Putting corannulene in its place. Reactivity studies comparing corannulene with other aromatic hydrocarbons

Article abstract of DOI:10.1039/c5ob01215e

A series of aromatic hydrocarbons were investigated so as to compare the reactivity of corannulene with planar aromatic hydrocarbons. Corannulene was found to be more reactive than benzene, naphthalene and triphenylene to Friedel-Crafts acylation whilst electrophilic aromatic bromination was also used to confirm that triphenylene was less reactive than corannulene and that pyrene, perylene and acenaphthene were more so. The stabilisation of a neighbouring carbocation by the various aromatic systems was investigated through consideration of the rates of methanolysis of a series of benzylic alcohols. The reactivity series was found to parallel that observed for the electrophilic aromatic substitutions and both series are supported by computational studies. As such, a reactivity scale was devised that showed that corannulene was less reactive than would be expected for an aromatic planar species of similar pi electron count.

Full text of DOI:10.1039/c5ob01215e

Organic &
Biomolecular Chemistry
View Article Online
View Journal | View Issue
PAPER
Putting corannulene in its place. Reactivity studies
comparing corannulene with other aromatic
hydrocarbons†
Cite this: Org. Biomol. Chem., 2015,
13, 9035
Stephen R. D. George,a Thomas D. H. Frith,a Donald S. Thomas*b and
Jason B. Harper*a
A series of aromatic hydrocarbons were investigated so as to compare the reactivity of corannulene with
planar aromatic hydrocarbons. Corannulene was found to be more reactive than benzene, naphthalene
and triphenylene to Friedel–Crafts acylation whilst electrophilic aromatic bromination was also used to
confirm that triphenylene was less reactive than corannulene and that pyrene, perylene and acenaphthene
were more so. The stabilisation of a neighbouring carbocation by the various aromatic systems was inves-
tigated through consideration of the rates of methanolysis of a series of benzylic alcohols. The reactivity
series was found to parallel that observed for the electrophilic aromatic substitutions and both series are
supported by computational studies. As such, a reactivity scale was devised that showed that corannulene
was less reactive than would be expected for an aromatic planar species of similar pi electron count.
Received 14th June 2015,
Accepted 21st July 2015
DOI: 10.1039/c5ob01215e
www.rsc.org/obc
acenaphthene 4, fluoranthene 5, pyrene 6 and triphenylene 7.
Introduction
Recently there has been significant interest3 in non-planar
Polycyclic aromatic hydrocarbons (PAHs) are a group of cyclic PAHs such as corannulene 8 and the chrysene 9,4 as potential
compounds consisting of fused aromatic rings that do not starting points for the rational synthesis of carbon nanotubes.
contain any heteroatoms; the carbon atoms are predominantly
Reactions of these species, particularly towards electro-
sp2 hybridised and the pi electrons of the system delocalised.1 philes, are well studied; these reactions include the Friedel–
First discovered in crude oils in the late 1800s,2 PAHs can Crafts acylation of species 1,5 2,5 3,6 5,7 78 and 8,9 and the bro-
either be a simple one ring extension of the monocyclic mination of acenaphthene 410 and pyrene 6.11 In terms of the
benzene 1 to give naphthalene 2 or form more complex poly- relative reactivity of some of these species, competitive sulfona-
cycles such as phenanthrene 3, the tethered napthyl system tion12 showed that reactivity increased in the order benzene 1,
phenanthrene 3, naphthalene 2 and, finally, pyrene 6 with the
order of 1, 3 and 2 also confirmed by Gore et al.13 and Pryor
et al.14 Whilst no competitive bromination studies have been
aSchool of Chemistry, University of New South Wales, UNSW Sydney, NSW 2052,
undertaken, a comparison of reaction conditions allows a reac-
tivity order to be developed. Species 1 and 2 do not react
with N-bromosuccinimide at room temperature in DMF.15 The
larger species 5 undergoes reaction at elevated temperature16
whilst species 4 and 6 react to completion in 215 and 2417
Australia. E-mail: j.harper@unsw.edu.au; Fax: +61 2 9385 6141;
Tel: +61 2 9385 4692
bNuclear Magnetic Resonance Facility, Mark Wainwright Analytical Centre,
University of New South Wales, UNSW Sydney, NSW 2052, Australia.
E-mail: donald.thomas@unsw.edu.au
†Electronic supplementary information (ESI) available: Preparation of the acy-
lated derivatives of species 1, 2 and 6–8, the alcohols 10-OH to 14-OH and the hours, respectively, at room temperature.
ethers 10-OCH3 to 14-OCH3; conditions used for the bromination of corannu-
Whilst the order of reactivity towards electrophilic aromatic
lene 8; spectra of all novel compounds; details of the competitive acylation reac-
tions involving species 1, 2 and 6–8; kinetic analyses of the methanolysis of the
alcohols 10-OH to 14-OH; energies of the species 1, 2 and 6–8 and the corres-
ponding acylated intermediates 1-Int, 2-Inta,b and 6-Int to 8-Int; reaction coordi-
substitution is relatively consistent across the reactions con-
sidered, it is clear that a number of factors, including the total
number of pi electrons and the potential for isolated aromatic
nate showing relative energies for the process shown in Scheme 1 (left); frontier cores18 need to be taken into account. Of particular interest
molecular orbitals for the species 1-Int, 2-Inta,b and 6-Int to 8-Int; charge den-
would be the position that corannulene 8 takes in this series
as complications arise as a result of the non-planarity of
this species.
The reactivity of non-planar aromatic hydrocarbons has
been considered, but typically this centres on the reactivity of
sities for the species 1-Int, 2-Inta,b and 6-Int to 8-Int; energies of the methylated
species S1–S5 and the corresponding acylated intermediates S1+–S5+; diagram
showing relative energies of S1–S5 and S1+–S5+; frontier molecular orbitals for
the species S1+–S5+; charge densities for the species S1+–S5+. See DOI: 10.1039/
c5ob01215e
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem., 2015, 13, 9035–9041 | 9035

View Article Online
Paper
Organic & Biomolecular Chemistry
Scheme 1 The Friedel–Crafts acylation (left) and the electrophilic bro-
mination (right) of the desired PAHs. Ar = phenyl (1), naphthyl (2), phe-
nanthrenyl (3), acenaphthenyl (4), fluoranthenyl (5), pyrenyl (6),
triphenylenyl (7) and corannulenyl (8).
Scheme 2 The methanolysis of benzylic alcohols 10-OH to 14-OH in
acidic methanol.
Further, the ability of these species to stabilise an incipient
carbocation on a carbon adjacent to the aryl system was inves-
tigated using an SN1 methanolysis pathway (Scheme 2).
Each of these contrasting reactions (an electrophilic aromatic
substitution and a reaction proceeding through a ‘benzylic’
carbocation) are of interest in determining the reactivity of
non-planar polycyclic aromatic species relative to the related
planar cases.
a double bond with distorted geometry; this is exemplified by
the work of Scott et al.,19 who showed that the reactivity of the
internal double bond of the chrysene derivative 9 with nucleo-
philes and to cycloadditions was enhanced relative to equi-
valent planar systems. Reactivity at the periphery has not been
considered to the same extent, with simply reports of success-
ful electrophilic aromatic substitutions to systems such as cor-
annulene 8 (including acylation20 and nitration21).
SN1 reactions adjacent to a non-planar aromatic, at the
‘benzylic’ position, have also not been considered. The corres-
ponding data22 for planar aromatic compounds based on
acetolysis of arylmethyl tosylates,‡ suggests a reactivity order
similar to that described for electrophilic aromatic substi-
tution but also highlights the importance of the site of substi-
tution.§ Whilst the bent systems would be expected to have
reduced pi orbital overlap compared to a planar counterpart
and hence reduced aromatic stabilisation, which is supported
by computational investigations,23 how this affects the reactiv-
ity of corannulene and its derivatives is unknown.
With the above in mind, the aim of the work described
herein is to rank corannulene 8 amongst the other polycyclic
aromatic hydrocarbons in terms of reactivity. In order to do
this, two electrophilic aromatic substitution processes,
Friedel–Crafts acylation and bromination (Scheme 1), were
used to determine the reactivity of corannulene 8 in compari-
son to a range of commercially available planar aromatic
species 1–7. Particularly, competition reactions between these
species allowed an order of reactivity to be determined.
Experimental
All chemicals used were purchased from either Sigma-Aldrich,
Alfa Aesar, Strem or Precious Metals Online and were used
without further purification. Corannulene 8 was the generous
gift of Prof. Jay Siegel (Tianjin University). Organic solvents
used in synthesis were either used as received from Ajax Fine-
chem or collected from a Pure Solv MD Solvent Purification
System.
NMR spectroscopy was performed using either a Bruker
Avance 300 (300.13 MHz, 1H; 75.5 MHz, 13C), an Avance III 400
(400.13 MHz, 1H; 100.6 MHz, 13C) with a Prodigy cryoprobe
cppbbo, an Avance III 500 (500.13 MHz, 1H; 125.7 MHz, 13C)
with a tbi probe or an Avance III 600 (600.13 MHz, 1H;
150.9 MHz 13C). NMR spectra were processed using the Bruker
TOPSPIN 3.0 software.
Authentic samples of the acylated derivatives of species 1,
2, 5, 6, 7 and 8 were prepared prior to the competition reac-
tions described below, using equivalent methodology. The cor-
annulene derivative 8-Ac had not been previously prepared
and hence was fully characterised, whilst physical and spectral
data for the remainder was consistent with that reported in lit-
erature (see ESI† for full details). The (aryl)phenylmethanols
10-OH to 14-OH were synthesised either through reduction of
the commercially available parent ketone (for alcohol 10-OH),
through reaction of the parent aldehyde with phenyl lithium
(for alcohol 11-OH), utilising lithiation of the available aryl
bromide and quenching with benzaldehyde (for alcohol
‡There is also some computational34,35 and experimental36 data for the relative
stability of carbocations adjacent to various planar aromatic species, with larger
planar systems general giving the more stable carbocation, though the impor-
tance of the site of substitution is further demonstrated.
§The symmetry of corannulene 8 means that issues of the site of substitution do
not arise with this non-planar aromatic.
9036 | Org. Biomol. Chem., 2015, 13, 9035–9041
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Paper
12-OH) or via a Friedel–Crafts addition of benzoyl chloride to sidered, a methodology was required in which each of the
the unsubstituted aromatic core followed by the aforemen- species 1–8 reacted to give products readily identified using
tioned reduction (for alcohols 13-OH and 14-OH). The alcohols NMR spectroscopy. Using conditions similar to those
12-OH, 13-OH and 14-OH had not been previously prepared so described for the trimethylacetylation of corannulene 820
were fully characterised, whilst the remainder had physical (addition of aromatic species to 2.5 equivalents of the pre-
and spectral data matching that reported in literature (see ESI† formed acetyl cation at −78 °C) allowed isolation of previously
for full details). The corresponding methyl ethers (10-OMe to unreported monoacetylated corannulene 8-Ac. This species
1
14-OMe) were isolated after being treated with acidic methanol was readily identified using H NMR spectroscopy with a ratio
at reflux for 3 hours (full details can be found in the ESI†).
of 1 : 3 noted for the relative integrations of the acetyl singlet
at δ 2.82 and the signals due to the aromatic protons in the
range δ 7.57–8.57. Under the same conditions, acetylation of
species 1, 2, 6 and 7 also proceeded smoothly to give products
readily identifiable using 1H NMR spectroscopy; acetophe-
none25 1-Ac, 1-acetylnaphthalene26 2-Ac-a, 2-acetylnaphtha-
lene27 2-Ac-b, 1-acetylpyrene28 6-Ac and 2-acetyltriphenylene29
7-Ac) indicating that, in all cases, mono-acylation had
occurred, with only reaction of naphthalene 2 giving a mixture
of regioisomers. Unfortunately, under these conditions, fluor-
anthene 5 underwent diacetylation and reaction of phenan-
threne 3 and acenaphthene 4 gave mixtures in which no
product could be readily identified using 1H NMR spec-
troscopy. Altering either the concentration of the aromatic
starting material (from 0.2 mM to 0.05 mM) or the order of
addition of reagents had no effect upon this result. It is worth
noting that phenanthrene 3 has been reported previously to
undergo reversible Friedel–Crafts acylations to form a complex
mixture of isomers,30 which may account for the complexity of
the product mixture in that case.
Acylation of polycyclic aromatic compounds – competition
reactions
To a stirred suspension of aluminium(III) trichloride (51.2 mg,
384 μmol) and the two aromatic species being studied (ca.
60 μmol each) in dichloromethane (5 mL) at 0 °C under
dynamic nitrogen was added acetyl chloride (8.0 μL, 112 μmol)
as a solution in anhydrous dichloromethane (0.1 mL). The
extents of conversion for each compound were determined
through comparison of the integrations of the signals due to
the starting material and the product in the 1H NMR spectrum
(see ESI†). This data allowed determination of the relative rates
of reaction of the two aromatic species. Each competition reac-
tion was carried out in triplicate.
Kinetic analysis of the methanolysis of alcohols 10-OH to 14-OH
A solution of sulfuric acid (98% w/w, 35 mg, 360 μmol) in deut-
erated methanol (0.6 mL) was generated. Of this, a portion
(0.5 mL) was added to an NMR tube that contained the alcohol
(28 μmol) being investigated. The reaction mixture was held at
Whilst the above results gave a clear indication of which
systems could be directly monitored using 1H NMR spec-
troscopy, and which conditions could be readily used, they
also highlighted a problem with this reaction in terms of
determining absolute rate constants; the reactions went to
completion in under an hour, which, when considered along-
side the heterogeneous nature of the aluminium catalyst,
made monitoring the individual reaction rates very difficult.
As an alternative, competition reactions were used in which
two aromatic species were placed in the same reaction vessel
with ca. 0.5 equivalents (relative to the total amount of compet-
ing substrates) of acetyl chloride and ca. 3 equivalents of alu-
minium trichloride. The extents of conversion of the two
1
23.8 °C in an NMR spectrometer and H NMR spectra taken at
appropriate intervals until at least three half lives had been
measured. The extent of reaction at a given time was deter-
mined through comparing the integration due to a signal
corresponding to the starting material relative to the inte-
gration of a signal due to the product ether (see ESI†). This
allowed calculation of observed first order rate constants for
the methanolysis of the alcohols 10-OH to 14-OH under these
conditions. The kinetic analysis for each alcohol was carried
out in triplicate.
Computational parameters
1
systems were monitored using H NMR spectroscopy and this
Compounds 1, 2 and 6–8 along with the corresponding non-
isolable intermediates in the reaction outlined in Scheme 1
(left) were modelled using Gaussian 0324 at an RHF/6-311++G
level of theory. Geometry optimized structures were obtained
and energies determined. Fine grain visualization of the fron-
tier molecular orbitals was performed using GaussView 3.0. As
models for the reaction outlined in Scheme 2, the methylated
derivatives S1–S5 and the corresponding benzylic cations
S1+–S5+ were also treated in the same fashion.
data converted into a ratio of the rate constants of the two
species present (Scheme 3, Chart 1).31 As described previously,
this is a particularly robust method in which the data is inde-
pendent of the ratio of the starting materials or the extent of
reaction.
Using this technique, the five systems (1, 2 and 6–8) that
mono-acetylate to give products readily identified using NMR
spectroscopy were investigated using competition experiments.
The resulting 1H NMR spectra were compared to authentic
samples of the starting materials and products and the result-
ing ratio of rate constants tabulated below (Table 1).
The first result to note is the complete selectivity under the
Results and discussion
Initially, Friedel–Crafts acetylation of the aromatic species 1–8 reaction conditions with naphthalene
2 reacting whilst
was considered. Before competition reactions could be con- benzene 1 did not. This result was not unexpected as the
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem., 2015, 13, 9035–9041 | 9037

View Article Online
Paper
Organic & Biomolecular Chemistry
To find the order of reactivity of naphthalene 2, tripheny-
lene 7 and corannulene 8, the two former species were tested
against corannulene 8. The resulting 1H NMR spectra indi-
cated that corannulene 8 was the most reactive of the three
species (as its extent of conversion was higher than either of
the other two species) and, through the use of Chart 1, had a
rate constant that was 3.50(20) times greater than triphenylene
7 and at least 50 times greater than naphthalene 2.
The acylation results clearly showed that (with respect to
reaction with the acetyl cation) corannulene 8 reacts at a faster
rate than triphenylene 7, naphthalene 2 and benzene 1 and at
a slower rate than pyrene 6. The order of pyrene 6, naphtha-
lene 2 and benzene 1 in this reactivity series is in good agree-
ment with the sulfonation12 results previously mentioned and
is not unreasonable given the increasing number of pi elec-
trons which may be involved in delocalisation of the positive
charge in the intermediates. The three larger systems 6–8 have
comparable numbers of pi electrons, so comparison of their
reactivity demonstrates the importance of the structure of the
systems. The significant reactivity of triphenylene 7 and coran-
nulene 8 is perhaps not overly surprising considering the
larger potential pi orbital conjugation though there are fewer
resonance forms in which Kekule sextets are disrupted in the
corannulene 8 case suggesting curvature may negate (to some
extent) the energetic cost of such disruption. The significantly
greater reactivity of the pyrene in these cases can be correlated
to the increased extent of delocalisation in the phenalenyl type
intermediate.
In order to further understand this order of reactivity, com-
putational studies were carried out. The aromatic species 1, 2
and 6–8 along with the corresponding acylated cationic inter-
mediates were modelled at RHF/6-311++G level of theory and
their energies determined in the gas phase (see Table S3† for
full details). As the relative energies of the intermediates can
be correlated to the relative energies of the transition state
leading to them, these data allowed the activation barriers for
the acylation of aromatic species 1, 2 and 6–8 to be ranked (see
Fig. S1†). These were found to parallel the order of reactivity
outlined in Table 1, though the subtle difference between tri-
phenylene 7 and corannulene 8 was not reproduced.
Scheme 3 An example competition reaction between naphthalene 2
and triphenylene 7.
Chart 1 A measure of the ratio of the respective reaction rates as cal-
culated from their extents of conversion as per Yau et al.31
Table 1 The ratio of the rate constants as determined using Chart 1,
for the competition reactions between aromatic systems shown in
Scheme 1 at 0 °C. Where reported the uncertainty in the final result is
half the range of triplicate data
Ratio of rates
(k1/k2)
Compound 1
Compound 2
Benzene 1
Naphthalene 2
Pyrene 6
Corannulene 8
Triphenylene 7
Corannulene 8
Corannulene 8
<0.02a
<0.02a
<0.02a
>50a
Naphthalene 2
Naphthalene 2
Pyrene 6
Pyrene 6
Triphenylene 7
>50a
0.29(2)
a This assumes a detection limit for the extent of conversion of either
compound of 5%.
Investigation of the frontier molecular orbitals of the inter-
mediates in all of these cases shows significant delocalisation
across the whole of the molecule in each case; this is particu-
larly notable in the pyrene 6 (see Table S4†), once again con-
enhanced reactivity of naphthalene 2 compared to benzene 1 sistent with the phenalenyl cation. The one exception is the
towards electrophilic aromatic substitution, due to the lower LUMO in the triphenylene case, which is localised on the ring
decrease in the aromatic stabilisation energy on reaction, is that is being substituted. Examination of the Mulliken charges
well noted in the literature.12,32
(Table S5†) further supports the delocalisation of charge in the
With one ‘end’ of the reactivity scale identified (in this case intermediates and further highlights the localisation of charge
the lower), it became of interest to find the most reactive of the on one ring in the triphenylene case.
aromatic species being considered. Pyrene 6 showed complete
Confirmation of the reactivity order observed for acylation
selectivity regardless of the competition partner chosen, was was highly desirable and, as such, electrophilic bromination
the most reactive of the systems chosen to the Friedel–Crafts using N-bromosuccinimide was examined; this reagent has
acylation, likely due to the significant amount of delocalisation been used extensively for bromination of a range of PAHs.16,33
of positive charge possible in the acylated intermediate which
is a phenalenyl cation.
The reaction of corannulene 8 with N-bromosuccinimide
over 24 hours resulted in a complex mixture of species as
9038 | Org. Biomol. Chem., 2015, 13, 9035–9041
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Paper
noted using 1H NMR spectroscopy (see ESI† for conditions
and analysis). Importantly a large amount of unreacted coran-
nulene 8 was present at the end of the reaction (ca. 50% as
determined using 1H NMR analyses).¶ The small extent of con-
version demonstrates that corannulene 8 is far less reactive
than pyrene 6 which, under the same conditions, resulted in a
greater than 80% isolated yield of the mono-brominated
product 8-Br after 24 hours, as is consistent with literature
reports.15 Triphenylene 7, benzene 1 and naphthalene 2 did
not react with N-bromosuccinimide under the same conditions
(with the latter two cases being consistent with literature
reports15) whilst acenaphthene 4 gave the monobrominated
product 5-bromoacenaphthene 4-Br in 3 hours in an 84%
yield. The extent of conversion of phenanthrene 3 was ca. 38%
after 24 h to a complex mixture of brominated isomers, indi-
cating that it has comparable reactivity to corannulene 8 under
these conditions.
Scheme 4 The methanolysis of an alcohol, which proceeds through a
protonated alcohol and subsequent carbocation intermediate.
The higher reactivity of corannulene 8 and pyrene 6 com-
pared to benzene 1 and naphthalene 2 for electrophilic bromi-
nation is consistent with all previous data. The lack of
reactivity of triphenylene 7 is particularly surprising indicating
that the threshold reactivity required for reaction in this case
lies between triphenylene 7 and corannulene 8. The much
higher reactivity of pyrene 6 and acenaphthene 4 as compared
to corannulene 8 is also an expected result, particularly when
one considers the donating effect of the ethane bridge in ace-
naphthene 4. The high reactivity of phenanthrene 3 is the
most contradictory result, as literature precedent indicates
reactivity comparable to naphthalene 2.
Chart 2 The observed rate constant (kobs) expressed as a function of
the first-order rate constant (k1), the acid concentration ([H+]) and the
pre-equilibrium constant (K).
alcohols would invalidate any comparison of kobs between the
systems. The easiest way to consider whether the position of
equilibrium would vary with the structure of the alcohol might
be to consider any changes in the pKa values of the corres-
ponding protonated species. Unfortunately, these values have
not been reported for the protonated forms of the alcohols of
interest. Irrespective, any effects of changing the structure of
the alcohol upon the pKa value of the protonated form of that
alcohol (and ultimately the position of the equilibrium) would
be expected to be significantly less than any effects upon the
stabilisation of the intermediate carbocation (due to its
increased proximity to the adjacent aromatic system) and
affect the observed rate in the same way. Therefore, any
changes in the observed rate constant can be attributed mainly
to changes in the stabilisation of the intermediate carbocation,
so comparing values for the observed rate constants is
reasonable.
Clearly, further consideration of the systems was required
and, as discussed in the Introduction, an SN1 methanolysis
pathway (Scheme 4) was also considered. Once again, the
difference between these two reactions (an electrophilic aro-
matic substitution and a process which proceeds through a
benzylic carbocation) should be noted but this provides
further insight into the reactivity of curved polycyclic systems
relative to their planar counterparts. The reactivities of the
derivatives 10-OH to 14-OH allowed the effectiveness of the
various PAHs at stabilising developing adjacent positive
charges to be probed. The reaction was simple to perform and
1
monitor in situ using H NMR spectroscopy allowing rate con-
The observed rate constants for the methanolysis of the
benzylic alcohols are presented in Table 2. In all cases the
reaction proceeded irreversibly to give the corresponding
methyl ester.
stants to be determined. It is worth noting that the observed
first order rate constant (kobs) is a function of the first-order
rate constant for the formation of the intermediate cation (k1),
the concentration of acid ([H+]) and the extent of protonation
of the alcohol as determined by the position of equilibrium (K)
(Scheme 4, Chart 2).
Given the acid concentration ([H+]) is controlled through
the methodology, it is important to consider whether the equi-
librium constant (K) will vary for the different alcohols. Whilst
it would be simplest to assume that the value for K does not
change, any significant alteration of this value between the
Table 2 The observed rate constant for the methanolysis of each of
the alcohols 10-OH to 14-OH in acidic methanol at 23.8 °C. Uncertain-
ties are reported as half the range of triplicate experiments
Alcohol
kobs/10−5 s−1
Benzhydrol 10-OH
6.16(0.6)
21.0(1.4)
19.3(1.9)
33.8(7.9)
38.6(2.7)
(2-Naphthyl)phenylmethanol 11-OH
(9-Phenanthrenyl)phenylmethanol 12-OH
(Triphenylenyl)phenylmethanol 13-OH
(Corannulenyl)phenylmethanol 14-OH
¶It is worth noting that the monobromination of corannulene has been reported
using bromine/iron(III) bromide at low temperatures.37
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem., 2015, 13, 9035–9041 | 9039

View Article Online
Paper
Organic & Biomolecular Chemistry
The observed rate constant for the methanolysis of that the triphenylene derivative 13-OH is as reactive under
2-naphthyl-1-phenyl methanol 11-OH is ca. 3.5 times greater these conditions as the naphthalene derivative 12-OH (it is
than that for benzhydrol 10-OH. The effect is similar to that faster, just outside experimental error) and less reactive than
described previously in the acetolysis of arylmethyl tosylates,22 the corrannulene derivative 14-OH (same within uncertainty).
though the magnitude of the change is less; this may either be The latter may simply be the result of the larger uncertainty in
the result of a different degree of charge build-up or steric the rate constant in that case.
interactions between the two aromatic systems limiting orbital
The frontier molecular orbitals of the species S1–S5
overlap in this case. Irrespective, it is also consistent with the and the corresponding benzylic cations were also examined
results described above for electrophilic aromatic substitution (Table S7†). As for the previous case, there was significant
and is due to the lower decrease in aromatic stabilisation delocalisation of these orbitals across the whole molecule in
energy on delocalisation of the incipient positive charge and almost every example, with the LUMO in the triphenylene case
the greater pi system over which it may delocalised.
being (again) a notable exception. Once again, the Mulliken
The rate constant for the reaction of the phenanthrene charges (Table S8†) support significant delocalisation of the
derivative 12-OH was found to be the same as for the naphtha- charge and the importance of not disrupting Kekule sextets in
lene derivative 11-OH, indicating that the phenanthrenyl cases such as triphenylene.
system was able to stabilise the intermediate carbocation only
Given both the experimental and the computational data,
to a similar extent as the naphthyl system. This is somewhat the larger rate constant for methanolysis for the corannulenyl
surprising given the significant rate enhancement seen case 14-OH as compared to (say) the naphthyl case 11-OH may
between 2-naphthyl and 9-phenanthrenyl systems in the aceto- be expected simply due to the size of the available conjugation
lysis of arylmethyl tosylates, though this may be the result of pathway, any contribution to the larger observed rate constant
the greater potential steric interactions between the aryl due to either the curvature of the corannulenyl portion of the
systems in this case. It is also consistent with a similar overall molecule or its rapid inversion at room temperature cannot be
number of pi electrons in the molecule.
differentiated.
Methanolysis of the triphenylene derivative 13-OH gave a
rate constant ca. 5 times faster than the parent benzhydrol
10-OH indicating that the triphenylene 7 core stabilised the
incipient carbocation to a greater degree than either naphthalene
Conclusions
2 or phenanthrene 3, a result consistent with the previously Utilising Friedel–Crafts reaction conditions, the order of reac-
studied Friedel–Crafts reactions and the increased size of the tivity for species 1, 2, 6, 7 and 8 to electrophilic substitution
pi system.
was readily determined. Electrophilic bromination reactions
Lastly, the rate constant for the methanolysis of (corannule- further confirmed this data whilst including the substituted
nyl)phenylmethanol 14-OH was found to be the same as for naphthalene 4 in overall order. Importantly, corannulene 8
the triphenylene derivative 13-OH and hence greater than the was found to be more reactive than benzene 1, naphthalene 2
other cases considered, indicating that corannulene is the and triphenylene 7 and less reactive than pyrene 6 and ace-
(equally) most effective cation at stabilising adjacent incipient naphthene 4. Comparison of the rate constants for acylation
positive charges. This result is consistent with the similar rate suggests that corannulene 8 was most similar to triphenylene
constants found in the Friedel–Crafts competition reactions 7 in terms of stability of the intermediate. The order of reactiv-
and the similar sizes of the pi systems.
ity was paralleled in computational studies.
Once again, computational studies were carried out to
SN1 methanolysis of the benzylic alcohols 10-OH to 14-OH
support this observed order of reactivity. In this case, rather was successfully performed, and showed the greater stabilising
than model the starting materials 10-OH to 14-OH (and the ability (of an incipient carbocation) of corannulene 8 com-
corresponding benzylic cations) the methylated derivatives of pared to naphthalene 2 and benzene 1, as well as confirming
species 1, 2 and 6–8 (compounds S1–S5, see ESI†) along with the similarity between triphenylene 7 and corannulene 8
the corresponding benzylic cations were considered; this was noted above. Once again, computational studies were consist-
anticipated to not only reduce computational cost but also ent with these outcomes.
focus on the extent to which the (poly)cyclic aromatic substitu-
The overall goal of this study was to compare corannulene 8
ent stabilised the developing positive charge. The relative ener- with other common planar PAHs in order to construct a reac-
gies of the methyl derivatives S1–S5 and their corresponding tivity scale. This has been achieved with the most notable
benzylic cations (Table S6, Fig. S2†) generally paralleled the outcome being the similar results for triphenylene 7 and
order of reactivity seen for species 10-OH to 14-OH. The differ- corannulene 8.
ences in relative energies were much smaller than observed for
the acylation of species 1, 2 and 6–8, as would be expected
based on the similarities in observed rates. Note that the calcu-
lated data suggests that the phenanthrene derivative 12-OH
Acknowledgements
would react faster than the naphthalene derivative 11-OH SRDG acknowledges the support of the Australian government
(though they are observed to be the same experimentally) and through the receipt of an Australian Postgraduate Award. JBH
9040 | Org. Biomol. Chem., 2015, 13, 9035–9041
This journal is © The Royal Society of Chemistry 2015

View Article Online
Organic & Biomolecular Chemistry
Paper
acknowledges financial support from the Australian Research 21 H. Boedigheimer, G. M. Ferrence and T. D. Lash, J. Org.
Council Discovery Project Funding Scheme (Project Chem., 2010, 75, 2518–2527.
DP130102331). The authors would like to acknowledge the 22 A. Streitwieser, Jr., H. A. Hammond, R. H. Jagow,
NMR Facility within the Mark Wainwright Analytical Centre at
the University of New South Wales for NMR support.
R. M. Williams, R. G. Jesaitis, C. J. Chang and R. Wolf,
J. Am. Chem. Soc., 1970, 92, 5141–5150.
23 M. A. Dobrowolski, A. Ciesielski and M. K. Cyranski, Phys.
Chem. Chem. Phys., 2011, 13, 20557–20563.
24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian
03, Revision B.05, Gaussian, Inc., Pittsburgh PA, 2003.
25 R. J. Abraham, M. Mobli and R. J. Smith, Magn. Reson.
Chem., 2003, 41, 26–36.
Notes and references
1 J. C. Fetzer, Polycyclic Aromat. Compd., 2007, 27, 143–162.
2 C. Glaser, Ber. Dtsch. Chem. Ges., 1872, 5, 12.
3 C. B. Larsen and N. T. Lucas, Chem. N.Z., 2012, 76, 49–55.
4 L. T. Scott, M. M. Boorum, B. J. McMahon, S. Hagen,
J. Mack, J. Blank, H. Wegner and A. d. Meijere, Science,
2002, 22, 1500–1503.
5 H. Sharghi, M. Jokar, M. M. Doroodmand and R. Khalifeh,
Adv. Synth. Catal., 2010, 352, 3031–3044.
6 E. Mosettig and J. van de Kamp, J. Am. Chem. Soc., 1930,
52, 3704–3720.
7 J.-H. Li, D. V. Ferraris, P. W. Kletzly, W. Li, E. Y. Wang,
A. D. Xing, W. Xu and J. Zhang, Preparation of symmetri-
cally disubstituted aromatic compounds and pharma-
ceutical compositions for the inhibition and/or modulation
of human poly(ADP-ribose) glycohydrolase (PARG) activity,
WO2002057211A1, Guilford Pharmaceuticals Inc., USA,
2002, 92 pp.
8 M. J. Tanga, R. F. Davis and E. J. Reist, J. Heterocycl. Chem.,
1987, 24, 39–41.
9 S. Nishida, Y. Morita, A. Ueda, T. Kobayashi, K. Fukui,
K. Ogasawara, K. Sato, T. Takui and K. Nakasuji, J. Am. 26 J. Ruan, X. Li, O. Saidi and J. Xiao, J. Am. Chem. Soc., 2008,
Chem. Soc., 2008, 130, 14954–14955.
130, 2424–2425.
10 G. Felix, M. Laguerre, J. Dunogues and R. Calas, J. Org. 27 J. Zhao, C. O. Hughes and F. D. Toste, J. Am. Chem. Soc.,
Chem., 1982, 47, 1423–1427. 2006, 128, 7436–7437.
11 R. S. Kathayat and N. S. Finney, J. Am. Chem. Soc., 2013, 28 J. Weng, Q. Mei, Q. Ling, Q. Fan and W. Huang, Tetra-
135, 12612–12614. hedron, 2012, 68, 3129–3134.
12 A. R. Katritzky, M. S. Kim, D. Fedoseyenko, K. Widyan, 29 T. T. Jayanth and C.-H. Cheng, Chem. Commun., 2006, 894–
M. Siskin and M. Francisco, Tetrahedron, 2009, 65, 1111–1114.
896.
13 P. H. Gore, C. K. Thadani, S. Thorburn and M. Yusuf, 30 F. Fernandez, G. Gomez, C. Lopez and A. Santos, J. Prakt.
J. Chem. Soc. C: Org., 1971, 2329–2330. Chem., 1989, 331, 15–21.
14 W. A. Pryor, G. J. Gleicher, J. P. Cosgrove and D. F. Church, 31 H. M. Yau, A. K. Croft and J. B. Harper, Chem. Commun.,
J. Org. Chem., 1984, 49, 5189–5194. 2012, 48, 8937–8939.
15 R. H. Mitchell, Y.-H. Lai and R. V. Williams, J. Org. Chem., 32 Y. Fujiwara, R. Asano, I. Moritani and S. Teranishi, J. Org.
1979, 44, 4733–4735.
Chem., 1976, 41, 1681–1683.
16 T. Morofuji, A. Shimizu and J.-i. Yoshida, Angew. Chem., Int. 33 H. Eguchi, H. Kawaguchi, S. Yoshinaga, A. Nishida,
Ed., 2012, 51, 7259–7262.
T. Nishiguchi and S. Fujisaki, Bull. Chem. Soc. Jpn., 1994,
67, 1918–1921.
34 W. C. Herndon and N. S. Mills, J. Org. Chem., 2005, 70,
8492–8496.
17 J.-Y. Hu, X. Feng, N. Seto, J.-H. Do, X. Zeng, Z. Tao and
T. Yamato, J. Mol. Struct., 2013, 1035, 19–26.
18 I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, Academic Press, 2nd edn, 1971.
35 G. P. Ford and P. S. Herman, J. Mol. Struct., 1991, 236, 269.
19 H. E. Bronstein and L. T. Scott, J. Org. Chem., 2007, 73, 88–93. 36 F. T. Ogata, M. Nojima and S. Kusabayashi, J. Am. Chem.
20 N. Niamnont, N. Kimpitak, K. Wongravee, P. Rashatasakhon, Soc., 1981, 103, 1145–1153.
K. K. Baldridge, J. S. Siegel and M. Sukwattanasinitt, Chem. 37 T. J. Seiders, E. L. Elliott, G. H. Grube and J. S. Siegel, J. Am.
Commun., 2013, 49, 780–782.
Chem. Soc., 1999, 121, 7804–7813.
This journal is © The Royal Society of Chemistry 2015
Org. Biomol. Chem., 2015, 13, 9035–9041 | 9041