Asymmetric Polymerization and Oligomerization of 3-Phenylpropanal with Grignard Reagent−(−)-Sparteine Complexes with Termination by Tishchenko Reaction

3-Phenylpropanal (3-PPA) was polymerized with Grignard reagent−(−)-sparteine complexes, such as ethylmagnesium bromide−(−)-sparteine (EtMgBr−Sp) and n -octylmagnesium bromide−(−)-sparteine (OctMgBr-SP) complexes, in toluene at low temperature. The poly(3-PPA) obtained showed optical activity with negative rotation ([α] 25 365 −33° to −56°), which may be based on a predominant one-handed helical conformation of the main chain, and also exhibited a signal due to an ester carbonyl group at 1738 cm -1 in its IR spectrum. This means that the polymer has an ester group at the ω-end which is formed by a Tishchenko-type termination reaction between the growing chain end and the 3-PPA monomer. The poly(3-PPA) is stable at room temperature, whereas the poly(3-PPA) obtained with EtMgBr alone at −78 °C degrades slowly even in the solid state to 3-PPA monomer at room temperature. 1 H and 13 C NMR spectra of the poly(3-PPA) showed rather sharp resonances, suggesting that the polymer may be stereoregular, although the tacticity is not clear. In order to determine the stereostructures of the poly(3-PPA) and the polymerization mechanism, the oligomerization of 3-PPA was carried out with the EtMgBr−Sp and OctMgBr−Sp complexes in toluene at −78 °C. The oligomers of 3-PPA were isolated by fractionation using GPC, HPLC, and supercritical fluid chromatography techniques. The main fraction was 3-phenylpropyl 3-phenylpropanoate, and the other oligomers were found to have a 3-phenylpropoxy group at the α-end and a (2-phenylethyl)carbonyl group at the ω-end, while a very small amount of oligomers contained an octyl group at the α-end. These results clearly indicate that the Grignard reagents are not the real initiator of most of the molecules but that the (1-phenylpropoxy)magnesium bromide formed by the Tishchenko-type termination reaction between the oligomeric growing chain end and the 3-PPA monomer mainly initiated the polymerization of 3-PPA. The propagation should be terminated by the Tishchenko reaction again to afford the poly(3-PPA) having the ester terminal ω-end and (3-phenylpropoxy)magnesium bromide as a cyclic mechanism. The 2-mer consisted of meso and racemo diastereomers, and the ratio was determined to be 85/15 by 1 H and 13 C NMR spectroscopies. Both the 1- and 2-mers were successfully resolved into optical isomers by chiral HPLC separation using cellulose tris[(3,5-dimethylphenyl)carbamate] as a chiral stationary phase. On the basis of these results, the mechanism of the polymerization of 3-PPA was elucidated.