Helical conformation chiral crystallization

Epitaxy of Isotactic Poly (1-butene) Isotactic poly(1-butene) (iPBul) is an archetypical polymorphic polymer with three different structures that differ by the chain conformation, and thus by the unit-cell geometry and symmetry (cf Chapter 2).The three crystal phases could be obtained by epitaxial crystallization on appropriate substrates [48-50]. Most interesting among them are the epitaxy of Form I (trigonal unit cell, threefold helical conformation, racemic phase) and that of Form III (orthorhombic unit cell, fourfold helical geometry, chiral crystal phase). 

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

Chiral Crystallization of Polymers with Helical Chain Conformations References... 

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 ... 

All the 311 molecules adopt a P-helical conformation in the central hexatriene part (this crystal is named (P)-311 NpF). Single crystals with the opposite chirality, (M)-311 NpF, in which 311 is in an M-helical conformation, were also obtained from the same sample batch. The chiral cocrystals underwent photochromism. Upon irradiation with 365 nm light, the colorless crystal turned blue. 

Molecules of 43c adopt chiral packing (space group P2 ) and a helical molecular conformation, and crystallize in (E,Z) conformation which is unfavorable for the oxetane formation. The solid-state irradiation of 43c was found to give the oxetane 44c and a 3-lactam derivative 45c. The (3-lactam 45c was revealed to be enantiomerically enriched to 88% ee, whereas the other photoproduct 44c was racemic. The occurrence and the mechanism of transformation of 43c to 45c involve hydrogen abstraction by the alkenyl carbon atom. 

Absolute asymmetric synthesis was observed in the solid-state photoreaction of benzoylbenzamide 59 to phthalide 60 however, the reaction mechanism was completely different from that of thioester 57. [35] Recrystallization of these amides 59a-c from the chloroform-hexane solution afforded colorless prisms in all cases. X-ray crystallographic analysis revealed that all prochiral amides 59a-c adopted orthorhombic chiral space group P2 2 2 and were frozen in chiral and helical conformation in the crystal lattice. 

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

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