ISSN:
0894-3230
Schlagwort(e):
Organic Chemistry
;
Physical Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
The alkylation, silylation and other electrophilic reactions of 1-lithio-1,3-bis(2-pyridyl)butane (2a) in THF at -78°C leads to meso-like products that may be accounted for by intramolecular coordination of Li ion with the penultimate 2-pyridyl group. This is also demonstrated by proton-transfer equilibria between 1-lithio-2-ethylpyridine and 2a, allowing the determination of the ΔH and ΔS of intramolecular chelation in 2a. The ΔS of chelation is nearly zero, probably as a result of chelation-induced desolvation of Li ion. Surprisingly, however, the corresponding addition of 2-vinylpyridine to 2a and higher P2VPLi homologs in THF is not stereoselective. In toluene at -78°C the t-BuLi-initiated polymerization of 2-VP is actually somewhat syndiotactic (rr = 0·44). The presence in this polymerization of at least one equivalent of t-BuOLi or other tertiary ROLi surprisingly leads to isotactic P2VP. SEC viscometric and other studies indicate that both the P2VPLi- and P2VPLi-ROLi complexes are monomeric in toluene. The stereochemistry is apparently due to a 1 : 1 P2VPLi-ROLi complex that discriminates between the two pro-chiral faces of the anion by preferntial coordination of the (ROLi2) moiety to the meso-prochiral face. Isotactic-like addition now occurs by electrophilic attack of 2-VP that is syn with respect to the (ROLi2) ‘counter ion.’
Zusätzliches Material:
2 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/poc.610080405
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