Enantioselective Synthesis of Complementary Double‐Helical Molecules that Catalyze Asymmetric Reactions

Abstract

The double helix of DNA is one of the most attractive targets in organic and supramolecular chemistry because of its key biological structures and functions. It is composed of complementary strands derived from the homochiral component (d-sugars) and as a consequence, results in the overall righthanded double-helical structure. The metal-directed doublestranded helicates [1] and hydrogen-bonding-driven assemblies of some aromatic oligoamides [2] are known to form double helices. In contrast with DNA, these double helices lack the complementarity between the two strands and have not yet been used for supramolecular catalysis.[3] However, recent discoveries of DNA-based biocatalysts, DNAzymes, [4] and metal-bound DNA hybrid catalysts [5] for enantioselective reactions imply the potential ability of double helices as a promising chiral framework for enantioselective catalysis. Herein we show that complementary double-helical molecules showing optical activity owing to its helicity can be enantioselectively synthesized and can catalyze an asymmetric reaction in the presence of a metal ion; the double-helix framework with controlled helicity is essential for its high enantioselectivity. This approach suggests the broad potential of double-helix catalysis in asymmetric synthesis, which will provide an important step toward more-effective DNAzymelike supramolecular catalysts with sequential information.The design and synthesis of enantiomeric double helices is based on our recently developed strategy with amidinium carboxylate salt bridges, which assist the intertwining of the two crescent-shaped complementary molecular strands, [6] as illustrated in Scheme 1. The two complementary molecular strands 1 and 2 bear achiral amidine and carboxy groups, respectively, and the m-terphenyl units are linked through Pt II acetylide complexes with chiral (R)-or (S)-2-diphenylphosphino-2'-methoxy-1,1'-binaphthyl (MOP) or achiral triphenylphosphine (PPh 3 ) ligands. Upon complexation, 1 and 2 are intertwined with each other through amidinium carboxylate salt bridges, resulting in the double helix 1·2 on which the bias in the twist sense could be induced and controlled by the chiral phosphine groups (MOP) on the Pt II atoms. The interstrand ligand-exchange reaction on the Pt II atoms by using an achiral diphosphine ligand, such as bis(diphenylphosphino)methane (dppm), leads to the bridged double helix 3, which no longer has any chiral components except for helicity. We anticipated that the bias in the twist sense of 3 would be retained if the bridging diphosphine bind the two complementary molecular strands strongly enough to retard the racemization.To this end, the diamidine strand (R)-1 a was complexed with the achiral dicarboxylic acid strand 2 through the saltbridge formation in CDCl 3 . The duplex formation was confirmed by cold-spray ionization mass spectrometry (CSI-MS) measurements; the CSI-MS spectrum of a 2+ , respectively (see Figure S1 in the Supporting Information).[7] The 31 P NMR spectrum (200 MHz, CDCl 3 ) of (R)-1 ...

Planar Chirality and Helical Polymers: Ferrocenyl-Substituted Amidinium–Carboxylate Salts

“…In contrast, when R 00 = CF 3 the hydrogen-bonded polymer contains alternating enatiomeric forms of the cation. (4). Compound 4 was synthesized as described for 2, using FcC{NCy}{NHCy} (0.25 g, (0.64 mmol) and mesCO 2 H (0.11 g, 0.64 mmol).…”

Chiral tether-mediated stabilization and helix-sense control of complementary metallo-double helices

“…The obtained T c , along with the chemical shift difference between the split signals (Δ ν = 313 Hz), enables us to estimate that the free energy of activation for the helix inversion (Δ G ‡ ) of the duplex 3d is 42.7 kJ mol –1 at 25 °C, which corresponds to the lifetime of the one-handed helical state ( τ ) of 4.98 × 10 –6 s. The Δ G ‡ value obtained for 3d appears to be much higher than that for the Pt II -linked duplex bearing the PEt 3 ligands, 14 k but lower than that of the bridged double helix by the DPPM ligands (Scheme 1A), because the enantiomerically-enriched DPPM-bridged double helix retains its optical activity for a long time. 12 …”

Homo‐double helix formation of an optically active conjugated polymer bearing carboxy groups and amplification of the helicity upon complexation with achiral and chiral amines

“…The SEC measurements were carried out using a JASCO PU-2080 liquid chromatograph equipped with a UVVisible (254 nm; JASCO UV-2070) detector. Two Tosoh TSKgel Multipore H XL -M (30 cm) SEC columns (Tosoh, Tokyo, Japan) were connected in series using THF containing tetrabutylammonium bromide (TBAB) (0.1 wt %) as the eluent at a flow rate of 1.0 mL min 21 . The molecular weight calibration curves were obtained with polystyrene standards (Tosoh).…”