Rearrangements

By

Dr Vandna Thakur

Assistant professor

Department of Chemistry

HMV

SCHMIDT REACTION

Introduction

Reaction of carboxylic acid with N3H in presence of H2SO4.

Yields both aliphatic and primary amines.

Reaction given by aldehydes, ketones, alcohols & alkenes too.

MECHANISM

EXAMPLES:1.

2.

4.

3.

5.

Dakin Rearrangement

The dakin reaction is an organic redox reaction in which an ortho or parahydroxylated phenyl aldehyde or ketone reacts with hydrogen peroxide in baseto form a benzenediol and a carboxylate .

The reaction works best if the aromatic aldehydes or ketones is electron rich.

Introduction

MECHANISMThe mechanism of this reaction is related to

baeyer-villiger oxidation.the carbonyl compound

undergoes nucleophilic addition with

hydroperoxide anion to give tetrahedral

intermediate.

EXAMPLES:

The Pinacol Rearrangement is Acid

Catalyzed Dehydration of glycols which

converts the glycol into an aldehyde or a

ketone.

For Example:-

Introduction

Mechanism of Pinacolic Transformation:-

This rearrangement is intramolecular rearrangement which is

confirmed by crossover experiment

Migratory Aptitude:-The ease with which any particular group will undergo

nucleophillic 1,2-Shift is known as its migratory aptitude. All

groups do not migrate with equal ease.

Order of Decreasing migratory aptitude:-

It is been found that migratory

aptitude of phenyl group is

more than hydrogen which is

more than the alkyl group .

Stereochemical And Conformational Factors:-

The most favourable Streochemistry for pinacolic

rearrangement is that the migrating group and the leaving group

are aligned in an anti- periplaner rearrangement easily in dilute

H2SO4 with migration of a methyl group whereas the trans

derivative undergoes rearrangement under similar conditions with

contraction of the ring.

Examples Of Pinacol-Pinacolone

Rearrangement:-

1.

2.

3.

4.

5.

7.

6.

8.

9.

10.

CURTIUS REARRANGEMENT

INTRODUCTION THE THERMAL/ PHOTOCHEMICAL REARRANGEMENT OF

ACYL AZIDES TO ISOCYANATES

ACYL AZIDES ARE USUALLY PREPARED EITHER BY REACT-

ION OF SODIUM AZIDE WITH A REACTIVE ACYLATING

AGENT OR BY A DIAZOTISATION OF AN ACYL HYDRAZIDE.

IT IS INTRAMOLECULAR REARRANGEMENT

THIS REACTION IS VERY GENERAL REACTION APPLICABLE

TO ALL CARBOXYLIC ACIDS-ALIPHATIC, ALICYCLIC,

HETROCYCLIC AS WELL AS AROMATIC

IT GIVES GOOD YIELD OF ISOCYANATE

PREPERATION OF ACYL AZIDE

REACTION

MECHANISM

Imp. Points about this rearrangement :-

In this rearrangement the key step is the thermal or

photochemical conversion of acyl azide to isocyanate .

This rearrangement is almost certainly concerted and does

not involve the intermediacy of a nitrene .

When curtius rearrangement is carried out in the presence of

vinyl monomers such as styrene ,it does not undergo

polymerisation .This rules out the formation of free radical ,

that is nitrene.

However ,there is evidence to support the existence of nitrene

when tertiary alkyl azides undergo curtius rearrangement to

form imines .

Difference b\w curtius and hofmann

rearrangement :-

Both reactions gives isocyanates ,but isocyanates can be isolable in curtius but not in hoffmannrearrangement ,because base is present and the isocyanate can not survive .

Stereochemistry of curtiusrearrangement :-

Studies of this rearrangemnt of optically active acyl azides in which the chiral carbon is directly bonded to this carbonyl group have shown that these reactions occur with retention of configuration .

Thus the chiral centre attach to the carbonyl group migrates to nitrogen with its electron but without inversion .

example

Beckmann rearrangement

Introduction

The acid-catalysed conversion of ketoximes to amides is

known as Beckmann Rearrangement .A variety of protic

acids , Lewis acids , acid anhydrides and acyl halides can

cause the reaction to occur . The mechanism of reaction is

as shown :

.

The role of catalysts like protic acids , Lewis

acids, acid anhydrides and acyl halides is to

convert the hydroxyl group into a better leaving

group. some acids (generally proton acids,

H2SO4,HCl and H3PO4) catalysed by simply

protonating the oxime. Other acids may esterify

the oximes.

EXAMPLE:

The reaction is catalysed by arene sulphonyl chlorides

which react with oxime giving the corresponding sulphonate

R ArSO2Cl R

C=N-OH C=N-O-SO2-Ar

R R

R-C=N-R

O-SO2-Ar

O H2O

R-C –NH-R R-C=N-R

OSO2-Ar

The oxime of ketone shows geometricalisomerism and it has two geometrical isomers ,syn and anti. When this reaction is carried outunder non-isomerising conditions , it is alwaysthe groups anti to the –OH that migrates.EXAMPLE:

NO2 PCl5 NO2

Cl C-C6H5 Cl C-NH-C6H5

N O

HO

C6H5-C- - -Br (i) O

N-O NO2 (ii) H2 O Br- - CNHC6H5

NO2

NO2

O

Br C-C6H5 (i) P h-C-NH- -Br

N NO2 (ii) H2 O

O NO2

NO2

HO-N H

CH3 H2S04 O N CH3

H3C ArSO2Cl H3C

Pyridine NH

H N H O

OH O

N-OH HCl Me3C-C-NH-

-C-CMe3 CH3COOH

CH3 O

C=N-OH ArSO2Cl NH-C-CH3

Pyridine

H H

ArSO2Cl O

O Pyridine NH

N-OH

O H

N-OH H3PO4 N

N

N-OH O H

Neber rearrangement

Introduction REARRANGEMANT GIVEN BY KETOXIME

TOLUENESULPHONATES IN THE PRESENCE OF ALKOXIDESOF PYRIDINE TO GIVE α-AMINO KETONES.

MECHANISM:- DEPROTONATION OF THE MIGRATION ORIGION.

THE CARBANION OBTANINED BY THE DEPROTONATION UNDERGOES INTERNAL DESPLACEMENT OF THE LEAVING GROUP TO FORM AN INTERMEDIATE AZIRINE.

AZIRINE ON HYDROLYSIS GIVES THE PRODUCT.

SIMILAR REARRANGMENT IS GIVEN BY N, N-DICHLOROAMINES.

• When both aldehyde and ketone are oxidised by per acid

known as baeyer villiger rearragement.

• This oxidation is the important reaction involing 1,2-

migration to the electro deficient oxygen.

•This oxidation is specially useful with ketone because it

convert them into carboxylic ester. Cyclic ketones

generate lactones .

Salient Features

C O

MMPP