Chapman rearrangement

Whis is Chapman rearrangement?

Mumm et al. reported the occurrence of a reaction in 1915, which was subsequently studied extensively by Chapman in the 1920s and 1930s. This reaction involves the thermal rearrangement of aryl imidates into N,N-diaryl amides and is commonly known as the Chapman rearrangement. It is occasionally referred to as the Beckmann-Chapman rearrangement as well.

Chapman rearrangement - general reaction scheme - Beckmann-Chapman rearrangement
Chapman rearrangement

The Chapman rearrangement follows approximately first-order kinetics and exhibits high yields when carried out in either polar or nonpolar solvents. However, the presence of polar solvents accelerates the reaction. Interestingly, the Chapman rearrangement can even take place in a solid state at a slower rate compared to its solution counterpart.

The Chapman rearrangement is classified as an intramolecular nucleophilic aromatic substitution, where the migration of the aryl ring attached to oxygen occurs in a stereospecific manner. The presence of ortho substituents on the aryl ring connected to the oxygen atom enhances the migration rate due to steric acceleration resulting from hindered rotation (SAHR) effect. This effect arises because free rotation restricts the formation of a four-membered ring in the transition state.

Moreover, it has been observed that the migration rate is increased by a para electron-donating group, while a para electron-withdrawing group (such as Cl or NO2) decreases the reaction rate. The Chapman rearrangement has been utilized for the synthesis of substituted N-phenylanthranilic acids.

Notably, if the migrating aryl moiety possesses an α-acidic hydrogen at the ortho position, an “abnormalChapman rearrangement takes place.

References

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