Intramolecular Cyclization of Nitrile Imines. Synthesis of Indazoles, Fluorenes, and Aza Analogues1a

Abstract

Flash thermolysis of 2,5-diaryltetrazoles 2 at 400-500°C (10-3 mm) gives 3-arylindazoles 5 in yields of 96-100%. Thus, 2,5-diphenyl-, 2-(p-tolyl)-5-phenyl-, 2-phenyl-5-(p-tolyl)-, 2,5-di(p-tolyl)-, and 2-phenyl-5-(4-pyridyl)tetrazole furnish 3-phenyl-, 3-phenyl-5-methyl-, 3-(p-tolyl)-, 3-(p-tolyl)-5-methyl-, and 3-(4-pyridyl)indazole, respectively. Indazoles 5 are formed also by heating the tetrazoles 2 in tetralin at 207°C for 15min. Flash thermolysis of the same tetrazoles 2 at 800°C (10-3 mm) gives 2,6-disubstituted fluorenes 7 (2,6 substituents=H or CH3) or 3-azafluorene (7e) in yields of 90-100%. The thermolysis of 3-phenylpyrazolo[3,4-b]pyridine (8) at 770°C resulted in a 49% conversion to 4-azafluorene (9). Thermolysis of 2,4-diphenyl-1,3,4-oxadiazolin-5-one (16a) at 500°C (10-2 mm) gave 3-phenylindazole (94%); at 750°C fluorene was obtained (84%). 2-Methyl-4-phenyl-l,3,4-oxadiazolin-5-one (16b) gave at 450°C 3-methylindazole (89%) and at 650°C styrene (94%). The results are interpreted in terms of the; intermediate formation of nitrile imines by loss of N2 from 2 and of CO2 from 16. The nitrile imines are regarded as a resonance hybrid of bent dipolar and carbene structures (23) which cyclize unto the remote aromatic ring. © 1978, American Chemical Society. All rights reserved.