Pinacol-pinacoloneRearrangements♦ Conversion of an alcohol having two adjacent OH groups (pinacol) to a ketone (pinacolone) by the action of acid
Mechanism ♦ Mechanism of the pinacol rearrangement of a symmetrical glycol CH3 CH3 CH3 CH3 _ OH CH3 CH H2SO4 2 3H3C CH3 H3C CH3 H3C + OH OH OH OH2 OH CH3 + 1. Protonation of 2. Loss of water pinacol a hydroxyl group 3. Methyl migration 4. Deprotonation H3C CH3 H3C CH3 H3C CH3 + CH3 CH3 _ + CH3 .. O H H O+ H O CH3 CH3 CH3 pinacolone Resonance-stablized carbocation
Facts♦ 1, 2 shift takes place in carbonium ion which is already a tertiary carbonium ion♦ It is probably due to stabilization of rearranged ion that can be affected by delocalization involving the electron pair on the oxygen atom, and ready loss of proton from the oxygen to produce a stable end product
The stable cation formed superior forstabilization Ph2C CH2 H+ Ph C CH2 + 2+ Ph2CH CH OH OH OH OH Ph2CH CH +Ph2CH CH +OH H O
Bromohydrin to pinacolone
Hydroxyamine to pinacolone
Facts♦ It seems that migration of alkyl group is extremely fast on the loss of leaving group, taking place simultaneously♦ That migrating group is from the opposite side of the leaving group – Because rearrangement is much more easy in compounds having trans configuration than that having cis configuration
Pinacol rearrangement of an asymmetricalglycol Me Ph Me Ph O Me H2SO4HO OH HO + Ph Me Ph Me Ph Me Ph A B C Me Ph + OH Me Ph D Et Ph Et Ph O Et H2SO4HO OH HO + _ + Ph Ph H Et Ph Et Et Ph E F
Which group migrates? H+ PhHC CHPhPhHC CHPh + OH OH OH +Ph2CH CH Ph2CH CH + Ph2CH CH +OH H OH O Phenyl group move first because of greater chances of stabilization due to delocalization
Which group migrates?
♦ The steric chemistry of Pinacol RearrangementsIt is found that o-MeOPh migratemore than a 1000 time less readilythat p-substituent and even muchless than phenyl. This is due tointerference in the transition statewith non-migrating group
Solved problem Explain the following experimental facts ： R R R R R C R + H2SO4 O O OH OH (A) (B) (C)When R=CH3, （ B ） and （ C ） are formed ；When R=Ph, only （ C ） is formed.
When R=Me R R + C R C R +O H 2 OH OH (1) (2) R R + + -H C R R C R +H+ :O H O C R (3) (B) OH OH R R (A) + C R C R + OH2 OH (4) OH (5) R + R -H + R R :O H (6) O (C)