Sommelet-Hauser rearrangement

What is Sommelet-Hauser rearrangement?

The Sommelet-Hauser rearrangement, which involves the intramolecular [2,3]-sigmatropic rearrangement of benzyl quaternary ammonium salt (particularly, benzyl dimethylalkyl ammonium salts) to orthosubstituted benzyl dialkylamines using a strong base like alkali metal amide, was first documented by Sommelet in 1937.

Sommelet-Hauser rearrangement - general reaction scheme - Sommelet rearrangement
Sommelet-Hauser rearrangement

Hauser later extended this reaction in 1951, which is why it is sometimes referred to as the Sommelet rearrangement. Additionally, it has been further expanded to transform corresponding sulfur ylides.

Sommelet-Hauser rearrangement - general reaction scheme - Sommelet rearrangement
Sommelet-Hauser rearrangement

When a strong base such as NaNH2, NaOMe, or KO-t-Bu deprotonates the α-carbon of the alkyl group connecting to the quaternary nitrogen atom, the resulting carbanion attacks the ortho-position of the aromatic ring to create the rearranged product through an ortho-dialkylamino methylene cyclohexadiene intermediate. If one of the methyl groups on the quaternary nitrogen atom is substituted with any other alkyl group with at least one α-proton, then isomers arising from the deprotonation at different α-carbon atoms will form. Moreover, if an allyl group or a second benzyl group exists on the quaternary nitrogen atom, then the allyl or benzyl group will migrate to the ortho-position of the aromatic ring. In the event that the nitrogen atom is part of a cyclic ring, a ring-enlarged product may be resolved. It is worth noting that the migratory group can also move to the para-position, and the rearrangement intermediate that undergoes re-aromatization to produce the final product also produces the competitive cyclopropanation product.

The Sommelet-Hauser rearrangement, an intramolecular [2,3]-sigmatropic rearrangement of benzyl quaternary ammonium salt, can be complex due to the competing Stevens rearrangement. However, the use of potassium amide NaNHor sodium amide NaNH2 in liquid ammonia has been found to be an effective system that yields products almost exclusively from the Sommelet-Hauser rearrangement pathway. The selectivity between the two rearrangements depends on various factors such as the electronic and steric effects of substituents on the aromatic ring, the choice of solvent, and the presence of specific additives.

The Sommelet-Hauser rearrangement dominates in the reaction of N-methylides with electron-donating or weak electron-withdrawing groups at the para– or ortho-position of the benzyl moiety, while the Stevens products are favored with strong electron-withdrawing groups.

Under certain conditions, the rearrangement of ylides to an electron-rich aromatic ring may remain at the intermediate step. Steric factors also control the rearrangement, and the Hofmann elimination can complicate the desired Sommelet-Hauser rearrangement if an alkyl group attaching to the quaternary nitrogen contains both α and β protons.

Although the Sommelet-Hauser rearrangement is a general reaction, it fails under certain circumstances, such as in the treatment of (2-furylmethyl)ammonium or (2-thienylmethyl)ammonium salt and the highly hindered (phenyl-tert-butylmethyl)trimethyl-ammonium bromide.

References

  • Sommelet, M., Compt. Rend., 1937, 205, 56
  • Kantor, S. W. and Hauser, C. R., J. Am. Chem. Soc., 1951, 73, 4122
  • Hauser, C. R. and Weinheimer, A. J., J. Am. Chem. Soc., 1954, 76, 1264
  • Hauser, C. R.; Manyik, R. M.; Brasen, W. R. and Bayless, P. L., J. Org. Chem., 1955, 20, 1119
  • Hauser, C. R. and van Eenam, D. N., J. Am. Chem. Soc., 1956, 78, 5698
  • Beard, W. Q. and Hauser, C. R., J. Org. Chem., 1960, 25, 334
  • Beard, W. Q. and Hauser, C. R., J. Org. Chem., 1961, 26, 371
  • Jones, F. N. and Hauser, C. R., J. Org. Chem., 1962, 27, 1542
  • Jones, G. C. and Hauser, C. R., J. Org. Chem., 1962, 27, 3572
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