Inherent chirality

The first silver clusters made using DNA as template were reported by Dickson et al. in 2004 [32], The paper describes the time-dependent formation of silver clusters in a 12-base (5 -AGGTCGCCGCCC-3 ). The clusters have intense absorption in the region 400-550 nm (Fig. 2b) and emission at around 630 nm. The latter band could be decomposed as the emission bands of two distinct excitations at 540 and 580 nm, indicating the existence of two different emitters. As the clusters do not have inherent chirality, the induced circular dichroism associated with the silver cluster electronic transitions is evidence that the clusters are bound to DNA (Fig. 2c). 

This system allows also for the indication of the absolute configuration of inherently chiral fullerenes by introducing the stereodescriptors f C) and f A) ( P = fullerene s = systematic numbering C = clockwise A = anti-clockwise). 

Adducts with an Inherently Chiral Addition Pattern... 

Figure 10.9 Schematic representation of some examples of inherently chiral bis- and tris-addition patterns of Qq adducts.

The relative configuration within the precursor trisadducts is retained in the inherently chiral C -symmetrical hexakisadducts, while the absolute configuration has changed. The Cotton effects of such hexakisadducts are much less pronounced than those of their precursor tris-adducts, since the local symmetry of the fullerene core is very close to Tjj. 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

Figure 13.5 Bisadducts of Cy obtained from twofold cyclopropanation reactions. Both addends with chiral and achiral substituents have been used. Adducts 2 and 3 exhibit an inherently chiral addition pattern and are formed together with the corresponding enantiomer or diastereomer involving the mirror image addition pattern.

A helical unit is inherently chiral. Its chirality sense or helicity can assume two values that correspond to the sign of the torsional angle. Although the definition of the sign is arbitrary (sec Section 1.1.1.). a universal scientific convention2 is followed (Klyne Prelog convention) ... 

Allenes are versatile synthetic precursors in organic chemistry and the synthesis of many natural products involves the use of allenic compounds [27]. However, they have received much less attention than alkenes or alkynes for transition metal catalyzed reactions. The explanation lies in the problems of selectivity that these substrates display as a result of their reactivity and their inherent chirality [28]. 

It is important to note that not all space groups that can accommodate chiral molecules are necessarily chiral. For example, it is clearly possible to place 2 nonchiral molecules in a monoclinic unit cell in, say P2, and have a nonchiral crystal. On the other hand there are 11 enantiomorphous pairs of space groups that must give chiral crystals because they are inherently chiral, regardless of what is in them. These are the following, which are all based on screw axes, and the pairs simply have axes of the same type spiraling in opposite directions ... 

The disulfide group is inherently chiral, with the preferred torsion angle about the S-S bond being -90°. Both right- and left-handed disulfide bridges occur in proteins.305 306... 

Another type of synthetic polymer-based chiral stationary phase is formed when chiral catalyst are used to initiate the polymerization. In the case of poly(methyl methacrylate) polymers, introduced by Okamoto, the chirality of the polymer arises from the helicity of the polymer and not from any inherent chirality of the individual monomeric subunits (109). Columns of this type (eg, Chiralpak OT) are available from Chiral Technologies, Inc., or J. T. Baker Inc. 

Attempts to induce asymmetry in the cyclisation of 57 using the chiral lithium amides, which had worked well with simple benzamides, were frustrated by the inherent chirality, at low temperature, of the naphthamide itself. This feature is common to all 2-substituted tertiary aromatic amides, which may become atropisomeric at low temperature due to slow rotation about the Ar-CO bond.48 We therefore sought to... 

Finally, it should be noted that L is a rotational operator. Therefore, any type of transition that involves exciting an electron between orbitals that are transformed into one another by a rotation (e.g., the n —> n transition of an inherently chiral carbonyl)20,21 will be magnetic dipole allowed and have g > 0.01. 

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