2-(3,5-Dinitrophenyl)-1,3-dithiane carbanion: A benzylic anion with a low energy triplet state

Article abstract of DOI:10.1021/ja204711a

Calculations at the DFT level predict that benzyl anions with strong π-electron-withdrawing groups in the meta position(s) have low energy diradical or triplet electronic states. Specifically, the 2-(3,5-dinitrophenyl)- 1,3-dithiane carbanion is predicted

Full text of DOI:10.1021/ja204711a

ARTICLE
pubs.acs.org/JACS
2-(3,5-Dinitrophenyl)-1,3-dithiane Carbanion: A Benzylic Anion with a
Low Energy Triplet State
Raffaele R. Perrotta,† Arthur H. Winter,‡ William H. Coldren,† and Daniel E. Falvey*,†
†Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
‡Department of Chemistry, Iowa State University, 2101d Hach Hall, Ames, Iowa 50011, United States
S Supporting Information
b
ABSTRACT: Calculations at the DFT level predict that benzyl anions with strong
π-electron-withdrawing groups in the meta position(s) have low energy diradical or
triplet electronic states. Specifically, the 2-(3,5-dinitrophenyl)-1,3-dithiane carba-
nion is predicted to have nearly degenerate singlet and triplet states at the
(U)B3LYP level as a free anion. Its lithium ion pair is predicted to be a ground-state
triplet with a substantial (26 kcal/mol) singletÀtriplet energy gap. Experiments on
this anion using chemical trapping, NMR, and the Evans method strongly suggest
that this anion is either a triplet or a ground-state singlet with a very low energy
triplet state.
’ INTRODUCTION
Scheme 1. Electronic Effect of Meta π-Donors with Exocyc-
lic Cationic and Anionic Substituents
Identifying and characterizing organic species with high-spin states
(triplet and higher) is a problem that continues to attract attention
for both fundamental and practical reasons. Accurately predicting
electronic states in high-spin molecules continues to challenge
current theoretical and computational tools, although substantial
progress has been made in the past decade.1À11 High-spin molecules
hold promise as building blocks for new materials having interesting
electronic and magnetic properties.12À19 However, a variety of
practical problems, not the least of which is the high chemical
reactivity of such species, will need to be overcome before commer-
cial products based on these concepts become widely available.
Density functional theory (DFT) has served as a highly
valuable tool for organic chemists in predicting various physical
properties that would prove otherwise difficult to measure or
determine experimentally. For example, DFT has been shown to
accurately predict the electronic states,1À5 spectroscopic,1 and
thermodynamic6,7 properties of high-spin diradicals.5À11
The m-xylylene diradical system is known to be a nondisjoint
and open-shell species with a triplet ground state (ΔEST = +9.6 (
0.2 kcal/mol).11,20,21 Its physical and chemical properties have
been studied both computationally and experimentally.13,22,23
The m-xylylene diradical and its derivatives has proven to be
somewhat of a challenge to study due to their high reactivity.
These intermediates tend to be generated and studied in
low temperature solid-state matrices or solutions by ESR
spectroscopy.13 Nonetheless, the m-xylylene diradical has
served as a robust template for designing new high-spin
building blocks. Rajca and co-workers have shown that aza-
m-xylylene diradical derviatives are stable and persistent at
room temperature.24
carbenes.25À31 Substituted triplet carbenes have been a conveni-
ent high-spin intermediate to study by many investigators, due to
their ease of generation from the thermal decomposition or
photolysis of the corresponding diazo precursors. Carbenes
are known to selectively add to alkenes and insert into activated
CÀH bonds. The predictable chemical reactivity of singlet and
triplet carbenes is yet another feature that demonstrates why
carbenes have been extensively studied by chemists as reactive
intermediates. However, other stable heteroatom-based radical
systems that possess high-spin states are known to have interesting
magnetic properties32À37 and are currently being investigated as
an alternative source of high-spin building blocks to carbenes.
To date, most approaches to high-spin organic molecules have
favored building blocks derived from neutral biradicals,13 carbenes,38
or nitrenes.12 However, recent computational studies, including
multireference methods, have identified a new class of ion diradicals,
described in Scheme 1, wherein a cationic acceptor group (e.g., O+,
One particular reactive intermediate that has been widely
investigated as having a triplet ground state are aryl-substituted
Received: May 23, 2011
Published: August 26, 2011
r
2011 American Chemical Society
15553
dx.doi.org/10.1021/ja204711a J. Am. Chem. Soc. 2011, 133, 15553–15558
|

Journal of the American Chemical Society
ARTICLE
Scheme 2. Predicted SingletÀTriplet Energy Gaps with DFT (B3LYP 6-31+G(d,p)) for Several Benzylic Carbanions
NR+, CR2 ) is placed on a benzene ring meta with respect to one or
two strong neutral π-electron donors (e.g., NMe2, NH2).39
In favorable cases, this can result in a species with a π,π*-triplet
ground state. The latter can be viewed as resulting from a formal
transfer of a single electron from the donor substituent to the
cationic center. A recent experimental study provided evidence
for the triplet state of the 3,5-bis(dimethylamino)benzyl carbe-
nium ion.40 Likewise, β-donor-substituted vinyl cations are also
predicted to show low energy triplet states.41
singletÀtriplet energy gaps (kcal/mol) include ZPVE, and a
+
negative value indicates a singlet ground state.
As expected, simple benzylic anions lacking strong π-acceptors
in the meta positions (5 and 11) are straightforward ground-state
singlets having negative ΔEST values. Substitution of π-acceptors
in the meta positions does have the effect of moving ΔEST in
favor of the triplet. However, in cases with modest acceptors 6À7
or a single acceptor 12, the singlet is still clearly the ground state.
In fact, even the 3,5-dinitrobenzyl anion is predicted to be a
singlet at the DFT level, although at this point the gap is very
small. Only when additional π-donating groups (8À10 and
13À14) are included on the anionic center does DFT predict
a triplet ground state (Scheme 2).
Analysis of the computed NBMOs in which the unpaired electrons
of structure 7 reside gives a qualitative picture showing clear overlap
of electron density. This overlap occurs largely on the positions ortho
to the nominally, carbanionic center and slightly on the para positions
and nitro groups (see the Supporting Information). Such overlap
establishes the nondisjointness of the nonbonding molecular orbitals,
reinforcing the conclusion that a triplet state is favored. Similar results
are seen with structure 8. However, the orbitals are more complex
due to the lower symmetry of this anion.
The following study was designed to determine if the same
reasoning could be applied to anionic species. Specifically, we
have generated and characterized a benzylic carbanion having
two nitro groups substituted meta with respect to the (formally)
anionic center. Calculations using density functional theory
(DFT) suggest that this species has a diradical ground state.
NMR, UV, as well as chemical trapping experiments provide
evidence that this species is both persistent and paramagnetic.
On the basis of these calculations and experiments, it is argued
that 8 either has a triplet ground state or has a singlet ground state
with a higher energy, but thermally accessible, triplet.
It should be noted that DFT is a single-reference method and
so does not explicitly account for nondynamical correlation in
any rigorous way. Because nondynamical correlation is usually
more important for singlet states than for triplet states, DFT
often underestimates the singlet stability relative to the triplet by
several kcal/mol.
To benchmark the accuracy of B3LYP 6-31+G(d,p) methods,
both CASSCF and CBS-QB3 calculations were performed on the
parent benzyl anion. CASPT2/cc-pVDZ and the CBS-QB3
method yielded results of À46.3 and À44.0 kcal/mol, respec-
tively. These numbers are in reasonable agreement with previous
calculations. While still a single reference method, CBS-QB3
has been known to give extremely accurate energetic values by
’ RESULTS AND DISCUSSION
DFT Calculations. To identify anionic species having low
energy open-shell and/or triplet electronic states, the anions
shown in Scheme 2 were considered. These include the parent
benzyl anion 5, its 3,5-dicyano 6, and 3,5-dinitro derivatives 7 and
8. Also investigated were benzylic anions wherein the formally
anionic center was substituted with either two sulfur atoms 8, two
oxygen atoms 9, or one of each 10. For each anion, geometries of
the singlet and triplet states were optimized at the (U)B3LYP/6-
31+G(d,p) level, and the reported geometries were verified
as local minima on the basis of vibrational energy calcula-
tions, which showed no imaginary frequencies. The reported
15554
dx.doi.org/10.1021/ja204711a |J. Am. Chem. Soc. 2011, 133, 15553–15558

Journal of the American Chemical Society
ARTICLE
accounting for both static and dynamic electron correlation as
well as extrapolating to an infinite basis set limit.
observing the incorporation of deuterium into the benzylic position
after the solutions were subsequently quenched with CD3OD. For
example, dithiane 15 in anhydrous THF was allowed to equilibrate
with KH at room temperature for 1 h. This mixture was then
quenched with CD3OD. The resulting 1H NMR spectrum showed
littletonosignals for thebenzylic proton,and86%H/Dincorpora-
tion was determined from this spectrum (Scheme 3). Essentially
identical results were obtained with other strong bases including
CH3Li, n-BuLi, as well as in other aprotic media including
benzene and dioxane (see the Supporting Information for
NMR spectra). For comparison purposes, the unsubstituted
analogue 16 was also investigated, and similar results were
obtained. On the basis of these experiments, we conclude that the
targeted anions are generated through straightforward deprotona-
tion of the corresponding dithianes.
It is also the case that the calculations shown in Scheme 2
represent free gas-phase anions. We considered the possibility
that ion pairing might affect ΔEST. Specifically, calculated charge
distributions for 13S-Li+À13T-Li+ show that the singlet state
favors negative charge on the benzylic carbon, whereas the triplet
state accumulates more negative charge on the nitro groups. This
picture is consistent with the general structure depicted in
Scheme 1. Moreover, itimplies that differential counterionbinding
to these sites might affect ΔEST.
DFT calculations, therefore, were carried out on the singlet
and triplet states of ion pairs consisting of anion 8 and lithium or
potassium cations. While several local minima were located with
respect to the positioning of the cations, the lowest energy
minima for each spin state are depicted in Figure 1. As expected,
the triplet state has the Li+ or K+ counterion associated with the
nitro group, whereas in the singlet state the same counterions are
associated with the carbon.
In contrast, similar attempts to generate the corresponding
O-substituted anions 9 and 10 resulted in exchange of the
aromatic proton that is between the two nitro groups, rather
than the desired benzylic proton.
More surprising is the effect of counterion binding on ΔEST.
For lithium, this value increases to +26 kcal/mol as compared to
nearly degenerate for the free anion. For potassium, the effect is
smaller (+12 kcal/mol), presumably due to its weaker coordina-
tion to the anionic sites. Of course, coordinating solvents, like
ethers, would be expected to bind to the counterions competi-
tively with the anions. As such, we would expect the magnitude of
the counterion effect on ΔEST to be diminished relative to what is
calculated in the gas phase. Indeed, reoptimization of 13S and
13T using the Polarizable Continuum model (Integral Equation
Formalism-PCM) in Gaussian 03 at the 6-31+G(d,p) level with
THF as solvent showed a decrease in ΔEST to a value of +17 kcal/mol
(including correction for ZPVEs). Nonetheless, it is reasonable
to expect that the lithium salt of ion 8 will be a triplet or at least
have a low-energy triplet state if it is generated in solution.
Generation of the Anions. Benzylic anion 8 was generated
from deprotonation of the corresponding dithiane 15 using various
strong bases (for the synthesis and characterization of 15, see
the Supporting Information). Anion formation was verified by
Chemical Trapping Experiments. Carbanions derived from
1,3-dithianes are well-known to carry out SN2 reactions with
primary alkyl halides.42À51 The anion generated from dithiane
16 is no exception. Generation of the latter and treatment with
methyl iodide or 1-bromopropane results in good yields of the
alkylated products (60%) and (57%), respectively (see the Sup-
porting Information for characterization details). However, the
dinitro-derivative 15, when subjected to the same conditions,
provided no detectable alkylation products, even after 24 h of
reaction time (see the Supporting Information). While this
experiment does not unambiguously identify the electronic con-
figuration of 8, it does illustrate that this species differs qualitatively
from the unsubstituted dithiane in its chemical behavior.
UVÀVis Spectra of Anion 8. Shown in Figure 2 is the spectrum
of the anion generated through the deprotonation of dithiane 15
using CH3Li/TMEDA. The main features are a maximum at
390 nm along with a shoulder at ca. 425 nm. These features only
appear when a base is added and are rapidly quenched upon
addition of water or alcohols. Thus, these signals are confidently
attributed to the anion. Similar spectra were obtained with other
bases and/or in other solvents (specific bands are listed in a table in
the Supporting Information). Unfortunately, the UVÀvis bands
computed using TD-DFT for the singlet and triplet anion are not
sufficiently different to make a definitive assignment of the spin state
by absorption spectroscopy, given the inherent approximations in
the computational method and that the absorption spectra that we
obtained for the anion are highly dependent on additives (such as
TMEDA), solvent, and counterion. Nonetheless, these experiments
indicate that anion 8 can be generated and is persistent in the
absence of oxygen, alcohols, or water.
Figure 1. Computed DFT (B3LYP-6-31+G(d,p)) geometries for the
singlet 13S-Li+ (left) and 13T-Li+ triplet(right) withlithiumcounterions.
Scheme 3. H/D Exchange Experiment of Dithianes 15 and 16 in Anhydrous THF with Excess KH at Room Temperature (1 h) and
Quenching with Excess CD3OD
15555
dx.doi.org/10.1021/ja204711a |J. Am. Chem. Soc. 2011, 133, 15553–15558

Journal of the American Chemical Society
ARTICLE
Figure 2. UVÀvis spectrum of 2-(3,5-dinitrophenyl)-1,3-dithiane (15)
and 156 mM TMEDA/benzene with no base added (A). Difference
UVÀvis absorption spectra of 15 (0.061 mM) in benzene with TMEDA
(164 mM) and titration of excess base (addition of 25 μL of 3.0 M MeLi/
DEM) (C), which is subtracted from a blank solution (B) containing
164 mM TMEDA, benzene, and 36.1 mM MeLi/DEM ((D) benzyl
anion 8 difference spectrum). See the Supporting Information for
additional UVÀvis spectra.
1H NMR Spectra of the Anion. Several years ago, Eliel et al.
demonstrated that the benzylic anion 11, derived from deprotona-
tion of 2-phenyl-1,3-dithiane, could be directly detected and
characterized using 1H NMR spectroscopy.52 Similar experiments
were carried out on 16, and the results are shown in Figure 3.
Following the earlier work, the protons meta and para with
respect to the anionic center show a substantial upfield shift
relative to the conjugate acid, while the ortho protons show a
downfield shift. The latter is presumably due to the coordination
of the lithium counterion at the benzylic site. Quenching with
CD3OD restores the original dithiane. The broadening of the
peaks in the latter spectrum is attributed to the formation of
insoluble lithium salts from the quenching reaction.
The dinitro derivative 15 shows qualitatively different behavior.
In this case, deprotonation of the dithiane using n-BuLi in dioxane-
d8 results in the broadening of the aromatic resonances to the point
where they disappear (Figure 3e). As with 16, quenching with
CD3OD restores the aromatic signals of the dithiane (Figure 3f).
Similar results were obtained with other bases (CH3Li) and using
other solvents (TMEDA/C6D6). This behavior is consistent with
what would be expected for a paramagnetic species having sig-
nificant spin density localized on the phenyl ring. In general,
paramagnetic species are observed to be NMR silent due to rapid
spinÀspin relaxation. Other causes, such as chemical exchange,
aggregation, and formation of precipitates, could also explain such
peak broadening. However, these causes would also have to be
consistent with nearly quantitative reformation of dithiane 15 upon
quenching with proton donors.
Figure 3. Room-temperature 1H NMR (400 MHz) spectra of the
generation of the 2-phenyl-1,3-dithiane anion and the 2-(3,5-dinitro-
phenyl)-1,3-dithiane anion in a sealed Young tube in d8-dioxane. (a) 16
(124 mM) in d8-dioxane (no base added). (b) 16 (124 mM) in d8-
dioxane after 0.45 mL of 2.5 M n-butyllithium/hexanes was added. (c)
16 (124 mM) in d8-dioxane after 0.45 mL of 2.5 M n-butyllithium/
hexanes was added and quenched with 0.2 mL of CD3OD. (d) 15
(89.0 mM) in d8-dioxane (no base added). (e) 15 (89.0 mM) in d8-
dioxane after 0.20 mL of 2.5 M n-butyllithium/hexanes was added. (f) 15
(89.0 mM) in d8-dioxane after 0.2 mL of 2.5 M n-butyllithium/hexanes
was added and quenched with 0.2 mL of CD3OD.
compartment contains solvent (C6D6) alone, and the outer
compartment was charged with a solution containing the anion
in the same solvent.
In the absence of anion, there is a single solvent peak (Figure 4a).
However, as the concentration of anion is increased, solvent peaks
for the inner and outer two begin to separate, signaling the
formation of a paramagnetic species (see the Supporting Informa-
tion for more detailed NMR spectra and a table that summarizes all
Evans method experiments on 15 in C6D6/TMEDA with n-BuLi).
Control experiments wherein n-BuLi/TMEDA but no dithianewas
added to the outer compartment showed no chemical shift
differences (see the Supporting Information).
To determine whether the observed peak broadening was due
to the formation of a paramagnetic species, an Evans experiment
was carried out. In addition to relaxation effects, paramagnetic
additives can also cause deshielding of species that are in the same
solution. Through comparison of chemical shifts of the solvent in
the presence and absence of a paramagnetic solute, it is possible to
determine if the anion shows paramagnetic character. For the
present study, this was accomplished using the Evans method
wherein 1H NMR spectra were carried out using a two-compart-
ment NMR sample tube.53À64 In these experiments, the inner
Figure 4 shows the 1H NMR resonances in the region
6.5À7.5 ppm from an Evans experiment wherein the anion
was generated in C6D6 using n-BuLi/TMEDA. The peaks
shown in these figures correspond to the residual protonated solvent.
15556
dx.doi.org/10.1021/ja204711a |J. Am. Chem. Soc. 2011, 133, 15553–15558

Journal of the American Chemical Society
ARTICLE
Third, the same peak broadening and Evans shift effects are
seen with a variety of bases (CH3Li, n-BuLi, KH) as well as in
different solvents (THF, benzene, dioxane). While it is not
strictly impossible that similar trace amounts of anion radical
could be formed under these different conditions, we regard this
as far less likely than a more straightforward assignment of the
paramagnetic effects to a thermally accessible triplet state of 8.
’ CONCLUSION
The calculations and experiments described provide evidence
that anion 8 is a persistent ground state triplet or has a singlet
ground state with a triplet state that is very low in energy. The
DFT calculations summarized in Scheme 2 are consistent with
the general picture developed for analogous cationic intermedi-
ates. Combining an anionic center with two strong π-electron-
withdrawing groups results in a low energy triplet state. This can
be viewed as a formal electron transfer, creating a diradical that is
similar to the well-characterized meta-xylylene system. H/D
exchange experiments establish that the anion is being formed,
and the 1H NMR and Evans experiments show that this species is
paramagnetic. Future efforts will be directed at identifying other
carbanions with this electronic configuration and characterizing
these species by X-ray crystallography.
Figure 4. Representative 1H NMR (400 MHz) Evans method experi-
ments at room-temperature expansions (7.5À6.5 ppm). (a) 85.0 mM 15
in C6D6 with 132 mM TMEDA. (b) 54.8 mM 15 in C6D6 with 132 mM
TMEDA and 0.1 mmol of 2.5 M n-butyllithium/hexanes. (c) 85.0 mM
15 in C6D6 with 132 mM TMEDA and 0.125 mmol of 2.5 M n-
butyllithium/hexanes. (d) 113.5 mM 15 in C6D6 with 258 mM TMEDA
and 0.250 mmol of 2.5 M n-butyllithium/hexanes.
’ ASSOCIATED CONTENT
S
Supporting Information. Detailed syntheses and char-
b
acterization of benzyl anion precursor 15 as wellasadditional H/D
exchange spectra, chemical trapping characterization, UVÀvis/
NMR titration spectra, and Evans method spectra of benzyl
anion 8 under a variety of different conditions. An Evans method
control spectrum and a detailed table summarizing the Evans
method results from the deprotonation of 15. Computational
data including Z-matrices, ZPVEs, singletÀtriplet energy gaps
for anions 8À14, and TD-DFT data for anion 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
A quantitative analysis of the chemical shift change data suggests
a lower limit of 1% triplet is formed under these conditions. This
could indicate that the singlet is the ground state with a slightly
higher energy triplet or a lack of quantitative formation of anion 8
(see the Supporting Information for more details).64À66
Also considered was the possibility that a radical anion
resulting from one electron reduction of dithiane 15 is respon-
sible for the 1H NMR peak broadening and Evans shift seen in
Figures 3 and 4. While such an intermediate could, in principle,
account for these observations, it is difficult to reconcile with
three observations.
’ AUTHOR INFORMATION
Corresponding Author
falvey@umd.edu
First, the high level of deuterium incorporation at the benzylic
position is inconsistent with the formation of an anion radical as
the latter would retain the hydrogen at the benzylic position.
Second, no products attributable to the anion radical were
’ ACKNOWLEDGMENT
We would like to thank Dr. Wei-Ho Huang for providing us
with ample samples of 2-(3,5-dinitrophenyl)-1,3-dithiane benzyl
anion precursor 15 used throughout this work. R.R.P.
(University of Maryland) is supported by the U.S. Department
of Education through a GAANN Fellowship.
1
detected by GC or H NMR. Similar nitroarene anion radicals
have been generated in aprotic media. These species seem to
decay by protonation on the nitro group followed by dispropor-
tionation to form hydroxylamines.67 To verify this, we generated
the anion radical of 15. Samarium(II) diiodide (SmI2) is known
to carry out one electron reduction reactions with aliphatic and
aromatic nitro groups to yield hydroxylamines, amines, and
various other reduced adducts.68,69 Therefore, dithiane 15 was
combined with an excess of SmI2/THF (ca. 6À10 equiv) at
reflux for 20.5 h. Following the quenching of this reaction with
CD3OD, the formation of 3À4 major products was observed by
GC, GCÀMS, and NMR (see the Supporting Information).
We did not observe any of these products in the deprotonation
experiments (Scheme 3). While it is impossible to exclude trace
formation of anion radical under these conditions, we feel that
the data are much more consistent with the formation of anion 8.
’ REFERENCES
(1) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J.
J. Phys. Chem. 1994, 98, 11623–11627.
(2) Perdew, J. P.; Yue, W. Phys. Rev. B. 1996, 33, 8800–8802.
(3) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988,
37, 785–789.
(4) Krylov, A. I. Acc. Chem. Res. 2006, 39, 83–91.
(5) Bally, T.; Borden, W. T. Rev. Comput. Chem. 1999, 13, 1–97.
(6) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(7) Poutsma, J. C.; Upshaw, S. D.; Squires, R. R.; Wenthold, P. G.
J. Phys. Chem. A 2002, 106, 1067–1073.
(8) Cramer, C. J. J. Am. Chem. Soc. 1999, 120, 6261–6269.
15557
dx.doi.org/10.1021/ja204711a |J. Am. Chem. Soc. 2011, 133, 15553–15558

Journal of the American Chemical Society
ARTICLE
(9) Andersson, K.; Malmqvist, P. A.; Ross, B. O. J. Chem. Phys. 1992,
96, 1218–1226.
(45) Seebach, D.; Jones, N. R.; Corey, E. J. J. Org. Chem. 1968,
33, 300–305.
(10) Ess, D. H.; Johnson, E. R.; Hu, X. Q.; Yang, W. T. J. Phys. Chem.
A 2011, 115, 76–83.
(11) Wright, B. B.; Platz, M. S. J. Am. Chem. Soc. 1983, 105, 628–630.
(12) Lahti, P. M. Magnetic Properties Of Organic Materials; Marcel
Dekker: New York, 1999.
(46) Seebach, D.; Corey, E. J. Org. Synth. 1968, 50, 72.
(47) Seebach, D.; Corey, E. J. J. Org. Chem. 1975, 40, 231–237.
(48) Stowell,M.H.B.;Rock,R.S.;Rees,D.C.;Chan,S.I.Tetrahedron Lett.
1996, 37, 307–310.
(49) Hassan, H. H. A. M.; Tamm, C. Helv. Chim. Acta 1996,
(13) Borden, W. T. Diradicals; Wiley: New York, 1982.
(14) Nozawa, T.; Nagata, M.; Ichinohe, M.; Sekiguchi, A. J. Am.
Chem. Soc. 2011, 133, 5773–5775.
79, 518–526.
(50) Lee, H. B.; Balasubramian, S. J. Org. Chem. 1999, 64, 3454–
3460.
(15) Shil, S.; Misra, A. J. Phys. Chem. A 2010, 114, 2022–2027.
(16) Shultz, D. A.; Fico, R. M.; Bodner, S. H.; Kumar, R. K.;
Vostrikove, K. E.; Kampf, J. W.; Boyle, P. D. J. Am. Chem. Soc. 2003,
125, 11761–11771.
(51) Br€auer, M.; Weston, J.; Anders, E. J. Org. Chem. 2000, 65,
1193–1199.
(52) Eliel, E. L.; Abatjoglou, A. G.; Kuyper, L. F. J. Am. Chem. Soc.
1977, 99, 8262–8269.
(17) Oka, H. Org. Lett. 2010, 12, 448–451.
(53) Gunther, H. J. Braz. Chem. Soc. 1999, 10, 241, general example
(18) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001, 294,
1503–1505.
(19) Lochmann, L.; Bauer, W. J. Am. Chem. Soc. 1992, 114, 7482–
of anions in proton NMR behavior.
(54) Carlin, R. L. Magnetochemistry; Springer: Heidelberg, Germany,
1986; pp 5À18.
7489.
(55) Dowd, P. J. Am. Chem. Soc. 1966, 88, 2587.
(56) Dowd, P. Acc. Chem. Res. 1972, 5.
(20) Wenthold, P. G.; Kim, J. B.; Lineberger, W. C. J. Am. Chem. Soc.
1997, 119, 1354–1359.
(21) Neuaus, P.; Grotte, D.; Sander, W. J. Am. Chem. Soc. 2008,
(57) Burns, G. J. Appl. Phys. 1961, 32, 2048.
(58) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance Elementary
Theory and Practical Application; McGraw-Hill: New York, 1972;
Chapter 10.
(59) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763.
130, 2993–3000.
(22) Wang, T.; Krylov, A. I. J. Chem. Phys. 2005, 123, 6.
(23) Lenington, M. J.; Wenthold, P. G. J. Phys. Chem. A 2010,
114, 1334–1337.
(24) Rajca, A.; Olankitanit, A.; Rajca, S. J. Am. Chem. Soc. 2011,
133, 4750–4753.
(25) Nemirowski, A.; Schreiner, P. R. J. Org. Chem. 2007, 72,
9533–9540.
(26) Wang, J.; Kubicki, J.; Peng, H.; Platz, M. S. J. Am. Chem. Soc.
2008, 130, 6604–6609.
(27) Woddcock, H. L.; Moran, D.; Brooks, B. R.; Schleyer, P. V.;
Schaefer, H. F. J. Am. Chem. Soc. 2007, 129, 3763–3770.
(28) Hirai, K.; Itoh, T.; Tomioka, H. Chem. Rev. 2009, 109,
3275–3332.
(29) Tomioka, H.; Nakane, N.; Tatsugi, J. Bull. Chem. Soc. Jpn. 2008,
81, 1629–1637.
(60) Gordy, W. In Theory and Applications of Electron Spin Resoance;
West, W., Ed.; John Wiley and Sons, Inc.: New York, 1980, Vol. 15, p
589 (A Weissberger series ed.).
(61) Loliger, J.; Schefford, R. J. Chem. Educ. 1972, 49, 646–647.
(62) Ostfeld, D.; Cohen, A. J. Chem. Educ. 1972, 49, 829.
(63) Piguet, C. J. Chem. Educ. 1997, 74, 815–816.
(64) Crawford, T. H.; Swanson, J. J. Chem. Educ. 1971, 48, 382–386.
(65) Grant, D. H. J. Chem. Educ. 1995, 72, 39–40.
(66) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(67) Cu, A.; Testa, A. C. J. Am. Chem. Soc. 1974, 96, 1963–1965.
(68) Banik, B. K.; Mukhopadhyay, C.; Venkatraman, M. S.; Becker,
F. F. Tetrahedron Lett. 1998, 39, 7243–7246.
(69) Ankner, T.; Hilmersson, G. Tetrahedron Lett. 2007, 48, 5707–
5710.
(30) Tomioka, H.; Moss, R. A.; Platz, M. S.; Jones, M., Jr. Triplet
Carbenes in Reactive Intermediate Chemistry; John Wiley and Sons:
Hoboken, NJ, 2004; pp 375À462.
(31) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88–94.
(32) Gilroy, J. B.; McKinnon, S. D.; Kennepohl, P.; Zsombor, M. S.;
Ferguson, M. J.; Thompson, L. K.; Hicks, R. G. J. Org. Chem. 2007,
72, 8062–8069.
(33) Fukuzaki, E.; Nishide, H. Org. Lett. 2006, 8, 1835–1838.
(34) Lee, E. C.; Choi, Y. C.; Kim, W. Y.; Singh, N. J.; Lee, S.; Shim,
J. H.; Kim, K. S. Chem.-Eur. J. 2010, 16, 12141–12146.
(35) Rajca, A. Advances in Physical Organic Chemistry; Elsevier
Academic Press, Inc.: San Diego, CA, 2005; Vol. 40, pp 153À199.
(36) Ichino, T.; Vilano, S. M.; Gianola, A. J.; Goebbert, D. J.; Velarde,
L.; Sanov, A.; Blanksby, S. J.; Zhou, X.; Hrovat, D. A.; Borden, W. T.;
Lineberger, W. C. Angew. Chem., Int. Ed. 2009, 48, 8509–8511.
(37) Amiri, S.; Schreiner, P. R. J. Phys. Chem. A 2009, 113, 11750–
11757.
(38) Hirai, K.; Itoh, T.; Tomioka, H. Chem. Rev. 2009, 109, 3275–
3332.
(39) Winter, A. H.; Falvey, D. E.; Cramer, C. J. J. Am. Chem. Soc.
2004, 126, 9661–9668.
(40) Perrotta, R. R.; Winter, A. H.; Falvey, D. E. Org. Lett. 2011, 13,
212–215.
(41) Winter, A. H.; Falvey, D. E. J. Am. Chem. Soc. 2010, 132,
215–222.
(42) Corey, E. J.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1965, 4,
1075–1077.
(43) Corey, E. J.; Seebach, D. Angew. Chem., Int. Ed. Engl. 1965,
4, 1077–1078.
(44) Corey, E. J.; Seebach, D. J. Org. Chem. 1966, 31, 4097–4099.
15558
dx.doi.org/10.1021/ja204711a |J. Am. Chem. Soc. 2011, 133, 15553–15558