Some Synthetically Useful Carbonium-Ion Reactions

Skeletal rearrangements that accompany SnI substitution reactions will not be emphasized here, since a discussion of these types of reactions was presented in Part A, Chapter 5. Rather, the focus will be on reactions with special structural features that promote a particularly useful skeletal change. 

One such reaction is the acid-catalyzed conversion of diols to ketones. This reaction is sometimes referred to as the pinacol rearrangement. The classic example of the reaction is the conversion of 2,3-dimethylbutane-2,3-diol (pinacol) to methyl t-butyl ketone (pinacolone)  

The mechanism involves carbonium ion formation and substituent migration, assisted by electron release from the remaining hydroxyl group  

Another method for carrying out the same net rearrangement involves synthesis of a glycol monosulfonate ester. These compounds rearrange under the influence of 

Aminomethylcarbinols yield ketones when treated with nitrous acid. This reaction has been used synthetically to form ring-expanded cyclic ketones, a procedure known as the Tiffeneau-Demjanov reaction The diazotization procedure generates the same type of /3-hydroxycarbonium ion that is formed in the 

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