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  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. This migration step is rate-determining.
|Published (Last):||22 September 2017|
|PDF File Size:||10.46 Mb|
|ePub File Size:||15.45 Mb|
|Price:||Free* [*Free Regsitration Required]|
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  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.
Benzilic Acid Rearrangement
Benzilic acid rearrangement