Spectroscopic and structural characterization of the CyNHC adduct of B 2pin2 in solution and in the solid state

Article abstract of DOI:10.1021/jo202127c

The Lewis base adduct of B₂pin₂ and the NHC (1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed to act as a source of nucleophilic boryl groups in the β-borylation of α,β- unsaturated ketones, has been isolated, and its solid state structure and solution behavior was studied. In solution, the binding is weak, and NMR spectroscopy reveals a rapid exchange of the NHC between the two boron centers. DFT calculations reveal that the exchange involves dissociation and reassociation of NHC rather than an intramolecular process.

Full text of DOI:10.1021/jo202127c

Note
pubs.acs.org/joc
Spectroscopic and Structural Characterization of the CyNHC Adduct
of B2pin2 in Solution and in the Solid State
Christian Kleeberg,† Andrew G. Crawford,† Andrei S. Batsanov,† Paul Hodgkinson,*,† David C. Apperley,†
Man Sing Cheung,‡ Zhenyang Lin,*,‡ and Todd B. Marder*,†
†Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom
‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
S
* Supporting Information
ABSTRACT: The Lewis base adduct of B2pin2 and the NHC
(1,3-bis(cyclohexyl)imidazol-2-ylidene), which was proposed
to act as a source of nucleophilic boryl groups in the β-
borylation of α,β-unsaturated ketones, has been isolated, and
its solid state structure and solution behavior was studied. In
solution, the binding is weak, and NMR spectroscopy reveals a
rapid exchange of the NHC between the two boron centers.
DFT calculations reveal that the exchange involves dissociation
and reassociation of NHC rather than an intramolecular
process.
Sp2−sp3 diboron compounds have been reported previously1
and were postulated as intermediates in both transition metal
and organo-catalyzed borylation reactions.2 However, their
reactivity has neither been studied in great detail, nor have they
been used explicitly as reagents until recently.3 Hoveyda et al.
postulated that a neutral, intermolecular Lewis base adduct of
B2pin2 (1, pin = OCMe2CMe2O) and the N-heterocyclic
carbene CyNHC (2, 1,3-bis(cyclohexyl)imidazol-2-ylidene)
acts as a boron nucleophile in the NHC-catalyzed β-borylation
of α,β-unsaturated ketones4 (Scheme 1), a novel organo-
ppm (sp3-B).7 The report of this intriguing adduct triggered us
to investigate in detail its structural and spectroscopic
properties in solution and in the solid state.
As suggested by Hoveyda et al., the highly Lewis basic
carbene 2 does react with 1, cleanly forming the intermolecular
adduct 3 (Scheme 2, Figure 1), which can be crystallized from
PhMe as a 1:1 solvate.
Scheme 2. Formation of the sp2−sp3 Diboron Compound 3
Scheme 1. NHC-Catalyzed Borylation Reported by Hoveyda
et al.
For comparison, we redetermined the crystal structure of 1 at
low temperature, as the previously reported8a one appears to
contain unresolved disorder resulting in inaccuracies in key
bond lengths.8b,9 The B−B distance in 3 is 0.039 Å longer than
in the low temperature structure of 1 (1.704(1) Å),9 as
expected from the change in hybridization of one of the boron
atoms.1b,c,3 The sp3 boron center B2 exhibits a distorted
tetrahedral geometry, whereas B1 is virtually planar (maximum
deviation from the B1, B2, O1, O2 plane: 0.009(2) Å), with
B1−O1 and B1−O2 distances comparable to those in related
catalytic process distinct from those previously reported
involving Cu-catalysis.2a,5 While DFT calculations suggested
the existence of this adduct, Hoveyda et al. initially reported
only 11B NMR signals at 4.5 and 6.3 ppm as experimental data
supporting its formation.4
The initial 11B NMR data were suspicious, as they are not in
agreement with our own studies on related neutral adducts of
1b,c
B2(O2C6H4)2 and B2(S2C6H4)2 or ionic sp2−sp3 adducts of
16 or with the data reported for the intramolecular adduct
pinacolato diisopropanolaminato diboron.3 Generally, two well-
separated signals are observed at >20 ppm (sp2-B) and <10
Received: November 3, 2011
Published: November 29, 2011
© 2011 American Chemical Society
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Note
leads to the appearance of two signals, resulting from the
tetrahedral (2.4 ppm) and the trigonal-planar (37.2 ppm)
boron sites. This is in agreement with isotropic chemical shifts
of 36 and 2 ppm that we observed in the solid-state 11B NMR
spectrum9 and with the corrected solution NMR data
reported4b by Hoveyda et al. while this work was in progress.
Because of the quadrupolar nature of the 11B nucleus (I = 3/
2, Q = 4.059 × 1026 m2, N.A. = 80.1%), it is subject to
quadrupolar coupling, and the quadrupolar relaxation pathways
typically results in very short transverse relaxation times (T2),
and hence broad line widths. The observation of such broad
signals is further complicated by the presence of borosilicate
glass in standard NMR probe heads and NMR tubes.
Therefore, a very broad background signal (ca. 100 ppm
Figure 1. Molecular structure of 3 (in 3·PhMe). Thermal ellipsoids are
drawn at the 50% probability level; hydrogen atoms and solvent are
omitted for clarity. Selected bond lengths (Å) and angles (°): B1−B2
1.743(2), O1−B1 1.379(2), O2−B1 1.386(2), O3−B2 1.490(2), O4−
B2 1.483(2), C13−B2 1.673(2), O3−B2−O4 104.6(1), O3−B2−C13
107.6(1), O4−B2−C13 111.7(1), O3−B2−B1 116.6(1), O4−B2−B1
111.1(1), C13−B2−B1 105.4(1).
arylboronates.10 The B2−C13 distance of 1.673(2) Å is similar
to that of 1.683(6) Å found in a triethylborane iPrNHC (1,3-
bis(isopropyl)imidazol-2-ylidene) adduct.11,12
The experimental solid-state structure of 3 is in good
agreement with that calculated by DFT methods in the gas
phase by Hoveyda et al.4 Using the simplified model
(MeNHC)B2pin2 (MeNHC = 1,3-bis(methyl)imidazol-2-yli-
dene), we also obtained a very similar optimized geometry
(B3LYP/6-31G*; B1−B2 1.734 Å, O1−B1 1.419 Å, O2−B1
1.407 Å, O3−B2 1.483 Å, O4−B2 1.490 Å, C13−B2 1.671 Å;
gas phase).
Figure 3. 11B{1H} NMR (160 MHz) spectrum of 3 in THF-d8 at 20
°C, a background spectrum under virtually identical conditions and the
difference spectrum. A 10 Hz exponential window function was
included in the processing.
Having verified that adduct 3 can be formed, its 11B NMR
spectrum deserves closer attention, as the data reported initially
by Hoveyda et al. are not in agreement with expected values
(vide supra). The variable temperature 11B NMR spectra give
wide) arising from the glass must be suppressed (Figure 3).
This is typically done by deletion of the first few data points of
the FID, followed by reconstruction using back linear
prediction (LP). This suppresses especially broad signals
from the glass that decay very quickly, but also any genuinely
broad signals resulting from chemical exchange or fast T2
relaxation of the sample.
An alternative approach that suppresses only the boron signal
from the glass is to subtract a background spectrum measured
under identical conditions. Using this method, the spectrum of
3, at 20 °C, shows a very broad, double humped signal at ca. 20
ppm, indicative of being close to the coalescence temperature
for the dynamic process (Figure 3).
Hoveyda et al. proposed the in situ formation of 3 on the
basis of 11B NMR spectroscopic data and DFT calculations.
Sharp 11B NMR signals at 6.3 and 4.5 ppm (solvent not
reported) in a mixture of 1 and 2 were initially assigned to 34
and likely correspond to the unusually sharp signals between 10
and 4 ppm observed here (major signals at 9.0 and 6.6 ppm at
20 °C). These signals, which clearly result from impurities or
decomposition products in the sample, were observed
repeatedly to a varying extent despite taking every precaution
to ensure the purity of the analyzed sample of 3 and the NMR
solvent.13 The integrated intensity of these signals is small
compared to those of 3 (integral ratio 11B NMR at 20 °C:
100:8) and is consistent with a signal in the 1H NMR spectrum
(integral ratio: 100:6 at 20 °C, Figure 4). Additionally, the
chemical shifts and peak widths of these signals contradict all
expectations for an sp2−sp3 diboron compound (vide supra), in
Figure 2. Selected variable temperature 11B{1H} NMR (160 MHz)
spectra of 3 in THF-d8 (∗ impurity, see text). The data were processed
using back linear prediction (LP) of the initial 11 data points and an
exponential window function (10 Hz).
evidence for the dynamic behavior of 3 in solution (Figure 2).
At 50 °C, one averaged 11B signal at 20.4 ppm is detected. At
20 °C, no signal is detected because of the broadening of the
signal close to the coalescence temperature and the applied data
processing (vide infra). Decreasing the temperature to 5 °C
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Note
constant, k, is assumed to follow the Eyring equation: k =
(kbT/h)exp(−ΔG⧧/RT). The set of spectra from −80 to 10 °C
were simultaneously fit to a single set of parameters using the
protocol described in the Supporting Information.9 The fitting
is very satisfactory given the assumptions involved, particularly
the linear extrapolatation of the temperature dependence of the
chemical shifts from the low temperature spectra over the full
temperature range. However, significant deviations are
observed at the higher temperatures, which are consistent
with a partial dissociation of 3, i.e., the single resonance
frequency is a weighted average of the Me shifts in adduct 3 and
free 2. The degree of dissociation, i.e., [3]/[2], is estimated
very approximately to be 5% at 20 °C, on the basis of the
deviation between the expected and observed mean shifts and a
measurement of the Me shift of free 2 (1.18 ppm at 20 °C).
This estimate is extremely sensitive to the extrapolation of the
shift values noted above. The fitted enthalpy and entropy of
activation for the exchange process are: ΔH⧧ = 51.1 0.6 kJ
mol−1 and ΔS⧧ = 2.6 2.4 J K−1 mol−1.
The spectral noise level is too low to limit the accuracy of the
results, and so the 2σ error bars quoted are based on the rms
deviation between fitted and experimental spectra. The
correlation coefficient between the entropy and enthalpy
parameters is extremely high (>0.999), so these individual
values should be treated with caution; the decomposition into
ΔH⧧ and ΔS⧧ contributions is much less reliable than the ΔG⧧
Figure 4. Details of selected variable temperature 1H NMR (500
MHz) spectra of 3 in THF-d8 and simulated spectra. The exchange
rates at each temperature for complexation/decomplexation, k,
calculated from the fitted activation parameters are shown.
value for this type of band shape analysis.15a ΔG⧧
is
298
calculated to be 50.3 kJ mol−1. While the 11B NMR spectra are
unsuitable for detailed line shape analysis, ΔG⧧ is estimated
298
to be ca. 50 kJ mol−1 on the basis of the signal separation at low
temperature (5 °C) and the coalescence temperature (25
°C),15 in excellent agreement with the value obtained via
contrast with the chemical shifts we observe in both solution-
state and solid-state 11B NMR spectra of 3. They are, however,
typical in position and line width of highly symmetric, four-
coordinate boron (i.e., tetrahedral borate with a minimal
electric field gradient at the nucleus) suggesting a species such
as [Bpin2]−. Thus, adopting a procedure reported for related
alkali metal salts of [Bpin2]−,9,14 we synthesized and
characterized spectroscopically and structurally [nBu4P]+
[Bpin2]−. The observed 11B NMR chemical shift of 8.6 ppm
in THF-d8 suggests, considering the different counterions and
the different dielectric constant of the solvent, that the borate
[Bpin2]− is indeed a decomposition product of the adduct 3,
although the nature of the corresponding cation is as yet
unknown.
1
analysis of the H NMR spectra.
To understand better the nature of the dynamic exchange
process, DFT calculations were performed on the simplified
As a quantitative study of 3 in solution via 11B NMR is
difficult, 1H NMR was used to obtain kinetic data of its
dynamic behavior. Figure 4 shows the signals from the
pinacolate methyl groups as a function of temperature. The
spectrum at −80 °C is consistent with the solid state-structure
of 3 (Figure 1). There are three resolved methyl signals (δ =
1.04, 1.03, 0.84 ppm) in a ratio of 12:6:6. The two smaller
signals correspond to the four methyl groups at the tetrahedral
boron atom, two facing toward the CyNHC moiety and the
other two facing away from it. Only one signal is observed for
the pinacolate moiety at the trigonal-planar boron atom,
implying that rotation around the B−B bond is fast even at −80
°C.9 Increasing the temperature, e.g., to −40 °C, leads to a
broadening of the signals and small shifts until only one signal
at an averaged chemical shift is observed (δ = 1.00 ppm at 20
°C), implying equivalence of the pinacolate moieties on the
NMR time scale.
Figure 5. Energy profiles calculated for the association process of
NHC and B2pin2. Relative free energies (ΔG298) and electronic
energies (ΔE298, dotted line) are given in kJ mol−1, and selected bond
lengths and bond angles are given in Å and °, respectively.
model adduct (MeNHC)B2pin2.9 Figure 5 shows the energy
profiles calculated for the association process of NHC and
B2pin2. NHC and B2pin2 form a complex of van der Waals type,
from which overcoming a very small barrier leads to the
formation of the adduct (MeNHC)B2pin2. The small barrier is
related to the pyramidalization process at the boron center to
which the NHC is bonded. From Figure 5, it can be seen that a
transition state corresponding to an intramolecular exchange of
the NHC between the two boron centers could not be located.
Figure 4 also shows the result of modeling this portion of 1H
VT NMR spectra in terms of an exchange process that
interconverts the pinacolate moieties. The exchange rate
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Note
(overlapping with THF signal), 2H), 1.51 (quint., J = 11 Hz, 8H),
Instead, the profiles suggest a dissociation−reassociation
mechanism for the dynamic exchange process.
1
1.28−1.16 (m, 2H), 1.02 (br s, 24H); H NMR (500 MHz; THF-d8,
−10 °C) 7.19 (s, 2H), 5.49 (br s, 2H), 2.03 (br s, 4H), 1.86−1.63 (m
(overlapping with THF signal), 6H), 1.56−1.43 (m, 8H), 1.28−1.16
The computed kinetic values for the adduct dissociation in
the gas phase are ΔH⧧ = 41.8 kJ mol−1, ΔS⧧ = 2.5 J K−1
298
298
1
(m, 2H), 1.01 (v br s, 24H); H NMR (500 MHz; THF-d8, −80 °C)
mol−1, and ΔG⧧298 = 41.1 kJ mol−1, in good agreement with the
experimental values. The computed thermodynamic values for
adduct formation in the gas phase are ΔH298 = −57.4 kJ mol−1,
ΔS298 = −188.4 J K−1 mol−1, and ΔG298 = −1.4 kJ mol−1. The
very small value of ΔG298 suggests that the (MeNHC)B2pin2
adduct is relatively weakly bound. Indeed, solution NMR
spectroscopy (vide supra) shows that the adduct is partially
dissociated at this temperature.
7.44 (s, 2H), 5.42 (t, J = 12 Hz, 2H), 2.00−1.96 (m, 4H), 1.79 (t, J =
12 Hz, 4H), 1.69 (d, J = 12 Hz, 2H), 1.62−1.39 (m, 8H), 1.26−1.14
(m, 2H), 1.04 (s, 12H), 1.03 (s, ov, 6H), 0.84 (s, 6H); 13C{1H} NMR
(125 MHz; THF-d8, RT) δ 169.3, 117.1, 79.7, 56.4, 34.7, 26.7, 26.5,
25.8; 13C{1H} NMR (125 MHz; THF-d8, −85 °C) δ 167.5, 117.7,
83.6, 81.2, 55.8, 34.7, 34.4, 26.4, 26.3, 25.4; 11B{1H} NMR (160 MHz;
THF-d8, 50 °C) δ 20.4 (br); 11B{1H} NMR (160 MHz; THF-d8, 20
°C) no signal detected (see text); 11B{1H} NMR (160 MHz; THF-d8,
5 °C) δ 37.2 (v br s), 2.4 (br s); 11B{1H} NMR (160 MHz; THF-d8,
−85 °C) δ 1.5 (v br s); 13C{1H} solid-state NMR (100 MHz, RT) δ
168.4 (br), 119.4, 114.3, 80.9, 77.6, 56.4, 55.2, 35.3, 34.7, 33.8, 27.5,
27.0, 26.1, 25.1; 11B{1H} solid-state NMR (160 MHz, RT) δ 36, 2.
Found: C, 66.96; H, 10.10; N, 6.10%. Calcd. for C27H48O4N2B2 (3):
C, 66.68; H, 9.95; N, 5.76%.
In conclusion, the existence of the adduct 3 has been verified,
both in solution and in the solid state. Calculated and
experimentally determined thermodynamic data show that 3
is only weakly bound. Kinetic data indicate the presence of a
dynamic equilibrium in THF solution. Theoretical and
experimental studies to develop an understanding of the
mechanism of the borylation of α,β-unsaturated ketones
reported by Hoveyda et al. are the subject of ongoing research.
[nBu4P]+ [Bpin2]−. Pinacol (3.00 g, 25.4 mmol, 2.0 equiv) and
boric acid (0.75 g, 12.7 mmol, 1.0 equiv) were added to water (20
mL). After addition of [nBu4P]OH as a 40% aqueous solution (9.00 g,
12.7 mmol, 1.0 equiv), the mixture was stirred at 75 °C for 16 h. After
removal of the solvent in vacuo, the residue was dissolved in THF (7
mL) and the solution extracted with n-hexane (2 × 10 mL). Upon
standing, crystals separated from the THF layer, which were collected
and recrystallized from hot THF/n-hexane to form crystals suitable for
an X-ray diffraction study. The bulk material was dried in vacuo over
P4O10 for several days: mp 239−244 °C; m/z (ES+) 259 (nBu4P)+,
(ES−) 243 (Bpin2)−; HRMS (ES−) found 242.1827, calcd. for
(C12H2410BO4) 242.1804; found 243.1785, calcd. for (C12H2411BO4)
243.1768; 1H NMR (400 MHz; MeCN-d3, RT) δ 2.08−2.00 (m, 8H),
1.54−1.39 (m, 16H), 0.94 (t, J = 8 Hz, 12H), 0.92 (s, 24H); 13C{1H}
NMR (100 MHz; MeCN-d3, RT) δ 77.1, 26.6, 24.9 (d, J = 15 Hz),
24.3 (d, J = 5 Hz), 19.4 (d, J = 48 Hz), 14.0 (d, J = 1 Hz); 31P{1H}
NMR (162 MHz; MeCN-d3, RT) δ 33.8 (s); 11B{1H} NMR (128
MHz; MeCN-d3, RT) δ 8.6 (s); 11B{1H} NMR (128 MHz; THF-d8,
RT) δ 8.6 (s); 11B{1H} NMR (128 MHz; DMF-d7, RT) δ 8.9 (s).
Found: C, 67.29; H, 12.12; N, 0.00%. Calcd. for C28H60O4B: C, 66.92;
H, 12.03; N, 0.00%.
X-ray Crystallography. Single crystals coated with perfluoropo-
lyether oil were each mounted on a human hair and cooled using an
open-flow N2 gas cryostat. A full sphere of data were collected on a
diffractometer with a CCD detector (Mo Kα radiation, λ = 0.71073 Å,
ω scans, 0.3° wide) using the SMART17a software. The data were
integrated using SAINT,17b and the structures were solved by direct
methods and refined by full-matrix least-squares against F2 of all data
using SHELXTL programs.17c,d All non-hydrogen atoms were refined
with anisotropic displacement parameters and hydrogen atoms were
included in calculated positions and refined using a riding model.
Analyses of the structures and the preparation of graphic were
performed using ORTEP 3 and PLATON.17e−g
EXPERIMENTAL SECTION
■
Unless otherwise noted, all manipulations were performed using
standard Schlenk or glovebox techniques under dry nitrogen. Reagent
grade solvents were nitrogen saturated and were dried and
deoxygenated using a solvent purification system and further
deoxygenated by using the freeze−pump−thaw method. THF-d8 was
distilled from potassium/benzophenone followed by deoxygenation
using the freeze−pump−thaw method. CyNHC was prepared
according to a modification of the literature procedures.16 B2pin2
was kindly provided by AllyChem Co. Ltd. (Dalian, China). All other
commercial reagents were checked for purity by GCMS, elemental
analyses, and/or NMR spectroscopy and used as received. They were,
if appropriate, stored over thoroughly dried 4 Å molecular sieves. The
solution state NMR spectra were recorded at H 500 MHz, 13C 125
1
MHz, 11B 160 MHz or 1H 400 MHz, 13C 100 MHz, 31P 162 MHz, 11
B
1
128 MHz. H NMR chemical shifts are reported relative to TMS and
were referenced via residual proton resonances of the corresponding
deuterated solvent (THF-d8 1.73, 3.58 ppm; MeCN-d3 1.94 ppm;
DMF-d7 2.73, 2.91, 8.01 ppm), whereas 13C NMR spectra are reported
relative to TMS using the carbon signals of the deuterated solvent
(THF-d8 67.6, 25.4 ppm; MeCN-d3 118.3, 1.3 ppm; DMF-d7 30.1,
35.2, 162.7 ppm). The 11B and 31P NMR chemical shifts are reported
relative to external BF3·Et2O and 85% H3PO4 in D2O, respectively. All
13C, 11B, and 31P NMR spectra were recorded with 1H decoupling. Air
sensitive NMR samples were handled under nitrogen using NMR
tubes equipped with Teflon valves. The solid-state magic-angle
spinning (MAS) NMR spectra were recorded at 128.30 MHz for
1
11B and 100.56 MHz for 13C (399.88 MHz for H) using a 4 mm
(rotor o. d.) MAS probe. The 11B spectra were obtained using direct
X-ray data for B2pin2(CyNHC)·PhMe (3·PhMe). C34H56B2N2O4:
Mr = 578.43, crystal size 0.46 × 0.34 × 0.22 mm3, monoclinic, P21/c, a
= 10.3682(3), b = 16.4505(5), c = 19.7145(7) Å, β = 91.028(1)°, V =
3362.0(2) Å3, Z = 4, ρcalcd = 1.143 g/cm3, μ = 0.07 mm−1, T = 120(2)
K, 2θ ≤ 52.0°, 32658 reflections with 6610 unique [R(int) = 0.104],
R1= 0.041 (for 4582 unique data with I > 2σ(I)), wR2 = 0.105 (all
data), max peak/hole = 0.25/−0.23 e−/Å3.
1
excitation with a 30° pulse and H decoupling, with a 0.2 s recycle
delay and at a spin-rate of 10 kHz. 11B isotropic chemical shifts were
estimated by simulating the observed spectrum using the Varian
STARS program. Melting points were determined in flame-sealed
capillaries filled with nitrogen.
B2pin2(CyNHC) (3). Under an atmosphere of nitrogen, 2 (65 mg,
280 μmol) and 1 (71 mg, 280 μmol) were mixed in dry toluene (2
mL). After 20 min, the mixture was concentrated to ca. 0.8 mL and left
at −20 °C to crystallize. The mother liquor was decanted while cold
(−20 °C), and the colorless, X-ray quality single crystals were washed
with 5 mL of cold (−20 °C) n-hexane and dried in vacuo: mp 90−95
°C; m/z (EI) 486 (M+), 471 (M − CH3)+, 422, 403 (M − C6H11)+,
X-ray Data for B2pin2 (1). C12H24B2O4: Mr = 253.93, crystal size
0.8 × 0.7 × 0.3 mm3, monoclinic, P21/c, a = 10.2834(3), b =
7.4809(5), c = 10.1672(8) Å, β = 110.48(1)°, V = 732.7(1) Å3, Z = 2,
ρcalcd = 1.151 g/cm3, μ = 0.081 mm−1, T = 120(2) K, 2θ ≤ 60.0°,
13019 reflections with 2143 unique [R(int) = 0.024], R1= 0.040 (for
1842 unique data with I > 2σ(I)), wR2 = 0.1194 (all data), max peak/
hole = 0.41/−0.16 e−/Å3.
1
359 (M − Bpin)+, 345, 297, 254 (B2pin2)+, 239, 155, 84; H NMR
X-ray Data for [nBu4P]+ [Bpin2]−. C28H60BO4P: Mr = 502.54,
crystal size 0.38 × 0.16 × 0.12 mm3, monoclinic, P2/n, a = 16.6584(4),
b = 10.4920(2), c = 19.4579(4) Å, β = 112.753(1)°, V = 3136.2(1) Å3,
Z = 4, ρcalcd = 1.064 g/cm3, μ = 0.116 mm−1, T = 120(2) K, 2θ ≤
(500 MHz; THF-d8, 20 °C) δ 7.13 (br s, 2H), 5.49 (br s, 2H), 2.06 (br
s, 4H), 1.79 (br s, 4H), 1.68 (br s, 2H), 1.50 (br s, 8H), 1.23 (br s,
2H), 1.00 (br s, 24H); 1H (500 MHz; THF-d8, 50 °C) δ 7.10 (s, 2H),
5.44 (br s, 2H), 2.08 (s, 4H), 1.84−1.76 (m, 4H), 1.73−1.67 (m
788
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Note
52.0°, 40430 reflections with 6161 unique [R(int) = 0.0453], R1=
0.041 (for 4667 unique data with I > 2σ(I)), wR2 = 0.108 (all data),
max peak/hole = 0.36/−0.25 e−/Å3. Crystallographic data for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication
No. CCDC 847767 to 847769. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, U. K. Fax: +44(1223) 336-033; E-mail: deposit@ccdc.cam.ac.uk.
2010, 49, 1194. (j) Mantilli, L.; Mazet, C. ChemCatChem 2010, 2, 501.
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Dang, L.; Wang, J.; Lin, Z.; Marder T. B. manuscript in preparation;
Presented in part at IMEBORON XIV, Niagara Falls, Canada,
September 2011.
(7) Noth, H.; Wrackmeyer, B. NMR Basic Princ. Prog. 1978, 14, 1.
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(8) (a) Noth, H. Z. Naturforsch., B: J. Chem. Sci. 1984, 39, 1463.
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ASSOCIATED CONTENT
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S
* Supporting Information
(9) See Supporting Information for details.
(10) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. Angew. Chem., Int.
Ed. 2009, 41, 5350.
Details of the NMR simulations, 13C and 11B solid-state NMR
spectra, DFT computations, and crystal data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.;
Nakamura, Y.; Kobayashi, K.; Ito, T. Chem. Commun. 2004, 2160.
(12) For selected NHC−B-adducts see: (a) Arduengo, A. J., III;
Davidson, F.; Krafczyk, R.; Marshall, W. J.; Schmutzler, R. Monatsh.
Chem. 2000, 131, 251. (b) Phillips, A. D.; Power, P. P. Acta Crystallogr.
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L.; Malacria, M.; Lacote, E.; Curran, D. P. J. Am. Chem. Soc. 2010, 132,
15072. (d) Braunschweig, H.; Chiu, C.; Radacki, K.; Kupfer, T. Angew.
Chem., Int. Ed. 2010, 49, 2041. (e) Holschumacher, D.; Bannenberg,
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Kupfer, T.; Radacki, K.; Wagner, K. J. Am. Chem. Soc. 2011, No. DOI:
10.1021/ja208372k.
(13) The purities of the samples of isolated 3 used for NMR
experiments were verified by elemental analyses. Furthermore, a solid-
state 13C NMR spectrum confirmed the purity and identity of one
sample.
(14) (a) Dale, J. J. Chem. Soc. 1961, 922. (b) Kalacheva, V. G.; Svarcs,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.hodgkinson@durham.ac.uk (P.H.); chzlin@ust.
hk (Z.L.); todd.marder@durham.ac.uk (T.B.M.).
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ACKNOWLEDGMENTS
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C.K. thanks the Deutsche Forschungsgemeinschaft (DFG) for a
postdoctoral fellowship. T.B.M. thanks AllyChem Co. Ltd. for a
generous gift of B2pin2, the Royal Society for a Wolfson
Research Merit Award, the Alexander von Humboldt
Foundation for a Research Award, EPSRC for an Overseas
Research Travel Grant, and the Royal Society of Chemistry for
a Journals Grant for International Authors. Z.L. thanks the
Research Grants Council of Hong Kong for support (HKUST
603711). We thank the EPSRC National Solid-State NMR
Research Service (Durham) for recording the solid-state NMR
spectra and for valuable discussions, the EPSRC National Mass
Spectrometry Service Centre (Swansea) for mass spectra, and J.
Magee (Durham) for performing the elemental analyses.
(15) (a) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press
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