A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis

Article abstract of DOI:10.1039/c4sc03675a

This manuscript probes the steric and electronic attributes that lead to "frustrated Lewis pair" (FLP)-type catalysis of imine hydrogenation by borenium ions. Hydride abstraction from (ItBu)HB(C₆F₅)₂ 2 prompts intramolecular C-H bond activation to give (CHN)₂(tBu) (CMe₂CH₂)CB(C₆F₅)₂ 3, defining an upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene-borane (NHC-borane) adducts ((R′CNR)₂C)(HBC₈H₁₄) (R′ = H, R = dipp 4a, Mes 5a, Me 8a; R = Me R′ = Me 9a, Cl, 10a) and ((HC)₂(NMe)(NR)C)(HBC₈H₁₄) (R = tBu, 6a, Ph 7a) are prepared and converted to corresponding borenium salts. These species are evaluated as catalysts for metal-free imine hydrogenation at room temperature. Systematic tuning of the carbene donor for the hydrogenation of archetypal substrate N-benzylidene-tert-butylamine achieves the highest reported turn-over frequencies for FLP-catalyzed hydrogenation at amongst the lowest reported catalyst loadings. The most active NHC-borenium catalyst of this series, derived from 10a, is readily isolable, crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional group-containing imines and N-heterocycles.

Full text of DOI:10.1039/c4sc03675a

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ARTICLE
A Family of N-heterocyclic Carbene-Stabilized
Borenium Ions for Metal-Free Imine Hydrogenation
Catalysis†
Cite this: DOI: 10.1039/x0xx00000x
Jeffrey M. Farrell,a Roy T. Posaratnanathana and Douglas W. Stephana,b*
Received 00th January 2012,
Accepted 00th January 2012
This manuscript probes the steric and electronic attributes that lead to “frustrated Lewis pair” (FLP)-
DOI: 10.1039/x0xx00000x
type catalysis of imine hydrogenation by borenium ions. Hydride abstraction from (I
tBu)HB(C6F5)2 2
www.rsc.org/
prompts intramolecular C-H bond activation to give (CHN)2(tBu)(CMe2CH2)CB(C6F5)2 3, defining an
upper limit of Lewis acidity for FLP hydrogenation catalysis. A series of seven N-heterocyclic carbene-
borane (NHC-borane) adducts ((R’CNR)2C)(HBC8H14) (R’ = H, R = dipp 4a, Mes 5a, Me 8a; R = Me R’ = Me
9a, Cl, 10a) and ((HC)2(NMe)(NR)C)(HBC8H14) (R = tBu, 6a, Ph 7a) are prepared and converted to
corresponding borenium salts. These species are evaluated as catalysts for metal-free imine
hydrogenation at room temperature. Systematic tuning of the carbene donor for the hydrogenation of
archetypal substrate N-benzylidene-tert-butylamine achieves the highest reported turn-over
frequencies for FLP-catalyzed hydrogenation at amongst the lowest reported catalyst loadings. The
most active NHC-borenium catalyst of this series, derived from 10a, is readily isolable,
crystallographically characterized and shown to be effective in the hydrogenation catalysis of functional
group-containing imines and N-heterocycles.
activity relationships. Indeed, although studies involving
Introduction
families of closely related FLP hydrogenation catalysts are
29, 38, 39
rare,42,
some examples in the literature suggests that
As the global consciousness awakens to the environmental
and fiscal costs associated with energy and material-intensive
chemical processes, the development of new and effective
catalytic strategies grows ever more significant. The
hydrogenation of unsaturated bonds is one such chemical
transformation that is employed on a terrific industrial scale.1-4
Currently these processes employ highly effective transition
metal catalysts despite oft associated high cost, toxicity and
significant carbon footprint. These drawbacks have led to the
intense pursuit of alternative or complimentary technologies.
For example hydrogenation catalysis by cheap and non-toxic
subtle changes to FLP catalysts can have dramatic impact on
activity and selectivity. For example, the group of Soós and co-
workers have shown that substitution of one C6F5 in B(C6F5)3
with the bulkier mesityl group effects selectivity control
through size exclusion.34, 35 Moreover, careful choice of Lewis
base in FLP hydrogenation catalysis has extended the substrate
33
scope to include silyl enol ethers,27 olefins19,
and most
iPr
H
R
N
R
NH
R"
NH
B
transition metals such as iron5-8 and cobalt,9, 10 as well as early
N
R'
R'
R"
12
iPr
metals such as titanium11,
and calcium13 has drawn
[B(C6F5)4]
[B(C6F5)4]
considerable attention. Our group14-23 and others24-40 have
focused on main-group alternatives motivated by our report of
metal-free hydrogen activation by a linked phosphino-borane.41
Indeed, soon after this initial report, we described the use of
“frustrated Lewis pairs” (FLPs) in the catalytic hydrogenation
of imines and protected nitriles.23 This prompted a flurry of
developments in FLP hydrogenation catalysis. While substrate
scope has since been dramatically broadened, the catalytic
activities of FLP systems and the catalyst loadings required are
not yet competitive with transition metal catalysts. Perhaps
more importantly, the synthetic challenge of preparing
electrophilic boranes limits the range of potential catalysts that
are readily accessible for a systematic evaluation of structure-
H2
1a
R
H
R
N
N
H
R'
R"
R'
R"
i
Pr
N
B
N
iPr
[B(C6F5)4]
1b
recently ketones and aldehydes.43, 44
Scheme 1. Hydrogenation catalysis by an NHC-borenium ion.
Borenium ions are three-coordinate boron cations.45-47
These relatively underexplored Lewis acids have attracted
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recent attention for use in catalysis48-55 and selective [Ph3C][B(C6F5)4], Me3SiOTf or HOTf showed no reaction. This
carboborations and haloborations.56-66 In an earlier stands in contrast to the facile hydride donation typically
communication, our group showed that an N-heterocyclic demonstrated by NHC-boranes.68, 69 However, upon heating
2
carbene-stabilized borenium salt [(I
(I
iPr)BC8H14] [B(C6F5)4] with HNTf2 in toluene to 100˚C for four days the clean
i
Pr = 1,3-di-iso-propylimidazol-2-ylidene) can be used as a conversion to a new product was evident from the appearance
catalyst for the metal-free hydrogenation of imines and of the 11B resonance at –14.8 ppm. 1H NMR spectroscopy
enamines.67 In this case, the borenium cation and the imine act showed sharp singlet resonances at 0.86 ppm and 1.04 ppm and
as an FLP to cleave H2. This affords an NHC-borane that a broad singlet resonance at 1.80 ppm integrating in a 9: 6: 2
delivers hydride to a transient iminium cation (Scheme 1). ratio. These combined NMR data suggest the new species
Borenium-based catalyst 1b derives its Lewis acidity from a (CHN)2(
cationic charge rather than electron-withdrawing fluoroaryl activation of
groups on boron. Moreover, the precursor NHC-borane adduct crystallographic study of
is robust and easily accessible. During the review process of (b)). This species is similar to compounds
this paper Crudden and coworker described triazolium derived (CHN)2(
borenium cations as catalysts CB(
Bu)Cl71 recently reported by Braunschweig and co-
Herein, we exploit our previous findings to access a family workers. The formation of is thought to proceed via transient
t
Bu)(CMe2CH2)CB(C6F5)2
tert-butyl substituent (Scheme 2).
confirmed its bicyclic nature (Figure
3 is derived from C-H
a
A
3
1
70
t
Bu)(CMe2CH2)CBBr2 and (CHN)2(tBu)(CMe2CH2)
t
3
of FLP catalysts. The reactivity of these borenium cations is generation of a cation and subsequent C-H activation (Scheme
evaluated in the metal-free hydrogenation catalysis of imines 2). Similar borylations of aliphatic groups by donor stabilized
and N-heterocycles. This systematic study of the steric and borenium ions have been reported by Prokofjevs and Vedejs.72
electronic attributes of NHC-borenium catalysts provides
insight into the structure-activity relationship of this new class
of FLP hydrogenation catalyst.
Results and discussion
Our initial efforts to enhance reactivity with respect to
previously reported catalyst 1b focused on the incorporation of
electron withdrawing C6F5 substituents in NHC-borenium ions.
To this end, the reaction of 1,3-di-tert-butylimidazol-2-ylidene
(I
tBu) with HB(C6F5)2 led to the formation of NHC-borane
adduct (I
t
Bu)HB(C6F5)2 (Scheme 2), which was isolated in
2
76% yield. Crystallographic characterization revealed the
anticipated pseudo-tetrahedral geometry about boron, an
average CNHC-B bond length of 1.645(4) Å and an average B-
CC6F5 bond length of 1.639(4) Å (Figure 1 (a)). The expected
Figure 1. POV-ray depiction of (a) 2 and (b) 3. C: black, B: yellow-green, N:
blue, F: pink, H: grey. H-atoms except for borohydride omitted for clarity
.
upfield doublet for
2
is observed by 11B NMR spectroscopy at -
The C-H activation that yields
C6F5-substituted borenium ion derived from
3
suggests that the proposed
is too Lewis
1
1
22.9 ppm with a JB-H coupling of 88 Hz. The H NMR and 19F
2
NMR spectra indicate hindered rotation about the CNHC-B bond
acidic for application in catalysis. This prompted us to further
examine 9-BBN based borenium ions. To this end 9-BBN was
in
2 on the NMR time scale. Broad and inequivalent resonances
were observed for the tert-butyl protons and fluorine atoms at
room temperature; however, these were resolved upon cooling
to -40ºC.
reacted
with
1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-
ylidene (Idipp) at 60ºC for one hour to afford Idipp-borane
adduct 4a in 79% yield (Scheme 3). Compound 4a exhibits a
broad 11B NMR signal at -15.3 ppm. Reaction of 4a with
[Ph3C][B(C6F5)4] at 45ºC overnight results in the generation of
Ph3CH and the quantitative conversion of the NHC-borane to a
new species as evidenced by 11B NMR signals at 82.6 ppm and
-16.6 ppm. These are consistent with the formation the
borenium-borate
Alternatively, treatment of 4a with
addition of a stoichiometric equivalent of [
tBu
tBu
HB(C6F5)2
toluene
N
N
B(C6F5)2
H
N
N
tBu
tBu
salt
[(Idipp)BC8H14][B(C6F5)4]
-BuN=CHPh with the
-Bu3PH]][B(C6F5)4]
4b.
2 (76% yield)
HNTf2, 100 C
4 days
t
-H2
t
results in generation of 4b with concurrent reduction of the
imine as evidenced by H NMR spectroscopy. This observation
1
tBu
N
tBu
N
prompted efforts to employ 4b in an FLP hydrogenation of
t-
B(C6F5)2
H
BuN=CHPh. However combination of excess -BuN=CHPh
t
B(C6F5)2
+ HNTf2
N
N
and 4b under 102 atm H2(g) showed no evidence of reduction of
the imine (Table 1, Entry 2). Thus while the steric bulk of 4a
does not deter hydride delivery, it does preclude H2 activation
by the corresponding borenium/imine FLP.
3
NTf2
To further probe the steric and electronic factors impacting
on the reactivity of NHC-borenium cations, a series of NHC-9-
BBN adducts were prepared exercising judicious variation of
the NHC. This was achieved by either directly reacting 9-BBN
dimer with the isolated carbene or by reacting 9-BBN dimer
with a carbene generated in situ through the combination of an
Scheme 2. Synthesis and reactivity of NHC borane 2.
Attempts to generate an NHC-borenium ion derived from
via treatment with the hydride abstraction reagents
2
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4SCA03R6T75ICALE
imidazolium salt with K[N(SiMe3)2]. This latter one-pot the B center in the cation with a B-CNHC bond length of
approach is similar to that described by Brahmi et al. to prepare 1.5768(3) Å similar to that observed for 1b (1.580(3) Å).67
a series of NHC-BH3 compounds 73
.
A series of seven adducts
including ((R’CNR)2C)HBC8H14 (R’ = H, R = dipp 4a, Mes (Me3P)(HBC8H14) (11) was also synthesized and isolated as
5a 74 Me 8a 50; R = Me R’ = Me 9a, Cl, 10a) and colorless crystals in 82% yield. The 11B NMR signal was
((HC)2(NMe)(NR)C)HBC8H14 (R = Bu, 6a, Ph 7a) were observed at –14.9 ppm and exhibited both B-H coupling of 88
prepared (Scheme 3). The NHC-borane adducts 4a
10a were Hz and B-P coupling of 48 Hz. The 31P{1H} resonance for 11 is
For comparative purposes the phosphine-borane adduct
,
t
-
isolated and purified via recrystallization from pentane or at –13.0 ppm and possesses similar B-P coupling. Single crystal
toluene in yields ranging from 72-95%. The spectral data X-ray diffraction confirmed the formulation (See SI). In
reported for these compounds were as expected and contrast to the carbene complexes described above, treatment of
crystallographic data for 5a, 7a, (See SI) and 8a-10a (Figure 2) 11 with stoichiometric [Ph3C][B(C6F5)4] gave a complex
further corroborated these formulations.
mixture of products as evidenced by 31P{1H} and 11B NMR-
spectroscopy.
Figure 3: POV-ray depiction of 10b C: black, B: yellow-green, N: blue, F:
pink, Cl: green. H-atoms omitted for clarity.
Hydrogenation Catalysis
Compounds 4b-10b were tested for catalytic activity using
the hydrogenation of t-BuN=CHPh as a comparative screen. A
Scheme 3. General syntheses of NHC-boranes 4a-10a and generation of
borenium salts 4b-10b.
solution of each was generated in situ, added to the imine
substrate and pressurized with 102 atm H2(g) for 30 minutes.
After the reaction, the extent of conversion to amine was
assessed by 1H NMR spectroscopy. These data reveal an
inverse correlation between the steric demands of the NHC and
the hydrogenation activity of the borenium catalyst (Table 1).
The bulkiest catalyst 5b shows no catalytic activity, while the
slightly less bulky catalyst 6b shows only trace conversion of
imine to amine at 1 mol% catalyst loading after 30 minutes of
reaction time. A sharp increase of catalytic activity is observed
as the steric demands of the catalyst are further reduced. The
previously reported catalyst 1b allows for 35% conversion to
amine at 1 mol% catalyst loading after 30 minutes while
catalysts 7b and 8b show quantitative conversion.
Reducing the loadings of these catalysts to 0.5 mol% under
otherwise identical conditions reduced the conversions and
demonstrated that the least bulky catalyst 8b effects 68%
conversion while 7b and 9b reach only 35% and 21%
conversion, respectively. In contrast, 10b gave complete
conversion. Even when the loading was dropped to 0.25 mol%
under otherwise identical conditions 10b gave complete
conversion of imine to amine. Further reduction to 0.1 mol%
gave 47% conversion representing a turn-over frequency (TOF)
of 940 h-1. A slight increase of catalyst loading to 0.15 mol%
and an extension of the reaction time to 2 h at room temperature
Figure 2: POV-ray depiction of (a) 8a (b) 9a (c) 10a; C: black, B: yellow-
green, N: blue, H: grey, Cl: green. H-atoms except BH omitted for clarity.
Each of these adducts reacts with [Ph3C][B(C6F5)4] to give
the corresponding borenium salts [((R’CNR)2C)BC8H14]
[B(C6F5)4] (R’ = H, R = dipp 4b, Mes 5b 7576
,
Me 8b; R = Me
R’ = Me 9b, Cl, 10b) and [((HC)2(NMe)(NR)C)HBC8H14]
[B(C6F5)4] (R = Bu, 6b, Ph 7b) concomitant with the
t
generation of a stoichiometric amount of Ph3CH (Scheme 3).
The most diagnostic spectroscopic change in each case is the
appearance of a broad 11B resonance in the range of 81-88 ppm
attributable to a three-coordinate B center. The expected
resonances for the [B(C6F5)4]- anion were seen at -16.7 ppm.
The species 10b was isolated as colorless crystals in 72% yield
via recrystallization from CH2Cl2/pentane at -35˚C.
Crystallographic data revealed trigonal planar geometry about
under 102 atm H2(g) led to complete conversion to
t-
BuNHCH2Ph and the product could be isolated in 83% yield
(Table 1, Entry 13).
These observations reveal that the least sterically
encumbered NHCs stabilize the most active borenium catalysts
despite the fact that FLP reactivity hinges upon the steric
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4SC03675A
protection of an acidic center. This suggests that the bulkier
Comparison of the isosteric catalysts 8b-10b reveals that
catalysts impede either H2 activation or hydride delivery in the reduced donation from the NHC78 to the B center has a positive
catalytic cycle. Since bulky NHC-borane 4a readily delivers impact on the catalytic activity. This is thought to result from
hydride to an iminium ion it seems most likely that the bulkiest an increase in the Lewis acidity at B. That being said, further
borenium ions are sterically prevented from generating the reduction of the donor ability of the stabilizing ligand79
“encounter complex” with the imine that is required for H2 jeopardizes the stability of the borenium cation as evidenced by
activation. Similarly diminished reactivity has been observed the efforts to abstract hydride from 11. Apparently the donor
for FLPs incorporating excessively bulky boranes.34 It is ability and steric demands of the NHC are suitably balanced in
noteworthy that computations suggest that a donor-boron 10b as it provides, to our knowledge, the highest TOF reported
distance of 4.2 Å is necessary to effect heterolytic cleavage of to date for the metal-free hydrogenation of imines. With the
H2.77 Thus, it is reasonable to suggest that bulky peripheral optimized catalyst 10b in hand, a variety of N-containing
substituents inhibit such a close approach.
unsaturated substrates were reduced affording products in high
isolated yields (Table 2). In these cases a catalyst loading of 5
mol% was employed to ensure high conversions in 30 minutes
and to overcome the impact of adventitious water. The imine o-
Table 1. Catalyst screening for the hydrogenation of N-benzylidene-tert-
butylamine.
ClC6H4CH=N
(MeO2C)C6H4CH=N
t
Bu, is readily reduced (Table 2, entry 1) as is p-
Bu (Table 2, entry 2). The latter stands in
t
contrast to previous FLP hydrogenations where sterically
unencumbered esters preclude or inhibit reductions using the
borane B(C6F5)3.20 While the steric demands of
C6H5CH=NCHPh2 slow imine reduction (Table 2, entry 3), the
Entry Cat.(mol%)
%yield[a]
35
0
0
trace
100
35
Entry
8
9
10
11
12
13
Cat.(mol%)
8b (0.5)
9b (0.5)
10b (0.5)
10b (0.25)
10b (0.1)
10b (0.15)
%yield[a]
67
21
100
aniline-derived
EtOC6H4(Me)C=NPh
ketimines
are
Ph(Me)C=NPh
readily hydrogenated
and
(
p-
to
1
2
3
4
5
6
7
1b (1)
4b (5)
5b (1)
6b (1)
7b (1)
7b (0.5)
8b (1)
corresponding amines (Table 2, entries 4 and 5). In stark
contrast, no hydrogenation of Ph(Me)C=NCH2Ph was observed
(Table 2, entry 6). This was attributed to the greater basicity
and lesser steric demands about the N-donor. 1,3,3-trimethyl-2-
methylideneindoline is hydrogenated to afford 1,2,3,3-
tetramethylindoline (Table 2, entry 7), however, 2,3,3-
trimethylindolenine is not reduced (Table 2, entry 8).
100
47
100 (83) [b]
100
Borenium salts were generated in situ by addition of [Ph3C][B(C6F5)4] to the
corresponding borohydride precursor. Isolated 10b was used in entries 11-13.
[a] Determined by 1H NMR spectroscopy, isolated yields in parentheses. All
reaction times were 30 min., except: [b] 2 h reaction time.
Nonetheless, in contrast to 1b 67
10b smoothly catalyzes the
,
hydrogenation of 8-methylquinoline to 1,2,3,4-tetrahydro-8-
methylquinoline (Table 2, entry 9), illustrating the subtlty of
steric and electronic effects on substrate scope.
Table 2. Hydrogenation of N-containing substrates catalyzed by 10b
Conclusions
In this manuscript we have probed the electronic and steric
parameters that impact on the ability of ligand stabilized
borenium cations to act as metal-free hydrogenation catalysts.
Although this catalysis proceeds via an FLP mechanism,
perturbations that enhance the Lewis acidity at B or the steric
demands of the NHC ligand can serve to deactivate the catalyst.
At the same time, sterically unencumbered NHCs bearing
electron withdrawing substituents enhance catalyst activity.
Crudden and coworkers80 have very recently described related
triazolium derived borenium cations and their use as catalysts
for hydrogenation. Nonetheless, the present systematic
examination of NHC stabilized borenium ion has led to
catalysts that are highly efficient. Indeed the isolable catalyst
10b is an effective catalyst for imine and N-heterocycle
reduction at low catalyst loadings and it affords the highest
TOF yet reported for metal-free hydrogenation catalysis.
Efforts are continuing to systematically develop borenium-
based metal-free hydrogenation catalysts and to further broaden
their applications.
Acknowledgements
Yields determined by 1H NMR spectroscopy, isolated yields in parentheses.
All reactions were carried out using 0.500 mmol substrate in CH2Cl2
Reaction times were 30 minutes. Catalyst loadings: 5 mol% except a 2.5
mol%.
The authors acknowledge the financial support from NSERC of
Canada and DWS is grateful for the award of a Canada
Research Chair.
Notes and references
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4SC03675A
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