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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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