Helices, chirality

Yamamoto et al. have reported a chiral helical titanium catalyst, 10, prepared from a binaphthol-derived chiral tetraol and titanium tetraisopropoxide with azeotropic removal of 2-propanol [16] (Scheme 1.22, 1.23, Table 1.9). This is one of the few catalysts which promote the Diels-Alder reaction of a-unsubstituted aldehydes such as acrolein with high enantioselectivity. Acrolein reacts not only with cyclo-pentadiene but also 1,3-cyclohexadiene and l-methoxy-l,3-cyclohexadiene to afford cycloadducts in 96, 81, and 98% ee, respectively. Another noteworthy feature of the titanium catalyst 10 is that the enantioselectivity is not greatly influenced by reaction temperature (96% ee at... 

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

Likely, this idea will continue to be explored because new chiral matrices are created frequently. Examples of published possible stable chiral matrices are chiral microporous cross-linked polymers5 and nanoscale chiral helical columns created from various inorganic materials.6 However, the interface problem will still exist. 

FIGURE 3.6 Chiral helical hexagonal structure of urea and thiourea adducts. A chiral guest generates one helical form. (Straight chain alkyl guests fit in the urea adduct channel larger molecules such as trans-decalin fit in thiourea adduct channel.)... 

Table 3 Data relative to selected crystalline polymers for which only chiral crystalline phases, characterized by chiral helical conformations, are known... 

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. 

Macroscopical helical structures formed by cholesteric liquid crystals have been used as chiral helical media for the asymmetric synthesis of helicenes. 

A new type of chiral helical titanium reagent has been designed based on Ti(OPr )4 and a chiral ligand derived from enantiopure binaphthol. These Ti reagents have been successfully utilized as efficient chiral templates for the conformational fixation of u,fi-unsaturated aldehydes, thereby allowing excellent enantioselective recognition of the substrates to achieve... 

High enantioselectivity has been achieved on addition of diethylzinc to benzaldehyde catalysed by a chiral diamine, (,S )-2-(A,A -disubstitutcd aminomethyl)pyrrolidine,116 and by chiral helical titanate complexes of tetradentate ligands.117 Enantioselective additions of dialkylzinc reagents to A,-(diphcnylphosphiiioyl)imines, promoted by aziridino alcohols,118 and to the carbon-nitrogen double bond of the nitrone 3,4-dihydroisoquinoline A-oxide, promoted by dicyclopentyl(R,R)-tartrate,119 have also been reported. 

J. Elguero et al., Failed attempt to induce chirality using a magnetic field The case of chiral helicity of tris(2-methylbenzimidazol-l-yl) methane. Heterocycl. Commun. 1, 102-103 (1994)... 

Figure 11.2. Chiral ligand that forms a chiral helicate with metal ions.

When carbodiimides are subjected to anionic polymerisation, the oligomers are formed with repeating type 4 biguanide units (Figure 5). The polymer chain has restrictions in its conformational degreees of freedom and adopts an helical conformation.7 If the N-substituents of the carbodiimide are chiral, there is intensive induction to yield one of the two diasteroisomeric chiral helices, as shown by optical studies. 

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