3187-74-4Relevant academic research and scientific papers
Diastereoselective Thermal or Acid-catalyzed Rearrangement of N,N'-Dibenzylidene-1,2-cyclopropanediamines to cis-2,3-Diaryl-2,3-dihydro-1H-1,4-diazepines
Quast, Helmut,Seidenspinner, Hubert-Matthias,Stawitz, Josef
, p. 1207 - 1229 (2007/10/02)
The trans-N-N'-dibenzylidene-1,2-cyclopropanediamines t1a-g and t6a, b are obtained in the reaction of the trans-1,2-cyclopropanediamines t4 and t5, respectively, with aromatic aldehydes.In contrast, only the cis-N,N'-disalicylidene-1,2-cyclopropanediamines c1b and c6b can be isolated when the aromatic aldehydes are allowed to react with the cis-1,2-cyclopropanediamines c4 and c5, respectively.In the other cases, the intermediate cis-bisimines isomerize in situ to yield the cis-2,3-diaryl-2,3-dihydro-1H-1,4-diazepines c3a-d and 7a, respectively.In bromobenzene solution, the isolated bisimines t1a, c-e, g and c6b, t6a rearrange above 90 deg C to give the corresponding dihydrodiazepines c3a, c-e, g and 7a,b, in the neat, molten state above 120 deg C.Surprisingly, in acetate-buffered methanol solution the dihydrodiazepines c3a,d, are also formed in the reaction of trans-1,2-cyclopropanediammonium bromide (t4*2HBr) with aldehydes at temperatures as low as 20 deg C.The diastereoselective step of these isomerizations, i. e. the Cope rearrangement of the cis-bisimines c1 and c6, is fast even at 20 deg C.The cis-bisimines c1 and c6 arise via a thermal or acid-catalyzed trans -> cis diastereomerization when the sequence starts from trans-1,2-cyclopropanediamine derivatives.All 2,3-diaryl-2,3-dihydrodiazepines which were obtained through rearrangement possess exclusively the cis configuration as shown by comparison with the authentic pair of diastereomers c3a, t3a and by 1H NMR spectroscopy.The 1H NMR spectra of the cis-2,3-dihydrodiazepinium cations c3* H(1+) in trifluoroacetic acid indicate slow ring inversion up to 20-25 deg C.The free enthalpies of activation for the diasteretopomerization of 2-H and 3-H of the cis--2,3-dihydrodiazepinium catiions in trifluoroacetic acid range from ΔG313 ca. 60 (for c3b) to ΔG361= 75.5 kJ/mol (for c3g).Besides slow ring inversion, the free base c3g as well as the cation c3g*H+ exhibit slow rotation of the 2,4,6-trimethylphenyl groups at C2 and C3.
