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Polydonor‐Substituted 1‐Oxa‐ and 1‐Thia‐3,5‐diazahexatrienes: Synthesis, Structures, Ring−Chain Tautomerism and Theoretical Calculations
Author(s) -
Maier Oliver,
Fröhlich Roland,
Würthwein ErnstUlrich
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200101)2001:1<83::aid-ejoc83>3.0.co;2-z
Subject(s) - chemistry , tautomer , ring (chemistry) , alkoxy group , alkylation , carbenium ion , chain (unit) , stereochemistry , quantum chemical , computational chemistry , condensation , crystallography , ion , molecule , organic chemistry , catalysis , alkyl , physics , astronomy , thermodynamics
Previously unknown 1‐oxa‐2,4‐diazahexatrienes 1a,b and c , substituted with alkoxy groups, have been prepared by carboxylation of N ‐alkylideneisoureas 2 with chloroformates 5 . X‐ray analyses of compounds 1b and c show nonplanar, open‐chain structures. In contrast, thiocarboxylation of 2 with thiochloroformate 6 leads to the spiro compounds 7‐ring , as verified by spectroscopic data and X‐ray analysis. A tetradonor‐substituted 1‐oxa‐2,4‐diazahexatriene 1d was obtained from the reaction of the 1‐oxa‐3‐aza‐butadiene 10 with the triply donor‐substituted carbenium ion 9 . Attempts at further chain elongation of 1a by an alkylation/condensation sequence failed because of preferential N ‐alkylation (compound 11 ). Quantum chemical calculations (AM1, PM3, RHF/3−21G, B3LYP/6−31+G*) on model systems of 1 predict the predominance, in the gas phase, of twisted, nonplanar chain structures for the oxa systems, and cyclic structures for the corresponding thia system, as found experimentally in the condensed or solid phase. In the series of oxa systems, the barrier towards ring‐opening of the cyclic form of the tautomeric equilibrium decreases with increasing number of donor groups; the preference for the chain structures increases simultaneously.