium salts lead to chemiluminescence emission. Acridans, in their reduced forms, are able to react directly with oxygen in aprotic solvents with 4>cl up to 10% . Scheme 31 shows the proposed mechanism for chemiluminescence of 9-cyano-lO-methylacridan and 9-cyano-lO-methylacridinium salt in the presence of oxidant and base, which postulates the cyclic peroxidic intermediate 44. 

Based on the kinetic results of experiments with photoinitiated peroxyoxalate chemiluminescence, Milofsky and Birks proposed, for the first time, the involvement of a six-membered cyclic peroxide (51) as HEI. On the basis of this suggestion, Hadd and coworkers, using conventional chemiluminescent kinetic studies with 47, also proposed the involvement of two HEIs, 48 and another six-membered cyclic peroxide 52 similar to 51. 

The present volume comprises 17 chapters, written by 27 authors from 11 countries, and deals with theoretical aspects and structural chemistry of peroxy compounds, with their thermochemistry, O NMR spectra and analysis, extensively with synthesis of cyclic peroxides and with the uses of peroxides in synthesis, and with peroxides in biological systems. Heterocyclic peroxides, containing silicon, germanium, sulfur and phosphorus, as well as transition metal peroxides are treated in several chapters. Special chapters deal with allylic peroxides, advances in the chemistry of dioxiranes and dioxetanes, and chemiluminescence of peroxide and with polar effects of their decomposition. A chapter on anti-malarial and anti-tumor peroxides, a hot topic in recent research of peroxides, closes the book. 

Chemiluminescence can occur when a thermal (dark) reaction is so exothermic that its energy exceeds that of the electronically excited state of one of the product molecules. The major pathway for these reactions is the decomposition of cyclic peroxides, and this is at the basis of most bioluminescence processes. There are some other physico-chemical processes which can lead to the formation of excited states and thereby to the emission of light these are based on the bimolecular recombination of high-energy species such as free radicals and radical ions. 

There are many different light-emitting dyes used for bioluminescence, usually called luciferins . The overall chemical reaction is rather complex and involves at least one enzyme-catalysed step in the formation of the excited dye molecule, but the energy is always derived from the decomposition of a cyclic peroxide. In this respect it is quite similar to the process of chemiluminescence (Figure 5.26). 

Cyclic peroxides may serve as a source of singlet oxygen. Wasserman et reacted 9,10-diphenylanthracene peroxide (238, conveniently prepared as in Nilsson and Kearns ) with 138 to give 140 rubrene peroxide proved to be considerably less efficient. Decomposition of anthracene peroxide alone takes another course. When 13 is treated with phthaloyl peroxide (239), 140 is isolated in 59% yield the reaction is accompanied by a weak chemiluminescence. A bright yellow chemiluminescence has been observed when a solution of 240 in 1,2,4-trichlorobenzene is treated with dibenzoyl peroxide at about 210°C. The generation of visible light from 138 under conditions where peroxides may present has been described. 

Dioxiranes, three-membered-ring cyclic peroxides, are known as highly efficient and selective oxidants, capable of performing a variety of transformations for synthetic purposes. It is known that some reactions of these peroxides are accompanied by chemiluminescence due to the release of singlet oxygen. For instance, infra-red chemiluminescence (IR-CL) of O2 at A, 1270 nm is emitted in the reaction of tertiary amines and N-oxides with dimethyldioxirane (DMD) and methyl(trifluoromethyl)dioxirane (TFD), as well as during the anion-catalyzed breakdown of the dioxiranes. Furthermore, IR-CL emission is produced in the ketone-catalyzed decomposition of the monoperoxysulfate ion HSOs through the intermediary dioxirane. ... 

Fig. 31. Thermal decomposition of the cyclic peroxide, 3,3,4-trimethyl-l,2-dioxetane, to give chemiluminescence.

As pointed out already, one of the most characteristic properties of the four-membered ring cyclic peroxides is their ability to chemiluminesce on thermal decay into carbonyl products [Eq. (14)1. 

The thermal decompositions are first order and usually unimolecular. A variety of experimental methods can. be used to follow the rates, which include direct chemiluminescence of the excited carbonyl product (A),14 50,93,96 activated chemiluminescence by energy transfer of the excited carbonyl to an efficient fluorescer (B),14c,l2a,94 9< dioxetane consumption or carbonyl product formation by NMR spectroscopy (C),l2M4b c iodometric titration of the cyclic peroxide (DVJ, Ub,c and infrared spectroscopy of a-peroxylactone consumption or carbonyl product formation (E).2,22,38 The method of choice depends on the particular system, but usually several techniques can be employed. 

Chemiluminescence.—It has been suggested that problems which occur in the determination of yields of bio- and chemi-luminescence may be due to the sample cell. Errors of 25% may be caused by reflection and refraction from interfaces, and, consequently, frosted containers and point-source geometries were recommended. Several authors have concentrated on the use of sensitizers for the enhancement of chemiluminescence. The heavy-atom effect was found to operate in the energy transfer from enzyme-generated acetone to xanthene dyes. 9,10-Diphenylanthracene (9,10-DP A) has been suggested to be a poor singlet counter for chemiluminescence as some triplet states were also counted. In another report, 9,10-dibromoanthracene was found to be a more effective enhancer, when compared with 9,10-DPA, for chemiluminescence from a cyclic peroxide. Luminol chemiluminescence was employed in the analysis of Cr" ions in sea-water. Enhancement with bromide ions enabled detection limits of 3.3 X 10 m to be achieved. 

Such cyclic peroxides have been proposed as intermediates in photochemical and chemiluminescent reactions of aromatic compounds with oxygen (4, 5, 44) and in the biological hydroxylation of aromatic compounds (22, 23, 34). In dilute ferrous ion solution the cyclic peroxides could decompose ionically to give a predominantly electrophilic distribution of substituted phenols, while at higher concentrations the ferrous ion would cleave the peroxide bond homolytically this, followed by loss of water, would give a more random pattern of substituted phenols. 

Dioxetanes are four-membered cyclic peroxides and their relative stability depends on the types of substituent groups present. Certain 1,2-dioxetanes are stable at room temperature but can be chemically triggered to produce chemiluminescence, these have an adamantyl group on one side of the ring and a... 

More control over the reaction conditions results in isolable peroxides, or at the very least, allows the exact structure of the peroxide to be inferred with some certainty. The reaction mechanisms which form the intellectual foundations of the phenomenon of organic chemiluminescence (and of bioluminescence) are all to be discoved here. In Chapter IV cyclic peroxides display a variety of mechanism, culminating in V with the very important dioxetans. Practical applications of these ideas must not be forgotten, and the chemistry of the active oxalates in Chapter VI brings together previous mechanistic concepts with the most well developed of all the chemiluminescent systems, the active oxalates. 

---