Water-mediated deracemization of a bisporphyrin helicate assisted by diastereoselective encapsulation of chiral guests

Abstract

Deracemization is a powerful method by which a racemic mixture can be transformed into an excess of one enantiomer with the aid of chiral auxiliaries, but has been applied only to small chiral molecular systems. Here we report a deracemization of a racemic double-stranded spiroborate helicate containing a bisporphyrin unit upon encapsulation of chiral aromatic guests between the bisporphyrin. The chiral guest-included helicate is kinetically stable, existing as a mixture of right- and left-handed double helices, which eventually undergo an inversion of the helicity triggered by water resulting from the water-mediated reversible diastereoselective B-O bond cleavage/reformation of the spiroborate groups, thus producing an optically-active helicate with a high enantioselectivity. Quantum chemical calculations suggest that the stereospecific CH-π interactions between the porphyrin hydrogen atoms of the helicate and an aromatic pendant group of the chiral guest play a key role in the enhancement of the helical handedness of the helicate.

Deracemization of Carbohelicenes by a Chiral Perylene Bisimide Cyclophane Template Catalyst

“…Whereas the earlier examples of synthetic cyclophane‐type supramolecular hosts typically were of high symmetry, recent studies have also explored the construction of cavities of lower symmetry for enantioselective recognition and sensing [8, 9] . Furthermore, Yashima and co‐workers recently reported double‐stranded spiroborate helicates that can be transformed into their optically active forms by the complexation of chiral guest molecules [10] . However, to the best of our knowledge, despite the long history of research on the deracemization of organic molecules [11] by dynamic kinetic resolutions via inclusion complexes [12] and chromatography on chiral phases, [13] deracemization by a templating chiral cyclophane host has not yet been demonstrated.…”

Stimuli-responsive Molecular Springs Based on Single- and Multi-stranded Helical Structures

“…An optically-active helicate 19 Na2 with up to a 72% helixsense excess can also be obtained by the deracemization reaction of the racemic helicate (rac-19 Na2 ) upon encapsulation of a chiral aromatic guest, such as (R)-or (S)-22, between the bisporphyrin via water-mediated reversible diastereoselective B-O bond cleavage/reformation of the spiroborate groups, whereas no deracemization took place in the absence of water (Figure 13b). 77 This water-mediated deracemization reaction of rac-19 Na2 proceeds under thermodynamic control, in which an energetically-disfavored diastereomeric inclusion complex is transformed into a thermodynamically more stable one through an inversion of the helicity that requires high temperatures to facilitate the interconversion between the (P)-and (M )-helicates. Interestingly, when organic acids, such as p-toluenesulfonic acid, were used as a proton source, the deracemization reaction of rac-19 TBA2 complexed with the enantiopure 22 readily proceeded even below room temperature in the presence of a catalytic amount of p-toluenesulfonic acid, thus producing a highly enantio-enriched 19 Na2 with up to 93% ee.…”

“…The diastereomeric excess (de) increased with the decreasing temperature and reached 98% and >99% de in THF at ¹35 and ¹50°C, respectively (Figure 13c). 77 Attractive edge-to-face CHπ interactions between the pendant aromatic rings of the encapsulated chiral guests and the porphyrin protons at the meso and its neighboring β-positions of the helicate play a crucial role in the observed highly-efficient diastereoselective inclusion complexation, resulting in an excellent helixsense selectivity during the water-mediated and further acidcatalyzed deracemization as revealed by 1 H NMR analyses in combination with theoretical calculations.…”

Recognition and Sensing of Chiral Organic Molecules by Chiral Porphyrinoids: A Review

“…Two or more molecules, able to non-covalently interact and to display binding-site complementarity, may exhibit molecular recognition [1,2]. In this respect, the chiral recognition is a specific process in which a host molecule selectively binds, through supramolecular forces (e.g., hydrogen bonding, metal coordination, hydrophobic effects, van der Waals forces, π-π and/or electrostatic interactions) to a precise enantiomer of the guest molecule compared with mirror stereoisomer [3][4][5][6][7][8][9][10][11]. Nevertheless, this latter phenomenon represents a key process in a wide range of chemical systems and biological occurrences in the natural world [12][13][14][15][16][17].…”