Abstract
The cascade process of a dynamic chirality transmission from the permanent chirality center to the stereodynamic triphenylmethyl group has been studied for series of optically active trityl derivatives. The structural analysis, carried out with the use of complementary methods, enabled us to determine the mechanism of chirality transfer. The process of chirality transmission involves a set of weak but complementary electrostatic interactions. The induction of helicity in a trityl propeller is revealed by rising non-zero cotton effects in the area of trityl UV-absorption. The presence of an additional stereogenic center in close proximity to the trityl-containing stereogenic center significantly affects the sign and, to a lesser extent, magnitude of the respective cotton effects. Despite the bulkiness of the trityl, in the crystalline phase, the molecules under study strictly fill the space. In the crystal, molecules form aggregates stabilized by OH•••O hydrogen bonds. However, the presence of two trityl groups precludes formation of OH•••O hydrogen bonding. Additionally, the trityl group seems to be responsible for the formation of the solid solutions by e.g., racemates of trans- and cis-2-tritylcyclohexanol. Therefore, the trityl group acts as a supramolecular protective group, which in turn can be used in the crystal engineering.
Highlights
The triphenylmethyl group and its congeners are frequently used in synthetic organic chemistry for protecting alcohols, thiols, and amines [1]
We have previously shown that the use of hybrid functionals, B3LYP for geometry and CAM-B3LYP for excited states calculations and for compounds containing trityl groups, gives results closest to the experimental ones [17,18]
The helicity of the trityl group for all molecules observed in the crystal phase, both in α and β forms, is MPM, which corresponds to the conformation of the calculated low-energy structure
Summary
The triphenylmethyl group (trityl, Tr) and its congeners are frequently used in synthetic organic chemistry for protecting alcohols, thiols, and amines [1]. Apart from usability in synthesis, the trityl, and related groups of the general Ar3 X type, represents a unique example of a molecular fragment whose structural dynamics and mode of action resemble that of the macroscopic propeller. For the prototypical example of O-trityl alcohols, the transfer of structural information proceeds through “a bevel gear” mechanism, which assumes direct steric interaction between the permanently chiral part of the system (the inductor) and the stereodynamic trityl unit (the reporter part). ECD spectroscopy, supported by theoretical DFT calculations, seems to be the most suitable for testing the dynamic chirality induction process in the trityl chromophore [21] This combination of experimental and theoretical approaches enables an accurate perception of the structure of the molecular entity and the mechanism of the optical activity induction. We will expand our study on the behavior of trityls to its association modes in the crystalline phase and to possible crystalline forms differing in densities and capable of hosting inclusions
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