Organic Chemistry II: Rearrangement Reactions – Practice Problems and Mechanisms

Rearrangement Reactions in Organic Chemistry

Introduction

Rearrangement reactions are a fundamental class of organic transformations in which the carbon skeleton of a molecule is reorganized to yield a structural isomer. These reactions often proceed via migration of atoms or groups within the molecule, and are crucial for the synthesis of complex organic compounds. The following study notes summarize key rearrangement mechanisms, illustrated by practice problems and their solutions.

Practice Problem 1: Beckmann Rearrangement

Mechanism and Features

• Beckmann Rearrangement converts oximes to amides under acidic conditions.

• The group anti to the leaving group migrates to the nitrogen atom.

• General reaction: $\text{Oxime} \xrightarrow{H_2SO_4,\ 180^\circ C} \text{Amide}$

• Key steps: Protonation, migration, loss of water, and rearrangement.

Example: Conversion of a cyclic oxime to a lactam (cyclic amide) under microwave heating.

Practice Problem 2: Beckmann Rearrangement with Tautomerism

Mechanism and Features

• Oxime formation from a ketone and hydroxylamine.

• Activation with TsCl (tosyl chloride) and pyridine facilitates rearrangement.

• Tautomerism may occur, leading to migration and formation of a new amide.

• General reaction: $\text{Ketone} + \text{NH}_2\text{OH} \rightarrow \text{Oxime} \xrightarrow{TsCl, Pyridine} \text{Amide}$

Example: Rearrangement of a methyl-substituted ketone to a corresponding amide.

Practice Problem 3: Staudinger Reaction and Rearrangement

Mechanism and Features

• Staudinger Reaction involves the reduction of azides to amines using triphenylphosphine (PPh3).

• Subsequent rearrangement leads to migration and formation of a new amide.

• Key steps: Azide activation, nitrogen extrusion, phosphine addition, and rearrangement.

• General reaction: $\text{Azide} + \text{PPh}_3 \rightarrow \text{Imine} \rightarrow \text{Amide}$

Example: Transformation of a complex azido ester to an amide via Staudinger reaction.

Practice Problem 4: Hypervalent Iodine(III) Mediated Rearrangement

Mechanism and Features

• Use of PIFA (phenyliodine(III) bis(trifluoroacetate)), a hypervalent iodine reagent, to induce rearrangement.

• Facilitates migration and cyclization, often with loss of a leaving group (e.g., PhI).

• General reaction: $\text{Amide} + \text{PIFA} \rightarrow \text{Cyclic Amide}$

Example: Cyclization of a hydroxy amide to a lactam using PIFA in acetonitrile/water.

Additional info: Hypervalent iodine reagents are mild oxidants used for oxidative rearrangements and cyclizations.

Practice Problem 5: Curtius Rearrangement

Mechanism and Features

• Curtius Rearrangement involves thermal decomposition of acyl azides to isocyanates, which can be trapped by alcohols to form carbamates.

• Key steps: Nitrogen extrusion, migration, isocyanate formation, nucleophilic trapping.

• General reaction: $\text{Acyl Azide} \xrightarrow{\Delta} \text{Isocyanate} \xrightarrow{R-OH} \text{Carbamate}$

Example: Conversion of a cyclic acyl azide to a tert-butyl carbamate.

Practice Problem 6: Hofmann Rearrangement

Mechanism and Features

• Hofmann Rearrangement converts primary amides to primary amines with one fewer carbon atom.

• Reagents: Bromine and base (e.g., NaOH).

• Key steps: Formation of N-bromoamide, rearrangement, loss of CO2, and amine formation.

• General reaction: $\text{RCONH}_2 \xrightarrow{Br_2, NaOH} \text{RNH}_2 + CO_2$

Example: Transformation of a cyclopropyl amide to a cyclopropyl amine.

Practice Problem 7: Wolff Rearrangement (Photochemical)

Mechanism and Features

• Wolff Rearrangement involves photolysis of α-diazoketones to generate ketenes, which react with nucleophiles.

• Key steps: Diazoketone activation, nitrogen loss, ketene formation, nucleophilic addition.

• General reaction: $\text{Diazoketone} \xrightarrow{hv} \text{Ketene} \xrightarrow{R-OH} \text{Ester}$

Example: Photochemical rearrangement of a diazoketone to a methyl ester.

Practice Problem 8: Wolff Rearrangement (Thermal)

Mechanism and Features

• Thermal activation of α-diazoketones leads to ketene formation and subsequent nucleophilic trapping.

• Key steps: Nitrogen extrusion, ketene formation, nucleophilic addition.

• General reaction: $\text{Diazoketone} \xrightarrow{\Delta} \text{Ketene} \xrightarrow{R-OH} \text{Ester}$

Example: Rearrangement of a diazoketone to a methyl ester in methanol.

Practice Problem 9: NBS-Mediated Rearrangement and Staudinger Reaction

Mechanism and Features

• NBS (N-bromosuccinimide) is used for selective bromination.

• Subsequent Staudinger reaction with PPh3 and base leads to rearrangement and formation of a new product.

• General reaction: $\text{Brominated Compound} \xrightarrow{PPh_3, K_2CO_3} \text{Rearranged Product}$

Example: Bromination followed by phosphine-mediated rearrangement of a cyclic ester.

Practice Problem 10: [1,2]-Wittig Rearrangement

Mechanism and Features

• [1,2]-Wittig Rearrangement involves base-induced migration of an alkoxy group to a carbanion center.

• Reagents: Strong base (e.g., LDA) in THF at low temperature.

• Key steps: Deprotonation, rearrangement, and hydrolysis.

• General reaction: $\text{Alkoxybenzyl Ether} \xrightarrow{LDA} \text{Rearranged Alcohol}$

Example: Rearrangement of a methoxybenzyl ether to a new alcohol.

Practice Problem 11: Lossen Rearrangement

Mechanism and Features

• Lossen Rearrangement converts hydroxamic acids to isocyanates, which can be trapped by nucleophiles to form ureas or amines.

• Key steps: Activation (e.g., tosylation), rearrangement, nitrogen extrusion, nucleophilic trapping.

• General reaction: $\text{Hydroxamic Acid} \xrightarrow{TsCl} \text{Isocyanate} \xrightarrow{R-NH_2} \text{Urea/Amine}$

Example: Rearrangement of a tosylated hydroxamic acid with an aryl amine to form a substituted urea.

Summary Table: Key Rearrangement Reactions

Reaction Name	Starting Material	Key Reagent(s)	Product	Migration Feature
Beckmann	Oxime	Acid (H2SO4), TsCl	Amide	Anti group migrates
Staudinger	Azide	PPh3	Amine/Amide	Phosphine addition, N2 loss
Curtius	Acyl Azide	Heat, Alcohol	Carbamate	R group migrates
Hofmann	Amide	Br2, NaOH	Amine	Loss of CO2
Wolff	Diazoketone	hv or Heat	Ketene/Ester	Migration, N2 loss
Lossen	Hydroxamic Acid	TsCl, Amine	Urea/Amine	Migration, N2 loss
[1,2]-Wittig	Ether	LDA	Alcohol	Alkoxy migration
Hypervalent Iodine	Amide	PIFA	Cyclic Amide	Oxidative migration

Conclusion

Rearrangement reactions are essential tools in organic synthesis, enabling the construction of new carbon frameworks and functional groups. Understanding their mechanisms, migration features, and applications is crucial for advanced study and practical use in organic chemistry.