Stereochemical investigation of N-formyl-2,7-diphenyl-1,4-diazepan-5-ones by NMR spectroscopy and single crystal X-ray diffraction studies

Abstract A series of N -formyl-2,7-diphenyl-1,4-diazepan-5-ones 11 – 15 are synthesized in significant yields (86–92%) from their corresponding 2,7-diphenyl-1,4-diazepan-5-ones 6 – 10 using acetic-formic anhydride. Synthesized compounds 11 – 15 are characterised by their analytical and spectral (IR, Mass, 1D & 2D NMR) data. All the N– formyl compounds 11–15 show doubling of NMR signals due to the existence of syn and anti rotamers at RT which are unambiguously assigned by HETCOR spectra and ASIS method. The preferred conformation has been arrived by considering the chemical shifts, coupling constants, dihedral angles and destabilizing strain factors. The N -formyl-2,7-diphenyl-1,4-diazepan-5-ones 11 – 15 are best accommodated in a coplanar orientation of formyl function and flattened boat conformation of the ring where the phenyl groups are occupying pseudo-axial orientations in solution state. Further, dynamic NMR study is carried out for 14 from 28 °C to 116 °C and the calculated average energy barrier is 84.4 kJ mol −1 . The single crystal X-ray diffraction study on N -formyl-2,7-diphenyl-1,4-diazepan-5-one 13 shows that the molecule adopts distorted boat conformation with coplanar orientation of formyl group.