Envelope-chair conformation

The introduction of three trigonal atoms in a wing of the bicyclo[3.3.1] nonane skeleton leads to the planarity of the wing. Such molecules tend usually to adopt envelope-chair conformations exemplified by 16. In the case of six such trigonal atoms, the envelope-envelope form 17 would be expected to become the only stable one. 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Three new spirans, including 131 and 132, incorporating a 1,5-benzodithiepin system have been synthesised from methyl benzynes. The structure of 131 was confirmed by X-ray crystallography, and the seven-membered ring was shown to have a chair conformation and the five-membered ring an envelope conformation . 

The structures of the salts (131)168 and (132),159 a chelated ylide,160 and the insecticide coroxon161 have been determined. The structures of the cis- and trans-isomers of the cyclic phosphonamide (133) differed in that the cw-isomer has a half-chair conformation whereas the trans-isomer has an envelope conformation. Both compounds had hydrogen bonds of approximately 269 pm.162 The phosphadiazane (134)... 

All other homoatomic Pn cluster anions are cage molecules with localized two-center-two-electron bonds. The cyclic [PnJ anion has been characterized by NMR spectroscopy for Pn = P [210], and structures are known for Pn = P, As, Sb, Bi (Table 2). A planar [Ps] anion has only been detected in solution by P NMR spectroscopy [210]. Envelop-shaped five- and six-membered rings are found for [Pns] and [Pe]", whereas [Asg]" deviates only slightly from planarity and has a chair conformation. The aromaticity of the planar anions [P4] and [Pe]" is distinct from that of regular 671-aromatic hydrocarbons and has been described... 

Electron diffraction measurements of gaseous 1,2,4-trioxolane point to either a C2 symmetry, as depicted at the bottom of Appendix A, or a Cj symmetry, corresponding to an envelope conformation. However, consideration of steric interactions favors the twisted chair conformation (288c) for the molecule. Table 5 presents the structural parameters derived from electron diffraction, assuming that both C—O distances are equal and that the H atoms are symmetrically placed with respect of the neighboring atoms. Assumptions on the vibrational amplitudes of the ring bonds do not affect the calculated values of Table 5, except for the H—C—H angle . ... 

The optimum glucocorticoid structure shows a la, 2P-half-chair conformation for ring A, with ring D as a 13-envelope (C-13 is bent up) or a half-chair. Halogenation is most effective in positions 6, 7, 9, and 12. The compounds bind on their p face by hydrophobic binding forces. 

In the envelope conformation four of the carbon atoms are coplanar. The fifth carbon is out of the plane of the other four. There are three coplanar carbons in the half-chair conformation, with one carbon atom displaced above that plane and another below it. In both the envelope and the half-chair conformations, in-plane and out-of-plane carbons exchange positions rapidly. Equilibration between conformations of cyclopentane is very fast and occurs at rates similar to that of rotation about the carbon-carbon bond of ethane. 

In compounds where the ion acts as a bidentate ligand (that means forming a five-membered MS4 ring system), an envelope as well as a half-chair conformation is possible. In [Cp2M(S4)j (M = Mo, W) (complexes with tetrahedral coordination of the metal ion)128 and in [M(S4)2]2 (M = Ni (44), Pd) the half-chair conformation is found,129 whereas several complexes with a square-pyramidal coordination of Mo and W show the envelope conformation (for details see Section 16.1.7.4).130,131... 

Fig. 12. Envelope (left) and half-chair conformations of five-membered rings...

The substituents at C-3 and C-5 may also have stacking interactions in the crystal and thus packing forces can dictate the preferential conformation of the 1,2,4-trioxolane ring as the C-5 and peroxide envelope or even the half chair conformations are energetically close. For instance in the 6,7,8-trioxa-3-thiabicyclo[3.2.1]octane 21, centrosym-metrical pairs are formed by 71-71 stacking between two phenyl rings combined with two weak C-H- -n interactions <2000AXC1510>. 

See Section 1.5.4. The saturated six membered rings are normally in a chair conformation whilst the saturated five membered rings have an envelope conformation. 

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