The thermolysis of polyazapentadienes. Part 3. Cyclisation of C-methyl-1,2,5-triazapentadienes and related compounds: the role of an intermediate spirodienyl radical

Abstract

A series of C-methyl-1,2,5-triazapentadienes and 2,5-diaza-1 -oxapentadienes are prepared from pyruv-aldehyde phenylhydrazones and oxime anils respectively. The 4-methyl derivatives show E/Z isomerism about the 4,5-double bond. Pyrolysis of the 5-phenyl derivatives (1), (3), (5), (7), and (10) causes cyclisation to 2-methylquinoxaline, though 2-phenyliminopropanonitrile is obtained from the oxime ester (9). Pyrolysis of the 5-p-tolyl derivatives (2), (4), (6), (8), and (11) gives mixtures of 2,6- and 2,7-dimethylquinoxaline. These are formed predominantly by a rearrangement reaction which involves the spirodienyl radical (19), but also by direct cyclisation of the iminyl radicals (17) and (18).