Fragmentation reactions photochemical

More recent studies have shown that monomeric metaphosphates such as 147 are just as unisolable as the metaphosphate ion 102, and are even more electrophilic. Generation of metaphosphates is accomplished mainly in two ways, i.e. by thermal or photochemical fragmentation reactions, on the one hand, and by decomposition of suitably activated phosphates on the other. 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

Photochemically Induced Fragmentation Reactions of Heterocyclic Compounds , R. M. Kellogg, in Photochemistry of Heterocyclic Compounds , ed. O. Buchardt, vol. 4 of General Heterocyclic Chemistry Series , Wiley, New York, 1976, pp. 367-455. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

Thermal and Photochemical Reactions, Not Formally Involving Other Species 3.5.2.1 Fragmentation Reactions... 

An unusual photochemical fragmentation reaction of a / -keto vinylogous amide has recently been observed by Siegel50. Irradiation of 148 in acetonitrile through a pyrex... 

Photochemical reactions—Mannich ketobases, mainly, are submitted to photochemical reactions leading to ring formation (see Figs. 121 and 128). Irradiation in methanol of several aminoaryl ketones, however, gives the imines 371 (Fig. 148) and dcaminoal-kylated compounds are also formed as byproducts, due to fragmentation reactions. " ... 

Turro and his coworkers were among the first to recognize the generality of the correlation between photochemical reactivity and mass spectrometric fragmentation reactions. s.ise) "phe photochemical reactivity of ketene dimers. Benzylcyclopropane and nortricyclanone g... 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

Fig. 2.100. Mechanism proposed for the reaction of the [Rh(Tp )(CNneopentyl)] fragment, obtained photochemically from the carbodiimide complex, with chloro-substituted alkanes.

Electron impact-induced fragmentations and photochemical reactions (see review, Bentley and Johnstone132) of V-imines show but little similarity analogous results are found only in the N-N bond cleavage.133 The same applies to the thermal and mass spectrometric process.128... 

Oxidation Reactions. Hypoiodite intermediates may be generated from the reaction of simple alcohols with NIS. When conducted under photochemical irradiation, the products of Barton-type or fragmentation reactions of alkoxyl radical intermediates may be obtained. Aldehydes are oxidized to methyl esters via hemiacetal intermediates by reaction with NIS in methanol at rt. However, such conditions are not effective for the oxidation of simple alcohols. The combination of NIS and Tetrabutylammonium Iodide in dichloromethane has been developed for the oxidation of a variety of alcohols to the corresponding carbonyl corrpounds (eq 8). This reagent system is most widely used for the oxidation of lactols to lactones, in which near-quantitative yields are generally obtained under mild conditions (eq 9). ... 

A detailed study of the fragmentation of the cyclohexenones (21a and b) has been made. Analogies between the mass spectral fragmentation and photochemical reaction of 5-diketones have been proposed in a study of some 27 such compounds. 

We ve seen loss of CO from ketones under certain circumstances (Eq. 16.61), and expulsion of a small stable molecule is a common photochemical reaction. Dinitrogen, N2, is an extremely stable fragment, and photochemical expulsion of dinitrogen can occur from several different types of structures. Photochemical elimination of N, has been especially useful in a wide array of studies of reactive intermediates. Whether under conventional conditions or cryogenic, matrix isolation conditions, photolysis with loss of N2 has been used to generate many types of reactive intermediates. We saw examples of this in Chapter 10 when radical additions were discussed and in Chapter 13 in the context of radical polymerizations. 

The general problem of the relationship between fragmentation reactions in a mass spectrometer and photochemical or thermochemical reactivity has fascinated mass spectroscopists for many years. The correlation in both cases is quite general and scores of examples are known. The scope and limits of these correlations have recently been reviewed from the point of view of PMO theory, and we will not discuss them further here. 

Another important photochemical reaction of both aliphatic and aromatic ketones is the fragmentation reaction. Unconjugated ketones on photoexcitation undergo a-cleavage followed by decarbonylation and subsequent reactions of alkyl radicals to give product(s). All these processes are collectively known as Norrish type-1 cleavage reactions [5]. These reactions take place both in gaseous and liquid phases. 

Aryl-tellurides undergo radical fragmentation reactions in the presence of methyl radical generated by photochemical decarboxylation of 0-acetyl thiohydroxamate. The driving force of this reaction is the formation of the relatively strong methyl-tellurium bond at the expense of the R-tellurium bond. Conventional radical traps can react with the formed alkyl radical. Scheme 16 shows an application of this reaction in the synthesis of showdomycind ... 

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