Overman Rearrangement

What Is Overman Rearrangement?

The Overman rearrangement is a [3,3]-sigmatropic rearrangement that converts allylic trichloroacetimidates into allylic trichloroacetamides, which upon hydrolysis afford allylic amines. It is widely regarded as the aza-variant of the Claisen rearrangement, enabling the stereospecific 1,3-transposition of alcohol and amine functionalities. The reaction was first reported in 1974 by Larry E. Overman at the University of California, Irvine, after whom it is named.

Since its discovery, the Overman rearrangement has evolved from a purely thermal process into a versatile, catalytic transformation. The original thermal protocol required elevated temperatures (80-140 °C) and suffered from limited functional group tolerance. The introduction of mercury(II) catalysts by Overman shortly thereafter significantly lowered the reaction temperature. A landmark advance came with the development of palladium(II) catalysts bearing chiral cobalt oxazoline palladacycle (COP) ligands, which enabled the first highly enantioselective variants. More recently, gold(I) catalysis has further expanded the scope, and the reaction has been integrated into tandem and cascade processes for the rapid assembly of complex nitrogen-containing architectures.

• Reagents: Trichloroacetonitrile (Cl₃CCN) and base (DBU, NaH, or K₂CO₃) for imidate formation; thermal activation or transition-metal catalysts: Hg(II) salts [Hg(OCOCF₃)₂, Hg(OAc)₂], Pd(II)-COP complexes (chiral cobalt oxazoline palladacycles), Au(I) complexes; basic hydrolysis (NaOH or K₂CO₃ in MeOH/H₂O) to cleave the trichloroacetamide.
• Reactants: Allylic alcohols (primary, secondary, or tertiary), converted to allylic trichloroacetimidates.
• Products: Allylic amines (after rearrangement and hydrolysis). The C-N bond is installed at the γ-position with overall allylic transposition and complete chirality transfer.
• Reaction Type: Concerted, thermal or metal-catalyzed [3,3]-sigmatropic rearrangement; aza-Claisen rearrangement.
• Related Reactions: Claisen rearrangement (oxygen counterpart, [3,3]-sigmatropic); Ireland-Claisen rearrangement; Cope rearrangement ([3,3]-sigmatropic of 1,5-dienes).
• Experimental Tips: a) Imidate preparation: Treat the allylic alcohol with trichloroacetonitrile (1.1-2.0 equiv) and DBU (0.1-0.2 equiv) in CH₂Cl₂ at 0 °C to rt for 1-2 h.  b) Thermal vs. catalytic: For simple substrates, thermal conditions (xylenes, 80-140 °C) suffice. For sensitive substrates, use Pd(II)-COP (rt) or Au(I) catalysis.

Fig 1. Overman rearrangement reaction and its mechanism. [1]

Mechanism of Overman Rearrangement

Step 1: Formation of Allylic Trichloroacetimidate

The reaction sequence begins with the conversion of an allylic alcohol to the corresponding allylic trichloroacetimidate. This is accomplished by treating the alcohol with trichloroacetonitrile in the presence of a catalytic amount of base, typically DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or NaH. The base deprotonates the alcohol, and the resulting alkoxide undergoes nucleophilic addition to the electrophilic nitrile carbon of trichloroacetonitrile, generating the imidate. This step is generally high-yielding (>85%) and proceeds under mild conditions.

Step 2: The [3,3]-Sigmatropic Rearrangement

The key transformation is a concerted, pericyclic [3,3]-sigmatropic rearrangement proceeding through a six-membered, chair-like transition state. The allyl fragment migrates from the oxygen atom to the nitrogen atom with suprafacial stereochemistry. The thermodynamic preference for the amide (C=O-N) over the imidate (C=N-O) functional group provides the driving force. Critically, the stereochemical information of the starting allylic alcohol—both the chirality at the carbinol center and the (E)/(Z) geometry of the double bond—is faithfully transferred to the product. The reaction is highly stereospecific: a chiral, enantioenriched allylic alcohol delivers the corresponding enantioenriched allylic amine with complete retention of stereochemistry.

Step 3: Hydrolysis to Allylic Amine

The resulting allylic trichloroacetamide is stable and can be isolated, but in most synthetic applications it is hydrolyzed directly to the free allylic amine. Hydrolysis is typically accomplished with aqueous sodium hydroxide or potassium carbonate in methanol-water mixtures. The overall three-step sequence—imidate formation, [3,3]-rearrangement, and hydrolysis—constitutes a net 1,3-transposition of an alcohol to an amine with complete stereochemical fidelity.

Comparison with the Claisen Rearrangement

The Overman rearrangement is the nitrogen analogue of the Claisen rearrangement. The table below summarizes the key distinctions:

Feature	Claisen	Overman
Migrating group	Allyl (C→C)	Allyl (O→N)
Substrate	Allyl vinyl ether	Allylic trichloroacetimidate
Product (after work-up)	γ,δ-Unsaturated carbonyl	Allylic amine
Transition state	Chair-like, 6-membered	Chair-like, 6-membered
Thermal conditions	150-250 °C	80-140 °C
Catalytic variants	Lewis acids (BCl3, AlMe3)	Hg(II), Pd(II)-COP, Au(I)
Stereochemistry	Chirality transfer	Chirality transfer
Key application	Carbonyl homologation	Stereodefined allylic amines

Application Examples of Overman Rearrangement

• Tetrodotoxin: The Overman rearrangement has been employed as a pivotal stereodefining step in the total synthesis of tetrodotoxin (TTX), one of the most celebrated targets in natural product synthesis. The reaction installs the crucial C-N bond of the densely functionalized cyclohexane core with complete stereocontrol. Several independent syntheses of TTX, including those by the Isobe, Du Bois, and Sato groups, have relied on the Overman rearrangement for constructing the guanidine-bearing quaternary carbon center. [2]
• Saxitoxin and Paralytic Shellfish Toxins: The Overman rearrangement has served as a key transformation in the total synthesis of saxitoxin, gonyautoxins, and related paralytic shellfish toxins. The reaction enables the stereocontrolled introduction of nitrogen at the bridgehead position of the tricyclic core—a challenge elegantly addressed through the thermal or palladium(II)-catalyzed Overman rearrangement of appropriately functionalized allylic alcohols.
• Chiral Allylic Amine Building Blocks: The asymmetric Overman rearrangement using Pd(II)-COP catalysts has become a routine method for the preparation of enantioenriched allylic amines. These versatile chiral building blocks are widely employed in medicinal chemistry for the synthesis of bioactive molecules, including α-amino acids, amino sugars, and nitrogen-containing heterocycles. The high enantioselectivity and broad substrate scope make this protocol a method of choice for accessing chiral amines. [3]
• Gephyrotoxin and Indolizidine Alkaloids: The Overman rearrangement has been featured in the synthesis of gephyrotoxin and related indolizidine alkaloids, providing efficient access to the characteristic bicyclic framework. The stereospecific nature of the [3,3]-shift ensures the correct relative configuration of substituents on the pyrrolidine ring.

Fig 2. Synthetic examples via Overman rearrangement reaction.

Related Products

CAS No.	Structure	Product
544-06-2		Trichloroacetonitrile,99%
5208-93-5		3-Methyl-1-(2,6,6-trimethylcyclohex-1-en-1-yl)penta-1,4-dien-3-ol
110351-94-5		1H-Pyrano[3,4-f]indolizine-3,6,10(4H)-trione,4-ethyl-7,8-dihydro-4-hydroxy-,(4S)-
497-06-3		3,4-Dihydroxy-1-butene
922-65-6		1,4-Pentadien-3-ol
616-25-1		1-Penten-3-ol
4187-86-4		1-Pentyn-3-ol
1517-66-4		2-Butanol,3-methyl-,(2S)-
64584-92-5		4-Penten-2-ol,(2R)-
115-18-4		2-Methyl-3-buten-2-ol
822-67-3		2-Cyclohexen-1-Ol
21378-21-2		3-Methyl-2-Cyclohexen-1-Ol
3391-86-4		1-Octen-3-ol

References

1. Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 415-417.
2. Nishikawa, Toshio, et al. Synlett 26.14 (2015): 1930-1939.
3. Fernandes, Rodney A., et al. Organic & biomolecular chemistry 15.13 (2017): 2672-2710.

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