Premium
Darstellung und Struktur von 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrienen. Die 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrien/2 H ‐1,3,5‐Oxadiazin‐Tautomerie
Author(s) -
Buhmann Martin,
Würthwein ErnstUlrich,
Möller Manfred H.
Publication year - 1993
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19931260417
Subject(s) - tautomer , chemistry , conformational isomerism , amide , ring (chemistry) , ab initio , stereochemistry , enthalpy , ab initio quantum chemistry methods , computational chemistry , resonance (particle physics) , double bond , crystallography , molecule , polymer chemistry , organic chemistry , physics , particle physics , quantum mechanics
Synthesis and Structures of 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatrienes. The 1‐Oxa‐3,5‐diaza‐1,3,5‐hexatriene/2 H ‐1,3,5‐Oxadiazine Tautomerism Reaction of N 1‐lithio‐or N 1‐trimethylsilyl‐substituted imines 1 with N 1‐acylimidic acid derivatives 2 provides 1‐oxa‐3,5‐diaza‐1,3,5‐hexatrienes 3 in fair to good yields. Depending on the substitution pattern, the acyclic compounds 3 form an equilibrium with their cyclic tautomers, e.g. 2 H ‐1,3,5‐oxadiazines 4 . Aromatic substituents at C‐6 favor the acyclic form 3 , whereas aliphatic groups (e.g. tert 1‐butyl) lead to the exclusive formation of the ring tautomer 4 . For a typical example ( 3d/4d ) the free enthalpy of activation for the ring/chain tautomerism was determined to be ca. 12.5 kcal/mol. Detailed spectroscopic data ( 1 H‐, 13 C‐NMR, IR, UV) for both tautomers are given. The X‐Ray analysis of the acyclic compound 3a shows a central CN bond with Z configuration and with s‐gauche conformations of the adjacent CO and CN systems. According to quantum mechanical ab initio calculations (MP2/6‐31G*/6‐31G*) for the C 3 H 4 N 2 O model compounds, the ring tautomer 6 is by ca. 6 kcal/mol lower in energy than the acyclic forms 7 . Within the series of open‐chain tautomers 7 , gauche forms are lower in energy than s‐trans conformers, indicating that amide resonance is more important for these systems than polyene delocalization, s‐cis Forms were found to be transition states. The acyclic conformers 7 are rather flexible; barriers of rotation for both C – N bonds are calculated to 4 – 5 kcal/mol only (RHF/ 3‐21G/3‐21G).