Hydrogenations at room temperature and atmospheric pressure with mesoionic carbene-stabilized borenium catalysts

Article abstract of DOI:10.1002/anie.201409250

1,2,3-Triazolylidene-based mesoionic carbene boranes have been synthesized in a convenient one-pot protocol from the corresponding 1,2,3-triazolium salts, base, and borane. Borenium ions are obtained by hydride abstraction and serve as catalysts in mild hydrogenation reactions of imines and unsaturated N-heterocycles at ambient pressure and temperature.

Full text of DOI:10.1002/anie.201409250

Angewandte
Chemie
DOI: 10.1002/anie.201409250
Hydrogenation Reactions
Hydrogenations at Room Temperature and Atmospheric Pressure with
Mesoionic Carbene-Stabilized Borenium Catalysts**
Patrick Eisenberger,* Brian P. Bestvater, Eric C. Keske, and Cathleen M. Crudden*
Abstract: 1,2,3-Triazolylidene-based mesoionic carbene bor-
anes have been synthesized in a convenient one-pot protocol
from the corresponding 1,2,3-triazolium salts, base, and
borane. Borenium ions are obtained by hydride abstraction
and serve as catalysts in mild hydrogenation reactions of
imines and unsaturated N-heterocycles at ambient pressure and
temperature.
sary high Lewis acidity to split H2 heterolytically is a signifi-
cant challenge and has delayed the development of new
catalysts in this area.[6]
An interesting alternative is the use of borenium ions,
which have made sporadic appearances in the literature in
other contexts.[7] However, several groups have recently
shown that these species possess interesting and novel
properties as reagents in direct electrophilic aromatic bor-
ylations,[8] the haloboration[9] and carboration of alkynes,[10]
the enantioselective reduction of ketones,[11] and even in
catalytic reduction schemes.[12] Imidazole-based N-heterocy-
clic carbene (NHC) borane adducts provide a particularly
interesting platform for the generation of borenium ions by
hydride abstraction.[11] Building on this work, the Stephan
group reported the use of NHC-stabilized borenium ions as
catalysts for hydrogenation of imines and N-heterocycles at
elevated pressures.[12e]
H
omogeneous transition-metal-catalyzed hydrogenations
have been a key tool in synthetic organic chemistry since their
discovery more than half a century ago.[1] Since that time,
great strides have been made to improve upon the reaction in
terms of chemo- and stereoselectivity as well as functional
group tolerance.[2] Significant mechanistic advances include
the use of frustrated Lewis pairs (FLPs) as catalysts. In FLP-
type hydrogenations, main-group elements conspire to split
H2 heterolytically by the cooperative action of bulky, strong
Lewis acid/base pairs, and the resulting acids/borohydrides
transfer H2 to organic compounds, regenerating the FLP
catalyst (Scheme 1).[3]
1,2,3-Triazolylidenes, the mesoionic cousins of NHCs[13]
can be prepared using a Huisgen cycloaddition between
azides and alkynes, greatly simplifying their synthesis.
Because the carbenic carbon is flanked by only one hetero-
atom, triazolylidenes are electronically unique.[13a] We pre-
viously demonstrated that borane adducts of simple triazole-
derived mesoionic N-heterocyclic carbenes (MIC) have
significantly greater hydricity compared to their NHC con-
geners,[14] which are already known to be among the most
reactive neutral hydride donors.[15] The increased sigma donor
capacity of the mesoionic carbene unit also provides greater
stabilization to the electron-deficient borenium ion. Finally,
the ability to easily access MICs with low steric hindrance is
another important feature of these previously unreported
MIC-based borenium ions.
In their most common incarnation, FLP reductions
employ N-heterocycles or imines as the Lewis base compo-
nents, with B(C6F5)3 as the most commonly employed Lewis
acid.[4] Recent advances have widened the scope of this
reaction to include enantioselective catalysts;[5] however,
preparing analogues of B(C6F5)3 that still possess the neces-
Scheme 1. Basic mechanism for reduction using FLP chemistry.
Herein, we report the first examples of MIC-stabilized
borenium ions, which have unprecedented reactivity in the
hydrogenation of imines and N-heterocycles. These reactions
can be carried out at room temperature and low pressures
without rigorous scrubbing of H2, representing a significant
advantage over typical B(C6F5)3-mediated reductions
(Figure 1).
[*] Dr. P. Eisenberger, B. P. Bestvater, E. C. Keske, Dr. C. M. Crudden
Department of Chemistry, Queen’s University
90 Bader Lane; Kingston, Ontario; K7L 3N5 (Canada)
E-mail: cruddenc@chem.queensu.ca
Dr. C. M. Crudden
Institute of Transformative Bio-Molecules (WPI-ITbM)
Nagoya University
Chikusa, Nagoya 464-8602 (Japan)
[**] The Natural Sciences and Engineering Research Council of Canada
(NSERC) is gratefully acknowledged in terms of operating, equip-
ment, accelerator, and strategic grants to C.M.C. J. Lu and Dr. G.
Schatte are thanked for the single-crystal studies. J. Song is thanked
for experimental contributions and Dr. K. Itami is thanked for useful
suggestions. E.C.K. thanks NSERC for a PGSD scholarship. E.C.K.
and B.P.B. thank Queen’s University for QGS Fellowships.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409250.
Figure 1. Classical versus borenium-based Lewis acids.
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The synthesis of 1,3,4-triphenyl-triazolylidene-5-(9-
borabicyclo[3.3.1]nonane) (2a) was accomplished by in situ
deprotonation of triazolium salt 1a with NaHMDS in the
presence of (9-BBN)2 (9-BBN = 9-borabicyclo(3.3.1)nonane.
This gives 2a directly in high yield under noncryogenic
conditions due to the increased stability of the free carbene
derived from 1a compared to related N-alkyl derivatives
(Scheme 2).
Figure 2. ORTEP-III representation of MIC-9-BBN 2a and 2d. Thermal
ellipsoids are drawn at 50% probability and hydrogen atoms other
than bound to boron are omitted for clarity. Select bond lengths [ꢀ]
and angles [8]: 2a Ccarbene–B 1.641(3), ] C-Ccarbene-N 101.28(18), ꢀ]
Ccarbene 360.0(2); 2d Ccarbene–B 1.616(4), ] C-Ccarbene-N 100.9(2), ꢀ]
Ccarbene 360.0(2).
tal error for 2a) to the values found in 2g (1.640(3) ꢀ,
1.645(3) ꢀ).[12e]
Reaction of 2a with an equimolar amount of Ph3C+B-
(C6F5)4À resulted in complete disappearance of the doublet at
À17.4 ppm and the appearance of a new broad resonance at
82.0 ppm in the 11B NMR spectrum, consistent with the
formation of borenium salt 3a. Similar 11B NMR chemical
shifts have been found for NHC-9-BBN borenium salts IMes-
9-BBN+OTfÀ (dB = 81.4 ppm),[11a] and 3g (dB = 83.8 ppm).[12e]
Hydride abstraction by the tritylium cation proceeds
smoothly and rapidly, independent of the steric crowding
about the boron center.
Scheme 2. Synthesis of MIC-9-BBN adducts and representative bore-
nium ions. Note: The synthesis of 2d was carried out at À788C,
starting from the corresponding triazolium iodide 1d. DiPP=2,6-
diisopropylphenyl.
We next set out to investigate the activity of these
borenium ions in the catalytic hydrogenation of aldimine 4a
and ketimine 4b (Figure 3, Table 1). A convenient in situ
protocol was developed, in which borenium ions were
In 11B NMR spectra, the boron atom of 2a appears as
a doublet centered at À17.4 ppm with a coupling constant of
1JB,H = 71.2 Hz, similar to reported values for IMes-9-BBN
(dB = À16.6 (d, 1JB,H = 59.6 Hz); IMes = 1,3-bis(2,4,6-trime-
thylphenyl)-imidazolylidene)[11b] and IiPr-9-BBN (dB =
À16.64 (d, 1JB,H = 80.0 Hz); IiPr= 1,3-bis(2,6-diisopropyl-
phenyl)-imidazolylidene).[12e] The borohydride signal appears
generated by hydride abstraction using Ph3C+B(C6F5)4
À
prior to addition of the substrate and exposure to H2.
For a direct comparison, two NHC-stabilized borenium
ions, 3g and 3h, were included. As shown in Figure 3, MIC-
1
as a broad resonance in the H NMR spectrum centered at
2.09 ppm. Replacing NaHMDS with the more atom-econom-
ical NaH resulted in similarly efficient formation of 2b in
78% yield. Interestingly, when isolated Na(9-BBN)H2 was
mixed with triazolylium 1b, the expected carbene borane 2b
was not formed.
With these convenient synthetic methods in hand,
a number of sterically and electronically diverse triazolyli-
den-9-BBN adducts 2b–e were synthesized and isolated.
Related borohydride 2 f was also prepared featuring a so-
called abnormal carbene.[16] When asymmetrically substituted
1d was exposed to an adapted protocol developed for our
MIC-borohydride synthesis,[14] 2d was isolated with high
selectivity for the 5-ylidene over the 4-ylidene (> 10:1 by
1H NMR spectroscopy), albeit in moderate yield.
Crystals suitable for single-crystal X-ray diffraction were
obtained by layering a CH2Cl2 solution of the zwitterionic
MIC-borohydrides (2a or 2d) with pentanes at À248C
(Figure 2). Compound 2e was isolated as a single regioisomer.
The metrics about the boron atom range within expected
values for related structures including the expected decreases
in N-Ccarbene-C bond angles (2a: 101.28(18)8, 2d: 100.9(2)8)
À
Figure 3. Catalyst development. Reaction conditions: 0.25 mmol imine
in 0.25 mL CH2Cl2 in a vial inserted into a Parr bomb, 700 rpm stir
rate, 4 h reaction time except for 1 atm runs which were left for 4 or
19 h as indicated.
and characteristic Ccarbene
1.641(3) ꢀ, 2d: 1.616(4) ꢀ), comparable (within experimen-
B single-bond lengths (2a:
2
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Table 1: Hydrogenation of 4b with MIC- and NHC-based borenium ions.
higher catalyst loading (entry 8). Finally, under standard
conditions, B(C6F5)3 gave no reaction.
In terms of substrate scope, the reaction is sensitive to
catalyst inhibition by imines and product amines lacking steric
bulk. Thus, although N-Ph ketimines are effective substrates,
the corresponding N-Bn compounds provide only trace
product. However, the benzhydryl protecting group was
a useful alternative, giving 26% yield at 1 atm, and 90% at
5 atm pressure.[17] To assess functional group compatibility,
the hydrogenation of 4b was run concurrently with 1 equiv of
fenchone and showed 82% product by NMR spectroscopy
and no reduction of fenchone, indicating that the ketone is not
a competitive substrate. However, less hindered methoxycar-
bonyl substituents do inhibit the reduction of imine function-
alities.[17]
Entry[a] Catalyst R1/R2
Loading [%] t [h] Conversion [%][b]
1
2
3
4
5
6
7
8
9
3b
3b
3b
3g
3c
3c
3e
3d
Ph/Ph
Ph/Ph
Ph/Ph
iPr/iPr
anisyl/anisyl
anisyl/anisyl 2.5
DiPP/Ph
Me/Ph
–
10
5
2.5
5
19
23
24
23
100 (87)
100
40
37
5
17.5 100
24
17.5 33
0
5
5
5
6.5
40
0
Next we investigated the ability of MIC-borenium-based
catalysts to reduce N-heterocycles 6a–k (Figure 4). Remark-
B(C6F5)3
17.5
[a] 0.25 mmol imine in 0.25 mL CH2Cl2 in a Schlenk tube, 1 atm H2,
1200 rpm stir rate. [b] Conversion estimated by 1H NMR spectroscopy,
yield of isolated products in brackets.
borenium 3a reduced imine 4a quantitatively in less than 4 h
at 100 atm hydrogen pressure. Decreasing the pressure to
38 atm and then 22 atm under otherwise identical conditions,
gave conversions of 50% and 30%. Isosteric NHC-derivative
3h gave only 16% conversion under 38 atm of hydrogen
illustrating the importance of the electronic effect of the MIC
group and the higher inherent reactivity of MIC-stabilized
borenium ions in this reaction.
To further assess the effect of sterics on the reaction,
compound 3b with only one flanking phenyl group was
examined and found to be the most active of all compounds
tested (Figure 3). Complete reduction of the aldimine was
observed in all cases, although longer reaction times were
required at low pressures. Previously reported NHC-bore-
nium 3g had a reactivity similar to that of MIC-borenium 3b,
but at low conversion the MIC-borane was superior (pale blue
bar, Figure 3). Having developed conditions under which
metal-free hydrogenations can be affected in regular glass-
ware under mild conditions, we turned to the more challeng-
ing ketimine 4b.
Figure 4. Mild hydrogenation of N-heterocycles catalyzed by 3b. Reac-
tions at 1 atm: 6 (0.25 mmol) in CH2Cl2 in a Schlenk tube (0.5–1 m),
1200 rpm stir rate. Reactions at 5 or 102 atm: as above, but performed
in an autoclave, 700 rpm stir rate. Yields of isolated products after
purification by column chromatography; yields in parentheses were
1
determined by H NMR spectroscopy against an internal standard.
To our delight, using 10 mol% 3b under atmospheric
pressure at 258C, the reduction of 4b proceeded to full
conversion in 19 h giving 5b in 87% yield (Table 1, entry 1).
Lowering the catalyst loading to 5 mol% gave full conversion
in less than 24 h, but at 2.5 mol% the conversion was
diminished (entry 3). Comparison with NHC-based borenium
ion 3g (entry 4) illustrated the advantage of the MIC-
stabilized borenium ion 3b, because only 37% conversion
was obtained compared with 100%.
Whereas the p-anisyl-derived borenium 3c gave full
conversion at 5% loading in 17.5 h, it was clearly inferior to
3b at 2.5% loading (entries 3 versus 6). Remote steric effects
also impacted reactivity such that 3e, in which the distal N-
substituent is a diisopropyl phenyl group, showed decreased
reactivity (entry 7). Consistent with this, unsymmetrical MIC-
borenium ion 3d, with a methyl group at the distal position,
has the highest activity observed yet, giving the same yield as
diphenyl derivative 3b in one fourth of the time, albeit at
ably, 2- and 8-arylated quinoline derivatives were hydro-
genated at near ambient pressure and room temperature with
catalyst 3b, giving the corresponding isolated 1,2,3,4-tetrahy-
droquinolines 7a–g in good to excellent yields. Although
unfunctionalized quinoline or 3-arylated quinoline deriva-
tives failed to undergo reduction even at 100 atm of H2, 8-
methyl quinoline (6d) reacted quantitatively at only 5 atm of
H2. The lack of reactivity with less hindered systems is likely
due to strong coordination to the borenium-catalyst impeding
H2 activation.
At elevated H2 pressure (102 atm), even 2,6-disubstituted
pyridine derivatives were reactive giving products 7h–j as
single diastereomers. Despite the higher pressure, this system
is still mild compared to the most effective FLP-type systems
reported to date (1008C, 50 atm H2, 20 h, 10 mol% B-
((CH2)2C6F5)(C6F5)2).[18] 9,10-Phenanthroline was also reac-
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tive giving tetrahydro-9,10-phenanthroline 7k in 47% yield
with no evidence of overreduction. At extended reaction
times, however, reduction of the second heterocycle was
observed.
more active. Steric effects on the MIC have a significant effect
on reactivity, even in distal positions.
Stoichiometric reactions were conducted to better under- Experimental Section
In a glove box, a vial was charged with 2b (8.5 mg, 25 mmol) and
stand the high reactivity of 3b. A modified Gutmann–Beckett
study on carbene-borenium ions 3a, 3b, 3g, and 3h showed
that all had similar inherent Lewis acidity as B(C6F5)3 (3b,
AN = 78.9; 3g, AN = 78.4; B(C6F5)3, AN = 79.8; AN =
acceptor number).[6,17,19] However, reacting MIC-borohydride
2a with B(C6F5)3 led to clean, quantitative hydride transfer to
B(C6F5)3, thereby producing 3a’ and demonstrating the
greater hydride affinity of B(C6F5)3. This observation qual-
itatively explains the observed high activity of MIC-based
borenium ions in hydrogenation catalysis, because these
species have high Lewis acidity as required for H2 activation,
and are still potent hydride donors.
Upon mixing MIC-borenium ion 3a with imine 4b under
catalytic or stoichiometric conditions in the presence of
atmospheric H2, only the borenium ion could be detected,
indicating that hydrogen activation is most likely the turn-
over-limiting step, followed by fast reduction of the resulting
iminium ion by 2a, consistent with the high hydride transfer
ability of 2a. When less hindered borenium ion 3b was
employed, only boronium ion 8b was observed. Thus in this
case, either equilibration to the free borenium or hydrogen
activation would be the slow step. Interestingly, the reduction
product 5b displays minimal interaction with 3b as demon-
strated by 11B NMR spectroscopy.[17] These data support the
mechanism shown in Scheme 3, in which imine 4b and
Ph3C+B(C6F5)4 (22.4 mg, 24 mmol). After adding a stir bar, CH2Cl2
À
(0.1 mL) was added via syringe and the resulting homogenous
mixture was stirred for 2–3 min until the solution turned colorless.
A second vial was charged with N-(1-phenylethylidene)aniline (4b,
49.4 mg, 0.26 mmol), dissolved in CH2Cl2 (0.05 mL) and added to the
catalyst solution. The contents of the vial were transferred to
a Schlenk tube equipped with a teflon-stoppered side-arm, both
vials rinsed with CH2Cl2 (0.1 mL), the tube sealed, and removed from
the glove box. The flask was attached to a Schlenk line with a mercury
bubbler, and the reaction mixture degassed by three freeze–pump–
thaw cycles, backfilled with H2 (supplier: Praxair, 5.0), and sealed.
The combination of a mercury bubbler and back filling with H2 after
FPT likely leads to pressures slightly greater than 1 atm. The reaction
was stirred (1200 rpm) at room temperature for 18 h. The reaction
mixture was subjected to flash column chromatography giving
product 5b. Yield: 44 mg (0.22 mmol, 87%).[17]
Received: September 18, 2014
Published online: && &&, &&&&
Keywords: borenium · catalysis · hydrogenation · Lewis acids ·
.
mesoionic carbenes
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4
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Hydrogenation Reactions
Size does matter after all: Tunable, com-
paratively robust mesoionic borenium
ions catalyze the mild hydrogenation of
N-containing unsaturated organic func-
tionalities at ambient temperature and
ambient pressure. These reactions pro-
ceed through a mechanism reminiscent
of frustrated Lewis pair chemistry.
P. Eisenberger,* B. P. Bestvater,
E. C. Keske,
C. M. Crudden*
&&& — &&&
Hydrogenations at Room Temperature
and Atmospheric Pressure with
Mesoionic Carbene-Stabilized Borenium
Catalysts
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!