Barrier puckering

Cyclobutane adopts a puckered conformation in which substituents then occupy axial-like or equatorial-like positions. 1,3-Disubstituted cyclobutanes show small energy preferences for the cis isomer since this places both substituents in equatorial-like positions. The energy differences and the barrier to inversion are both smaller than in cyclohexane. 

It is well-documented that the thietane ring is puckered and an energy barrier exists to planarity. Hence two conformations must be considered for each isomer of the cis- and... 

Dioxathiolane 2,2-dioxide (21) adopts a puckered conformation in the solid state, as shown by x-ray diffraction (Section 4.15.3.1). In solution, the H NMR spectrum of (21) indicates that the compound undergoes a rapid pseudorotation between twist-envelope forms (Section 4.15.3.3.1) the O NMR spectra of simple derivatives also indicate a rapid conformational equilibrium by pscudorotation, although substitution may act as a barrier to complete ring inversion (Section 4.15.3.3.3). The solution-phase dipole moment of 1,3,2-dioxathiolane 2,2-dioxide is consistent with a nonplanar conformation (Section 4.15.3.8). 

Azetidine, a liquid, has been known since 1899 it smells of ammonia and is a strong base. For azetidine and its derivatives the ring is puckered to reduce eclipsing interactions, the extent depending upon the substituents. The barrier to inversion is low, which helps equilibration between the two conformers (Scheme 8.4). 

Structure a Bond lengths (A) b c d da Bond angles (°) ab be cd Pucker angle o Ring inversion barrier (kJ mol-1) Section number/ref. 

Substitution of alkyl and aryl groups on the oxetane ring, on the other hand, increases the barrier of the puckering vibration. 2-Methyloxetane and 3-methyloxetane are non-planar, existing as mixtures of stable equatorial and axial conformers at room temperature (79JST(56)157>. 

Table 5 Potential Barrier for Ring Puckering Vibration in Oxetanes...

Table 5 lists values of the potential barrier for the ring puckering vibration in various oxetanes (see also Section 5.13.2.3.2). 

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