Poly(quinoxaline-2,3-diyl)s Bearing (S)-3-Octyloxymethyl Side Chains as an Efficient Amplifier of Alkane Solvent Effect Leading to Switch of Main-Chain Helical Chirality

Abstract

Poly(quinoxaline-2,3-diyl) containing (S)-3-octyloxymethyl side chains was synthesized to investigate the induction of a single-handed helical sense to the main chain in various alkane solvents. The polymer showed an efficient solvent dependent helix inversion between n-octane (M-helix) and cyclooctane (P-helix). After a screening of alkane solvents, it was found that linear alkanes having large molecular aspect ratios induced M-helical structure, and branched or cyclic alkanes having small molecular aspect ratios induced P-helical structure. A polymer ligand containing (S)-3-octyloxymethyl side chains and diphenylphosphino pendants also exhibited solvent-dependent helical inversion between n-octane and cyclooctane, leading to the highly enantioselective production of the both enantiomeric product in a palladium-catalyzed asymmetric hydrosilylation reaction of styrene (R-product 94% ee in n-octane and S-product 90% ee in cyclooctane).

Helical Polymers Showing Inverse Helicity and Synergistic Effect in Chiral Catalysis: Catalytic Functionality Determining Enantioconfiguration and Helical Frameworks Providing Asymmetric Microenvironment

“…The effect of solvent on the yield and enantioselectivity of the aldol reaction was also observed ( Table 2 , run 5 vs run 7, run 8 vs run 10). Referring to recent studies on the switch of enantioselection by changing reaction solvent [ 59,60 ] and our previous work, [ 49 ] it can be inferred that decreased content of helices in THF affected the catalytic effi ciency, resulting in lower yield and ee value.…”

Poly(quinoxaline-2,3-diyl)s: A Fascinating Helical Macromolecular Scaffold for New Chiral Functions

“…Furthermore, by introduction of (S) 3 octyl chiral side chains, hydrosilylation of styrene afforded opposite enantiomers in high enantioselectivities in n octane (94% ee, R) and in cyclooctane (90% ee, S) (Scheme 28). 29 We have shown that the majority rule effect also play a role in achieving chiral ampli cation. 52,53 First of all, we prepared separately two enantiomeric dl monomers bearing two (S) or (R) 2 butoxymethyl side chains.…”

“…29 In addition to helix inversion between chloroform and 1,1,2 TCE, it shows inversion between n alkanes and cycloalkanes. Indeed, the 250 mer PQX adopted pure left handed helical structure in n octane, whereas it takes pure right handed helical structure in cyclooctane.…”

Kinetic Traps to Activate Stereomutation in Supramolecular Polymers

“…[4] Controlling the formation of these entities aids to clarify the role of chiral organization and also opens venues for new applications.T he dynamic switching of the helical sense,o rs tereomutation, is of special relevance to tuning the helicity of adaptable ensembles. Although stereomutation has been amply studied in covalent polymers, [5] it is scarcely reported in supramolecular polymers,d espite their inherent reversibility that could favor switching helicity.S tereomutation in supramolecular assemblies can be achieved using two main approaches:1 )making use of external stimuli such as pH, [6a] guests, [6b] or solvents; [6c,d] and 2) through kinetic traps or pathway complexity. [7] Herein, the chiral features of the supramolecular copolymerization of carbonyl-bridged triarylamines (CBTs) 1-2 is described (Figure 1a and b).…”