This paper reports the synthesis of a series of piperidones 1–8 by the Mannich reaction and analysis of their structures and confor-mations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-ones, compounds 1 and 2, adopt a chair–boat conformation, while those in 2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9-ones, com-pounds 3–8, adopt a chair–chair conformation because of stereoelectronic effects. These stereoelectronic effects were analyzed by the 1 J C–H coupling constants, which were measured in the 13 C satellites of the 1 H NMR spectra obtained with the hetero-dqf pulse sequence. In the solid state, these stereoelectronic effects were investigated by measurement of X-ray diffraction data, the molec-ular geometry (torsional bond angles and bond distances), and inter-and intramolecular interactions, and by natural bond orbital analysis, which was performed using density functional theory at the ωB97XD/6311++G(d,p) level. We found that one of the main factors influencing the conformational stability of 3–8 is the interaction between the lone-pair electrons of nitrogen and the anti-bonding sigma orbital of C(7)–H eq (n N →σ* C–H(7)eq), a type of hyperconjugative interaction.
Garcias-Morales, C., Ortegón-Reyna, D., & Ariza-Castolo, A. (2015). Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction. Beilstein Journal of Organic Chemistry, 11, 1973–1984. https://doi.org/10.3762/bjoc.11.213