Chiral helicate

Interestingly, Reggelin et al. [147] prepared helical chiral polymers by helix-sense selective anionic polymerization of methacrylates, using an asymmetric base mixture as initiator (Scheme 61). 

These polymers, as helical chiral ligand lacking any other elements of chirality, were tested as chiral Hgands for the palladium-catalyzed allyhc substitution, and in the test reaction, enantiomeric excesses of up to 33% were obtained (higher than those reached with monomer 115 as unique Hgand). 

Lewis and Johnson compared the c.d. spectra of amylose and cyclomaltohexaose, and showed that amylose is helical in aqueous solution. Cyclomaltohexaose is chromophorically equivalent to amylose, and it is known to assume a pseudohelix having zero pitch, and thus, no helical chirality. The conformation of amylose is clearly different from that of cyclomaltohexaose, as their c.d. spectra are very different (see Fig. 9). The difference in conformation was considered to be a matter of helical chirality. To confirm this, these workers measured the c.d. spectrum of an amylose-1-butanol complex presumed to have the V-form of helical conformation with the 1-butanol complexed in the channel of the helix. The c.d. spectrum of the complex is identical to that of amylose in aqueous solution. 

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. 

Self-assembled hexanuclear arene ruthenium metallo-prisms with 2,4,6-tripyridyl-l,3,5-triazine (ppt) subunits showed unexpected double helical chirality <06CC4691>. 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

This phenomenology is, of course, consistent with optically active domains in a conglomerate. It should be pointed out that this kind of rotation of plane polarized light in a 4-. im path cannot derive from molecular chirality but should be due to some kind of macroscopic helical chirality. 

Helical Chirality. Helicity is a special case of chirality in which molecules are shaped as a right- or left-handed spiral like a screw or spiral stairs. The configurations are designed M and P, respectively, according to the helical direction. Viewed from the top of the axis, a clockwise helix is defined as P, whereas a counterclockwise orientation is defined as M. Thus, the configuration of example 9 is defined as M. 

PSS) helical chirality in the attached PMHS part, a phenomenon which the authors termed helical sense... 

Two precedent examples had been reported of the enantioselective [2+2+2] cycloaddition of alkynes. In one case, an enantioposition-selective intermolecular reaction of a triyne with acetylene generated an asymmetric carbon at the benzylic position of a formed benzene ring [19]. In the other case, an intramolecular reaction of a triyne induced helical chirality [20]. Both reactions were developed by chiral Ni catalysts. 

The bicapped tris-binaphthol (273), and its earlier-synthesized tris-biphenol analogue,are closely related to tris-catechol cages. The Fe + complex of (273) exists in A and A forms, derived from the two enantiomers—the six hydroxy groups impose helical chirality. 

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]. 

A cholesteric, or chiral nematic (N ) phase. This is a positionally disordered fluid in which the constituent molecules align on average their axes along a common direction called the nematic director. Being the DNA helices chiral, the orientational order develops an additional macro-helical superstructure with the twist axis perpendicular to the local director. The phase thus consists of local nematic layers continuously twisted with respect to each other, with periodicity p/2 (where p is the cholesteric pitch see Fig. 8a) [27,28]. For 150-bp helices, the N phase appears at a concentration around 150 mg/mL in 100 mM monovalent salt conditions. This LC phase is easily observed in polarized optical microscopy. Since the N pitch extends to tens of micrometers (that is, across... 

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 =... 

Because the overcrowded nature of the structure of the E form of fulgides forces them to adopt the helical configuration of the photoreactive 1,3,5-hexatriene moiety, it generates helical chirality (Scheme 5). The two enantiomeric helicities are abbre-... 

Kaftory performed X-ray crystallographic analyses of 20E, 20Z, and 20C, and found that crystals of 20E and 20Z were composed of single enantiomers of helical chirality, while crystals of 20C comprised a racemic mixture of enantiomers about the quaternary chiral carbon atom.1171 Irradiation of a crystal of 20E with UV light to induce photocoloration in the crystalline state resulted in the coloration only of the surface of the crystal. 

Distorted Square Planar Four-Coordinate The first example of helical chirality for a square planar complex with achiral ligands is cis-bis(2,6-diphenyl-... 

Annelation of nonracemic intermediate giving nonracemic [njhelicene Several methods for the transfer of center or axial chirality into helical chirality have been described. In many cases, practically complete transfer of enantiomeric excess was observed. 

Two approaches to nonracemic [5]helicenes, starting from axially chiral binaphthyls, provided an early stereochemical correlation between axial and helical chiralities. The correlation was based on comparison of the signs of optical rotations the absolute configurations of binaphthyls were obtained from chemical... 

Fig. 15.9 Syntheses of nonracemic [5]helicene 3 stereochemical correlation between axial and helical chiralities.

Benzoin acetate, methylben-zoin, Troger s base, tra/w-stilbene oxide and 1,1 -binaphthyl-2,2 -diol Aminopropyl silica gel (LiChrospher 1000, 5 pm) coated with helically chiral poly(diphenyl-2-pyridylmethyl methacrylate) (PDPM) Methanolic solution of ammonium acetate (2.5 mM, pH 4.5) 300 mm x 100 pm i.d. 200 mm effective length, chiral separation 163... 

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