Copolymerization of N-propargylphosphonamidates. Helicity control of the copolymers by P-chirality

The copolymerization of diastereomeric pairs of N-propargylphosphonamidate monomers 1–3 ((1R,2S,5R)-HCCCH2NHP(O)R-O-menthyl, 1: R = CH3, 2: R = C2H5, 3: R = n-C3H7) carrying P- and C-chiral centers was carried out with (nbd)Rh+[η6-C6H5B−(C6H5)3] as a catalyst in CHCl3 for 24 h to afford the copolymers with number-average molecular weights ranging from 1400 to 12 000 in 65–85% yields. The cis contents of the copolymers were above 80%. The copolymers consisting of a pair of diastereomeric monomer units with a large diastereomeric excess showed a large specific rotation and an intense Cotton effect in CHCl3, indicating that they take a helical conformation with predominantly one-handed screw sense. On the other hand, the copolymer with a small diastereomeric excess showed a small specific rotation and a CD signal. The copolymerization of P-(R)-1 (1a) with an achiral N-propargylphosphoramidate monomer, HCCCH2NHP(O)(OPh)2 (4) was also carried out with various feed ratios. Among the obtained copolymers with various compositions, poly(1a95-co-45) showed the largest specific rotation and CD intensity in CHCl3, and poly(1a50-co-450) did so in THF. The copolymers decreased the CD intensity upon raising the temperature.