Chiral compounds helical

Introduction of the tricarbonylchromium group into cyclophane chemistry has great potential for the preparation of planar- (or helical-)chiral compounds. Besides structure/chiroptics relationships, carrying out stereoselective reactions is of major importance. Recently, inclusion of ferrocenes and benzene-Cr(CO)3 into the cavity of a cyclodextrine was achieved and this makes the connection to supramolecular chemistry [183]. 

A monomeric selenolato complex of zinc (65) was synthesized with a chelating oxazoline ligand.587 The complex was characterized by X-ray crystal structure, 1H, 13C, and 77Se NMR. These studies demonstrate that the compound is helically chiral and solution studies appear to show retention of chirality in solution. The zinc complex is tetrahedral with a Zn—Se average bond length of 2.378(1) A. 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

The deepwater stalked crinoid Gymnocrinus richeri contained the gymnochromes C (659) and D (660) and isogymnochrome D (661). These compounds have a helical chirality and chiral atoms in the sidechains give rise to isomers [524]. 

The Cahn-Ingold-Prelog rules work for inorganic compounds too but coordination complexes often have coordination numbers greater then four and may exhibit helical chirality, for example, denoted A and A (or Pand Min the Cahn-Ingold-Prelog system). The formal condition for chirality is that the molecule should not have an improper axis of rotation (i.e. a rotation + reflection axis, 5n =... 

Fig. 14 Schematic illustration of helicity induction on poly(phenylacetylene)s upon com-plexation with chiral compounds...

Another interesting group of chiral compounds results when molecules are forced to adopt a helical geometry. Like the turn of a screw, the turn of the helix can be either right-handed or left-handed. One example is the compound known as hexahelicene, which has six aromatic rings fused together. The molecule is forced to adopt a helical... 

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

The synthesis of several high molecular-weight platinum polyyne polymers carrying chiral (R)-l,l -bi-2-naphthol bridge, P17, was described.48 These represent the first examples of metal acetylide polymer with an optically active backbone. They display much larger specific negative optical rotations than both the metal-free alkyne precursors and their corresponding dinuclear model compounds. These results support the fact that the main chain of P17 adopts a one-handed helical conformation and induces the helical chirality of the polymers. 

A helical chain such as Se o is easy to recognize, but it is not always such a facile task to identify a chiral compound by attempting to convince oneself that it is, or is not, non-superposable on its mirror image. Symmetry considerations come to our aid a chiral molecular species must lack an improper (S ) axis of symmetry. 

Therefore, the above reaction path provides a convenient way of preparing helical-(planar-) chiral compounds, which show interesting circulardichroisms [45, 153]. These results contribute to a better understanding of the relation between structure and chiroptical properties [153, 154],... 

When the nematic phase is composed of optically active materials (either a single component or a multicomponent mixture made up of chiral compounds or chiral compounds mixed with achiral materials), the phase itself becomes chiral and has reduced environmental space symmetry. The structure of the chiral nematic (or cholesteric) modification is one where the local molecular ordering is identical to that of the nematic phase, but in the direction normal to the director the molecules pack to form a helical macrostructure, see Fig. 5. As in the nematic phase the molecules have no long-range positional order, and no layering exists. The pitch of the helix can vary from about 0.1 x 10 m to almost infinity, and is dependent on optical purity and the degree of molecular... 

The aim of this chapter is to discuss the recent examples of chiral recognition in organometallic and coordination compounds. Most of the examples presented are related to octahedral tris(bidentate) compounds with helical chirality. The homo-chiral... 

A nonphotochromic chiral compound, chiral (Fig. 10.7), which was derived with (1 s, 2R, 5S)-( + )-menthol, was found to give a right-handed helix by addition in E44. Contrary to chiral, m-azo-8 gave a left-handed helical helix within E44. Therefore, when both chiral compounds are added in E44, the opposite helical senses of chiral and m-azo-8 each compensate the other s EITP, leading to a compensated nematic phase. 

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