3,4‐Dihydro‐3 H ‐pyrrol‐2‐imines as Conformationally Restrained 1,3‐Diazabutadienes: Synthesis, Structural Properties and Protonation

5‐Aryl‐3,3,4,4‐tetramethyl‐3,4‐dihydro‐3 H ‐pyrrol‐2‐imines,conformationally restrained 1,3‐diazabuta‐1,3‐diene derivatives, were easily prepared by treating aryllithium species with 2,2,3,3‐tetramethylsuccinonitrile ( 1 ). Trapping the reaction intermediate with chlorotrimethylsilane gave N ‐silylated compounds 2a – e , whereas aqueous workup gave N‐H derivatives 3a , b . Pyrenyl‐substituted compound 3b was characterised by X‐ray diffraction studies, revealing the presence of both intermolecular aromatic face‐to‐face contacts and the formation of homodimers by twofold H‐bonding. N ‐Silylated derivatives 2a – d were used successfully as nucleophilic components in palladium‐catalysed C–N bond‐forming reactions to obtain N ‐arylated compounds 5b – h , j , k , m and 7a – d . The UV spectra of compounds 5 and 7 exhibit long wavelength absorptions up to 462 nm for 7d , thus indicating extended π–π* conjugation. Dihydropyrrolimine‐based compounds with larger conjugated aryl substituents in the 5‐position react with Brønsted and Lewis acids displaying a significant colour change that could be used to estimate the p K b of 3a to a value of –4.5. Derivatives 2c , e and 3a , b , which are not N ‐arylated, are fluorescent with a Stokes shift of 107 nm (6034 cm –1 ) for 3a . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)