Sulfone “Geländer” Helices: Revealing Unexpected Parameters Controlling the Enantiomerization Process

Abstract

The two sulfonyl-bridged Geländer helices 1a and 2a are obtained by oxidation of the corresponding sulfide bridged precursors 1b and 2b. Both Geländer structures are fully characterized by NMR, high-resolution mass spectrometry, and optical spectroscopies. X-ray diffraction with a single crystal of 2a provides its solid-state structure. Both Geländer helices 1a and 2a are separated into enantiomers, and their racemizations are monitored by circular dichroism. For 1a, consisting of two equally sized macrocycles, a substantial increase in the enantiomerization barrier is observed upon going from the sulfide to the sulfone, and only a subtle rise is detected for the constitutional isomer 2a with two macrocycles of different size during the same transformation. This results not only in 1a with the highest configurational stability in the series of hitherto investigated Geländer structures but also challenges the so far hypothesized correlations between bridging structures and the Gibbs free energy of enantiomerization. The simulation of the enantiomerization process in the macrocyclic subunits suggests the proximity of the endotopic hydrogens as parameter responsible for the heights of the enantiomerization barrier.

Geländer Molecules with Orthogonal Joints: Synthesis of Macrocyclic Dimers

“…H) New "Geländer" macrocycles 1 and 2 with rectangular arrangements between axle and rung, as well as between rung and banister molecule concept to ladder-type oligomers with length mismatch in both ladder stringers. In the last years, we synthesized various "Geländer" structures with oligophenylene (OP) backbones, wrapped by benzylic ethers, [3][4][5] sulfones, [6] or oligophenylene ethanes (Figure 1D). [7] Recently, we even enlarged the radius of the design by interlinking both oligomers by alkynes and wrapped para-oligophenylene diyne around the OP backbone as a conjugated banister (Figure 1E).…”

Tuning Supramolecular Polymer Assembly through Stereoelectronic Interactions

“…To provide a self-assembling [n.n]­pCp with stereoelectronic tunability, the central methylene groups of previously synthesized [3.3]­pCpTA were replaced with a sulfur to give 2,11-dithia[3.3]­paracyclophane tetracarboxamide ( dt­[3.3]­pCpTA ; see Figure ). It was our hypothesis that conformational, structural, electronic, and stereoelectronic aspects of a self-assembling dt­[3.3]­pCpTA system could be influenced remotely through the oxidation of the sulfide bridge ( dt­[3.3]­pCpTA- S , (±)-1 ) to a sulfone bridge ( dt­[3.3]­pCpTA- SO 2 , (±)-2 ). , An example of the profound influence of sulfur oxidation state on Geländer macrocycle structure and flexibility was recently studied by Mayor and co-workers . In the sulfide bridged dt­[3.3]­pCpTA- S (±)-1 , the bridging sulfur is engaged in a weak n → π* interaction with the proximal amide carbonyls.…”

“…30,31 An example of the profound influence of sulfur oxidation state on Gelander macrocycle structure and flexibility was recently studied by Mayor and coworkers. 32 In the sulfide bridged dt [3.3]pCpTA-S(±)-1, the bridging sulfur is engaged in a weak n → π* interaction with the proximal amide carbonyls. When the sulfide is oxidized to a sulfone (dt [3.3]pCpTA-SO 2 (±)-2), the sulfone oxygen engages the amide carbonyl in a stronger n → π* interaction while not significantly altering the structure of the dt [3.3]pCp skeleton.…”

An Ortho‐Tetraphenylene‐Based “Geländer” Architecture Consisting Exclusively of 52 sp2‐Hybridized C Atoms

“…These pioneering Geländer structures not only interconvert between enantiomers easily, the combination of the molecule's symmetry with the independence of both interlinking junctions also allows their meso forms, having opposed axial chirality along the molecule's axis. We developed the concept of Geländer molecules further by attaching a second phenyl‐based oligomer of different step‐size alongside the para ‐terphenyl axle (Figure 1c) [25–29] . While the improved molecular design avoids meso ‐forms by propagating the axial chirality along the oligomers backbone, it came at the prize of considerably increased synthetic challenges.…”

“…We developed the concept of Geländer molecules further by attaching a second phenyl‐based oligomer of different step‐size alongside the para ‐terphenyl axle (Figure 1 c). [ 25 , 26 , 27 , 28 , 29 ] While the improved molecular design avoids meso ‐forms by propagating the axial chirality along the oligomers backbone, it came at the prize of considerably increased synthetic challenges. Furthermore, also these improved Geländer structures enantiomerize at room temperature and are thus rather model compounds to investigate the parameters controlling their racemization than stable enantiopure materials for chiroptical applications.…”