Enantioselective Conjugate Additions of Radicals

Transfer of axial chirality in radical cyclization reported by Curran [11] 

Although the larger body of work in the area of conjugate additions involves carbanionic intermediates, some remarkable examples of enantioselective addition of radicals have also been documented. Stereocontrol is effected through the use of a chiral Lewis acid that activates the electrophilic acceptor [38, 47). Sibi has reported that acceptor 212 undergoes enantioselective addition of iso-propyl radical in the presence of Mgl and bisoxazoline 213 to give 214 in 97% ee and 91% yield (Equation 40) [160]. A clear correlation between the catalyst loading and the level of asymmetric induction was observed, with 30 mol % identified as optimal. 

Interestingly, a-substituted acceptors such as 215 can be employed to furnish the anti product 216 selectively as a result of diastereoselective reduction of the Ccx radical generated in the conjugate addition (Equation 41) [161]. The model substrate-catalyst complex 217 has been formulated to account for the high enantioselectivity of the process, entailing addition of the radical from the exposed top face of the chelated substrate [161, 162]. The high diastereoselectivity observed for the formation of the anti adduct 216 (dr= 99 1) has been suggested to be controlled by the j3-t-Bu substituent [161, 163]. 

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