von Auwers rearrangement

What is von Auwers rearrangement?

The von Auwers rearrangement is a reaction that involves the thermal or acid-promoted rearrangement of alkylidene-4-(dichloromethyl)-4-methyl cyclohexadienes to produce corresponding benzene derivatives. The reaction was first reported by von Auwers (University of Marburg) in 1922 and is named after him.

von Auwers rearrangement - general reaction scheme
von Auwers rearrangement

X = CHCl2, CCl3, etc.
R = H, CO2H, alkyl, aryl (see list of acronyms)

The starting material for this reaction can be easily prepared by using a suitable o– or p-alkyl phenol with chloroform CHCl3 and alkali under the conditions of the Reimer-Tiemann reaction, which converts the cyclohexadienone made from the p-substituted phenol into a carbinol by the Grignard reaction. Alternatively, the starting material can also be obtained from the nucleophilic addition of a carbanion to the Zincke-Suhl product followed by dehydration.

The activation energy for this reaction is estimated to be ∼40 Kcal/mol. The driving force for the von Auwers rearrangement is likely attributed to aromatization. It has been observed that the von Auwers rearrangement usually involves either a 1:2 shift of the methyl group or a 1:3 or 1:5 shift of the trichloromethyl group, with only one exception of the 1:2 shift for the trichloromethyl group.

One example of this reaction involves the conversion of 1-phenyl-4-methyl-4-trichloromethylcyclohexadienol into p-ethylbiphenyl by the treatment with formic acid at low temperature. Additionally, 3-methyl-4-trichloromethylbiphenyl and 2-methyl-4-phenylbenzoic acid are also obtained from this rearrangement.

von Auwers rearrangement usually requires an induction period, which can be reduced by adding benzoyl peroxide. However, the rearrangement can be inhibited in the presence of duroquinone, indicating the involvement of radical species in this reaction.

References

von Auwers, K. and Jülicher, W., Ber., 1922, 55, 2167

Shares