Indazoles, pyrolysis

All the examples quoted in this section concerning fragmentations or rearrangements involve photochemistry. An interesting thermal reaction has been described (72TL2235) in which the pyrolysis of indazole between 700 and 800 °C leads to a mixture of (197) and (198 Scheme 15). A mechanism involving the 3// tautomer and the carbene seems reasonable. 

High-temperature gas-phase pyrolysis of pyrazole and indazole derivatives 99MI27. 

Under flash vacuum pyrolysis conditions 1- and 2-(l-adamantyl)indazoles as well as 1-, 2- and 3-tritylindazoles are mutually interconverted (91BSF592, 89BSB349). 

N-Oxides 230 and 231 are stable benzo[o/]indazole derivatives. A mixture of these compounds was obtained on pyrolysis of l-azido-8-nitronaphthalene 228 or on oxidation of 1,8-naphthalenediamine 229. It... 

Representatives of l,2-dihydrobenzo[cdifferent substituents at the nitrogen atoms are formed on pyrolysis... 

The pyrolysis of indazoles 113-115 with bulky substituents R has also given some interesting results. For R = Ph3C, FVP of 113 at 400 °C leads to partial conversion into 114, while at 300 °C 114 gives 113. The less stable isomer 115 is converted at 300 °C into a mixture of 113 and 11465. For R = 1-adamantyl, 113 and 114 are again interconverted at 600 °C, while at 700 °C the new product 117 is formed from either by way of the azirine intermediate 11666. 

The pyrolysis of 4-arylhydrazono-3-methylisoxazolone-5 forms isocyanoam-ines, which undergo rearrangement to cyanoamides and corresponding indazoles (Scheme 2.35). Among other compounds 5-nitroindazole was obtained in an analogous way [323],... 

NH2- condensed pyridines (Skraup reaction) NH2-+N2 -+H, Cl, I, CN,... C02H->Br (Hunsdiecker reaction) Br-+H (Na/Hg) etc. As a matter of fact, the whole spectrum of aromatic reactivity can be transferred to indazoles substituted at the fused benzene ring. Compare, for example, the reactions described by Suschitzky et al. (68JCS(C)1937) for 2-azidonaphthalene (432) and for 5-azidoindazole (433). Pyrolysis of these compounds in a mixture of acetic and polyphosphoric acid yields (434) and (435), respectively. 

In some cases the indazole and spiroindazole route did not lead to isolable cycloproparenes. Thus, the attempted synthesis of a 1,1-diphenylbenzocyclopropene or 1-phenylbenzocyclo-propene-l-carbonitrile by this route resulted instead in isolation of fluorene derivatives. The photolysis or pyrolysis of spiro[fluorene-9,3 -3//-indazole] gave fluoradene (4b//-indeno-[l,2,3-y, ]fluorene) as the major product (80%). The cycloproparene, spiro[17/-bcnzocyclo-propene-l,9 -fluorene], although formed as a reaction intermediate, was not isolable. Similarly, the thermolysis and photolysis of 9,10-dihydrospiro[anthracene-9,3 -3 /f-indazole]-10-one gave spirocycloproparenes as (non-isolable) reactive intermediates. ... 

Flash vacuum pyrolysis of 1- (71), 2- (72) and 3-tritylindazole (73) was carried out at 300 °C and 400°C. The experimental results show that (73) isomerizes to (71) and (72) (72) isomerizes to (71) at 300°C, and (71) remains unchanged at 300°C and at 400°C gives (72) and by-products <89BSB349>. A reaction profile consistent with these observations was proposed which involves indazole aromatic and nonaromatic (3H) tautomers. Under FVP conditions iV-adamantylindazoles (59) and (60) interconverted mutually <91BSF592>. 

A rearrangement of the type shown in Scheme 28 cannot be observed with C,N-diphenylnitrilimines, because these cyclize onto the aromatic ring bound to N, giving indazoles (Scheme 29). The reaction is virtually quantitative and therefore of synthetic value for the preparation of 3-arylindazoles and pyrazolopyridines (149).193,194 When the same reaction is carried out at 800°C (flash pyrolysis), a further molecule of N2 is extruded, giving a new carbene which cyclizes to fluorenes or azafluorenes (150), again in quantitative yields (Scheme 29).193... 

Aminoethyl)-l,2,4-oxadiazoles (606 R = alkyl or aryl) readily isomerize to 3-acylamino-2-pyrazolines (607) on heating. Treatment of the salt (608) with triethylamine yields the transient ylide (609), which decomposes to methyl-phenylcarbodi-imide (610). 2,3-Diphenyl-l,2,4-oxadiazolin-5-one (611) rearranges photochemically to a mixture of the oxadiazolinones (612) and (613) pyrolysis gives 2-phenylbenzimidazole with extrusion of carbon dioxide. The oxadiazole (614) rearranges thermally to the indazole (615). ... 

Flash vacuum pyrolysis of 5-(2-furyl)-2-phenyltetrazole (667) at ca. 400 °C yields the indazole (670), whereas at higher temperatures the benzofulvene (671) is obtained. The authors have suggested that (668) and (669) are key intermediates in these reactions. Another intriguing process is the formation of 2-phenylbenzotriazole (675) when the tetrazole (672) is thermolysed the proposed mechanism (see Scheme 8) receives support from the isolation of the benzotriazinone oxide (673b) and the spectroscopic detection of the isocyanate (614). ... 

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