BENZILIC ACID REARRANGEMENT PDF

Reaction mechanism[ edit ] The reaction is a representative of 1,2-rearrangements. The long-established reaction mechanism was first proposed in its entirety by Christopher Kelk Ingold , and has been updated with in silico data [6] as outlined below. The reaction is second order overall in terms of rate, being first order in diketone and first order in base. A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to form the alkoxide 2. The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group.

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This diketone reaction is related to other rearrangements: the corresponding keto-aldehyde one alkyl group replaced by hydrogen rearranges in a Cannizzaro reaction , the corresponding 1,2-diol reacts in a pinacol rearrangement. Reaction mechanism The reaction is a representative of 1,2-rearrangements.

These rearrangements usually have migrating carbocations but this reaction is unusual because it involves a migrating carbanion.

The long established reaction mechanism updated with in silico data [2] is outlined in scheme 2. A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to the hydroxyl anion 2. The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group in a concerted step with reversion of the hydroxyl group back to the carbonyl group.

This sequence resembles a nucleophilic acyl substitution. Calculations show that when R is methyl the charge build-up on this group in the transition state can be as high as 0. Calculations show that an accurate description of the reaction sequence is possible with the participation of 4 water molecules taking responsibility for the stabilization of charge buildup.

They also provide a shuttle for the efficient transfer of one proton in the formation of intermediate 5. From a molecular orbital point of view this rearrangement may at a first glance not obvious. Contrary to a carbocationic rearrangement as in the Wagner-Meerwein rearrangement in which the empty carbocationic orbital interacts positively and symmetry allowed with the filled pi orbital HOMO of the central C-C bond situation A in scheme 3 , a filled carbanionic orbital should not be able to escape a symmetry forbidden MO overlap with the LUMO which is the empty antibonding pi orbital having one node situation B.

In reality a 1,2-diketone LUMO is a 4 electron system without any nodes in the central C-C bond and a symmetry allowed transition is possible Situation C. References Liebig, J. Annalen der Chemie Further reading Donald A. Ballard and William M. Dehn Category : Rearrangement reactions.

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