Chair structure

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

One of the two chair structures of cis- l-chloro-3-mefhylcyclohexane is more stable than the other by 15.5 kj/mol (3.7 keal/mol). Which is it What is the energy cost of a 1,3-diaxial interaction between a chlorine and a methyl group ... 

Figure 4.8 Cubane and chair structures adopted by complexes (R3P)AgX.

Phosphonic acid analogues of aspartic acid, glutamicacid C-benzy1g1ycine,191 and the phosphor inane (70)1 92 have been studied, also the oxime (71)193 a phosphonopiperidinol - with a very distorted chair structure,194 the phosphonate (72),195 a pyrazole,196 a thiocarbamoy1phosphonate, 97 and two iminobis-(methylphosphonic acids).198 Additionally there have been studies of the new bicyclic sulphur heterocycle (73),199 the oxazaphospho1idine (74)200 and a triaminophosphetanium aluminium betaine.201... 

Per-O-acylated glycosyl iodides are stable at room temperature and can be purified on a silica gel column and stored at 0 °C. Stachulski and coworkers [202] synthesized methyl 2,3,4-tri-O-pivaloyl-glucopyranuroate iodide, which is a stable solid at 20 °C and can be stored for months at room temperature or for more than a year at 0 °C. The X-ray crystal structure of this compound, the first one of this class, shows a typical chair structure. Importantly, such a disarmed and stable iodide can be coupled with primary and secondary steroidal alcohols using I2 as a promoter, as demonstrated by the synthesis of morphine-6-glucuronide, an analgesic [202], The glycosyl donor ability... 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

Perhaps one of the most elegant object lessons demonstrating the importance of careful compositional analysis comes from the work by Beinert et al. (1983) on the 3Fe cluster. Even with structural analysis by numerous sophisticated spectroscopic methods and by X-ray diffraction, there was substantial controversy about whether the cluster was a modified cubane structure with four inorganic sulfurs and three ligands or a more open chair structure with three sulfurs and six ligands. The analysis of aconitase indicated four sulfurs and resolved the question in favor of the modified cubane. Further X-ray diffraction studies confirmed the cubane 3Fe 4S cluster for aconitase and ferredoxins from A. vinelandii and Desulfovibrio pgas (Stout et al., 1988 Stout 1988 Kissinger et al., 1989). 

For 1,4-dithiane 17, the chair structure is more stable than the corresponding boat structure by 10.3 kcal mol . The corresponding radical cation is calculated to be more stable in the boat form <2002JA8321>. 

Concerning 1,4-dioxane 16, its inversion has been studied by using ab initio molecular orbital theory at the HF/6-31G and BLYP/6-31G levels. The chair conformation is the lowest in energy, followed by the two twist-boats. The transition state connecting the chair and the twist-boats is a half-chair structure, in which four atoms in the ring are planar <1997PCA3382>. 

Correspondingly, the interconversion of the twist-boat intermediate into the other chair form can be viewed as rotation about the opposite ring bond. Overall, two independent bond rotations , pausing at the high-energy (but stable) twist-boat intermediate, effect conversion of one chair structure into another equivalent chair, and at the same time switch axial and equatorial hydrogens. 

The overall barrier to ring inversion is well established experimentally, and is believed to correspond to the energy difference between chair and half-chair structures. Less certain are the relative energies of chair and twist-boat conformers and the energy of the boat transition structure, although a small range of values for each of these quantities has been established experimentally. Comparison of the experimental data with the results of calculations is provided in Table 8-5. The... 

In six-membered chelate rings such as those formed by 1,3-propanediamine (tn), at least two conformational energy minima, which correspond to an achiral chair structure (23a) and the chiral skew- or twist-boat forms (25b) and (25c),166-168 occur and both have been observed in crystal structures.169 176 The possible conformational isomerism in tn complexes is therefore even more involved than in en complexes.177 In (OC-6-12)-[CoCl2(tn)2]+ (tratns-[CoCl2(tn)2]+), for example, there are two achiral bis(chair) forms (26a and 26b), two enantiomeric (chair, skew-boat) forms (26c and 26d), one achiral form (26e) and two enantiomeric bis (skew-boat) forms (26f and 26g). Several of these have been isolated in conformationally locked systems.178 180... 

Inspection of the two sets of chair structures reveals that in one compound the all-equatorial conformer is overwhelmingly favored. In the other compound both chair structures have comparable energies so both will be populated significantly. In the all-equatorial isomer, the carbon-chlorine bond dipole moments reinforce one another leading to a large molecular moment. In the other compound the chlorines are both equatorial part of the time but part of the time they are trans diaxial where the carbon-chlorine bond dipole moments tend to cancel one another. Thus the average dipole moment of these two conformations will be less than the first compound, which exists virtually completely in the all-equatorial conformer. 

Now, if in a ring one dihedral angle (e.g. 6-1-2-3) opens, others elsewhere (e.g. 3—4—5—6) must close. A well-known example of this effect concerns the cyclohexane molecule. In its chair structure, all dihedral angles are 60°, whereas in a boat structure, two of the dihedral angles are zero and the others are greater than 60°. 

Tetrahydrothiopyran adopts a chair structure similar to that of cyclohexane but it is slightly more puckered to accommodate the sulfur atom. The calculated and experimental (microwave MW, electron diffraction ED) bond lengths and bond angles are generally in good agreement (Table 21). 

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