Unsaturated Hetero Chains, II. 1‐Oxa‐3,5,7‐triaza‐1,3,5,7‐octatetraenes – Syntheses and Structures

Three independent routes for the synthesis of the hitherto unknown substituted 1‐oxa‐3,5,7‐triazaoctatetraenes 4 have been developed. Method 1 utilizes N‐lithiated imines 1 and 1,3,5‐oxadiazinium salts 2 for the synthesis of 4b–e. Method 2 takes advantage of the new 6‐alkoxy‐1‐oxa‐3,5‐diazahexatrienes 5, which are allowed to react with imino lithium compounds to produce 4f. Method 3 is based on the combination of the lithiated 1,3‐diazabutadiene 7 with the N‐acylimidate 8 and gives access to 4g–j. Depending on the substitution pattern, the title compounds show ring‐chain tautomerism; in the case of the pentaphenyl system, only the triazine derivative 3a is observed. An X‐ray analysis of 4f reveals a non‐planar structure with (+)‐gauche‐Z‐(–)‐gauche‐Z‐(+)‐gauche conformation, resulting from amide‐like n/π interactions between C=N–π systems and the lone pair of the adjacent nitrogen atoms. Detailed spectroscopic data for all open‐chain and cyclic compounds are given. Quantum mechanical calculations (MP2/6‐31G*//6‐31G*) of model compounds 13 predict that non‐planar isomers with gauche‐configurated subunits are favored by more than 8 kcal/mol over a planar all‐trans structure as in polyenes. Most planar isomers correspond to low‐lying transition states for the rotation around the C–N bonds, but not to local minima, indicating high molecular flexibility of these oligonitrile chains.