Solvent effect in the stereoselective polymerization of α-amino acid N-carboxyanhydride and some considerations on the mechanism of stereoregulation

✍ Scribed by Yutaka Hashimoto; Yukio Imanishi; Toshinobu Higashimura

• Publisher: Wiley (John Wiley & Sons)
• Year: 1978
• Tongue: English
• Weight: 656 KB
• Volume: 17
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1978.360171104

✦ Synopsis

In the polymerization of phenylalanine N-carboxyanhydride (NCA) in N02Ph initiated by MeNHBzl, L-, D-, and DL-NCAS were polymerized at the same rate, and no stereoselectivity was observed. When the same experiment was carried out in HCONEt2, however, L-and D-NCAS were both polymerized a t a rate which was about twice as large as that of DL-NCA. In this case, the polymerization is stereoselective, ascribable to a preferable reaction between the optical enantiomorphs of the terminal residue of the growing chain and the NCA of the same chirality. On the other hand, the polymerizations initiated by SarNMez and MeNH(CH2)2CONMe2 were stereoselective in NOzPh and HCONEtz, but they were not stereoselective in rn-(MeO)nPh. These findings indicate that the polymerizations initiated by a strong base in highly dipolar solvents are stereoselective. Apparently, the reaction brtween a chiral, cyclic terminal of growing chain and a chiral, cyclic activated NCA in the activated-NCA mechanism is highly stereoselective. In addition, from a kinetic investigation on the copolymerization between L-and D-NCAS, the penultimate chiral centers were also suggested to contribute to the stereoselection. Stereoselection by the a-helical conformation of the growing chain and by a chiral, linear terminal amine have been considered so far, and the contribution from the present type of stereoselection must have been overlooked.