Template-bound radical

These data can be recast to expose the differing effects of solvent on yield suppression with each terminator (Scheme 8-30). With either 106 or 107, the yield of macrocycle 94 was greater in benzene than in methyl isobutyrate. Thus, intermolecular trapping of an intermediate template-bound radical is less significant in benzene than in methyl isobutyrate. In these experiments, the differences are large (-20%) and suggest that a template-bound radical (e.g., 99) is not as accessible to either added terminator, 106 or 107, in benzene as it is in methyl isobutyrate. 

Scheme 8-2 Oligomerization control via template-bound monomers. I =radical initiator, T-1 = chain transfer agent.

Scheme 8-3 Oligomerization control via template-bound initiator and terminator. I = radical initiator, T = radical terminator.

This curious observation refocuses concerns about yield loss to a critical choice that a template-bound pMMA radical 99 faces macrocyclization to afford the controlled-... 

Let us consider the case in which monomer is strongly bound to template. Initiation proceeds in a different way below and above the critical concentration of the template used. If concentration of the template is small and coils of the template are separated, primary radicals appear mainly outside the coils and then migrate to the sur-... 

Diaiyl sulfide templates have also been used to direct chlorinations. The selectivities indicate that the chlorine atom is bound to the sulfur, but the yields are not as good as those with aryl iodide templates. The problem is that the sulfur gets oxidized under the reaction conditions. As expected, a thiophene ring is more stable to oxidation and its sulfur atom can still bind chlorine in a radical relay process." The best sulfur template so far examined is the thioxanthone system (Scheme 20). Thus with 3 equiv. PhICh compound (18) undergoes directed C-9 chlorination in 100% conversion, affording a 71% yield of the A iO-alkene sifter base treatment, along with some polar products from excessive chlorination. The thioxanthone template can be recovered unchanged. 

One approach to oligomer control in a free-radical polymerization utilizes bound monomers and relies on templated radical macrocyclization reactions. Successful execution of this strategy requires that cyclotelomerization effectively compete with intermo-lecular chain transfer. Scheme 8-2 in Section 8.1 depicts this chemistry schematically wherein radical addition (A), cyclization (C), and chain transfer (T) provide an =3 telomer. The key macrocyclizations (cyclotelomerizations) must precede chain transfer. These transformations are well precedented by systematic investigations of free-radical macrocyclizations that appeared in the 1980s [19-23] and by the seminal contributions of Kammerer, Scheme 8-4 [24-34]. 

The templates can be covalently bound to the electrophores/substrates and serve as tethers or are more weakly attached to the reaction partners by coordinative bonds. In electroorganic synthesis, this concept was successfully applied and demonstrated in several cases. The templates work if in the course of the redox transformation either a compensation of charge is achieved or a radical center is stabilized in the neighborhood... 

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