Asymmetric polymerization and oligomerization of 5-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(S-PPA) obtained showed optical activity with negative rotation ([alpha](25)(365)-33 degrees to -56 degrees), 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 omega-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 degrees C degrades slowly even in the solid state to 3-PPA monomer at room temperature. H-1 and C-13 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 S-PPA was carried out with the EtMgBr-Sp and OctMgBr-Sp complexes in toluene at -78 degrees 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 alpha-end and a (2-phenylethyl)carbonyl group at the omega-end, while a very small amount of oligomers contained an octyl group at the ct-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 omega-end and (3-phenylpropoxy)magnesium bromide as a cyclic mechanism. The 2-mer consisted of meso and raceme diastereomers, and the ratio was determined to be 85/15 by H-1 and C-13 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.