2,5‐Disubstituted Pyrrolidines as Chiral Auxiliaries in Radical Reactions: A Theoretical Approach

The radical addition to amides derived from chiral 2,5-disubstituted pyrrolidines has been theoretically studied by the use of density functional methods and the results compared with known experimental data. The results agree quite well with those obtained experimentally and allow the full rationalization of the factors influencing the diastereoselectivity. Steric effects are the main factors determining the selectivity, but electronic interactions can also be very important when the attacking alkene is α,β-conjugated, as in acrylic esters or acrylamides. Additions at the α- and β-positions of the amide chain in the auxiliary are subject to different rules, with the former usually yielding high diastereoselectivities both experimentally and theoretically, whereas the latter is theoretically predicted to occur with low selectivity. We fully rationalize these two opposite behaviours and suggest several ways to circumvent this limitation, thus strongly increasing the interest of this type of structure as chiral auxiliaries in radical reactions.