Acid-Catalyzed Skeletal Rearrangements in Arenes: Aryl versus Alkyl Ring Pirouettes in Anthracene and Phenanthrene

Article abstract of DOI:10.1021/acs.joc.7b02058

In 1 M triflic acid/dichloroethane, anthracene is protonated at C9, and the resulting 9-anthracenium ion is easily observed by NMR at ambient temperature. When heated as a dilute solution in triflic acid/dichloroethane, anthracene undergoes conversion to phenanthrene as the major volatile product. Minor dihydro and tetrahydro products are also observed. MALDI analysis supports the simultaneous formation of oligomers, which represent 10-60% of the product. Phenanthrene is nearly inert to the same superacid conditions. DFT and CCSD(T)//DFT computational models were constructed for isomerization and automerization mechanisms. These reactions are believed to occur by cationic ring pirouettes which pass through spirocyclic intermediates. The direct aryl pirouette mechanism for anthracene has a predicted DFT barrier of 33.6 kcal/mol; this is too high to be consistent with experiment. The ensemble of experimental and computational models supports a multistep isomerization process, which proceeds by reduction to 1,2,3,4-tetrahydroanthracene, acid-catalyzed isomerization to 1,2,3,4-tetrahydrophenanthrene with a predicted DFT barrier of 19.7 kcal/mol, and then reoxidation to phenanthrene. By contrast, DFT computations support a direct pirouette mechanism for automerization of outer ring carbons in phenanthrene, a reaction demonstrated previously by Balaban through isotopic labeling.

Full text of DOI:10.1021/acs.joc.7b02058

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Article
Acid Catalyzed Skeletal Rearrangements in Arenes: Aryl
vs. Alkyl Ring Pirouettes in Anthracene and Phenanthrene
Sarah L Skraba-Joiner, Jeffrey W. Brulet, Min K Song, and Richard P. Johnson
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02058 • Publication Date (Web): 14 Nov 2017
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Acid Catalyzed Skeletal Rearrangements in Arenes:
Aryl vs. Alkyl Ring Pirouettes in Anthracene and Phenanthrene
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Sarah L. SkrabaꢀJoiner, Jeffrey W. Brulet, Min K. Song and Richard P. Johnson*
Department of Chemistry
and Materials Science Program
University of New Hampshire
Durham, NH 03824
Corresponding Author: Richard.Johnson@unh.edu
Image for TOC Graphical Abstract
Abstract.
In 1 M triflic acid/dichloroethane, anthracene is protonated at C9 and the resulting 9ꢀanthracenium ion is
easily observed by NMR at ambient temperature. When heated as a dilute solution in triflic
acid/dichloroethane, anthracene undergoes conversion to phenanthrene as the major volatile product.
Minor dihydro and tetrahydro products are also observed. MALDI analysis supports the simultaneous
formation of oligomers, which represent 10 – 60% of the product. Phenanthrene is nearly inert to the
same superacid conditions. DFT and CCSD(T)//DFT computational models have been constructed for
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isomerization and automerization mechanisms. These reactions are believed to occur by cationic ring
pirouettes which pass through spirocyclic intermediates. The direct aryl pirouette mechanism for
anthracene has a predicted DFT barrier of 33.6 kcal/mol; this is too high to be consistent with experiment.
The ensemble of experimental and computational models support a multistep isomerization process, first
suggested by Cook and Colgrove in 1994, which proceeds by reduction to 1,2,3,4ꢀtetrahydroanthracene,
acid catalyzed isomerization to 1,2,3,4ꢀtetrahydrophenanthrene, with a predicted DFT barrier of 19.7
kcal/mol, and then reꢀoxidation to phenanthrene. By contrast, DFT computations support a direct
pirouette mechanism for automerization of outer ring carbons in phenanthrene, a reaction demonstrated
previously by Balaban through isotopic labeling.
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INTRODUCTION
The acid catalyzed isomerization of substituted aromatic compounds by substituent migrations (Scheme
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1) has been known for over a century. Interconversion of isomers (1 – 3, R = alkyl or aryl) is believed
to occur through intermediate ipso arenium ions which result from protonation at sites bearing
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substituents. Fused saturated rings also migrate, as has been demonstrated through equilibration of
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octahydroanthracene (4) and octahydrophenanthrene (5), and through automerization of an isotopic label
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in tetrahydrophenanthrene (6). Skeletal rearrangements of fully aromatic polycyclic arenes might also
be acid catalyzed. The first reported example was the AlCl catalyzed isomerization of benz[a]anthracene
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(
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8) to chrysene (9), which was described by Dansi and Salvioni in 1941. Small amounts of phenanthrene
10) were also reported as a product in this reaction. Several more complex examples in longer chains
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were later reported by Zander and by BuuꢀHoi , who noted the generality and potential importance of
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this type of rearrangement. Following an erroneous report on automerization of C labeled naphthalene,
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Balaban demonstrated C label automerization in phenanthrene (11 → 15). Spirocyclic carbocation 13
is the most logical intermediate to explain transposition of the isotopic label. Carbon automerization in
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biphenyl occurs by 1,2ꢀphenyl shifts and not by rearrangements within the benzene rings. Several more
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complex skeletal rearrangements have recently been described. We have previously studied proton
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catalyzed aryl group migrations.
In the present work, we focus on the mechanisms for skeletal
rearrangements in naphthalene and anthracene.
The simplest nondegenerate arene skeletal rearrangement interconverts anthracene (16) and
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phenanthrene (10), the latter more stable thermodynamically by ca. 5 kcal/mol. This reaction does not
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occur during flash vacuum pyrolysis at 1200 °C but has been reported in medium pressure pyrolysis
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experiments, presumably as a consequence of radical catalysis. No mechanism was suggested in the
conversion of 16 to 10 and a similar isomerization of tetracene to chrysene which were observed as a
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consequence of high temperature passage of samples over an alumina/CrO catalyst. In the most
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detailed earlier study, Cook and Colgrove reported in 1994 the isomerization of anthracene by “fluid cat
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cracking” in which anthracene dissolved in petroleum was passed over a silica/alumina catalyst at 482
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C. The observation that 17 and 18 are also formed led Cook and Colgrove to postulate that acid
catalyzed skeletal rearrangement occurs in these reduced structures (17 → 18) rather than by direct
protonation of anthracene. This requires multistep reduction and oxidation to complete rearrangement of
16 to 10.
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Scheme 1. Acid Catalyzed Rearrangements in Arenes
Our work began with the expectation that aryl ring “pirouettes” such as the mechanism proposed
by Balaban (12 → 13 → 14) would provide a universal mechanism for skeletal isomerizations. As will be
detailed below, our experimental and computational results support this automerization mechanism in
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phenanthrene; however, with anthracene, a more complex process is better supported by theory and
experiment.
RESULTS AND DISCUSSION
In strongly acidic media, anthracene (16) is protonated to give a green solution of the 9ꢀanthracenium ion
(19). Olah previously reported the NMR spectrum of this cation which was generated under stable ion
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conditions. We find that 19-d is easily observed at ambient temperature; the well resolved H NMR
spectrum (Figure 1) indicates complete cation formation in 1 M TfOD/DCEꢀd solution. Chemical shifts
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are in good agreement with predictions from DFT computation (Figure SIꢀ1) and the earlier report which
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did not show the resonance for Hf. The upfield singlet at δ 5.2 is attributed to the H isotopologue while
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the adjacent triplet (J = 4.7 Hz) is assigned as the HD isotopologue, with this multiplicity arising from Hꢀ
D germinal coupling. Solutions of phenanthrene under the same conditions are bright redꢀorange but the
NMR spectrum at ambient temperature is only broadened, indicating incomplete protonation (Figure SIꢀ
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). We note that the facile formation of 19 does not exclude the formation at equilibrium of higher energy
isomeric carbocations which are critical to the mechanisms described below.
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Figure 1. 400 MHz H NMR Spectrum of the 9ꢀAnthracenium Ion
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When dilute (1 mg/mL) solutions of anthracene (16) in 1 M TfOH/DCE were heated to reflux (84
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C) or up to ca. 150 °C in a microwave reactor, the main volatile product recovered after neutralization
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was phenanthrene (10). MALDI spectroscopy of the crude product mixture showed that oligomerization
also occurs, giving primarily dimeric material. (Fig. SIꢀ4) Given the site of protonation and usual
behavior of anthracene in electrophilic additions, the dimers are expected to be 9,9’ꢀbianthryls in various
oxidation states. Minor volatile products 17 – 18, 20 and 21 were identified in the product mixture by
NMR and capillary GC, with comparison to authentic samples. With the use of an internal standard added
after reaction, the absolute yield of 10 did not exceed ca. 40%, with the balance of materials apparently
oligomeric. In these experiments, we attribute no unusual effects to microwaves; as we have shown
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earlier, the microwave reactor provides a safe means of heating superacid solutions. Under these same
conditions, heating phenanthrene did not lead to anthracene or to significant amounts of oligomer, with a
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3% recovery after chromatography. In addition to recovered starting material, small amounts of 21 were
observed as the sole volatile product. A typical distribution of volatile reaction products in both reactions
is shown in Scheme 2.
Scheme 2. Acid Catalyzed reactions of Anthracene and Phenanthrene: Volatile Product
Distributions
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When tetrahydro isomers 17 and 18 were subjected to the same reaction conditions, a similar
mixture of volatile products was observed (Scheme 3). Notably, tetrahydroanthracene (17) afforded
primarily rearranged products 10, 18 and 21 (total 67% of volatile material) while 18 gave primarily 10
and dihydro isomers 21 and 22, accompanied by ca. 10% of linear ring products. These experiments
reveal the dominant direction of rearrangement (17 to 18) and support reꢀoxidation to fully aromatic
products under our reaction conditions.
Scheme 3. Superacid Catalyzed Reactions of Tetrahydroarenes: Volatile Product Distributions
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In order to distinguish among potential mechanisms, we measured the effect of initial anthracene
concentration on reaction rate and found a nonꢀfirst order relationship, as measured by loss of reactant. In
a series of experiments, the initial concentration of anthracene was varied and the volatile product
distribution from reaction at 150 °C was measured by gas chromatography after a constant 15 min of
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reaction. At the lowest reactant concentration (10 M), most of the remaining volatile material was
reactant, while tetrahydro products 17 and 18 were barely detectable. As reactant concentration was
increased (Figure SIꢀ3) to 0.07 M, the product mixture was primarily phenanthrene with ca. 10% each of
17 and 18. The total yield of these volatile products, as determined by addition of an internal standard
after reaction, decreased as the initial concentration of anthracene increased, resulting from more material
lost to oligomerization. These results support a mechanism in which oligomerization seeds formation of
17 and 18 as a route to phenanthrene. Direct isomerization of 16 to 10 may occur but this clearly is not the
major route.
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COMPUTATIONAL STUDIES ON POTENTIAL MECHANISMS
Two mechanisms seem likely in the rearrangement of anthracene to phenanthrene. The first mechanism,
which arises from direct protonation of anthracene at a ring juncture carbon, is an aryl ring pirouette
(Scheme 4) to give cation 28 and then on to 29. A similar process beginning with cations 24 or 25 can
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lead to automerization of phenanthrene ring carbons, as reported by Balaban and coꢀworkers in 1989.
The second mechanism is best described as an alkyl ring pirouette (Scheme 5) which would interconvert
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tetrahydro structures 17 and 18. This indirect mechanism was proposed by Cook and Colgrove in 1994.
We carried out computations to assess the comparative energetics of these two mechanisms. The
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potential surface for rearrangements was investigated with density functional theory using Spartan and
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Gaussian.
All structures were optimized at the B3LYP/6ꢀ31+G(d,p) level of theory. This density
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functional has been widely used to model carbocation chemistry. The polarizable continuum model
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PCM) was employed to assess implicit solvation in dichloroethane. In general, we find that DCE
solvation provides significant absolute stabilization of the carbocation but does not significantly change
relative energies of stationary points. Single point CCSD(T)/ccꢀpVDZ calculations were carried out at
DFT optimized geometries. The CCSD(T) energies were corrected with DFT zeroꢀpoint energies. In the
rearrangements of naphthalene, MP2 and MP4//MP2 computations were also carried out for comparison.
Results of DFT and (in brackets) CCSD(T)//DFT computations on the direct rearrangement
surface which connects anthracene and phenanthrene are summarized in Scheme 4. As a reference point,
cation 28 was arbitrarily set at zero since rearrangement should begin at this point. This is at significantly
higher energy than 19, which is observed in solution (Fig. 1). In the rearrangement mechanism,
protonation of 16 at a ring juncture carbon yields 28, which proceeds through spirocyclic cation 29 and
then on to protonated phenanthrene 25. An alternative higher energy bond migration from 28 would give
30. Structure 30 is not an energy minimum but opens spontaneously to give 31 which should deprotonate
to give benz[a]azulene. We have seen no experimental evidence for this pathway. The initial barrier to
rearrangement through TS5 is 33.6 kcal/mol according to DFT calculations and 27.8 kcal/mol from
CCSD(T)//DFT. Energetics along the 28 to 25 pathway are consistent with a unidirectional process in
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which anthracene is converted to phenanthrene. On the phenanthrene side, the lowest energy pathway for
automerization of the outer ring carbons, a rearrangement observed by Balaban and coꢀworkers, passes
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through cations 24 and 27, as previously suggested. The predicted barrier between 24 and 27 is 19.5
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kcal/mol according to DFT calculation; this estimate is diminished to 14.2 kcal/mol by CCSD(T)//DFT.
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Scheme 4. Computational Results for the Aryl Pirouette Mechanism.
a) B3LYP/6ꢀ31+G(d,p) relative free energies (kcal/mol) with solvation in 1,2ꢀdichloroethane (DCE)
b) In brackets, CCSD(T)/ccꢀpVDZ//DFT + ZPVE.
Scheme 5 summarizes results for pirouette rearrangements in tetrahydro derivatives 17 and 18. In each
case, migration of either an alkyl group or an aryl group is possible. The lower energy pathway clearly is
alkyl migration which connects cations 32 and 34 through TS7, 33 and TS8. Predicted barriers through
spirocycle 33 are ca. 20 kcal/mol at both levels of theory. Reaction energetics again favor the
phenanthrene side of the reaction coordinate.
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a,b
Scheme 5. Computational Results for the Alkyl Pirouette Mechanism.
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a) B3LYP/6ꢀ31+G(d,p) relative free energies (kcal/mol) with solvation in 1,2ꢀdichloroethane (DCE)
b) In brackets, CCSD(T)/ccꢀpVDZ//DFT + ZPVE.
Aryl vs. Alkyl Pirouette Mechanisms. The acid catalyzed rearrangement of anthracene to phenanthrene
can be explained either by an aryl ring pirouette (Scheme 4) or an alkyl ring pirouette (Scheme 5, top
pathway). The latter mechanism requires a multistep redox process passing through 17 and 18, which
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was first proposed by Cook and Colgrove.
Thus, including only the rearrangement process, our
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computations support feasibility of the CookꢀColgrove mechanism.
Two literature results are also supported by these computations. First is Balaban’s observation of
isotopic label automerization in phenanthrene (Schemes 1) which likely proceeds through cations 24 and
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27. The predicted barriers through TS2 are less than 20 kcal/mol, which is consistent with the reported
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reaction conditions. Scott’s report of a similar skeletal rearrangement of isotopically labelled 18 would
pass through 34, TS8 and 33. Our computations predict a barrier of ca. 16 kcal/mol for this process.
The redox chemistry which is central to the CookꢀColgrove mechanism has ample precedent. In
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documented.
Scheme 6 shows a putative scheme for double reduction of anthracene. In this scheme,
RH is a dihydroaromatic hydride donor and R+ is an arenium ion. Anthracene protonation at C9 provides
a mechanism for interconversion with 9,10ꢀdihydroanthracene (20), one of the minor products observed
here. This is likely to be rapid but reversible. Alternatively, protonation at C1 will lead through hydride
transfer to dihydro derivatives 39 or 40 which can then proceed to 17 by a second hydride transfer to 41.
For 39, this also requires a 1,2ꢀhydride shift after protonation. Oxidation reverses the process. The most
likely initial hydride donors to seed this chemistry are dihydroaromatic intermediates which are generated
during anthracene dimerization, a competitive process which reduces isomeric product yields. Each new
arylꢀaryl bond releases one proton and one hydride. This assertion is supported by MALDI
characterization of oligomeric products.
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9
0
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9
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7
8
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2
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7
8
9
0
Scheme 6. Redox Mechanisms for Formation of Tetrahydroanthracene.
On the Absence of Rearrangement in Naphthalene. Balaban coined the term “automerization” to
describe the degenerate rearrangement of atoms in naphthalene, a reaction which ultimately proved
8a
elusive. Why? In order to confirm that this was not a consequence of low reaction temperature or
13
26
insufficient acidity, we prepared C ꢀ C labeled naphthalene (44) by a known route, as shown in
1
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Scheme 7, and subjected this material to our TfOH/DCE microwave reaction conditions. After 60 min at
180 °C, no label migration was detected in recovered starting material. Even heating with pure triflic acid
o
at 220 C for 30 min failed to demonstrate rearrangement in recovered naphthalene 44.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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40
41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 7. Attempted Automerization of Naphthalene
o
a) polyphosphoric acid, 90 C b) NaBH c) TsOH/CH Cl d) DDQ
4
2
2
The rearrangement of protonated naphthalene (Scheme 8) was modeled at several different levels
of theory, with results summarized in Table 1. PCM solvation (DCE solvent) shows only a small effect on
the relative energies for stationary points. The predicted DFT barrier from cation 45 is ca. 33 kcal/mol;
this is very similar to TS5 (Scheme 4) for rearrangement of anthracene. CCSD(T)//DFT, MP2/MP2 and
MP4SDTQ//MP2 computations all predict a barrier which is ca. 6 kcal/mol lower.
Scheme 8. Mechanism for Naphthalene Automerization
Table 1: Relative Free Energies (kcal/mol) for Automerization in Naphthalene
a
b
c
d
e
Structure
DFT (PCM)
ꢀ19.4
DFT
CCSD(T)//DFT
MP2
MP4//MP2
ꢀ17.6
0.0
47
ꢀ19.4
0.0
ꢀ17.9
0.0
ꢀ17.7
0.0
45
0.0
TS11
33.4
32.8
26.9
26.8
22.6
27.3
21.2
26.4
46
27.1
23.0
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6
a
b
ZPE corrected B3LYP/6ꢀ31+G(d,p) energy with PCM solvation in 1,2ꢀdichloroethane (DCE). ZPE corrected
c
B3LYP/6ꢀ31+G(d,p) energy. Corrected energy at the CCSD(T)/ccꢀpVTZ//B3LYP/6ꢀ31+G(d,p) + ZPE (B3LYP/6ꢀ
3
pVTZ//MP2/ccꢀpVTZ + ZPE (MP2/ccꢀpVTZ) level).
d
e
1+G(d,p) level). Corrected energy at the MP2/ccꢀpVTZ. Corrected energy at the MP4(SDTQ)/ccꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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4
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9
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8
9
0
1
2
3
4
5
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9
0
Apparently, the cumulative barriers to protonation of naphthalene at Cꢀ9 followed by
rearrangement cannot be surmounted under our reaction conditions. In this case, the absence of
polymerization or detectable reduced naphthalene suggests that the polyhydroaromatic route is not
8
b
8c
available to naphthalene. This would seem to explain why neither Balaban, nor Staab, nor our group
have seen evidence for naphthalene automerization.
CONCLUSIONS
Acidꢀcatalyzed rearrangements of arenes can occur (Scheme 1) by both substituent migration and skeletal
isomerization. The earliest example for an acidꢀcatalyzed arene skeletal rearrangement is Dansi and
5
Salvioni’s report in 1941 that benz[a]anthracene (8) isomerizes to chrysene (9) upon heating with AlCl3.
Scattered reports since that time indicate some general patterns of rearrangement but reaction mechanisms
6ꢀ7,16ꢀ17
have been poorly characterized.
The acid catalyzed automerization of phenanthrene (11 Scheme 1)
9
by interconversion of outer ring carbons reported in 1989 by Balaban suggested that protonation and an
arene ring pirouette might provide a general mechanism.
Isomerization of anthracene to more
thermodynamically stable phenanthrene has been reported under a number of reaction conditions and
1
5ꢀ16
seemed likely to proceed by a similar pirouette mechanism.
Contrary to that expectation, our
experimental studies and computations support what seems an improbable multiꢀstep process originally
17
proposed by Cook and Colgrove in 1994. In this mechanism, anthracene is doubly reduced on one outer
ring to 17, rearranges to 18 by ipsoꢀprotonation and an alkyl ring pirouette (Scheme 5) and is then reꢀ
oxidized to phenanthrene! The oxidation/reduction steps are all mediated by protonation and then hydride
transfer to carbocations (Scheme 6). Initial hydride sources are likely to be dihydroaromatic intermediates
which are produced by parallel oligomerization. This results in a low absolute yield of phenanthrene. We
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cannot rule out a minor contribution from direct isomerization of anthracene but this clearly is not the
major path. This conclusion is supported by three main pieces of evidence: 1) isolation and behavior of
intermediate tetrahydroarenes 17 and 18 under the same reaction conditions; 2) kinetic studies showing an
non firstꢀorder effect of anthracene concentration on rate of reaction; 3) computational models showing
significantly lower barriers for alkyl ring pirouette. DFT and CCSD(T)//DFT results provide modestly
different quantitative results but predict the same preferred reaction paths. The formation of oligomers
provides a logical source of hydride through dihydroaromatic intermediates. Phenanthrene does not
significantly oligomerize under these reaction conditions; however, barriers predicted for Balaban’s
automerization mechanism (Schemes 1 and 4, 24 → TS2 → 27) are consistent with the reported reaction
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9
conditions. Our computations support an aryl pirouette mechanism in this case. It thus appears that two
mechanisms are operative for acid catalyzed arene skeletal isomerization. The absence of observed
8
automerization in naphthalene, reconfirmed here, is consistent with a high barrier to an aryl pirouette.
Earlier literature and the results presented here indicate that acidꢀcatalyzed skeletal isomerizations
should be observed in many classes of arenes and can proceed by at least two general mechanisms. We
will soon report results on higher homologues of anthracene and phenanthrene.
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
1
under a nitrogen atmosphere. H NMR spectra were measured in CDCl at 400 MHz. Capillary analytical
3
gas chromatography was performed using a DBꢀ3 column (30 m x 0.320 mm), with temperature
programming from 75 to 250 °C. Retention times were as follows: 10: 34.8 min, 16: 35.2 min, 17: 33.8
min, 18: 34.2 min, 20:31.2 min, 21: 31.7 min. Microwave reactions were conducted using a singleꢀmode
microwave reactor in 10 mL vessels with temperature monitoring by an external sensor. Absolute yields
were obtained via capillary GC with fluorene added after reaction as an internal standard, or by isolation
using column chromatography with hexane elution. Anthracene, phenanthrene, 9,10ꢀdihydroanthracene
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and 9,10ꢀdihydrophenanthrene were commercial samples. 1,2,3,4ꢀTetrahydroanthracene (17) and 1,2,3,4ꢀ
27
tetrahydrophenanthrene (18) were prepared as reported previously.
General Procedure for Rearrangement in Microwave (MW) Reactor. Under a nitrogen atmosphere,
the substrate (ca. 4 – 50 mg) and 1,2ꢀdichloroethane (DCE, 4 mL) were added to a 10 mL reaction vessel.
Trifluoromethanesulfonic acid (TfOH) (0.40 mL, 4.5 mmol) was added dropwise by syringe; this
typically caused formation of a bright color. The reaction mixture was purged with nitrogen, capped and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
heated in a microwave reactor. Products were isolated by careful neutralization with saturated aqueous
1
NaHCO and extraction with dichloromethane, then analyzed using H NMR and capillary GC. The
3
presence of oligomeric material was assessed on crude isolated product through timeꢀofꢀflight matrix
assisted laser desorption ionization (MALDIꢀTOFꢀMS) mass spectrometry, using sulfur as a matrix.
Isomerization of Anthracene (16). Concentration 14 mg/mL: 16 (56 mg, 0.31 mmol) in DCE/TfOH
was heated using a MW reactor (150 °C, 30 min) according to the general rearrangement procedure.
Analysis of volatile products via capillary GC showed 10 (80%), 17 (14%), 18 (6%), and <1% 16, 20, and
21. The crude product mixture was purified via column chromatography with hexanes, resulting in 16 mg
(
28% recovery) of product mixture. Concentration 1 mg/mL: 16 (4.0 mg, 0.02 mmol) was heated using a
MW reactor (150 °C, 1 hour) according to the general rearrangement procedure. Analysis of volatile
products via capillary GC showed 10 (61%), 16 (31%), 17 (8%), and <1% 18 and 20. 21 was not
observed. The crude product mixture was filtered over a silica plug with hexanes, resulting in 1.2 mg
(
29% recovery) of product mixture.
Acid Catalyzed Reaction of Phenanthrene (10). Phenanthrene (10) (63 mg, 0.35 mmol) was heated
using a MW reactor (150 °C, 30 min) according to the general rearrangement procedure. Analysis of
volatile products via capillary GC showed 10 (96%) and 21 (4%) Chromatography over silica gel with
hexane as eluent gave 59 mg (93% recovery) of product mixture.
Isomerization of 1,2,3,4-Tetrahydroanthracene (17).
27a
An authentic sample of 17 was prepared as described in the literature. Pure 17 (40 mg, 0.02 mmol) was
heated using a MW reactor (150 °C, 30 min) according to the general rearrangement procedure. Analysis
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of volatile products via capillary GC showed 10 (21%), 21 (30%), 17 (20%), 18 (16%) and <1% 16 and
20. Small amounts of unidentified products were also present. Chromatography over silica gel with
hexane as eluent gave 22 mg (56% recovery) of product mixture.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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Isomerization of 1,2,3,4-Tetrahydrophenanthrene (18). An authentic sample of 18 as prepared as
2
7b
described in the literature. Pure 18 (10 mg, 0.05 mmol) was heated using a MW reactor (150 °C, 30
min) according to the general rearrangement procedure. Analysis of volatile products via capillary GC
showed 10 (36%), 21 (26%), 22 (16%), 18 (9%), 17 (9%), 20 (1%), and <1% 16. Small amounts of
unidentified products were also present. Chromatography over silica gel with hexane as eluent gave 6.2
mg (62% recovery)of product mixture.
Reaction of 9,10-Dihydroanthracene (20).
A sample of 20 (40 mg, 0.02 mmol) was heated using a MW reactor (150 °C, 30 min) according to the
general rearrangement procedure. Analysis of volatile products via capillary GC showed 10 (48%), 18
(
19%), 17 (16%), 16 (8%), 21 (7%) and <1% 20. Small amounts of unidentified products were also
present. Chromatography over silica gel with hexane as eluent gave 34 mg (54% yield) of product
mixture.
Acid-Catalyzed reaction of 9,10-Dihydrophenanthrene (21).
A sample of 21 (4.0 mg, 0.02 mmol) was heated using a MW reactor (150 °C, 30 min) according to the
general rearrangement procedure. Analysis of volatile products via capillary GC showed 10 (34%), and
2
1 (65%). 16, 17, 18, and 20 were not observed. Chromatography over silica gel with hexane as eluent
gave 3.7 mg (97 % recovery) of product mixture.
Rearrangement of Anthracene (16) at Varying Concentrations. Anthracene (0.100 g – 0.002 g) and
and 1,2ꢀdichloroethane (DCE) (4 mL) were added to a 10 mL reaction vessel. Trifluoromethanesulfonic
acid (TfOH) (0.40 mL, 4.5 mmol) was added dropwise by syringe. The reaction vessel was capped and
o
heated in a microwave reactor (150 C, 15 min). Products were isolated by careful neutralization with sat.
aqueous NaHCO and extraction with dichloromethane and analyzed using capillary GC. Yields were
3
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5
6
determined by capillary GC with the addition of fluorene as an internal standard after reaction. The
distribution of major volatile products is shown in Figure SIꢀ3.
1
H NMR of Protonated Anthracene (19). A sample of 16 (0.01 g, 0.06 mmol), DCEꢀd (0.4 mL), and
4
1
o
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
TfOD (0.04 mL) were added to an NMR tube. The H NMR spectrum (Fig. 1) was observed at 21.7 C.
1
H NMR (400 MHz) δ 9.90 (s, 1H), 8.52 (t, 3H), 8.40 (t, 3H), 8.16 (d, 2H), 7.99 (t, 3H), 5.17 – 5.07 (m,
2H).
13
13
Synthesis of [1 C] Naphthalene. [1ꢀ C]Naphthalene was synthesized with a 20% isotopic label
2
6
28
utilizing routes (Scheme 7) described by Auger and by Badger.
13
Attempted Automerization of [1- C] Naphthalene 1 M TfOH. A 10 mL microwave tube was charged
with labeled naphthalene (35.0 mg, 0.24 mmol) and purged with nitrogen. Approximately 300 µL of 1.1
o
M TfOH in DCE was added and the mixture was heated in a microwave reactor (180 C, 1 hour). The
reaction mixture was quenched with saturated sodium bicarbonate, extracted into ether and concentrated.
13
The C NMR spectrum showed no change from starting material.
13
Attempted Automerization of [1- C] Naphthalene in pure TfOH. A 10 mL microwave tube was
charged with 35.0 mg (0.27 mmol) of labeled naphthalene and purged with nitrogen. Approximately 250
o
µL of TfOH was added and the solution was heated in a microwave reactor (220 C, 30 min). The
reaction mixture was quenched with saturated sodium bicarbonate, extracted into ether and concentrated.
13
The C NMR spectrum showed no change from starting material.
Acknowledgements. We are grateful for support from the National Science Foundation ( CHEꢀ9616388
and CHEꢀ1362519). This research used the Extreme Science and Engineering Discovery Environment
(XSEDE), supported by NSF Grant No. OCIꢀ1053575.
Supporting Information Available. NMR data for protonated anthracene and phenanthrene, kinetic plot
for rearrangement, MALDI spectrum of product mixture, Cartesian coordinates, and summary energetics
for all stationary points.
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