chapter 7_arene reactions_arene hydrogenation
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CHAPTER 7
ARENE REACTIONS
Professor Bassam El Ali 2
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling
Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Professor Bassam El Ali 3
INTRODUCTION
Benzene and its derivatives form a broad range of it-
complexes and -aryl derivatives with transition metal
ions.
These arene and aryl complexes are intermediates in
many catalytic reactions that have potential
application in industry and in laboratory syntheses.
Some palladium(II)-catalyzed reactions accomplish
substitutions or couplings of arenes that are difficult to
effect by conventional methods, but these have found
little industrial use.
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Professor Bassam El Ali 4
INTRODUCTION
Copper complexes catalyze some unique reactions
that are used on a moderate scale.
Ziegler catalysts related to those used in olefin
polymerization have become important in thehydrogenation of benzene.
A variety of soluble catalysts are used in synthesis of
specialty chemicals, especially in the venerable dyes
industry.
Professor Bassam El Ali 5
ELECTROPHILIC AROMATIC SUBSTITUTION
One major limitation on use of the metal-catalyzed
reactions is that classical organic methods for
electrophilic aromatic substitution work so well.
Many large-scale arene reactions (Figure 7.1) are
catalyzed by simple Lewis or Bronstd acids.
The primary interaction of the acid is with the
attacking reagent rather than with aromatic substrate.
Professor Bassam El Ali 6
ELECTROPHILIC AROMATIC SUBSTITUTION
One In chlorination, for example, FeCl3 reacts with
chlorine to form a polarized complex which behaves
as though free Cl+ were formed.
This activated reagent attacks benzene to replace H
with Cl.
Similar mechanisms generate incipient NO2 from nitric
acid or C2H5 from ethylene.
The Monsanto process for AlCl3-ethylation of benzene
as a route to styrene has been described.
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Professor Bassam El Ali 7
ELECTROPHILIC AROMATIC SUBSTITUTION
Figure 7.1 Some important industrial processes based on electrophilic substitutionreactions of benzene.
Professor Bassam El Ali 8
ELECTROPHILIC AROMATIC SUBSTITUTION
The palladium-catalyzed reactions seem to involve
electrophilic attack of Pd on the aromatic ring.
The resulted arylpalladium compounds react by
conventional organometallic mechanisms like those
observed for olefins.
-complex involving one double bond is believed to
be transformed to a -aryl complex via electrophilic
metallation or oxidative addition processes:
Professor Bassam El Ali 9
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
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Professor Bassam El Ali 10
PALLADIUM-CATALYZED REACTIONS
Palladium(II) salts, especially the acetate, catalyze
many oxidative substitution reactions of benzene and
other aromatic hydrocarbons.
These reactions are not used commercially, but havebeen studied as potential processes for manufacture
of styrene, phenol, and substituted biphenyls.
Like the olefin reactions, the arene reactions are very
sensitive to reaction conditions. A change in acetate
concentration in the benzene Pd(OAc)2 reaction
makes biphenyl the major product.
Professor Bassam El Ali 11
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
When styrene and Pd(OAc)2 are heated in benzene that
contains acetic acid, trans-stilbene is a major product:
The phenyl groups in the stilbene comes from benzene.
The reaction is catalytic when it is conducted under oxygen
pressure.
Similarly, ethylene can be phenylated catalytically to form
styrene, stilbene, and higher derivatives.
Professor Bassam El Ali 12
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
The arene-olefin coupling reactions do not give high
yields of a single product.
Arene-arene coupling is not observed in the presenceof olefin, but addition of acetic acid to the C=C bond
occurs.
The styrene-PdCl2 complex in benzene/acetic acid that
contains sodium acetate gives -phenylethyl acetate inaddition to stilbene and a little -acetoxystyrene.
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Professor Bassam El Ali 13
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
This lack of selectivity has reduced the appeal of the
benzene-ethylene reaction as a potential industrial
process for styrene production.
The mechanism of arene-olefin coupling is unclear.
Two very plausible proposals appear in the literature.
Both involve metallation of the benzene ring by a Pd2+
electrophile as a key step.
Professor Bassam El Ali 14
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
It seems likely that the electrophile coordinates to the
face of the ring (perhaps off-center) and displaces a
proton as illustrated for toluene:
Professor Bassam El Ali 15
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
The formation of the -aryl derivative appears to be a
common step in arene-olefin coupling, arene-arene
coupling, and arene acetoxylation.The para specificity observed with toluene in some of
these reactions can be interpreted as evidence for
attack by a bulky electrophile.
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Professor Bassam El Ali 16
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
The coupling proceeds through coordination of the
olefin to the Ar-Pd compound to give a complex 1.
Figure 7.2 shows two conventional mechanisms forformation of styrene from an ethylene complex.
In the upper pathway ethylene inserts into the Ar-Pd
bond to form a -phenethyl complex 2.
Hydrogen elimination by transfer to the metal gives
styrene and an unstable Pd-H species which
decomposes to palladium metal. Alternatively (lower
equation), the ethylene is metallated to form a -vinyl
derivative 3.
Professor Bassam El Ali 17
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
Figure 7.2. Two pathways for styrene synthesis from an ethylene complex of anarylpalladium compound..
Professor Bassam El Ali 18
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
This compound then undergoes reductive elimination
of the two organic ligands to form the styrene directly.
There is good precedent for both pathways. Preformed-vinyl complexes react with benzene to form styrenes
as in the lower pathway.
On the other hand, preformed -phenylpalladium
compounds react with olefins to form similar products.
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Professor Bassam El Ali 19
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
In a related reaction that may be useful in organic
synthesis, aryl halides and olefins react to form
styrenes.
For example, o-bromotoluene and ethylene react at
125C and 9 atmospheres pressure to form o-
methylstyrene in 86% yield.
Professor Bassam El Ali 20
PALLADIUM-CATALYZED REACTIONSArene-Olefin Coupling
A palladium complex formed in situ from palladium
acetate and triphenyl is used as a catalyst rather than
a reagent.
It appears that the reaction proceeds via an
arylpalladium complex formed by oxidative addition of
aryl bromide to a palladium(0) complex.
Once the aryl-Pd bond is formed, coordination,
insertion, and coupling of the olefin proceed as shown
in Figure 7.2.
Professor Bassam El Ali 21
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
Benzene, palladium chloride, and sodium acetate in
acetic acid react to form biphenyl in high yield.
This reaction uses the palladium salt as a stoichiometric
oxidant, but the reaction becomes catalytic in palladium
when it is carried out under oxygen pressure.
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Professor Bassam El Ali 22
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
The stoichiometric coupling to form biphenyl is
ordinarily accompanied by phenyl acetate formation,
but acetoxylation is almost completely suppressed by
50 atmospheres oxygen pressure.
Heteropolymolybdate ions serve as catalysts to
couple the palladium and O2 redox systems.
With a Pd(OAc)2/Hg(OAc)2/H5(Mo10V2PO40) catalyst,
toluene is converted to dimethylbiphenyls with good
rates and yields at 1.5 atmospheres pressure and 50-
90C.
Professor Bassam El Ali 23
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
The oxidative coupling occurs with a variety of aromatic
compounds.
Much of the industrial interest in this reaction lay in its
potential use to couple toluene to 4,4-dimethylbiphenyl
and o-xylene to 3,4,3,4-tetramethylbiphenyl.
These compounds are possible precursors of
biphenyldi- and tetracarboxylic acids, which yield
polyamides and polyimides with interesting physical
properties.
Professor Bassam El Ali 24
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
Ube Industries makes Upirex, a high-performance
polyimide, from the tetracarboxylic acid obtained by
oxidative coupling of dimethyl phthalate to the
tetraester:
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Professor Bassam El Ali 25
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
Neat dimethyl phthalate is treated with a palladium(II)
chelate, copper(II) acetate and oxygen.
The reaction may be conducted at atmospheric
pressure. With the 1,10-phenanthroline complex of
Pd(OAc)2, 93-94% selectivity to the desired isomer is
obtained at approximately 10% conversion.
When the reaction is carried out under more severe
conditions (160-180C, 10 atmospheres pressure) to
get commercially acceptable reaction rates, the
selectivity is about 82% at 9% conversion.
Professor Bassam El Ali 26
PALLADIUM-CATALYZED REACTIONSArene-Arene Coupling
As in the palladium-catalyzed arene-olefin coupling, it
seems likely that a -arene-palladium compound 4
forms in the initial reaction of benzene with a palladium
(2+) salt, but the subsequent C-C bond formation
mechanism is. unclear.
Kinetic studies suggest formation of intermediate
phenylpalladium 5 and diphenylpalladium 6
intermediates which lead to coupling by reductive
elimination of two C-Pd bonds (upper sequence in
Figure 7.3).
Professor Bassam El Ali 27
PALLADIUM-CATALYZED REACTIONSOxidative Substitution
Figure 7.3. Alternative pathways for arene-arene coupling.
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Professor Bassam El Ali 28
PALLADIUM-CATALYZED REACTIONSOxidative Substitution
Arenes react with palladium (2+) salts in the presence
of anionic nucleophiles to form substitution products.
The most studied reaction is acetoxylation.
The acetoxylation was of interest as a potential phenol
synthesis because phenyl acetate is easily hydrolyzed
to phenol and acetic acid.
Professor Bassam El Ali 29
PALLADIUM-CATALYZED REACTIONSOxidative Substitution
An analogous oxidation of phenyl acetate has been
studied us a route to the acetates of catechol,
resorcinol, and hydroquinone.
The acetoxylation of benzene can be made catalytic in
palladium by addition of inorganic oxidants such as
K2Cr2O7, but it is repressed by oxygen.
The best results are from the use of a heterogeneous
catalyst, as in the closely analogous vinyl acetate
synthesis.
Professor Bassam El Ali 30
PALLADIUM-CATALYZED REACTIONSOxidative Substitution
Nearly quantitative yields of phenyl acetate and phenol are
obtained by passing benzene and acetic acid vapors in a
dilute oxygen stream over a supported palladium metal
catalyst at 130-190C.Palladium(II) trifluoroacetate reacts readily with electron-rich
arenes such as anisole and toluene to form trifluoroacetoxyderivatives with preference for ortho and para substitution.
The reaction can be made catalytic in palladium by use of
K2S2O8 as a cooxidant, just as was done earlier in
acetoxylations with Pd(OAc)2 in acetic acid.
The reaction is moderately fast and clean with a range of
arene substituents from CH3 to CO2CH3.
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Professor Bassam El Ali 31
PALLADIUM-CATALYZED REACTIONSOxidative Substitution
In the acetoxylation, meta products predominate even
with toluene, for which electrophilic attack by Pd2+
might be expected to give ortho and para isomers.
A possible explanation for this phenomenon and for the
delicate balance between substitution and coupling
involves competitive reactions of an arene -complex 4
as in arene-arene coupling.
An alternative explanation is that the palladation of
arenes to form -arylpalladium species like 5 in Figure7.3 is simply a nonselective aromatic substitution.
Professor Bassam El Ali 32
PALLADIUM-CATALYZED REACTIONSOxidative Carbonylation
The reaction of palladium(II) salts with CO and an arene
is a potentially interesting synthesis of arylcarboxylic acid
derivatives.
Carbonylation of arylmercury and arylthallium
compounds in the presence of palladium acetate-, metal-
metal exchange forms an arylpalladium complex which
carbonylates readily.
Professor Bassam El Ali 33
PALLADIUM-CATALYZED REACTIONSOxidative Carbonylation
The acylpalladium species formed reacts with alcohols
to give esters.
Carboxylation of toluene by this route because themetallation is highly para-specific.
Both the mercury and thallium reactions yield over 90%
methyl para-toluate, an intermediate in terephthalic
acid synthesis for polyester manufacture.
Difficulties in reoxidizing the reduced metal salts have
inhibited industrial use of this chemistry.
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Professor Bassam El Ali 34
PALLADIUM-CATALYZED REACTIONSOxidative Carbonylation
For laboratory syntheses of benzoic acid derivatives, a
closely related carbonylation of aryl halides may be
useful.
Palladium complexes such as PdBr2(PPh3)2 catalyze
the reaction of bromobenzene with CO and butanol
under mild conditions (100C, 1 atmosphere):
Professor Bassam El Ali 35
PALLADIUM-CATALYZED REACTIONSOxidative Carbonylation
With bromo- and iodoarenes, yields are high and many
kinds of functional groups are tolerated by the catalyst.
The mechanism of the reaction is not clearly
established.
It seems likely that the palladium(II) complex is reduced
by CO or the alcohol.
The resulting zero-valent complex reacts with the aryl
halide to form an arylpalladium complex:
Professor Bassam El Ali 36
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
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Professor Bassam El Ali 37
COPPER-CATALYZED OXIDATIONS
Copper(II) salts catalyze several synthetically usefuloxidations of aromatic compounds.
An oxidative decarboxylation of benzoic acid yields phenol.
Oxidative coupling of 2,6-disubstituted phenols producespolymers.
These reactions differ in mechanism from the analogousoxidative substitution and coupling reactions with palladiumcatalysts.
The palladium-catalyzed oxidations seem to involveorganometallic intermediates like the olefin oxidations.
The copper-catalyzed reactions are commonly described asfree-radical processes, although organocopperintermediates may be present.
Professor Bassam El Ali 38
COPPER-CATALYZED OXIDATIONSDecarboxylation
Copper salts catalyze both decarboxylation and
oxidative decarboxylation of benzoic acid and its
derivatives:
Professor Bassam El Ali 39
COPPER-CATALYZED OXIDATIONSDecarboxylation
The simple decarboxylation is often used in organic
synthesis and the oxidative decarboxylation has
been used commercially for manufacture of phenol.
In both reactions, a copper(I) salt catalyzes CO2elimination but, in the oxidative process, copper(II)
also plays an important part.
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Professor Bassam El Ali 40
COPPER-CATALYZED OXIDATIONSDecarboxylation
The simple decarboxylation is very clean with arene
carboxylic acids.
When cuprous benzoate is heated above 200C in a
high-boiling solvent such as quinoline, benzene is
formed in 99% yield.
The salt need not be preformed, but can be prepared
in situ by reaction with CuO2CCH3, CuO2CCF2, or an
arylcopper(I) complex.
The copper(I) compound may be used in catalytic
quantities and is quite tolerant of other functional
groups.
Professor Bassam El Ali 41
COPPER-CATALYZED OXIDATIONSDecarboxylation
For example, 0.1 equivalent of [CuC6F5]4 catalyzes
the decarboxylation of an indolecarboxylic acid in
high yield:
Copper(II) benzoate, in contrast to the Cu(I) salt,
undergoes oxidative decarboxylation. This reaction
is the basis for phenol syntheses developed by Dow
and by Lummus.
Professor Bassam El Ali 42
COPPER-CATALYZED OXIDATIONSDecarboxylation
Steam and air are blown through a solution of
copper(II) and magnesium salts in molten benzoic
acid at 230-240C.
Carbon dioxide evolves rapidly and phenol distills
from the mixture in about 80% yield.
One particularly interesting feature of the oxidative
decarboxylation is that the phenolic hydroxyl group
occupies a position ortho to that of the original
carboxyl group.
For example, p-toluic acid yields m-cresol. Similarly,
1-14C-benzoic acid gives 2-14C-phenol.
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Professor Bassam El Ali 43
COPPER-CATALYZED OXIDATIONSDecarboxylation
The origin of the ortho placement of the enteringsubstituent comes from study of the stoichiometricpyrolysis of copper(II) benzoate.
Heating this salt in mineral oil at 250C produces amixture of copper(I) salts and some free benzoic acid:
Professor Bassam El Ali 44
COPPER-CATALYZED OXIDATIONSDecarboxylation
The ortho-benzoatobenzoate and the salicylate salts
almost certainly result from intramolecular attack on
an ortho-C-H of the benzene ring.
The attack is usually described as the result of two
one-electron transfers:
Professor Bassam El Ali 45
COPPER-CATALYZED OXIDATIONSDecarboxylation
This formulation of the mechanism is based on
attack of the ortho position by an incipient benzoate
radical.
The arenium radical 7 thus formed is oxidized by a
second copper(II) ion to produce the observed
copper(I) o-benzoatobenzoate 8 (X = PhCO2).
No organocopper intermediates are involved in this
description of the reaction.
The catalytic phenol synthesis is a combination of
several reactions as shown in Figure 7.4.
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Professor Bassam El Ali 46
COPPER-CATALYZED OXIDATIONSDecarboxylation
The first is the pyrolysis of copper(II) benzoate. The
copper(I) o-benzoatobenzoate decarboxylates rapidly
at 230-250C to form pl benzoate.
This ester is hydrolyzed under the phenol synthesis
conditions to give phenol and benzoic acid.
The copper(I) salts are reoxidized by air to regenerate
copper(II) benzoate.
Professor Bassam El Ali 47
COPPER-CATALYZED OXIDATIONSDecarboxylation
Figure 7.4. Reactions involved in phenol synthesis by oxidative decarboxylationof benzoic acid.
Professor Bassam El Ali 48
COPPER-CATALYZED OXIDATIONSPhenol Coupling
The oxidation of phenols by air in the presence of
copper(I) salts call take several different pathways as
shown in Figure 7.5.
Phenol itself is oxidized by oxygen top-benzoquinone in
80% yield in a reaction catalyzed by copper(I) chloride
in acetonitrile.
When a pyridine complex of copper(I) chloride is used
in methanol solution, a major product is the
monomethyl ester ofcis,cis-muconic acid 10.
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Professor Bassam El Ali 49
COPPER-CATALYZED OXIDATIONSPhenol Coupling
The same product is obtained from 1,2-dihydroxybenzene.
Probably both oxidations involve o-benzoquinone 9 as anintermediate prior to ring cleavage.
The copper(I)-catalyzed oxidations of phenols show
considerable ortho-para specificity.
When the ortho positions are blocked by alkyl or halo
substituents, reaction occurs at the para position, even
with the CuCl/pyridine complex as a catalyst.
The With very bulky ortho substituents such as tert-butyl,
two phenol molecules couple to form the quinoid dimer12
shown in Figure 7.5.
Professor Bassam El Ali 50
COPPER-CATALYZED OXIDATIONSPhenol Coupling
Figure 7.5. Oxidation of phenols by copper(I) chloride/amine/oxygen.
Professor Bassam El Ali 51
COPPER-CATALYZED OXIDATIONSPhenol Coupling
From a technological viewpoint, the most important
oxidation in this class is that of 2,6-xylenol.
This oxidation produces a para-phenylene oxidepolymer11 by coupling an oxygen of one phenol
molecule to the para carbon of another.
This aromatic polyether is a high melting plastic which
is very resistant to heat and to water.
It has found wide use as an engineering thermoplastic
under the trade name PPO.
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Professor Bassam El Ali 52
COPPER-CATALYZED OXIDATIONSPhenol Coupling
An even more rigid and higher melting material is
obtained by the analogous oxidation of 2,6-
diphenylphenol.
The all-aromatic character of this material imparts
outstanding thermal stability.
The oxidation of 2,6-xylenol is easy.
In a semiworks experiment that may simulate
commercial practice, 2,6-xylenol is reacted with oxygen
in toluene at 40C in the presence of copper(II) chloride,
dibutylamine, NaBr, and a quaternary ammonium
dispersing agent.
Professor Bassam El Ali 53
COPPER-CATALYZED OXIDATIONSPhenol Coupling
A high molecular weight polymer is formed in about 80
minutes.
The polymerization process is reversible in the
presence of catalyst.
The catalyst is deactivated with a chelating agent or
with HCl in order to stabilize the polymer.
The latter approach is used in a laboratory synthesis of
poly(2,6-dimethyl- 1,4-phenylene ether).
Professor Bassam El Ali 54
COPPER-CATALYZED OXIDATIONSPhenol Coupling
The course of the oxidation is sensitive to the
amine:copper ratio in the catalyst.
High ratios of amine to copper produce thepolyphenylene oxide polymer.
However, at low ratios, the major product is a quinoid
dimer13.
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Professor Bassam El Ali 58
COPPER-CATALYZED OXIDATIONSPhenol Coupling
The simplest chain-growth mechanism is coupling of a
monomeric phenolate radical with a similar radical 14
generated from a polymer chain (Figure 7.6).
Addition of the polymeric radical to the pant position ofa monomeric radical forms a coupling product 15 witha cyclohexadienone end group.
Tautomerization of the end group to a phenol structure
yields the enlarged polymer.
The phenol end group can react with copper(II) again
to form another polymeric radical that can undergo
chain growth by a similar mechanism.
Professor Bassam El Ali 59
COPPER-CATALYZED OXIDATIONSPhenol Coupling
Figure 7.6. A chain-growth step in the copper-catalyzed oxidative polymerizationof 2,6-xylenol.
Professor Bassam El Ali 60
COPPER-CATALYZED OXIDATIONSPhenol Coupling
In all the phenol oxidation processes shown in Figure
7.5.
It seems likely that a copper(II) species is the actual
oxidant even though a copper(I) salt is often the
preferred catalyst precursor.
The role of copper(II) as a one-electron oxidant can
also be filled by a cobalt or manganese complex that
bears a tightly bound chelating ligand.
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Professor Bassam El Ali 61
COPPER-CATALYZED OXIDATIONSPhenol Coupling
In particular, the cobalt complex 16 known as
Salcomine or cobalt(salen) is very efficient for the
oxidation of phenols top-benzoquinones.
Professor Bassam El Ali 62
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling
Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Professor Bassam El Ali 63
COUPLING REACTIONS OF ARYL HALIDES
Many reactions of halobenzenes are catalyzed by
soluble transition metal complexes.
The Ar-X bond is notoriously sluggish toward direct
substitution even by powerful nucleophiles such asRS.
Traditionally such nucleophilic substitutions have been
catalyzed by copper salts, as in the ammonolysis of p-
chlorobenzotrifluoride to form p-aminobenzotrifluoride,
an important industrial intermediate.
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Professor Bassam El Ali 64
COUPLING REACTIONS OF ARYL HALIDES
A more broadly useful synthesis tool is the coupling of
aryl halides with organometallic compounds such as
Grignard reagents.
The coupling catalysts or reagents usually arecomplexes of copper or nickel although many other
metals are active.
These coupling reactions have bee reviewed
extensively.
Professor Bassam El Ali 65
COUPLING REACTIONS OF ARYL HALIDES
Salts and complexes of Fe, Co Ni, Pd, and Cu catalyze
the reactions of halobenzenes with Grignard reagents
to form alkylbenzenes and biphenyls:
This reaction does not proceed well in the absence of
the transition metal compound, but it becomes rapid
when the proper catalyst is present.
Alkyllithinm, zinc, and aluminum compounds can often
be substituted for the Grignard reagent.
Professor Bassam El Ali 66
COUPLING REACTIONS OF ARYL HALIDES
Nickel complexes have received the broadest study in
this reaction.
The most effective catalysts are NiCl2(PR3)2
complexes although the yields vary with the nature ofthe phosphine and of the Grignard reagent.
Generally, highest yields are obtained with complexes
of chelating phosphines such as Ph2P(CH2)3PPh2(DPPP).
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Professor Bassam El Ali 67
COUPLING REACTIONS OF ARYL HALIDES
For example, o-dichlorobenzene reacts with n-
butylmagnesium bromide in the presence of NiCl2(DPPP)
to form o-dibutylbenzene in about 80% yield.
Professor Bassam El Ali 68
COUPLING REACTIONS OF ARYL HALIDES
This product would be difficult to prepare by conventional
organic syntheses.
Similar yields are obtained with a wide range of n-alkyl
and aryl Grignard reagents.
However, with sterically hindered aryl Grignard reagents
such as 2,4,6-trimethyl-phenylmagnesium bromide, the
nonchelate complex NiCl2(PR3)2 is most effective.
Professor Bassam El Ali 69
COUPLING REACTIONS OF ARYL HALIDES
The ligand effect is illustrated in the coupling of
(CH3)2CHMgCl and chlorobenzene with various NiCl2L2complexes.
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COUPLING REACTIONS OF ARYL HALIDES
The chelating phosphine ligand DPPP gives almost
entirely the simple coupling product.
However, the methyl chelate ligand leads to extensive
isomerization.The triphenylphosphine complex produces extensive
reduction of the chlorobenzene to benzene.
These seemingly anomalous results can be
accommodated by a mechanism proposed for the
coupling reaction. The mechanism is illustrated in Figure
7.7.
Professor Bassam El Ali 71
COUPLING REACTIONS OF ARYL HALIDES
As in the stoichiometric coupling described above, a
nickel(0) complex is a key intermediate and provides
entry to the catalyst cycle.
Oxidative addition of chlorobenzene to NiL2 gives a -phenyl complex 17.
A metathetical reaction of the Grignard reagent gives a
complex 18 containing both -aryl and -alkyl ligands.
In the normal coupling process, reductive elimination of
the two organic ligands from nickel gives
isopropylbenzene and NiL2 to complete the catalytic
cycle.
Professor Bassam El Ali 72
COUPLING REACTIONS OF ARYL HALIDES
Figure 7.7 A sirnplitied mechanism for the coupling of chlorobenzene andisopropyl niagnesium (L = R3P)
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Professor Bassam El Ali 73
COUPLING REACTIONS OF ARYL HALIDES
However, when the phosphine ligands are small or
weakly bound, other reactions can occur.
-Hydrogen elimination from the isopropyl ligand forms
a hydrido olefin complex:
Professor Bassam El Ali 74
COUPLING REACTIONS OF ARYL HALIDES
As indicated in the equation, the hydrido complex is in
equilibrium with both n- and i-propyl compounds.
If isomerization of the alkyl group is faster than
reductive elimination of the alkyl and aryl ligands, n-
propylbenzene is the major product.
Reductive elimination of the Ni-H and Ni-Ph bonds in
the hydrido complex gives benzene.
Professor Bassam El Ali 75
COUPLING REACTIONS OF ARYL HALIDES
The reaction pathways described above account for the
observed products simply, but they do not represent a
detailed mechanism.
They fail to account for the observations that oxidizingagents such as O2 and ArBr accelerate the coupling
reaction and electron acceptors such as nitroarenes
inhibit it.
The accelerating effect of simple oxidizing and alkylating
agents is probably due to depletion of the phosphine
ligand which, in turn, opens coordination sites on the
metal ion.
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Professor Bassam El Ali 76
COUPLING REACTIONS OF ARYL HALIDES
A more important effect, however, is probably access
to odd electron species such as Ni(I) and Ni(III) that
accelerate the reductive elimination process.
Mechanisms that involve free radicals or one-electrontransfers have been suggested for catalysts based on
Fe, Co, and Cu complexes.
As with nickel, phosphine complexes of palladium
catalyze coupling of halobenzenes with
organomagnesium and zinc reagents.
Professor Bassam El Ali 77
COUPLING REACTIONS OF ARYL HALIDES
The high yields and mild conditions of these reactions
often justify the use of the precious metal catalysts.
The reaction pathways seem similar to those of nickel.
Zero-valent complexes such as Pd(PPh3)4 are quite
effective, but the more stable PdCl2(PPh3)2 or
Pd(Ar)(I)(PPh3)2 complexes are more convenient for
most purposes.
Professor Bassam El Ali 78
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
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ARENE HYDROGENATION
Hydrogenation of aromatic compounds with soluble
catalysts has received concerted scientific scrutiny
only in recent years.
Homogeneous catalysts for the hydrogenation ofarenes have been known since the early 1950s,
but they have received little attention because
heterogeneous catalysts are extraordinarily effective
for this reaction.
Professor Bassam El Ali 80
ARENE HYDROGENATION
For the organic laboratory, Adams catalyst (brown
PtO2) hydrogenates aromatics at 25C and 3
atmospheres pressure.
In commercial practice, palladium-on-carbon or high
surface area nickel catalysts are used to hydrogenate
benzene to cyclohexane on a very large scale.
Professor Bassam El Ali 81
ARENE HYDROGENATION
In recent years, however, two major developments in
industrial benzene hydrogenation have occurred.
A seemingly soluble nickel catalyst developed by the
Institute Franais du Ptrole (IFP) has been appliedextensively in Europe for hydrogenation of benzene to
cyclohexane.
The selectivity of ruthenium catalysts for hydrogenation
of benzene to cyclohexane has been increased to the
point that industrial use is likely. The potentially practical
catalysts are heterogeneous, but soluble rutheniumcomplexes provide useful models for the chemistry.
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Professor Bassam El Ali 82
ARENE HYDROGENATIONIFP Process
Benzene is hydrogenated to cyclohexane on a scale of
millions of tons per year to provide the feedstock for makingadipic acid, a major intermediate in production of nylon.
The process is customarily carried out with a Raney nickel
heterogeneous catalyst, but the conventional technology is
being displaced by IFPs soluble nickel catalyst.
Apparently the mechanical and thermal advantages of
working with a soluble catalyst rather than a slurry
compensate the problems of catalyst separation andrecycle that usually handicap homogeneous catalysts.
In this instance, the volatility of cyclohexane facilitates
separation of the product from the catalyst.
Professor Bassam El Ali 83
ARENE HYDROGENATIONIFP Process
It has been known for many years that the Ziegler
olefin polymerization catalysts catalyze the
hydrogenation of arenes.
Complexes prepared by reaction of triethylaluminum
with a cobalt or nickel salt catalyze the hydrogenation
of benzene and its derivatives.
For example, benzene is reduced to cyclohexane
rapidly and quantitatively at 150-190C and about 75
atmospheres pressure with Al(C2H5)3 and Ni(2-
ethylhexanoate)2 as the catalyst.
Professor Bassam El Ali 84
ARENE HYDROGENATIONIFP Process
Similarly, a combination of Co(2-ethylhexanoate)2 and
excess alkylaluminum compound reduces the xylenes
to dimethylcyclohexanes.
The cis-dimethylcyclohexanes are favored overtransby about 2:1, consistent with a predominant cis
addition of hydrogen.
A significant problem with the simple Ziegler systems
is the instability of the soluble catalysts.
It appears that IFP has solved this problem with careful
control of reaction conditions, control of the Al:Ni ratio,
and, possibly, use of adjutants to stabilize the soluble
nickel species.
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Professor Bassam El Ali 85
ARENE HYDROGENATIONIFP Process
In a typical patent example, benzene is hydrogenated to
cyclohexane at 155C and 10 atmospheres hydrogen
pressure with a catalyst prepared from a mixture of
nickel and zinc octanoates and excess triethylaluminum
(Al:Ni = 4.2).
The catalysts, even though they are prepared from
highly moisture-sensitive organometallic compounds,
tolerate the presence of hydroxyl groups in the material
to be hydrogenated, for example, Bisphenol A:
Professor Bassam El Ali 86
ARENE HYDROGENATIONIFP Process
The nature of the Ziegler-type catalysts is poorly
defined.
The reaction of Al(C2H5)3 with either a cobalt or nickel
salt gives a dark brown or black solution.
In the case of nickel, the mixture is neither pyrophoric
nor paramagnetic and does not yield solids on
ultracentrifugation.
It seems likely that the solutions contain metal hydride or
alkyl species that are stabilized by coordination to
aluminum.
Professor Bassam El Ali 87
ARENE HYDROGENATIONIFP Process
A model catalyst system produced by interaction of
NiCl2(PEt3)2 with Al2Me3Cl3 at -40C was characterized
by EXAPS.
Many species were present, but the average structurecorresponded to:
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Professor Bassam El Ali 88
ARENE HYDROGENATIONIFP Process
Relatively stable catalysts are obtained by the reaction
of triethylaluminum with cobalt(II) acetylacetonate in the
presence of tributylphosphine.
This catalyst system effects cohydrogenation of olefinsand arenes under mild conditions.
At 30C and 1,5 atmospheres hydrogen pressure,
styrene and benzene are hydrogenated to form primarily
ethylbenzene and cyclohexane.
Hydrogenation of the olefin is much faster than reduction
of the arenes.
Professor Bassam El Ali 89
ARENE HYDROGENATIONAllyl Cobalt Catalysts
The -allyl complex Co(C3H5)(P(OMe)3)3 hydrogenates
benzene to cyclohexane at room temperature and
atmospheric pressure.
The hydrogenation is slow but very stereoselective.
When D2 is the reducing agent, all-cis-cyclohexane-d is
formed in over 95% yield at low conversion.
Similarly, naphthalene and anthracene give the cis-
perhydro derivatives. In contrast to the Ziegler systems,
benzene is hydrogenated more rapidly than naphthalene
and anthracene with the allyl catalyst.
Professor Bassam El Ali 90
ARENE HYDROGENATIONAllyl Cobalt Catalysts
With alkylbenzenes, the rates fall in the order:
benzene> toluene > xylenes > mesitylene> durene
Little or no cyclohexene is produced from benzene
under ordinary conditions.
The major drawbacks of tins catalyst axe the low
hydrogenation rate and the limited catalyst life.
A catalyst derived from reaction of RhCl3 and a
quaternary ammonium salt [(C8H17)3NCH3]+Cl- produces
results very similar to those obtained with the allyl cobalt
catalyst except that it is more stable and functions in the
presence of water.
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Professor Bassam El Ali 91
ARENE HYDROGENATIONAllyl Cobalt Catalysts
It seems likely that the whole family of soluble cobalt
and nickel catalysts function by a mechanism like that
sketched in Figure 7.8.
The precursor complex designated "Co" reacts withhydrogen to form a dihydride complex, "CoH2" which
may be stabilized by Lewis acidic ligands in the case of
the IFP catalyst.
In the allyl cobalt series, such dihydrides have been
observed spectroscopically.
In the interaction of benzene with the CoH2 species, the
benzene ligand is probably complexed through a single
double bond.
Professor Bassam El Ali 92
ARENE HYDROGENATIONAllyl Cobalt Catalysts
Addition of a Co-H bond to this C=C bond gives a
cyclohexadienyl complex.
Transfer of a second Co-H gives 1,3-cyclohexadiene,
which is still coordinated to the cobalt.
In the normal operation of the catalyst, it probably
remains coordinated to the metal for two more H
addition cycles that ultimately yield cyclohexane.
The overall process is very similar to the hydrogenation
of an olefin by Wilkinson's catalyst.
Professor Bassam El Ali 93
ARENE HYDROGENATIONAllyl Cobalt Catalysts
Figure 7.8 Catalytic cycle for hydrogenation of one double bond in benzene.
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Professor Bassam El Ali 94
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
The hydrogenation of benzene to cyclohexene has been
a major target of industrial research because the
oxidation of cyclohexene to adipic acid may proceed
more cleanly than the current cyclohexane oxidation.
Most of the research has centered on ruthenium and its
complexes because this metal seems to hydrogenate
benzene in preference to cyclohexene.
Professor Bassam El Ali 95
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
When used in the presence of aqueous NaOH,
ruthenium- on-magnesia gives about 50% cyclohexene
at moderate conversion of benzene.
Recently issued patents and papers point the way to
even greater selectivity with aqueous slurries of metallic
ruthenium catalysts.
Professor Bassam El Ali 96
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
While industrial cyclohexene production will probably
use heterogeneous catalysts, research on soluble
ruthenium and osmium catalysts may be instructive in
understanding the operation of the metallic catalysts.Bis(hexamethylbenzene) ruthenium(0) hydrogenates
benzene rapidly at 90C and 2-3 atmospheres pressure.
The reaction resembles that catalyzed by ruthenium
metal in that substantial amounts of cyclohexene are
formed (40-55% dimethylcyclohexenes from the
xylenes).
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Professor Bassam El Ali 97
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
The bis(hexamethylbenzene) ruthenium catalyst differs
in several respects from the allyl cobalt catalysts
discussed above, but is said to operate by a similar
mechanism.It differs in giving cyclohexene as a substantial product
and in producing extensive H/D exchange when D2 is
the reducing agent.
When xylene is treated with D2, deuterium appears in
the methyl groups of the unreduced xylene.
Professor Bassam El Ali 98
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
In addition, the hexamethylbenzene ligands of recovered
catalyst undergo methyl H/D exchange.
It was proposed that this exchange occurred via a -
benzyl intermediate.
The bis(hexamethylbenzene)ruthenium(0) is interesting
in that one ligand is symmetrically complexed (6) but
the other is coordinated through only two C=C bonds
(4).
Professor Bassam El Ali 99
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
Recent research on the hydrogenation of arene
complexes of osmium sheds new light on the selective
hydrogenation process.
Osmium like ruthenium, binds arenes tightly even whenonly one or two localized C=C bonds are coordinated to
the metal.
2-Arene complexes such as the anisole complex 19
have been characterized crystallographically.
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Professor Bassam El Ali 100
ARENE HYDROGENATIONPartial Hydrogenation of Benzene
When this complex is hydrogenated with a heterogeneous
rhodium catalyst, the two noncoordinated double bonds
are reduced and a methoxycyclohexene complex 20 is
isolated:
Professor Bassam El Ali 101
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling
Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Professor Bassam El Ali 102
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
some homogeneous catalytic reactions that are used to
produce specialty chemicals on a scale of less than 5000
tons per year (in North America).
These reactions are particularly important in theproduction of dyes, colored pigments, and agrochemicals.
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AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
One especially valuable property of metal ions is to
steer an electrophile or a nucleophile to a position
ortho to a substituent on a benzene ring.
This effect is seen with widely varying metal ions from
(Al+3, Zr+4) to d10 (Hg+2).
This family 0f reactions embraces a wide range of
mechanisms, but it is generally assumed that the metal
ion coordinates to an electron-donating substituent
such as -OH or NH2.
Professor Bassam El Ali 104
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
This effect is illustrated in Figure 7.9, which outlines a
speculative mechanism for the ortho-ethylation of
aniline.
Aluminum alkyls react with aniline to form the trianilide,
which contains Al-N bonds.
In the postulated mechanism, interaction of filled -
orbitals of an ethylene molecule with a vacant p-orbital
on aluminum guides the olefin into a location in which
a series of electron pair migrations 21 create a new C-
C bond ortho to the nitrogen substituent on thebenzene ring.
Professor Bassam El Ali 105
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Subsequent aminolysis of the Al-C bond in 22 leads toregeneration of an aluminum anilide and formation of
intermediate 23, which subsequently tautomerizes to ortho-ethylaniline.
While the postulated mechanism accounts for the observed
products, it is tempting to propose an alternative in which
the aluminum ion migrates from the n itrogen of the anilide
to the ortho position on the benzene ring.
Insertion of ethylene into the ortho C-Al bond, a well-
documented reaction, would yield 22, which then proceedsas shown in Figure 7.9 to give ortho-ethylaniline.
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AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Figure
Professor Bassam El Ali 107
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Whatever the mechanism, the ortho-ethylation of
anilines has become an important industrial process
for the manufacture of the widely used herbicides,
Lasso and Dual.
A critical intermediate for the production of Lasso is
2,6-diethylaniline, which is produced by reaction of
aniline with ethylene in the presence of aluminum
trianilide.
The catalyst can be generated in situ by reaction of
triethylaluminum or activated aluminum turnings withexcess aniline.
Professor Bassam El Ali 108
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
The solution of aluminum trianilide in aniline is heated withethylene at 300-400C and 30-65 atmospheres pressure.
Distillation of the reaction mixture gives over 90% 2,6-
diethylaniline and leaves a residue of aluminum trianilide,which can be used as a catalyst for a subsequent reaction
batch.
A similar process is used to convert o-toluidine to 2-ethyl-6-methylaniline, a key for production of Dual.
The solution of aluminum trianilide in aniline is heated with
ethylene at 300-400C and 30-65 atmospheres pressure.
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Professor Bassam El Ali 109
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Distillation of the reaction mixture gives over 90% 2,6-diethylaniline and leaves a residue of aluminum trianilide,
which can be used as a catalyst for a subsequent reactionbatch.
A similar process is used to convert o-toluidine to 2-ethyl-6-
methylaniline, a key for production of Dual.
Professor Bassam El Ali 110
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Chemistry closely analogous to that of aniline
ethylation is used to produce 2,6-di-t-butylphenol
which is widely used in formulating antioxidants and
ultraviolet stabilizers for polymers.
The reaction of isobutene with phenol in the presence
of a protonic acid catalyst ordinarily gives a mixture of
t-butyl phenyl ether and para-t-butylphenol along with
minor quantities of ortho-t-butylphenol.
The course of the reaction changes dramatically when
aluminum phenoxide is used as the catalyst.
Professor Bassam El Ali 111
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
No significant quantity of the para-t-butylphenol is
formed.
The reaction can be driven through the use of excess
isobutene to produce largely 2,4,6-tri-t-butylphenol
which is also useful as a component of stabilizing
formulations for polymers.
In recent years, another ortho substitution of phenol has
received attention as a potential process for making
agrichemical intermediates.
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AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Many of Du Ponts sulfonylurea herbicides, such as
chlorosulfuron, are characterized by strong species
selectivity in controlling weeds (in addition to their
innocuous relationship to life forms other than plants).
Professor Bassam El Ali 113
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
The selectivity for particular plant species is determined
both by the nature of the nitrogen heterocycle at one
end of the molecule and the substituents on the arene at
the other end.
It is often desirable to introduce a substituent ortho to
the sulfonyl group on the benzene ring.
Professor Bassam El Ali 114
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
One approach to placing a hydroxyl group and a sulfur
side by side on a benzene ring is the ortho-
alkylthiolation of phenol.
The acid-catalyzed reaction of phenol with
dialkyldisulfides generally gives a mixture of ortho and
para substituted products:
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Professor Bassam El Ali 115
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
It has been observed that aluminum phenoxide
catalyzes the alkylthiolation of phenol with greater
ortho selectivity than that attained with simpleBronstd or Lewis acid catalysts.
For example, an aluminum phenoxide solution,
prepared by in situ reaction of aluminum powder with
excess phenol, reacts with dimethyl disulfide at 123-
170C to produce 2-(methylthio)phenol in 40% yield
after distillation.
Professor Bassam El Ali 116
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
The crude reaction mixture contains the ortho- and
para-methylthiophenols and two bis(thiophenols) in a
ratio of 17:7:3:1.
More recently it has been reported that zirconium
phenoxide is effective in catalyzing the ortho
methylthiolation of phenol.
Professor Bassam El Ali 117
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
With both the aluminum and zirconium catalysts, it is
plausible to suggest that the metal ion steers the
dialkyl disulfide reagent to the ortho position on thephenol ring through an assemblage such as:
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Professor Bassam El Ali 118
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Another ortho-substitution process of industrial
importance is the mercury(II) catalyzed sulfonation of
anthraquinone to produce antraquinone-1-sulfonicacid.
This acid is a major intermediate in making
anthraquinone dyes, the second largest class of texthe
dyes.
The effect of Hg2+ ion as a catalyst in this process is
dramatic.
Professor Bassam El Ali 119
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
The uncatalyzed reaction of anthraquinone with oleum
containing 45% SO3 at 150C produced the 2-sulfonic
acid almost exclusively.
When the reaction is carried out in the presence of a
small amount of a mercuric salt, the product is
primarily anthraquinone-1-sulfonic acid.
Professor Bassam El Ali 120
AROMATIC INTERMEDIATES FORSPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution
Continued reaction leads to introduction of a second
sulfonic acid group.
The second group is directed to an ortho position (5 or
8) of the other benzenoid ring.
In mercury- catalyzed reactions such as these, it is
likely that the first step is mercuration of the arene ring
to form a C-Hg bond.
Subsequent reaction of the aryl-Hg function with
another reagent (e.g., SO3) places the incoming
substituent at the site of the initial attack by mercuric
ion.
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Professor Bassam El Ali 124
CHAPTER 7OBJECTIVES
INTRODUCTION
ELECTROPHILIC AROMATIC SUBSTITUTION
PALLADIUM-CATALYZED REACTIONS
Arene-Olefin Coupling
Arene-Arene Coupling
Oxidative Substitution
Oxidative Carbonylation
COPPER-CATALYZED OXIDATIONS
Decarboxylation
Phenol Coupling
COUPLING REACTIONS OF ARYL HALIDES
ARENE HYDROGENATION
IFP Process
Allyl Cobalt Catalysts
Partial Hydrogenation of Benzene
AROMATIC INTERMEDIATES FOR SPECIALTY CHEMICALS
Metal Ion-Directed Ortho Substitution