UNSATURATED HETEROCHAINS .2. 1-OXA-3,5,7-TRIAZA-1,3,5,7-OCTATETRAENES- SYNTHESES AND STRUCTURES

Three independent routes for the synthesis of the hitherto unknown sub stituted 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 th e synthesis of 4b-e. Method 2 takes advantage of the new 6-alkoxy-1-ox a-3,5-diazahexatrienes 5, which are allowed to react with imino lithiu m 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 acc ess to 4g-j. Depending on the substitution pattern, the title compound s show ring-chain tautomerism; in the case of the pentaphenyl system, only the triazine derivative 3a is observed. An X-ray analysis of 4f r eveals a non-planar structure with (+)-gauche-Z-(-)-gauche-Z-(+)-gauch e conformation, resulting from amide-like n/pi interactions between C= N-pi systems and the lone pair of the adjacent nitrogen atoms. Detaile d spectroscopic data for all open-chain and cyclic compounds are given . Quantum mechanical calculations (MP2/6-31G//6-31G*) of model compou nds 13 predict that non-planar isomers with gauche-configurated subuni ts are favored by more than 8 kcal/mol over a planar all-trans structu re as in polyenes. Most planar isomers correspond to low-lying transit ion states for the rotation around the C-N bonds, but not to local min ima, indicating high molecular flexibility of these oligonitrile chain s.