Te. Hogenesch et al., STEREOCHEMICAL CONTROL IN ANIONIC-POLYMERIZATION OF 2-VINYLPYRIDINE AND RELATED VINYL MONOMERS, Journal of physical organic chemistry, 8(4), 1995, pp. 222-230
The alkylation, silylation and other electrophilic reactions of 1-lith
io-1,3-bis(2-pyridyl)butane (2a) in THF at -78 degrees C leads to meso
-like products that may be accounted for by intramolecular coordinatio
n of Li ion with the penultimate 2-pyridyl group, This is also demonst
rated by proton-transfer equilibria between 1-lithio-2-ethylpyridine a
nd 2a, allowing the determination of the Delta H and Delta S Of intram
olecular chelation in 2a, The Delta S of chelation is nearly zero, pro
bably as a result of chelation-induced desolvation of Li ion, Surprisi
ngly, however, the corresponding addition of 2-vinylpyridine to 2a and
higher P2VPLi homologs in THF is not stereoselective. In toluene at -
78 degrees C the t-BuLi-initiated polymerization of 2-VP is actually s
omewhat syndiotactic (rr=0.44). The presence in this polymerization of
at least one equivalent of t-BuOLi or other tertiary ROLi surprisingl
y leads to isotactic P2VP, SEC viscometric and other studies indicate
that both the P2VPLi- and P2VPLi-ROLi complexes are monomeric in tolue
ne, The stereochemistry is apparently due to a 1:1 P2VPLi-ROLi complex
that discriminates between the two pro chiral faces of the anion by p
referential coordination of the (ROLi(2)) moiety to the meso-prochiral
face, Isotactic-like addition now occurs by electrophilic attack of 2
-VP that is syn with respect to the (ROLi(2)) 'counter ion.'