Induction of One-Handed Helix Sense in Achiral Poly(N-propargylamides)

Achiral N-propargylamides, i.e., N-propargyl-3-methylbutanamide (<BO>1</BO>), N-propargyl-2-ethylbutanamide (<BO>2</BO>), and N-propargyl-3,3-dimethylbutanamide (<BO>3</BO>), were polymerized with (nbd)Rh⁺[η⁶-C<INF>6</INF>H<INF>5</INF>B-(C<INF>6</INF>H<INF>5</INF>)<INF>3</INF>] to afford polymers with moderate molecular weights (M<INF>n</INF> = 6000−22 000) in good yields. The ¹H NMR and UV−vis spectra demonstrated that the polymers, poly(<BO>1</BO>)−poly(<BO>3</BO>), have stereoregular structures (cis = 100%) and equally populated right- and left-handed helical conformation. A predominant helix sense was induced in these polymers by the addition of chiral alcohols or amine, which was confirmed by CD and UV−vis spectroscopies. ¹H NMR and CD spectroscopic studies strongly suggested that the poly(N-propargylamides) interacted with the chiral alcohols by hydrogen bonding at the amide groups of the polymer side chain. Chiral terpenes could also induce single-handed helical conformation. It is likely that hydrophobic interaction led to the one-handed helical conformation in the case of the chiral terpenes because the addition of n-hexane decreased the CD signal.