Equatorial puckering

Although Dsh symmetry requires the equatorial plane in the average to be planar, the low frequency of Vi 1 combined with the large Fax. -Feq an )litude of vibration m s the effective equatorial puckering comparable to that found for the lower symmetries (see text). The actual displacements of the five equatorial fluorines from the ideal plane in a clockwise sense were (deg) +0.35, -3.15, +6.65, -6.65, +3.25 with the axial fluorines being bent toward the equatorial fluorine with the smallest (0.35) equatorial displacement... 

Nature of the Equatorial Puckering in IOF5 . As mentioned above, the... 

N(CH3)4+I0F6 do not appear to be noticeably affected by the slight equatorial puckering, they were assigned (see Table V) in point group Csv which is the lowest energy structure of the free anion (see below). 

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. 

Cyclohexane is strain-free because it adopts a puckered chair conformation, in which all bond angles are near 109° and all neighboring C—H bonds are staggered. Chair cyclohexane has two kinds of positions axial and equatorial. Axial positions are oriented up and down, parallel to the ring axis, whereas equatorial positions lie in a belt around the equator of the ring. Each carbon atom has one axial and one equatorial position. 

The conformation of thietane-1-oxide (185), determined from its H NMR spectrum taken in a nematic solvent, was found to have the oxygen equatorial and to be strongly puckered with an angle of puckering of about 38°, in agreement with the results of a microwave study170. 

The proton spectra analysis of thietane, thietane oxide and thietane dioxide at 100 and 300 MHz in the temperature range — 140 to 190 °C confirmed the puckered structure for the oxide (5a) with the sulfinyl oxygen in the equatorial orientation, as inferred from chemical-shift considerations180. It appears that the repulsive-type 1,3-interactions between the oxygen and the 3-substituents184 are operating between oxygen and the axial proton on C-3 in the unsubstituted thietane oxide (5a). For the thietane dioxide (5b ... 

Interestingly, the crystal structures of 3-substituted thietane and thietane dioxides180 showed that in the solid state they exist in the puckered structure, with the S—O bond equatorial in the oxides and the 3-substituent axial for the trans-isomers, contrary to what has been quoted before66,195. Thus, the claim that conformer 5c is predominant in the solutions of the trans-isomer needs to be re-examined. 

It might well be that, compared with other thietane oxide systems, the larger pucker angle here is due to the two bulky 2,4-phenyl substituents that tend toward equatorial conformation. 

The gas-phase structure of 1,3-dithietane 1-oxide (189) has been determined from its microwave spectrum and the spectra of eight isotopic modifications192. The ring is puckered, the angle between the two CSC planes being 39.3° with the oxygen equatorial. 

Following a detailed NMR study of the 3-substituted thietane dioxides 188 it was concluded that the three-bond coupling constants can be safely used for stereochemical assignments in this series in particular the 7rih4<4Hz (Table 7, R =H, X = C) is consistent with an equatorial-equatorial interaction. This indicates an axial preference for the 3-substituent R (i.e. 188b) in both liquid and solid phases, and also suggests that the ring is puckered . 

In this scheme, a highly puckered metallocycle was envisioned, possessing pseudoaxial and equatorial substituents, and reaction pathways were said to be favored which minimized the following effects (a) 1,3-diaxial interactions of substituents on the two a-carbons (b) axial substituent interactions with juxtaposed ring carbons and (c) 1,2-diequatorial interactions. This scheme predicts relatively nonstereospecific metathesis of rra/j.v-olefins but highly stereospecific metathesis of ra-olefins. For example, the following pathways for reactions of m-olefins were proposed ... 

Figure 3.82 The puckered (D2d) equilibrium geometry of C4H8, showing canting of methylene units to give alternating axial and equatorial C—H bonds.

Why does C4H8 adopt puckered D2d geometry As shown in Fig. 3.82, the skeletal carbon atoms twist out of planarity (with dihedral cccc = 17.9°), allowing each methylenic hydrogen to be distinguished as axial (with cccH(a) = 94.9°) or equatorial (with cccH(e) = 138.7°). The puckered equilibrium structure lies only 0.8 kcal mol-1 below the transition-state D4h structure. 

The metallacyclobutane ring is not flat, but puckered, with carbon-2 above the plane formed by the metal and the other two carbon atoms. This leads to equatorial and axial positions for the substituents. In the ring 1,2 and 1,3... 

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