Q2(c). Acetophenone pinacol and the pinacol–pinacolone rearrangement

Introduction. Magnesium transfers one electron to the C=O of acetophenone to give a ketyl radical anion. Two ketyls couple at carbon (bimolecular reduction, pinacol coupling). Acid then converts the 1,2-diol into a ketone by a 1,2-shift: the pinacol–pinacolone rearrangement.

Scheme: Two ketyl radicals couple tail to tail; the chelated Mg alkoxide is hydrolysed.

I and J

1. Mg gives one electron to each of two ketone molecules: PhC(Me)=O + e⁻ → PhC•(Me)-O⁻. The radicals couple (C–C bond) while Mg²+ holds both oxygens: I = magnesium 2,3-diphenylbutane-2,3-diolate (five-membered chelate).
2. Hydrolysis gives J = 2,3-diphenylbutane-2,3-diol (C₁₆H₁₈O₂), as the meso and (dl) pair.

K: pinacol–pinacolone rearrangement

Scheme: Protonation, loss of water, phenyl migration to the cation, loss of H⁺.

1. Cold conc. H₂SO₄ protonates one OH; loss of water gives a tertiary, benzylic carbocation (rate-determining).
2. A group on the neighbouring carbinol carbon shifts with its electron pair. Phenyl migrates in preference to methyl: migratory aptitude Ph > Me, because the shift passes through a bridged phenonium-like transition state.
3. The positive charge moves to the carbon bearing OH, where it is stabilised as an oxocarbenium ion (C=O+H); loss of H⁺ gives the ketone.
4. K = 3,3-diphenylbutan-2-one, CH₃COC(C₆H₅)₂CH₃ (C₁₆H₁₆O).

Conclusion. I = Mg pinacolate, J = 2,3-diphenylbutane-2,3-diol, K = 3,3-diphenylbutan-2-one. The rearrangement step is the pinacol–pinacolone rearrangement (phenyl 1,2-shift to a carbocation).

Application. The same rearrangement of pinacol gives pinacolone (tert-butyl methyl ketone), an intermediate for the fungicide triadimefon; 1,2-shifts of this type also build spiro ketones from cyclic diols.