Isotactic telomer

Scheme 8-12 ACT reaction maximizing formation of the = 4 isotactic telomer.

An investigation into the preparation of n = 2 telomers was successful in showing that the ACT strategy with templates of type 14 is a viable means for producing isotactic 1,3 stereocenters as exemplified in the production of 16 (n=2) (Scheme 8-5). The oxazolidine unit has documented success as a stereocontrol element in acyclic radical reactions [35-37], and thus its incorporation into this template provides, in effect, a chiral auxiliary to control the configuration of new stereogenic centers formed in the sequence. 

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

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