Shape substituents

Terminal Crown Ethers with One Taper-Shaped Substituent. 144... 

In this section, crown ethers will be discussed which are equipped with one taper-shaped substituent (Fig. 10). Columnar hexagonal mesophases are the most commonly observed phases in this kind of molecule. 

Fig. 10 General design of liquid crystalline crown ethers with a taper-shaped substituent...

Terminal crown ethers with taper-shaped substituents often possess columnar phases. Sometimes, the columnar order is only observed after the complexation of specific salts. The uptake of salts often results in the crown ethers assembling side by side forming supramolecular cylinders with the crown moieties on the inside and the alkyl chains on the outside leading to possible ion-conducting arrangements within an insulating jacket. Indeed, it was shown that matrix-fixed tubular arrangements are ion conductors. 

Fig. 14 Schematic representation of crown ethers with several taper- or disc-shaped substituents...

Crown Ethers with Several Taper- or Disc-Shaped Substituents... 

We should not assume that all alkyl substituents prefer the equatorial conformation. Goren and Biali reported that the preferred conformation of all -frans-l,2,3,4,5,6-hexaisopropylcyclohexane has all six isopropyl groups axial rather than equatorial. They concluded that severe torsional and steric interactions between the equatorial isopropyl groups are responsible for the surprising conformational preference. Moreover, Biali determined that, with an appropriately shaped substituent, it might be possible for a monoalkylcy-clohexane to be more stable with the substituent in the axial rather than in the equatorial position. ... 

Ring-shaped substituents are produced when bifunctional reagents react with neighboring monofunctional groups on polymers. An example... 

Discotic Phases. Molecules which are disk-shaped rather than elongated also form thermotropic Hquid crystal phases. Usually these molecules have aromatic cores and six lateral substituents, although the predominance of six lateral substituents is solely historical molecules with four lateral substituents also can form Hquid crystal phases. Although the flatness of these molecules creates a steric effect promoting alignment of the normal to the disks, the fact that disordered side chains are also necessary for the formation of these phases (as is often the case for Hquid crystallinity in elongated molecules) should not be ignored. 

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

Besides the shapes adopted by the rings, considerable attention has been paid to the conformational preferences of substituents, both on carbon and on the heteroatoms (nitrogen and sulfur). The reader is referred to the relevant monograph chapters for details. Noteworthy points are as follows ... 

The influence of nuclear substituents on the properties of a homopolymer depends on the nature, size and shape of the substituent, the number of the substituents and the position of entry into the benzene ring. 

Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

There are a number of important kinds of stereogenic centers besides asymmetric carbon atoms. One example is furnished by sulfoxides with nonidentical substituents on sulfur. Sulfoxides are pyramidal and maintain dieir configuration at room temperature. Unsymmetrical sulfoxides are therefore chiral and exist as enantiomers. Sulfonium salts with three nonidentical ligands are also chiral as a result of their pyramidal shape. Some examples of chiral derivatives of sulfur are given in Scheme 2.1. 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

The same phenomenon was observed for oxidation of la and 2a2 in the presence of 7. When a series of branched alkyl groups are used as R4, the selectivity shows a minimum at the isopentyl group (Fig. 4)21). The r value of 1.4 for R4 = n-C5Hu differs largely from that for R4 = i-C5Hn (r = 0.30)21 . These results demonstrate that the more closely the substituent R4 of 7 resembles the isopentyl group of 2 in geometrical shape, the more remarkably the selectivity is lowered. 

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

In practice simple alkenes give poor cycloaddition so diene (49) was the chosen starting material and the aldol was protected and hydrogenated to give (48). Note that (50) is emphatically bowl shaped (Chapter T 34) so that substituents (OR) or reagents (Hg) prefer to be on the outside of the bowl. The regioselectivity of the addition of Cl2C=C=0 to the diene (49) is correct. Synthesis ... 

The shape-persistent, structurally well-defined nature of PAMs and PDMs make them attractive models for binding guest molecules within their cavities. In 1995, Hoger and Enkelmann reported the construction of the first meta/para-PAM/PDM hybrid designed to possess hydrophobic and hydrophilic substituents for subsequent use in host guest chemistry [71]. Macrocyclic amphiphile 116 was assembled via the straightforward manner depicted in Scheme 26. 

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