Reactions of 2,2-Diphenyl-1-picrylhydrazyl (DPPH) with Two Syringylic Phenols or One Aroxide Derivative

The reactions between excess 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH) and either 4‐hydroxy‐3,5‐dimethoxybenzaldehyde (syringaldehyde, 1) or methyl 4‐hydroxy‐3,5‐dimethoxybenzoate (methyl syringate, 2) afford different products from those obtained with the corresponding aroxide (3) of 1. In the former case (with the free phenols) the aryloxy (syringyl) group substitutes the 4‐nitro group of DPPH, yielding dinitro products 7 and 8 (new stable free radicals) and 9 and 10 (corresponding hydrazines), whereas in the latter case (with the sodium aroxide 3) the aryloxy (syringyl) group substitutes one of the phenyl groups of DPPH in the para position, forming trinitro products 14 (new stable free radical) and 15 (corresponding hydrazine). Accompanying products were isolated by TLC and identified by their 1H NMR, 13C NMR, UV/Vis, and EPR spectra. The reaction mechanisms involving hydrazyl and aroxyl free radicals are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) Syringaldehyde or methyl syringate substitute the 4‐nitro group of 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH). By contrast, the complexed sodium salt of syringaldehyde substitutes one of the phenyl groups of DPPH yielding another product.