Spontaneous chiral crystallization approach

Solid-state asymmetric photoreactions have been already reviewed from various aspects [7-20]. However, the reviews do not seem to have given us a complete understanding of solid-state chiral photochemistry. In this chapter, solid-state asymmetric photoreactions are systematically reviewed by classification into supramolecular approaches and spontaneous chiral crystallization approaches since the beginning in the 1970s to the present. 

VI. ABSOLUTE ASYMMETRIC PHOTOREACTIONS BY THE SPONTANEOUS CHIRAL CRYSTALLIZATION APPROACH... 

Solid-state chiral photochemistry has been developed based on spontaneous chiral crystallization and supramolecular approaches. Since the report of solid-state [2 4- 2] photocycloaddition by Cohen and Schmidt in 1964 [11], the Weiz-mann Institute Group has been also a pioneer of solid-state chiral photochemistry. The research group first succeeded in absolute asymmetric [2 + 2] photocycloaddition by using chiral crystals spontaneously formed from achiral dienes in 1973 [12]. Thereafter, more than twenty successful examples of absolute asymmetric photoreactions have been reported by several research groups [9,10,13,14]. The... 

This kind of chiral crystallization of an achiral molecule always gives both right- and left-handed crystals. However, under ordinary crystallization conditions, enantiomorphous control is not possible. We have reported that a cocrystal is formed by the crystallization of a solution of a 1 1 mixture of 3-indolepropionic acid and phenanthridine in acetonitrile [28]. When the acetonitrile solution was divided into six parts and spontaneously evaporated in six containers, P-crystals appeared in four containers and M-crystals in two. If such chiral crystallization is carried out using a much larger number of flasks, the ratio of the formation of both enantiomeric crystals will approach 1 1. 

The earliest approach to explain tubule formation was developed by de Gen-nes.168 He pointed out that, in a bilayer membrane of chiral molecules in the Lp/ phase, symmetry allows the material to have a net electric dipole moment in the bilayer plane, like a chiral smectic-C liquid crystal.169 In other words, the material is ferroelectric, with a spontaneous electrostatic polarization P per unit area in the bilayer plane, perpendicular to the axis of molecular tilt. (Note that this argument depends on the chirality of the molecules, but it does not depend on the chiral elastic properties of the membrane. For that reason, we discuss it in this section, rather than with the chiral elastic models in the following sections.)... 

Coumarin photochemistry has been recently employed to demonstrate that a frozen axial chirality can be used to induce the absolute configuration of stereogenic centers. Coumarin 103 was obtained as a single atropisomer by spontaneous crystallization (Scheme 6.37). Upon warming powdered crystals of 103 in MeOH to —20 °C, sensitized [2 + 2]-photocydoaddition to ethyl vinyl ether gave the almost enantiomerically pure products 104. The approach to the coumarin double bond occurred preferentially from the less-shielded face to which the amide carbonyl group... 

Chiral smectic ferroelectric liquid crystals are liquids that possess spontaneous polar order. Combined with their excellent processibility on silicon integrated circuits, these liquid crystals provide an attractive potential approach to synthesis of materials for second order nonlinear optics, provided adequate second order susceptibility can be obtained. Unfortunately, the second-order nonlinear optical susceptibility of the ferroelectric liquid crystals are usually low and their thermal stability is limited. Several, very interesting approaches to utilization of liquid crystals, however, were carried out. 

One optical feature of helicoidal structures is the ability to rotate the plane of incident polarized light. Since most of the characteristic optical properties of chiral liquid crystals result from the helicoidal structure, it is necessary to understand the origin of the chiral interactions responsible for the twisted structures. The continuum theory of liquid crystals is based on the Frank-Oseen approach to curvature elasticity in anisotropic fluids. It is assumed that the free energy is a quadratic function of curvature elastic strain, and for positive elastic constants the equilibrium state in the absence of surface or external forces is one of zero deformation with a uniform, parallel director. If a term linear in the twist strain is permitted, then spontaneously twisted structures can result, characterized by a pitch p, or wave-vector q=27tp i, where i is the axis of the helicoidal structure. For the simplest case of a nematic, the twist elastic free energy density can be written as ... 

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