In chiral crystals

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

Penzien, K. and G. M. J. Schmidt (1969). Reactions in chiral crystals - an absolute asymmetric synthesis. Angew. Chem. Int. Ed. Engl, 8, 608. 

Systematic studies of topochemical reactions of organic solids have led to the possibility of asymmetric synthesis via reactions in chiral crystals. (A chiral crystal is one whose symmetry elements do not interrelate enantiomers.) (Green et al, 1979 Addadi et al, 1980). This essentially involves two steps (i) synthesis of achiral molecules that crystallize in chiral structures with suitable packing and orientation of reactive groups and (ii) performing a topochemical reaction such that chirality of crystals is transferred to products. The first step is essentially a part of the more general problem of crystal engineering. An example of such a system where almost quantitative asymmetric induction is achieved is the family of unsymmetrically substituted dienes ... 

Absolute Asymmetric Transformation from Nonchiral Molecules in Chiral Crystals... 

Irie et al. reported on the absolute reversible photocyclization reactions of achiral diarylethene 52 in chiral crystal leading to 53 in up to 99% ee. The asymmetric photochromic material has potential application in nonlinear optics, for example, in switchable second harmonic generation devices. [32]... 

II. ABSOLUTE ASYMMETRIC TRANSFORMATION FROM NONCHIRAL MOLECULES IN CHIRAL CRYSTALS... 

Addadi, L., Vanmil, J., Lahav, M., Photo polymerization in Chiral Crystals. 4. Engineering of Chiral Crystals for Asymmetric [2ji + 2n] Photo polymerization Execution of an Absolute Asymmetric Synthesis with Quantitative Enantiomeric Yield, J. Am. Chem. Soc. 1982, 104, 3422 3429. 

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]. 

Di-7C-Methane Type Solid State Photorearrangements. Asymmetric Induction in Chiral Crystals of an Achiral Compound... 

Reviews of general interest in this area highlights microreactors that can be used for a variety of photochemical reactions such as the synthesis of large ring ketones. Interest in the control that can be exercised on the outcome of photochemical reactions in constrained environments continues to increase and reviews dealing with the enantioselective photoreactions of achiral compounds in chiral crystals and inclusion crystals have been published. ... 

Scheme 37. Enantioselective and solid state [2 + 2] photocycloadditions of unsaturated esters in chiral crystals and enones in chiral inclusion complexes.

Penzien K. and Schmidt G.M. (1969) Reactions in chiral crystals an absolute asymmetric synthesis, Angew. Chem. Int. Ed. Engl. 8, 608-609. Saeva F.D., Sharpe P.E. and Olin G.R. (1975) Asymmetric synthesis in a cholesteric liquid crystal solvent, J. Amer. Chem. Soc. 97, 204-205. Thiemann W. and Teutsch H. (1990) Possible amplification of enantiomeric excess through structural properties of liquid crystals-model for origin of optical activity in the biosphere. Origin ofLife Evolution of Biosphere 20, 121-126 1986,16,420. 

This breaking can be used to determine unambiguously the absolute configuration of the structure (in chiral crystals) calculating Fc hkl) for a number of Friedel (Bijvoet) pairs, one of the enan-tiomorphs will fit better, and this will be the correct absolute configuration. 

The macrocycles in chiral crystals ( 1 CHC13) are all "R"-conformers or all S"-conformers. 

Subsequently, with the aim of amplifying the chirality created in the system in this first generation step, the effect of the chiral products on the crystallization of the two enantiomeric parent crystals, A and A, was investigated. A number of disubsti-tuted phenylene-diacrylates packing in chiral crystals with the structural motif of Scheme 1 were crystallized, either from the melt or from solution, in the presence of enantiomerically pure dimers, trimers or oligomers generated photochemically from the same phases in a previous step. It was observed in all experiments carried out... 

Gridnev, I. D. Chiral Symmetry Breaking in Chiral Crystallization and Soai Autocatalytic Reaction. Chem. Lett. 2006,148-153. 

Green BS, Lahav M, Rabinovich D (1979) Asymmetric synthesis via reactions in chiral crystals. Acc Chem Res 12 191-197. doi 10.1021/ar50138a001... 

SYNTHESIS OF CHIRAL NON-RACEMIC DIMERS AND POLYMERS VIA TOPOCHEMICAL REACTIONS IN CHIRAL CRYSTALS AN EXAMPLE OF AN ABSOLUTE ASYMMETRIC SYNTHESIS... 

Abstract. The asymmetric synthesis of chiral polymers by topochemically controlled polymerization in chiral crystals is discussed. Following a short survey of topochemical polymerization in the solid state and some comments on chiral crystals, we present the requirements for the performance of asymmetric polymerization based on [2+2]-photocycloaddition. The planning and execution of two successful polymerizations of this sort are described. In the first, we start with a chiral non-racemic monomer and obtain optically active cyclobutane oligomers. The optical yields of the dimer and trimer were quantitative on the scale of N.M.R. sensitivity. In the second reaction we start with a racemate, and by the processes of crystallization in a chiral structure and solid-state reaction we generate an optically active polymer, in the absence of any outside chiral agent. The possible application of this novel method to additional systems is discussed. 

The interest of our group in the systematics of the reactivity of organic solids has led us in recent years to the investigation and exploitation of reactions in chiral crystals for the performance of asymmetric syntheses [1,2]. In classical asymmetric transformations [3, 4] there is always the necessity of the presence of an outside chiral agent e.g. a chiral catalyst or chiral handle ). On the other hand, in the asymmetric syntheses of the type we are investigating the induction is applied by the chiral environment which the crystal itself provides at the reaction site. 

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