Reactive intermediates. Part XXVI. Flash vacuum pyrolysis of phenyl-substituted 1,2,4-triazoles; a new synthesis of isoindoles

Abstract
The nine mono-, di-, and tri-phenyl-1,2,4-triazoles have been subjected to flash vacuum pyrolysis at 650–800°, and the products have been isolated and identified. 1,3,5- and 3,4,5-Triphenyl-1,2,4-triazole both gave 1,3-diphenylisoindole in good yield. The formation of this product requires the extrusion of nitrogen from the triazoles, for which a mechanism is suggested involving a [1,5] phenyl shift and the generation of a non-aromatic 3H-1,2,4-triazole intermediate. The four diphenyl-1,2,4-triazoles reacted similarly to give 1-phenylisoindole, which was oxidised during work-up to 3,3′-diphenylbi-1H-isoindol-1-ylidene, and 1-phenyl-1,2,4-triazole gave isoindole, which was isolated as its Diels–Alder adduct with N-phenylmaleimide. 3-Phenyl-1,2,4-triazole underwent fragmentation in a different way, giving benzonitrile and cyanamide. From 4-phenyl-1,2,4-triazole a highly unstable product, thought to be N-cyano-N-phenylformamidine, was isolated at low temperatures. This product reacted with N-phenylmaleimide to give 2-(N-cyanoanilino)-N-phenyl-succinimide (7) for which a mechanism involving the transient formation of monophenylcarbodi-imide is suggested. An independent synthesis of the cyanamide (7) depended upon the formation of the lithium salt of phenylcyanamide from 1-phenyltetrazole and butyl-lithium. 4-(2,4,6-Trimethylphenyl)-1,2,4-triazole gave (2-amino-4,6-dimethylphenyl)acetonitrile (11) on pyrolysis, probably through 2-amino-5,7-dimethyl-3H-indole (10) as intermediate. This amino-indole readily isomerised on pyrolysis to give the phenylacetonitrile (11).

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