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Experimental and Theoretical Characterization of the Valence Isomerization of Bi‐2 H ‐azirin‐2‐yls to Diazabenzenes
Author(s) -
Banert Klaus,
Grimme Stefan,
Herges Rainer,
Heß Kirsten,
Köhler Frank,
MückLichtenfeld Christian,
Würthwein ErnstUlrich
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600318
Subject(s) - isomerization , chemistry , homolysis , valence (chemistry) , bond cleavage , photochemistry , stereochemistry , quantum chemical , computational chemistry , crystallography , medicinal chemistry , radical , catalysis , molecule , organic chemistry
3,4‐Diazidocyclobutenes 16 were prepared from the corresponding dihalides. Some of these diazides, such as parent compound 16 d and phenyl‐substituted derivatives 16 c , f , underwent spontaneous stereoselective electrocyclic ring opening below room temperature, whereas the tetraalkyl derivatives of 16 had to be heated to force the same reaction. In most cases, the resulting 1,4‐diazidobuta‐1,3‐dienes 8 were isolated to study their photochemical transformation into bi‐2 H ‐azirin‐2‐yls 9 via intermediate mono‐azirines 17 . Except for starting materials with a low number of substituents such as 9 d and 9 f , title compounds 9 underwent a thermal valence isomerization which led exclusively to pyridazines 18 at surprisingly low temperatures. Based on quantum‐chemical calculations for the parent bi‐2 H ‐azirinyl‐2‐yl 9 d at the UB3LYP/6‐31+G(d) and MR‐MP2/TZV(2df,2p) levels, the valence isomerization process is best explained by simultaneous homolytic cleavage of both CN single bonds of 9 to generate energetically favorable N , N ′ diradicals 26 , which cyclize to 18 . The theoretical studies indicate also that one stereoisomer of 9 , namely, the rac compound, should undergo valence isomerization more easily than the other, which is in conformity with different rates of these rearrangement reactions found experimentally. For the tetramethyl‐bi‐2 H ‐azirin‐2‐yls 9 g , which are better models for the experimentally studied compounds, simultaneous homolytic cleavage of both CN single bonds is also predicted by the calculations, although the intermediate diradicals 26 g are significantly higher in energy than those of the parent system 9 d .