16 Ion Pair Structure and Stereochemistry in Anionic Oligomerization and Polymerization of Some Vinyl Monomers T . E . H O G E N - E S C H , W . L . J E N K I N S , R . A . S M I T H , and C . F . T I E N
Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch016
Chemistry Department, University of Florida, Gainesville, FL 32611
We have recently (1,2) reported the stereoregular anionic oligomerization of 2-vinylpyridine and various alkylacrylates in THF and similar low dielectric constant media (Eq. 1):
a) R = 2-pyridyl, b) R = 4-pyridyl, c) R = carboalkoxy. [2],[6] n = 1; [3],[7], n = 2; [4],[8] n = 3; [5],[9] n > 4. M = Li, Na, Κ, Rb One of the interesting features of these reactions is the notent i a l for comparing the stereochemistry of oligomerization with that of alkylation protonation or other organic reactions of general interest. For instance, Table 1 shows that the methyla tion of the Li and Na salts of anion [2a] is highly stereo selective, Table 1 Methylation Stereochemistry of Anion [3] As a Function of Cation, and Solvent or Coordinating Agent f
Cation
Solvent/Coordinating Agent
Electrophyle T°C
% Meso
Li
THF
CH I
-78°
>99
Li
THF
CH3I
-30°
98
Li
THF
CH3I
0°
95
CH3I
-78°
96
Na
THF
3
0097-6156/81/0166-0231$05.00/0
© 1981 American Chemical Society
McGrath; Anionic Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
ANIONIC POLYMERIZATION
232
Table 1 (Cont'd.)
Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: November 30, 1981 | doi: 10.1021/bk-1981-0166.ch016
Cation
a. c. e. g.
Solvent/Coordinating Agent
Electrophyle
T°C
CH I
-78°
70
CH3I
-78°
57
CH3I
-78°
83
CH3I
-78°
93
CH3I
-78°
92
CH3I
-78°
>99
-78°
60
-78°
--e
-78°
>99
-78°
58
Κ
THF
Rb
THF
Li
THF/Pyridine
Li
THF/TG
Li
THF/12-crown-4
Li
Toluene
Li
Toluene
CH3I
Li
THF
CH S0 F
Li
THF
Na
THF/18-crown-6
3
a
b
c
d
g
3
2
(CH ) S0 3
2
4
CH3I
c
% Meso
50% by volume, b. Triglyme 20% excess over carbanion, 20% excess of crown ether, d. Trace of THF present, Mixture of pentane and butane products, f. About .2M Ethyl carbanion salt prepared in toluene without the presence of THF.
but the stereoselectivity decreases sharply with increasing cation size and -coordination. The results also suggest that the methylation stereochemistry is kinetically determined, and this is confirmed by epimerization (k-tBuO/DMSO) of the mesopentane which yields approximately equal proportions of the meso- and racemic 2,4-di-(2-pyridyl)pentane. The place of sub stitution in both the pyridine rings of [2] is a key factor in the stereoselectivity. The methylation of the Li salt of [2b] does yield approximately equal proportions of meso and racemic [6b] under these conditions, and the methylation of the 1lithio-1-(4-pyridyl)-3(2-pyridyl)butane likewise yields a 50/50 mixture of diastereomers. A comparison of the stereochemistry of the 2-vinylpyridine addition to [2a] with that of methylation addition is particu l a r l y instructive. Table 2 shows the stereochemistry of forma tion of [7a] as a function of counterion size and coordination. Table 2 Stereochemistry of Formation of 2,4,6-Tri(2-pyridyl)-heptane As a Function of Cation and Cation Coordination in THF at -78°C I(m,m) Li, THF Na, THF
d
>95(98.0)
a
>95(92.2)
Na, THF; Na, Ce, THF
b
H(mr)
