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GOMBERG REACTION AND APPLICATIONSIN THE CHEMICAL INDUSTRY

VEDAVEDA VV.. KISARAKISARA

FINALFINAL YEARYEAR BB..TECHTECH

DYESDYES

ICTICT


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MOSES GOMBERGRussian-born American chemist who initiated

the study of free radicals in chemistry when in

1900 he prepared the first authentic one,

Triphenylmethyl radical.

C


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WHAT IS GOMBERG REACTION??? Base Promoted radical coupling between an aryl

diazonium salt and an arene to form a diaryl

compound.

N+

N R

+ ROH

-


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Reaction Scheme

N+

N H+HN N

The Diazonium species in aqueous alkaline

solution is in equilibrium withcorrespondingdiazohydroxide


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Reaction Scheme

DEPROTONATIONANDANHYDRIDE FORMATION

N+

N +OH NN

NN

O

N N


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Reaction Scheme

NNN N

N N

+ N N +C

Anhydride Intermediate Decomposes to give

phenyl radical and phenyl diazoxy radical.


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Reaction Scheme

C

R

CH

R

H

Reaction of phenyl radical and aromatic substrate


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Reaction Scheme

Subsequent loss of Hydrogen radical, which

combines with phenyl diazoxy radical to givebiphenyl derivative and diazohydroxide

N N OCH

R

H

+R

OHN N

+


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Overview Yields obtained not more than 40 50 %

Two phase reaction system :

Aqueous solution of diazonium salt

Liquid Aromatic compound or solid compound in

organic solvent

Ortho product more predominant

Side reactions formation of linear polyaryls, parent arene

and arylazo compounds.


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Overview

Substituted arenes affect the rate of reaction

EWG Slow reaction and less yield

EDG - Relatively fast reaction improves yield

Extraction of diazo hydroxide from the aqueous to

the organic phase

Solubility of the compound

Nature of the compound (eg pyridine)


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Modifications

Use of alternate bases

Non aqueous mediums

Stable arene diazonioum salts

Phase transfer catalysed Gomberg Bachmann

Synthesis


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Alternate basesBases like sodium acetate and pyridine have made the reaction more clean

and chemoselective.

Formation of different diazo intermediates which decompose by slowerrate to aryl radicals

Substrates capable of generating aryl radicals:

N-nitroso acetanilides: Reaction of acetanilides with nitrous fumes and

rearrangement on heating Aryltriazines and pyridine under acidic conditions

Arylazotriphenyl methanes

Arylhydrazines under oxidative conditions


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Alternate Bases

N+

N X-

CH3COO-

+

N N OOCCH3

+ X-

N N OOCCH3

CH3COO-

+

N N O-

+ (CH3CO)2O

N

+

N X

-

+N N O

-

ON N NN

ON N NN N N O

+ +N2C

Eg: Sodium acetate as the most efficient base


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Stable Diazonium Salts

Stable arenediazonium salts used in aromatic hydrocarbon solvents in non

aqueous medium

Prepared in high yield and purity and safe storage

Mild reaction conditions give higher yield in Gomberg Synthesis

Examples: Dry arylazonium salts of -

Naphthalene-1-sulfonic acid

Naphthalene -1,5-disulfonic acid

ZnCl2 Complex

Hexafluorophosphoric acid

Tetrafluoroboric acid


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Phase Transfer CatalystsThe pt Gomberg reaction is carried out at room

temperature by stirring the ArN2+ -BF4- or ArN2+-PF6-

salt in aromatic hydrocarbon solvent with solid

KOAc and a pt catalyst (crown ether ) for 1 2

hours.

5 mol% 18 - C - 6 / 2 e . KOAc

ben ene / T / 1.5 hrR

N2+ BF4

CH3


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Phase Transfer catalystsUse of stable and dry diazonium species and

highly effective bases

Reaction conducted at room temperature ortho isomer is predominant

Safe and convenient method for preparation of

unsymmetrical biarenesTypes rown Ether , Glymes , Aliquat 336 ,

Bu4NHSO4 etc


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Survey of pt Gomberg Reaction

ARENE TOTAL

YIELD %

ISOMER %

2- 3- 4-

FLUOROBENZENE 68 59 31 10

CHLOROBENZENE 75 85 14 1

ANISOLE 55 91 2 7

BROMOBENZENE 60 75 25 0

BENZONITRILE 63 60 14 26

METHYL BENZOATE 29 80 7 13

4 Cl - PYRIDINE 51 78 15 7

4 Cl - FURAN 75 100 0 NA

4 Cl - THIOPHENE 62 100 0 NA

4 Br p - XYLENE 26 100 0 0


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APPLI ATIONS

IN HEMI AL INDUS

TRY


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AROMATIC COMPONENTSSYMMETRICAL AND UNSYMMETRICAL BIPHENYLS

HETEROCYCLIC COMPOUNDS

R1 = F, Cl, Br, NO2, CN, COOR, Ar etc.

R1 != lower alkyls, CHO

AR = , Z = O , S

R1

R2

R1R

4

R3

AR

Z


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Pharmaceutical industry Fluoroaryl moieties are often found in many pharmaceuticals

such as non-steroidal antiinflammatory drugs, antifungal agents,

and human antiparasitic agents and biphenyl skeletons have

played key roles in several therapeutic areas.

Preparation by directcross coupling between arylmetallics and a

fully functionalizedaryl halide.

ross coupling reactions of the organometallics with a variety of

organic electrophiles using transition metals have provided useful

methods for - bond formation.


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Diffunisal

F

F

OH

COOH

Gomberg reaction of difluorophenyl diazonium salt with benzene

The resulting coupling product ( , 4 difluorophenyl )has to be

functionalised to give Diffunisal.


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Spiro compounds as LASER Dyes

A dye laser is a laser which uses

an organic dye as the lasing medium,

usually as a liquid solution. Compared

to gases and most solid state lasing

media, a dye can usually be used for

a much wider range of wavelengths.

The wide bandwidth makes them

particularly suitable for tunable

lasers and pulsed lasers.

R , Ka , Na , M , L = phenyl , biphenyl, triphenyl ,heterocyclicetc systems

Dye obtained by Gomberg reaction starting from 9,9 - spirobifluorene


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PHOTOCHROMIC NAPHTHOPYRANS These compounds are used in synthetic resins of all types

particularly for opthalmic uses

When energy is supplied in the form of heat,the dye molecules

change over into a stimulated colour state and turn colourless

when energy supply is interrupted

These dyes have an inadequate longwave absorption in the

excited as well as non excited state


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2- H Naphtho-[1,2-b]pyransGomberg synthesis of

spiro-9-xanthene- -[2-(4-

methoxyphenyl)-2-phenyl-,6,7, -tetrahydro-6,6,7,7-

tetramethyl-

phenanthro[9,10-b]pyran]


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Tolan DerivativesComponents of

Liquid Crystal and

Electro opticaldisplay elements.

R1= H or alkyl radical 1-12 C

atoms, substituted with

halo,CN etc

R2 = 1-10 C atoms alkyl,alkoxy,

-CF,OCF3,CN ,OCHFCF3

L1-L6 = INDEPENDENT H OR F

Q= -O- , SINGLE BOND

Z=COO,C2H2,-O- ETC

A= trans cyclohexylene

radical


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Miscellaneous usesHalogenation of the asphaltenes can be achieved by use of via

the Gomberg reaction , whereby products of tar sands can be

obtained from bitumen.

Gomberg reaction forSilane Preparation

Si2OCl6 is irradiated to form solid SiO which is isolated and

reacted with HCl at 00 deg C.

Tetrachloro silane,water aHnd Hydrogen gas is produced and

no activation of catalysts is required


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CONCLUSIONS The pt catalysed Gomberg reaction is one of the most efficient

methods for the synthesis of biaryls.

Simple method for cross coupling of various compounds using

organometallic reagents Starting materials are easily available and inexpensive

Excess of arene can be regenerated by the distillation of the

filtrate after separation of potassium tetrafluoroborate.

It can also be reused as a precipitating reagent for preparation of

parent diazonium species.

On contrary to organometallics , Gomberg reaction doesnot

involve toxic salts(Ni) , dry solvents, expensive catalysts (Pd),

inert conditions and complicated procedures.


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THANK YOU

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