Radical chain processes stereoselective additions

Eollowing a radical process, radiation induced chain addition of allylbenzene to 1,4-dioxane 16. Efficiency of the addition depends on the concentration of the monomer <1999JRN953>. Alcohols also react, albeit in low yields (10%), with 16 in the presence of (diacetoxyiodo)benzene, probably via a radical pathway, to afford 2-alkoxy-l,4-dioxanes <2004SL2291 >. Free radicals have also been generated by decarboxylation of dimethoxydioxanecarboxylic acids 101 and added to some maleimides and acrylates with high stereoselectivity (Scheme 9) <20020L2035>. 

Generally, free radical addition reactions or homolytic chain propagation reactions are not stereoselective. However, Okamoto determined that Lewis acids can induce stereocontrol in the RP of acrylic monomers. Conditions for CRP in the presence of Lewis acids were developed. Therefore, all three major CRP processes, ATRP, RAFT, and NMP, were investigated... 

To develop better stereocontrol agents, it is crucial to understand stereoselection at a fundamental mechanistic level and appreciate the subtle but crucial differences between tacticity determination in ionic and coordination versus radical processes. As previously outlined, the propagation step of Hving radical polymerization is identical to that of free-radical polymerization and so the mechanism of tacticity determination in these processes is also identical. In radical polymerization, the planar (sp hybridized) carbon radical of the polymer terminus is prochiral, and its stereochemistry is determined during subsequent monomer addition. The addition of monomer occurs exclusively to the anti-h.ce of the radical, as the syn-face is shielded by the steric bulk of the penultimate group (Scheme 2). Hence, the prochiral polymer terminus can adopt either pro-meso or pro-racemo arrangements defined by the relative orientation of the terminal and penultimate side-chains with respect to the macromolecular C—C backbone. 

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