Synthesis and Chiroptical Properties of Helical PolyallenesBearing Chiral Amide Pendants.

Two allene derivatives, l- and d-N-(1-(octylamino)-1-oxopropan-2-yl)-4-(propa-1,2-dien-1-yloxy)benzamide(l-1and d-1), bearingchiral amide pendants were designed and synthesized. Living polymerizationsof l-1and d-1with allylnickelcomplex as a catalyst afforded poly-l-1mand poly-d-1mwith controlledmolecular weights and narrow molecular weight distributions. Thesepolymers were found to possess a stable helical conformation witha preferred handedness in aprotic solvents on the basis of their circulardichroism (CD) spectra and specific rotation as well as computer simulation.The helical conformation of the polymers was revealed to be stabilizedby elongation of the repeating unit until the degree of the polymerizationreaches 80. The slightly influence of temperature on the CD spectraof poly-l-1100in CHCl3indicated the helical conformation was quite stable at least inthe range of 0–55 °C. Although poly-l-1100showed similar CD spectra in different aprotic solvents,remarkable decrease was observed upon the addition of protic solventssuch as methanol due to the weakened hydrogen bonding interactionsbetween the adjacent repeating units. The poly-l-1100behaves as a pH-responsive property; the helical structureof the main chain can be transformed to random coil by addition oftrifluoroacetic acid to the THF solution which again switches backto helical conformation by neutralization with triethylamine. It wasconfirmed that the copolymerization of l-1and d-1obeyed the majority rule as supported by thenonlinear correlation between the enantiomeric excess of monomer 1with the CD intensities of the generated copolymers. Atomicforce microscope (AFM) and scanning electron microscope (SEM) studiesrevealed poly-l-1100self-assembledinto well-defined helical fibrils with distinct handedness. [ABSTRACT FROM AUTHOR]