Stereocontrol in radical-mediated allylation of acyclic α-bromo-β-siloxy esters by complexation with lanthanide shift reagents Ln(fod)3

Abstract

Stereoselectivity in the radical-mediated allylation of α-bromo-β-siloxy esters 2 yielding α-allyl-β-siloxy esters 3 (syn) and 4 (anti) was remarkably affected when the reaction was conducted in the presence of Ln(fod)₃[= tris-(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionato)lanthanide]. In the allylation of α-bromo-β-siloxysuccinate esters 2c and 2d affording preferentially syn-diastereoisomers 3c and 3d through chelated transition states a stoichiometric amount of the Lewis acid [Eu(fod)₃ or La(fod)₃] was required in order to maximize the stereoselectivities, whereas in the reaction of α-bromo-β-siloxybutanoate esters 2g and 2h and α-bromo-β-siloxy-β-phenylpropanoate ester 2i the effect induced by the coordination of Eu(fod)₃ to the ester group was catalytic.