Charge delocalization and enhanced acidity in tricationic superelectrophiles

Article abstract of DOI:10.1021/ja2046364

This Article presents the results from studies related to the chemistry of tricationic superelectrophiles. A series of triaryl methanols were ionized in Bronsted superacids, and the corresponding tricationic intermediates were formed. The trications are found to participate in two types of reactions; both are characteristic of highly charged organic cations. One set of reactions occurs through charge migration. A second set of reactions occurs through deprotonation of an unusually acidic site on the tricationic species. One of the tricationic intermediates has been directly observed by low temperature NMR spectroscopy. These highly charged ions and their reactions have also been studied using density functional theory calculations. As a result of charge migration, electron density at a carbocation site is found to increase with progression from monocationic to pentacationic structures.

Full text of DOI:10.1021/ja2046364

ARTICLE
pubs.acs.org/JACS
Charge Delocalization and Enhanced Acidity in Tricationic
Superelectrophiles
Rajasekhar Reddy Naredla,† Chong Zheng,† Sten O. Nilsson Lill,‡ and Douglas A. Klumpp*,†
†Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States
‡Department of Chemistry, University of Gothenburg, SE-412 96 Gothenburg, Sweden
S Supporting Information
b
ABSTRACT: This Article presents the results from studies
related to the chemistry of tricationic superelectrophiles. A
series of triaryl methanols were ionized in Brønsted superacids,
and the corresponding tricationic intermediates were formed.
The trications are found to participate in two types of reactions;
both are characteristic of highly charged organic cations. One
set of reactions occurs through charge migration. A second set of reactions occurs through deprotonation of an unusually acidic site
on the tricationic species. One of the tricationic intermediates has been directly observed by low temperature NMR spectroscopy.
These highly charged ions and their reactions have also been studied using density functional theory calculations. As a result of
charge migration, electron density at a carbocation site is found to increase with progression from monocationic to pentacationic
structures.
’ INTRODUCTION
prepared readily from reactions of the appropriate ketones or
esters with organolithium reagents. Two types of conversions
were observed, depending on whether benzene was present in
the mixture. In the presence of C6H6, alcohol 4 gives product 9 in
89% yield at 60 °C (entry 1). If the reaction was done at lower
temperatures, then the starting material was recovered. Similar
results were obtained with imidazole and benzimidazole-based
substrates (5, 7, 8; entries 2, 4, 5). When the reaction was done
with a substrate (6) bearing the 2-naphthyl substituent, nucleo-
philic attack occurred at the C-1 position of the naphthyl ring to
give product 11 in good yield. The structure of compound 11 was
established by 1H and 13C NMR spectra, mass spectrum analysis,
and X-ray crystallography.5
In the absence of benzene, reactions of the triarylmethanols
lead to cyclization products (entries 6À10). For example, com-
pound 4 reacts in CF3SO3H at 60 °C to provide the pyrido[1,2-
a]indole derivative (16) in 81% yield. Benzimidazole derivatives
5, 6 also react to give cyclization products (17 and 18). In the
reaction of 6, cyclization occurs regioselectively at the 1-position
of the naphthyl ring to give product 18 (confirmed by X-ray
crystal structure).5 The pyrido[1,2-a]indole derivatives 19 and
20 were formed by reactions of compounds 14 and 15, respec-
tively. Product 20 is the only major product from 15, indicating a
preference for cyclization at the pyridine ring. The vinyl-substituted
derivative (21) does not undergo cyclization in CF3SO3H; how-
ever, a low yield of cyclization product (22) may be obtained
from reaction in the stronger acid system CF3SO3HÀSbF5 (ca.
10:1; eq 2). These types of cyclizations involving N-heterocycles
are not well-known, as the only other example was described by
During the mid-1970s, Olah proposed the concept of super-
electrophilic activation.1 Several published studies had previously
described the high reactivities of monocationic electrophiles in
superacidic media. This suggested that the monocationic elec-
trophiles were interacting with the acidic media to enhance their
reactivities. Thus, protonation of a monocationic species such as
the nitronium ion (1) could produce an ion with an increasing
positive charge (2, eq 1). In the limiting cases, even dicationic
species (3) are considered to be involved in the chemistry. While
many examples of dicationic superelectrophiles have been de-
scribed,2 few examples of tricationic species have been studied.
Most tricationic and tetracation species are formed as resonance
stabilized ions.3 Little is known about the reactivities of small
organic ions bearing such a large amount of positive charge.4 In
this Article, we describe studies in which a reactive carbocation
site interacts with two adjacent cationic charges. Reactions of the
tricationic species with arenes lead to products consistent with
charge migration. Under some conditions, these reactive inter-
mediates undergo cyclizations to produce novel heterocyclic ring
systems. The tricationic intermediates are further studied by
spectroscopic and theoretical methods.
’ RESULTS AND DISCUSSION
Our studies began with an examination of the chemistry of
triarylmethanols (4À8, 14, 15) by reaction in superacidic
CF3SO3H (Table 1). The triarylmethanol substrates were
Received: May 26, 2011
Published: July 11, 2011
r
2011 American Chemical Society
13169
dx.doi.org/10.1021/ja2046364 J. Am. Chem. Soc. 2011, 133, 13169–13175
|

Journal of the American Chemical Society
ARTICLE
Table 1. Reactions of Heterocyclic Alcohols in CF3SO3H at 60 °C (with and without C6H6)
Posselt (involving cyclization between a 4-methoxyphenyl group
and a pyridine or pyridinium ring).6 Nevertheless, our results
indicate that novel heterocyclic ring systems are prepared by the
superacid promoted cyclizations of the triarylmethanols.
showed high selectivity for the toluene with kt/kb of about 30:1.
Similar results are obtained from reactions of 4 with bromo-
benzene and 4-(3-phenylpropyl)pyridine. Products 29 and 30
are formed in good yields with high regioselectivities.
In addition to arene nucleophiles, we also attempted to capture
the electrophilic intermediate(s) with n-type nucleophiles, in-
cluding carbon monoxide, fluoride, and water. Although carbon
monoxide often reacts efficiently with carbocationic intermediates,8
no adducts of this nucleophile could be isolated in reactions with
4 in superacid. When compound 4 is reacted in CF3SO3H and
KHF2 is added, then fluoride adduct 31 may be isolated in 51%
yield (eq 3; compound 4 is also recovered). Interestingly,
nucleophilic attack occurs at the methine carbon rather than at
the ring carbon. A similar reaction of 4 with CF3SO3H, followed
by water, leads to formation of the starting material 4, rather than
functionalization at the remote position.
When triarylmethanols 23 or 24 are reacted under similar
conditions, the carbocations 25 and 26 are produced (Figure 1).
However, with benzene, no FriedelÀCrafts products are formed
from the monocation 25 or dication 26. With simple heating to
60 °C (no benzene), cyclization products are also not obtained.7
Compound 4 generates the tricationic intermediate 27 in heated
superacid. Thus, trication 27 exhibits enhanced reactivity as
compared to 25 and 26. When substrate 4 is reacted with sub-
stituted arenes, product formation (28À30) occurs with good
regioselectivity. Reaction of compound 4 with toluene in CF3-
SO3H gives 28 as the major product, isolated in 72% yield. Gas
chromatography analysis of the crude product mixture indicated
a para,ortho/meta ratio of about 10:1. A competition experiment
was also done with substrate 4 in a CF3SO3H-promoted reaction
with toluene/benzene (1/1 ratio). Analysis of the product mixture
In our mechanistic studies, the intermediates and transition
states were studied using density functional theory (DFT).
13170
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175

Journal of the American Chemical Society
ARTICLE
Geometries for these structures were initially optimized in the
gas phase at the M06/6-31G(d) level of theory9 using Jaguar.10
Gibbs’ free energy corrections (ΔG) were calculated, and struc-
tures were verified to be minima or transition states by inspection
of the number of imaginary frequencies. In a second step, single-
point energies of the optimized structures were calculated using
M06/cc-pVTZ(-f),11,12 giving a basis set correction term (ΔBS).
In a final step, solvent phase geometry optimizations were done.
Using a PoissonÀBoltzmann solver as implemented in Jaguar,13
electrostatic solvation effects from the surroundings were calcu-
lated using the SCRF method with CF3SO3H as a solvent (di-
electric constant = 77.4, probe radius = 2.5985274).14 The free
energy (ΔG) and basis set correction terms (ΔBS) were added to
the solution phase energy. NMR chemical shifts were calculated
using B3LYP/IGLO-II as implemented in Gaussian 03,15À17 on
M06/6-31G(d) gas-phase optimized structures with benzene set
to the experimental value of 127.7 ppm as the NMR reference.
NBO charges were calculated using NBO 5.0 as implemented in
Jaguar.18 To verify that the M06/6-31G(d) compounds were
able to properly describe the σ- and π-complexes, tests were also
performed using B3LYP-DCP/6-31+G(d,p), a dispersion-cor-
rected DFT method,19,20 resulting in optimized structures similar
to those with M06/6-31G(d).
In considering the mechanisms of the reactions, we propose a
mechanism involving formation of tricationic intermediates with
highly delocalized carbocation centers and equilibria with a dica-
tionic species from deprotonation. The alcohol substrates (4À8)
react with benzene in the superacid, and nucleophilic attack oc-
curs at the remote position on the aryl substituent group (Figure 2).
Thus, compound 4 ionizes in superacid, and loss of water pro-
duces the tricationic superelectrophile 27. With formation of the
carbocation 27, positive charge is highly delocalized into the
adjacent phenyl ring. Nucleophilic attack occurs at the remote
position on the phenyl group to give intermediate 32, when ben-
zene is present in the reaction mixture. Several examples of charge
migration and remote functionalization have previously been
reported involving dicationic superelectrophiles.21
Product 31 is formed by fluoride attack at the methine carbon,
suggesting that some positive charge remains at the methine center.
This raises an obvious question: why would the n-type nucleo-
philes (fluoride and water) react at the methine carbon while π-
type nucleophiles (benzene and substituted arenes) react at the
remote para-position of the phenyl group? First, there is likely
some degree of steric effects influencing the regiochemistry of
nucleophilic attack. Nucleophilic attack at the methine carbon
(by benzene) would form the highly congested tetraarylmethane.
Second, formation of the π-complex may enhance charge se-
paration (Figure 3). Formation of the toluene π-complex (33a),
and subsequent Wheland intermediate or σ-complex (34a), ef-
fectively moves the positive charge away from the pyridinium
rings. Density functional theory (DFT) calculations were done
(vide infra) and showed the toluene ring bears +0.62 charge in
the π-complex (33a) and +0.97 charge in the σ-complex (34a).
In the case of toluene, steric effects in the π-complex may also be
important in the high para selectivity.22
Although high positional selectivity is often due to a large
energy barrier between the π-complex and σ-complex,23 this
does not appear to be the case with 33a and 34a. DFT calculations
have indicated that the π-complex resides in a very shallow
potential energy well, as small perturbations in structure 33a
easily lead to formation of the σ-complex 34a as found by DFT
geometry optimizations. In the competition experiment, high
substrate selectivity was observed (kt/kb ≈ 30:1). This value
likely reflects the relative stabilities of the σ-complexes with ben-
zene and toluene.23 When the π- and σ-complexes are examined,
the σ-complex from toluene (34a) is 7.9 kcal molÀ1 more stable
than the π-complex from toluene (33a), while the σ-complex
from benzene (34b) is only 4.6 kcal molÀ1 more stable than its π-
complex with benzene (34a).
In reactions of compound 6, products 11 and 18 were formed.
While charge migration occurred into the napthyl ring system, it
did not delocalize to the most remote position (across both
rings). Ionization of the benzimidazole rings and the hydroxy
group leads to trication 35a (eq 4). Formation of product 11
Figure 1. Ionization products (25À27) and FriedelÀCraft reaction
products 28À30.
Figure 2. Proposed mechanism for the reactions of trication 27, leading to product 9.
13171
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175

Journal of the American Chemical Society
ARTICLE
Figure 3. Relative energies of Wheland- and π-complexes with toluene (33a, 34a) and benzene (M06/6-31G*-optimized structures, 33b, 34b).
Table 2. Observed 13C NMR Data from Isoelectronic Carbocations 25À27 and Calculated 13C NMR Data for 25À27 and 36,37a
a (a) Spectrum taken at À60 °C. (b) Spectrum taken at 0 °C.
occurs by nucleophilic attack at the 1-position of the naphthyl
group. This implies a greater contribution from resonance struc-
ture 35b, instead of structure 35c (the structure with greater
charge separation).
Supporting Information). It is notable that the para and ortho
carbons are significantly deshielded. This is consistent with the
charge delocalization driven by chargeÀcharge repulsive effects.
The dicationic ion 26 and the trityl cation (25)24 were also
studied in solution by low temperature 13C NMR, with compar-
ison to calculated spectra. Interestingly, the chemical shift at the
methine carbon moves progressively downfield from the mono-
cationic trityl cation (25), to the dication (26), and to the
trication (27). This is an indication that charge delocalization be-
comes enhanced with increasing charge. The tetra- and penta-
cationic structures (36, 37) were also calculated, and the results
show further deshielding of the para-position and shielding of the
methine carbon. While the trityl cation has a calculated chemical
shift of δ 144 at the para position of the phenyl ring, the pentacation
37 has a calculated chemical shift of δ 231 at the para position of
the phenyl ring.
To gain further insight into these reactions, low temperature
13C NMR experiments were done. Compound 4 was ionized in
FSO3HÀSbF5ÀSO2ClF at À60 °C, and the observed spectrum
is consistent with the formation of the tricationic species 27
(Table 2). For comparison, the DFT calculated spectrum of
trication 27 is in reasonably good agreement with the results
from solution (for complete assignments of 13C signals, see the
As a measure of charge delocalization in these systems, natural
bond order (NBO) charges, bond lengths, and dihedral angles
were analyzed from the calculated structures. In a series of related
13172
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175

Journal of the American Chemical Society
ARTICLE
Table 3. Calculated Properties of Isoelectronic Ions 25À27 and 36,37
NBO charge
structure
charge
C-methine
C-para
dihedral angle, δ
CÀC bond length (methine-ipso), Å
25
26
27
36
37
1+
2+
3+
4+
5+
+0.24
+0.15
+0.12
À0.01
À0.14
À0.15
À0.10
0.00
33.2°
31.1°
24.0°
19.5°
11.2°
1.441
1.429
1.402
1.394
1.391
+0.10
+0.20
Figure 4. Calculated structures related to the cyclization of trication 27.
structures, 13C NMR chemical shifts may be roughly correlated
to charge densities.25 Down field chemical shifts (or deshielding)
often correspond to electron-deficient carbon atoms.25 Within
the isoelectronic series from the trityl cation (25) to the penta-
cation (37), experimental and calculated NMR spectral data
suggest that the carbocationic center moves progressively from
the methine carbon to the para position of the phenyl ring. The
calculated NBO charges also show this trend (Table 3). Re-
markably, there is almost a complete transfer of positive charge
from the methine carbon to the para-carbon. The methine carbon
carries a +0.24 NBO charge in the trityl cation (25) and a À0.14
NBO charge in the pentacation (37), while the para-carbon
carries a À0.15 NBO charge in 25 and a +0.20 NBO charge in 37.
As the positive charge is moved to the para position, there is also
increasing double bond character between the methine carbon
and the phenyl ring. This is apparent from the decreasing bond
dihedral angle (δ) and CÀC bond length as charge is added to
the system. Thus, the trityl cation 25 has a dihedral angle of 33.2°
and CÀC bond length of 1.441 Å, while the pentacation 37 has a
dihedral bond angle of 11.2° and CÀC bond length of 1.391 Å.
13173
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175

Journal of the American Chemical Society
ARTICLE
The dihedral angle and NBO charge are strongly correlated to the
overall charge on the cationic structure. Increasing charge leads to
progressively greater positive charge at the para-carbon, while
simultaneously increasing electron density at the methine carbon
(evident from the negative NBO charges). Although it might be
expected that a carbocationic center would exhibit increasing
positive charge as neighboring groups become more electron
deficient, the opposite occurs in this case. The increasing charge
of the N-heterocycles leads to charge migration across the phenyl
ring to the remote para position (maximizing the chargeÀcharge
separation).
As described in Table 1, compound 4 reacts in CF3SO3H to
give the pyrido[1,2-a]indole derivative (16) in the absence of an
arene nucleophile. Because ionization of 4 should provide trication
27, theoretical calculations were also done to study the cycliza-
tion of the tricationic species 27 (Figure 4). Calculations suggest
that direct cyclization of the trication 27 is unlikely, as the transition
state 38 leading to 39 is estimated to be 41 kcal/mol above 27.
The cyclized trication 39 itself is 38 kcal/mol higher in energy.
A lower energy pathway was found involving initial deprotona-
tion at the pyridinium ring, proceeding through two minima (40
and 41) and giving dication 42 in a slightly exothermic step.
Cyclization then occurs with a barrier of 9.8 kcal/mol (43), and it
gives stabilized species 44 and 45. Upon basic workup, cation 45
provides the observed product 16. Although triflate normally
would not be expected to deprotonate a pyridinium ring, the high
charge density of the trication 27 must lead to enhanced acidity
of the pyridinium hydrogen. Previous studies have shown super-
electrophiles to possess highly acidic protons.26 With trication
27, the carbocationic site cannot readily lead to a deprotonation
equilibrium, so instead the pyridinium ring undergoes deproto-
nation.
Ionization of compound 4 in FSO3HÀSbF5ÀSO2ClF gives
NMR spectral data that differs somewhat from the calculated
spectrum for trication 27. While experimental NMR spectra often
differ slightly from the calculated spectra, the present case may be
the result of an equilibrium between trication 27 and the dication
42. Rapid proton transfer should average the spectral data from
27 and 42, and this would account for the differences between
the calculated and experimental spectra for trication 27. As
shown in Table 2, the experimental data for the compound 4
ionization product, presumably trication 27, showed a methine
resonance at δ 176.8 and para-carbon at δ 169.4. The calculated
spectrum for 27 has a methine resonance at δ 176.0 and para-
carbon at δ 178.2. When dication 42 was studied computation-
ally, the calculated spectrum has a methine resonance at δ 179.1
and para-carbon at δ 161.6 (for a full list of peaks, see the
Supporting Information). If a numerical average is made from the
calculated chemical shift values of trication 27 and dication 42,
then an averaged spectrum should have methine resonance at
δ 177.6 and para-carbon at δ 169.9. These averaged values are
very close to the experimentally determined values from ioniza-
tion of 4 in magic acid. Moreover, the calculated energies of 27
and 42 are within 0.5 kcal/mol, suggesting a significant concen-
centration of these two species from the low temperature ionization.
This raises an obvious question: could all of the chemistry
simply involve equilibria between dications? This question was
examined computationally by considering the step in which the
alcohol ionizes to the carbocation. Assuming the alcohol group
must be protonated for ionization, this means the dicationic
intermediate 46 would of necessity be involved in the chemistry.
However, calculations indicate that 46 is about 35 kcal/mol less
Figure 5. Cyclization of trication 46 to compounds 20 and 51.
stable than the dication in which both pyridyl rings are proto-
nated (47). This strongly suggests the formation of the tricationic
oxonium ion 48, leading to the proposed tricationic superelec-
trophile 27.
As described in Table 1, compound 15 reacts in superacid to
give the pyrido[1,2-a]indole 20. This product is somewhat
surprising, as a dicationic cyclization might be expected to cyclize
into the benzothiazole ring. Assuming ionization of 15 leads to
the trication 49, deprotonation could lead to either 50 or 51, but
only dication 50 should lead to product 20 (Figure 5). However,
the pKa data for the parent heterocycles suggest that the benzo-
thiazole ring should be more acidic (pKa for pyridine 5.2; pKa for
benzothiazole 1.2).27 This apparent inconsistency may be ex-
plained by considering the stabilities of the intermediates 52 and
53. DFT calculations have shown that cyclization through the
pyridine ring leads to an intermediate (52) that is 4.6 kcal/mol more
stable than a similar intermediate with benzothiazole cyclization
(53). Likewise, product 20 is almost 4 kcal/mol more stable than
its isomer 54.
13174
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175

Journal of the American Chemical Society
ARTICLE
’ CONCLUSIONS
(6) (a) Posselt, K. Arzneim. Forsch. 1978, 28, 1056–1065. (b) See also:
Alarcon, H. A. R.; Soldatenkov, A. T.; Soldatova, S. A.; Samalyoa, A. I.;
Obando, H. U.;Prostakov, N. S. Khim. Geterotsikl. Soedin. 1993, 1233–1238.
(7) Decomposition products are obtained.
(8) Sommer, J.; Bukala, J. Acc. Chem. Res. 1993, 26, 370–376.
(9) Zhao, Y.; Truhlar, D. Theor. Chem. Acc. 2008, 120, 215–241.
(10) Jaguar, version 7.7; Schrodinger, LLC: New York, 2010.
(11) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007–1023.
(12) Woon, D. E.; Dunning, T. H. J. Chem. Phys. 1993, 98, 1358–1371.
(13) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.;
Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100,
11775–11788.
(14) Lira, A. L.; Zolotukhin, M.; Fomina, L.; Fomine, S. J. Phys.
Chem. A 2007, 111, 13606–13610.
(15) Frisch, M. J. T.; et al. Gaussian 03, revision D.02; Gaussian, Inc.:
Wallingford, CT, 2004.
(16) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623–11627.
We have studied the superacid-promoted reactions of a series
of triarylmethanols, which generate tricationic intermediates. As
compared to analogous mono- and dicationic species, the tri-
cationic intermediates exhibit new reactions resulting from the
effects of the closely oriented positive charges. These effects in-
clude the tendency for cationic charge to “migrate” across a phenyl
ring and the tendency for neighboring protons to exhibit very
high acidities. The migration of charge across organic structures
is important in material science applications and in the design of
organic-based electronics. Our results show that densely charged
structures may have high charge mobilities. As a consequence,
these tricationic superelectrophiles react with arene nucleophiles
at a remote site. Without a nucleophilic reactant, cyclizations oc-
cur to provide novel N-heterocyclic products. The functionalized
heterocycles are produced in good to excellent yields. Computa-
tional studies indicate that this chemistry arises from the deprotona-
tion of an unusually acidic pyridinium ring.
(17) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR, Basic
Principles and Progress; Diehl, P., Fluck, E., G€unther, H., Kosfield, R.,
Seelig, J., Eds.; Springer-Verlag: Berlin, 1990; Vol. 23, pp 165À262.
(18) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0; Theoretical
Chemistry Institute, University of Wisconsin, Madison, WI, 2001;
http://www.chem.wisc.edu/∼nbo5.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental proce-
b
dures, characterization data, H and 13C NMR spectra for new
1
(19) Mackie, I. D.; DiLabio, G. A. J. Phys. Chem. A 2008, 112,
10968–10976.
compounds, 13C NMR spectra for ions 26 and 27, computational
methods and results, and crystallographic data for compounds 11
and 18. This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Nilsson Lill, S. O. J. Mol. Graphics Modell. 2010, 29, 178–187.
(21) (a) Ohwada, T.; Okabe, K.; Ohta, T.; Shudo, K. Tetrahedron
1990, 46, 7539–7555. (b) Shudo, K.; Ohta, T.; Okamoto, T. J. Am.
Chem. Soc. 1981, 103, 645–653. (c) Nakamura, S.; Sugimoto, H.;
Ohwada, T. J. Am. Chem. Soc. 2007, 129, 1724–1732. (d) O’Connor,
M. J.; Boblak, K. N.; Topinka, M. J.; Kindelin, P. J.; Briski, J. M.; Zheng,
C.; Klumpp, D. A. J. Am. Chem. Soc. 2010, 132, 3266–3267.
(22) Nelson, K. L.; Brown, H. C. J. Am. Chem. Soc. 1951, 73,
5605–5607.
’ AUTHOR INFORMATION
Corresponding Author
dklumpp@niu.edu
(23) Olah, G. A. Acc. Chem. Res. 1971, 4, 240–248.
(24) Abarca, B.; Asensios, G.; Ballesteros, R.; Luna, C. Tetrahedron
Lett. 1986, 27, 5657–5660.
(25) (a) Forsyth, D. A.; Sandel, B. B. J. Org. Chem. 1980, 45,
2391–2394. (b) Forsyth, D. A.; Spear, R. J.; Olah, G. A. J. Am. Chem.
Soc. 1976, 98, 2512–2518.
(26) (a) Nenajdenko, V. G.; Shevchenko, N. E.; Balenkova, E. S.
Chem. Rev. 2003, 103, 229–282. (b) Olah, G. A.; Grant, J. L.; Spear, R. J.;
Bollinger, J. M.; Serianz, A.; Sipos, G. J. Am. Chem. Soc. 1976, 98, 2501.
(c) Larsen, J. W.; Bouis, P. A. J. Am. Chem. Soc. 1975, 97, 6094–6102.
(d) Olah, G. A.; Calin, M. J. Am. Chem. Soc. 1968, 90, 4672–4675.
(e) Olah, G. A.; Reddy, V. P.; Rasul, G.; Prakash, G. K. S. J. Am. Chem.
Soc. 1999, 121, 9994–9998.
(27) (a) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous
Solution; Butterworths: London; 1965. (b) Eicher, T.; Hauptmann, S. The
Chemistry of Heterocycles; Wiley-VHC: Weinheim, 2003.
’ ACKNOWLEDGMENT
We gratefully acknowledge the support of the National
Science Foundation (CHE-0749907 and CRIF MU-0840504)
and the NIH-National Institute of General Medical Sciences
(GM085736-01A1). S.O.N.L. gratefully acknowledges the finan-
cial support from the Åke Wiberg Foundation.
’ REFERENCES
(1) Olah, G. A.; Germain, A.; Lin, H. C.; Forsyth, D. J. Am. Chem. Soc.
1975, 97, 2928–2929.
(2) Olah, G. A.; Klumpp, D. A. Superelectrophiles and Their
Chemistry; Wiley: New York, 2008.
(3) (a) Prakash, G. K. S.; Rawdah, T. N.; Olah, G. A. Angew. Chem.,
Int. Ed. Engl. 1983, 22, 390–401. (b) Ito, S.; Morita, N.; Asao, T. Bull.
Chem. Soc. Jpn. 2000, 73, 1865–1874. (c) Head, N. J.; Prakash, G. K. S.;
Bashir-Hashemi, A.; Olah, G. A. J. Am. Chem. Soc. 1995, 117, 12005–12006.
(d) Rathore, R.; Burns, C. L.; Green, I. A. J. Org. Chem. 2004, 69,
1524–1530. (e) Reddy, V. P.; Rasul, G.; Prakash, G. K. S.; Olah, G. A.
J. Org. Chem. 2003, 68, 3507–3510. (f) See also: Pagni, R. M. Tetrahedron
1984, 49, 4161–4215.
(4) For recent examples of reactive tricationic species, see: (a)
Ohwada, T.; Yamagata, N.; Shudo, K. J. Am. Chem. Soc. 1991, 113,
1364–1373. (b) Kurouchi, H.; Sugimoto, H.; Otani, Y.; Ohwada, T.
J. Am. Chem. Soc. 2010, 132, 807–815. (c) Zhang, Y.; Sheets, M. R.;
Raja, E. K.; Boblak, K. J.; Klumpp, D. A. J. Am. Chem. Soc. 2011, 133,
8467–8469.
(5) Crystallographic data are found in the Supporting Information.
13175
dx.doi.org/10.1021/ja2046364 |J. Am. Chem. Soc. 2011, 133, 13169–13175