Photochemical and thermal

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

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). 

Thermal and photochemical decomposition of peroxides (4) and (5) lacking a-hydrogens (those derived from ketones) produces macrocycHc hydrocarbons andlactones (119,152,153). For example, 7,8,15,16,23,24-hexaoxatrispiro [5.2.5.2.5.2] tetracosane (see Table 5) yields cyclopentadecane and oxacycloheptadecan-2-one. 

The alkyl peroxyesters undergo homolysis, thermally and photochemically, to generate free radicals (168,213,229—232) ... 

Trichloroethane is also a coproduct in the thermal and photochemical chlorination of 1,1-dichloroethane to produce 1,1,1-trichloroethane. Vapor chlorination favors the 1,1,1-isomer, whereas reaction in the Hquid phase may give much higher ratios of 1,1,2-trichloroethane. V-type 2eohtes have been used in vapor-phase chlorination of 1,1-dichloroethane to produce 1,1,2-trichloroethane in high selectivity (100). 

Thermal and Photochemical Reactions Involving No Other Species... 

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... 

Azomethine ylides (Section 4.03.6.1.1) have been generated from a wide variety of aziridines using both thermal and photochemical methods. With carbon-carbon unsaturated dipolarophiles, pyrrolines or pyrrolidines are obtained. With hetero double bonds, however, ring systems of interest to this discussion result. 

Thermal and photochemical reactions formally involving no other species... 

Thermal and Photochemical Reactions involving No Other Species 147... 

THERMAL AND PHOTOCHEMICAL REACTIONS, NOT FORMALLY INVOLVING OTHER SPECIES... 

Ring cleavage (probably homolytic), thermal and photochemical (Section 5.05.3.2)... 

Reactions Formally Involving No Other Species (Except as Trapping Agents) S.OS.3.2.1 Thermal and photochemical reactions... 

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). 

There are few reports of the thermal and photochemical decomposition of simple azetidines. 

---