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).