Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene

Abstract

We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]­paracyclophane by integrating it into a macrocycle. The [2.2]­paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]­paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.

An Enantiopure Propeller‐Like Trityl‐Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter

“…Variable‐temperature ECD, also known as the dynamic ECD (i.e., dECD) technique was chosen for evaluating the stability of the enantiomers in solution, since some overestimated thermodynamic data might be obtained from the analysis of the elution profiles resulting from the dynamic (CSP)HPLC method . The evolution with time of the ECD signal of the second eluted (CSP)HPLC fraction was recorded at different temperatures to quantify the kinetic parameters of the enantiomerization barriers.…”

“…Variable-temperature ECD, also known as the dynamic ECD (i.e.,d ECD)t echnique was chosen for evaluating the stability of the enantiomersi ns olution,s ince some overestimated thermodynamic data might be obtained from the analysis of the elution profiles resulting from the dynamic (CSP)HPLC method. [30] The evolution with time of the ECD signal of the second eluted (CSP)HPLC fractionw as recorded at different temperatures to quantify the kinetic parameters of the enantiomerizationb arriers. Interestingly,n oh ints of racemization were observed at any temperature up to 60 8Cf or the TTBrM radical( see Section S5 of the Supporting Information), in contrast to the previously observed behaviour for the TTM radical, with a t 1/2 of 18 sa tt hat given temperature.…”

Beyond Simple Substitution Patterns – Symmetrically Tetrasubstituted [2.2]Paracyclophanes as 3D Functional Materials

“…While for that purpose a twofold functionalized PC like 69 would be enough, only a fourfold substituted PC like 97 enables the decoration with bulky substituents slowing down the racemization process to a level enabling the isolation of the helical chiral enantiomers. As first model compound the macrocycle 98 exposing two para ‐(methoxycarbony)phenylethynyl substituents (Scheme ) stabilizing both enantiomers to a level, enabling their isolation was assembled …”

“…For clarity exclusively one enantiomer (( M )‐ 98 ) is displayed. Bottom : ECD spectra of both enantiomers ( M )‐ 98 (red) and ( P )‐ 98 (blue) …”

Bicyclic Phenyl–Ethynyl Architectures: Synthesis of a 1,4‐Bis(phenylbuta‐1,3‐diyn‐1‐yl) Benzene Banister

“…A particularly beautiful example is the entirely sp‐hybridized macrocyclic carbon allotrope reported by Anderson and co‐workers recently [6] . While most macrocycles are formally achiral (without a chiral center), their spatial arrangement may induce topological chirality, with axial chirality as a prominent example related to helical structures [7–11] …”