Intramolecular chelation as a factor in the stereochemistry of anionic vinyl polymerization

Abstract

AbstractThe methylation (CH3I) of 1,3‐bis(2‐pyridyl)butyllithium in THF at −78°C is highly meso‐selective (>98%) but the selectivity decreases with increasing cation‐size or ‐coordination. The reaction of 1,3‐bis(2‐pyridyl)butyllithium with other electrophiles such as i‐C3H7Br, PhCH2Cl, Me3SiCl (CD3)2CO and 4‐vinylpyridine is also stereoselective under these conditions giving meso‐like products. On the other hand, addition of 2‐vinylpyridine is only slightly selective (64%) and this is consistent with the 65% meso content of P2VP formed by polymerization in the presence of Li ion. The chemistry of the above reactions is rationalized by intramolecular coordination of Li or other cations by the penultimate 2‐pyridyl group and this is supported by equilibria involving proton abstraction of 1,3‐bis(2‐pyridyl)butane and similar compounds by bases having Li, Na or K counterions. It is shown that the tendency for intramolecular chelation is highest for Li and lowest for K ion. Temperature dependence of the above equilibrium shows that intramolecular chelation of Li and Na ions is exothermic (−1.4 and −1.3 kcal respectively) whereas the AH for K ion is very small (−0.5 kcal). Entropies of chelation are slightly positive for Li (0.8 e.u.) and negative for Na and K ions (‐2.60 and −0.40 e.u. respectively). The lack of stereoregular polymerization of 2‐VP in the presence of Li ion is most likely due to the requirement that the Li ion of the newly formed 2‐pyridyl anion is coordinated with the 2‐pyridyl group of the previous asymmetric center. Thus it would appear that intramolecular coordination of metal ion by penultimate 2‐pyridine does not necessarily lead to isotactic‐polymerization.