Enantiomorphs achiral/chiral structures

The first chiral bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecific polymerization catalyst was racemic ethylenebis-(4,5,6,7-tetrahydro-l-indenyl)zirconium dichloride (see Structure 9) [45]. Ewen showed that the analogous ethylenebis(l-indenyl)titanium dichloride (a mixture of the meso form and the racemate) produces a mixture of isotactic and atactic polypropylene [46]. The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic site control in the case of the titanocene, the achiral meso structure causes the formation of atactic polymer. 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

Stereoselectivity in the insertion of olefins is brought about by a chiral environment of the coordination sphere. When the selectivity is predominantly governed by chirality of the catalyst itself, stereospecificity shows catalytic-site control , also called catalyst control or enantiomorphic-site control . Catalysts must have chiral structures (but need not be homochiral) in this case. Even though a catalyst is achiral, an inserted a-olefin can make a chiral center at the end of a polymer chain. When this chirality controls the stereoselectivity of the next monomer insertion, it is called chain-end control . 

In the envelope conformation (A) the peroxide bond and the two carbon atoms are all coplanar (with the C-O-O-C dihedral angle being close to 0°) while the ethereal oxygen atom can be displaced by as much as 0.65 A to either side of this plane. In conformation B the peroxide bond straddles the plane of the remaining three atoms and this dihedral is around 50°. While conformation A is achiral, B has C.y symmetry. Usually ozonides crystallize in chiral space groups however, both enantiomorphic forms of B are usually encountered in the crystal lattice. Furthermore, disorder of the peroxide oxygen atoms over several occupancies is frequent, and in recent analyses, due mostly to improvement in the structure refinement algorithms, this disorder could be taken into account and suitably refined models could be built from the diffraction data. 

Fig. 16 STM images (b 11.7x 11.7 nm2, c 11.5 x 11.5 nm2) and structure models (d,e) for the enantiomorphous lamella structures induced by adsorption of ISA (a) on hopg from a 1-heptanol solution [47]. For opposite enantiomers, opposite lamella tilt angles (0) are observed. The large lamella ALi is built up from pure ISA enantiomers, while the smaller AL2 lamella consists of coadsorbed achiral 1-heptanol molecules. The ISA chirality is transferred to the coadsorbed solvent molecules via opposite alignment angles (p (f,g). Reprinted with permission from Wiley...

R,R)-TA crystallizes in different enantiomorphous superstructures on Cu( 110), but at a coverage of 0.25 molecules per substrate atom, the monotartrate species forms an achiral c(4 x 2) or (4 0,2 1) structure [71]. In contrast to the bitartrate in its sawhorse geometry, only a single molecular site is connected to the substrate and chirality is not transferred into the lattice structure. Under these conditions, chiral resolution cannot be expected (see below) [72],... 

Chirality is a concept well known to organic chemists and to all chemists concerned in any way with structure. The geometric property that is responsible for the nonidentity of an object with its mirror image is called chirality. A chiral object may exist in two enantiomorphic forms that are mirror images of one another. Such forms lack inverse symmetry elements, that is, a center, a plane, and an improper axis of symmetry. Objects that possess one or more of these inverse symmetry elements are superimposable on their mirror images they are achiral. All objects belong to one of these categories. 

When two enantiomorphous right- and left-crystals are separately obtained, one can conveniently use each crystal for the seeding of the selective chiral crystallization to either one of the two enantiomorphous crystals. More elegant pseudoseeding, based on utilizing different crystals with similar crystal structure as seed crystals, can enantiocontrol crystallization from solutions of tryptamine and achiral carboxylic acids [35]. 

Similarly to other molecules with isoconstitutional chiral substructures, the combination of distinct fullerene addition patterns with enantiomorphic or homomorphic chiral addends can lead to steric arrangements with a certain complexity. However, these can be easily analyzed with the substitution test delineated in Figure 1.1. In the case of the 1,4-addition pattern of Cgo we may consider the following cases (Figure 1.34) If both chiral addend moieties are homomorphic (structure A), the resulting addition pattern is achiral... 

Synthetic peptide-nucleic acids (PNA, Scheme 14C) consisting of poly-N-(2-aminoethyl)glycine which is derivatized with nucleotides, are an interesting hybrid class of compounds, as they are found to form Watson-Crick base pairs with a complimentary peptide-nucleic acid, RNA, and DNA. The polypeptide backbone is achiral but chirality can be induced in the molecule or its assemblies in various ways. Tagging of the peptide-nucleic acid duplex with either L- or D-lysine leads to enantiomorphic structures with opposite CD spectra, analogous to the so-called sergeants-and-soldiers effect for polyisocyanates. When paired with RNA, the peptide-nucleic acid assumes the A-structure typical of RNA (see above). ... 

Xiong et al. also demonstrated that the chiral 2D framework [Cu(PPhj)(A,A( -(2-pyridyl-(4-pyridyl methyl)-amine)),5] C10 (9), with triangular cavities, synthesized from achiral components, [Cu(MeCN)j(PPh3)J[C10 ] and N,N -(2-pyridyl-(4-pyridyl methyl)-amine) can enantioselectively include 2-butanol [76]. Spontaneous resolution produced crystalline inclusion compound 91.5(2-butanol), which was structurally characterized. They manually separated the enantiomorphic forms of 91.5(2-butanol) according to their CD spectra in solid state and evacuated at 100°C to collect enantiopure 2-butanol. Although this work provides an economical route to enantioselective separation of racemic small diols, the separation... 

The stereochemical similarity between the additive and the crystal structure of one of the enantiomorphic substrates was found to be of paramount importance [8], while parameters like temperature, concentration or nature of the medium had only a quantitative effect on the induction in this system. Further kinetic and mechanistic studies resulted in the formulation of a mechanism according to which the additive is enantioselectively adsorbed in small amounts at the surface of the growing crystal of the same absolute configuration. The adsorption of the chiral additive causes a drastic decrease in the rate of growth of this same crystal, thus shifting the crystallization equilibrium towards the unaffected enantiomorphous phase. This is illustrated in Scheme 2, where the achiral monomer is represented as a fast racemizing... 

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