Electron-rich dioxetans

The rare earth chelate Eu(fod)3, does not catalyse the decomposition of simple dioxetans but acts as an excellent acceptor of the energy from the triplet carbonyl product. However it is an effective catalyst for the electron rich dioxetan (11), and the complex formed emits the characteristic bright red light of the europium chelate [48]. 

Nonetheless, a dioxetan decomposition mechanism for lucigenin chemiluminescence, based on the exergonic processes described in Chap. V, seems well established [3]. A direct demonstration of the intermediacy of this dioxetane was first made [4] in 1969 by treating 10,10 -dimethyl-9,9 -biacrylidene (4) with singlet oxygen from several sources. Emission from N-methyl acridone was unequivocally shown. The lifetime of the intermediate was characteristic of the supposed dioxetane. Intramolecular electron transfer has been suggested as the excitation mechanism in the decomposition of this and other electron-rich dioxetans. 

This intramolecular version of CIEEL has been invoked for the luciferins [16b] and electron rich dioxetans [16 a],... 

Although there are many components in a mechanistic description of a chemiluminescent reaction, the heart of the matter is the actual excitation step. Several such steps have been identified. Some are molecular in character e. g. the decomposition of dioxetans and some are intermolecular electron transfer steps. There is an intermediate class in which the step can be formulated as an /n ramolecular electron transfer. Many luminescent reactions have been ascribed to this category with varying degrees of confidence. Cyclic hydrazides such as luminol belong rather uncertainly here. Electron rich dioxetans and dioxetanones and the luciferins with such intermediates on the pathway are a little more reasonably assigned to an intramolecular electron transfer mechanism. Even here however caution is required in that direct evidence for discrete electron transfer will by its very nature be almost impossible to obtain and will probably remain circumstantial. 

A related phenomenon is seen in the easy decomposition and extraordinarily high quantum yield (0cl = 0.13) of the electron rich dioxetan (18) [35] see alsop. 63). 

Here solvent effects confirm the electron transfer or charge transfer character of the decomposition. Other electron rich dioxetans are amino-substituted [36], such as (19) and (20)... 

Dioxetanes are obtained from an a-halohydroperoxide by treatment with base (41), or reaction of singlet oxygen with an electron-rich olefin such as tetraethoxyethylene or 10,10 -dimethyl-9,9 -biacridan [23663-77-6] (16) (25,42). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Reaction of triethylsilyl hydrotrfoxide with electron-rich olefins to gh/e dioxetanes that react IntrarTMlecularly with a keto group in the presence of t-txrtyidimethyl silyl triflateto afford 1,2,4 Inoxanes also oxydatnre cleavage ol alkenes Also used in cleavage ol olefins... 

Singlet oxygen reacts with electron rich or highly strained alkenes to form 1,2-dioxetanes. These four-membered ring peroxides decompose on warming to two carbonyl compounds (or moieties), usually with appearance of light emission (chemiluminescence). The macrocyclic bis-lactone in (6.17)608>, a musk fragrance, has been synthesized via such a sequence. 

Posner and coworkers reported that triethylsilyl hydrotrioxide is an efficient reagent for direct conversion of electron-rich alkenes to 1,2-dioxetanes 72 (equation 80) . 

A mechanism was proposed to account for the reaction of triethylsilyl hydrotrioxide with electron-rich alkenes as a dioxetane-forming process (route a equation 82) and with inactivated alkenes as a nondioxetane carbonyl-forming process (route b equation 83). 

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. 

With electron rich olefins 1,2 cycloaddition forms relatively unstable dioxetanes which cleave to give carbonyl fragments ... 

The reaction with enol ethers, enamines, and electron-rich alkenes, without allylic hydrogens of proper orientation, to yield 1,2-dioxetanes... 

The singlet oxygen ( C ) cycloaddition to electron-rich alkenes is by far the most prevalent method used for the construction of 1,2-dioxetanes. The Kopecky method, which relies on the cyclization of a /3-halo hydroperoxide, is rarely utilized these days but was heavily relied upon in the past. The base-catalyzed cyclization of /3-epoxy hydroperoxides also appears to becoming more popular. There are also several miscellaneous methods that have been utilized for specific dioxetane examples and these are summarized in Section 2.16.7.1.3. 

Kopecky s synthesis of trimethyldioxetane employed the base-mediated dehydrohalogenation of 2-methyl-2-hydroperoxy-3-bromobutane. Subsequently, this type of eliminative cyclization (14) has been applied to the preparation of scores of dioxetanes. Additionally, many dioxetanes have been prepared by the addition of singlet oxygen to electron-rich olefins which do not possess allylic hydrogens (15), a method discovered first by Bartlett and Schaap... 

However, in recent years, it has become apparent that several photoinduced oxidation reactions do not involve singlet oxygen as the reactive intermediate, and, consequently, the reaction products cannot be accounted for the mechanisms shortly reported above. Moreover, since the first report on the photochemical stereospecific synthesis of the most fascinating peroxide derivatives, i.e., 1,2-dioxetanes [32], it clearly appeared, with a few notable exceptions [33,38], that only electron-rich olefins, such as enamines, enol ethers, and thio-substituted... 

In a number of groundbreaking explorations,46 Wudl, Hummelen, and coworkers showed that photoinduced [2 + 2] cycloaddition of singlet oxygen to one of the electron-rich enamine-type double bonds of azahomo [60] fullerene derivative 53 (Scheme 1.8), followed by decomposition of the 1,2-dioxetane intermediate (not shown), led to ketolactam ( )-54, the first cluster-opened fullerene with a free orifice.153 Under synthetic conditions mimicking events observed in the gas phase,154 acid-induced loss of 2-methoxyethanol from... 

Since [4 + 2]cycloaddition and ene reactions are generally assumed to proceed in a concerted manner via isopolar activated complexes, they should exhibit virtually the same small, often negligible, response to changes in solvent polarity. This is what, in fact, has been found cf. for example [138, 682, 683]. However, two-step [2 + 2]-cycloaddition reactions of singlet oxygen to suitably substituted electron-rich alkenes proceed via dipolar activated complexes to zwitterionic intermediates (1,4-dipoles or perepoxides). In this case, the relative amounts of 1,2-dioxetane and allylic hydroperoxides or e do-peroxides should vary markedly with solvent polarity if two or even all three of the reaction pathways shown in Eq. (5-145) are operative [681, 683, 684]. 

Further evidence in support of zwitterionic intermediates in the [2 + 2]-cycloaddition of singlet oxygen to electron-rich alkenes has been obtained by Jefford et al. [684]. The photo-oxygenation of 2-(methoxymethylidene)adamantane creates a zwitterionic intermediate (peroxide or perepoxide), which can be captured by acetaldehyde to give 1,2,4-trioxanes in addition to 1,2-dioxetanes cf. Eq. (5-147). 

It has been suggested" that the formation of the keto-aldehyde (137) in relatively high yield from the sensitized photo-oxidation of thujopsene (138) can best be explained in terms of a dioxetan intermediate (139), similar examples of which have recently been found in singlet oxygen addition to electron-rich double bonds. An extensive analysis of the products of acid-catalysed rearrangement of thujopsene (138) has been carried out." Under different acid conditions ten products have been isolated and identified these include the known compounds, chami-grene (140), cuparene (141), and widdrol (142 R = H) together with the previously unknown compounds (142 R = Et) and (143)—(148). The authors have put forward a mechanistic scheme to explain the formation of all these compounds based on interconversions of cyclopropylcarbinyl and homoallyl cations. 

Electron-rich olefins, especially vinyl ethers, " ketene acetals, thioalkyl-substituted olefins,and enamines, " react readily with singlet oxygen to give dioxetane cleavage products. Under carefully controlled temperature conditions, the intermediary 1,2-dioxetanes can be isolated. The first 1,2-dioxetanes prepared in this manner were the cis and trans isomers (8a) and (8b), respectively (Eq. 14). 

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