Benzyl group conformation in some 4-benzyl-4-hydroxypiperidines
Manimekalai A. 1, Maruthavanan T. 2, Selvaraju K. 1, Ibon Alkorta3
1Department of Chemistry, Annamalai University, Annamalainagar-608 002, India
2Department of Chemistry, SONASTARCH, Sona College of Technology, Salem-636 005, India
3Instituto de Quimica Medica, CSIC, Juan de la Clerva, 3, E-28006 Madrid, Spain
Ключевые слова: 4-Benzyl-4-hydroxypiperidines, 1H NMR, 13C NMR, configuration, conformation
Страницы: 1104-1112
Подраздел: ИССЛЕДОВАНИЕ СТРОЕНИЯ МОЛЕКУЛ ФИЗИЧЕСКИМИ МЕТОДАМИ
Аннотация
The high resolution 1H and 13C NMR spectra of eight 4-benzyl-4-hydroxypiperidines 1—8 have been recorded in CDCl3 and analyzed. In 2, the conformation of the equatorial benzyl group at C(4) was established as an equilibrium mixture of A [phenyl group is gauche with respect to OH and C(5)] and B [phenyl group is gauche with respect to OH and C(3)], whereas in 3-alkyl-4-benzyl-4-hydroxypiperidines 3—8 the favored conformation of the benzyl group at C(4) is A. In 1, the axial benzyl group at C(4) adopts the gauche conformations A? [phenyl group is gauche with respect to OH and C(3)] and B? [phenyl group is gauche with respect to OH and C(5)] in which the phenyl ring of the benzyl group is gauche with respect to OH group. Hybrid HF/DFT B3LYP/6-3G* calculations of model systems 1? —3? also support these conformations. 13C data reveal that the equatorial methyl group at C(3) exerts a shielding influence on the methyl-bearing carbon and the magnitude of a -effect was found to be approximately –1.5 ppm. The 13C substituent parameters for the benzyl group reveal that the a -effect of the equatorial benzyl group is considerably higher in 3-ethyl tertiary alcohol 7 than in 3-methyl tertiary alcohol 3 and 4-benzyl-t(4)-hydroxypiperidine 2. This may be explained taking into account different conformations of ethyl group in t(4)-hydroxy-3-ethyl-2,6-diphenylpiperidine 12 and 3-ethyl tertiary alcohol 7.
|
