Scholl Cyclizations of Aryl Naphthalenes: Rearrangement Precedes Cyclization

Article abstract of DOI:10.1021/acs.joc.5b01559

In 1910, Scholl, Seer, and Weitzenbock reported the AlCl₃-catalyzed cyclization of 1,1′-binaphthyl to perylene. We provide evidence that this classic organic name reaction proceeds through sequential and reversible formation of 1,2′- and 2,2′-binaphthyl isomers. Acid-catalyzed isomerization of 1,1′-binaphthyl to 2,2′-binaphthyl has been noted previously. The superacid trifluoromethanesulfonic acid (TfOH), 1 M in dichloroethane, catalyzes these rearrangements, with slower cyclization to perylene. Minor cyclization products are benzo[k]fluoranthene and benzo[j]fluoranthene. At ambient temperature, the observed equilibrium ratio of 1,1′-binaphthyl, 1,2′-binaphthyl, and 2,2′-binaphthyl is <1:3:97. DFT calculations with the inclusion of solvation support a mechanistic scheme in which ipso-arenium ions are responsible for rearrangements; however, we cannot distinguish between arenium ion and radical cation mechanisms for the cyclization steps. Under similar reaction conditions, 1-phenylnaphthalene interconverts with 2-phenylnaphthalene, with the latter favored at equilibrium (5:95 ratio), and also converts slowly to fluoranthene. Computations again support an arenium ion mechanism for rearrangements.

Full text of DOI:10.1021/acs.joc.5b01559

Article
pubs.acs.org/joc
Scholl Cyclizations of Aryl Naphthalenes: Rearrangement Precedes
Cyclization
Sarah L. Skraba-Joiner, Erin C. McLaughlin, Aida Ajaz, Rajesh Thamatam, and Richard P. Johnson*
Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, United
States
S
* Supporting Information
ABSTRACT: In 1910, Scholl, Seer, and Weitzenbock reported
the AlCl3-catalyzed cyclization of 1,1′-binaphthyl to perylene.
We provide evidence that this classic organic name reaction
proceeds through sequential and reversible formation of 1,2′-
and 2,2′-binaphthyl isomers. Acid-catalyzed isomerization of
1,1′-binaphthyl to 2,2′-binaphthyl has been noted previously.
The superacid trifluoromethanesulfonic acid (TfOH), 1 M in
dichloroethane, catalyzes these rearrangements, with slower
cyclization to perylene. Minor cyclization products are benzo-
[k]fluoranthene and benzo[j]fluoranthene. At ambient temper-
ature, the observed equilibrium ratio of 1,1′-binaphthyl, 1,2′-
binaphthyl, and 2,2′-binaphthyl is <1:3:97. DFT calculations
with the inclusion of solvation support a mechanistic scheme in
which ipso-arenium ions are responsible for rearrangements; however, we cannot distinguish between arenium ion and radical
cation mechanisms for the cyclization steps. Under similar reaction conditions, 1-phenylnaphthalene interconverts with 2-
phenylnaphthalene, with the latter favored at equilibrium (5:95 ratio), and also converts slowly to fluoranthene. Computations
again support an arenium ion mechanism for rearrangements.
rearrangements are believed to proceed by the formation of an
ipso-arenium ion,6 followed by a 1,2-shift of the substituent.
With disubstituted benzenes, equilibrium favors meta sub-
stitution (2), a consequence of greater cationic product
stability.
INTRODUCTION
■
It has long been known that alkyl-substituted1 and aryl-
substituted2 benzenes (1−3) can interconvert (Scheme 1) in
Scheme 1. Acid-Catalyzed Rearrangements
Much less studied have been similar rearrangements of
substituted naphthalenes, which interconvert isomers 4 (alpha)
and 5 (beta), presumably through ipso cations 6 and 7. On this
potential energy surface, the lowest-energy cation 8 is accessible
by an additional 1,2-H shift in cation 7; this is consistent with
the preference for beta product described in the modest
number of known examples. In 1976, Olah reported that alkyl
naphthalenes isomerize during Friedel−Crafts alkylation,
showing that substitution in the beta position was favored at
equilibrium.7 1,2-Aryl migration catalyzed by BCl3 has been
reported in a congested 5,6-diaryl-substituted acenaphthene.8
Kovacic and Koch first described the rearrangement (Scheme
2) of 1,1′-binaphthyl (9) to 2,2′-binaphthyl (10) upon
exposure to AlCl3 or MoCl5.9 Kuivila and co-workers later
reported the same rearrangement when 9 was reacted at
ambient temperature with TfOH in dichloromethane.10
Binaphthyl derivatives have also been observed upon heating
naphthalene with pure TfOH.11 The high-temperature isomer-
ization of binaphthyl isomers has also been reported.12 The
the presence of acid catalysts or reagents, such as aluminum
chloride (AlCl3), which generate a protic acid through reaction
with adventitious water.3 We recently studied the rearrange-
ments of phenyl-substituted benzene derivatives through
catalysis by 1 M trifluoromethanesulfonic acid (TfOH) in
1,2-dichloroethane (DCE).4 This superacid5 medium provides
a more reliable and predictable alternative to AlCl3. These
Received: July 7, 2015
© XXXX American Chemical Society
A
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rapidly converts 9 to 10, followed by slower formation of 11
and other minor products. As in our recent study with phenyl-
substituted benzenes, 1 M TfOH in DCE proved to be optimal
for observing these rearrangements.4 Reactions were monitored
by capillary GC and NMR with similar observed results. Under
these conditions, 1,1′-binaphthyl (9) rearranged rapidly at
ambient temperature to an equilibrium mixture (Scheme 3)
Scheme 2. Reported Reactions of Binaphthyl
Scheme 3. Binaphthyl Rearrangement and Cyclization
solution-phase acid-catalyzed isomerization of phenylnaphtha-
lene has not been reported; however, heating 1-phenyl-
naphthalene to 450 °C with acidic zeolites in the gas phase
resulted in formation of 2-phenylnaphthalene as the major
product, accompanied by small amounts of fluoranthene and
naphthalene.13 Structurally related cyclizations to fluoranthene
are also known to occur upon heating with a H atom donor12c
or with a chromia-alumina catalyst.14
In 1910, Scholl reported the formation of perylene (11)
when either 9 or naphthalene was heated to 140 °C with AlCl3
(Scheme 2).15 This classic and often cited oxidative
cyclization16 involves the formation of an aryl−aryl bond and
the elimination of two aryl-bound hydrogen atoms.17 Scholl
reaction conditions originally used Friedel−Crafts catalysts,
with atmospheric oxygen likely to act as an oxidant.15 Other
conditions were later adopted to include an acid with an added
oxidant; those include AlCl3/CuCl2,18 FeCl3,19 MoCl5,20
SbCl5,21 Tl(O2CCF3)3 in CF3CO2H or BF3−OEt2,22 and
CH3SO3H/DDQ.23 As noted most clearly by King, acid-
catalyzed rearrangements are well-known to occur under Scholl
reaction conditions and can be problematic in designing the
synthesis of polyarenes.24 Arenium ion25 and radical cation23,26
mechanisms have been proposed for oxidative cyclizations
similar to the Scholl reaction; it is likely that both mechanisms
exist.16,27
containing 9, 1,2′-binaphthyl (12), and 2,2′-binaphthyl (10) in
a ratio of <1:3:97. Essentially, the same equilibrium mixture
resulted when this reaction was carried out beginning with any
of the three isomers. Reduced acidity led to slower isomer-
ization. These results support a sequential 9 to 12 to 10
transformation. At ambient temperature under nitrogen, no 11
was detected. When the reaction was run under 1 atm of
oxygen, we observed 3% of 11 after 72 h at ambient
temperature.
At reflux under nitrogen (84 °C), isomerization of 9 to 10
was too rapid to measure. Longer reaction times at reflux as
well as heating in a microwave reactor to higher temperatures
(100−160 °C) yielded primarily perylene (11) as product,
accompanied by smaller amounts (9%) of benzo[k]-
fluoranthene (14), the apparent product of cyclization of 12.
Benzo[j]fluoranthene (13) might also be formed by cyclization
of 9 or 12; only trace amounts (<1%) of 13 were observed by
NMR analysis of product mixtures.
The independent observations (Scheme 2) that 1,1′-
binaphthyl isomerizes to 109,10 but also cyclizes to perylene15
would appear to be in conflict, implying a dynamic process. Our
goal in the present study was to resolve this conflict and
elucidate reaction mechanisms for both processes.
RESULTS AND DISCUSSION
■
We recently described the technique of microwave flash
pyrolysis (MFP), in which high-temperature pyrolytic con-
ditions are approached by heating mixtures of organic
substances with graphite or carbon nanotubes in a microwave
reactor.28 MFP reaction of 1,1′-binaphthyl gave very little
perylene, with naphthalene the major product. When we
investigated either the solution- or solid-phase microwave
reaction of 1,1′-binaphthyl with AlCl3, 2,2′-binaphthyl proved
to be the major product instead of perylene. This unexpected
result prompted a more thorough investigation of rearrange-
ment by acid catalysis.
Along with isomerization and cyclization, minor dihydroar-
omatic products were formed at elevated temperature. 3,4-
Dihydro-2,2′-binaphthyl was observed in small amounts from
reaction at 130 °C in a microwave reactor, as identified by
11
1
comparison to previously reported H NMR data. A more
significant secondary product under MW or prolonged reflux
conditions was assigned as 2,3-dihydroperylene (15). This has
been reported previously as a product of electrochemical
reduction of 11.29 The same product could be produced in ca.
50% yield upon heating pure 11 in 1 M TfOH/DCE at 160 °C
(Scheme 4). Attempts at separating 15 from perylene were
unsuccessful, and this substance easily aromatized. DDQ
Acid-Catalyzed Rearrangement and Cyclization of
Binaphthyl Isomers. The original 1910 procedure described
by Scholl involved heating 9 for 1 h at 140 °C with AlCl3 under
an atmosphere of dry air followed by cautious addition of water,
boiling the crude product with concentrated aqueous HCl to
complete hydrolysis, and then atmospheric pressure sublima-
tion of the product at 350 °C! This procedure reportedly gave a
15% yield of yellow crystals which were assigned as perylene
and represented its first synthesis.15
Scheme 4. Formation of 2,3-Dihydroperylene
In repeating the initial steps in the Scholl reaction
conditions,15 we find that the heating stage of this reaction
B
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Figure 1. Energetics of protonation with TfOH and rearrangements of binaphthyl isomers in DCE.
Scheme 5. Energetics of Arenium Ion and Radical Cation Mechanisms for Cyclization to Perylene
oxidation of the 11 + 15 mixture yielded only 11. We have
recently shown that heating 11 with TfOH and DDQ in
dichloromethane affords a high yield of quaterrylene, a product
of further Scholl coupling.30
Computational Models for Binaphthyl Rearrange-
ments. The energetics of protonation and the potential
surface for rearrangements were investigated with density
functional theory using Spartan 1031 and Gaussian 09.32 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.33 The polarizable continuum model
(PCM) was employed to assess solvation in dichloroethane.34
The relative free energies of stationary points on this energy
surface, with DCE solvation, are summarized in Figure 1.
Energetics for the rearrangement of binaphthyl isomers follow a
trend similar to those we have previously reported.4 Solvation
in DCE using PCM greatly stabilizes the carbocations for
binaphthyl isomers compared to the energetics in vacuo,
supporting a cationic mechanism in this medium. Because we
are focusing on rearrangements, the combined solvated
energies of cation 9b + triflate anion have been chosen as an
energy reference point, thus both the protonation steps and
rearrangements share an energy scale. Predicted energetics for
rearrangements are probably more reliable than absolute
energies for protonation. The top portion of this figure
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represents interconversions among the ipso cations 9b, 12c,
12d, and 10b, which have modest barriers for naphthyl
migration. 1,2-H shifts in arenium ions have low barriers4 and
should provide facile pathways to the lower-energy non-ipso
cations 9a, 12a, 12b, and 10a.
14 are predicted (Schemes S1−S3) to have barriers that are 7−
9 kcal/mol higher than those for arenium ions.
Rearrangement and Cyclization of Phenylnaphtha-
lene. Experimental results for the rearrangement and
cyclization of phenylnaphthalene are outlined in Scheme 6.
In these reactions, all of the rearrangement steps must occur
through higher-energy ipso-arenium ions.4 A stepped sequence
of naphthyl migrations with decreasing barriers follows initial
formation of the ipso-arenium ion 9b. Although cations 12c and
12d might interconvert by an inter-ring 1,2-shift, computations
show a high barrier to this process, and thus neutral binaphthyl
12 must be an intermediate. The energetics in Figure 1 are
consistent with the observed rapid isomerization of 9 and even
faster isomerization of isomer 12 to 10, with the latter structure
favored at equilibrium. The lowest-energy carbocation 10a is
ultimately formed through an exothermic 1,2-H shift from 10b.
Energetics of the neutral isomers would favor 10 at equilibrium,
as observed, but we believe it is more likely that relative cation
stability is product-determining. Overall, Figure 1 must be
considered to be a simple model for a very complex dynamic
process; nevertheless, this seems to capture the essential
features of our observed chemistry.
Scheme 6. Rearrangement and Cyclization of
Phenylnaphthalene
As expected, triflic acid catalysis at ambient temperature under
nitrogen results in rapid phenyl migration, yielding a 5:95
mixture of 20 and 21. At reflux, this mixture slowly transforms
into fluoranthene (19).13
The energetics for initial rearrangement of phenylnaphtha-
lene are summarized in Figure 2. The chosen reference point is
Although rearrangements almost certainly pass through ipso-
arenium ions, the final cyclization to perylene can, in principle,
occur through either intramolecular cyclization of an arenium
ion or a radical cation. This is an ongoing debate in the
mechanism for Scholl reactions.16 Scheme 5 shows a direct
comparison of the cyclization steps to intermediates of trans
stereochemistry. This step is likely to be rate-determining in
either mechanism. Computations for both pathways were
carried out at the UB3LYP/6-31+G(d,p) level of theory with
DCE solvation.
On the arenium ion pathway, protonation of 9c followed by
cyclization to 16 and deprotonation would proceed to a
dihydroperylene. Aromatization can then occur through loss of
hydride and subsequent deprotonation to yield 11. There is
ample precedent for proton and hydride transfers under
strongly acidic conditions.35 The radical ion mechanism is
expected to proceed by cyclization of 17 to 18, followed by
deprotonation, a second one-electron oxidation of the radical,
and then deprotonation. The barriers to these two cyclizations
are very similar, but the cyclized radical cation 18 lies in a very
shallow energy minimum. Electron transfer might also be
coupled with deprotonation, and this could diminish the
effective barrier for the radical ion mechanism. Given the many
unknowns about other reaction steps, our calculations cannot
distinguish between arenium ion and radical cation mechanisms
for the final cyclization to perylene.
The same argument can be made about minor cyclization
products 13 and 14. Scholl products containing five-membered
rings can be formed through cyclization of 1,1′-binaphthyl or
1,2′-binaphthyl to yield benzo[j]fluoranthene (13) or benzo-
[k]fluoranthene (14). The barriers for these cyclizations were
calculated to be slightly higher than those for the cyclization to
11 (21−26 kcal/mol, Supporting Information Schemes S1−
S3); however, the relative activation energies did not correlate
with the minor amount of 14 formed and the trace of 13.
Overall product stability of the six-membered ring product
versus that of the five-membered ring product is likely dictating
the observed product formation. Starting with 10 or 12 did not
change the ratio of cyclized products formed under microwave
reaction conditions. Radical cation cyclizations to give 13 and
Figure 2. Energetics of protonation with TfOH and rearrangement of
phenylnaphthalenes in DCE.
the ipso cation of 1-phenylnaphthalene (20b) + triflate anion in
order to directly compare both rearrangement and protonation
steps. The net barrier for this rearrangement from 20 to 21 is
20.6 kcal/mol (8.0 + 12.6), while the major product is formed
through an exothermic 1,2-H shift from 21b to 21a.
Energetics for the cyclization to 19 have not been calculated;
however, a mechanism similar to that of the cyclization of
binaphthyl seems likely. Protonation of 1-phenylnaphthalene,
followed by cyclization, would afford a protonated, dihydro
derivative of fluoranthene. Deprotonation and loss of hydrogen
would yield fluoranthene (19).
CONCLUSIONS
■
The cyclization of 1,1′-binaphthyl (9) to perylene (11)15 is
often cited as a classic Scholl reaction. As might have been
deduced from earlier reports on the chemistry of 9,9,10 this
D
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and <1% 1,1′-binaphthyl, 1,2′-binaphthyl, benzo[j]fluoranthene, and
3,4-dihydro-2,2′-binaphthyl by capillary GC and 1H NMR analysis
(89% mass recovery).
reaction proceeds (Scheme 3) through interconversion with
1,2′- and 2,2′-binaphthyl isomers (12 and 10), with cyclization
necessarily occurring from the low equilibrium concentration of
9. Our experiments and DFT computational models support
arenium ion mechanisms for rearrangements and are consistent
with the rapid isomerization of 9 to 10, with the intermediacy
of 12. We cannot distinguish between arenium ion and radical
cation mechanisms for the final cyclization step to perylene.
The presence of molecular oxygen effects a modest rate
acceleration for cyclization, presumably by assisting the
oxidation of a dihydroaromatic intermediate. Phenylnaphtha-
lene also rearranges easily, with slower cyclization to
fluoranthene. This has previously been observed over zeolites
in the gas phase.13 It is increasingly clear that arene
rearrangements and oxidative cyclizations are inextricably
linked.16,24 In the present case, rearrangement provides an
efficient synthesis of 2,2′-binaphthyl (10).
Rearrangement and Cyclization of 1,1′-Binaphthyl under O2
Atmosphere. When the rearrangement of 1,1′-binaphthyl at ambient
temperature was repeated under an O2 atmosphere (1 atm), the
formation of perylene (11) was observed after 24 h. After 4 days, 11
(3%), 10 (89%), 12 (7%), and 9 (1%) were observed by capillary GC
1
and H NMR analysis.
Observation of 1,2′-Binaphthyl (12) Intermediate with
Reduced Acidity. 9 (0.02 g, 0.1 mmol), DCE (2 mL), and TfOH
(18 μL, 2.0 mmol) were purged with N2 and stirred at ambient
temperature. After 1 h, the solution was neutralized with saturated
NaHCO3 and extracted using dichloromethane. Capillary GC analysis
showed 9 (87%), 12 (2%), and 10 (11%).
Rearrangement and Cyclization of 2,2′-Binaphthyl (10). A
mixture of 10 (0.05 g, 0.2 mmol), TfOH (0.40 mL, 4.5 mmol), and
DCE (4 mL) was heated in a microwave reactor under N2 (100 °C, 15
min). The resulting product distribution (90% mass recovery) was
1
obtained by capillary GC and H NMR analysis: 10 (83%), 11 (9%),
12 (8%), and 9 (<1%).
EXPERIMENTAL SECTION
■
Formation of 2,3-Dihydroperylene (15). Perylene (11) (0.10 g,
0.40 mmol), DCE (4 mL), and TfOH (0.4 mL, 4.5 mmol) were mixed
in a 10 mL reaction tube and purged with N2. The reaction vessel was
capped and placed in a microwave reactor (160 °C, 90 min). After the
reaction time, the green/blue solution was neutralized with saturated
NaHCO3 and extracted using dichloromethane. Product was isolated
as an orange solid (69% mass recovery) and was found to contain 11
(23%) and 15 (77%) by H NMR analysis. 2,2-Dihydroperylene: H
NMR (500 MHz, CDCl3) δ 8.06 (d, 1H), 7.98 (d, 1H), 7.91 (d, 1H),
7.71 (dd, 2H), 7.48 (t, 2H), 7.30 (t, 1H), 7.16 (d, 1H), 6.73 (t, 1H),
General Methods. Trifluoromethanesulfonic acid (99% purity)
and dichloroethane (99+%) were used as received from commercial
sources. Glassware was oven-dried, and all reactions were run under a
1
nitrogen atmosphere. H NMR spectra were measured in CDCl3 at
400 MHz and reported relative to TMS unless otherwise noted.
Capillary analytical gas chromatography was performed using a DB-3
column (30 m × 0.320 mm), with temperature programming from 75
to 250 °C. Retention times were as follows: 9, 18.4 min; 10, 27.9 min;
11, 43.0 min; 12, 22.1 min; 14, 35.6 min; 15, 38.5 min; 19, 17.2 min;
20, 14.7 min; 21, 15.8 min. Percentages are uncorrected for response
factors. Microwave reactions were conducted in 10 mL vessels using a
CEM Discover single-mode microwave reactor. 1,1′-Binaphthyl,
perylene, and 1-phenylnaphthalene were commercial samples. The
syntheses and NMR spectral data for 1,2′-binaphthyl,36 2,2′-
binaphthyl,37 and 2-phenylnaphthalene38 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. 0.05 g) and DCE (4 mL) were charged to the flask.
Trifluoromethanesulfonic acid (0.40 mL, 4.5 mmol) was added
dropwise by syringe; this typically caused formation of a bright color
due to formation of arenium ions.4 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 saturated aqueous NaHCO3 and extraction with
dichloromethane unless otherwise noted.
General Procedure for Rearrangement in a Microwave
Reactor. Under a nitrogen atmosphere, the substrate (ca. 0.05 g) and
DCE (4 mL) were added to a 10 mL tube. Trifluoromethanesulfonic
acid (0.40 mL, 4.5 mmol) was added dropwise by syringe; this
typically caused formation of a bright color. The reaction mixture was
capped and heated by microwave. Products were isolated by careful
neutralization with saturated aqueous NaHCO3 and extraction with
dichloromethane, followed by analysis using 1H NMR or capillary GC,
unless otherwise noted.
Rearrangement and Cyclization of 1,1′-Binaphthyl (9).
Ambient Temperature. 1,1′-Binaphthyl (9) rearranged rapidly (10
min) to 1,2′-binaphthyl (12) (3%) and 2,2′-binaphthyl (10) (97%) by
capillary GC and 1H NMR analysis. This equilibrium remained
unchanged after 5 days at room temperature.
1
1
1
2.90 (t, 2H), 2.56 (td, 2H). Calculated versus experimental H NMR
shifts can be seen in the Supporting Information (Figure S1).
Rearrangement and Cyclization of 1-Phenylnaphthalene
(20). Microwave. 20 (0.10 g, 0.49 mmol), DCE (4 mL), and TfOH (9
μL, 1.0 mmol) were mixed in a 10 mL reaction tube and purged with
N2. The reaction vessel was capped and placed in a microwave reactor
(115 °C, 30 min). After the reaction time, the solution was neutralized
with saturated NaHCO3 and extracted using dichloromethane.
Capillary GC analysis showed 20 (5%) and 2-phenylnaphthalene
(21) (95%).
Reflux. 20 rapidly rearranged to 21 via general refluxing conditions.
After 4 h, the reaction mixture consisted of fluoranthene (19) (41%),
21 (57%), and 20 (2%) by capillary GC analysis.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b01559.
Summary table of total energies and Cartesian
coordinates for stationary points and selected spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: richard.johnson@unh.edu.
Notes
The authors declare no competing financial interest.
Reflux in DCE (84 °C). 9 rapidly reached initial equilibrium of
binaphthyl isomers, followed by cyclization. After 24 h, the reaction
mixture consisted of perylene (11) (73%), 10 (27%), and <1% 9, 1,2′-
binaphthyl (12), 3,4-dihydro-2,2′-binaphthyl, benzo[k]fluoranthene
ACKNOWLEDGMENTS
■
We are grateful for generous support from the National Science
Foundation (CHE-9616388 and CHE-1362519). This work
used the Extreme Science and Engineering Discovery Environ-
ment (XSEDE), which is supported by National Science
Foundation Grant Number OCI-1053575. Thanks to Professor
1
(14), and 2,3-dihydroperylene (15) by capillary GC and H NMR
analysis (82% mass recovery).
Microwave. Heating the reaction mixture in a microwave reactor
(130 °C, 30 min) resulted in 11 (72%), 10 (11%), 14 (9%), 15 (8%),
E
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DOI: 10.1021/acs.joc.5b01559
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