Oligomerization Stereochemistry of Vinyl Monomers. 7. Diastereomeric Ion Pairs as Intermediates in the Stereoregular Anionic Oligomerization of 2-Vinylpyridines. A proposed Mechanism

Abstract

Oligomers of 2-vinylpyridine have been prepared by addition of 2-vinylpyridine to T H F solutions of alkali salts of 2-ethylpyridine followed by termination with CH31. With Li and Na as counterions, these oligomers were highly (>95%) isotactic as determined with 'H and 13C NMR. With K and larger counterions, the formation of dimers is not stereoselective. Epimerization of these types of compounds yields statistically expected mixtures of stereoisomers. Thus stereoselection appears kinetically controlled. Dimerization of 4-vinylpyridine or addition of kinylpyridine t o lithio-2-ethylpyridine followed by methylation is not stereoselective. The results particularly with the Li and Na salts indicate that the stereochemistry of methylation and 2-vinylpyridine addition is identical. A mechanism is proposed taking the chirality of the ion pair itself into account. For the Li and Na salts of the living oligomers, the cation appears to be coordinated with the nitrogen lone pair of the penultimate 2-pyridine unit. In such a case, one of the two possible diastereomeric ion pairs is expected to be favored, and this is most likely the reason for the observed stereoselectivity. Electrophilic attack is apparently occurring in a "syn" fashion.

Oligomerization of vinyl monomers 18. Stereochemistry of anionic oligomerization of 4‐vinylpyridine

“…The methyl resonances were also sensitive to stereochemistry in the 'H NMR spectrum, with the doublet for the meso compound occurring somewhat downfield from the racemic doublet, as observed in the 2-pyridyl analog. These NMR spectra are similar to those reported for the (m) and (r) stereoisomers of 2,4diphenylpentane(2,4-DPP)l0 and 2,4-di(2-pyridyl)pentane (2,2J4 (Table I).…”

“…For the tetramer terminated by protonation [eq. (2)] four stereoisomers are possible: mm, mr, rm, and rr. In the '3c NMR spectrum of this tetramer four methyl absorptions of approximately equal intensity, which corresponded to the four stereoisomers, were observed.…”

“…Oligomers may be conveniently isolated by preparative liquid chromatography'" or recycle-size exclusion chromatography,P8 and their stereochemistry may be determined accurately by 1 H 1 -3 , 5 9 9 -1 1 and 13CwJ2J3 nuclear magnetic resonance (NMR) gas chromatography (GC). ' We have already reported a study of the stereochemistry of the anionic oligomerization of 2-vinylpyridine,'s2 the results of which indicate that the high degree of stereoselectivity observed on methylation of oligomeric anions [2]- [4] is due to an intramolecular coordination of the alkali metal counterion by the nitrogen lone pair of the penultimate pyridine group: a 4-pyridyl group is present in the penultimate pasition and would be expected to produce oligomers of a stereochemistry different from that of the 2-pyridyl analogue. Therefore we extended our study to the anionic oligomerization of 4-vinylpyridine7 which is the subject of the present article.…”

Structure d'un tétramère de (pyridyl-2)-2 propène [tétraméthyl-2,4,6,8 tétra(pyridyl-2)-2,4,6,8 nonane]

“…Introduction. I1 est bien connu que les polym~risations et oligom~risations anioniques des vinyl-2 pyridines et monom~res analogues, amorc6es par les bases fortes en milieu faiblement polaire, aboutissent g~n6ralement ~t des produits pr&~rentiellement isotactiques* (Natta, Mazzanti, Longi, D'AII Asta & Birnardini, 1961;Matsuzaki, Uryu, Ishida & Takeguchi, 1967;Brigodiot, Cheradame, Fontanille & Vairon, 1976;Hogen-Esch & Tien, 1980). On a montr~ que ces r6sultats sont compatibles avec la ch61ation intra-mol6culaire du contre ion m6tallique par le doublet libre de l'atome donneur situ~ en fl du carbone m~thine asym&rique p~nulti~me (Cram & Kopecky, 1959;Bovey, 1969).…”

“…On a montr~ que ces r6sultats sont compatibles avec la ch61ation intra-mol6culaire du contre ion m6tallique par le doublet libre de l'atome donneur situ~ en fl du carbone m~thine asym&rique p~nulti~me (Cram & Kopecky, 1959;Bovey, 1969). Un tel processus de coordination aboutit pr&~rentiellement ~ la formation d'un cycle de type cyclohex~ne contenant l'ion m&allique, dans lequel le groupe m~thyle ou polym~re du centre d'asym6trie p~nulti~me est en position pseudo ~quatoriale (Hogen-Esch & Tien, 1980). L'attaque ~lectrophile sur le * si la configuration relative des centres chiraux est la m~me le long de la cha[ne, le polyrn6re est dit 'isotactique'.…”