Stereochemistry of nucleophilic substitution reactions of sterically rigid phosphoranes

The stereomutation barriers of diastereomeric spirophosphoranes, [o- O(CF 3 )(CH 3 )C ∗ C 6 H 4 ][o- O(CF 3 ) 2 C 6 H 4 ] P ∗ X (X=OMe, SMe) were determined to be high enough to freeze the mutation at ambient temperatures [ ΔG ≠ (298K)=25.0, 30.4 kcal mol −1 ; respectively]. The nucleophilic substitution reaction of SMe compounds with alkyllithium reagents resulted in inversion of configuration, whereas that of OMe compounds gave various ratios of inversion and retention products depending on the stereochemistry of the diastereomeric reactant phosphoranes and solvent. However, the use of OCH 2 CH 2 NMe 2 as substituent lead to almost exclusive formation of the retention product regardless of these conditions.