Wittig Rearrangement of Lithiated Allyl Aryl Ethers: A Mechanistic Study

At –75 °C, α‐lithiated allyl phenyl ether undergoes mainly the [1,2] Wittig rearrangement to afford, after acidic hydrolysis, 1‐phenyl‐2‐propen‐1‐ol as the main product. A second metalation taking place at one of the ortho positions is the sole competing side reaction. Both, the significant decrease of the isomerization rate upon the introduction of a tert ‐butyl substituent in the para position of the aromatic ring and the complete absence of [1.4] rearrangement products suggest an intramolecular addition/elimination process bringing about the aryl migration. The first step, a nucleophilic attack of the α‐ to the ipso ‐carbon atom generates a spiro‐connected oxiranylidene‐cyclohexadienyllithium species. This short‐lived intermediate collapses to the final product, a lithium alkoxide, by the nucleofugal departure of the oxygen atom which simultaneously binds the metal atom.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)