Migrations to Carbonyl Carbon

Liebig observed the first intramolecular rearrangement in 1838 when he found that benzil in basic solution forms a new compound.136 In 1870 Jena correctly established the product of the reaction as benzilic acid, but proposed an incorrect structure for the starting material to avoid postulating a skeletal rearrangement.137 In 1928 Ingold proposed the mechanism shown in Equation 6.50, which today is solidly supported by experimental evidence.138 

An interesting aspect of this rearrangement is that the phenyl group with the lower electron-donating ability usually migrates. For example, in 104 the 

If the second step is rate-determining, then the observed rate is given in Equation 6.51, 

If the substituted phenyl is to migrate, then the intermediate 103a must be formed migration of the phenyl requires 103b. Electron-withdrawing substituents will increase K1 for the formation of 103a if Kx is increased more than k2 is decreased, more substituted phenyl will migrate than unsubstituted phenyl. 

Our consideration of rearrangements to electron-deficient heteroatoms must be brief. In discussing migrations to electron-deficient nitrogen, we first discuss three rearrangements that occur in carbonyl derivatives, the Beckmann, Hofmann, and Schmidt rearrangements, and then consider rearrangements of nitrenium ions. 

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In the reaction scheme proposed, the Lewis acidity of the reductant is stressed, and it is suggested that the initial interaction involves BH3 coordination at the carbonyl oxygen followed by hydride migration to carbonyl carbon to give a species such as 5 ... 

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