The Overman Rearrangement allows the conversion of readily available allylic alcohols into allylic amines by a two-step synthesis involving the rearrangement of an allylic trichloroacetimidate to an allylic trichloroacetamide with clean 1,3-transposition of the alkenyl moiety.
Allylic amines are useful precursor of a variety of nitrogen-containing molecules, such as alkaloids, antibiotics and unnatural amino acids.
Mechanism of the Overman Rearrangement
The deprotonated alcohol ads to trichloroacetonitrile to give a trichloroacetimidate anion. As this latter intermediate can readily deprotonate the starting alcohol, only a catalytic amount of a strong base is needed.
The formation of the allylic amine can involve a thermal [3,3]-sigmatropic rearrangement - comparable to the Claisen Rearrangement - that prefers to proceed via a chair-like transition state:
Alternatively, the rearrangement can be induced by a transition metal catalyst such as Pd(II) or Hg(II):
Some chiral transition metal catalysts bind with face-selectivity toward the substrate, which enables the enantioselective conversion of prochiral starting materials:
For a more detailed explanation of the reaction mechanism and the stereoselection induced by transition metal catalysts along with some early examples of Pd-catalyzed chiral conversions, please refer to a recent publication by L. E. Overman (J. Org. Chem. 1997, 62, 1449. DOI).
Catalytic Asymmetric Rearrangement of Allylic Trichloroacetimidates. A Practical Method for Preparing Allylic Amines and Congeners of High Enantiomeric Purity
C. E. Anderson, L. E. Overman, J. Am. Chem. Soc., 2003, 125, 12412-12413.