Photochemical fragmentation

Thermal fragmentation Photochemical fragmentation Equilibria with open-chain compounds Rearrangement to other heterocyclic species Polymerization... 

Farrens, D.L., Cordonnier, M.M., Pratt, L.H., and Song, P.-S., The distance between the phytochrome chromophore and the N-terminal chain decreases during phototransformation. A novel fluorescence energy transfer method using labeled antibody fragments, Photochem. Photobiol, 56, 725, 1992. 

The contribution of form B increases with the growth of the electron-acceptor properties of the heteroatom X in the series CO < CH2 < O < S < Se < NMe, which promotes the formation of strong IMHB. 2-(Af-Phenylaminomethylene)-l-methylindol-3(2//)-one (X = NMe) exists exclusively in the E-ketoenamine form with a strong IMHB and does not exhibit photochromic properties. Other compounds 14 display in solutions equilibrium of E- and Z-forms, the position of which depends on the nature of the heterocycle annelated to the ketoenamine fragment. Photochemical Z/ -isomerization is observed for 14 (X = 0), and /Z-isomerization takes place in the case of compounds with X = S and also... 

The main features are the molecular ions as the base peak and the M-t-1 ions arising from another species. For 2-aminothiazole the m/e 73 ion (M-HCN) is shifted to m/e 75 in the spectrum of the dideuteroamino derivative and, therefore, largely arises via rupture of 2-3 and 4-5 bonds (Scheme 18). This fragmentation process could involve the kind of intermediates postulated in photochemical rearrangements (see Chapter III, Section IX.3.B). The other fragments fit well the general pattern of fragmentation proposed by Clarke (136). 

Thermal and Photochemical Reactions. Unsubstituted ethyleneimine has astonishing thermal stabihty. The reaction of ethyleneimine diluted with argon proceeds to give a mixture of unidentified compounds only at temperatures above 400°C (339). In a flow pyrolysis system under pressures of <1.33 kPa (<10 mm Hg) on quartz wool, isomerization to give /V-methylenemethylamine and ethylideneimine was observed only ia the temperature range 510—535°C. Higher temperatures result ia fragmentation (340). 

There are correlations between mass spectral fragmentations and thermal and photochemical fragmentations and rearrangements see Sections 4.02.1.2.1 and 4.02.1.2.2. 

Azolinones, azolinethiones, azolinimines N-Oxides, N-imides, N-ylides of azoles Thermal and Photochemical Reactions Formally Involving No Other Species 2.1 Thermal fragmentation... 

Reactivity of Five-membered Rings with Two or More Heteroatoms 4.02.1.2.2 Photochemical fragmentation... 

Mesoionic compounds undergo a variety of photochemical fragmentations. Examples are shown in which CO2 or PhNCO is extruded (Schemes 3 and 4, respectively) (76AHC(i9)l). 

In addition to fragmentation on electron impact (Section 4.04.1.3.8), most of the references in this section involve photochemical studies and an excellent account of these has been given by Lablache-Combier (B-76PH123). 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

The total synthesis of caryophyllene and its Z-isomer involved a photochemical [2 + 2] cycloaddition reaction to generate the 4-membered ring and a fragmentation process Helv. Chim. Acta, 1951, 34, 2338) to establish the 9-membered ring. Caryophyllene and various oxygenated derivatives protect plants against insects. 

A study of the photochemical reactions of some ylides of compound 1 showed the expected fragmentation to give, from compound 258 for example, 3-methyl-triazolopyridine and the products 259 and 260 postulated as derived from a carhene intermediate (00MI2). Ester 261 gives a hydrazone, and ylide 262 an indolizine. Thermal reactions of ylides are in Section IV. I. 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Morishita, H., et al. (2002). Cloning and characterization of an active fragment of luciferase from a luminescent marine alga, Pyrocystis lunula. Photochem. Photobiol. 75 311-315. 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

The photolysis of dimethyl sulphoxide (at 253.7 nm) in a wide range of solvents has been studied in detail176. Three primary reactions occur, namely (i) fragmentation into methyl radicals and methanesulphinyl radicals, equation (60), (ii) disproportionation into dimethyl sulphone and dimethyl sulphide, equation (61) and (iii) deactivation of the excited state to ground state dimethyl sulphoxide. All chemical processes occur through the singlet state. Further chemical reactions of the initial photochemical products produce species that have been oxidized relative to dimethyl sulphoxide. 

Dimethyl a-disulfone forms the fragments shown in equation (35) under electron impact at 70 eV. The fragmentation processes are closely related to the thermal and photochemical results reported for diaryl a-disulfones. In contrast to MeSOjSMe, MeSOjSOjMe shows no indication of the formation of MeS(OH)2 ... 

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