2,6‐Di‐tert‐butyl‐4‐(3,3‐diarylpropadienylidene)‐2,5‐cyclohexadien‐1‐ones, the First Stable p‐Quinopropadienes

Abstract

AbstractTreatment of 2,6‐di‐tert‐butyl‐4‐ethynylphenol with 2 equivalents of nBuLi, followed by addition of diaryl ketones, afforded the corresponding p‐quinopropadienes 2a−f upon appropriate dehydrative treatments. These compounds are fairly stable, orange to purple, crystalline substances, exhibiting intense absorptions at longer wavelengths. In particular, the bis(dimethylamino) derivative 2a absorbs across the whole of the visible spectrum and into the near infrared. Unlike the quinone methides 1, compounds 2 exhibited irreversible oxidation waves together with irreversible reduction waves on cyclic voltammetry. The very small E1sum value of 2a also indicates the highly amphoteric redox properties of 2a; however, both the anion and cation radicals of 2 are highly reactive. The molecular structures of 2a and 2e exhibit relatively small dihedral angles between two aromatic ring planes (2a for 47° and 2e for 52°), and each has a short central double bond and long side double bonds. The short central double bond (2a for 1.228 Å and 2e for 1.235 Å) indicates the importance of the resonance contribution of the dipolar structure with an acetylenic bond. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)