AROMATIZATION is a chemical reaction in which an aromatic system is
formed. It can also refer to the production of a new aromatic moiety
in a molecule which is already aromatic. Theoretically, this can be
achieved by dehydrogenation of existing cyclic compounds (such as in
converting cyclohexane into benzene ) or by formation of new cyclic
system (such as in the cyclotrimerization of acetylene to benzene);
practically, other moieties are typically required to carry out such
conversion, and other approaches like applying condensation reactions
Aromatization includes the formation of any aromatic
system (including heterocyclic systems), and is not restricted to
benzene and its derivatives.
* 1 History
* 1.3 Characteristics of aromatic systems
* 2 Significance of aromatic systems
* 3 Formation of new cyclic systems
* 3.2 Other cyclizations
* 4 From existing cyclic systems
* 4.1 Ionic systems
* 5 See also
* 6 References
Main article: benzene
The substance now called benzene , C6H6, was known as a component of
Southeast Asian aromatic resins used in perfumery since the 15th
century. It was first isolated and identified by
Michael Faraday in
1825 who named it bicarburet of hydrogen. In 1845, Charles Mansfield
, working under
August Wilhelm von Hofmann
August Wilhelm von Hofmann , isolated benzene from
coal tar and began the industrial-scale production based on this
method four years later. Over time, consensus developed that other
substances were chemically related to benzene, comprising a diverse
chemical family; Hofmann used the word "aromatic " for this family for
the first time in 1856. Historic benzene formulae (from left to
right) by Claus (1867), Dewar (1867), Ladenburg (1869), Armstrong
(1887), Thiele (1899), and Kekulé (1865).
Dewar benzene and prismane
are different chemicals that have Dewar's and Ladenburg's structures.
Thiele and Kekulé's structures are used today. The various
representations of benzene
Historic interest in benzene arose from the uncertainty over its
structure and reactivity. It was well established that it had a carbon
to hydrogen ratio of 1:1 which suggested the presence of double or
triple bond carbon-to-carbon bonds, yet such unsaturated compounds
typically undergo addition reactions , which benzene does not.
Numerous potential structures were proposed, including by Claus ,
Dewar , Ladenburg , Armstrong , and Thiele . The most influential
proposal was that of Kekulé who in 1865 suggested a cyclic structure
of six carbon atoms with alternating single and double bonds Kekulé
based his argument on the now well-known observations of arene
substitution patterns , namely that there are always only one isomer
of any monoderivative of benzene and three isomers of every
disubstituted derivative. Critics noted that Kekulé's proposal
implied that there should be two distinguishable isomers for an
ortho-substitution (a 1,2-disubstituion) depending on whether a single
or double bond joined these two carbons. Kekulé suggested in reply
that benzene had two complementary structures and that these forms
rapidly interconverted, which would also explain the lack of addition
reactions (as the valency of each carbon atom is not that expected of
a double bond). Though this interconversion is incorrect —in fact,
they are both resonance contributors to the actual structure (which is
closest to Thiele's structure)— the geometry and structure proposed
by Kekulé are correct.
Benzene is the prototypical example of an
aromatic compound, a hitherto unknown class of compounds, and its
synthesis was the first example of aromatization, the process and
chemical reactions whereby an aromatic system is formed. The new
understanding of benzene and all aromatic compounds was so important
for both pure and applied chemistry that in 1890 the German Chemical
Society organized an elaborate appreciation in Kekulé's honor,
celebrating the twenty-fifth anniversary of his first benzene paper;
its 150th anniversary was also marked. The cyclic nature of benzene
was confirmed crystallographically by
Kathleen Lonsdale in 1929.
X-ray diffraction confirms that all six carbon-carbon bonds in
benzene are the same length, 140 pm (1.40 Å ) – intermediate
between the bond lengths of single (154 pm) and double (134 pm)
carbon-carbon bonds. This is consistent with delocalization of the
electrons from the double bonds in Kekulé's structures equally
between each of the six carbon atoms which form a planar ring. The
molecular orbital theory description involves the formation of three
delocalized π orbitals spanning all six carbon atoms, while the
valence bond theory description involves a superposition of resonance
structures. This electronic structure provides benzene with unusual
stability and contributes to the molecular and chemical properties
which are now known as aromaticity . To accurately reflect the nature
of the bonding as having neither single nor double bonds, but rather
something in between, benzene is often depicted with a circle inside a
hexagonal arrangement of carbon atoms, similar to the structure first
illustrated by Thiele.
Main article: aromaticity Crystal structure of a fullerene
bound in a buckycatcher through π-stacking.
In organic chemistry , the term aromaticity has come to refer to
cyclic (ring-shaped), planar (flat) organic compounds with a ring of
resonance bonds that exhibit unusually high stability relative to
other geometric or connective arrangements with the same set of atoms.
Aromatic molecules consequently do not easily break apart and react
with other substances (they have low chemical reactivity ) and exhibit
special physical characteristics such as π stacking (the
understanding of which is still the subject of ongoing investigation
). In terms of electronic structure, aromaticity describes a
conjugated system made of alternating notionally single and double
bonds, plus in some cases a lone pair from an occupied p-orbital
perpendicular to the plane of the ring. The term "aromatic sextet" for
the electronic system which resists disruption (and hence possesses
unusual stability) is attributed to Sir Robert Robinson though the
idea can be traced further to Crocker and Armstrong. This
configuration allows for the delocalization of π electrons around the
ring, increasing the molecule's stability. Such molecules cannot be
accurately represented by a single structure and are understood to
exist as a combination of resonance hybrids, though single
representations are frequently chosen as a short-hand for convenience.
As an example, the cyclohexatriene structures Kekulé proposed should
have alternating single and double bonds of different lengths, yet
each is the same and has a length consistent with benzene having six
"one-and-a-half" bonds which is logically the superposition of each
resonance contributor. A more accurate representation of the
electrons following the merging of p orbitals into π-bonds (see
illustration above) is Armstrong's inner cycle model as the electron
density is evenly distributed around the aromatic ring in molecular
orbitals considered to have π symmetry, though this representation
is inconvenient in arrow pushing in mechanistic organic chemistry. The
quantum mechanical origins of aromaticity were first modelled by
Hückel in 1931.
CHARACTERISTICS OF AROMATIC SYSTEMS
An aromatic (or aryl ) compound contains a set of covalently bound
atoms with specific characteristics:
Pyrrole (left) is a
much weaker base than pyridine (right), despite both heterocycles
having a lone pair on the nitrogen atom
* A delocalized conjugated π system, most commonly an arrangement
of alternating single and double bonds
Coplanar structure, with all the contributing atoms in the same
* Contributing atoms arranged in one or more rings
* A number of π delocalized electrons that is even, but not a
multiple of 4 . That is, 4n + 2 π-electrons, where n = 0, 1, 2, 3,
and so on. This is known as Hückel\'s rule .
Systems following Hückel's rule, having 4n + 2 π-electrons, are
aromatic, but if there are 4n π-electrons along with characteristics
1–3 above, the molecule is said to be antiaromatic . Antiaromatic
systems are destabilized and sometimes adopt distorted structures to
avoid planarity. An atom in an aromatic system can have other
electrons that are not part of the system, and are therefore ignored
for the 4n + 2 rule. For example, in furan , the oxygen atom is sp2
hybridized. One lone pair (in the pz orbital) contributes to the π
system and the other in the plane of the ring (in the non-bonding sp2
orbital and analogous to the C–H bond in the other positions) does
not; consequently, there are 6 π-electrons in furan and it is
aromatic. Comparing the nitrogen heterocycles pyrrole and pyridine ,
each has a 6 π-electron system: the lone pair on the nitrogen atom is
required to complete the aromatic sextet in the case of pyrrole while
the lone pair of in pyridine is not required. The difference can be
seen in the basicity of the two systems, as pyridine can be easily
protonated (its lone pair is available for reaction) whilst pyrrole
does not react readily. This can be seen in the pKa values of the
respective conjugate acids – 8.25 for pyridine compared with −0.27
for pyrrole. In the case of oxazole and isoxazole , the lone pairs
contributed to the aromatic sextet come from the occupied pz orbital
of oxygen atom rather than the nitrogen lone pairs in hybridized sp2
Electrostatic potential surfaces illustrate the difference
in electronegativity of the heteroatom bearing an electron pair
depending on whether or not it forms part of the aromatic system. In
the diagram below, the nitrogen atom in pyridine is markedly more
electronegative than nitrogen atom in indole , as indicated by the red
regions on the surfaces, because the pyridine nitrogen is free to act
Lewis base whereas the indole nitrogen is not. The surfaces also
illustrate that heteroatoms influence the ability to form cation–pi
interactions as cations will be attracted to the aromatic system much
more strongly in indole and in benzene than will be the case in
pyridine where a direct interaction with the lone pair is more
Electrostatic potential surfaces for indole, benzene,
and pyridine (left to right), available from the Spartan software
suite of computational chemistry tools. Shading colors vary from blue
(electropositive areas) to red (electronegative areas) and are
determined by calculating the energy of interaction of a positive
spherical point charge (a proton ) as it moves over the surface of the
molecule. An aromatic ring current (orange) interacts with an
applied magnetic field, B0 (red arrow) and induces a magnetic field
(purple) which changes the chemical shifts in
Molecules which can possess an aromatic system will tend to undergo
any electronic or conformational structure changes needed to attain
aromaticity. Chemical changes which occur as a consequence include a
tendency to undergo electrophilic aromatic substitution and
nucleophilic aromatic substitution reactions, but not electrophilic
addition reactions as happens with carbon–carbon double bonds. This
chemical behaviour offers one way to confirm experimentally that an
aromatic system is present. The nuclear magnetic resonance (NMR)
phenomenon offers a spectroscopic method for demonstrating and
investigating aromatic systems as the circulating π-electrons produce
ring currents which oppose the applied magnetic field and lead to
changes in the measured chemical shifts for both 1H and 13C nuclei.
X-ray crystal structures confirm the existence of π-π interactions
in aromatic systems, such as the dimer of benzene; Perpendicular and
offset parallel configurations can be observed in the crystal
structures of many simple aromatic compounds. More recently, direct
measures of electron delocalization allows direct quantification of
the extent of aromaticity.
SELECTION OF SIMPLE HYDROCARBON AND HETEROCYCLIC AROMATIC SYSTEMS
SIGNIFICANCE OF AROMATIC SYSTEMS
Zwitterionic form of histidine , with the aromatic side chain
including an imidazole functionality shown in blue
Research work undertaken to understand the benzene system opened a
new field in organic chemistry leading to the modern understanding of
aromatic systems as common throughout the chemical world. Benzene
itself is a useful solvent as well as a gasoline additive due to its
high octane number and ability to reduce engine knocking , although
methylbenzene is now being used as an alternative.
Benzene is also a
precursor used in the manufacture of styrene , nylon , and epoxy
Naphthalene is the active ingredient in traditional mothballs
and is used to make the insecticide carbaryl .
Aromatic systems appear
in many other important industrial products including
polystyrene , and PET plastics .
Aromatic systems are common in biological systems. Amongst the 20
standard naturally occurring amino acids necessary for protein
synthesis , four are aromatic – histidine , phenylalanine ,
tryptophan , and tyrosine . (
Histidine is aromatic at all pH levels
due to its imidazole side-chain but is also basic and so is often
grouped with the two other basic amino acids arginine and lysine . )
Aromatic amino acids also serve as the starting materials for
biosynthesis of vitamins , neurotransmitters and hormones including
dopamine and norepinephrine from tyrosine, serotonin and niacin from
tryptophan, and tyrosine, thyroxine , and N-methylphenethylamine
from phenylalanine. Pharmaceuticals with many different applications
possess aromatic moieties, as illustrated below.
SELECTION OF PHARMACEUTICALS CONTAINING AROMATIC SYSTEMS
Beta blocker and antihypertensive
Leukemia chemotherapeutic agent
All five of the bases found in
RNA are aromatic – adenine
and guanine (purine core structures), and cytosine , thyamine , and
uracil (pyrimidine core structures). Biologically active substances
including caffeine and uric acid (responsible for gout ) also possess
RNA have polymeric structures with
repeating nucleotide units, each possessing a phosphate group, a
ribose or deoxyribose sugar, and a nucleobase. Nucleobases are
Watson-Crick base paired through hydrogen bonding , and these pairs
are stacked through the center of the double helix through π-π
interactions of their aromatic systems. The possibility for extended
planar aromatic systems such as acridine (dibenzopyridine) to insert
between base pairs was recognised by Lerman in a process now known as
intercalation . Intercalating agents have applications in
biochemical research as stains (ethidium bromide ) and in medicine
as chemotherapeutic agents (doxorubicin ). Polycyclic aromatic
hydrocarbons like benzopyrenes and dibenzanthracene are some of the 43
known carcinogens in tobacco smoke (the Environment Protection Agency
classes tobacco smoke as a class A carcinogen).
Benzopyrene has been
shown to form preferentially in lung cancer hotspots and is known to
be transformed into a metabolite ,
(+)-Benzopyrene-7,8-dihydrodiol-9,10-epoxide , which forms mutagenic
DNA . The epoxide and group on the metabolite alkylate
the N2 of guanine bases while the pyrene system intercalates between
base pairs, disrupting the structure. Intercalation interferes with
DNA replication which can inhibit tumour growth but can also cause
changes in the base pair sequence which explains its potential for
(+)-Benzopyrene-7,8-dihydrodiol-9,10-epoxide , the metabolite of
benzopyrene which binds with DNA
(Left): A segment of
DNA with nucleobase pairs π-stacked through the
center of the double helix.
(Center): Intercalation distorts the
DNA structure by inserting
suitable extended aromatic systems between base pairs. Ethidium
bromide is shown along with an example of its intercalation.
(Right): Tobacco smoke carcinogen and mutagen benzopyrene
metabolises to (+)-Benzopyrene-7,8-dihydrodiol-9,10-epoxide which
reacts with a guanine base and intercalates the pyrene system between
FORMATION OF NEW CYCLIC SYSTEMS
Bönnemann cyclization to produce pyridine derivatives
Many aromatization processes start with two or three acyclic
molecules which are fused into a new aromatic ring. Alkyne
trimerization is one simple case whereby the cycloaddition of three
alkynes yields a substituted benzene – cyclization of 2-butyne
forming hexamethylbenzene , for example.
Benzene itself can be
prepared by Reppe synthesis with the ligands on the nickel catalyst
dictating whether production of benzene or cyclooctatetraene
predominates. If an unsymmetric alkyne is used, a variety of products
Rhodium -catalysed cyclotrimerization of phenylacetylene
yields both the 1,2,4- and 1,3,5- isomers of triphenylbenzene along
with substantial quantities of acyclic enyne products, though use of
an alternative catalyst system can produce 97% selectivity for the
Antiaromatic cyclobutadiene -derivatives and
non-aromatic cyclooctatetraene products are also observed
side-products of cyclotrimerizations, and when a mixture of
substituted acetylenes is used regioselectivity becomes especially
problematic. Replacement of one acetylene by a nitrile allows
formation of pyridine or one of its derivatives by Bönnemann
Alkyne trimerization involving an aryne
generated in situ
Cyclisation carried out with a diyne and a separate alkyne affords
greater control as well as high atom economy . Using commercially
available cyclopentadienylcobalt dicarbonyl , CpCo(CO)2, as catalyst,
bis(trimethylsilyl)acetylene (BTMSA) will react with a
diyne-1,2-disubstituted benzene to form an anthroquinone aromatic
system as shown below:
Benzyne , generated in situ from a bezene ring
bearing ortho -distributed triflate and trimethylsilyl substituents,
can be used to generate an aryne in place of an acetylene and combined
with a suitable diyne. Such a benzene derivative reacts with
1,7-octadiyne in the presence of a suitable catalyst to generate a
naphthalene system. This is an example of a hexadehydro Diels-Alder
If three alkyne moieties are tethered together, it is possible to
create three rings in a single step without the problems with side
products mentioned above. An example is seen in the synthesis of
calomelanolactone using Wilkinson\'s catalyst to achieve an
Bergman cyclization is conceptually similar, using an enediyne
with (Z)-stereochemistry plus a hydrogen donor. (Z)-hex-3-en-1,5-diyne
cyclizes to benzene after reduction of the intermediate diradical, for
example. The enediyne moiety can be included within an existing ring,
allowing access to a bicyclic system under mild conditions as a
consequence of the ring strain in the reactant.
Cyclodeca-3-en-1,5-diyne reacts with
1,3-cyclohexadiene to produce
benzene and tetralin at 37 °C, the reaction being highly favorable
owing to the formation of two new aromatic rings:
Wulff–Dötz reaction an alkyne, carbon monoxide and a
chromium carbene complex are the reactants and the product is a
chromium half-sandwich complex . The phenol ligand can be isolated
by mild oxidation with ceric ammonium nitrate . Diels-Alder
synthesis of hexaphenylbenzene from tetracyclone
Diels–Alder reactions of alkynes with
cyclopentadienone with expulsion of carbon dioxide or carbon monoxide
, respectively, also form arene compounds.
Tetracyclone reacts with
diphenylacetylene to form hexaphenylbenzene after loss of carbon
monoxide, for example. The
Wagner-Jauregg reaction involves a double
Diels-Alder addition of maleic anhydride to a 1,1-diarylethylene,
resulting in an aryl-substituted naphthalene product. The reaction is
unusual in that the aromaticity of the starting material is lost as
the aryl-substituted styrene moiety is sufficiently activated to act
as a dienophile, but the intermediate undergoes re-aromatization with
Huisgen cycloaddition , an azide is combined with an alkyne to
generate a triazole , and is a specific example of a 1,3-dipolar
cycloaddition to a form a heterocyclic five-membered aromatic rings
with regio- and stereoselectivity . American chemist Barry Sharpless
has described this cycloaddition as "the cream of the crop" of click
chemistry and "the premier example of a click reaction."
Synthesis of 2,3-diphenylquinoxaline
Condensation reactions are another route to the formation of
heterocyclic aromatic systems. The Chichibabin pyridine synthesis
involves condensation of acrolein (from a Knoevenagel reaction of
acetaldehyde with formaldehyde ) with acetaldehyde and ammonia,
followed by catalysed oxidation of dihydropyridine to pyridine .
Hantzsch\'s approach to pyridines dates from 1881 and involves
condensation of a β-keto acid or ester with an aldehyde and a
nitrogen donor (typicallyammonia or one of its salts . Knoevenagel
modified this process for asymmetrically substituted pyridine
Quinoxaline systems can be formed by the condensation of
1,2-diketones with o-diaminobenzene, quinoxaline itself forming from
glyoxal and o-phenylenediamine . Combining benzil with
o-phenylenediamine in the presence of
2-iodobenzoic acid (IBX,
catalyst) yields 2,3-diphenylquinoxaline in high yield.
Synthesis of pentaphenylarsole from diphenylacetylene
Another effective means of generating heterocyclic systems is to
effect ring-closure using the heteroatom. "Moderately aromatic" arsole
derivatives can be formed by dimerising diphenylacetylene with lithium
to form 1,4-dilithiotetraphenyl-1,3-butadiene. Phenylarsenous
dichloride is added to close the ring to form pentaphenylarsole. The
diiodo analogue of the lithium salt can be used in its place, and
other pnictogen heterocycles can be produced by replacing the arsenic
compound as appropriate.
The high stability of aromatic systems was demonstrated as early as
1880 in a one-pot synthesis of hexamethylbenzene from methanol. Though
the authors did not understand what the process occurring, they
interpreted it as a condensation of methylene (CH2) units followed by
Friedel-Crafts alkylation with chloromethane (generated in situ) to
exhaustively methylate the resulting ring. At 283 °C, the melting
point of the zinc chloride catalyst, the reaction has a ΔG of −1090
kJ mol−1. 15 CH
3OH → C
6 + 3 CH
4 + 15 H
FROM EXISTING CYCLIC SYSTEMS
Semmler-Wolff synthesis of aniline
Non aromatic rings can be aromatized through a variety of simple
transformations. Dehydration allows the Semmler-Wolff transformation
2-cyclohexenone oxime to aniline under acidic conditions. In
applying redox processes, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and an acid catalyst has been used to synthesise a steroid with
a phenanthrene core by oxidatation accompanied by a double methyl
migration . In the process, DDQ is itself reduced into an aromatic
hydroquinone product. The p-cymene ruthenium(II) dichloride
Redox reactions involving transition metals can also be used to
produce aromatic systems which then serve as ligands in organometallic
is oxidised to p-iso-propyltoluene with the reduction of ruthenium
trichloride . The resulting ruthenium(II) dimer 2] can be cleaved to
a monomeric adduct with a suitable
Lewis base – is formed with
triphenylphosphine , for example. Exchange reactions occur with other
arenes and so the p-cymene product can be recovered.
Ciamician-Dennstedt rearrangement of a pyrrole to a pyridine
The 10 π-electron system cyclodecapentaene is non-aromatic as it is
non-planar, but aromatic derivatives can be prepared. In the following
reaction sequence, naphthalene is converted through a series of
non-aromatic intermediates to 1,6-methanoannulene by
Birch reduction ,
carbene addition, and then DDQ oxidation. Many other conversions of
one aromatic system to another are known. One simple example is the
industrial synthesis of pyrrole by exchanging the oxygen atom in furan
for a nitrogen moiety.
Ammonia and solid acid catalysts (like SiO2 and
Al2O3) are required.
Pyrrole can be converted to 3-chloropyridine by
insertion of a carbene into the five-member ring.
to form a strained bicyclic cyclopropane system which then opens to
form the six-member pyridine product, a transformation known as the
Ciamician-Dennstedt rearrangement .
Aromatase is an enzyme which converts some enones with a six membered
ring to aromatic rings - specifically testosterone to estradiol and
androstenedione to estrone . Each of these aromatizations involves the
oxidation of the C-19 methyl group to formic acid to allow for the
formation of the aromatic system, conversions which are necessary
parts of estrogen tumorogenesis in the development of breast cancer
and ovarian cancer in postmenopausal women and gynecomastia in men.
Aromatase inhibitors like exemestane (which forms a permanent and
deactivating bond with the aromatase enzyme) and anastrozole and
letrozole (which compete for the enzyme) have been shown to be more
effective than anti-estrogen medications such as tamoxifen likely
because they prevent the formation of estradiol.
Structural formula of the tropylium cation
Aromaticity can be found in individual ions which satisfy Hückel\'s
rule , which can be accessed by simple redox or acid-base reactions. A
Hückel n = 0 cation can be prepared from cyclopropene systems, an
example being the bromide salt of the triphenylcycloprenium cation .
Ionic Hückel n = 1 systems are known both for anions
(cyclopentadienyl ) and cations (tropylium ). Tropylium, C
7, was first reported in 1881 though its identity was not recognized
for over 60 years. Tropylium appears regularly in mass spectrometry
, given a signal at m/z = 91. Compounds containing the benzyl moiety
produce the molecular fragment PhCH+
2, which isomeizes to the stable tropylium cation. Cycloheptatriene
(non-aromatic) undergoes a redox reaction with bromine to produce
tropylium with the release of hydrogen bromide ; it can also be
oxidised with chromic acid and a ring-contracting rearrangement occurs
in which benzaldehyde is produced: C
8 + Br
2 → C
7 + Br−
+ HBr C
7 + HCrO−
4 → C
5CHO + CrO
2 + H
Thallium cyclopentadienide , an organometallic polymer of
the cyclopentadienyl anion
Cyclopentadiene is more acidic than a typical hydrocarbon (pKa ~
15-16 compared with cyclopentane's pKa ~ 45) and so can be readily
deprotonated to its aromatic conjugate base, an approach used to form
compounds like thallium cyclopentadienide , an example of an
organometallic polymer . A direct redox reaction of sodium metal with
cyclopentadiene yields sodium cyclopentadienide , though a convenient
base like sodium hydride can also be used. Compounds like these can
be used to prepare metallocene compounds such as ferrocene where two
cyclopentadienyl ligands coordinated to a metal centre.
Freshly-cracked cyclopentadiene will react with iron(II) chloride and
a base to effect deprotonation to produce ferrocene in good yield.
Even a weak bases such as amines are sufficient to deprotonate
cyclopentadiene for this reaction. Like most aromatic substances, the
product here undergoes substitution reactions rather than addition
reactions . As an example,
Friedel-Crafts acylation of ferrocene with
acetic anhydride yields acetylferrocene just as acylation of benzene
yields acetophenone under similar conditions.
Corannulene , a circulene , is a fragment of buckminsterfullerene
which is found in the buckycatcher shown above and often called a
buckybowl due to its bowl-like shape. Motivation for its synthesis was
the hypothesis that it would display annulene-within-an-annulene
aromaticity, with a 6 π-electron cyclopentadienyl anion core within a
14 π-electron annulenyl cation. Later work has questioned the
validity of this model though the molecule is definitely aromatic.
The dianion has been demonstrated to be antiaromatic and the
tetraanion is aromatic. Sandwich compound uranocene ,
Cyclooctatetraene in its native "tub-shaped" conformation
Annulene-within-an-annulene model of corannulene aromaticity
Cyclooctatetraene has been shown to be non-aromatic and to possess a
bent "tub-shaped" conformation with alternating double and single
bonds. Its planar conformation would be antiaromatic and is thus
disfavoured. By contrast, its dianion C
8 is both planar and aromatic and has a Hückel electron count of 10.
It is formed readily by direct reaction with potassium metal and
uranocene , an analogue of ferrocene, has been prepared from
dipotassium cyclooctatetraenide and uranium tetrachloride in
tetrahydrofuran at 0 °C:
Equilibrium between phenol and its cyclohexadienone tautomer
1,4-Dioxotetralin in equilibrium with 1,4-naphthalenediol
An isomerization reaction involves the rearrangement of atoms within
a single molecule to one of its isomers . Some isomerization processes
may occur spontaneously, resulting in an equilibrium between the
isomers, and the position of the system generally favors an aromatic
product if the process is itself an aromatization. Amongst keto-enol
tautomers , the keto form is generally favoured on by thermodynamics,
but in the case of cyclohexa-2,4-dienone its aromatic isomer phenol is
strongly favoured. The equilibrium constant for this system has been
determined to be 10−13, meaning that in phenol there are 10 trillion
molecules in the aromatic enol form for every molecule in the keto
Isomerization reactions are temperature dependent, for example
melting 1,4-naphthalenediol at 200 °C produces a 2:1 mixture with its
keto form, 1,4-dioxotetralin. The conversion to the 1,4-dioxotetralin
tautomer can be driven to completion when bound in a chromium
tricarbonyl piano stool complex . Representation of two resonance
forms of azulene
A non-tautomeric example of isomerization can be seen in the azulene
–napthalene system, a thermal rearrangement in which each of the
compounds is stable at room temperature; such interconversion
aromatization reactions are rare.
Carbon-13 isotope studies have
shown scrambling of the α- and β- positions of naphthalene through
an azulene intermediate at elevated temperatures though naphthalene
has roughly double the aromatic stabilization of azulene. The
synthesis of the azulene system is an example of a multi-step organic
synthesis , in this case from cycloheptatriene, as shown below.
Azulene is unusual in that it is a polar hydrocarbon – the molecular
dipole moment of azulene is 1.08 D compared with naphthalene's 0 D.
This can be understood by recognizing that one of the important
resonance contributors for azulene has the tropylium cation fused to
the cyclopentadienyl anion; reactivity studies confirm the
electrophilic and nucleophilic characteristics of these rings,
* ^ A B Reppe, W. ; Schlichting, O.; Klager, K.; Toepel, T. (1948).
"Cyclisierende Polymerisation von Acetylen I Über Cyclooctatetraen".
Justus Liebigs Annalen der Chemie (in German). 560: 1–92. doi
* ^ Morris, E. T. (1984). Fragrance: The Story of Perfume from
Cleopatra to Chanel. Charles Scribner\'s Sons . p. 101. ISBN
* ^ Faraday, M. (1825). "On New Compounds of Carbon and Hydrogen,
and on Certain Other Products Obtained During the Decomposition of Oil
Philosophical Transactions of the Royal Society
Philosophical Transactions of the Royal Society . 115:
440–466. doi :10.1098/rstl.1825.0022 .
* ^ Kaiser, R. (1968). "Bicarburet of Hydrogen. Reappraisal of the
Benzene in 1825 with the Analytical Methods of 1968".
Angewandte Chemie International Edition in English . 7 (5): 345–350.
doi :10.1002/anie.196803451 .
* ^ Hofmann, A. W. von (1845). "Ueber eine sichere Reaction auf
Annalen der Chemie und Pharmacie (in German). 55 (2):
200–205. doi :10.1002/jlac.18450550205 .
* ^ Mansfield, C. B. (1849). "Untersuchung des Steinkohlentheers".
Annalen der Chemie und Pharmacie (in German). 69 (2): 162–180. doi
* ^ Hofmann, A. W. von (1856). "On Insolinic Acid". Proceedings of
the Royal Society . 8: 1–3. doi :10.1098/rspl.1856.0002 .
* ^ Claus, A. K. L. (1867). "Theoretische Betrachtungen und deren
Anwendungen zur Systematik der organischen Chemie". Berichte über die
Verhandlungen der Naturforschenden Gesellschaft zu Freiburg im
Breisgau (in German). 4: 116–381.
* ^ Dewar, J. (1867). "On the Oxidation of Phenyl Alcohol, and a
Mechanical Arrangement Adapted to Illustrate Structure in the
Proceedings of the Royal Society of
Edinburgh . 6: 82–86. doi :10.1017/S0370164600045387 .
* ^ Ladenburg, A. (1869). "Bemerkungen zur aromatischen Theorie".
Berichte der Deutschen Chemischen Gesellschaft (in German). 2:
140–142. doi :10.1002/cber.18690020171 .
* ^ A B Armstrong, H. E. (1887). "An Explanation of the Laws which
Govern Substitution in the case of Benzenoid Compounds". Journal of
the Chemical Society . 51: 258–268. doi :10.1039/CT8875100258 .
* ^ A B Thiele, J. (1899). "Zur Kenntnis der ungesättigten
Justus Liebigs Annalen der Chemie (in German). 306:
87–142. doi :10.1002/jlac.18993060107 .
* ^ Kekulé, F. A. (1865). "Sur la Constitution des Substances
Bulletin de la Société Chimique de Paris (in French).
* ^ Kekulé, F. A. (1866). "Untersuchungen über Aromatische
Annalen der Chemie und Pharmacie (in German).
137 (2): 129–36. doi :10.1002/jlac.18661370202 .
* ^ Read, J. (1995). From Alchemy to Chemistry. New York: Dover
Publications . pp. 179–180. ISBN 9780486286907 .
* ^ Rocke, A. J. (2015). "It Began with a Daydream: The 150th
Anniversary of the Kekulé
Benzene Structure". Angewandte Chemie
International Edition in English . 54 (1): 46–50. doi
* ^ Lonsdale, K. (1929). "The Structure of the
Benzene Ring in
Proceedings of the Royal Society A . 123 (792): 494–515.
Bibcode :1929RSPSA.123..494L. doi :10.1098/rspa.1929.0081 .
* ^ Lonsdale, K. (1931). "An X-Ray Analysis of the Structure of
Hexachlorobenzene, Using the Fourier Method". Proceedings of the Royal
Society A . 133 (822): 536–553.
Bibcode :1931RSPSA.133..536L. doi
* ^ A B "Bonding in
Benzene – the Kekulé Structure".
www.chemguide.co.uk. Retrieved 31 May 2016.
* ^ Moran, D.; Simmonett, A. C.; Leach, F. E.; Allen, W. D.;
Schleyer, P. V.; Schaefer, H. F. (2006). "Popular Theoretical Methods
Benzene and Arenes to be Nonplanar". Journal of the American
Chemical Society . 128 (29): 9342–9343. doi :10.1021/ja0630285 .
PMID 16848464 .
* ^ Cooper, D. L.; Gerratt, J.; Raimondi, M. (1986). "The
Electronic Structure of the
Benzene Molecule". Nature . 323 (6090):
Bibcode :1986Natur.323..699C. doi :10.1038/323699a0 .
* ^ Pauling, L. (1987). "Electronic Structure of the Benzene
Molecule". Nature . 325 (6103): 396.
Bibcode :1987Natur.325..396P. doi
* ^ Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.;
Olmstead, M. M. (2007). "A Double Concave Hydrocarbon Buckycatcher".
Journal of the American Chemical Society . 129 (13): 3842–3843. doi
:10.1021/ja070616p . PMID 17348661 .
* ^ McMurry, J. (2007). Organic
Chemistry (7th ed.).
p. 515. ISBN 0495112585 .
* ^ A B Hunter, C. A.; Sanders, J. K. M. (1990). "The Nature of
Journal of the American Chemical Society . 112
(14): 5525–5534. doi :10.1021/ja00170a016 .
* ^ McGaughey, G. B.; Gagné, M.; Rappé, A. K. (1998).
"Pi-Stacking Interactions. Alive and Well in Proteins". Journal of
Chemistry . 273 (25): 15458–15463. doi
:10.1074/jbc.273.25.15458 . PMID 9624131 .
* ^ Wheeler, S. E.; Bloom, J. W. G. (2014). "Toward a More Complete
Understanding of Noncovalent Interactions Involving
Journal of Physical Chemistry A
Journal of Physical Chemistry A . 118 (32): 6133–6147. Bibcode
:2014JPCA..118.6133W. doi :10.1021/jp504415p .
* ^ Armit, J. W.; Robinson, R. (1925). "CCXI. Polynuclear
Aromatic Types. Part II. Some Anhydronium Bases". Journal
of the Chemical Society, Transactions . 127: 1604–1618. doi
* ^ Crocker, E. C. (1922). "Application Of The Octet Theory To
Aromatic Compounds". Journal of the American Chemical
Society . 44 (8): 1618–1630. doi :10.1021/ja01429a002 .
* ^ Armstrong, H. E. (1890). "The structure of Cycloid
Proceedings of the Chemical Society (London) . 6 (85):
95–106. doi :10.1039/PL8900600095 .
* ^ Housecroft, C. E.; Sharpe, A. G. (2005). Inorganic Chemistry
Pearson Prentice Hall
Pearson Prentice Hall . pp. 29–33. ISBN 0130399132 .
* ^ Hückel, E. (1931). "Quantentheoretische Beiträge zum
Benzolproblem I. Die Elektronenkonfiguration des Benzols und
Zeitschrift für Physik . 70 (3–4):
Bibcode :1931ZPhy...70..204H. doi :10.1007/BF01339530 .
* ^ Hückel, E. (1931). "Quanstentheoretische Beiträge zum
Benzolproblem II. Quantentheorie der Induzierten Polaritäten".
Zeitschrift für Physik . 72 (5–6): 310–337. Bibcode
:1931ZPhy...72..310H. doi :10.1007/BF01341953 .
* ^ Hückel, E. (1932). "Quantentheoretische Beiträge zum Problem
der Aromatischen und Ungesättigten Verbindungen. III". Zeitschrift
für Physik . 76 (9–10): 628–648.
doi :10.1007/BF01341936 .
* ^ Ouellette, R. J.; Rawn, J. D. (1996). Organic Chemistry.
Prentice Hall . p. 473. ISBN 0023901713 .
* ^ Ghatak, K. L. (2014). "Properties of Molecules II". A Textbook
Chemistry and Problem Analysis. PHI Learning Pvt. Ltd. p.
380. ISBN 9788120347977 .
* ^ Lawrence, S. A. (2004). "An Introduction to Heterocyclic
Amines". Amines: Synthesis, Properties and Applications. Cambridge
University Press . p. 120. ISBN 9780521782845 .
* ^ Ma, J. C.; Dougherty, D. A. (1997). "The Cation-π
Chemical Reviews . 97 (5): 1303–1324. doi
:10.1021/cr9603744 . PMID 11851453 .
* ^ A B C Seager, S. L.; Slabaugh, M. R. (2013). "Properties and
Aromatic Compounds". Organic and Biochemistry for Today (8th
Cengage Learning . pp. 65–66. ISBN 9781285605906 .
* ^ Merino, G.; Heine, T.; Seifert, G. (2004). "The Induced
Magnetic Field in Cyclic Molecules". Chemistry: A European Journal .
10 (17): 4367–4371. doi :10.1002/chem.200400457 .
* ^ Gomes, J. A. N. F.; Mallion, R. B. (2001). "
Chemical Reviews . 101 (5): 1349–1384. doi
* ^ Sinnokrot, M. O.; Valeev, E. F.; Sherrill, C. D. (2002).
"Estimates of the Ab Initio Limit for π-π Interactions: The Benzene
Journal of the American Chemical Society . 124 (36):
10887–10893. doi :10.1021/ja025896h . PMID 12207544 .
* ^ Feixas, F.; Matito, E.; Poater, J.; Solà, M. (2015).
Aromaticity with Electron Delocalisation Measures".
Chemical Society Reviews . 44 (18): 6434–6451. doi
* ^ A B Gropper, S. S.; Smith, J. L. (2012). "Amino Acid
Classification". Advanced Nutrition and Human Metabolism (6th ed.).
Cengage Learning . pp. 183–186. ISBN 9781133104056 .
* ^ Mrozek, A.; Karolak-Wojciechowska, J.; Kieć-Kononowicz, K.
(2003). "Five-Membered Heterocycles. Part III.
Imidazole in 5+n Hetero-Bicyclic Molecules". Journal of Molecular
Structure . 655 (3): 397–403.
Bibcode :2003JMoSt.655..397M. doi
* ^ A B Broadley, K. J. (2010). "The Vascular Effects of Trace
Amines and Amphetamines".
Pharmacology & Therapeutics . 125 (3):
363–375. doi :10.1016/j.pharmthera.2009.11.005 . PMID 19948186 .
* ^ Schaechter, J. D.; Wurtman, R. J. (1990). "
varies with brain tryptophan levels".
Brain Research . 532 (1-2):
203–210. doi :10.1016/0006-8993(90)91761-5 . PMID 1704290 .
* ^ Ikeda, M.; Tsuji, H.; Nakamura, S; Ichiyama, A.; Nishizuka, Y.;
Hayaishi, O. (1965). "Studies on the
Biosynthesis of Nicotinamide
Adenine Dinucleotide. II. A Role of Picolinic Carboxylase in the
Biosynthesis of Nicotinamide
Adenine Dinucleotide from
Journal of Biological Chemistry . 240 (3):
1395–1401. PMID 14284754 .
* ^ Lindemann, L.; Hoener, M. C. (2005). "A Renaissance in Trace
Amines Inspired by a Novel GPCR Family". Trends in Pharmacological
Sciences . 26 (5): 274–281. doi :10.1016/j.tips.2005.03.007 . PMID
* ^ "Lamotrigine".
Drugs.com . Retrieved 1 June 2016.
* ^ "Tenofovir".
Drugs.com . Retrieved 1 June 2016.
* ^ "Ciprofloxacin".
Drugs.com . Retrieved 1 June 2016.
* ^ "Oxprenolol".
Drugs.com . Retrieved 1 June 2016.
* ^ "Codeine".
Drugs.com . Retrieved 1 June 2016.
* ^ A B "Doxorubicin".
Drugs.com . Retrieved 1 June 2016.
* ^ "Clonazepam".
Drugs.com . Retrieved 1 June 2016.
* ^ "Lornoxicam".
Drugs.com . Retrieved 12 June 2016.
* ^ Spiller, H. A.; Winter, M. L.; Weber, J. A.; Krenzelok, E. P.;
Anderson, D. L.; Ryan, M. L. (2003). "Skin Breakdown and Blisters from
Senna-Containing Laxatives in Young Children". Annals of
Pharmacotherapy . 37 (5): 636–639. doi :10.1345/aph.1C439 . PMID
* ^ "Raltegravir".
Drugs.com . Retrieved 12 June 2016.
* ^ "Chloroquine".
Drugs.com . Retrieved 1 June 2016.
* ^ "
Drugs.com . Retrieved 1 June 2016.
* ^ Verma, S.; Eckstein, F. (1998). "Modified Oligonucleotides:
Synthesis and Strategy for Users".
Annual Review of Biochemistry . 67:
99–134. doi :10.1146/annurev.biochem.67.1.99 . PMID 9759484 .
* ^ Chen, L. X.; Schumacher, H. R. (2008). "Gout: an Evidence-Based
Journal of Clinical Rheumatology . 14 (5): S55–62. doi
:10.1097/RHU.0b013e3181896921 . PMID 18830092 .
* ^ Watson, J. D. ; Crick, F. (1953). "A Structure for Deoxyribose
Nucleic Acid". Nature . 171 (4356): 737–738. Bibcode
:1953Natur.171..737W. doi :10.1038/171737a0 . PMID 13054692 .
* ^ Protozanova, E.; Yakovchuk, P.; Frank-Kamenetskii, M. D.
(2004). "Stacked–Unstacked Equilibrium at the Nick Site of DNA".
Journal of Molecular Biology . 342 (3): 775–785. doi
:10.1016/j.jmb.2004.07.075 . PMID 15342236 .
* ^ Lerman, L. S. (1961). "Structural Considerations in the
DNA and Acridines".
Journal of Molecular Biology . 3
(1): 18–30. doi :10.1016/S0022-2836(61)80004-1 . PMID 13761054 .
* ^ Luzzati, V. F.; Lerman, L. S. (1961). "Interaction of
Proflavine: A Small-Angle X-Ray Scattering Study". Journal of
Molecular Biology . 3 (5): 634–639. doi
:10.1016/S0022-2836(61)80026-0 . PMID 14467543 .
* ^ Lerman, L. S. (1963). "The Structure of the DNA-Acridine
Complex" . Proceedings of the National Academy of Sciences of the
United States of America . 49 (1): 94–102. Bibcode
:1963PNAS...49...94L. doi :10.1073/pnas.49.1.94 . PMC 300634 . PMID
* ^ Sabnis, R. W. (2010). Handbook of Biological Dyes and Stains:
Synthesis and Industrial Application. Wiley . ISBN 9780470407530 .
* ^ Denissenko, M. F.; Pao, A.; Tang, M.-S.; Pfeifer, G. P. (1996).
"Preferential Formation of
Benzopyrene Adducts at Lung Cancer
Mutational Hotspots in P53". Science . 274 (5286): 430–432. Bibcode
:1996Sci...274..430D. doi :10.1126/science.274.5286.430 . PMID 8832894
* ^ A B Pradhan, P.; Tirumala, S.; Liu, X.; Sayer, J. M.; Jerina,
D. M.; Yeh, H. J. C. (2001). "Solution Structure of a Trans-Opened
in a fully Complementary
DNA Duplex: Evidence for a Major Syn
Conformation". Biochemistry . 40 (20): 5870–5881. doi
:10.1021/bi002896q . PMID 11352722 .
* ^ Fornari, F. A.; Randolph, J. K.; Yalowich, J. C.; Ritke, M. K.;
Gewirtz, D. A. (1994). "Interference by
in MCF-7 Breast Tumor Cells".
Molecular Pharmacology . 45 (4):
649–656. PMID 8183243 .
* ^ Weber, S. R.; Brintzinger, H. H. (1977). "Reactions of
Bis(hexamethylbenzene)iron(0) with Carbon Monoxide and with
Unsaturated Hydrocarbons". Journal of Organometallic
Chemistry . 127
(1): 45–54. doi :10.1016/S0022-328X(00)84196-0 .
* ^ Ardizzoia, G. A.; Brenna, S.; Cenini, S.; LaMonica, G.;
Masciocchi, N.; Maspero, A. (2003). "Oligomerization and
Polymerization of Alkynes Catalyzed by Rhodium(I) Pyrazolate
Complexes". Journal of Molecular Catalysis A: Chemical . 204–205:
333–340. doi :10.1016/S1381-1169(03)00315-7 .
* ^ Hilt, G.; Vogler, T.; Hess, W.; Galbiati, F. (2005). "A Simple
Cobalt Catalyst System for the Efficient and Regioselective
Cyclotrimerisation of Alkynes".
Chemical Communications . 2005 (11):
1474–1475. doi :10.1039/b417832g . PMID 15756340 .
* ^ Behr, A. (2008). Angewandte homogene Katalyse (in German).
Wiley-VCH . ISBN 9783527316663 .
* ^ Hillard, R. L.; Vollhardt, K. P. C. (1977). "Substituted
Benzocyclobutenes, Indans, and Tetralins via Cobalt-Catalyzed
Cooligomerization of α,ω-diynes with Substituted Acetylenes.
Formation and Synthetic Utility of Trimethylsilylated
Journal of the American Chemical Society . 99
(12): 4058–4069. doi :10.1021/ja00454a026 .
* ^ Hsieh, J.-C.; Cheng, C.-H. (2005). "Nickel-Catalyzed
Cocyclotrimerization of Arynes with Diynes; A Novel Method for
Chemical Communications . 2005
(19): 2459–2461. doi :10.1039/b415691a .
* ^ Neeson, S. J.; Stevenson, P. J. (1988). "
Cycloadditions – An Efficient Regiospecific Route to
Tetrahedron Letters . 29 (7): 813–814. doi
* ^ Mohamed, R. K.; Peterson, P. W.; Alabugin, I. V. (2013).
"Concerted Reactions that Produce Diradicals and Zwitterions:
Electronic, Steric, Conformational and Kinetic Control of
Chemical Reviews . 113 (9):
7089–7129. doi :10.1021/cr4000682 . PMID 23600723 .
* ^ Waters, M.; Wulff, W. D. (2008). "The Synthesis of Phenols and
Quinones via Fischer
Organic Reactions . 70 (2):
121–623. doi :10.1002/0471264180.or070.02 .
* ^ Dötz, K. H. (1983). "Carbon-Carbon Bond Formation via
Carbene Complexes". Pure and Applied
Chemistry . 55 (11). doi
* ^ Woodard, B. T.; Posner, G. H. (1999). "Recent Advances in
Diels-Alder Cycloadditions Using 2-Pyrones". In Harmata, M. Advances
in Cycloaddition. 5. JAI Press. pp. 47–83. doi
:10.1002/chin.199935304 . ISBN 9780762303465 .
* ^ Fieser, L. F. (1966). "Hexaphenylbenzene".
Organic Syntheses .
46: 44. doi :10.15227/orgsyn.046.0044 . ; Collective Volume, 5, p. 604
* ^ Bergmann, F.; Szmuszkowicz, J.; Fawaz, G. (1947). "The
Condensation of 1,1-Diarylethylenes with Maleic Anhydride". Journal of
the American Chemical Society . 69 (7): 1773–1777. doi
:10.1021/ja01199a055 . PMID 20251415 .
* ^ Huisgen, Rolf (1963). "1.3-Dipolare Cycloadditionen Rückschau
Angewandte Chemie (in German). 75: 604–637. doi
* ^ Kolb, H. C.; Finn, M. G.; Sharpless, K. B. (2001). "Click
Chemistry: Diverse Chemical Function from a Few Good Reactions".
Angewandte Chemie International Edition . 40 (11): 2004–2021. doi
:10.1002/1521-3773(20010601)40:113.0.CO;2-5 . PMID 11433435 .
* ^ Kolb, H. C.; Sharpless, B. K. (2003). "The Growing Impact of
Chemistry on Drug Discovery".
Drug Discovery Today . 8 (24):
1128–1137. doi :10.1016/S1359-6446(03)02933-7 . PMID 14678739 .
* ^ Frank, R.L.; Seven, R. P. (1949). "Pyridines. IV. A Study of
the Chichibabin Synthesis".
Journal of the American Chemical Society .
71 (8): 2629–2635. doi :10.1021/ja01176a008 .
* ^ Shimizu, S.; Watanabe, N.; Kataoka, T.; Shoji, T.; Abe, N.;
Morishita, S.; Ichimura, H. (2005). "
Pyridine and Pyridine
Derivatives". Ullmann's Encyclopedia of Industrial Chemistry.
Wiley-VCH . doi :10.1002/14356007.a22_399 .
* ^ Hantzsch, A. (1881). "Condensationsprodukte aus Aldehydammoniak
und ketonartigen Verbindungen". Berichte der Deutschen Chemischen
Gesellschaft (in German). 14 (2): 1637–1638. doi
* ^ Knoevenagel, E. ; Fries, A. (1898). "Synthesen in der
Pyridinreihe. Ueber eine Erweiterung der Hantzsch'schen
Dihydropyridinsynthese". Berichte der Deutschen Chemischen
Gesellschaft (in German). 31 (1): 761–767. doi
* ^ Jones, R. G.; McLaughlin, K. C. (1950).
Organic Syntheses . 30: 86. doi
* ^ Heravi, M. M.; Bakhtiari, K.; Tehrani, M. H.; Javadi, N. M.;
Oskooie, H. A. (2006). "Facile Synthesis of
Using o-Iodoxybenzoic Acid (IBX) at Room Temperature".
Arkivoc . 2006
(16): 16–22. doi :10.3998/ark.5550190.0007.g02 .
* ^ Johansson, M. P.; Juselius, J. (2005). "
Revisited". Letters in Organic
Chemistry . 2: 469–474. doi
:10.2174/1570178054405968 . Using quantum chemical methodology, we
reinvestigate the aromaticity of the much debated arsole, using the
newly developed gauge-including magnetically induced currents (GIMIC)
method. GIMIC provides a quantitative measure of the induced ring
current strength, showing arsole to be moderately aromatic.
* ^ Braye, E. H.; Hübel, W.; Caplier, I. (1961). "New Unsaturated
Heterocyclic Systems. I".
Journal of the American Chemical Society .
83 (21): 4406–4413. doi :10.1021/ja01482a026 .
* ^ Chang, Clarence D. (1983). "Hydrocarbons from Methanol". Catal.
Rev. - Sci. Eng. 25 (1): 1–118. doi :10.1080/01614948308078874 .
* ^ Horning, E. C.; Stromberg, V. L.; Lloyd, H. A. (1952).
"Beckmann Rearrangements. An Investigation of
Special Cases". Journal
of the American Chemical Society . 74 (20): 5153–5155. doi
* ^ Brown, W.; Turner, A. B. (1971). "Applications of
High-Potential Quinones. Part VII. The Synthesis of Steroidal
Phenanthrenes by Double
Methyl Migration". Journal of the Chemical
Society C: Organic : 2566–2572. doi :10.1039/J39710002566 .
* ^ Bennett, M. A.; Huang, T. N.; Matheson, T. W.; Smith, A. K.
(1982). "(η6-Hexamethylbenzene)ruthenium Complexes". Inorganic
Syntheses . 21: 74–78. doi :10.1002/9780470132524.ch16 .
* ^ Vogel, E.; Klug, W.; Breuer, A. (1974). "1,6-Methanoannulene".
Organic Synthesis . 54: 11. doi :10.15227/orgsyn.054.0011 .
* ^ Harreus, A. L. (2002). "Pyrrole". Ullmann's Encyclopedia of
Wiley-VCH . doi :10.1002/14356007.a22_453 .
* ^ Skell, P. S.; Sandler, S. R. (1958). "Reactions of
Electrophilic Reagents. Synthetic Route
for Inserting a Carbon Atom Between the Atoms of a Double Bond".
Journal of the American Chemical Society . 80 (8): 2024–2025. doi
* ^ A B Avendaño, C.; Menéndez, J. C. (2008). "Aromatase
Chemistry of Anticancer Drugs.
Elsevier . pp.
65–73. doi :10.1016/B978-0-444-52824-7.00003-2 . ISBN 9780080559629
* ^ Jasek, W., ed. (2007). Austria-Codex (in German) (62nd ed.).
Vienna: Österreichischer Apothekerverlag. pp. 656–660. ISBN
* ^ Dinnendahl, V.; Fricke, U., eds. (2007). Arzneistoff-Profile
(in German). 4 (21st ed.). Eschborn, Germany: Govi Pharmazeutischer
Verlag. ISBN 9783774198463 .
* ^ Xu, R.; Breslow, R. (1997). "1,2,3-Triphenylcyclopropenium
Organic Syntheses . 74: 72. doi :10.15227/orgsyn.074.0072 .
* ^ Merling, G. (1891). "Ueber Tropin". Berichte der Deutschen
Chemischen Gesellschaft (in German). 24 (2): 3108–3126. doi
* ^ Doering, W. von E. ; Knox, L. H. (1954). "The
Cycloheptatrienylium (Tropylium) Ion". Journal of the American
Chemical Society . 76 (12): 3203–3206. doi :10.1021/ja01641a027 .
* ^ Balaban, A. T.; Oniciu, D. C.; Katritzky, A. R. (2004).
Aromaticity as a Cornerstone of Heterocyclic Chemistry". Chemical
Reviews . 104 (5): 2777–2812. doi :10.1021/cr0306790 .
* ^ Agarwai, O. P. (2009). Reactions and Reagents (46th ed.).
Krishna Prakashan Media . pp. 614–615. ISBN 9788187224655 .
* ^ Nielson, A. J.; Rickard, C. E. F.; Smith, J. M. (1986).
"Cyclopentadienylthallium (Thallium Cyclopentadienide)". Inorganic
Syntheses . 24: 97–99. doi :10.1002/9780470132555.ch31 .
* ^ Olbrich, F.; Behrens, U. (1997). "Crystal Structure of
Catena-Cyclopentadienyl Thallium, ". Zeitschrift für Kristallographie
– New Crystal Structures . 212 (1): 47–47. doi
* ^ Cotton, F. A. ; Wilkinson, G. (1999). Advanced Inorganic
Chemistry (6th ed.).
John Wiley and Sons
John Wiley and Sons . ISBN 9780471199571 .
* ^ Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. (1999).
Synthesis and Technique in Inorganic Chemistry. University Science
Books. ISBN 0935702482 .
* ^ Wilkinson, G. (1956). "Ferrocene".
Organic Syntheses . 36: 31.
doi :10.15227/orgsyn.036.0031 .
* ^ Bozak, R. E. (1966). "Acetylation of Ferrocene: A
Chromatography Experiment for Elementary Organic Laboratory". Journal
of Chemical Education . 43 (2): 73.
Bibcode :1966JChEd..43...73B. doi
* ^ Barth, W. E.; Lawton, R. G. (1966). "Dibenzofluoranthene".
Journal of the American Chemical Society . 88 (2): 380–381. doi
* ^ Sygula, A.; Rabideau, P. W. (1995). "Structure and Inversion
Barriers of Corannulene, its Dianion and Tetraanion. An ab initio
Study". Journal of Molecular Structure: THEOCHEM . 333 (3): 215–226.
doi :10.1016/0166-1280(94)03961-J .
* ^ Monaco, G.; Scott, L.; Zanasi, R. (2008). "Magnetic Euripi in
Journal of Physical Chemistry A
Journal of Physical Chemistry A . 112 (35):
Bibcode :2008JPCA..112.8136M. doi :10.1021/jp8038779 .
PMID 18693706 .
* ^ Johnson, A. W. (1947). "Organic Chemistry".
Science Progress .
35 (139): 506–515.
JSTOR 43413011 .
* ^ Kaufman, H. S.; Fankuchen, I.; Mark, H. (1948). "Structure of
Cyclo-octatetraene". Nature . 161 (4083): 165. Bibcode
:1948Natur.161..165K. doi :10.1038/161165a0 .
* ^ Thomas, P. M.; Weber, A. (1978). "High Resolution Raman
Spectroscopy of Gases with Laser Sources. XIII – The Pure Rotational
Spectra of 1,3,5,7-
Cyclooctatetraene and 1,5-Cyclooctadiene". Journal
of Raman Spectroscopy . 7 (6): 353–357. Bibcode
:1978JRSp....7..353T. doi :10.1002/jrs.1250070614 .
* ^ Katz, T. J. (1960). "The Cyclooctatetraenyl Dianion". Journal
of the American Chemical Society . 82 (14): 3784–3785. doi
* ^ Streitwieser, A.; Mueller-Westerhoff, U. (1968).
"Bis(cyclooctatetraenyl)uranium (Uranocene). A New Class of Sandwich
Complexes that Utilize Atomic f Orbitals". Journal of the American
Chemical Society . 90 (26): 7364–7364. doi :10.1021/ja01028a044 .
* ^ Clayden, J. ; Greeves, N.; Warren, S. ; Wothers, P. (2001).
Chemistry (1st ed.).
Oxford University Press
Oxford University Press . p. 531. ISBN
* ^ Capponi, M.; Gut, I. G.; Hellrung, B.; Persy, G.; Wirz, J.
(1999). "Ketonization Equilibria of
Phenol in Aqueous Solution".
Canadian Journal of
Chemistry . 77 (5-6): 605–613. doi
* ^ Kündig, E. P.; Garcia, A. E.; Lomberget, T.; Bernardinelli, G.
(2005). "Rediscovery, Isolation, and Asymmetric Reduction of
1,2,3,4-Tetrahydronaphthalene-1,4-dione and Studies of its Complex".
Angewandte Chemie International Edition . 45 (1): 98–101. doi
* ^ Scott, L. T. (1982). "Thermal Rearrangements of Aromatic
Accounts of Chemical Research
Accounts of Chemical Research . 15 (2): 52–58. doi
* ^ Scott, L. T.; Kirms, M. A. (1981). "
Rearrangements. 13C–Labeling Studies of Automerization and
Isomerization to Naphthalene". Journal of the American Chemical
Society . 103 (19): 5875–5879. doi :10.1021/ja00409a042 .
* ^ Scott, L. T.; Agopian, G. K. (1977). "Automerization of
Journal of the American Chemical Society . 99 (13):
4506–4507. doi :10.1021/ja00455a053 .
* ^ Carret, S.; Blanc, A.; Coquerel, Y.; Berthod, M.; Greene, A.
E.; Deprés, J.-P. (2005). "Approach to the Blues: A Highly Flexible
Route to the Azulenes".
Angewandte Chemie International Edition . 44
(32): 5130–5133. doi :10.1002/anie.200501276 .
* ^ Anderson, A. G.; Stecker, B. M. (1959). "Azulene. VIII. A Study
of the Visible Absorption Spectra and Dipole Moments of Some 1- and
Journal of the American Chemical Society .
81 (18): 4941–4946. doi :10.1021/ja01