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Subject Chemistry

Paper No and Title CH-402: Organic Synthesis

Module No and Title 5. Metallocenes, Nonbenzenoid Aromatic and Polycyclic

Aromatic Compounds

Module Tag PG-331

Content Writer:-

Dr. Archana Chahar

Asst. Professor and Head,

Dept. of Chemistry,

Kisan PG College, Simbhaoli

TABLE OF CONTENTS:

1. General considerations of Metallocenes

2. Ferrocene

3. Chrysene

4. Azulene

1. General considerations of Metallocene

Metallocene

A metallocene is a compound typically consisting of two cyclopentadienyl

anions (C5H5−, abbreviated Cp) bound to a metal center (M) in the oxidation

state II, with the resulting general formula (C5H5)2M. Certain metallocenes and

their derivatives exhibit catalytic properties, although metallocenes are rarely

used industrially. Cationic group 4 metallocene derivatives related to

[Cp2ZrCH3]+ catalyze olefin polymerization.

Some metallocenes consist of metal plus two cyclooctatetraenide

anions (C8H82−

, abbreviated cot2−

), namely the lanthanocenes and

the actinocenes (uranocene and others).

Metallocenes are a subset of a broader class of compounds called sandwich

compounds. In the structure shown below, the two pentagons are the

cyclopentadienyl anions with circles inside them indicating they

are aromatically stabilized. Here they are shown in a staggered conformation.

Figure: General chemical structure of a metallocene compound, where M is

a metal cation

The first metallocene to be classified was ferrocene, and was discovered

simultaneously in 1951 by Kealy and Pauson, and Miller et al. Kealy and

Pauson were attempting to synthesize fulvalene through the oxidation of

a cyclopentadienyl salt with anhydrous FeCl3 but obtained instead the substance

C10H10Fe. At the same time, Miller et al reported the same iron product from a

reaction of cyclopentadiene with iron in the presence of aluminum, potassium,

or molybdenum oxides. The structure of "C10H10Fe" was determined by

Wilkinson et al. and by Fischer et al. These two were awarded the Nobel Prize

in Chemistry in 1973 for their work on sandwich compounds, including the

structural determination of ferrocene. They determined that the carbon atoms of

the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that

bonding occurred due to the metal d-orbitals and the π-electrons in the p-

orbitals of the Cp ligands. This complex is now known as ferrocene, and the

group of transition metal dicyclopentadienyl compounds is known as

metallocenes. Metallocenes have the general formula [(η5-C5H5)2M]. Fischer et

al. first prepared the ferrocene derivatives involving Co and Ni. Often derived

from substituted derivatives of cyclopentadienide, metallocenes of many

elements have been prepared.[5]

One of the very earliest commercial manufacturers of metallocenes was

Arapahoe Chemicals in Boulder, Colorado.

Definition:

The general name metallocene is derived from ferrocene, (C5H5)2Fe or Cp2Fe,

systematically named bis(η5-cyclopentadienyl)iron(II). According to

the IUPAC definition, a metallocene contains a transition metal and two

cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two

cyclopentadienyl anions are on parallel planes with equal bond lengths and

strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5

carbon atoms of a cyclopentadienyl ring is denoted as η5, pronounced

"pentahapto". There are exceptions, such as uranocene, which has

two cyclooctatetraene rings sandwiching a uranium atom.

In metallocene names, the prefix before the -ocene ending indicates

what metallic element is between the Cp groups. For example, in ferrocene,

iron(II), ferrous iron is present.

In contrast to the more strict definition proposed by IUPAC, which requires a d-

block metal and a sandwich structure, the term metallocene and thus the

denotation -ocene, is applied in the chemical literature also to non-transition

metal compounds, such as barocene (Cp2Ba), or structures where the aromatic

rings are not parallel, such as found in manganocene or titanocene

dichloride (Cp2TiCl2).

Some metallocene complexes of actinides have been reported where there are

three cyclopendadienyl ligands for a monometallic complex, all three of them

bound η5.

Classification:

There are many (η5-C5H5)–metal complexes and they can be classified by the

following formulas:

Formula Description

[(η5-C5H5)2M] Symmetrical, classical 'sandwich' structure

[(η5-C5H5)2MLx] Bent or tilted Cp rings with additional ligands, L

[(η5-C5H5)MLx]

Only one Cp ligand with additional ligands, L ('piano-stool'

structure)

Metallocene complexes can also be classified by type:

1. Parallel

2. Multi-decker

3. Half-sandwich compound

4. Bent metallocene or tilted

5. More than two Cp ligands

2. Ferrocene

Ferrocene is an organometallic compound with the formula Fe(C5H5)2.

The molecule consists of two cyclopentadienyl rings bound on opposite sides of

a central iron atom. Ferrocene is a p-complex in which reactions between the d-

orbitals of the Fe2+

metal centre with the p-orbitals of the two planar

cyclopentadienyl ligands (C5H5-) form the metal-ligand bonds. Hence there is

equal bonding of all the carbon atoms in the cyclopentadienyl rings to the

central Fe2+

ion. It is an orange solid with a camphor-like odor,

that sublimes above room temperature, and is soluble in most organic solvents.

Ferrocene shows aromatic properties and is very stable.

Figure: Ferrocene [Fe(η-C5H5)2]

Synthesis

1. Via Grignard reagent

The first reported syntheses of ferrocene were nearly simultaneous. Pauson and

Kealy synthesised ferrocene using iron(III) chloride and a Grignard reagent,

cyclopentadienyl magnesium bromide. Iron(III) chloride is suspended

in anhydrous diethyl ether and added to the Grignard reagent. A redox

reaction occurs, forming the cyclopentadienyl radical and iron(II) ions.

Dihydrofulvalene is produced by radical-radical recombination while the

iron(II) reacts with the Grignard reagent to form ferrocene. Oxidation of

dihydrofulvalene to fulvalene with iron(III), the outcome sought by Kealy and

Pauson, does not occur.

2. Gas-metal reaction

The other early synthesis of ferrocene was by Miller et al., who reacted metallic

iron directly with gas-phase cyclopentadiene at elevated temperature.

3. Cracking of dicyclopentadiene

4. An approach using iron pentacarbonyl was also reported.

Fe(CO)5 + 2 C5H6(g) → Fe(C5H5)2 + 5 CO(g) + H2(g)

5. Via alkali cyclopentadienide

More efficient preparative methods are generally a modification of the

original transmetalation sequence using either commercially available sodium

cyclopentadienide or freshly cracked cyclopentadiene deprotonated

with potassium hydroxide and reacted with anhydrous iron(II) chloride in

ethereal solvents.

Modern modifications of Pauson and Kealy's original Grignard approach are

known:

Using sodium cyclopentadienide:

2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl

Using freshly-cracked cyclopentadiene:

FeCl2·4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O

Using an iron(II) salt with a Grignard reagent:

2 C5H5MgBr + FeCl2 → Fe(C5H5)2 + 2 MgBrCl

Even some amine bases (such as diethylamine) can be used for the

deprotonation, though the reaction proceeds more slowly than when using

stronger bases:

2 C5H6 + 2 (CH3CH2)2NH + FeCl2 → Fe(C5H5)2 + 2 (CH3CH2)2NH2Cl

Direct transmetalation can also be used to prepare ferrocene from other

metallocenes, such as manganocene:

FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2

Reactions

1. With electrophiles

Ferrocene undergoes many reactions characteristic of aromatic compounds,

enabling the preparation of substituted derivatives. A common undergraduate

experiment is the Friedel–Crafts reaction of ferrocene with acetic

anhydride (or acetyl chloride) in the presence of phosphoric acid as a catalyst.

Under conditions for a Mannich reaction, ferrocene gives N,N-

dimethylaminomethylferrocene.

Figure: Important reactions of ferrocene with electrophiles and other reagents

2. Synthesis of [Fe(η-C5H5)(η-C6H6)]PF6

Figure: Ligand exchange of ferrocene with benzene

3. Lithiation

Ferrocene reacts with butyllithium to give 1,1′-dilithioferrocene, which is a

versatile nucleophile. Tert-Butyllithium produces monolithioferrocene.

Dilithioferrocene reacts with S8, chlorophosphines, and chlorosilanes. The

strained compounds undergo ring-opening polymerization.

4. Protonation of ferrocene allows isolation of [Cp2FeH]PF6.

5. In the presence of aluminium chloride Me2NPCl2 and ferrocene react to

give ferrocenyl dichlorophosphine, whereas treatment

with phenyldichloro-phosphine under similar conditions forms P,P-

diferrocenyl-P-phenyl phosphine.

6. Ferrocene reacts with P4S10 forms a diferrocenyl-dithiadiphosphetane

disulfide.

7. Some transformations of dilithioferrocene.

8. The phosphine ligand 1,1'-bis(diphenylphosphino)ferrocene (dppf) is

prepared from dilithioferrocene.

9. Reaction of [Fe(η-C5H5)(η-C6H6)]PF6 with Nucleophiles

The reaction of the iron benzene p-complex with LiAlH4 and LiAlD4, sources of

H and D- ions respectively. Arenes, for instance benzene, are more susceptible

to attack by electrophiles than nucleophiles. However, associating with a metal

often alters the reactivity of organic ligands. Thus, the reactivity of the benzene

ligand in the iron p-complex towards the nucleophiles H- and D- is examined.

3. Chrysene

Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular

formula C18H12 that consists of four fused benzene rings. It is a natural

constituent of coal tar, from which it was first isolated and characterized. It is

also found in creosote at levels of 0.5-6 mg/kg. Chrysene is a constituent of

tobacco smoke.

The name "chrysene" originates from Greek Χρύσoς (chrysos), meaning "gold",

and is due to the golden-yellow color of the crystals of the hydrocarbon, thought

to be the proper color of the compound at the time of its isolation and

characterization. However, high purity chrysene is colorless, the yellow hue

being due to the traces of its yellow-orange isomer tetracene, which cannot be

separated easily.

Figure: Chrysene

Synthesis

1. Via acid-catalyzed rearrangement of cyclobutanone derivatives

2. Jutz synthesis of 4-cyanochrysene

3. Other synthesis reactions

Reactions

1. The reaction of dimethyldioxirane with chrysene: Formation of a

trioxide

2. Photochemical reaction of chrysene in acetonitrile/water

The photochemical reaction of chrysene under UVA light irradiation in

acetonitrile/water yields 1,4- and 5,6-chrysenequinones and one lactone, H-

benzo[d]naphtha[1,2-b]pyran-6-one. Solvent-dissolved oxygen is necessary for

the photolysis. The presence of TiO2, Al2O3, La2O3, KI, or I2 decreased the

photolysis rate. The free radical scavenger, Na2S2O3, greatly suppressed the

chrysene photolysis, indicating that a free radical process was involved.

3. Biodegradation of chrysene by Polyporus sp. S133 in liquid medium

4. Synthesis of 1-ethylchrysene.

5. Synthesis of chrysene phosphoramidite

Figure: Synthesis of chrysene phosphoramidite 5. Reagents and conditions: (a)

Br2, 1,2-dichloroethane, 85 oC, 18 h, 84%; (b) 5-hexyn-1-ol, Pd[PPh3]2Cl2,

CuI, THF–Et3N (1 : 1), 80 _C, 46 h, 42%; (c) DMTCl,Et3N, THF, 24 h, rt,

53%; (d) PAMCl, Et3N, DCM, 1 h, rt, 71%.

6. New polyaromatic compounds with a side chain

7. Electrophilic attack on chrysene occurs selectively in the 6-position. Thus

the reaction of chrysene with sulfuryl chloride and bromine give 6-

chloro- and 6-bromochrysene, respectively, and further halogenation

affords the 6,12-dichloro and 6,12-dibromo darivatives (Clar. 1964).

8. Halogenation of chrysene with TiCl4 or AlBr3 catalyzed by NO2 also

affords mainly the 6-halo derivatives (Sugiyama, 1982).

9. Nitration with nitric acid in acetic anhydride (Dewar et al. 1956) or

dinitrogen tetroxide (Radner, 1983) give 6-nitrochysene as the major

product.

10. Sulfonation of chrysene with chlorosulfonic acid affords chrysene-6-

sulfonic acid (Clar, 1964).

11. Acetylation with acetyl chloride and AlCl3 in CH2Cl2 furnishes 6-

acetylchrysene, similar reaction in nitrobenzene provides 2-, 3-, and 6-

acetylchrysene in the ration 3:2:1 (Carruthers, 1953)

4. Azulene

Azulene is an organic compound and an isomer of naphthalene. Whereas

naphthalene is colourless, azulene is dark blue. Two

terpenoids, vetivazulene (4,8-dimethyl-2-isopropylazulene) and

guaiazulene (1,4-dimethyl-7-isopropylazulene), that feature the azulene skeleton

are found in nature as constituents of pigments in mushrooms, guaiac wood oil,

and some marine invertebrates.

Azulene has a long history, dating back to the 15th century as the azure-

blue chromophore obtained by steam distillation of German chamomile. The

chromophore was discovered in yarrow and wormwood and named in 1863 by

Septimus Piesse. Its structure was first reported by Lavoslav Ružička, followed

by its organic synthesis in 1937 by Placidus Plattner.

Synthesis

1. From indane (ring-expansion method):

Synthetic routes to azulene have long been of interest because of its unusual

structure. In 1939 the first method was reported by St. Pfau and Plattner starting

from indane and ethyl diazoacetate. This is the most widely used method for the

azulene synthesis. In this method, indane (or indane derivatives) is treated drop-

wise with ethyl diazoacetate at 130-135°C for about 2 hr. Upon completion of

the addition, the temperature is raised to 160-165°C for several hours. During

the process, first carbene is generated in situ through nitrogen extrusion

reaction.

2. From methylamine derivative (Demjanow ring expansion):

3. From adipic acid:

Two molecules of adipic acid combine together by the loss of one molecule of

water and one molecule of carbon dioxide resulting in the formation of 6-oxo-

undecanedioic acid. Loss of one water molecule from 6-oxo-undecanedioic acid

results in the formation of 2-(4- carboxy-butyl)-cyclopent-1-enecarboxylic acid.

Again loss of one molecule of water and one molecule of carbon dioxide from

2-(4-carboxy-butyl)-cyclopent-1-enecarboxylic acid results in the formation of

2,3,5,6,7,8-hexahydro-1H-azulen-4-one. Further dehydration and

dehydrogenation from it results in the formation of azulene.

4. Hafner Azulene Synthesis

Synthesis of azulenes by condensation of cyclopentadienes with derivatives of

glutaric dialdehyde derivatives derived from pyridine.

5. From substituted tropone

6. An efficient one-pot route entails annulation of cyclopentadiene with

unsaturated C5-synthons. The alternative approach

from cycloheptatriene has long been known, one illustrative method

being shown below.

Procedure: step 1: cycloheptatriene 2+2 cycloaddition with dichloro ketene step

2: diazomethane insertion reaction step 3: dehydrohalogenation reaction

with DMF step 4: Luche reduction to alcohol with sodium borohydride step

5: elimination reaction with Burgess reagent step 6: oxidation with p-

chloranil step 7: dehalogenation with polymethylhydrosiloxane, palladium(II)

acetate, potassium phosphate and the DPDB ligand

Reactions

1. Friedel craft acylation of azulene using acetyl chloride and aluminium

chloride leads to the formation of 1-acetyl azulene and 1,3-

diacetylazulene

2.

3. Synthesis of azulene derivatives

Figure: Synthesis of azulene derivatives substituted at the 1-position (a and b) and the

2-position (c and d) by direct C@H activation. bpy=2,2’-bipyridine, cod=1,5-

cyclooctadiene, pin=pinacol, TFAA=trifluoroacetic anhydride

4. Reaction of azulenes with 1-trifluoromethanesulfonylpyridinium

trifluoromethanesulfonate (TPT)

5. Thermal rearrangement

6. Reaction with aryl glyoxal and 1,3-dicarbonyl

7. With acid and bases

8. Organometallic complexes

In organometallic chemistry, azulene serves as a ligand for low-valent metal

centers, which otherwise are known to form π-complexes with

both cyclopentadienyl and cycloheptatrienyl ligands. Illustrative complexes are

(azulene)Mo2(CO)6 and (azulene)Fe2(CO)5