Effect of chiral substituents on the secondary structure of poly(N-alkynylamides)

Optically active N-alkynylamides (S)-HC C(CH2)(a)NHCO(CH2)(b)CH(CH3)CH2CH3 (1: a=1, b=0; 2: a=1, b=1; 3: a=1, b=2; 4: a=1, b=3; 5: a=2, b=0; 6: a=2, b=1; 7: a=3, b=0), having several numbers of methylene spacers between the ethynyl group and chiral carbon or amide group, and (S)-HC C(CH2)(2)OCOCH2CH(CH3)CH2CH3 (8) were polymerized with (nbd)Rh+[eta(6)-C6H5B--(C6H5)(3)] to afford the corresponding polymers with moderate molecular weights (Mn = 11000-21000) in 67-99% yields. The H-1 NMR spectra demonstrated that the resulting polymers had stereoregular structures (cis content = 79-100%). CD, UV-vis, and IR spectroscopic studies revealed that the position of the amide group and chiral center strongly affects the pitch and stability of helical structure of the polymers, and intramolecular hydrogen bonding is formed between the amide groups in CHCl3. By comparison with the results of CD measurements and molecular orbital calculation, the sign of Cotton effect and the relationship between the screw sense of poly(N-propargylamides) was elucidated.