Thermal fragmentation

1-Vinylbenzotriazoles give indoles on flash pyrolysis at 600°C/10 2 Torr. However, depending on the vinyl substituents, side reactions leading to A-phenylketenimines or benzonitrile are also observed 

5-Oxadiazoles undergo thermal and photochemical ring cleavage at the 0(1)-N(2) and C(3)-C(4) bonds to yield nitrile and nitrile oxide fragments, and products derived therefrom. Thus, diphenylfurazan (30, X = 0) decomposes under flash vacuum pyrolysis conditions (600°C, 10-3 mm 

Hg) affording benzonitrile and benzonitrile oxide in nearly quantitative yields (81TH405-01). Benzofurazans are thermally more stable but can be cleaved photolytically. 

Although unsubstituted 1,2,5-thiadiazole is stable on heating at 220°C, 3,4-diphenyl-1,2,5-thiadia-zole 1,1-dioxide (30, X = S02) decomposes into benzonitrile and sulfur dioxide at 250°C (68AHC(9)107). 

Thermal degradation of 13C labeled 5-diazotetrazole (32) provides a source of atomic carbon for the investigation of its carbene reaction (79JA1301). 

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

The thermal fragmentation of unsaturated bicyclic 1,4-peroxides, often readily made from 1,4-dienes (Scheme 84), has become an important route to novel bis(oxiranes) (80T833, 81CRV91). 

Thiophene, 2-amino-3-cyano-5-phenyl-synthesis, 4, 888-889 Thiophene, 3-amino-4,5-dihydro-cycloaddition reactions, 4, 848 Thiophene, 2-amino-3-ethoxycarbonyl-ring opening, 4, 73 Thiophene, 2-amino-5-methyl-synthesis, 4, 73 Thiophene, 2-anilino-synthesis, 4, 923-924 Thiophene, aryl-synthesis, 4, 836, 914-916 Thiophene, 2-(arylamino)-3-nitro-synthesis, 4, 892 Thiophene, azido-nitrenes, 4, 818-820 reactions, 4, 818-820 thermal fragmentation, 4, 819-820 Thiophene, 3-azido-4-formyl-reactions... 

Thermal fragmentation of l,3-dioxin-4-ones or acylated Meldrum acids with generation of a-oxoketenes, hetero Diels-Alder reactions of the latter, and their transformations into lactones and lactams, among them macrocyclic 99YGK76. 

This thermal fragmentation is so facile that only under inert atmosphere and very low temperatures can the rate of decomposition be reduced sufficiently so as to make the systematic study of these molecules possible. 

The dominant pattern for the thermal fragmentation of thietane dioxides involves extrusion of sulfur dioxide leading to a 1,3-diradical (i.e. 242) which closes to final products, mainly cyclopropanes, accompanied by rearrangement products resulting from hydrogen migration within the diradical191,1930 230,256-258 (equation 92). 

An interesting sulfone to sulfinate rearrangement has been observed by Schank and Schmitt100 in the thermal fragmentation of a-alkoxysulfones, and is believed to involve an ion-pair mechanism (equation 32). 

A similar mechanism may also be suggested for the thermal fragmentation of cyclic five-membered a-sulfonyl ethers to sulfur dioxide, alkenes and carbonyl compounds (equation 33)101 103 as well as for the analogous rearrangement and fragmentation of trithioorthoacetate-S, S-dioxides (equation 34)104. 

The procedure described here is a modification of one involving the thermal fragmentation of 1-adamantyl hypoiodite and cycliza-tion of the resulting iodo ketone/ By means of this procedure, 4-protoadamantanone is obtained from 1-adamantanol with consistent yields in the range of 71 to 82% and a purity greater than 98%. This method is also applicable to the preparation of other polycyclic ketones from the related bridgehead alcohols with a-bridges of zero, one, or two carbon atoms (see Table I). 

In the presence of a suitably disposed /i-hydrogen—as in alkyl-substituted thiirane oxides such as 16c—an alternative, more facile pathway for thermal fragmentation is available . In such cases the thiirene oxides are thermally rearranged to the allylic sulfenic acid, 37, similarly to the thermolysis of larger cyclic and acyclic sulfoxides (see equation 9). In sharp contrast to this type of thiirane oxide, mono- and cis-disubstituted ones have no available hydrogen for abstraction and afford on thermolysis only olefins and sulfur monoxide . However, rapid thermolysis of thiirane oxides of type 16c at high temperatures (200-340 °C), rather than at room temperature or lower, afforded mixtures of cis- and trans-olefins with the concomitant extrusion of sulfur monoxide . The rationale proposed for all these observations is that thiirane oxides may thermally... 

Table 6.22 summarises the main characteristics of APCI-MS. Sometimes the heat burden of the APCI interface causes thermal decomposition, which is unwelcome if the requested information is only the molecular weight. On the other hand, studying the thermal fragmentation can provide additional data about the sample. Since the thermal and collisional (CID) fragmentation do not necessarily follow the same pathway, the two methods do complement each other. Therefore, even good use can be made of the thermal decomposition for structure elucidation during APCI experiments [145],... 

The A5-phosphorin 164 and the bicyclic compound 165 are precursors of isopropyl metaphosphate, 16). Thermal fragmentation of 165 leads via [2 -I- 2]cycloreversion to triphenyltoluene 166 and isopropyl metaphosphate 167. The latter is identified as isopropyl phosphate after reaction with water. The mass spectrum of 165 is also dominated by this fragmentation picture (m/e 442 (16%) = M + m/e 320 (100%) = M + — 167). 

The reactions of tetrafluorobenzyne with 9,10-dihydrophenanthrene and phenanthrene yield the expected adducts formed by cyclo-addition at the 1,4-positions 90). The reaction of tetrafluorobenzyne with 1,6-methanocyclodecapentaene (52) was carried out in order to study the mass spectral and thermal fragmentation of (53) 90>. In the event benzocyclopropene and 1,2,3,4-tetrafluoronaphthalene (54) were formed. 

Carbonaceous solids appear as a result of retrogressive reactions, in which organic thermal fragments recombine to produce insoluble semi-cokes (59,65). Coke formation is observed during liquefaction of all coals and its extent can vary widely according to the coal, the reaction solvent, and reaction conditions. The predominant inorganic species produced during the process of coal... 

The significance of the above-described work is that in all of the presently developing coal liquefaction processes, the initial step in the conversion is thermal fragmentation of the coal structure to produce very fragile molecules which are highly functional, low in solubility, and extremely reactive toward dehydrogenation and char formation. A more detailed discussion of the chemical nature of these initial products has been presented elsewhere (4). ... 

The formation of these thermal fragments is necessary to catalytic liquefaction processes before the catalysts can become effective for hydrogen introduction, cracking and/or heteroatom removal (10). ... 

There is a clear trend, however, in the content of aromatic carbon in a coal and its convertibility at short times. This is shown in Figure 13. It can be seen that high convertibility occurs for coals which are intermediate in aromatic carbon content. This observation is consistent with the common belief that thermal fragmentation occurs at aliphatic positions a or to aromatic rings. 

Nitrile oxides generated under neutral conditions by thermal fragmentation of nitrolic acids 32, were trapped in situ with alkenes to afford isoxazolines 33 in 53-97% yields <00TL1191>. Nitrile oxides were also produced by treating O-silylated hydroxamic acids 34 with triflic anhydride and TEA . ... 

Benzonitrile oxide and mesitonitrile oxide undergo 1,3-dipolar cycloaddition reactions with 1,3,5-triphosphinines under mild conditions to afford fused heterocyclic compounds (Scheme 1.33), for example, 192 and 193. Oxaphosphazoles and oxadiphospholes have become accessible by thermal fragmentation reactions of such fused heterocyclic compounds (358). 

DR. ANTHONY POE (University of Toronto) As Dr. Geoffroy mentioned, there is a fair bit of work being done on dinuclear metal-metal bonded carbonyls but rather less on metal clusters [Geoffroy, G. L. Wrighton, M. S., "Organometallic Photochemistry," Academic Press New York, 1979]. We have been interested for some time in the thermal fragmentation of metal clusters, and have recently looked at some photochemical reactions as well. I would like to present some results here today which are very preliminary. 

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