Fluorine center dot center dot center dot and pi center dot center dot center dot Alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: An experimental and theoretical study

The alpha-alkylation of amide enolates by using a pseudo-C-2 symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (> 99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F center dot center dot center dot metal and pi center dot center dot center dot metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 k cal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F center dot center dot center dot metal and pi center dot center dot center dot metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 k cal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.