CONFOR

FIGURE 3 14 (a) A ball and spoke model and (b) a space filling model of the boat confor mation of cyclohexane Torsional strain from eclipsed bonds and van der Waals strain involving the flagpole hydrogens (red) make the boat less stable than the chair... 

Draw or construct a molecular model of the most stable confor... 

Examine the models of 1 3 butadiene in Figure 10 6 on Learn mg By Modeling and com pare space filling models of the s CIS and s trans confor mation... 

FIGURE 10 6 Confor mations and electron delo calization in 1 3 butadiene The s CIS and the s trans con formations permit the 2p or bitalsto be aligned parallel to one another for maxi mum TT electron delocaliza tion The s trans conformation is more stable than the s CIS Stabilization resulting from tt electron de localization is least in the perpendicular conformation which IS a transition state for rotation about the C 2—C 3 single bond The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases... 

The various conformations of cyclohexane are in rapid equilibrium with one another, but at any moment almost all of the molecules exist in the chair confor mation. Not more than one or two molecules per thousand are present in the skew boat conformation. Thus, the discussion of cyclohexane confor-mational analysis that follows focuses exclusively on the chair confor mation. 

The confor mational features of six-membered rings are fundamental to organic chemistry, so it is essential that you have a clear- understanding of the directional properties of axial and equatorial bonds and be able to represent them accurately. Figure 3.17 offers some guidance on the drawing of chair cyclohexane rings. 

A potential energy diagran for ring inversion in cyclohexane is shown in Figure 3.18. In the first step the chair confor-rnation is converted to a skew boat, which then proceeds to the inverted chair in the second step. The skew boat confor-rnation is an intermediate in the process of ring inversion. Unlike a transition state, an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile. 

Other substituted cyclohexanes are similar- to methylcyclohexane. Two chair confonnations exist in rapid equilibrium, and the one in which the substituent is equatorial is more stable. The relative fflnounts of the two confor-rnations depend on the effective size of the substituent. The size of a substituent, in the context of cyclohexane confor-rnations, is related to the degree of branching at the atom connected to the ring. A single... 

Wedge-and-dash drawings fail to show confonnation, and it s important to remember that the rings of cis- and fran5-l,2-dimethylcyclohexane exist in a chair confor mation. This... 

Beginning with cycloheptane, which has four confomnations of similar energy, conformational analysis of cycloalkanes becomes more complicated. The sane fundamental principles apply to medium and large rings as apply to smaller ones—but there ar e more atoms and more bonds to consider and more confor-mational possibilities. 

Cyclopentane has two nonplanar- confor-mations that are of similar- stability the envelope and the half-chair. 

Section 3.7 Three confor-mations of cyclohexane have approximately tetrahedral angles at carbon the chair, the boat, and the skew boat. The chair- is by far- the most stable it is free of torsional strain, but the boat and skew boat are not. When a cyclohexane ring is present in a compound, it almost always adopts a chair- confor-mation. 

All the ring substituents in p-D-glucopyranose are equatorial in the most stable chair-confor-rnation. Only the anorneric hydroxyl group is axial in the a isomer all the other-substituents are equatorial. 

Proline is relatively compact because of the cyclic nature of its side chain. It has less confor-mational flexibility than the other anino acids, and the presence of proline affects the shape of a peptide more than other amino acids. 

The primary structure of a peptide is its anino acid sequence. We also speak of the secondary structure of a peptide, that is, the confonnational relationship of nearest neighbor fflnino acids with respect to each other. On the basis of X-ray crystallographic studies and careful examination of molecular- models, Linus Pauling and Robert B. Corey of the California Institute of Technology showed that certain peptide confor-rnations were more stable than others. Two anangernents, the a. helix and the P sheet, stand out as... 

Hydrogen bonding can occur when the N—H group of one amino acid unit and the C=0 group of another are close in space confor-mations that maximize the number of these hydrogen bonds are stabilized by them. 

These remarkable observations stimulated an investigation to understand the origin of the directing effect. To clarify the contribution of the axially- and equato-rially-oriented oxygen atoms in the ketal, a survey of the reaction of three confor-mationally biased t-butyl cyclohexenone ketals 78, 81 and 84 was undertaken (Scheme 3.26) [56]. In each case, careful conformational analysis provides critical clues to rationalizing selectivity. 

The influence of the solvent on the oxidation of film under conformational relaxation control is illustrated in Fig. 47, which shows chronoamperograms obtained by steps from -2000 to 300 mV vs. SCE at room temperature (25°C) over 50 s in 0.1 M LiC104 solutions of different solvents acetonitrile, acetone, propylene carbonate, (PC), dimethyl sulfoxide (DMSO), and sulfolane. Films were reduced over 120 s in the corresponding background solution. Despite the large differences observed in the relative shape of the curves obtained in different solvents, shifts in the times for the current maxima (/max) are not important. This fact points to a low influence of the solvent on the rate at which confor-... 

Fig. 2.36 The y-peptide 2.614-helical fold. (A) Stereo-view along the helix axis of the (P)-2.6i4-helical structure adopted by y -hexapep-tide 141 in pyridine. This low energy confor-mer was obtained by simulated annealing calculations under NMR restraints. Side-chains have been partially omitted for clarity.

Significant advance in the field of asymmetric catalysis was also achieved with the preparation of l,2-bis(phospholano)benzene (DuPHOS 4) and its confor-mationally flexible derivative (l,2-bis(phospholano)ethane, known as BPE) by Burk et al. [59]. Two main distinctive features embodied by these Hgands, as compared to other known chiral diphosphine ligands, are the electron-rich character of the phosphorus atoms on the one hand and the pseudo-chirality at phosphorus atoms, on the other. These properties are responsible for both the high activity of the corresponding metal complex and an enantioselection indepen-... 

Finally, it is worth while to consider the applicability of these relaxation methods to molecules having flexible conformations. Examples given in the previous Section demonstrated that relaxation rates are able to define either the most probable conformation of a flexible molecular segment, or to specify a range of allowed conformations from the total number of confor-... 

With less hindered hydride donors, particularly NaBH4 and LiAlH4, confor-mationally biased cyclohexanones give predominantly the equatorial alcohol, which is normally the more stable of the two isomers. However, hydride reductions are exothermic reactions with low activation energies. The TS should resemble starting ketone, so product stability should not control the stereoselectivity. A major factor in the preference for the equatorial isomer is the torsional strain that develops in the formation of the axial alcohol.117... 

The bond distortions that are required to transform the bound ground state substrate to its transition state structure and then to the product state structure(s) are commonly facilitated by introducing strain into the substrate molecule, through confor-... 

A crystal structure of the cation (248) shows the preferred S,S-confor-mation (26), the methyl groups being equatorially disposed to give the... 

Polymer Molecular weight3 Charge Charge Density13 Chain Confor- mation Backbone Functional Group Attachment Hydrogen Bonding... 

---