Heteroatom Containing Polycyclic Aromatic Hydrocarbons with Positive Charge

210 Pages·2008·27.74 MB·English

Heteroatom Containing Polycyclic
Aromatic Hydrocarbons with
Positive Charge - Synthesis and
Characterization
Dissertation
zur Erlangung des Grades
“Doktor der Naturwissenschaften”
dem Fachbereich Chemie und Pharmazie der
Johannes Gutenberg-Universität Mainz
vorgelegt von
Dongqing Wu
geboren in Henan Province / P. R. China
Mainz, 2008
Decan: Herr Prof. Dr.
1. Berichterstatter: Herr Prof. Dr.
2. Berichterstatter: Herr Prof. Dr.
Tag der mündlichen Prüfung:
Die vorliegende Arbeit wurde in der Zeit von August 2004
bis Mai 2008 im Max-Planck-Institut für Polymerforschung in
Mainz unter Anleitung von Herrn Prof. Dr. Müllen
ausgeführt.
Ich danke Herrn Prof. Dr. K. Müllen für seine
wissenschaftliche und persönliche Unterstützung sowie für
sein ständige Diskussionsbereitschaft.
Introduction Chapter1
Chapter 1
Introduction
1.1 Aromaticty and aromatic compounds
Aromaticity is a chemical property in which a conjugated ring of unsaturated
bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be
expected by the stabilization of conjugation alone. It can also be considered as a
manifestationofcyclicdelocalization andofresonance.1-7
The first known use of the word "aromatic" as a chemical term - namely, applied
to compounds that contain the benzene groups – occurred in an article by A. W.
Hofmann in 1855.8 Nevertheless, it is curious that Hofmann said nothing about why
he introduced an adjective indicating olfactory character to a group of chemical
substances, onlysomeof whichhavenotablearomas.
Figure1-1.a)Kekuléandb)Robinsonstructuresofbenzene.
First discovered by M. Faraday in 18259, the simplest, yet the most important
aromatic compound is benzene 1-1.The structure of benzene remained for a longtime
a centre of dispute in the scientific community until its cyclohexatriene structure
(Figure 1.1a) was first proposed by A. Kekulé in 1865. Over the next few decades,
1
Introduction Chapter1
most chemists readily accepted this structure, since it accounted for most of the
known isomeric relationships of aromatic chemistry. However, it was always puzzling
that the purportedly highly unsaturated molecule was so unreactive toward addition
reactions. An explanation for the exceptional stability of benzene was conventionally
attributed to Sir R. Robinson10, who was the first to coin the term aromatic sextet as a
group of six electrons that resists disruption (Figure 1.1b). In 1931 the quantum
mechanical origins of this stability, or aromaticity, were first modelled by E. Hückel
whowas thefirst toseparatethebondingelectrons into σ andπelectrons.11
An aromatic compound is an organic molecule which contains a set of
covalently-boundatoms withspecificcharacteristics:
a). A delocalized conjugated π-system, most commonly an arrangement of
alternatingsingleanddoublebonds;
b).Coplanarstructure,withall thecontributingatoms inthesameplane;
c).Contributingatoms arrangedinoneormorerings;
d). The number of π delocalized electrons that is even, but not a multiple of 4.
This is known as Hückel's rule. Permissible numbers of π electrons include 2, 6, 10,
14,andso on;
e). Special reactivity in organic reactions such as electrophilic aromatic
substitutionandnucleophilicaromaticsubstitution.
The key aromatic compounds of commercial interest are benzene, toluene,
ortho-xylene and para-xylene. About 35 million tons of these compounds are
produced worldwide every year. They are extracted from complex mixtures obtained
by the refining of oil or by distillation of coal tar, and are used to produce a range of
important chemicals and polymers, including styrene, phenol, aniline, polyester and
nylon12.Aromaticcompounds canusuallybeclassifiedintothreetypes:
a).Substitutedbenzenes:
Many chemical compounds contain simple benzene rings in their structure.
2
Introduction Chapter1
Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin),
1,3-benzodioxole(methylenedioxybenzene) andparacetamol.
b).Heterocyclics:
In heterocyclic aromatics, one or more of the atoms in the aromatic ring is of an
element other than carbon. This can alter the ring's aromaticity, and thus (as in the
case of furan) change its reactivity. Other examples include pyridine, imidazole,
pyrazole,oxazole,thiophene,andtheirbenzannulatedanalogs.
c).Polycyclicaromatichydrocarbons:
Polycyclic aromatic hydrocarbons (PAHs) are molecules containing two or more
simple aromatic rings fused together by sharing two neighboring carbon atoms such
as naphthalene, anthraceneandphenanthrene.
1.2 Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons (PAHs), which were first discovered in coal
tar in the 19th century, have become one of the most widely investigated compounds
in medical sciences, biology, organic chemistry, physics and material sciences in
recent years.1,5,6,13-15
PAHs are the first chemical carcinogens to be discovered. In 1775, the English
surgeon P. Pott found an association between exposure to soot and a high incidence of
scrotal cancers in chimney sweepers. The famous description of chemically induced
carcinogenesis found the experimental counterpart in the coal tar tumors induced in
rabbits byYamagiwa and Ichikawa in 1915. Later research indicated that it was PAHs
in the residue of combustion such as soot and coal tar which caused skin cancers of
human and animals. PAHs were regarded as the main carcinogens before 1950s.
Nowadays, PAHs are still one of the most important classes of carcinogens due to
theirabundanceintheenvironment.16,17
PAHs are also found in the interstellar medium, comets and meteorites. A team
led by A. Witt of the University of Toledo, Ohio studied ultraviolet light emitted by
3
Introduction Chapter1
the Red Rectangle nebula and found the spectral signatures of anthracene and pyrene.
This discovery was considered as the confirmation of the PAH world hypothesis. This
biological hypothesis proposes that PAHs served as basis for the origin of life in a
pre-RNAworld.18
To the interest of organic chemists and material scientists, the most attractive
property of PAHs is their aromaticity.19-25 The electron delocalization along the
polycyclic aromatic structures gives rise to interesting electronic and optical
properties of these PAH materials. The breakthrough discovery of conducting and
semiconductingorganic polymers in 1970s leads to promisingapplications in the field
of organic electronics nowadays.26-28 The intrinsic electronic properties and the
versatile functionalization qualified PAHs also are promising semiconducting
materials in organic devices such as light-emitting diodes (LED), field effect
transistors (FET), liquid crystal display (LCD) and solar cells.29,30 On the other hand,
these polycyclic aromatic molecules can form stable columnar mesophase after
attaching flexible chains, which are desirable for device processing due to their
self-assemblyandself-healingcapability.19,31,32
Furthermore, two-dimensional all-benzenoid PAHs can be viewed as model
compounds for graphite. Therefore, PAHs are also of special interest in theoretical
problems like the scope, limitation and effects of electron delocalization in aromatic
materials.33
1.2.1 Synthesis of PAHs
The natural and industrial sources of PAHs are coal tar, oil shale and the
side-products of the catalytic hydrocracking of petroleum. Due to the industrial scale
of the process, some PAHs which only exist in very small amounts in the crude
material are able to be collected in reasonable amounts. First contributions in the area
of direct synthesis and characterization of PAHs were pioneered by R. Scholl,E. Clar
and M. Zander.13, 34-38 However, the classical synthetic methods involved poor
selectivity and relatively vigorous reaction conditions such as high temperatures and
4
Introduction Chapter1
pressures (Scheme1-1).
Scheme1-1.UnselectiveSynthesisofPAHs.
Nowadays, research towards the synthesis of PAHs focuses on much milder
methods, with better regioselectivity and higher yields. Several widely used modern
syntheticmethods arelistedbelow:
1.2.1.1 Flash vacuum pyrolysis (Thermolysis)
The classic strategy toward pure PAHs is the conversion of appropriate
precursors to target PAHs at elevated temperature. A typical experimental process is
flash vacuum pyrolysis (FVP), in which high temperature gas-phase pyrolysis of
precursors with short contact time (tens of ms to several seconds) in the hot zone
result inelectrocyclizationwithloss ormigration ofhydrogens (orhydrogenhalide).
Key point of FVP is to design the precursors, which should have a good thermal
stability and proper reactive sites. Appropriate planar precursors with halogen
substituents in the fjord regions or at ortho-positions have been applied to synthesize
strained geodesic PAH in significantly higher yields because the C -X (X =
aryl
5
Introduction Chapter1
halogens) bonds have lower dissociation enthalpies than C -H bonds. A most
aryl
successful example is the rational chemical synthesis of the Buckminster fullerene C
60
1-9,inwhichthekeystepwas accomplishedbyFVP(Scheme1-2).39,40
Scheme1-2.SynthesisoffullereneC byFVPmethod.
60
Intramolecular carbene insertion during the pyrolysis process is another new
synthetic method to prepare curved PAHs. This method was firstlyreported byR. F. C.
Brown, which is based on the reversible rearrangement of terminal acetylenes to
vinylidenes undertheconditions ofFVP(Figure 1-2).41,42
Figure1-2.Thereversiblerearrangementofterminalacetylenestovinylidenes.
One example is the gram-scale three-step synthesis of the bowl-shaped
20-carbon fullerene fragment corannulene 1-14 (Scheme 1-3) from commercially
availablestartingmaterials, whichwas developed byL.T.Scott et al.43,44
6
Introduction Chapter1
Scheme1-3.Three-stepsynthesisofcorannulene.
1.2.1.2 Friedel-Crafts condensation (Haworth phenanthrene
synthesis)
Scheme1-4.Haworthphenanthrene synthesis.
Haworth synthesis provides a rational route to PAHs, as first illustrated by the
synthesis of alkylphenanthrene.45 The classic Haworth synthesis starts from
Friedel-Craftscondensation of succinic anhydrides 1-15 with a polyarene 1-16 to give
a keto-acid product 1-17, followed by reduction of the keto group to form the
butanoic acid 1-18. And the essential transformation in this synthesis is the
intramolecular Friedel-Crafts acylation of 1-18 to yield the ketone product 1-19,
whichcanbearomatized tothecorrespondingPAH1-20(Scheme1-4).
Scheme1-5.ModifiedHaworthsynthesistolargerPAHs
7