Unambiguous Proof for Alcoxycarbonyl‐group Migration in Wagner‐Meerwein Rearrangements

In HSO 3 F/SO 2 ClF the β‐hydroxy esters Ph‐CHOH‐CMe 2 ‐COOR ( 1 , RMe, Et) are doubly protonated, then transformed into the fluorosulfates 7 and (partly) into the fluorides 8. At −15°, both 7 and 8 undergo a rearrangement, forming derivatives of Me 2 CC(Ph)COOR ( 2 ). By labelling 1 with 13 C, singly ( 13 C(3)) and doubly ( 13 C(1,3)), it could be shown that exclusively the ROOC groups undergo a 1,2‐shift. Compound 2 is also formed in HSO 3 F/SO 2 ClF from the isomeric Me 2 COH‐CHPh‐COOR ( 3 ) by elimination, and less easily from the α‐hydroxy ester Ph‐CMe 2 ‐CHOH‐COOR (5) via a phenyl 1,2‐shift. Another isomer, Ph‐C(OH)Me‐CHMe‐COOR (4) gives products different from 2 . Using more acidic systems containing SbF 5 , the free carbenium ions 13 (Ph‐CH + ‐CMe 2 ‐COOR) can be stabilized; they do not form 2 , possibly because of complexation of the ester group with SbF 5 . The energy profile and the mechanism of the rearrangement 1 → 2 are discussed.