N-Methyl stereochemistry in tropinone: the conformational flexibility of the tropane motif.
The intrinsic conformational and structural properties of the tropinone azabicycle have been investigated in a supersonic jet expansion using rotational spectroscopy. The spectrum revealed the presence of equatorial and axial conformers originated by the inversion of the N-methyl group, with the tropane motif adopting a distorted chair configuration. The determination of substitution and effective structures for the two conformers reveals the flexibility and structural changes associated with the N-methyl inversion, mostly a flattening at the nitrogen atom and a simultaneous rising of the carbonyl group in the axial form. Relative intensity measurements indicate that the conformational equilibrium is displaced towards the equatorial form, with a relative population in the jet of N(eq)/N(ax) approximately 2/1, which would correspond to a relative energy of ca. 2 kJ mol(-1) in pre-expansion conditions. Supporting ab initio calculations of the molecular properties and inversion barrier complemented the experimental work.
Cocinero
Emilio J
EJ
Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV-EHU), Apartado 644, E-48080 Bilbao, Spain.
Lesarri
Alberto
A
Ecija
Patricia
P
Grabow
Jens-Uwe
JU
Fernández
José A
JA
Castaño
Fernando
F
D008968:Molecular Conformation; D013055:Spectrophotometry, Infrared; D013237:Stereoisomerism; D014326:Tropanes / Q000737:chemistry*
D014326:Tropanes; C016808:3-tropinone