Remarkable Structures Effects On Chiroptical Properties Of Polyisocyanides Carrying Proline Pendants

To have simplified chemical structures and possess high conformation stabilitiesfor synthetic helical polymers are important for their applications as chiral materialsand asymmetrical catalysts. We here report on the synthesis of a series of aliphatic andaromatic polyisocyanides carrying proline derivatives of different chiralities, andinvestigation of the effects of the pendants’ chemical structures on their chiropticalproperties. The configuration of the chiral center at4-position of proline pendants wasshifted from S to R to check its effects on the handedness of the helical conformation.To examine effects of the steric hindrance on the stabilities of helical conformation forthe aliphatic representatives, proline pendants carrying various substituents at bothcarboxyl and amine terminals were designed. In order to further examine the stericeffects of the proline pendants, the aromatic counterparts were also prepared. In thelatter case, effects of hydrogen bonds between pendant units on the enhancement andstabilities of helical conformation were investigated by switching the ester to amidelinkage. The Cotton effects and signal intensities of these polyisocyanides fromcircular dichroism spectroscopy were compared based on the bulkiness of pendantgroups, solvent polarities and solution temperatures. It was found that stable helicalconformations can be mediated by small bulky monoproline pendants forpolyisocyanides.