Determination of Helical Sense of Poly(N-propargylamides) by Exciton-Coupled Circular Dichroism

Optically active N- propargylamides bearing a porphyrin group, ( S )-HC⋮CCH 2 NHCOCH(CH 3 )OCOR ( 1 ) and ( S )-HC⋮CCH 2 NHCOCH(Ph)OCOR ( 2 ), where R = 4-(10,15,20-tris(4- tert -butylphenyl)porphyrin-5-yl)phenyl, were polymerized with (nbd)Rh + [η 6 -C 6 H 5 B - (C 6 H 5 ) 3 ] along with analogous monomers ( S )-HC⋮CCH 2 NHCOCH(CH 3 )OCOC 6 H 4 - p - tert -Bu ( 3 ) and ( S )-HC⋮CCH 2 NHCOCH(Ph)OCOC 6 H 4 - p - tert -Bu ( 4 ) for comparison to afford the corresponding polymers with moderate molecular weights ( M n = 17 000−24 000) in 28−76% yields. The 1 H 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.