Bis(oxazoline)–ligand-mediated asymmetric [2,3]-Wittig rearrangement of benzyl ethers: reaction mechanism based on the hydrogen/deuterium exchange effect

We have investigated the mechanism of chiral induction in the asymmetric [2,3]-Wittig rearrangement of allyl benzyl ether in the presence of a bis(oxazoline) chiral ligand [(S,S)-Box–tBu] by comparing the reaction of both enantiomers of monodeuterated benzyl ether 1a–d. As a result, we found that chirality was induced via enantioselective deprotonation followed by efficient chirality transfer of the resulting chiral benzyl carbanion with the inversion of stereochemistry. It was revealed that the chiral ligand facilitates selective deprotonation as well as prevents the chiral carbanion from racemization. Moreover, we examined the effect of the o-methoxy substituent on the benzene ring.