Catalytic Regioselective Isomerization of 2,2‐Disubstituted Oxetanes to Homoallylic Alcohols

The selective isomerization of strained heterocyclic compounds is an important tool in organic synthesis. An unprecedented regioselective isomerization of 2,2‐disubstituted oxetanes into homoallylic alcohols is described. The use of tris(pentafluorophenyl)borane (B(C6F5)3), a commercially available Lewis acid was key to obtaining good yields and selectivities since other Lewis acids afforded mixtures of isomers and substantial polymerization. The reaction took place under exceptionally mild reaction conditions and very low catalyst loading (0.5 mol %). DFT calculations disclose the mechanistic features of the isomerization and account for the high selectivity displayed by the B(C6F5)3 catalyst. The synthetic applicability of the new reaction is demonstrated by the preparation of γ‐chiral alcohols using iridium‐catalyzed asymmetric hydrogenation. Ring strain: 2,2‐Disubstituted oxetanes, easily prepared by a double Corey–Chaykovsky reaction, readily isomerize into homoallylic alcohols using B(C6F5)3 as a catalyst. DFT calculations show a key intramolecular proton migration. The synthetic potential was proven by the synthesis of several chiral disubstituted alcohols, with high enantiomeric excesses, by asymmetric hydrogenation of the homoallylic alcohols.