Determination of helical sense of poly(N-propargylamides) by exciton-coupled circular dichroism

Optically active N-propargylamides bearing a porphyrin group, (S)-HC equivalent to CCH2NHCOCH(CH3)OCOR (1) and (S)-HC equivalent to CCH2NHCOCH(Ph)OCOR (2), where R = 4-(10,15,20-tris(4-tert-butylphenyl)porphyrin-5-yl)phenyl, were polymerized with (nbd)Rh+[eta(6)-C6H5B-(C6H5)(3)] along with analogous monomers (S)-HC equivalent to CCH2NHCOCH(CH3)OCOC6H4-p-tert-Bu (3) and (S)-HC equivalent to CCH2NHCOCH(Ph)OCOC6H4-p-tert-Bu (4) for comparison to afford the corresponding polymers with moderate molecular weights (M. = 17 000-24 000) in 28-76% yields. The H-1 NMR spectra demonstrated that the resulting polymers had fairly stereoregular structures (cis content = 78-97%). CD and UV-vis spectroscopic studies revealed that poly(1)-poly(4) took one-handed helical structure. The relationship between the screw sense of poly(N-propargylamides) and the sign of Cotton effect originating from the main chain was elucidated by comparison of the Cotton effects of either poly(1) and poly(3) or poly(2) and poly(4). It was concluded on the basis of the exciton chirality method along with molecular modeling study that the pendent porphyrins of poly(1) and poly(2) form right-handed helical arrays and that the main chains form a right-handed helix.