Thermolysis and Photolysis

The formation of transient benzazetidinones (251) in the photolysis and thermolysis of benzotriazin-4-ones (250) is well established (76AHC215) and the highly hindered adamantyl derivative has actually been isolated from flash pyrolysis of 4-adamantylbenzotriazinone (73JCS(Pl)868). A second route to such hindered benzazetidinones involves cyclization of the iminoketene valence tautomer (252 R = Bu ), the latter being generated by deprotonation of the anthranilium salts (253) (71JA1543). 

Research has continued on the photolysis and thermolysis of such systems in recent years, particularly by Ishikawa. The silenes formed, most of which were trapped as confirmation of their formation, are referenced in Table I. 

Nalidixic acid is stable up to five years under reasonable conditions of temperature and humidity. Pawelczyk and Plotkowiakowa(17) subjected sodium nalidixate solutions to accelerated aging, but were unable to identify decomposition products. Detzer and Huber(lS) studied the photolysis and thermolysis of nalidixic acid in the presence of oxygen. Photolysis produced de-carboxylated nalidixic acid, structure A, and a diketone product, structure B, as well as carbon dioxide and ethylamine. 

Horton, D. and Philips, K.D., Diazo derivatives of sugars. Synthesis of methyl 2-deoxy-2-diazo-D-arahino-hexonate, its behaviour on photolysis and thermolysis, and conversion into a pyrazole derivative. Carbohydr. Res., 1972, 22, 151. 

Asymmetric induction using an optically active acrylate ester with O-ethylthiohydroxamate has also been reported (87TL4205). [See reaction (52).] A review on the photolysis and thermolysis of O-hydroxamic thio-... 

The behavior of the unconventional 3-diazo-2-oxoindoles 21c,d on photolysis and thermolysis needs to be mentioned separately because 21c,d show a reactivity that is somehow different from that observed in indole series. 

Ando and co-workers have carried out photolysis and thermolysis on 2-alkylidene-l,3,4-thia-diazolines (34) synthesized from the thioketene (35) and diazo compounds (36). Irradiation of a benzene solution of (34) (R,R = —Me2C(CH2)2CMe2—) with a medium pressure mercury lamp produced (37) in a 51% yield, the bicyclo derivative (38) in a 15% yield, and a smaller amount of (35). In contrast, thermolysis of (34) at 130°C gave the thioketone (39) and the ethyne (40) in a quantitative yield. The episulfide (37) was isomerized quantitatively into the thiadiazoline (34) when treated with a trace of acid. A number of mechanisms were suggested for these transformations <90TL3571>. 

The same has been observed by Dodson and Klose in previous work where cis- and trans-1,2-diphenylcyclopropane were formed by pyrolysis of 2,3-diphenylthietane 1,1-dioxide at 230°C. Analogous results have been achieved by both photolysis and thermolysis of l-phenyl-2-benzoylthiethane 1,1-dioxide to a cis-trans mixture of l-phenyl-2-benzoylcyclopropane. ... 

The photolysis and thermolysis of 3,3,6,6-tetraaryl-l,2-dioxanes have also been studied <1996TL8181>. 

Abramovitch and Takeuchi had explored the use of N-aminopyridinium ions as precursors to nitrenium ions in both photolysis and thermolysis reactions, but had not attempted to use these precursors under flash photolysis conditions. Moran and Falvey demonstrated that the LFP of 138 in CFI3CN (Scheme 57) generated a short-lived intermediate (1.5 jus) that had characteristics similar to the ions l31b-f. The transient was identified as the singlet ion 116. " Evidence from product analysis and spectrophotometric detection of the cation radical Ph2NH- as a long-lived intermediate was initially thought to indicate that there were parallel path-... 

The main difference in the thermolysis of tetrazolinones (44) and tetrazolinethiones (45) is the generation of sulfur (S2) and hydrogen sulfide from the latter compounds. The sulfur atom is split off from the starting molecules and not from a thermolysis product such as an isothiocyanate in a secondary reaction. The sulfurless tetrazole is then cleaved by cycloreversion, affording the corresponding isonitrile as a product that cannot be explained otherwise. There is little in common between photolysis and thermolysis of... 

Photolysis and thermolysis of 4-aryl-l,2,4-triazoline-3,5-diones (70, R = aryl) have been studied by Wamhoff and Wald (77CB1699). Photolysis produces the same products as were found in FVP. The thermal conversion of 70, R = Ph, into the corresponding s-triazolo[l,2-a]-s-triazole derivative, which takes place below the decomposition temperature of the educt, is assumed to proceed via a radical chain reaction (Scheme 8). 

The photolysis and thermolysis of hexa-t-butylcyclotrisilane in the absence of trapping reagents was investigated recently45. Heating the compound in a sealed tube to 185°C affords a 5-methylene-l,2,3,7,8,9-hexasilanonane derivative (Scheme 5). 

TABLE 10. H CIDNP effects observed in the photolysis and thermolysis of 17 in the presence of halogenated traps ... 

The nitrene 28 is not produced from the azide precursor, but from heterocycles via photolysis and thermolysis as shown in Sch. 11 [20]. Iminoacyl nitrenes react intramolecularly giving benzimidazoles with good yields (Sch. 11), and, dependending on the precursor used and the reaction conditions, varying amounts of carbodiimides are obtained. The reactivity of the acyl nitrenes is influenced by the substituent connected to the acyl group (see Sch. 10), however all acyl nitrenes are quite reactive and therefore rather unselective. Apart from cycloaddition reactions with Tt-bonds, insertion reactions into a-bonds, additions to lone pair electrons of... 

More unusual are the reactions between CO, CO2, and SO2 and [Ni(L)(S3)] complexes with L = N2 and N(SiMe3)2. The azide complex (NBu4)[Ni(N3) (S3)] is inert toward UV photolysis and thermolysis, however, it readily reacts with CO under standard conditions according to Eq. 37 (119). 

The previous discussion has referred to photolysis and thermolysis only. Many of the transformations described have also been observed on mass spectrometry of isoxazoles, although it is not always clear whether they occur as a result of electron impact or by prior thermal reaction.211-212 It should be noted that such reactions can create problems in the use of mass spectrometry for elucidating the structures of isoxazoles. 

During the past two decades, considerable attention has focused on several externally stimulated initiators, for example onium salts that undergo photolysis and thermolysis to initiate cationic polymerization. The major classes of externally stimulated initiators which can suceesfully be applied to initiate cationic polymerization are collected in Table 1. 

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