Photochemical elimination

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Zitko [20] has described a confirmatory method in which the chloroparaffins in sediments are reduced to normal hydrocarbons which are then analysed by gas chromatography. This method lacks sufficient sensitivity for trace (sub-ppm) analysis and the confirmatory method may be difficult to apply. Friedman and Lombardo [21] have described a gas chromatographic method applicable to chloroparaffins that are slightly volatile the method is based on microcoulometric detection and photochemical elimination of chlorinated aromatic compounds that otherwise interfere. 

Alkylidene-phosphapyrazolines 98-101 are much more thermally stable than their relatives 88, which do not possess the exo-methylene substitution. Dediazo-niation of 98 required heating in toluene at 110°C and gave one or more of the following products, probably via intermediate diphenylmethylene(vinylidene)phos-phoranes methylenephosphiranes, (2-siloxyvinyl)phosphanes, 2//-l,3-oxaphos-pholes, and l-alkylidene-2,3-dihydro-l//-benzo[c]phospholes (169). Thermolysis of 100 ( R = t-Bu, 1-adamantyl) afforded isolable 2-phosphabutadienes (169). The photochemical elimination of N2 from 98 generated cyclic azomethine imine dipoles 104 (Scheme 8.24), which rearrange to compounds 105 and 106 that could be further trapped with DMAD to form 107 (170). 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

Intramolecularly alkoxy base-stabilized bis(germylene)-, 54, or (germylene)(silylene)-, 55, iron complexes, of type IV, were obtained from the photolysis of digermyl or germylsilyl iron complexes (equation 43)14. These are the same type of complexes noted above for base-free tungsten analogs146 involving a-elimination subsequent to photochemical elimination of CO as noted below. 

Azirines are also made by carbene addition to nitriles (121 122) and by thermal or photochemical elimination of N2 from vinyl azides (123 124) . Vinyl azides are prepared by the Hassner reaction, where iodine azide is first added to an alkene and the resultant -iodoazide is dehydrohalogenated with a base this approach is illustrated by the preparation of haloazirines 125 (Scheme 66) <2001T6203>. 

However it was soon recognized that a one-bond scission (b) is in better agreement with experimental data 54>. Although a lot of work has been done in the field of photochemical decomposition of azo compounds the question of one-bond versus two-bond scission has only recently received more detailed attention. The photochemical elimination of nitrogen from an azo compound can in principle proceed in the following way ... 

The photochemical elimination of larger molecule fragments has also been described. 1,2,4.6.7-Pentakis(trifluoromethyl)-3,5-diazatricyclo[4.1.0.0 ]hept-3-ene can be converted photochemi-cally to its bicyclic isomer, 1,3,5,6,7-pentakis(trifluoromethyl)-2,4-diazabicyclo[3.2.0]hepta-2,6-diene (7). This can be cleaved by irradiation with a low-pressure mercury lamp to give 2.4,.5-tris(trifluoromethyl)imidazole (8) with loss of hexafluorobutyne. ... 

Scheme 3.9. Photochemical elimination of a Si-containing side group for positive-tone plasma development.

Photochemical elimination of N2 from bicyclic azo compounds produces cyclic c7.r-l,n-biradicals followed by stereospeciflc ring cleavage or cycliza-... 

Performing the photochemical elimination at — 30"C in alkane or arene solvent affords the product in reasonable yield. 

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