Chiral net

Chiral building blocks are the preferred choice for building homochiral frameworks. However, achiral molecular components are also capable of generating chiral frameworks by adopting chiral net structures. Both approaches are discussed... 

Nets may be used to create multi component systems with exact ordering. Non-centrosymmetric nets may be used for noii-linear optical materials. Chiral nets may be used for enantiomeric separation and enantioselective synthesis and catalysis. 

Chiral nets may also be used to obtain conglomerates (or spontaneous resolution) from racemic mixtures. 

This chiral net is named after the B Os structure [12] and is closely related to the much more common ths net ((10,3)-b). Like this net, the bto net has zigzag strips although not in layers, every second strip is turned 60° making the logons twisted rather than chair-formed. Also note the difference this makes in comparison with ths when the nets are turned, see Figure 6.6. The bto net has vertex symbol 10-102-102, and genus 4. 

Intrinsic chiral nets and induced chiral nets... 

It might therefore be interesting to turn to some other chiral nets that are simple enough (low genus) but that are not self dual. Figure 12.8 shows some of them that we have already encountered. Perhaps they will make suitable synthetic targets ... 

Figure 12.8 Some chiral nets ihree-contiecled bto (IO,3)-c and eta (8,3)-a, four-connected qtz (quarts) and three- and four-connected dmf.

Finally, we should note that there are only a few studies on the effects of either type of chiral nets (intrinsic and induced) on chiral processes, thus for the moment we cannot say if either type has any distinctive advantage. 

Optical A chiral net has two optical isomers. However, if it is constructed from chiral molecular building blocks these net-isomers are not polymorphs unless they rapidly interconvert in solution (and even then it is debatable), ... 

Optically Inactive Chiral Compounds. Although chirality is a necessary prerequisite for optical activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers, no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic and is designated as ( ) and not as dl. Racemic mixtures usually have melting points higher than the melting point of either pure enantiomer. 

Stereoselective All lations. Ben2ene is stereoselectively alkylated with chiral 4-valerolactone in the presence of aluminum chloride with 50% net inversion of configuration (32). The stereoselectivity is explained by the coordination of the Lewis acid with the carbonyl oxygen of the lactone, resulting in the typ displacement at the C—O bond. Partial racemi2ation of the substrate (incomplete inversion of configuration) results by internal... 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

The triazole 76, which is more accurately portrayed as the nucleophilic carbene structure 76a, acts as a formyl anion equivalent by reaction with alkyl halides and subsequent reductive cleavage to give aldehydes as shown (75TL1889). The benzoin reaction may be considered as resulting in the net addition of a benzoyl anion to a benzaldehyde, and the chiral triazolium salt 77 has been reported to be an efficient asymmetric catalyst for this, giving the products (/ )-ArCH(OH)COAr, in up to 86% e.e. (96HCA1217). In the closely related intramolecular Stetter reaction e.e.s of up to 74% were obtained (96HCA1899). 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

A combination of the Lewis acid zinc triflate and the bases NEt, or pyridine acted as an achiral catalyst for this reaction. Instead, using a chiral base which incorporates a bipy ligand to bind zinc gave 26% ee of the product (Scheme 5-42a). Alternatively, diethylzinc was an active precatalyst, but attempts to use chiral amino alcohols as ligands in this system gave low ees (Scheme 5-42b) [31]. 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

A less obvious explanation is that the observed residual structure is not due to attractive interactions, but rather to repulsive ones. The steric repulsion between atoms forced to partially overlap is a dominant, if not the dominant, force in all of chemistry. These highly local interactions are known to be important in polymer conformations (Flory, 1969 Ramachandran and Sasisekharan, 1968). For homopolymers or simple alternating polymers, they can often be safely neglected by assuming they confer no net directionality to the chain. Polypeptide chains, however, are chiral and support specific sequences of 20 differently shaped... 

Figure 5.14 (a) A beam of plane-polarized light encounters a molecule of (/ )-2-butanol (a chiral molecule) in a particular orientation. This encounter produces a slight rotation of the plane of polarization, (b) exact cancellation of this rotation requires that a second molecule be oriented as an exact mirror image. This cancellation does not occur because the only molecule that could ever be oriented as an exact mirror image at the first encounter is a molecule of ( S)-2-butanol, which is not present. As a result, a net rotation of the plane of polarization occurs. 

Certain S- and e-oxygenated allylic stannanes have been found to transmetallate with SnCU to give chiral pentacoordinated chloro stannane intermediates which add stereos-electively to aldehydes (Scheme 31)74. These reactions proceed with net 1,5-and 1,6-asymmetric induction. 

Moss and coworkers provided an early example of the way in which micellization can control the stereochemical course of a reaction. Deamination of chiral primary aliphatic amines in water proceeds with net inversion and extensive racemization, and the extent of racemization depends upon the lifetime of the carbocation-like intermediate. The situation changes dramatically if the salts of the primary amine can self-micellize, because now the nucleophile, typically water, is directed in from the front-side so that there is extensive retention of configuration (Moss et al., 1973). 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

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