Helix-Sense-Selective Polymerization of Phenylacetylenes Having a Porphyrin and a Zinc-Porphyrin Group: One-Handed Helical Arrangement of Porphyrin Pendants.

Newly synthesized two kinds of achiral phenylacetylenes having a free-base- or a zinc-porphyrin (1 and Zn1, respectively) were polymerized by using a chiral rhodium catalyst system, Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst and (R)-(+)- or (S)-(⁻)-1-phenylethylamine ((R)- or (S)-PEA, respectively) cocatalyst. Poly(1) and poly(Zn1) in THF showed a Cotton signal at the absorption region of the porphyrin and the main chain in the circular dichroism (CD) spectra. This result suggests that poly(1) and poly(Zn1) exist in a conformation with an excess of one-handed helix sense and the porphyrin moiety arranged in chiral helical fashion. The one-handed helical structure of poly(1) could be sustained in a mixture of THF/HMPA (10/2, v/v) due to stabilizing by stacking effect of porphyrin moieties along the main chain. This is the first example about helix-sense-selective polymerization by using Rh⁺(nbd)[(η⁶-C₆H₅)B⁻(C₆H₅)₃] catalyst. Additionally, poly(Zn1) showed about 10 times larger CD intensity in comparison with poly(1). This result suggests the regularity of arrangement of the porphyrin in poly(Zn1) is higher compared with poly(1). Spatial arrangement of porphyrins was achieved by utilizing a one-handed helical poly(phenylacetylenes) as a template.