Acid-catalyzed rearrangements in arenes: Interconversions in the quaterphenyl series

Article abstract of DOI:10.3762/bjoc.15.258

Arenes undergo rearrangement of phenyl, alkyl, halogen and other groups through the intermediacy of ipso arenium ions in which a proton is attached at the same carbon as the migrating substituent. Interconversions among the six quaterphenyl isomers have b

Full text of DOI:10.3762/bjoc.15.258

Acid-catalyzed rearrangements in arenes: interconversions in
the quaterphenyl series
Sarah L. Skraba-Joiner, Carter J. Holt and Richard P. Johnson*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 2655–2663.
Department of Chemistry and Materials Science Program, University
of New Hampshire, Durham, NH 03824, USA
doi:10.3762/bjoc.15.258
Received: 20 June 2019
Email:
Accepted: 18 October 2019
Published: 06 November 2019
Richard P. Johnson* - Richard.Johnson@unh.edu
* Corresponding author
This article is part of the thematic issue "Reactive intermediates –
carbocations".
Keywords:
arenium ion; carbocation; density functional theory; microwave
reaction; rearrangement; superacid
Guest Editor: S. R. Hare
© 2019 Skraba-Joiner et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Arenes undergo rearrangement of phenyl, alkyl, halogen and other groups through the intermediacy of ipso arenium ions in which a
proton is attached at the same carbon as the migrating substituent. Interconversions among the six quaterphenyl isomers have been
studied here as a model for rearrangements of linear polyphenyls. All reactions were carried out in 1 M CF3SO3H (TfOH) in
dichloroethane at 150 °C in a microwave reactor for 30–60 min, with product formation assessed by high field NMR analysis.
Under these reaction conditions, m,p'-quaterphenyl is the equilibrium product. This isomer is unchanged by the reaction conditions
and all other quaterphenyl isomers rearrange to m,p' as the dominant or sole product. DFT computations with inclusion of implicit
solvation support a complex network of phenyl and biphenyl shifts, with barriers to rearrangement in the range of 10–21 kcal/mol.
Consistent with experiments, the lowest energy arenium ion located on this surface is due to protonation of m,p'-quaterphenyl. This
supports thermodynamic control based on carbocation energies.
Introduction
Carbocations are enigmatic reactive intermediates of enduring Every student of organic chemistry is taught the importance of
importance in chemistry. No other reactive species displays arenium ions in the classic two step SEAr mechanism for elec-
such a complex and fascinating collection of molecular rear- trophilic aromatic substitution. Addition of an electrophile to an
rangements. Building on a long history, new synthetic applica- arene leads to a bound species, sometimes called a σ-complex,
tions [1,2] and explanations of carbocation reaction mecha- which then loses a proton at the site of substitution to yield the
nisms [3-6] continue to be discovered. Chemistry in superacid product [9]. Of course challenges to this simple mechanism
solutions has played a major role in this field [7,8].
exist [10-15], including the recent proposal of a one-step
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process [16]. Reaction dynamics of electrophile–arene π com- Our interest in this field arose from an accidental rediscovery of
plexes may also play a role in site selectivity [17]. It is less rearrangements in the terphenyl series (1–3; Scheme 1), by
commonly known that arenium ions, like many other types of heating 1 with AlCl3 – a reaction independently discovered
carbocations, often rearrange by 1,2-shifts. This leads to a fasci- twice before [22,23]! We confirmed earlier observations of
nating collection of rearrangements that can migrate hydrogen, facile acid-catalyzed interconversion, as well as the fact that
halogens or more complex substituents around the ring and m-terphenyl (2) is favored at equilibrium (observed ratio 1:2:3
even modify the carbon skeleton. Early reports by Baddeley is <1:69:31) and used theory to explain this product selectivity
[18] helped to explain odd results from Friedel–Crafts reactions [25]. This preference is consistent with formation of the most
[19] and this type of process is sometime referred to as a stable arenium ion intermediate, as shown by DFT computa-
Baddeley rearrangement. Many examples of alkyl group migra- tions.
tion have been described [19]. Phenyl groups migrate easily and
degenerate phenyl shifts in biphenyl were confirmed by isotopic Early studies in this field used AlCl3 as catalyst and it was gen-
labeling [20,21]. One classic example of arenium ion chemistry erally assumed that adventitious traces of water generated the
is the interconversion of terphenyl isomers 1–3 (Scheme 1). actual catalyst, precise identity unknown. We found these
This rearrangement was first reported by Allen and Pingert in "water-promoted" reactions with catalytic AlCl3 to be unreli-
1942 [22] and then independently rediscovered by Olah and able. To remove ambiguity about the catalyst and extend the
Meyer twenty years later [23]. Interconversion of isomers 1–3 temperature range, we developed a more reliable method for
is believed to occur through the intermediacy of ipso arenium studying higher temperature carbocation rearrangements. In our
ions 4–6 which connect through 1,2-phenyl shifts.
method, we use 1 M (ca. 20% by volume) trifluoromethane-
sulfonic acid (TfOH) as catalyst with dichloroethane as our
preferred solvent. Most importantly, reactions are conducted in
the capped tube of a microwave reactor. With this approach, we
can safely and reproducibly heat reactions to ca. 170 °C, so far
without incident.
Other more complex rearrangements are easily observed. In the
binaphthyl series 7–9, three sequential rearrangements occur at
ambient temperature favoring the 2,2'-isomer 9 (97%) at equi-
librium [26]. Aryl shifts occur readily in naphthalene, with beta-
substitution favored at equilibrium. Skeletal rearrangements of
fused arene rings are also possible and can proceed through
several mechanisms. The first example was reported by Dansi
and Salvioni in 1941 in the rearrangement of benz[a]anthracene
to chrysene [27]. We recently studied the rearrangement of
anthracene (10) to phenanthrene (11) [28], finding evidence to
support a complex process, suggested earlier [29], that involves
initial reduction to 1,2,3,4-tetrahydroanthracene, followed by a
pirouette rearrangement of the reduced ring through a spiro-
cyclic intermediate and then re-oxidation to phenanthrene.
These reactions involve a complex series of proton and hydride
transfers.
Results and Discussion
Beyond terphenyls, acid-catalyzed rearrangements pose limita-
tions in the synthesis of extended polyphenyls but the factors
controlling interconversion of isomers are poorly understood
[30-33]. As one recent example, Jasti and co-workers showed
Scheme 1: Acid-catalyzed rearrangements of arenes.
The term "ipso" was first proposed by Perrin and Skinner to that cycloparaphenylenes undergo rapid acid-catalyzed rear-
explain unusual results in electrophilic substitution reactions; rangement which precludes using Scholl-type chemistry in this
this refers to protonation at the site of a substituent [24]. Ipso series [30]. In the present work, we have explored rearrange-
protonation is the essential step in arenium ion rearrangements. ments of quaterphenyls, the next homolog in the paraphenylene
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Scheme 2: Rearrangement of quaterphenyl isomers by phenyl shifts.
series, now with six structural isomers. Scheme 2 summarizes
interconnections via 1,2-migration of a terminal phenyl group.
As will be shown below, an internal aryl–aryl bond can also be
transposed through 1,2-biphenyl migration, with a similar
network of interconversions. Based on the behavior of terphenyl
which favors meta substitution at equilibrium, we initially
hypothesized that m,m'-quaterphenyl (14) would likely be the
major isomer at equilibrium.
Only brief exploration of quaterphenyl rearrangements had been
described previously. Isomerization of o,o’-quaterphenyl (17)
with SnCl4/AlCl3 catalysis has been reported to yield a mixture
of p,p’- (12), m,p’- (13), and m,m’-quaterphenyl (14) [34].
p,p’-Quaterphenyl (12) and m,m’-quaterphenyl (14) were avail-
able commercially. To complete the series, samples of m,p’-
(13), o,p’- (15), o,m’- (16), and o,o’-quaterphenyl (17) were
synthesized as shown in Scheme 3. Suzuki–Miyaura coupling
was used to synthesize 13, 15, and 16 from the corresponding
aryl bromides and boronic acids [35]. o,o’-Quaterphenyl (17)
was synthesized by homo-coupling of 2-bromobiphenyl, as pre-
viously reported [36].
Scheme 3: Synthesis of quaterphenyl isomers.
with reaction times of 30 or 60 min. After chromatographic
Promoting the rearrangement of all quaterphenyl isomers at purification to remove oligomeric material, product distribu-
room temperature or in refluxing DCE (84 °C) proved to be tions were determined by high field 1H NMR, with integration
difficult because p,p’-quaterphenyl (12) is very poorly soluble of unique resonances for each isomer. In these reactions, we at-
in this solvent and higher temperatures were required in some tribute no special effect due to microwaves. This has been a
cases to reach equilibrium. In a solution of 1 M TfOH/DCE, 12 subject of some debate in the literature [37,38]. As we have
forms a colored solution more slowly (ca. 1–2 min) than demonstrated earlier, the microwave reactor simply provides a
previous examples we have studied. Heating these reaction mix- safe and reliable way to heat samples with superacids at temper-
tures in a microwave reactor allows for the substrate to dissolve atures well above the normal solvent boiling point.
in the reaction medium, resulting in a rearrangement. To
provide a consistent reaction environment, the rearrangement of The results of quaterphenyl isomerizations are summarized in
all isomers was studied using a microwave reactor at 150 °C, Table 1. Initially it was expected that m,m’-quaterphenyl (14)
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Table 1: Product distributions from rearrangement of quaterphenyl isomers.a,b.
Reactant
Time (min)
12
13
14
15
16
17
Yield
12
12
13
13
14
14
15
16
17
30
60
30
60
30
60
30
30
30
–
–
–
–
–
–
–
–
–
100
100
100
100
50
67
81
57
60
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
70
55
88
75
81
90
70
64
67
–
50
33
19
43
40
aAll reactions were carried out in 1 M CF3SO3H in dichloroethane at 150 °C in a microwave reactor. bYields are total isolated products after flash
chromatography.
might be the major isomer formed at equilibrium, as previous (Scheme 4a). Biphenyl shifts are also possible through proton-
examples have heavily favored meta-substitution patterns. Con- ation at the internal ipso sites (Scheme 4b). This results in a
trary to this expection, the major product observed in all cases very complex potential energy surface. Protonation of a termi-
was m,p’-quaterphenyl (13). In 30 min at 150 °C, p,p’-quater- nal phenyl group can also lead to terphenyl migration but this
phenyl (12) isomerized completely to 13. Under the same process is structurally degenerate and was not explored by com-
conditions, m,m’-quaterphenyl (14) isomerized more slowly, putations.
yielding 13 and 14 in a 50:50 mixture in 30 min. Increased reac-
tion time with m,m’-quaterphenyl (14) yielded 13 and 14 in a
67:33 ratio. Starting with o,m’- (16) or o,o’-quaterphenyl (17)
yielded a mixture of ca. 60% 13 and ca. 40% 14 in 30 min.
Similarly, o,p’-quaterphenyl (15) isomerized to an 81:19 mix-
ture of 13:14. When starting with m,p’-quaterphenyl, no rear-
rangement to 14 was observed, even at increased reaction times.
Minor products with more downfield chemical shifts were also
observed via 1H NMR of these crude product mixtures; these
were easily separated from quaterphenyl isomers using flash
column chromatography. Analysis of the crude product mix-
tures by MALDI–TOF–MS confirmed these minor products
could be attributed to oligomerization (m/z = 344, 673).
Our conclusion from this series of experiments is that m,p'-
quaterphenyl (13) is the equilibrium product from rearrange-
ment of all of the six isomers. This is most clearly demon-
strated in the rearrangement of 12 to 13 and the failure of 13 to
produce other isomers. In some cases, reaction times were
insufficient to fully reach equilibrium.
Scheme 4: Rearrangement of quaterphenyl isomers via (a) 1,2-phe-
nyl shift and (b) 1,2-biphenyl shift.
Computational models for quaterphenyl
Both phenyl and biphenyl rearrangement pathways were studied
by DFT methods, with inclusion of implicit solvation by the
rearrangements
While the energy surface for the interconversion of quater-
phenyl isomers initially seems straightforward, there are addi-
tional modes of rearrangement that must be considered. 1,2-
Phenyl shifts are possible to interconvert isomers, occurring
through protonation at the ipso site on the external phenyl rings
polarizable continuum model (PCM). As in our earlier research,
we employed B3LYP/6-31+G(d,p) theory with PCM solvation
in dichloroethane [25,26,28]. As noted earlier by Tantillo, the
B3LYP functional provides a very good description of carbo-
cation chemistry [3].
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Figure 1 shows the lowest energy pathways for 1,2-phenyl lar π stacking. Very similar results were obtained with M06-2X/
shifts, while Figure 2 shows those for a 1,2-biphenyl shifts. In 6-311+G(d,p) theory, which also placed 17 as the lowest energy
each case, the energy reference is arbitrarily chosen as the linear quaterphenyl isomer. Our results thus do not support thermo-
ipso cation 12c or 12d for phenyl or biphenyl shift, respective- dynamic control based on relative energies of the neutral
ly. Predicted barriers for rearrangements of ipso cations are all quaterphenyls.
in the range of 9–22 kcal/mol. Common to both potential
energy surfaces are the lowest energy non-ipso carbocations A more plausible scenario is that thermodynamic control
12a–17a which lie at the bottom on the energy scale. It is note- applies to carbocation intermediates, with the equilibrium prod-
worthy that m,p' cation 13a is the lowest energy species pre- uct determined by the energy of the lowest energy carbocation
dicted by our calculations. In each diagram, double-headed in solution. This is predicted to be 13a. The speed of equilibra-
vertical arrows show the energy difference between ipso cations tion for different isomers is determined by the number of re-
and their non-ipso counterpart which might be formed rapidly quired steps and their barriers. Thus 12 rearranges to 13 in a
by secondary 1,2-hydride shifts.
phenyl or biphenyl migration, passing through TS7 or TS13, re-
spectively. This rearrangement is complete in our standard
In principle, the equilibrium product in these complex reactions 30 min reaction time. By contrast, 17 requires three separate
might correlate with relative energies of the neutral quater- migration steps to arrive at 13, passing through 16 and 14; this
phenyl isomers. Reliable heats of formation for quaterphenyls rearrangement (Table 1) is incomplete during the same reaction
are unavailable in the literature but these values and other esti- period.
mates for relative energies can be predicted by theory. Starting
with the lowest energy conformer for each quaterphenyl, we An unknown factor is the degree of protonation, especially at
first computed heats of formation using the T1 method [39]. 150 °C. We observed earlier by NMR spectroscopy that
Predicted values (Supporting Information File 1) are narrowly anthracene is fully protonated at ambient temperature in 1M
clustered in a range of ca. 2 kcal/mol, with the lowest energy TfOH but 1,3,5-triphenylbenzene is not [28]. One experimental
isomer predicted to be o,o'-quaterphenyl (17). This low ranking observation is that the 1M TfOH/arene reaction solutions
for the most congested isomer may be attributed to intramolecu- invariably have a bright color at ambient temperature due to the
Figure 1: Pathways for terminal 1,2-phenyl shifts in quaterphenyl isomers calculated with IEFPCM(DCE)/B3LYP/6-31+G(d,p) theory. Relative free
energies are given in kcal/mol.
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Figure 2: Pathways for 1,2-biphenyl shifts in quaterphenyl isomers calculated with IEFPCM(DCE)/B3LYP/6-31+G(d,p) theory. Relative free energies
are given in kcal/mol.
carbocation [40,41]. As these solutions are heated, they become Conclusion
much darker in color, implying a higher level of protonation. Acid-catalyzed interconversions among the six quaterphenyl
Upon cooling, a more normal color is returned. Full proton- isomers have been studied in this work as a model for rear-
ation of 13 at 150 °C would explain thermodynamic control by rangements of linear polyphenyls. All reactions were carried out
cationic intermediates.
in 1 M CF3SO3H (TfOH) in dichloroethane at 150 °C in a
microwave reactor for 30 or 60 min, with product formation
Our naïve supposition at the outset of this project that m,m'- assessed by high field NMR analysis. Under these reaction
quaterphenyl 14 might be favored at equilibrium was unsup- conditions, m,p'-quaterphenyl (13) is the equilibrium product.
ported by experiment, which instead showed the m,p' isomer 13 This isomer is unchanged by the reaction conditions and all
to be preferred. A comparison of 5a, the lowest energy cation other quaterphenyl isomers rearrange to m,p' as the dominant or
from terphenyl rearrangements, with 13a, the corresponding sole product. DFT computations with inclusion of implicit
cation for quaterphenyls, provides a simple explanation. If a solvation support a complex network of phenyl and biphenyl
single ring is protonated, greater stability accrues from a para shifts, with barriers to rearrangement in the range of
phenyl substituent with meta a close second. The same effect 10–21 kcal/mol. Consistent with experiments, the lowest energy
should apply to longer polyphenylene chains, resulting in a arenium ion located on this surface is due to protonation of
chain of aryl groups that remains mostly para after generating a m,p'-quaterphenyl. This supports thermodynamic control based
more basic meta substituted site.
on carbocation energies. The same effect may apply to longer
polyphenylene chains, resulting in a chain of aryl groups that
does not fully rearrange but remains mostly para after gener-
ating a more basic meta substituted site.
Experimental
General methods. Trifluoromethanesulfonic acid (TfOH, 99%
purity) and dichloroethane (DCE, 99+%) were used as received
from commercial sources. Glassware was oven dried
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and all reactions were run under a nitrogen atmosphere. with DCM. The organic extract was washed with water and
1H NMR spectra were measured in CDCl3 using a Varian brine and dried with Na2SO4. The crude product was purified
XL-400 MHz spectrometer. Microwave reactions were by flash chromatography with hexanes to yield o,p’-quater-
conducted using a single-mode CEM microwave reactor phenyl as a white solid (0.55 g, mp 107–109 °C, lit 117–120 °C,
in 10 mL vessels with temperature monitoring by an external 90% yield). 1H NMR (400 MHz, CDCl3) δ 7.61–7.57 (m, 2H),
sensor.
7.50–7.40 (m, 8H), 7.35–7.29 (m, 1H), 7.26–7.17 (m, 7H).
General procedure for rearrangement in the microwave Suzuki–Miyaura coupling to form o,m’-Quaterphenyl (16)
(MW) reactor. In a similar manner as described in [28], under [35]: 2-Bromobiphenyl (0.20 g, 0.80 mmol), biphenyl-3-
a nitrogen atmosphere, the substrate (ca. 4 – 50 mg) and 1,2- boronic acid (0.25 g, 1.3 mmol), and Pd(PPh3)4 (0.02 g,
dichloroethane (DCE, 4 mL) were added to a 10 mL reaction 0.02 mmol) were dissolved in DMF (10 mL). The reaction mix-
vessel. Trifluoromethanesulfonic acid (TfOH) (0.40 mL, ture was cooled to 0 °C and purged with nitrogen. A solution of
4.5 mmol) was added dropwise by syringe; this typically caused K2CO3 (0.20 g, 1.5 mmol) and water (5 mL) was added via
formation of a bright color. The reaction mixture was purged syringe and the reaction mixture was stirred at 0 °C for 5 h. The
with nitrogen, capped and heated in a microwave reactor. Reac- product was extracted with DCM, and washed with water,
tion times represent hold times after a ramp time of ca. NaOH, and HCl. The organic extract was then washed with
10 minutes. After cooling, products were isolated by careful water and brine and dried with Na2SO4. The crude product was
neutralization with saturated aqueous NaHCO3 and extraction purified by flash chromatography with hexanes to yield o,m’-
with dichloromethane. The crude product mixture was purified quaterphenyl as a white solid (0.085 g, mp 84–86 °C, lit
by flash chromatography then analyzed using 1H NMR. The 90–91 °C, 33% yield). 1H NMR (400 MHz, CDCl3) δ
presence of oligomeric material was assessed on crude isolated 7.52–7.47 (m, 1H), 7.47–7.40 (m, 4H), 7.38–7.29 (m, 6H),
product through time-of-flight matrix assisted laser desorption 7.29–7.26 (m, 1H), 7.26–7.22 (m, 3H), 7.21–7.15 (m, 3H).
ionization (MALDI–TOF–MS) mass spectrometry, using sulfur
as a matrix.
Synthesis of o,o’-quaterphenyl (17) [36]: A solution of
2-bromobiphenyl (0.15 g, 0.65 mmol), magnesium turnings
Suzuki–Miyaura coupling to form m,p’-quaterphenyl (13) (0.02 g, 0.69 mmol), and THF (2 mL) was stirred at ambient
[35]: 4-Biphenylboronic acid (0.18 g, 0.91 mmol) and 1 M temperature overnight under a nitrogen atmosphere. Additional
K2CO3 (1.5 mL) were added to a 10 mL Pyrex microwave tube. THF (4 mL) was added and the reaction mixture was cooled to
3-Bromobiphenyl (0.07 mL, 0.42 mmol), ethanol (2.3 mL), and −78 °C via a dry ice/acetone bath. TiCl4 (0.14 g, 0.65 mmol)
Pd(PPh3)4 (28 mg, 0.24 mmol) were then added. The head- was added dropwise via syringe and the reaction mixture was
space was purged with nitrogen and the tube was capped before warmed to 0 °C via an ice/water bath and stirred for 1 h. The
the reaction mixture was placed in a microwave reactor product was extracted with ethyl acetate (3x) and dried with
(150 °C, 30 min hold time). After the reaction, the mixture was Na2SO4. The crude product was purified by chromatography
quenched with 1 M NaOH, extracted with dichloromethane, with hexanes to yield o,o’-quaterphenyl as a white solid
washed with water and brine, and dried with Na2SO4. The (0.040 g, mp 110–113 °C, lit 116–118 °C, 20% yield). 1H NMR
organic layer was then concentrated to a brown solid and puri- (400 MHz, CDCl3) δ 7.4–7.40 (m, 2H), 7.38–7.29 (m, 4H),
fied by chromatography with hexanes to yield m,p’-quater- 7.18–7.14 (m, 2H), 7.10–7.05 (m, 2H), 7.03–6.96 (m, 4H),
phenyl (13) as a white solid (0.058 g, mp 163–166 °C, lit 6.62–6.59 (m, 4H).
167–168 °C, 42% yield); 1H NMR (400 MHz, CDCl3) δ
Rearrangement of quaterphenyl isomers in a
7.87–7.85 (m, 1H), 7.76–7.69 (m, 4H), 7.69–7.64 (m, 4H),
7.64–7.58 (m, 2H), 7.57–7.52 (m, 1H), 7.50–7.45 (m, 4H), microwave reactor
7.41–7.35 (m, 2H). p,p’-Quaterphenyl (12): 150 °C, 30 min. Compound 12
(17 mg, 0.06 mmol) was heated in a MW reactor (150 °C,
Suzuki–Miyaura coupling to form o,p’-quaterphenyl (15) 30 min) according to the general rearrangement procedure. The
[35]: K2CO3 (0.91 g, 6.6 mmol) and water (7.7 mL) were crude product was purified via CombiFlash with hexanes to
combined in a 25 mL round bottom flask which was purged yield m,p’-quaterphenyl (13) as an off-white solid (12 mg, 70%
with nitrogen, and cooled to 0 °C. Biphenyl-4-boronic acid yield).
(0.89 g, 4.5 mmol) was added, followed by 2-bromobiphenyl
(0.50 g, 2.2 mmol), DMF (11.5 mL) and Pd(PPh3)4 (0.13 g, m,p’-Quaterphenyl (13): 150 °C, 30 min. Compound 13
0.1 mmol). The reaction mixture was stirred at 0 °C for 5 h. (17 mg, 0.05 mmol) was heated in a MW reactor (150 °C,
NaOH (1 M, 10 mL) was added and the product was extracted 30 minutes) according to the general rearrangement procedure.
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The crude product was filtered over a silica plug with hexanes
to yield 13 as an off-white solid (15 mg, 88% yield).
Supporting Information
Supporting Information File 1
150 °C, 1 h. 13 (16 mg, 0.05 mmol) was heated in a MW
reactor (150 °C, 1 h) according to the general rearrangement
procedure. The crude product was filtered over a silica plug
with hexanes to yield 13 as an off-white solid (12 mg, 75%
yield).
Selected NMR spectra, MALDI spectrum of the product
mixture, Cartesian coordinates, and summary energetics for
all stationary points.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-258-S1.pdf]
m,m’-Quaterphenyl (14): 150 °C, 30 min. Compound 14
(16 mg, 0.05 mmol) was heated in a MW reactor (150 °C,
30 minutes) according to the general rearrangement procedure.
The crude product was purified via CombiFlash with hexanes to
yield an off-white solid (13 mg, 81% yield) consisting of 13
(50%) and 14 (50%).
Acknowledgements
We are grateful for support from National Science Foundation
award CHE-1362519. This research used the Extreme Science
and Engineering Discovery Environment (XSEDE), supported
by NSF Grant No. OCI-1053575.
ORCID® iDs
150 °C, 1 h. Compound 12 (10 mg, 0.03 mmol) was heated in a
MW reactor (150 °C, 1 h) according to the general rearrange-
ment procedure. The crude product was filtered over a silica
plug with hexanes to yield an off-white solid (9 mg, 90% yield)
consisting of 13 (67%) and 14 (33%).
Sarah L. Skraba-Joiner - https://orcid.org/0000-0002-1545-0608
Carter J. Holt - https://orcid.org/0000-0001-7459-353X
Richard P. Johnson - https://orcid.org/0000-0002-1100-6235
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o,m’-Quaterphenyl: Compound 16 (28 mg, 0.09 mmol) was
heated in a MW reactor (150 °C, 30 minutes) according to the
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