The Journal of Organic Chemistry
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
dx.doi.org/10.1021/jo202127c | J. Org. Chem. 2012, 77, 785−789