Chapter 7_arene Reactions_arene Hydrogenation

8/6/2019 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


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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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.



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.



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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Figure 7.1 Some important industrial processes based on electrophilic substitutionreactions of benzene.



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:


CHAPTER 7OBJECTIVES

INTRODUCTION




Arene-Arene Coupling Oxidative Substitution



Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process






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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.


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.



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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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.



It seems likely that the electrophile coordinates to the

face of the ring (perhaps off-center) and displaces a

proton as illustrated for toluene:



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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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.



Figure 7.2. Two pathways for styrene synthesis from an ethylene complex of anarylpalladium compound..



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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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.



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.


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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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.



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.



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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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.



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).


PALLADIUM-CATALYZED REACTIONSOxidative Substitution

Figure 7.3. Alternative pathways for arene-arene coupling.


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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.



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.



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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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.


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.



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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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):



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:


CHAPTER 7OBJECTIVES

INTRODUCTION







Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process






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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.


COPPER-CATALYZED OXIDATIONSDecarboxylation

Copper salts catalyze both decarboxylation and

oxidative decarboxylation of benzoic acid and its

derivatives:



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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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.



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.



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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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:



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:



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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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.



Figure 7.4. Reactions involved in phenol synthesis by oxidative decarboxylationof benzoic acid.


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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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.



Figure 7.5. Oxidation of phenols by copper(I) chloride/amine/oxygen.



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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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.



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).



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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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.



Figure 7.6. A chain-growth step in the copper-catalyzed oxidative polymerizationof 2,6-xylenol.



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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In particular, the cobalt complex 16 known as

Salcomine or cobalt(salen) is very efficient for the

oxidation of phenols top-benzoquinones.


CHAPTER 7OBJECTIVES

INTRODUCTION








Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process







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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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.



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.



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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For example, o-dichlorobenzene reacts with n-

butylmagnesium bromide in the presence of NiCl2(DPPP)

to form o-dibutylbenzene in about 80% yield.



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.



The ligand effect is illustrated in the coupling of

(CH3)2CHMgCl and chlorobenzene with various NiCl2L2complexes.


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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.



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.



Figure 7.7 A sirnplitied mechanism for the coupling of chlorobenzene andisopropyl niagnesium (L = R3P)


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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:



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.



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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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.



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.


CHAPTER 7OBJECTIVES

INTRODUCTION







Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process






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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.


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.


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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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.



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.



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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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:



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.



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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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.


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.



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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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.



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.



Figure 7.8 Catalytic cycle for hydrogenation of one double bond in benzene.


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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.



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.



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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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.



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).



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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When this complex is hydrogenated with a heterogeneous

rhodium catalyst, the two noncoordinated double bonds

are reduced and a methoxycyclohexene complex 20 is

isolated:


CHAPTER 7OBJECTIVES

INTRODUCTION








Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process






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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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.




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.




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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Figure




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.




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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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.




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.




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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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).




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.




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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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.




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.




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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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.




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.




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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CHAPTER 7OBJECTIVES

INTRODUCTION








Decarboxylation

Phenol Coupling


ARENE HYDROGENATION

IFP Process





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