Solvent-manipulated guest binding and signaling of a fluorescent resorcin[4]arene cavitand with 1,3,2-benzodiazaboryl D-π-A conjugation flaps

Article abstract of DOI:10.1021/jo4006238

The conformation of resorcin[4]arene cavitand system 1 was controlled by DMSO through a hydrogen bonding network between benzodiazaborole NHs of the cavitand flaps and DMSO molecules to stabilize the vase form. Subsequently, a guest-binding cavity of 1 was formed to accommodate tetraalkylammonium guest 3, permitting the monitoring of the guest by the unaided eye as a result of a CH-π interaction between the benzodiazaborole π-donor group and the guest.

Full text of DOI:10.1021/jo4006238

Note
pubs.acs.org/joc
Solvent-manipulated Guest Binding and Signaling of a Fluorescent
Resorcin[4]arene Cavitand with 1,3,2-Benzodiazaboryl D‑π‑A
Conjugation Flaps
Kaoru Otsuka, Takuya Kondo, Ryuhei Nishiyabu, and Yuji Kubo*
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1
minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
S
* Supporting Information
ABSTRACT: The conformation of resorcin[4]arene cavitand
system 1 was controlled by DMSO through a hydrogen
bonding network between benzodiazaborole NHs of the
cavitand flaps and DMSO molecules to stabilize the vase
form. Subsequently, a guest-binding cavity of 1 was formed to
accommodate tetraalkylammonium guest 3, permitting the
monitoring of the guest by the unaided eye as a result of a
CH−π interaction between the benzodiazaborole π-donor
group and the guest.
uch attention has been devoted to π-conjugated systems
containing three-coordinate boryl substituents, in which
help us to monitor the host−guest interaction between the
benzodiazaborole wall of 1 and a guest accommodated with the
cavitand cavity by spectrophotometry.
M
the vacant pz orbital of boron is conjugated to the π* orbitals of
the adjacent π-framework.1 The resultant unique optical and
electronic properties of these compounds have motivated
chemists to formulate organic materials for applications in
nonlinear optical materials,2 two-photon excited fluorescence
materials,3 luminescence,4 and electron-transporting materials.5
Despite their superior properties as organic materials, the use of
boryl-functionalized π-systems in supramolecular chemistry has
been limited to being used as chemosensors for detecting Lewis
However, in the course of this study, we unexpectedly found
that the benzodiazaborolyl D-π-A walls in 1 could act as a
molecular “flap,” capable of responding to a hydrogen-bond-
acceptor solvent (DMSO). These conformational dynamics are
attributable to a resorcin[4]arene cavitand that can be switched
between two types of conformations: (i) closed “vase” and (ii)
opened “kite.”10a−c It was earlier reported that these
conformations were controlled by chemical stimuli such as
change in pH,13 metal ion concentration,14 solvent polarity,15
and light.16 In addition, only a few examples regarding DMSO-
induced conformational regulation have been reported.13a,17
Nevertheless, we have demonstrated here for the first time that
fluorescent signaling based on host−guest interactions in a
cavitand receptor is promoted by DMSO because of the
hydrogen-donor ability of benzodiazaborole-NH. This paper
also discusses the intriguing functionality of 1,3,2-benzodiaza-
borole from the viewpoint of conformational dynamics of the
cavitand-type receptors as well as its fluorescent signaling.
Resorcin[4]arene cavitand system 1 was synthesized by
condensing octaaminocavitand 49 with 4-cyanophenylboronic
acid in a solid-state reaction in 36% yield. The UV/vis
absorption and fluorescence spectra could be measured in a
CH2Cl2 solution of 1 (10 μM), which showed absorption (λmax
= 329 nm) and fluorescence bands (λmax = 473 nm, λex = 329
nm) with a high quantum yield (Φ) of 0.63 against a p-
terphenyl dye (ΦR = 0.92)18 (Supporting Information). A large
Stokes shift of 144 nm allowed us to detect a blue light
emission from the solution. Its specific properties were
bases such as fluoride ion6 and cyanide ion,6d 7 in part because
,
of the Lewis acid−base interactions in such π-systems based on
the affinity of boron atoms. Because boronic acids are widely
used in molecular-recognition events,8 it is worthwhile to
explore the possibility of using boron-containing π-conjugated
molecules as artificial receptors in such events. With this
objective, we recently synthesized resorcin[4]arene cavitand
systems by incorporating a fluorescent 1,3,2-benzodiazaborole
wall.9 The resorcin[4]arene cavitand motif has been recognized
as an excellent building block of artificial hosts.10 The interplay
of the benzodiazaborole segments based on conjugated three-
coordinate organoboron11 and cavitand led us to propose a new
type of fluorescence receptor for detecting tetraalkylammonium
guests that can be accommodated with the cavity. However, the
role of benzodiazaborole with respect to fluorescence-signaling
capability still remains unclear. Therefore, it is important to
understand the functionality to design new chemosensors. In
this paper, to study this phenomenon, we newly synthesized
resorcin[4]arene cavitand 1 with D-π-A benzodiazaboryl walls
by incorporating a CN group as an effective acceptor. Because
the 1,3,2-benzodiazaborolyl group serves as a π-donor, the π-
conjugation systems with acceptor groups will possess intra-
molecular charge transfer (ICT) characteristics,12 which would
Received: March 29, 2013
© XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Note
Chart 1. Chemical Structures of the Compounds 1 and 2 Investigated in This Study and Guest 3
attributed to the benzodiazaboryl walls of the cavitand. To
further investigate, 2-(4-cyanophenyl)-1,3,2-benzodiazaborole 2
was synthesized as the model fluorophore (Chart 1).
Table 1 summarizes the absorption and fluorescence data of
2 in which toluene, AcOEt, CH2Cl2, DMSO, and MeCN were
D by employing a Lippert-Mataga plot19 in Figure S9
(Supporting Information). The value is almost congruent
with that of trans-ethyl-p-(dimethylamino)cinnamate,20 imply-
ing that compound 2 has an efficient D-π-A character when the
benzodiazaborole group acts as an electron donor. On careful
observation, we noted that λmax values of both the UV/vis
absorption and fluorescence spectra obtained in DMSO were
slightly larger than those obtained in MeCN (higher polarity).
Further, we noticed that the quantum yield of the fluorescence
dramatically decreased in DMSO compared to other solvents
used, the value being 0.053, presumably because of a facile
nonradiative pathway. This suggests that DMSO could
efficiently interact with 2. This speculation was supported by
1H NMR measurements (Figure S8 in Supporting Informa-
tion); when CD2Cl2 was replaced with DMSO-d6, the
benzodiazaborole NH resonance was significantly low-field
shifted by 2.46 ppm. We reasoned that diazaborole NH would
act as a hydrogen donor, which plays an important role in the
conformational dynamics of the cavitand (vide infra).
Table 1. Absorption and Fluorescence Data of 2
λabs
λem
Stokes shift Stokes shift
a
solvent
ET
(nm) (nm)
(nm)
(cm−1
)
Φ
Toluene
AcOEt
CH2Cl2
DMSO
MeCN
33.9
38.1
40.7
45.1
45.6
316
316
312
324
313
416
437
443
473
464
100
121
131
149
151
7607
8762
9478
9723
10397
0.94
0.91
1.0
0.053
0.77
a
Values were determined against a p-terphenyl dye (ΦR = 0.92).
Conditions: [2] = 10 μM at 25 °C.
employed for the assessment. Although the observed changes in
the absorption spectra were smallup to 8 nm on moving
from toluene to MeCNthe fluorescence band was signifi-
cantly shifted to a longer wavelength as the polarity of the
solvent was increased. Thus, the change in the static dipole
moments of the fluorophore (Δμ) can be estimated to be 13.7
The use of NMR spectroscopic analysis is the most
convenient method to monitor the conformational preferences
of the cavitand.10a We thus investigated how the conformation
of 1 would be affected by varying the content of DMSO-d6 in
CD2Cl2. Figure 1 shows the 1H NMR spectra of 1 upon varying
Figure 1. 1H NMR spectra of 1 at 25 °C; (a) CD2Cl2 (85 μM), (b) CD2Cl2:DMSO-d6 99:1 v/v (2 mM), (c) CD2Cl2:DMSO-d6 95:5 v/v (2 mM),
(d) CD2Cl2:DMSO-d6 90:10 v/v (2 mM), (e) CD2Cl2:DMSO-d6 85:15 v/v (2 mM), (f) CD2Cl2:DMSO-d6 50:50 v/v (2 mM), (g) CD2Cl2:DMSO-
d6 25:75 v/v (2 mM), (h) DMSO-d6 (2 mM). Signals with an asterisk (*) represent residual solvent peaks. (i) A plausible structure of DMSO-
interacted 1.
B
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Note
1
Figure 2. H NMR spectra in CD2Cl2 at 25 °C. (a) 3 (2 mM), (b) 1 (85 μM), (c) 1 (85 μM) plus 3 (850 μM). Signals with an asterisk (*)
represent residual solvent peaks.
Figure 3. 1H NMR spectra in CD2Cl2:DMSO-d6 (99:1 v/v) at 25 °C. (a) 3 (2 mM), (b) 1 (2 mM), (c) 1 (2 mM) plus 3 (2 mM). (d) Plausible
complex structure of DMSO-interacted 1 and 3 where counterion of 3 are omitted for clarity.
the content of DMSO-d6 in CD2Cl2. Because of the low
solubility in CD2Cl2, its NMR measurement in CD2Cl2 was
carried out using a low concentration of 1 (85 μM). As inferred
from Figure 1a, although aromatic resonances were observed in
1, there were no detectable signals arising from the methine
protons, possibly because of a slow exchange rate in vase/kite
equilibration on the NMR time scale.21 This means that the
entropically unfavorable vase conformer was destabilized in
CD2Cl2.
of DMSO governed the conformation dynamics. The
observation of broadened and higher-field-shifted methine
resonances allowed us to assume that an exchange equilibrium
with the alternate kite conformations may be involved under
the conditions.17 Taken together, we proposed that the
cavitand with 1,3,2-benzodiazaborole flips could stabilize a
vase-like geometry influenced by a network of DMSO-mediated
hydrogen bonds along its upper rim.
The next stage was to examine how DMSO would influence
host−guest interactions of 1 with hexyltrimethylammonium
hexafluorophosphate 3. In CD2Cl2, taking into account a low
concentration of 1 (85 μM) for the measurement, an excess of
3 (10 equiv) was added to the solution of 1 (Figure 2).
However, there were no detectable signals at >0 ppm and no
change of signals for 3, indicative of no guest-encapsulation as a
result of the destabilization of the vase conformer in CD2Cl2.22
On the other hand, in CD2Cl2/DMSO-d6 (99:1 v/v), where
a vase conformation was stabilized with 70 equiv of DMSO, the
addition of 1 equiv. of 3 into a solution of 1 allowed us to
detect proton signals assignable to the encapsulated 3 at >0
ppm (Figure 3). A full assignment of the signals was
In contrast, in a CD2Cl2 solution with 1% vol DMSO-d6, the
1
solubility of 1 was remarkably improved and the H NMR
spectrum ([1] = 2 mM) at 25 °C suggested that 1 mostly
assumed a vase conformation despite somewhat broadened
signals (Figure 1b); the signal attributed to the methine proton
was detectable at 5.66 ppm. The proton resonance for
diazaborole NH was also observed as a singlet at 8.38 ppm.
Interestingly, increase in the fraction of DMSO-d6 up to 15%
(v/v) in CD2Cl2 provided a set of well-defined spectral peaks
assignable to the vase form (Figure 1c, d, e). This result
indicates that the addition of incremental amounts of DMSO-d6
into a CD2Cl2 solution of 1 promoted the stabilization of the
vase conformation. Taking into account a lower shift for
diazaborole NH proton, DMSO could participate in hydrogen
bonding interactions with the two donors of the diazaborole
NHs at the corner of the cavitand walls to complete the seam of
a hydrogen bond network, the plausible structure being
depicted in Figure 1i. However, the 1H NMR spectra
broadened when the content of DMSO was ≥50% (Figure
1f, g, h), indicative of the bulk environment where the polarity
1
accomplished by H−1H COSY experiments (Figure S11 in
Supporting Information). The most intense peak at −1.59 ppm
was ascribable to N(CH3)3 protons (HG), and the value of the
upfield shift compared to that of the free guest (Δδ = 4.71
ppm) was indicative of binding within the shielding aromatic
cavity of 1. Further evidence for the host−guest complexation
came from the 2D NOESY measurement (Figure S12 in
Supporting Information). NOE cross-peaks were obtained
C
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The Journal of Organic Chemistry
Note
Figure 4. Change in (a, b, c) UV/vis absorption and (d, e, f) fluorescence spectra of 1 upon the addition of incremental amounts of 3 in CH2Cl2 (a,
d), CH2Cl2/DMSO (99:1 v/v) (b, e), and CH2Cl2/DMSO (95:5 v/v) (c, f) at 25 °C. [1] = 10 μM, [4] = 0−100 μM. The values of λex are 329 nm
for (d), 331 nm for (e), and 332 nm for (f).
between not only Hf and HG but also between Hd and HG. This
indicates that CH3−N+ of the guest is adjacent to the
benzodiazaborole segment of the cavitand wall. Taking into
account the electron-donor character of benzodiazaborole,
significant CH-π interactions were presumed to be the driving
force for complexation. Indeed, lower field-shifted aromatic
(Hd, He, and Hf) and diazaborole NH resonances (Hc) (Figure
3c) strongly supported our speculation. Such interactions could
affect the optical properties of the diazaborole flap in 1 to
induce an optical response (vide infra). In this way, DMSO is
thought to allosterically mediate host−guest interactions of 1
with 3.
It is interesting to note that the guest signaling of 1 was
significantly mediated by DMSO. Indeed, 1 showed almost no
response to the guest in the UV/vis absorption and
fluorescence spectra in CH2Cl2 (Figures 4a and 4d), which
can be interpreted based on the fact that 1 cannot adopt the
closed vase form under the conditions. On the other hand,
when we set up conditions comprising a CH2Cl2 solution
containing 1% vol DMSO, a hypsochromic shift of 4 nm was
measured in the UV/vis absorption spectrum on adding 3 into
the solution (Figure 4b). The fluorescence response was
remarkable; a considerable short-wavelength shift of 32 nm was
detected with increasing fluorescence intensity (Figure 4e). For
the assessment, a nonlinear curve-fitting procedure carried out
under the assumption of a 1:1 stoichiometric complex
formation reproduced the fluorescence titrations data wherein
the high association constant Ka was estimated to be 1.44 × 107
M−1 for 3 (Figure S13 in Supporting Information). The optical
response of 3 in CH2Cl2/DMSO (95:5 v/v) (Figure 4c and f)
is the same as that observed in CH2Cl2 solution with 1% vol
DMSO. These results implied that DMSO should have a
significant effect toward guest signaling. Taken together,
benzodiazaborole plays a significant role not only in conforma-
tional dynamics but also in optical signaling; in the latter, the
benzodiazaborole group acts as an electron donor in the
fluorophore moiety on the cavitand wall, and therefore,
significant CH−π interactions between the benzodiazaborole
and guest encapsulated in the cavity diminish the strength of
the dipole moment of the D-π-A conjugated wall to induce an
optical response.
In conclusion, we fully evaluated the functionality of
benzodiazaborole for host−guest interactions based on cavitand
receptors as follows: (1) the hydrogen bonding donor
capability of benzodiazaborole NH was proven to be feasible
to stabilize a cavitand vase conformation through a seam of
hydrogen-bonding networks with DMSO, resulting in the
formation of a cavity for guest accommodation; (2) π-donor
characteristics of benzodiazaborole allows for participation in
CH−π interactions with electron-deficient guests to induce a
significant response in the emission, permitting the monitoring
of the guest by the unaided eye. We believe that the results
obtained in this study will pave the way for designing new
chemosensor systems based on cavitands and other receptor
skeletons.
EXPERIMENTAL SECTION
■
1
General. H and 13C NMR spectra were recorded on a magnetic
resonance spectrometer at 500 and 125 MHz, respectively. Chemical
shifts (δ) are reported downfield from the internal standard Me4Si.
Mass spectrometry data of 1 and 2 were taken by using a MALDI-
TOF mass spectrometer and a FAB mass spectrometer, respectively.
Dithranol and distilled THF were used as a matrix and solvent for
MALDI-TOF mass spectrometry and 3-nitrobenzyl alcoholwas used as
a matrix for FAB mass spectrometry. The absorption spectra and
fluorescence spectra were measured using a spectrophotometer and a
spectrofluorometer, respectively. Elemental analyses were performed
on an elemental analyzer.
Material. Reagents used for the synthesis were commercially
available and used as supplied. Hexyltrimethylammonium ion as
bromide salt purchased from a chemical supplier was exchanged to the
corresponding hexafluorophosphate salt 3 using KPF6.
Synthesis of Tetra(2-(4-cyanophenyl)-1,3,2-diazaborole)-ap-
pended Cavitand 1. The octaaminocavitand precursor (4·nHCl)
(487.7 mg, 0.270 mmol) dissolved in EtOH/H2O (1:1 v/v) (200 mL)
was basified with 1N NaOH aqueous solution. The product was
extracted with CH2Cl2 (180 mL × 2), dried with Na2SO4, and
evaporated under reduced pressure. Drying in vacuo for 1 h afforded a
brown solid. This solid (377.7 mg) and 4-cyanophenylboronic acid 5
(153.1 mg, 1.04 mmol) were coground in a mortar and the resultant
solid was stirred with zirconia balls (φ = 1.25 mm) under 60 Pa for 37
h at 140 °C. Dry-acetone/CH2Cl2 (1:1 v/v) was then added to the
reaction mixture and the solution was then filtrated to remove
insoluble impurities and zirconia balls. A solid was obtained by
removal of solvent in vacuum. The solid was dissolved in a small
D
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The Journal of Organic Chemistry
Note
amount of dry-acetone/CH2Cl2 (1:1 v/v) and the addition of MeCN
to the solution produced a precipitate. The reprecipitation procedure
was repeated several times until the resultant supernatant solution
became colorless. In this way, the entitled compound was obtained as a
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1
beige solid (175.3 mg, 36%). H NMR (500 MHz, CDCl3/DMSO-d6
(95:5 v/v)) δ (ppm) 8.41 (s, 8H), 7.43 (d, 8H, J = 8.0 Hz), 7.38 (s,
4H), 7.20 (d, 8H, J = 8.0 Hz), 7.17 (s, 4H), 7.07 (s, 8H), 5.68 (t, 4H, J
= 7.7 Hz), 2.21 (quart, 8H, J = 7.6 Hz), 1.44−1.28 (m, 72H,
(CH2)9CH3), 0.89 (t, 12H, J = 6.9 Hz); 13C NMR (125 MHz, CDCl3/
DMSO-d6 (95:5 v/v)) δ 156.1, 145.7, 134.7, 134.1, 133.1, 133.1, 130.6,
123.5, 118.3, 117.1, 112.1, 105.7, 33.2, 32.6, 31.8, 29.7, 29.6, 29.3, 28.1,
22.6, 14.1; MALDI-TOF-MS m/z = 1966.09 [M + H]+, 1988.08 [M +
Na]+ (Matrix: dithranol); Elemental Anal. Calcd for C124H136B4N12O8:
C, 75.76; H, 6.97; N, 8.55. Found: C, 75.31; H, 7.06; N, 8.27%.
Synthesis of 2-(4-Cyanophenyl)-1,3,2-benzodiazaborole 2. A
mixture of o-phenylenediamine (73.6 mg, 0.681 mmol) and 4-
cyanophenylboronic acid 5 (100.0 mg, 0.681 mmol) was ground in a
mortar and the resultant solid was stirred with zirconia balls (φ = 1.25
mm) under a N2 atmosphere for 72 h at 50 °C. To the reaction
mixture was added CH2Cl2 and the resultant solution was filtrated to
remove unreacted 5 and zirconia balls. The solution was concentrated
using a rotary evaporator and the addition of hexane to obtain 2 as a
1
yellow solid (56.4 mg, 38%). H NMR (500 MHz, CDCl3) δ (ppm)
7.82 (d, 2H, J = 8.1 Hz), 7.71 (d, 2H, J = 8.2 Hz), 7.16 (dd, 2H, J = 5.6
Hz, 3.3 Hz), 7.01 (dd, 2H, J = 5.8 Hz, 3.2 Hz), 6.87 (s, 2H); 13C NMR
(125 MHz, CDCl3) δ 135.9, 133.4, 131.6, 120.0, 118,9 113.1, 111.6;
FAB-MS m/z = 219 [M]+ (Matrix: 3-nitrobenzyl alcohol); Elemental
Anal. Calcd for C13H10BN3: C, 71.28; H, 4.60; N, 19.18. Found: C,
71.02; H, 4.62; N, 18.96%.
ASSOCIATED CONTENT
■
S
* Supporting Information
Spectroscopicand analytical data for new compounds of 1 and
2, and host−guest complexation of 1 with 3. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) (a) Weber, L. Coord. Chem. Rev. 2008, 252, 1−31. (b) Weber,
L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.; Brockhinke,
A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009, 1339−1351.
(c) Weber, L.; Werner, V.; Fox, M. A.; Marder, T. B.; Schwedler, S.;
Brockhinke, A.; Stammler, H.-G.; Neumann, B. Dalton Trans. 2009,
2823−2831. (d) Kojima, T.; Kumaki, D.; Nishida, J.; Tokito, S.;
Yamashita, Y. J. Mater. Chem. 2011, 21, 6607−6613. (e) Kuhtz, H.;
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yujik@tmu.ac.jp.
Notes
Cheng, F.; Schwedler, S.; Bohling, L.; Brockhinke, A.; Weber, L.;
̈
The authors declare no competing financial interest.
Parab, K.; Jakle, F. ACS Macro Lett. 2012, 1, 555−559.
̈
(12) (a) Weber, L.; Eickhoff, D.; Marder, T. B.; Fox, M. A.; Low, P.
J.; Dwyer, A. D.; Tozer, D. J.; Schwedler, S.; Brockhinke, A.; Stammler,
H.-G.; Neumann, B. Chem.Eur. J. 2012, 18, 1369−1382.
(b) Schwedler, S.; Eickhoff, D.; Brockhinke, R.; Cherian, D.; Weber,
L.; Brockhinke, A. Phys. Chem. Chem. Phys. 2011, 13, 9301−9310.
ACKNOWLEDGMENTS
■
We thank Associate Professor T. Fujiwara of Molecular
Analysis and Life Science Center, Saitama University for the
measurement of MADI-TOF MS.
́
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The Journal of Organic Chemistry
Note
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29, 465−470.
(20) Singh, T. S.; Mitra, S. J. Lumin. 2007, 127, 508−514.
(21) In order to get insight into the conformational exchange, we
carried out VT 1H NMR measurement of 1 (2 mM) in THF-d8
ranging from 25 to −50 °C (Figure S10 in Supporting Information).
At 25 °C, a broad signal resonating at 5.22 ppm was detected (Figure
S10a in Supporting Information). However, upon cooling to −50 °C,
the broad signal was found to split into two separate signals at 5.74 and
1
4.41 ppm. 2D H−1H COSY NMR measurement of 1 in THF-d8 at
−50 °C where cross peaks have been observed between each of the
signals and methylene protons (Hh) allowed us to assign the peaks to
methine protons. On the basis of a coalescence temperature, Tc, of 0
°C and Δμ = 664 Hz, the ΔG⧧ for the conformational exchange is
calculated to be 12.0 kcal mol1, being almost consistent with that of
cavitand with four quinoxaline flaps.10b Further, at −50 °C, the
detection of plural signals resonating at around 9 ppm suggests that
two types of kite conformers with C2v symmetry may be involved in
the solution (Figure S10d in Supporting Information). From these
results, we consider that the cavitand can adopt vase and kite
conformers where the signals at 5.74 ppm and at 4.41 ppm are
assignable to methine protons of a vase conformer and kite conformer,
respectively.
(22) On careful observation, we found that the low-field region (5.5−
8 ppm) in Figure 2c shows not only that the aromatic resonances (Hd,
He, and Hf) of 1 shifted lower by 0.41−0.47 ppm but also that new
signals that may be assigned to methane protons (Hg) appeared at 5.78
ppm. This may indicate that the addition of an excess of 3 could affect
the vase/kite conformational equilibrium. This perturbation may also
be responsible for a slight change in the UV/vis and florescence
spectra upon the addition of 3 to a CH2Cl2 solution of 1 (vide infra;
Figure 4a and d).
F
dx.doi.org/10.1021/jo4006238 | J. Org. Chem. XXXX, XXX, XXX−XXX