Dynamic Induction of Optical Activity in Triarylmethanols and Their Carbocations.

Bartosz Stasiak,Marcin Kwit,Agnieszka Czapik

doi:10.1021/acs.joc.0c02289

Bartosz Stasiak, Marcin Kwit + Show 1 more

Open Access

https://doi.org/10.1021/acs.joc.0c02289

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Abstract

A series of artificial triarylmethanols has been synthesized and studied toward the possibility of exhibiting an induced optical activity. The observed chiroptical response of these compounds resulted from the chiral conformation of a triarylmethyl core. The chirality induction from a permanent chirality element to the liable triarylmethyl core proceeds as a cooperative and cascade process. The OH···O(R) and/or (H)O···HorthoC hydrogen bond formation along with the C–H···π interactions seem to be the most important factors that control efficiency of the chirality induction. The position of chiral and methoxy electron-donating groups within a trityl skeleton affects the amplitude of observed Cotton effects and stability of the trityl carbocations. In the neutral environment, the most intense Cotton effects are observed for ortho-substituted derivatives, which undergo a rapid decomposition associated with the complete decay of ECD signals upon acidification. From all of the in situ generated stable carbocations, only two exhibit intense Cotton effects in the low energy region at around 450 nm. The formation of carbocations is reversible; after alkalization, the ions return to the original neutral forms. Unlike most triarylmethyl derivatives known so far, in the crystal, the triarylmethanol, para-substituted with the chiral moiety, shows a propensity for a solid-state sorting phenomenon.

Highlights

The exceptional structural features and a wide-scope of applications make triphenylmethane (TrH), and related compounds of the general Ar3X formula, interesting and still intensively explored objects of study
As has been mentioned above, the importance of trityls and trityliums in various aspects of chemistry is not related to the growing interest in their dynamic stereochemistry
Continuing our interest in the dynamic stereochemistry of chiral compounds, we have decided to expand our study on triarylmethyl systems that may exhibit an induced optical activity in the neutral and in the cationic form

Summary

Introduction

The exceptional structural features and a wide-scope of applications make triphenylmethane (TrH), and related compounds of the general Ar3X formula, interesting and still intensively explored objects of study. Adaptation of trityl to the structure of chiral secondary alcohols, through the bevel-gear mechanism, was demonstrated for the first time by Gawronski.[13] Since a process of chirality transmission from the permanent chirality element to the stereodynamic trityl has been exploited by us and others for stereochemical assignments of alcohols, amines, sulfides, and selenides.[14] For all systems studied so far, the final, observed effects have provided evidence for a dynamic induction of the optical activity,[15] the detailed mechanism of the chirality transfer phenomenon is rather case sensitive

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