Equilibria of 4-Spiropiperidines

The use of dipole moments and low-temperature NMR measurements on 4-spiro- 1-tert-butylpiperidines and 5-spiro-l,3-dioxanes has been developed 

The dipole moments of the two conformers 146 and 147 of 1-rm-butyl-piperidine-4-spiro-4 (l, 3 -dioxolan) were calculated by vector addition of the moments of 1-fert-butylpiperidine and 4,4-dimethyl-1,3-dioxolan, making reasonable assumptions regarding geometries. This gave 1.15 and 1.93 D as the moments of the two conformers. The observed moment of 1.47 D gave AG° of 0.37 kcal mol-1 in favor of the axial oxygen conformer 146 in cyclohexane solution. AG%2- of 0.26 kcal mol-1 was estimated from low-temperature H-NMR studies.121 The differences between the piperidine and 

Dipole-moment measurements (in benzene) and low-temperature NMR studies (CDCI3-CFCI3 or CDC13-CC14 at -72 to -79°C) gave AG° values for other spiro systems. The AG° values shown in Table XIV are from the NMR measurements,199,200 Buttressing and inductive effects are probably responsible for variations in AG° values. The l-fert-butylpiperidine-4-spiro-2 -oxiran and -2 -thiiran adopt equilibria favoring the axial heteroatom con-formers (dipole-moment measurements) (Table XIV).201 

The preceding work shows clearly that the steric requirements of an oxygen atom are less than those of a CH2 group where the probe is a CH at the /J-axial position of a six-membered ring. 

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