The radical allylation of a series of β-alkoxy
esters using allyltrimethylsilane in the presence of
MgBr2·
OEt2 is described. Under bidentate chelation-controlled
conditions, allyltrimethylsilane rivals allyltributyltin
in
efficiency and is a superior reagent from ecological and practical
perspectives. The reactions work with iodides and
bromides as well as phenylselenides. The isolation of
γ-phenylseleno intermediates indicates that the reaction
proceeds
by an atom transfer process. These reactions require initiation
with Et3B and can be inhibited by galvinoxyl, m-
and
p-dinitrobenzene indicating that this atom transfer sequence
involves the intermediacy of radicals.
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.
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