Synthesis and Lewis acidic behavior of a cationic 9-thia-10-boraanthracene

Article abstract of DOI:10.1021/om100415z

As part of our efforts in the chemistry of Lewis acidic organoboron compounds, we have synthesized a cationic borane ([1]⁺) featuring a 9-thia-10-boraanthracene moiety substituted at boron by the cationic anilium group [4-(Me₃N)-2,6-

Full text of DOI:10.1021/om100415z

5490 Organometallics 2010, 29, 5490–5495
DOI: 10.1021/om100415z
Synthesis and Lewis Acidic Behavior of a Cationic
9-Thia-10-boraanthracene†
Takeshi Matsumoto, Casey R. Wade, and Franc-ois P. Gabbaı*
Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843-3255
Received May 3, 2010
As part of our efforts in the chemistry of Lewis acidic organoboron compounds, we have synthesized a
cationic borane ([1]þ) featuring a 9-thia-10-boraanthracene moiety substituted at boron by the cationic
anilium group [4-(Me3N)-2,6-Me2-C6H2]þ. This new cationic borane has been fully characterized. Its
UV-vis spectrum features a low-energy band at λmax = 392 nm which, as confirmed by computational
studies, arises from a π-π* transition of the 9-thia-10-boraanthracene chromophore. As a result of the low
steric bulk present around the boron center and the inductive effects imparted by the anilium group, [1]þ
possesses unusual Lewis acidic properties and reacts with DMAP to form the corresponding adduct. This
derivative also binds both fluoride and cyanide anions in THF to afford 1-F and 1-CN whose stability
constants exceed 107 M-1. Although [1]þ is unstable in pure water, it can be used to selectively extract
cyanide under biphasic conditions in nitromethane/water.
Introduction
functionalities (Chart 1).9-14 Because of favorable Coulombic
effects, these boranes display a higher anion affinity and can
sometimes be used for the complexation of fluoride and cyanide
ions in aqueous environments. A common feature uniting these
cationic boranes concerns the use of sterically encumbered
ligands whose role is to protect the boron center from nucleo-
philic attack by water molecules. This steric protection can, for
The chemistry of Lewis acidic triarylboranes has experienced
a resurgence of activity fueled by the discovery of numerous
applications in the domains of catalysis1 and small molecule
activation2 as well as optoelectronics.3,4 Such organoboranes
have also been extensively studied as receptors for small anions
including fluoride and cyanide.3,5 Research carried out in the
past decade indicates that simple triarylboranes such as
Mes2PhB,6 Mes3B,7 and Ant3B8 (Ant =9-anthryl) form stable
fluoride complexes in organic solvents but not in water where
the high hydration energy of the fluoride anion prevents
complex formation. In an effort to overcome these limitations,
we and others have investigated the synthesis and anion binding
properties of triarylboranes decorated by peripheral cationic
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Aldridge, S. Inorg. Chem. 2010, 49, 157–173.
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4957–4960. Wade, C. R.; Gabbaï, F. P. Dalton Trans. 2009, 9169–9175. Wade,
C. R.; Gabbaï, F. P. Inorg. Chem. 2009, 49, 714–720. Xu, Z.; Kim, S. K.; Han,
S. J.; Lee, C.; Kociok-Kohn, G.; James, T. D.; Yoon, J. Eur. J. Org. Chem. 2009,
3058–3065. Kim, Y.; Hudnall, T. W.; Bouhadir, G.; Bourissou, D.; Gabbaï, F. P.
Chem. Commun. 2009, 3729–3731. Hudnall, T. W.; Kim, Y.-M.; Bebbington,
M. W. P.; Bourissou, D.; Gabbaï, F. P. J. Am. Chem. Soc. 2008, 130, 10890–
10891. Neelakandan, P. P.; Ramaiah, D. Angew. Chem., Int. Ed. 2008, 47,
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A. L.; Schatz, J.; Nau, W. M.; Pischel, U. J. Org. Chem. 2007, 72, 3889–3895.
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D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510–7511. Lee, M. H.; Gabbaï,
F. P. Inorg. Chem. 2007, 46, 8132–8138. Chiu, C.-W.; Gabbaï, F. P. J. Am.
Chem. Soc. 2006, 128, 14248–14249. Kubo, Y.; Ishida, T.; Minami, T.; James,
T. D. Chem. Lett. 2006, 35, 996–997. Bresner, C.; Aldridge, S.; Fallis, I. A.;
Jones, C.; Ooi, L.-L. Angew. Chem., Int. Ed. 2005, 44, 3606–3609. Badugu, R.;
Lakowicz, J. R.; Geddes, C. D. Sens. Actuators, B 2005, B104, 103–110.
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2001, 46, 3421–3429. Nicolas, M.; Fabre, B.; Chapuzet, J. M.; Lessard, J.;
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† Part of the Dietmar Seyferth Festschrift. This paper is dedicated to Prof.
Dietmar Seyferth in recognition of his scientific contribution and his role as
founding and long-time editor of Organometallics.
*To whom correspondence should be addressed. Phone: (þ1)979-845-
4719. Fax: (þ1)979-845-4719. E-mail: francois@tamu.edu.
(1) Piers, W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem.
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ꢀ
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Published on Web 07/01/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5491
Figure 1. Top left: Electronic absorption (;) and emission (---) spectra of [1]OTf in THF. Top right: Cyclic voltammogram of [1]OTf
at a scan rate of 100 mV s-1 with n-Bu4NPF6 as supporting electrolyte. Bottom: Optimized structure of [1]þ with an overlay of the
HOMO and LUMO (isodensity value = 0.03).
Chart 1
Scheme 1. Synthesis of [1]OTfa
a Conditions: (i) Et2O, rt, 24 h; (ii) MeOTf (excess), CH2Cl2, rt, 1 h.
of this reaction, which was not isolated, was treated with
MeOTf in CH2Cl2 to afford the anilium borane [1]þ as a triflate
salt (Scheme 1). This salt, which can be stored in air for extended
1
example, be achieved by the use of a single 2,4,6-triisopropyl-
phenyl (Tip) group as in D12 or two bulky mesityl groups as in
A,13 B,11 and C.9 Inspired by the work of Kobayashi and
Kawashima who have investigated less sterically hindered catio-
nic boranes such as E,14 we have now synthesized and studied a
cationic 9-thia-10-boraanthracene ([1]þ) featuring a sterically
accessible boron atom. In addition to reporting on the anion
binding properties of this new compound, we also show that
[1]þ can form adducts with neutral Lewis bases such as DMAP.
periods of time, has been fully characterized. The H NMR
spectrum recorded in CDCl3 shows a series of aromatic CH
resonances consistent with a perpendicular arrangement of the
anilium and 9-thia-10-boraanthracene moieties. Accordingly, a
single resonance is detected at 2.06 ppm for the two ortho-
methyl groups. The proton resonance of the trimethylammo-
nium appears at 3.69 ppm, confirming the formation of an
anilium species.15 The broad 11B NMR signal observed at 57.3
ppm agrees with the presence of a base-free, trigonal-planar
boron center. While efforts to grow single crystals of this com-
pound were unsuccessful, the structure of [1]OTf has been stu-
died computationally using density functional theory (DFT)
methods (B3LYP/6-31G(d) for S, C, and H atoms and 6-31þ
G(d0) for B and N atoms). The optimized structure of the ca-
tion is shown in Figure 1. The anilium and 9-thia-10-boraan-
thracene moieties adopt a perpendicular arrangement con-
Results and Discussion
Synthesis of Cationic Borane [1]OTf. The reaction of 2,6-
dimethyl-4-dimethylaminophenyllithium15 with 10-bromo-9-
thia-10-boraanthracene7 was carried out in Et2O. The product
(15) Chiu, C.-W.; Kim, Y.; Gabbaı, F. P. J. Am. Chem. Soc. 2009,
131, 60–61.
1
sistent with the H NMR spectrum. This structure is also

5492 Organometallics, Vol. 29, No. 21, 2010
Matsumoto et al.
Scheme 2. Complexation Reaction of [1]OTf with DMAP
analogous to that reported for 10-mesityl-9-thia-10-bora-
anthracene,16 a neutral analogue of [1]þ. The cyclic voltammo-
gram of [1]OTf recorded in THF at room temperature shows a
quasi-reversible reduction wave at E1/2red=-2.32 V (vs Fc/Fcþ)
(Figure 1), corresponding to the formation of a radical species.
This reduction potential, which may be compared to that
of Mes3B (-2.9 V vs Fc/Fcþ) or [4-(Me3N)-2,6-Me2-C6H2-
BMes2]þ (-2.55 V vs Fc/Fcþ),15 suggests that the anilium
substituent of [1]þ increases its electrophilicity.17 The UV-vis
spectrum of [1]OTf in THF features a distinct absorption band
centered at 392 nm reminiscent of that observed in the case
of 10-mesityl-9-thia-10-boraanthracene (λmax = 387 nm, cyclo-
hexane) (Figure 1).16 Time-dependent DFT calculations indi-
cate that this low-energy band arises from a HOMO-LUMO
excitation. This assignment is reinforced by the good match
obtained between the computed energy of this band (367 nm)
and that experimentally observed (392 nm). Inspection of the
frontier molecular orbitals indicates that this band is a π-π*
transition centered on the 9-thia-10-boraanthracene moiety
(Figure 1). The emission spectrum of [1]OTf mirrors the low-
energy band of the absorption spectrum, indicating that it
originates from the π-π* singlet excited state (Figure 1).
In order to probe the Lewis acidity of [1]OTf, we decided to
investigate its reactivity toward p-dimethylaminopyridine (DM-
AP) in chloroform at room temperature. This reaction produces
the triflate salt [1-DMAP]OTf, which has been isolated in 77%
yield (Scheme 2). Formation of [1-DMAP]OTf was confirmed
Figure 2. ORTEP drawing of [1-DMAP]OTf with thermal el-
lipsoid plots (50% probability). Solvent molecule, triflate counter-
anion, and hydrogen atoms are omitted for clarity. Selected bond
˚
lengths (A) and angles (deg): S(1)-C(6) 1.766(4), S(1)-C(7)
1.766(4), B(1)-C(1) 1.629(6), B(1)-C(12) 1.615(6), B(1)-C(13)
1.637(5); C(1)-B(1)-C(12) 110.5(3), C(1)-B(1)-C(13) 113.8(3),
C(12)-B(1)-C(13) 112.7(3), C(1)-B(1)-N(2) 100.6(3), C(12)-
B(1)-N(2) 105.4(3), C(13)-B(1)-N(2) 112.9(3).
Scheme 3. Reactions of [1]OTf with TBAF and KCN
room temperature to afford a crystalline precipitate of the
zwitterionic cyanoborate 1-CN (Scheme 3). Both 1-F and 1-
CN have been fully characterized. The 1H NMR spectra of these
derivatives suggest that they adopt Cs symmetry in solution. The
11B NMR signals detected at 2.7 ppm for 1-F and -16.6 ppm for
1-CN are in the expected range for four-coordinate boron
species.11,13 The crystal structures of these two compounds have
been determined (Figure 3). Compound 1-F crystallizes in the
space group P21/n with two independent molecules in the
asymmetric unit. These two molecules, denoted as molecules
A and B, have very similar structures. The B-F bond lengths
1
by integration of the H NMR spectrum, which indicates the
presence of one coordinated DMAP molecule, as well as by
detection of a 11B NMR signal at 3.46 ppm corresponding to the
four-coordinate boron center. The structure of [1-DMAP]OTf
has been studied by single-crystal X-ray diffraction (Figure 2).
˚
The B-NDMAP bond (B(1)-N(2)) of 1.642(5) A is almost
identical to that measured in the adduct Ph3B-DMAP
(1.636(2) A).18 Coordination of the DMAP molecule to the
˚
boron center leads to a distinct pyramidalization of the
latter, as indicated by the sum of the Caryl-B-Caryl angles
P
˚
˚
(1.480(7) A for molecule A and 1.479(8) A for molecule B) are
(
) of 337°. Formation of DMAP adducts with cationic
comparable to those found in other triarylfluoroborate species
11,13
boranes such as A and B has not been observed.11,13 Hence,
the contrasting behavior of [1]þ may be assigned to the low
steric protection of the boron center.
˚
(1.47 A).
Similarly, the B-CCN bond (B(1)-C(24)) of
1.626(5) A observed in 1-CN is comparable to those of other
˚
triarylcyanoborate anions such as [Ph3BCN] (1.65 A).19 As in
-
˚
Reactions of [1]OTf with Fluoride and Cyanide. Fluoride
and cyanide anion capture by [1]OTf in organic solvent was also
examined. Reaction of [1]OTf with n-tetrabutylammonium
fluoride (TBAF) in CHCl3 at room temperature results in the
rapid formation of the corresponding zwitterionic fluoroborate
1-F, which could be isolated as a colorless crystalline power
(Scheme 3). Similarly, [1]OTf reacts with KCN in MeOH at
[1-DMAP]OTf, the boron center of these derivatives is dis-
P
tinctly pyramidalized ( =338.7° for molecule A of 1-F, 338.4°
for molecule B of 1-F, 333.5° for 1-CN), implying a loss of
aromaticity of the 9-thia-10-boraanthracene unit.
In agreement with this loss of aromaticity, anion binding
induces aquenchingoftheπ-π*transition detected at 392nm
in THF.16 Titration experiments carried out by monitoring
the progressive quenching of this band upon incremental
addition of anions indicate that the fluoride and cyanide
(16) Agou, T.; Kobayashi, J.; Kawashima, T. Chem.;Eur. J. 2007,
13, 8051–8060.
(17) Chiu, C.-W.; Gabbaı, F. P. Dalton Trans. 2008, 814–817.
(18) Chisholm, M. H.; Gallucci, J. C.; Yin, H. Dalton Trans. 2007,
4811–4821.
(19) Kuz’mina, L. G.; Struchkov, Y. T.; Lemenovsky, D. A.;
Urazowsky, I. F. J. Organomet. Chem. 1984, 277, 147–151.

Article
Organometallics, Vol. 29, No. 21, 2010 5493
Figure 3. ORTEP drawings of 1-F (left) and 1-CN (right) with thermal ellipsoid plots (50% probability). Solvent molecules and
˚
hydrogen atoms are omitted for clarity in each case. In the case of 1-F, only molecule A is shown. Selected bond lengths (A) and angles
(deg) for 1-F (the metrical parameters of molecule B are provided in brackets): S(1)-C(6) 1.765(5) [1.760(6)], S(1)-C(7) 1.768(5)
[1.764(5)], B(1)-F(1) 1.485(6) [1.481(6)], B(1)-C(1) 1.630(7) [1.613(7)], B(1)-C(12) 1.617(7) [1.616(7)], B(1)-C(13) 1.640(7) [1.657(7)];
C(1)-B(1)-C(12) 111.1(4) [114.0(4)], C(1)-B(1)-C(13) 114.5(4) [111.3(4)], C(12)-B(1)-C(13) 113.1(4) [113.1(4)], F(1)-B(1)-C(1)
˚
102.4(4) [104.5(4)], F(1)-B(1)-C(12) 105.2(4) [104.3(4)], F(1)-B(1)-C(13) 109.6(4) [109.0(4)]. Selected bond length (A) and angles
(deg) for 1-CN: S(1)-C(7) 1.770(3), S(1)-C(6) 1.771(2), B(1)-C(1) 1.627(4), B(1)-C(12) 1.625(4), B(1)-C(13) 1.647(4), B(1)-C(24)
1.627(4), N(2)-C(24) 1.155(3); C(1)-B(1)-C(12) 112.7(2), C(1)-B(1)-C(13) 111.7(2), C(12)-B(1)-C(13) 109.1(2), C(24)-B(1)-
C(1) 103.8(2), C(24)-B(1)-C(12) 102.1(2), C(24)-B(1)-C(13) 117.2(2).
Figure 4. Top: UV-vis titration obtained upon addition of KF (left) and KCN (right) to a 4.8 ꢀ 10-5 M THF solution of [1]OTf.
Bottom: 1H NMR spectrum of [1]OTf in nitromethane-d3 before and after layering/shaking with KCN/D2O solution (right).
binding constants of [1]þ in THF exceed 107 M-1 (Figure 4).
Encouraged by these elevated values, which may be assigned
to the cationic nature of the borane and the accessibility of
the boron center, we decided to determine if anion binding
could also be observed in protic media. To this end, the
ability of [1]þ to capture fluoride and cyanide from water
under biphasic conditions was investigated. Shaking a bi-
phasic mixture consisting of KCN in D2O (6.1 ꢀ 10-2 M,
0.5 mL) and [1]OTf in nitromethane-d3 (1.6 ꢀ 10-2 M,
0.5 mL) resulted in a 75% conversion of [1]OTf into 1-CN
behavior is reminiscent of that observed for species such as E,
which also decomposes in water.14
Conclusion
In conclusion, we report the synthesis of an aromatic
boracycle ([1]þ) whose boron atom is substituted by a
cationic anilium substituent. This new derivative features a
sterically accessible boron center and reacts with DMAP to
form the corresponding adduct. This derivative also reacts
with fluoride and cyanide anions in THF to afford the
corresponding zwitterionic fluoroborate and cyanoborate
species. Titration experiments indicate that the stability
constants of these zwitterions exceed 107 M-1, thus reflecting
the accessibility of the boron atom as well as the favorable
inductive and Coulombic effects imparted by the anilium
substituent. The unusual anion affinity of [1]þ is also illu-
strated by its ability to selectively complex cyanide and not
1
after 30 min based on H NMR (Figure 3). An analogous
experiment carried out with fluoride showed no formation of
1-F, indicating that [1]þ can selectively extract cyanide
anions under these conditions. Attempts to use [1]þ in pure
water were complicated by its slow decomposition. This lack
of stability contrasts with the behavior of compounds such as
A and B (Chart 1) and provides additional evidence for the
increased reactivity of this exposed boron derivative. This

5494 Organometallics, Vol. 29, No. 21, 2010
Matsumoto et al.
Table 1. Crystal Data, Data Collection, and Structure Refinement for 1-F-0.5(Acetone), 1-CN-Acetone, and [1-DMAP]OTf-Acetone
1-F-0.5(acetone)
1-CN-acetone
[1-DMAP]OTf-acetone
Crystal Data
formula
Mr
C24.5H28BFNO0.5
406.35
0.17 ꢀ 0.15 ꢀ 0.12
monoclinic
P21/n
8.9480(3)
35.296(11)
13.556(3)
96.815(5)
4251(2)
S
C27H31BN2OS
442.41
0.24 ꢀ 0.19 ꢀ 0.13
orthorhombic
Pbca
16.585(4)
13.446(3)
21.832(5)
C34H41BF3N3O4S2
687.63
0.30 ꢀ 0.20 ꢀ 0.20
monoclinic
P21/n
12.900(6)
17.592(8)
16.313(7)
111.056(5)
3455(3)
cryst size (mm3)
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
β (deg)
3
V (A )
˚
4869(2)
8
1.207
0.154
1888
Z
F
8
1.270
0.174
1728
4
1.322
0.212
1448
calc (g cm-3
)
μ (mm-1
F(000)
)
Data Collection
T (K)
scan mode
hkl range
110(2)
ω
-10 f þ10
-40 f þ40
-15 f þ15
37 245
110(2)
ω
-18 f þ18
-15 f þ15
-22 f þ24
21 295
110(2)
ω
-14 f þ14
-20 f þ20
-18 f þ18
29 970
measd reflns
unique reflns [Rint
]
reflns used for refinement
6684 [0.0936]
6684
3817 [0.0929]
3817
5421 [0.0611]
5421
Refinement
refined params
GOF on F2
523
1.008
0.0658, 0.1764
0.699, -0.655
P
289
1.008
0.0531, 0.1279
0.319, -0.270
424
1.008
0.0560, 0.1451
0.577, -0.563
R1,a wR2,b all data
-3
˚
fin(max/min) (e A
F
)
P
P
P
a R1 = ||Fo| - ||Fc||/ |Fo|. b wR2 = {[ w(Fo2 - Fc2)2]/[ w(Fo2)2]}1/2
.
fluoride under biphasic conditions in nitromethane/water.
The Lewis acidic properties of [1]þ suggest that this cationic
borane could be used in sensors as a molecular recognition
unit for cyanide anions. Its ability to complex neutral Lewis
bases such as DMAP suggests that such cationic boranes
may also find a use as Lewis acid catalysts for organic
transformations.
Theoretical Calculations. Density functional theory calcula-
tions (full geometry optimization) were carried out with Gauss-
ian03 using the gradient-corrected Becke exchange functional
(B3LYP) and the Lee-Yang-Parr correlation functional. A
6-31G(d) basis set was used for S, C, and H atoms and a
6-31þG(d0) basis set for B and N atoms. Frequency calculations
were carried out on the optimized structure of each compound
to confirm the absence of negative frequencies.
Electrochemistry. Electrochemical experiments were per-
formed with an electrochemical analyzer from CH Instruments
(model 610 A) with a glassy carbon working electrode and a
platinum auxiliary electrode. The reference electrode was built
from a silver wire inserted into a small glass tube fitted with a
porous Vycor frit at the tip and filled with a THF solution
containing (n-Bu)4NPF6 (0.1 M) and AgNO3 (0.005 M). All
three electrodes were immersed in a THF solution (5 mL)
containing (n-Bu)4NPF6 (0.1 M) as a support electrolye and
the analyte (2.0 mM for [1]OTf). The electrolyte was dried under
vacuum prior to use. In all cases, ferrocene was used as an
internal standard, and all reduction potentials are reported with
respect to the E1/2Red of the Fc/Fcþ redox couple.
Experimental Section
General Procedures. Methyl triflate (MeOTf) was purchased
from TCI America and used without purification. TBAF (n-
Bu4NF 3H2O), KF, and KCN were purchased from Alfa Aesar
3
and used as received. Et2O was dried by reflux over Na/K. Hexane
was dried by reflux over potassium metal. CH2Cl2 was dried by
passing through a column charged with activated alumina, then
dried over CaH2 under N2 atmosphere and distilled prior to use.
4-Dimethylamino-2,6-dimethylphenyllithium was synthesized by
following a published procedure.15 10-bromo-9-thia-10-boraan-
thracene was synthesized by following a published procedure.7
Air-sensitive compounds were handled under N2 atmosphere,
using standard Schlenk and glovebox techniques. Electronic
absorption spectra were recorded using an Ocean Optics
USB4000 spectrometer with an Ocean Optics ISS light source.
Emission spectra were collected at room temperature in THF
solution using a PTI QuantaMaster 4 fluorescence spectrophot-
ometer equipped with a model 810 PMT detector. Elemental
analyses were performed at Atlantic Microlab (Norcross, GA).
NMR spectra were recorded on a Varian Unity Inova 400 FT
Crystallography. The crystallographic measurements were per-
formed by using a Bruker APEX2 diffractometer with graphite -
˚
monochromated Mo KR radiation (λ = 0.71069 A). Specimens of
suitable size and quality were selected and mounted onto glass
fibers with apiezon grease. The structures were solved by direct
methods, which successfully located most of the non-hydrogen
atoms. Subsequent refinement on F2 with use of the SHELXTL/
PC package (version 5.1) allowed location of the remaining non-
hydrogen atoms.
1
NMR spectrometer (399.59 MHz for H, 375.95 MHz for 19F,
Synthesis of Cationic Borane [1]OTf. A solution of 2,6-
dimethyl-4-dimethylaminophenyllithium (120.8 mg, 0.78 mmol)
in Et2O (20 mL) was slowly added to a solution of 10-bromo-9-
thia-10-boraanthracene (321.6 mg, 1.17 mmol) in Et2O (20 mL)
at room temperature. After stirring for 24 h, the solvent was
128.2 MHz for 11B, 100.5 MHz for 13C) by using an internal
deuterium lock. Chemical shifts δ are given in ppm and are
referenced against external Me4Si (H, 13C, δ = 0 ppm), BF3 Et2O
3
(11B, δ = 0 ppm), and CFCl3 (19F, δ = 0 ppm).

Article
Organometallics, Vol. 29, No. 21, 2010 5495
removed in vacuo. The resulting residue was extracted with hexanes
(20 mL) and filtered. Evaporation of solvent from the filtrate
afforded a yellow powder, which was used without further pur-
ification. MeOTf (500.0 mg, 3.05 mmol) was added to a solution of
the crude product in CH2Cl2 (15 mL). After stirring for 1 h, hexane
was added to the reaction mixture to afford a precipitate of [1]OTf
(231 mg, 0.46 mmol, 59%). [1]OTf: colorless powder, mp 335 °C
(dec); 1H NMR (400 Hz, CDCl3) δ 2.06 (s, 6H), 3.82 (s, 9H), 7.33
(dd, 3JH-H = 8.0 Hz, 2H), 7.38 (s, 2H), 7.69-7.71 (m, 4H), 7.83 (d,
3JH-H = 8.4 Hz, 2H); 13C NMR (100.5 Hz, CDCl3) δ 23.18, 57.28,
116.42, 120.67 (CF3), 124.96, 125.53, 132.46, 132.71, 138.90,
142.11, 144.78, 146.27, 147.56; 11B NMR (128.2 Hz, CDCl3) δ
57.3. Anal. Calcd for C24H27BF3NO4S2 ([1]OTf-H2O): C, 54.86;
H, 5.18. Found: C, 55.09; H, 5.01.
Reaction of [1]OTf with DMAP. Addition of a solution of
DMAP (14.6 mg, 0.12 mmol) in CHCl3 (2 mL) to a CHCl3
(3 mL) solution of [1]OTf (55.0 mg, 0.11 mmol) resulted in the
formation of a colorless precipitate. After stirring for 30 min, the
mixture was filtered and the solid was washed with Et2O (5 mL)
and dried under vacuum. Recrystallization from acetone/Et2O
vapor diffusion afforded [1-DMAP]OTf as colorless crystals
(53.3 mg, 0.08 mmol, 77%). [1-DMAP]OTf: colorless crystal,
mp 224 °C (dec); 1H NMR (400 Hz, CDCl3) δ 1.73 (s, 6H), 3.00
(s, 6H), 3.69 (s, 9H), 6.34 (br s, 2H), 7.05-7.12 (m, 6H), 7.16 (s,
2H), 7.37 (d, 3JH-H = 10.0 Hz, 2H), 7.54 (br s, 2H); 13C NMR
(100.5 Hz, acetone-d6) δ 24.53, 38.57, 56.58, 106. 48, 118.60,
121.32, 125.00, 125.45, 126.23, 134.30, 137.69, 144.50, 144.80,
146.13, 147.02, 153.53, 155.24; 11B NMR (128.2 Hz, acetone-d6)
δ 3.46. Anal. Calcd for C31H35BF3N3O3S2 ([1-DMAP]OTf-
(acetone)): C, 59.39; H, 6.01. Found: C, 59.19; H, 6.08.
Reaction of [1]OTf with TBAF. Addition of a solution of
TBAF (31.6 mg, 0.10 mmol) in CHCl3 (5 mL) to a CHCl3
solution of 1 (52.8 mg, 0.10 mmol) resulted in the formation of a
colorless precipitate. After stirring for 30 min, the mixture was
filtered and the solid was washed with Et2O (5 mL) and dried under
vacuum. Colorless crystals of 1-F (35.6 mg, 0.09 mmol, 94%) were
obtained by recrystallization using acetone/Et2O vapor diffusion. 1-
F: colorless crystal, mp 372 °C (dec); 1H NMR (400 Hz, CDCl3) δ
2.22 (s, 3H), 2.23 (s, 3H), 3.72 (s, 9H), 6.79 (dd, 3JH-H = 7.2 Hz,
2H), 6.91 (dd, 3JH-H=7.2 Hz, 2H), 7.05 (dd, 3JH-H=7.2 Hz, 2H),
7.13 (d, 3JH-H=8.4 Hz, 2H), 7.15 (s, 2H); 13C NMR (100.5 Hz,
acetonitrile-d3) δ 24.89, 24.96, 57.55, 116.98, 123.19, 123.84, 124.97,
134.57, 136.87, 136.90, 143.44, 144.42 (two signals corresponding to
the ipso-carbons of the boron atom could not be observed due to
broadening by a quadrupole moment of 11B nuclei); 11BNMR(128.2
Hz, acetone-d6) δ 2.68; 19F NMR (375.9 Hz, acetone-d6) δ -159.7.
Anal. Calcd for C24.5H28BFNO0.5S (1-F-0.5(acetone), two mole-
cules of 1-F and one acetone molecule were in the unit cell): C, 72.41;
H, 6.95. Found: C, 72.33; H, 6.85.
Reaction of [1]OTf with KCN. Addition of a solution of KCN
(13.5 mg, 0.21 mmol) in MeOH (10 mL) to a MeOH (10 mL)
solution of [1]OTf (104.9 mg, 0.21 mmol) resulted in the forma-
tion of a colorless precipitate. After stirring for 30 min, the
mixture was filtered and the solid was washed with methanol
and dried under vacuum. Colorless crystals of 1-CN (68.4 mg,
0.18 mmol, 86%) were isolated by recrystallization using acet-
one/hexane vapor diffusion. 1-CN: colorless crystal, mp. 350 °C
(dec); 1H NMR (400 Hz, acetone-d6) δ 2.23 (s, 6H), 3.79 (s, 9H),
6.81 (ddd, 3JH-H=7.2 Hz, 3JH-H=7.2 Hz, 4JH-H=1.2 Hz, 2H),
6.92 (m, 4H), 7.11 (dd, 3JH-H=8.0 Hz, 4JH-H=1.2 Hz, 2H), 7.30
(s, 2H); 13C NMR (100.5 Hz, acetonitrile-d3) δ 25.52, 57.01,
117.52, 117.60, 124.41, 124.80, 125.04, 133.47, 136.48, 136.51,
145.56 (two signals corresponding to the ipso-carbons of the
boron atom could not be observed due to broadening by a
quadrupole moment of 11B nuclei); 11B NMR (128.2 Hz,
acetone-d6) δ -16.58. Anal. Calcd for C25.2H27.4BN2O0.4S (1-
CN-0.4(acetone), approximately 60% of acetone molecules in
the crystalline product were lost upon drying in vacuo): C, 74.26;
H, 6.78. Found: C, 74.21; H, 6.53.
Biphasic Cyanide Capture. A solution of [1]OTf in nitro-
methane-d3 (0.5 mL, 1.6 ꢀ 10-2 M) was layered with a solution
of KCN in D2O (0.5 mL, 6.1 ꢀ 10-2 M), and the sample was
shaken for 30 min. After separation of the layers, the 1H NMR
spectrum of the nitromethane-d3 layer showed 75% conversion
of [1]þ into 1-CN.
Titration of [1]OTf with Fluoride and Cyanide in THF. Solu-
tions of [1]OTf (3.0 mL, 4.8 ꢀ 10-5 M, THF) were titrated with
incremental (5 μL) amounts of fluoride or cyanide anions by
addition of a 3.0 ꢀ 10-3 M solution of KF in MeOH or a 3.8 ꢀ
10-3 M solution of KCN in MeOH.
Acknowledgment. Support by the National Science
Foundation (CHE-0646916 and CHE-0952912), the Welch
Foundation (A-1423), Texas A&M University (Davidson
Professorship), and the Laboratory for Molecular Simula-
tion at Texas A&M University (software and computation
resources) is gratefully acknowledged.
Supporting Information Available: Crystallographic data for
1-F, 1-CN, and [1-DMAP]OTf in cif format. This material is
available free of charge via the Internet at http://pubs.acs.org.