RESEARCH FRONT
Carbene-based Lewis Pairs for Hydrogen Activation
1171
Hydrogen Activation using Ph CBF
and a precipitate. After letting the mixture stir for 2 h, Et O was
2
3
4
removed under vacuum and replaced with toluene. The mixture
was filtered to remove the precipitate and toluene was removed
from the filtrate under vacuum to produce a pale-yellow solid
Following the General Method above, carbene 1 (0.100 g,
0
0
.555 mmol) in THF (5 mL) was added to Ph CBF (0.183 g,
3 4
.555 mmol) in THF (5 mL) under pressure of hydrogen. The
(
(
(
1.4 g, 4.6 mmol, 74 % yield). Mp 61–658C. d ([D8]THF) 6.79
H
s, 4H), 5.60 (s, 2H), 4.94 (s, 2H), 2.33 (s, 12H), 2.19 (s, 6H). dC
solids that were obtained from the reaction were washed three
times with toluene (5 mL), which afforded the imidazolium salt
125.5 MHz, [D6]DMSO) 140.28, 137.79, 135.74, 130.12,
1
9
8 (0.075 g, 0.28 mmol, 50 % yield). d (500 MHz, [D6]DMSO)
H
1
.02 (s, 1H), 8.08 (s, 2H), 1.62 (s, 18H). F NMR (470 MHz,
9
117.33, 74.24, 20.85, 19.98.
[
1
D6]DMSO) À148.22 (s). d (125.5 MHz, [D6]DMSO) 132.62,
C
1
1
Synthesis of Aminal 13
20.91, 60.10, 29.52. B NMR (160 MHz, [D6]DMSO) À1.44
(s). HCPh was isolated from the toluene phase and characterized
by H and C NMR and TLC comparisons against an authentic
Imidazolium salt 12 (1.52 g, 4.07 mmol) was added as a solid
portionwise to LiAlH (0.174 g, 5.40 mmol) dissolved in Et O.
3
1
13
4
2
sample (R 0.84 in 60:40 hexane/ethyl acetate).
f
An immediate reaction was observed with the formation of a
white precipitate. After allowing the mixture stir for 3 h, Et O
2
was removed from the filtrate under vacuum and replaced with
benzene. The mixture was filtered to remove the precipitate and
the toluene was removed under vacuum to produce a white solid
(1.2 g, 3.5 mmol, 85 % yield). X-ray-quality crystals were grown
by cooling a saturated toluene/hexamethyldisiloxane solution to
Measurement of Imidazolium Borohydride 6 Equilibrium
Solid NaHB(OMe) (0.030 g, 0.024 mmol) was added portion-
3
wise to a solution of imidazolium chloride 7 (0.051 g,
0
.024 mmol) in [D3]acetonitrile (0.7 mL) in an NMR tube
within the dry-box. After letting the mixture sit for 2 h at room
temperature, the NMR was taken for analysis. The resulting
mixture contained 8 % aminal 8 in acetonitrile.
À258C. Mp 205–2088C. d ([D8]THF) 5.46 (s, 2H, HC=C), 4.26
H
(s, 2H, N CH ),2.03 (brs, 6H, C CH), 1.68 (brs, 12H, C CH ),
2 2 3 2 2
1
HC=C), 4.45 (s, 2H, N CH ), 1.93 (brs, 6H, C CH), 1.70 (brs,
.67 (br (unresolved ab), 12H, C CH ). d (C D ) 5.60 (s, 2H,
2 2 H 6 6
Imidazolium Borohydride 6: d (500 MHz, [D3]acetonitrile)
H
2
2
3
9
(
.26 (s, 1H), 7.74 (s, 2H), 1.69 (s, 18H), 1.67 (s, 9H), À0.15
1
q, 1H, J
12H, C CH ), 1.52 (br (unresolved ab), 12H, C CH ). d
2 2 2 2 C
81.3). d (125 MHz, [D3]acetonitrile) 132.89,
B–H C
1
1
15.66, 60.39, 48.01 (br), 25.95. B NMR (160 MHz, [D3]
(125.5 MHz, [D8]THF) 114.26, 63.58, 53.41, 40.37, 37.72,
1
1
acetonitrile) 3.08 (br, J
5
1
30.67. N NMR (30.414 MHz, [D8]THF) À293.47. (Anal. calc.
unresolved).
B–H
for: C 81.60, H 10.12, N 8.28. Found: C 81.72, H 10.01, N
8
.23 %.)
Synthesis of Aminal 8
Solid LiAlH (0.524 g, 13.8 mmol) was slowly added portion-
4
Hydrogen Gas Reduction of Triphenylmethanol to
Triphenylmethane
wise to imidazolium salt 7 (3.000 g, 13.80 mmol) in THF
(20 mL) via weigh-paper within the dry-box. The mixture was
˚
Dry Ph COH (0.250 g, 0.960 mmol) and 3-A molecular sieves
3
left to stir at room temperature overnight before it was filtered
through Celite under nitrogen. Removal of THF under vacuum
resulted in a clear colourless oil and white precipitate. The
mixture was extracted with hexane to afford a clear colourless
oil (1.755 g, 9.626 mmol, 70 % yield). dH (500 MHz, [D6]
(
3.0 g) in dry Et O (2 mL) were placed into a Schlenk flask
2
equipped with a stir bar and septum. Next, HBF4(aq) (48 %,
.124 mL, 0.960 mmol) was added to the mixture via syringe.
0
The white precipitate turned yellow on addition. The mixture
was left to stir for 2 h at room temperature before it was cooled to
À708C. Vacuum was used to remove nitrogen from the system
and the atmosphere was replaced with hydrogen via a hydrogen
balloon. After stirring the mixture for 30 min, carbene 1
DMSO) 5.49 (s, 2H), 4.09 (s, 2H), 1.02 (s, 18H). d (125.5 MHz,
C
[D6]DMSO) 116.24, 67.47, 52.59, 26.89.
Synthesis of Imidazolium Borohydride 9
(0.208 g, 1.15 mmol) in dry Et O (3 mL) was added to the
2
A solution of sodium borohydride (0.143 g, 3.77 mmol) in ace-
tonitrile (10 mL) was added to a stirred suspension of 1,3-
dimethylimidazolium chloride (0.5 g, 4 mmol) in acetonitrile
reaction via syringe. The reaction mixture turned cloudy from
the light-brown precipitate that was formed. After stirring for
30 min at À708C, the reaction was left to warm to room tem-
perature over 2 h before the solids were collected by filtration
and dried under vacuum. Proton NMR and comparison with an
(50 mL). A fine white precipitate was observed during addition.
After stirring overnight at room temperature, all solids were
removed from the solution by filtration and a white solid was
collected by the removal of acetonitrile under vacuum. The
white solid (0.37 g, 88 %) was recrystallized from a THF/tolu-
ene solution at À208C. X-ray-quality crystals were grown by
cooling an acetonitrile solution of 9 layered with toluene to
authentic sample by TLC (40 % ethyl acetate/hexane) con-
1
COH ( H NMR d 7.22–7.03 ppm
firmed the presence of both Ph
(m, 15H), 2.49 (s, 1H). R
7.27 (m, 15H), 5.47 (s, 1H). R
3
1
0.68) and Ph
CH ( H NMR d 7.52–
3
f
0.82) in a 6:1 ratio, as well as the
f
imidazolium tetrafluoroborate salt 18.
À208C for 24 h. d (500 MHz, [D6]DMSO) 9.08 (s, 1H), 7.68 (s,
H
1
2
H), 3.84 (s, 6H), À0.28 (q, 4H, J
81.4). d (500 MHz, [D3]
H
High-pressure Hydrogen Activation using Fe(CO)5
B–H
acetonitrile) 8.64 (s, 1H), 7.33 (s, 2H), 3.83 (s, 6H), À0.21 (q,
Carbene 1 (0.540 g, 3.00 mmol) in THF (0.7 mL) was sealed in a
glass ampoule under a nitrogen atmosphere. Within the dry-box,
the sealed ampoule was carefully placed in a steel bomb con-
1
H, J
4
3
8
81.4). d (125.5 MHz, [D3]acetonitrile) 138.2, 124.8,
B–H C
1
1
1
7.2. B NMR (160 MHz, [D6]DMSO) À37.0 (quin, J
B–H
1.4).
taining Fe(CO) (0.710 g, 3.62 mmol) in THF (3 mL) and a loose
5
steel bar that could be used to break the ampoule with agitation
once the entire vessel was sealed and pressurized. The bomb was
sealed with a needle valve and taken outside the dry-box, where
it was connected to a hydrogen cylinder and a Schlenk line using
a three-way vacuum adaptor and high-pressure tubing. The
Synthesis of Aminal 11
Imidazolium salt 10 (2.120 g, 6.20 mmol) was added as a solid to
LiAlH (0.240 g, 6.32 mmol) dissolved in Et O. An immediate
reaction was observed with the formation of a yellow solution
4
2