Phenyl shifts in substituted arenes via Ipso arenium ions

Article abstract of DOI:10.1021/jo301848g

The isomerization of substituted arenes through ipso arenium ions is an important and general molecular rearrangement that leads to interconversions of constitutional isomers. We show here that the superacid trifluoromethanesulfonic acid (TfOH), ca. 1 M in dichloroethane (DCE), provides reliable catalytic reaction conditions for these rearrangements, easily applied at ambient temperature, reflux (84 °C), or in a microwave reactor for higher temperatures. Interconversion of terphenyl isomers in TfOH/DCE at 84 °C gives an ortho/meta/para equilibrium ratio of 0:65:35, nearly identical to values reported earlier by Olah with catalysis by AlCl₃. For the three triphenylbenzenes, TfOH-catalyzed equilibration strongly (>95%) favors the 1,3,5-triphenyl isomer. Equilibration of the three possible tetraphenylbenzenes gives a 61:39 mixture of the 1,2,3,5- and 1,2,4,5-substituted isomers. Under the reaction conditions explored, none of these structures undergoes significant Scholl cyclization. DFT calculations with inclusion of solvation support a mechanistic scheme in which all of the phenyl migrations occur among a series of ipso arenium ions. In every case studied, the preferred isomers at equilibrium are those that yield highly stable cations by the most exothermic, hence least reversible 1,2-H shift.

Full text of DOI:10.1021/jo301848g

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Phenyl Shifts in Substituted Arenes via Ipso Arenium Ions
Aida Ajaz, Erin C. McLaughlin, Sarah L. Skraba, Rajesh Thamatam, and Richard P. Johnson*
Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United States
S
* Supporting Information
ABSTRACT: The isomerization of substituted arenes through ipso arenium ions is an
important and general molecular rearrangement that leads to interconversions of
constitutional isomers. We show here that the superacid trifluoromethanesulfonic acid
(TfOH), ca. 1 M in dichloroethane (DCE), provides reliable catalytic reaction conditions
for these rearrangements, easily applied at ambient temperature, reflux (84 °C), or in a
microwave reactor for higher temperatures. Interconversion of terphenyl isomers in
TfOH/DCE at 84 °C gives an ortho/meta/para equilibrium ratio of 0:65:35, nearly
identical to values reported earlier by Olah with catalysis by AlCl3. For the three
triphenylbenzenes, TfOH-catalyzed equilibration strongly (>95%) favors the 1,3,5-
triphenyl isomer. Equilibration of the three possible tetraphenylbenzenes gives a 61:39
mixture of the 1,2,3,5- and 1,2,4,5-substituted isomers. Under the reaction conditions
explored, none of these structures undergoes significant Scholl cyclization. DFT
calculations with inclusion of solvation support a mechanistic scheme in which all of the
phenyl migrations occur among a series of ipso arenium ions. In every case studied, the preferred isomers at equilibrium are those
that yield highly stable cations by the most exothermic, hence least reversible 1,2-H shift.
Scheme 1. Acid-Catalyzed Interconversions of Substituted
Benzenes (R = Alkyl)
INTRODUCTION
■
Soon after discovery of the Friedel−Crafts alkylation in 1877,1
it was noted that in some unusual cases, meta-disubstituted
products can be formed, instead of the usual ortho + para
product mixture.2−4 Early suggestions that this result may be
due to secondary isomerization were supported by Baddeley’s
report in 1935 that p-dipropylbenzene rearranges to the meta
isomer upon heating in the presence of AlCl3.5 Over the next
two decades, investigations by Baddeley,6 Nightingale,1b,7
Norris,8 Allen,9 McCaulay,10 Brown,11 and others established
the generality of acid-catalyzed alkyl group migrations in
substituted benzenes under Friedel−Crafts reaction conditions.
In these reactions, AlCl3 or AlBr3 usually are assumed to
generate a protic acid through reaction with of adventitious
water.12 It is commonly observed that disubstituted alkylben-
zenes favor the meta isomer at equilibrium (Scheme 1),
trisubstituted benzenes favor products with 1,3,5-substitution
patterns, and tetrasubstituted structures favor products with
1,2,3,5-substitution. Baddeley suggested a carbocation mecha-
nism,6 which was soon supported by Nightingale’s reports of
simultaneous side-chain isomerization of sec-butyl to tert-
butyl.7a McCaulay presented the first clear description of a
mechanism passing through ipso arenium ions.10 Key
mechanistic details of these rearrangements were next laid
out in a series of papers by Olah and co-workers, who ascribed
the common preference for meta-substitution in alkylbenzenes
to formation of the most stable cation and also described
similar isomerizations of halobenzenes.13 This general type of
rearrangement has been referred to as a “Baddeley isomer-
ization”14 or a “Friedel−Crafts isomerization”.13
carbocations (also known as σ complexes15) that are
interconvertible by 1,2-shifts, substituent migration can only
occur through the “ipso” arenium ions shown, in which a
substituent and hydrogen share a carbon atom. The lowest
energy cations on this surface are accessible by 1,2-H shifts in
the meta isomer; this clearly plays a major role in determining
the favored product.
Gas-phase electrophilic substitution reactions of alkylben-
zenes show some parallel behavior but may involve both σ and
π complexes; this is a subject of some current debate.16
Scheme 2 presents the essential mechanism for rearrange-
ments in a disubstituted benzene. Among the nine possible
Received: August 28, 2012
Published: October 12, 2012
© 2012 American Chemical Society
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Another early discovery in arenium ion chemistry was the
Scholl reaction31 in which arenes either oligomerize or cyclize
upon treatment with AlCl3 or protic acids.32 Variations on the
Scholl reaction have been widely applied in the synthesis of
polycyclic aromatic compounds,33 but the reaction mechanism
has remained somewhat mysterious and may be closely linked
to arenium ion rearrangements.34 As noted above, rearrange-
ment by phenyl migration can be a major impediment to
synthesis.28b
Scheme 2. Carbocation Interconversions in Disubstituted
Benzenes
RESULTS AND DISCUSSION
■
Previous work in this field has demonstrated a general and
synthetically useful reaction in which substituted arenes
interconvert through acid catalysis, with the intermediacy of
ipso arenium ions. As our first effort in a more systematic study,
we report here experimental and computational results on
phenyl migrations by the ipso arenium ion mechanism.
Substituent scrambling within arenes in mass spectrometry is
well-known and has been reviewed by Kuck.17 The existence of
ipso arenium ions has been amply demonstrated through their
trapping in electrophilic nitration reactions.18 Alkyl group
transfer (transalkylation) from ipso arenium ions is also well-
known.19
Initial Experiments. Our entry into this field was
serendipitous. We recently described the technique of micro-
wave flash pyrolysis (MFP) in which high-temperature
pyrolytic conditions are approached by heating mixtures of
organic substances with graphite or carbon nanotubes in a
microwave reactor.35 MFP reaction of o-terphenyl (1) on
graphite gave only a 10% yield of triphenylene, the major
product being naphthalene. Since the desired cyclization is
essentially an intramolecular Scholl reaction, we next explored
anhydrous AlCl3 as the solid phase. Heating mixtures of 1 with
AlCl3 in a microwave reactor (Scheme 3) led to m-terphenyl
Much is known about the ability of phenyl groups to migrate
in pinacol and other rearrangements.20 It is surprising that
phenyl migration in arenium ions has not been as well
investigated. Acid-catalyzed interconversion of terphenyl
isomers was first described by Allen and Pingert in 1939.21
These authors reported that o-terphenyl (1) is first converted
to meta (2) by heating with AlCl3, with slower conversion to
the para isomer 3, which was believed to be the eventual
product. Two decades later, with analysis by infrared
spectroscopy, Olah and Meyer reported a 0:63:37 ortho/
meta/para equilibrium product ratio upon heating any of the
three isomers with AlCl3 in sealed tubes.13a In one especially
revealing experiment reported in 1963, Wynberg used a 14C-
labeled substrate to demonstrate complete carbon atom
scrambling by phenyl migration in biphenyl.22 Scott and co-
workers later showed that this process occurs through stepwise
phenyl migrations on the opposing ring. An initial ipso carbon
label migrates successively to ortho, meta, and then para
positions.23 Tolbert and co-workers studied triflic acid catalyzed
rearrangements in dimethylbiphenyls, noting a high preference
for di-meta substitution at equilibrium, which was their
synthetic objective.24 In this case, it is not possible to
distinguish between phenyl or methyl migrations as the source
of rearrangement. More complex examples of reactions that
proceed through ipso arenium ions include automerization of
1,2,3,4-tetrahydrophenanthrene25 and phenanthrene,26 both
demonstrated through 13C labeling studies. Balaban’s report
of an acid-catalyzed automerization of naphthalene was later
shown to be in error,27 but this reaction seems likely to occur at
higher temperature. Examples of unexpected phenyl migrations
under conditions of acid catalysis or Scholl reaction are
scattered in the literature.28
Scheme 3. Microwave Rearrangement of o-Terphenyl
(2) as the major product, with a small amount of the para
isomer (3). We quickly learned that the AlCl3-catalyzed
interconversion of terphenyl isomers was reported by Allen
and Pingert in 194221 and shown by Olah and Meyer in 1962
to give primarily the meta isomer.13a
Reaction Conditions: Moving Beyond AlX3 Catalysis.
These isomerizations are usually assumed to occur by protic
acid catalysis, with the acid generated from reaction of
adventitious traces of water with AlCl3 or AlBr3. This can
generate HX (X = Cl or Br), but it seems more likely that the
acid is an AlX3−H2O or AlX3−HX complex, which is probably
a stronger acid. Previous reports24,36 and our own exper-
imentation with other protic acids led use to choose
trifluoromethanesulfonic acid (4, TfOH) in 1,2-dichloroethane
(DCE) as a more reliable alternative to AlCl3. The absolute pKa
of TfOH in DCE is not known, but it is considered to be a
superacid37 in this solvent, with a pKa of −11.4, vs picric acid as
zero.38 DCE is a non-nucleophilic thermally stable solvent that
is sufficiently polar to support cationic intermediates.39 We find
that TfOH-catalyzed reactions in DCE proceed most
conveniently at reflux (84 °C) but can also be run to at least
160 °C in a microwave reactor. Optimal conditions proved to
be 1.1 M TfOH in DCE (ca. 1:5 v/v).
There have been few theoretical studies on this type of
arenium ion rearrangement. Tolbert and co-workers used AM1
methods to study dimethylbiphenyls, showing high corre-
spondence between equilibrium product ratios and the relative
free energies of carbocations.24 Several groups have employed
Hartree−Fock methods to study related π complexes.16e,g,29
Motivated by results from gas phase and mass spectrometric
studies,17 Kolboe recently used DFT methods in a thorough
study of the complex interconversions among monoalkylben-
zenes and xylenes, including higher energy ring-expansion
reactions30 and a computational search for π complexes.16b−d
It is noteworthy that dissolution of any of the arenes studied
in TfOH/DCE immediately yields brightly colored solutions,
with each arene displaying a unique color. This color persists
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until the reaction is neutralized and may be due to low
equilibrium concentrations of the cations. Protonated arenes
are known to absorb in the visible region.40
As with most carbocations, arenium ions undergo facile
rearrangements. Table 1 summarizes free energy barriers to
Equilibrium Ratios and Relative Rates in Rearrange-
ments of Terphenyl Isomers. Our initial experiments using
either AlCl3 or TfOH as catalyst led to equilibrium mixtures in
which the meta isomer is heavily dominant. Although this result
is in agreement with earlier studies, it seemed essential to
establish the product ratios at prolonged reaction. Pure samples
of each terphenyl isomer (ca. 10−2 M) were refluxed (Scheme
4) with 1.1 M TfOH in DCE, with products monitored by
Table 1. B3LYP/6-31+G(d,p) Free Energy Barriers to
Rearrangement (kcal/mol) at 298 K
substituent (R)
ΔG⧧
ΔG⧧ (DCE solvated)
R = H
11.46
18.44
9.73
10.75
17.88
12.57
8.01
Scheme 4. Interconversions of Terphenyl Isomers
R = CH3
R = Ph
R = Br
6.67
degenerate rearrangements in benzenium ions for hydrogen
and other common migrating groups. The transition-state
structure for phenyl migration is shown in Figure 1. The effect
capillary GC analysis of neutralized aliquots. Under these
conditions, ortho isomer 1 reacts very rapidly. After 1 h at
reflux, the mixture was 97% meta isomer, followed by slower
formation of a meta/para mixture that equilibrated at 65:35,
consistent with Olah’s observation.13a Only <0.1% of the ortho
isomer was present at equilibrium. The para isomer 3
rearranged more slowly, again giving mostly meta and with a
slow evolution toward a 65:35 ratio. m-Terphenyl (2)
rearranged most slowly to give essentially the same equilibrium
ratio. The same product ratio was obtained starting with a
50:50 meta/para mixture. In every case, only a trace (<1%) of
ortho isomer is present at equilibrium. At prolonged reaction
times, minor byproducts, primarily biphenyl and triphenylene
(5), appeared in the chromatograms, but analysis of the final
1
products by H NMR showed almost exclusively terphenyls.
These conditions clearly do not favor Scholl cyclization of 1.
We did not see evidence for any significant quantities of
oligomers from 1, which were previously seen by King under
Scholl conditions (MoCl5/CH2Cl2).34b Not surprisingly,
heating triphenylene in TfOH/DCE under the same conditions
did not cause reversion to 1 or its isomers.
Carrying out the same reactions in a microwave reactor
allowed for higher temperatures and more rapid approach to
equilibrium. As one example, heating 1 at 140 °C in TFOH/
DCE for one hour gave a mixture that was <0.1% ortho/69%
meta/31% para. Longer reaction times or higher temperatures
did not significantly change this result.
Our results for terphenyl are thus in good agreement with
Olah’s earlier observation13a and clearly demonstrate protic acid
catalysis. An initial product distribution that heavily favors the
meta isomer may be preparatively useful if monitored closely.
This very slowly (1−3 days) evolves to a meta + para mixture
that is consistently ca. 65:35. Only traces of triphenylene are
formed by Scholl cyclization.
Computational Models for Terphenyl Rearrange-
ments. The energetics of protonation and the potential
surface for rearrangements were investigated with density
functional theory using Spartan 10,41 Gaussian 03, or Gaussian
09.42 All calculations reported here are at the B3LYP/6-
31+G(d,p) level of theory. This density functional has been
widely used to study carbocation chemistry.43 The polarizable
continuum model (PCM) was employed to assess solvation in
dichloroethane.44
Figure 1. B3LYP/6-31+G(d,p) transition state structure (TS7, R =
Ph) for phenyl migration.
of solvation on barriers to rearrangement is seen to be quite
modest. For R = H, Olah used NMR line shape analysis to
measure a barrier of 10
1 kcal/mol in superacid media.45
Radom and co-workers calculated a barrier of 8.2 kcal/mol
based on G2(MP2) theory.46
One critical question is the energetics of protonation.
Scheme 5 summarizes the free energies calculated for proton
transfer from TfOH (4) to form the lowest energy cation of m-
terphenyl in silico and using a PCM model in 1,2-dichloro-
ethane. The predicted change in solvation energies in this
modestly polar solvent (dielectric constant = 10.3) is
surprisingly large at 75.8 kcal/mol. If any doubt remained,
these results provide clear support for a carbocation
mechanism.
Figure 2 summarizes the free energies of relevant stationary
points on this energy surface, with inclusion of DCE solvation.
The combined energies of cation 1b + triflate anion have been
chosen as a reference point, thus both the protonation steps
and rearrangements share an energy scale. The top portion of
this figure represents interconversions among the ipso cations
1b, 2b, and 3b, which have modest barriers for phenyl
migration. We have not calculated the barriers, but 1,2-H shifts
should easily convert these with the lower energy non-ipso
cations 1a, 2a, and 3a.
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Scheme 5. Energetics of Protonation by Triflic Acid (4)
Figure 2. Energetics of protonation and rearrangements in terphenyl isomers.
In principle, true thermodynamic equilibration would reflect
low energy but the highly exothermic rearrangement of 2b to
2a by 1,2-H shift.
the relative energies of neutral isomers and give a ca. 1:1
mixture of meta and para isomers. It seems likely that this stage
is never reached. Interconversion of isomers requires an initial
pre-equilibrium which generates the ipso arenium ion, probably
through 1,2-H shift from a lower energy isomer. Phenyl
migrations are followed by direct deprotonation or 1,2-H shifts
to the lower energy cation, followed by deprotonation. A
detailed kinetic model would require a large assembly of
equilibrium and rate constants; however, results in hand allow
some important conclusions about relative rates of reaction and
the equilibrium product. After protonation of 1 to give its
lowest energy cation 1a, the net barrier to rearrangement is
18.34 kcal/mol (3.62 for 1,2-H shift +14.72 for rearrangement),
consistent with the observed rapid reaction at 84 °C. For 3,
which rearranges more slowly, the barrier is 23.87 kcal/mol
(8.53 + 15.34). The meta isomer 2 is easiest to protonate but
slowest to react because it has the highest barriers to
rearrangement; 28.0 and 28.6 kcal/mol by the same two step
path. In the equilibration of cationic species, the dominance of
2a and hence neutral product 2 is determined not just by its
Rearrangements of Tri- and Tetraphenylbenzenes.
Scheme 6 summarizes our experimental results for the isomeric
tri- and tetraphenylbenzenes. In each case, rearrangement was
carried out with pure isomers in refluxing TfOH/DCE or at
higher temperatures in a microwave reactor. Product mixtures
Scheme 6. Rearrangement of Tri- and Tetraphenylbenzenes
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Figure 3. Energetics of protonation and rearrangements in triphenylbenzenes.
1
were analyzed by 400 MHz H NMR spectroscopy since each
in the deepest energy well (23.97 + 5.99 kcal/mol) and gives
rearrangement product only under extreme conditions. Once
again, the equilibrium product is determined by the cation
which is not only most stable but has the most exothermic 1,2-
H shift (10b to 10a).
In the tetraphenylbenzene isomers, catalyzed equilibration
proceeds somewhat more slowly to a mixture of two products
(Scheme 6). According to Figure 4, the tetraphenyl isomers
should be most easily protonated but also show some of the
highest barriers to rearrangement; both effects are due to
enhanced resonance stabilization. Protonation of the center
ring of 11 directly yields the ipso cation needed for
rearrangement, thus 11 rearranges easily to 12. Cations 12b,
12c, and 13b have similarly exothermic rearrangements to
stable isomers; this explains why their neutral counterparts 12
and 13 dominate at equilibrium.
isomer is easily identified by characteristic resonances.47
In the triphenylbenzene series, the 1,2,3-isomer (8)
rearranged quickly (15 min) to isomer 9 which then rearranged
more slowly to 10. Not surprisingly, 10 proved to be nearly
inert in these reactions, generating only ca. 8% of the 1,2,4-
isomer at the highest temperature explored. We note that 1,3,5-
triarylbenzenes are often synthesized by the acid catalyzed aldol
condensation of acetophenones.48 An unrecognized key to
success in this chemistry is likely to be the absence of
rearrangement for 1,3,5-trisubstituted benzenes!
Tetraphenylbenzenes underwent somewhat slower reaction.
The 1,2,3,4-tetraphenyl isomer (11) rearranged in refluxing
TfOH/DCE to a 61:39 mixture of 12 and 13. These isomers
were separated by chromatography for characterization. A
similar product distribution was reached in a microwave reactor
at 130 °C, beginning with either pure 11 or 13.
Figures 3 and 4 present summaries of free energies (with
solvation in DCE) for relevant cations and transition states in
the tri- and tetraphenylbenzene series. As above, the chosen
reference point is a rearranging cation plus triflate anion, thus
both protonation and rearrangement are represented in these
diagrams. For the triphenyl isomers, energetics of cationic
species are consistent with the sequential isomerization 8 to 9
to 10. Rearrangement of protonated 8 proceeds through a
barrier of 20.72 kcal/mol (10.68 + 10.04); this is both rapid and
unidirectional toward 9. For protonated 9, the lower barrier
(22.2 vs 26.3) proceeds toward 10. This protonated structure is
CONCLUSIONS
■
The isomerization of substituted arenes through ipso arenium
ions is an important and general molecular rearrangement that
can lead to interconversions of ring substituents, as well as
more complex processes. This reaction has been known
empirically since the late 19th century from studies on
Friedel−Crafts reactions.2−4 Baddely and Kenner described
the first example of a para to meta rearrangement in 1935, thus
explaining the origin of unusual products in Friedel−Crafts
alkylations.5 This was followed by a series of papers by many
authors which demonstrated the generality of substituent
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Figure 4. Energetics of protonation and rearrangements in tetraphenylbenzenes.
rearrangement in alkylbenzenes.6−11 Olah explained the general
preference for meta-disubstituted products through the
intermediacy of ipso arenium ions, which lead through 1,2-H
shifts to the most stable carbocation.13 The majority of this
earlier work was carried out under Friedel−Crafts conditions
using aluminum trihalides as catalysts. This process has often
been referred to as the Baddeley or Friedel−Crafts rearrange-
ment.
We have shown here that the superacid trifluoromethane-
sulfonic acid (TfOH) in dichloroethane38 provides more
reliable catalytic reaction conditions, easily applied at ambient
temperature, reflux or in a microwave reactor for higher
temperatures. Rearrangements of terphenyl isomers have been
reported earlier;13a,21 our experiments confirm a preference for
meta terphenyl at equilibrium, giving ortho/meta/para equili-
brium ratios of 0:65:35, nearly identical to values reported
earlier by Olah. For triphenylbenzenes, acid catalyzed
equilibration strongly favors the 1,3,5-triphenyl isomer.
Equilibration among the three possible tetraphenylbenzenes
reproducibly gives a 60:40 mixture of the 1,2,3,5- and 1,2,4,5-
substituted isomers. Under the reaction conditions we explored,
none of these structures undergoes significant Scholl
cyclization.32 Not surprisingly, equilibrium product ratios with
phenyl substitution are similar to those described earlier for
alkyl-substituted benzenes (Scheme 1).
show barriers to phenyl migration that range from 10 to 27
kcal/mol and a wide range of stability among mechanistically
related non-ipso cations. In every case studied here, the
preferred isomers at equilibrium are those which yield highly
stable cations by the most exothermic, hence least reversible
1,2-H shifts.
One great mystery is why the fundamental molecular
rearrangements shown in Scheme 1 are essentially absent
from chemistry textbooks and are so little known among
chemists today. By contrast, the mechanistically related Scholl
cyclization is much better known.31,33c−h,34 Arenium ion
rearrangements have much unrealized potential in the synthesis
of polycyclic aromatic structures and should be suspected
whenever substituted arenes are subjected to Friedel−Crafts or
Scholl reaction conditions.
EXPERIMENTAL SECTION
■
General Methods. Trifluoromethanesulfonic acid (TfOH, 99%
purity) and dichloroethane (DCE, 99+%) were used as received from
commercial sources. Glassware was oven-dried, and all reactions were
run under a nitrogen atmosphere. 1H NMR spectra were measured in
CDCl3 at 400 MHz and reported relative to TMS. Capillary analytical
gas chromatography was performed using a DB-3 column (30 m ×
0.320 mm), with temperature programming from 75 to 250 °C. For
terphenyl isomers, retention times were as follows: 1, 14.7 min; 2, 17.9
min; 3, 18.7 min; 5, 25.5 min. Microwave reactions were conducted
using a CEM Discover single-mode microwave reactor in 10 mL
vessels with temperatures measured by an infrared sensor at the
bottom of the tube. Terphenyl isomers and 1,3,5-triphenylbenzene
(10) were commercial samples. The syntheses and NMR spectral data
DFT calculations support a mechanistic scheme in which all
of the phenyl migrations occur among a series of ipso arenium
ions. Computational models with inclusion of implicit solvation
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for 1,2,3-triphenylbenzene (8),47b 1,2,4-triphenylbenzene (9),47a and
1,2,3,4-tetraphenylbenzene(11)47a have been reported previously.
General Procedure for Rearrangement under Reflux. A 25
mL, two-neck round-bottom flask was equipped with a stir bar, water-
cooled condenser, and glass stopper. Under a nitrogen atmosphere,
the substrate (ca. 0.10 g) and 1,2-dichloroethane (DCE) (4 mL) were
charged to the flask. Trifluoromethanesulfonic acid (TfOH) (0.40 mL,
4.5 mmol) was added dropwise by syringe; this typically caused
formation of a bright color. The mixture was brought to reflux and
monitored periodically by removal and analysis (1H NMR or capillary
GC) of small aliquots. Products were isolated by careful neutralization
with satd aqueous NaHCO3 and extraction with diethyl ether unless
otherwise noted.
in vacuo. The residue was initially purified by column chromatography
over silica gel, eluting with 10% dichloromethane in hexanes giving a
mixture of 12 and 13 in a 61:39 ratio and an overall yield of 96%. This
isomeric mixture was separated by preparative TLC using 3%
dichloromethane in cyclohexane to isolate pure 12 and 13. A similar
reaction of 11 in a microwave reactor at 130 °C for 60 min gave a
mixture of 12 and 13 in a ratio of 60:40.
1,2,4,5-Tetraphenylbenzene (13). A solution of 13 (0.010 g, 0.030
mmol), DCE (2 mL), and TfOH (0.20 mL, 2.3 mmol) was subjected
to microwave reaction at 150 °C for 30 min. After cooling, the dark
red mixture was neutralized with satd NaHCO3 and extracted using
1
dichloromethane. A red solid was recovered (96%). H NMR analysis
indicated a product ratio of 64:36 12:13.
Rearrangement of Terphenyl Isomers. Ortho (1). By GC analysis
of neutralized aliquots, o-terphenyl isomerized rapidly (ca. 15 min) to
the meta isomer with slower appearance of p-terphenyl (see Figure S5
in the Supporting Information). After 5 days, the observed product
distribution was o- <1%, m- 63%, and p-terphenyl 37%. An orange
solid was recovered after 14 days (67%) and found to contain o- <1%,
ASSOCIATED CONTENT
* Supporting Information
Summary table of total energies and Cartesian coordinates for
stationary points. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
1
m- 66%, and p-terphenyl 34% by H NMR analysis.
Meta (2). By GC analysis, slow formation of p-terphenyl was
observed. After 5 days, the observed product distribution was o- <1%,
m- 73%, and p-terphenyl 27%. An orange solid was recovered after 14
days (56%) and found to contain o- <1%, m- 69%, and p-terphenyl
AUTHOR INFORMATION
Corresponding Author
*E-mail: richard.johnson@unh.edu.
Notes
■
1
31% by H NMR analysis.
Para (3). By GC analysis, p-terphenyl isomerized quickly to the m-
isomer. After 5 days, the observed product distribution was o- <1%, m-
63%, and p-terphenyl 37%. An orange solid was recovered after 14
days (59%) and was found to contain o- <1%, m- 62%, and p-terphenyl
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for generous support from the National Science
Foundation (CHE-9616388).
■
1
38% by H NMR analysis.
Rearrangement of o-Terphenyl via Microwave. o-Terphenyl (1)
(0.030 g, 0.13 mmol), DCE (2 mL), and TfOH (0.2 mL, 2.2 mmol)
was mixed in a 10 mL reaction tube. The reaction vessel was capped
and placed in a microwave reactor (140 °C, 90 min). After the reaction
time, the dark orange in solution was neutralized with satd NaHCO3
and extracted using diethyl ether. An orange solid was recovered
(72%) and was found to contain o- <1%, m- 64%, and p-terphenyl 32%
and triphenylene 3% by capillary GC analysis.
Rearrangement of Triphenylbenzene Isomers. 1,2,3-Triphenyl-
benzene47b (8) (0.080 g, 0.26 mmol), DCE (4 mL), and TfOH (0.4
mL, 4.5 mmol) were heated to reflux, becoming red in color. Aliquots
(0.5 mL) were removed periodically and neutralized. 1H NMR analysis
indicated that 8 isomerized rapidly to 9 (δ m 7.25−7.17 ppm, 10 H)
and 10 (δ s 7.79 ppm, 3 H). After 15 min, the observed product
distribution was ca. 1:1 9:10, with no starting material present. After
4.5 h, the observed product distribution was ca. 1:13 9:10.
DEDICATION
■
This paper is dedicated to the memory of Dr. Atena Necula, a
pioneer in this field.
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