✍ Scribed by Anna-Reine Fhal; Philippe Renaud
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We report here the first examples of enantioselective radical alkylations directed by chiral Lewis acids where complexated radicals are directly generated by a carbon-halogen bond homolysis. N-(2-Iodopropionyl)oxazolidinones are allylated with allylstannane in the presence of chiral aluminum based Lewis acids prepared from Me3AI and chiral diols/diamides ligands. The observed enantioselectivities are still modest (< 34% ee). By analogy to cycloaddition reactions, a model is proposed to rationalize the sense of the enantioselectivity. © 1997 Elsevier Science Ltd.
Chiral Lewis acids have been used for highly enantioselective Diels-Alder reactions using for instance 3acryloyl-1,3-oxazolidin-2-one 1 as dienophile. 1,2 In order to control the stereoselectivity, the Lewis acids should favor one geometry of the dienophile 1 (see possible rotation indicated by arrows) and mask one face of the alkene moiety. In our approach to devise enantioselective radical reactions, 3 we decided, by analogy of cycloaddition reactions, to investigate radicals of type 2.
The s-cis/s-trans isomerism of these radicals and the stereofacial approach are expected to be also controlled by Lewis acids. During our previous work, we have observed that aluminum derivatives are particularly efficient in radical reactions. 4 Thus, we decided to focus first on chiral Lewis acids containing aluminum as metal center.
Very high level of stereoselectivity have been observed in Diels-Alder reactions with this particular type of Lewis acids 5'6 and a model based on complexation of a single carbonyl group has been proposed. 5c
The radical precursors 3 and 4 have been prepared from 1,3-oxazolidin-2-one and 4,4-dimethyl-l,3oxazolidin-2-one by acylation (BuLi/CH3CHBrCOCI) and subsequent Finkelstein reaction (NaI/acetone).
Different chiral Lewis acids of general formula MeAI[R*(X)2] (R*(XH)2 = chiral diol or chiral diamide) have been prepared by mixing Me3AI with the ligands 5-7.
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