Synthesis and Properties of Serine- and Threonine-Based Helical Polyacetylenes

Serine- and threonine-based acetylene monomers carrying hydroxyl group, N-tert-butoxycarbonyl-l-serine N‘-propargylamide (1), N-tert-butoxycarbonyl-l-threoine N‘-propargylamide (2), and their O-silylated monomers, N-tert-butoxycarbonyl-O-triethylsilyl-l-serine N‘-propargylamide (3) and N-tert-butoxycarbonyl-O-triethylsilyl-l-threonine N‘-propargylamide (4), were polymerized with a rhodium zwitterion catalyst in THF, MeOH, CH2Cl2, and toluene to afford the corresponding optically active poly(N-propargylamides) with moderate number-average molecular weights (4200−12 800) in good yields. The polymers exhibited large specific rotations (−172° to −955°) and clear CD signals at the absorption region of polyacetylene main chains. The CD signals of poly(1) and poly(2) appeared around 270−350 nm, while those of poly(3) and poly(4) appeared around 400 nm. It is considered that these polymers take helical structures with predominantly one-handed screw sense, whose helical pitches are different. The presence of intramolecular hydrogen bonding of poly(1) and poly(2) was confirmed by IR spectroscopy measured in CHCl3. Desilylation of poly(3) and poly(4) satisfactorily proceeded to afford the polymers exhibiting the same CD spectroscopic patterns as those of poly(1) and poly(2).