Oligoselectivity

It should be noted that the ACT sequence and the standard telomer assay were employed to study oligoselectivity with this template and thus the products analyzed are the same methyl acrylate telomers identified in the previous studies. The ACT reaction of 40 under standard conditions with cyclohexyl iodide and allyltributyltin (2.5 /80V200 "), without the aid of a counter-ion, showed interesting results. The product histogram obtained with 40 after telomer assay is presented in Scheme 8-15. 

Template-imposed oligoselectivity in (anionic) group transfer polymerization was also of interest with this template system. A suitable initiator-terminator pair for effecting this chemistry was proposed to be the trimethylsilylacetate/fluoromethacrylate species shown in the (NBD)3-derived template 69 (Scheme 8-21). Under fluoride catalysis. 

Scheme 8-24 Oligoselective free-radical polymerization of MMA with the (NBDli-derived template effects of variation in solvent and radical leaving group.

The stereochemical outcome of this oligoselective polymerization is of some interest. In principle, eight stereoisomeric macrocycles 94 can be formed. However, HPLC analysis of the cyclized material revealed that only six of these possibilities are represented in the product mixture. In benzene as solvent, over half of the product mixture is a single stereoisomer, whereas in methyl isobutyrate as solvent the diastereomers are more evenly distributed. Preliminaty attempts to ascertain the relative stereochemistry of the major isomer within 94 via DNOE NMR measurements did not allow unambiguous assignment. Without this structural information in hand, further speculation on the relationship between chain stereochemistry and cyclization efficiency within 99 (see Scheme 8-27) is not warranted. Nevertheless, there must be some influence, given the non-statistical distribution of isomers. In comparison, the H-NMR spectrum of the pMMA portion of uncontrolled oligomer 95 is superimposable with that of atactic (i.e., random stereochemistry) pMMA. 

Finally, the ability of the silylacetate, fluoromethacrylate-bearing template 69 to enforce oligoselectivity on the group transfer polymerization of MMA was examined. However, no fluoride-initiated polymerization conditions could be identified which led to template-bound oligomer. Typically, off-template initiation (F"-i-MMA ) furnished uncontrolled polymer. In these cases, the template 69 suffered simple desilylation to deliver the acetate 70. Perhaps a template-bound silyl ketene acetal initiator would have performed in a more desirable manner, but that species remained elusive. 

Scheme 8-27 Presumed mechanistic course of the template mediated oligoselective polymerization of MMA.

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