Hydrogen-Bond-Assisted Asymmetric Radical Cyclopolymerization of N-allyl-N-tert-butylacrylamide in the Presence of Chiral Tartrates

Abstract: Low-temperature radical polymerization of achiral N-allyl-N-tert-butylacrylamide (AltBAAm) was conducted in toluene in the presence of chiral tartrates, such as diethyl L-tartrate (L-EtTar) and di-n-butyl L-tartrate. The 1H NMR spectra of the polymers obtained indicated progress of cyclopolymerization at low temperatures such as –80 °C. Optical properties of the poly(AltBAAm)s examined by optical rotation and circular dichroism measurements indicated asymmetric induction in the cyclopolymerization of achiral AltBAAm. For example, addition of L-EtTar at –80 °C provided the polymer with specific rotation of –4.8°. The tert-butyl groups of poly(AltBAAm)s were removed by treatment with CF3SO3H, transforming into poly(N-allylacrylamide)s [poly(AlAAm)s]. Then, stereochemistry in the stereorepeating unit was investigated by comparing 1H NMR signals of the poly(AlAAm)s with those of model compounds, cis- and trans-3-ethyl-4-methyl-2-pyrrolidones. Taking the negative specific rotation values of the poly(AltBAAm)s obtained with L-tartrates into account, it was revealed that (3S,4S)-trans-unit was predominantly formed through the hydrogen-bond-assisted complex formation of AltBAAm with L-tartrates.