(5‐Imino‐4,5‐dihydro‐3 H ‐pyrrol‐2‐yl)amines as Sterically Restrained 1,3,5‐Triazapenta‐1,3‐dienes: Useful Building Blocks for the Synthesis of Oligonitriles

(5‐Imino‐4,5‐dihydro‐3 H ‐pyrrol‐2‐yl)amines 2 may be considered as sterically restrained 1,3,5‐triazapenta‐1,3‐dienes. They were easily prepared from 2,2,3,3‐tetramethylsuccinonitrile ( 1 ) and lithium amides with subsequent aqueous workup to give the monocyclic compounds 2a – d and the bi‐ and tricyclic compounds 2e – g . X‐ray diffraction studies of 2c and 2d , which contain a primary NH 2 group, reveal the existence of homodimers held together with paired hydrogen bonds in the solid state. Trapping of the lithiated intermediate 2a‐Li by benzoyl chloride provided access to the 1‐oxa‐3,5,7‐triazahepta‐1,3,5‐triene 3b . It is characterized in the solid state by a planar heterocyclic subunit and a strongly twisted N ‐acylamidine part. With 1‐oxa‐3,5‐diazinium salts 4 , compounds 2a , b , d or 2a‐Li reacted to give 1‐oxa‐3,5,7,9,11‐pentaazaundeca‐1,3,5,7,9‐pentaenes 5a – d . Here again, the planar heterocyclic parts combine with longer, twisted (not helical) oligonitrile moieties. The X‐ray diffraction study of 5d indicates intermolecular N–H ··· O=C hydrogen bonding in the solid state, producing one‐dimensional strands. In a similar way as 1 the trinitrile 2,3,5‐trimethyl‐2,3,5‐tricyanohexane ( 6 ), a byproduct of the thermal decomposition of azoisobutyronitrile (AIBN), reacted with lithium benzylamide to give a bicyclic intermediate which was successfully converted into the corresponding 1‐oxa‐3,5,7,9,11,13‐hexaazatrideca‐1,3,5,7,9,11‐hexaene 7 when treated with 1‐oxa‐3,5‐diazinium salt 4b . Similar to 5d , this extended oligonitrile forms one‐dimensional hydrogen‐bonded polymers in the solid state. Its UV spectrum ( λ max = 286 nm) is interpreted in terms of a planar conjugated π–π* azapolyene substructure forced into planarity by the heterocycles.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)