title: Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction.
abstract: This paper reports the synthesis of a series of piperidones 1-8 by the Mannich reaction and analysis of their structures and conformations 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, compounds 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 molecular 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 antibonding sigma orbital of C(7)-Heq (nN→σ*C-H(7)eq), a type of hyperconjugative interaction.
authors.affiliation: Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida Instituto Politécnico Nacional 2508 Colonia San Pedro Zacatenco, C.P. 07360, México, D.F., Mexico.
authors.forename: Cesar
authors.initials: C
authors.lastname: Garcias-Morales
authors.affiliation: Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida Instituto Politécnico Nacional 2508 Colonia San Pedro Zacatenco, C.P. 07360, México, D.F., Mexico.
authors.forename: David
authors.initials: D
authors.lastname: Ortegón-Reyna
authors.affiliation: Departamento de Química, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Avenida Instituto Politécnico Nacional 2508 Colonia San Pedro Zacatenco, C.P. 07360, México, D.F., Mexico.
authors.forename: Armando
authors.initials: A
authors.lastname: Ariza-Castolo
keywords: NMR spectroscopy; X-ray diffraction structure; piperidone; spin–spin coupling constant; stereoelectronic effect