Wittig rearrangement of lithiated allyl aryl ethers: a mechanistic study

A -75 DegC, alpha-lithiated allyl Ph ether undergoes mainly the 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-Bu substituent in the para position of the arom. ring and the complete absence of rearrangement products suggest an intramol. addn./elimination process bringing about the aryl migration. The first step, a nucleophilic attack of the alpha- 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. [on SciFinder (R)]