Free radicals and Lewis acid. Chelation-controlled radical allylations of substituted α-halo- or α-phenylseleno-β-alkoxy esters. The endocyclic effect.

Abstract

The radical allylation of a series of α-halo or α-phenylseleno-β-alkoxy esters in the presence of MgBr2·OEt2 is reported and compared with analogous reactions under non-chelating conditions. The addition of MgBr2·OEt2 gives excellent selectivity favoring anti products; in some cases ratios >100:1 are obtained. Varying the substrate substituents reveals that these reactions are quite tolerant of alkyl functionalities at the β-position. Changes to the alkoxy function indicate that a chelate is involved in the reaction. The reactions are successful with secondary iodides, bromides, and phenylselenides, as well as tertiary iodides, which all give very high ratios under chelation control. Performing less well under the same conditions are substrates with a radical exocyclic to a tetrahydrofuran ring. EDTA titration is used to determine the amount of Mg2+ dissolved in the allylation reaction mixture, and 13C NMR is employed to better define the nature of the complex formed (chelate or monodentate) prior to the reaction. Competition experiments suggest that the chelate and monodentate pathways are in competition for the radical allylation with allyltributyltin.Key words: allylation, radicals, Lewis acid, stereoselectivity, 1,2-induction.