The Hofmann elimination is an interesting reaction that involves using an amine as a leaving group to create an alkene. The first step involves converting the amine into a tertiary amine, a good leaving group, using methyl iodide (CH3I). The second step involves treatment with silver oxide (Ag2O) and H2O, which together act as OH–, creating a base than can finish the elimination reaction. Interestingly enough, in asymmetric molecules, the least stable (and least substituted) alkene is the one that is formed, resulting in what is referred to as Hofmann’s Rule.
Another reaction that acts as an exception to a common rule is the hydroboration-oxidation reaction that can add H/OH across a double bond, but in an anti-Markovnikov fashion. This involves treating an alkene first with BH3(or similar compounds) which reacts with the double bond to add H and BH2 across the alkene. The second step is an oxidative workup with H2O2and H2O, effectively creating an OH– nucleophile that will substitute out the BH2.
A student carried out the Hofmann elimination using Compound 1 below with the intention of continuing the reaction through with the hydroboration-oxidation reaction to add a terminal hydroxyl. However, it turns out the reaction vessel had been cleaned improperly and contained some H2SO4 residue on its sides. This resulted in the unexpected byproduct Compound 3 in addition to the desired Compound 4.
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