THE USE OF LEWIS-ACIDS IN RADICAL CHEMISTRY - CHELATION-CONTROLLED RADICAL REDUCTIONS OF SUBSTITUTED ALPHA-BROMO-BETA-ALKOXY ESTERS AND CHELATION-CONTROLLED RADICAL-ADDITION REACTIONS

The radical reduction of a series of alpha-bromo-beta-alkoxy esters un der chelation-controlled conditions is reported. Proceeding with high stereoselectivity in the presence of MgBr2. OEt2, reductions give acce ss to syn, products. Systematic variations in substrate substituents s how that these reactions tolerate a wide variety of alkyl functionalit ies at positions 2 and 3 and are relatively unaffected by the nature o f the ester group. Changes to the alkoxy function indicate that a bide ntate chelate is involved in the reaction and that an excess of MgBr2. OEt2 is required for optimum selectivity by favoring this species in preference to the anti-selective monodentate or nonchelated pathways. Competition experiments suggest that the monodentate pathway is kineti cally favored over the bidentate one. The suppressibility of the react ion by radical chain inhibitors and the need for initiation indicate t he intermediacy of radicals. Further support for this mechanism includ es both radical addition to alpha,beta-unsaturated esters and reductio n of bromides conducted in the presence of a Lewis acid.