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.