Chiral crystal lattice packing

A related phenomenon can also occur when the crystal lattice packing is chiral. This intrinsic handedness can result in formation of a 1 1 mixture of enantiomeric crystals. In this case, although there has been self-resolution into (+)- and (—)-crystals, both molecular enantiomers remain unseparated in each crystal. The fundamental distinction is that a conglomerate single crystal contains only one molecular enantiomer and therefore would be optically active in solution, while, for the latter, a single crystal contains both molecular enantiomers and its solution would be optically inactive. 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

Fig. 7 Chiral packing of rod- and disk-shaped molecules in 2D crystal lattice...

We conclude this survey with extended systems, where the assembled structures show the translational symmetry characteristic of the crystalline state. The packing of chiral units into a crystal lattice will inevitably involve some type of diastereo-selectivity, either homochiral or heterochiral, although this is fiequently not discussed in crystal stucture reports. If the associations are all homochiral, then an enantiomerically pure crystal will be obtained, and a solution of the racemate will yield a racemic mixture (see section 3). If, on the other hand, heterochiral association (often related by a centre of inversion or glide plane) is favoured, a racemic compound will crystallize. The double helicate [ 02(51)2] " crystallizes with a homochiral association of complexes along the helieal axis (Figure 38). The homochiral columns are then arranged in pseudohexagonal arrays with the ehirality... 

There are several alternative methods for the synthesis of optically active polymers from achiral or racemic monomers that do not involve polymerization catalysts. Optically active polymers have been formed from achiral dienes immobilized in a chiral host lattices [ 106]. In these reactions, the chiral matrix serves as a catalyst and can be recovered following the reaction. For example, 1,3-penta-dienes have been polymerized in perhydrotriphenylene and apochoUc acid hosts, where asymmetric induction occurs via through-space interactions between the chiral host and the monomer [107,108]. The resultant polymers are optically active, and the optical purities of the ozonolysis products are as high as 36%. In addition, achiral monomers have been found to pack in chiral crystals with the orientations necessary for topochemical soHd-state polymerization [109]. In these reactions, the scientist is the enantioselective catalyst who separates the enantiomeric crystals. The oligomers, formed by a [27H-27i] asymmetric photopolymerization, can be obtained in the enantiomeric pure form [110]. 

The contact terms are largely responsible for the orientations adopted by nonspherical ions and molecules in crystal lattices, and must underlie the lattice energy discriminations in the packing of diastereoisomers, as in Pasteur s separations. For a long time they were thought to be the only interactions involved in asymmetric transformations and crystallizations of chiral systems. 

Synthesis of chiral enantiomerically pure materials from non-chiral reagents has been accomplished by crystallization of the symmetrical (in solution) substrate in appropriately packed chiral single crystals, followed by a lattice controlled reaction [6]. This concept is illustrated in Scheme 1 for the generation of chiral cyclobutane polymers from non-chiral dienes packing in engineered chiral crystals [7]. 

DNA, polypeptides (such as PBG mentioned above), and polysaccharides (such as xanthan) and many other biological and nonbiological polymers have a definite handedness due to the chiral centers. Rod-like long molecules of these materials in water solutions often crystallize into a hexagonal columnar phase so that the cross-section normal to the rods reveals a triangular lattice. Since the polymers are chiral, close hexagonal packing competes with the tendency to twist [25], [26]. Macroscopic twist can proliferate by... 

Scheme 8. Enantioselective Photoreactions in TADDOL Inclusion Compounds with a Cou-marin, a Methacryl Anilide, and an Oxocyclohexenyl-carboxamide. In the first case, the packing of the coumarin molecules in the mixed crystal is such that the double bonds are predisposed for the (2+2) cycloaddition. In the second example, a photochemical electrocychc reaction is followed by a sigmatropic H shift. The third reaction is an intramolecular (2+2) cycloaddition with dia- and enantioselective formation of three new stereogenic centers. There are several more reactions of this type, described in the literature [54], and the Toda group has determined the crystal structures of a number of inclusion compounds to show the correlation between the crystal packing and the configuration of the photoproducts. EMastereoselective solid-phase reactions of chiral guests in TADDOL-host lattices have also been described by the...

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