Expanding the scope of metal-free catalytic hydrogenation through frustrated Lewis pair design

Article abstract of DOI:10.1002/anie.201001518

No metal causes frustration: Based on a conceptual framework, novel frustrated Lewis acid-base catalyst systems with orthogonal reactivity have been developed. Aside from enhanced functional-group tolerance, unique chemoselectivity can be achieved in catalytic metal-free hydrogenations (see scheme).

Full text of DOI:10.1002/anie.201001518

Angewandte
Chemie
DOI: 10.1002/anie.201001518
Hydrogen Activation
Expanding the Scope of Metal-Free Catalytic Hydrogenation through
Frustrated Lewis Pair Design**
˝
Gꢀbor Eros, Hasan Mehdi, Imre Pꢀpai, Tibor Andrꢀs Rokob, Pꢁter Kirꢀly, Gꢀbor Tꢀrkꢀnyi, and
Tibor Soꢂs*
The further development of the field of catalysis is based on
the discovery, understanding, and implementation of novel
activation modes that allow unprecedented transformations
and open new perspectives in synthetic chemistry. In this
context, the recently introduced concept of frustrated Lewis
pair (FLP) from the Stephan research group represents a
fundamental and novel strategy to develop catalysts based on
main-group elements for small-molecule activation.[1] These
sterically encumbered Lewis acid–base systems are not able
to form a stable donor–acceptor adduct, nevertheless, an
intermolecular association of the Lewis acidic (LA) and basic
(LB) components to a unique “frustrated complex” was
proposed.[2,3] Our research group has also shown that this
encounter pair cleaves hydrogen in a cooperative manner and
the steric congestion implies a strain, which can be directly
utilized for bond activation.[2]
needed for successful catalytic hydrogenation reactions. This
synthetic limitation triggered us to develop FLP catalysts that
have a broader range of applications and possible selectivity
in reduction processes.
Our design concept for increased functional-group toler-
ance is based on the simple hypothesis that steric hindrance in
FLPs is a relative phenomenon (Figure 1): further increase of
Using steric hindrance as a critical design element, several
combinations of bulky Lewis acid–base pairs were effectively
probed for heterolytic cleavage of hydrogen.[4–6] Moreover,
this remarkable capacity of FLPs was exploited in metal-free
hydrogenation procedures.[7] Additionally, the bifunctional
and unquenched nature of the FLPs makes them capable of
reacting with alkenes,[8] dienes,[9] acetylenes,[10] and THF.[5f]
Although this type of reactivity represents a breakthrough in
main-group chemistry, its enhanced and non-orthogonal
nature obviously limits the synthetic applicability of FLPs.
Herein we report an attempt to develop frustrated Lewis pairs
with orthogonal reactivity and improved functional-group
tolerance for catalytic metal-free hydrogenation.
Figure 1. Strategy to develop FLP catalysts with enhanced functional-
group tolerance.
congestion around the boron center in FLP I and its parallel
decrease around the LB could lead to a Lewis pair (FLP II)
that may have a markedly higher tolerance for the function-
alities of common organic molecules. Thus, the steric
demands imposed on the boron center by additional ortho-
aryl substituents are such that they can prevent or markedly
decrease the complexation ability with normal Lewis bases
but still allow the cleavage of the small hydrogen molecule.
Additionally, we assumed that the increased shielding around
boron in FLP II could preclude its addition to olefins,
therefore creating a unique opportunity to investigate the
chemoselectivity of FLP-catalyzed hydrogenations.
The previously reported FLP-based hydrogen activation
relied mostly on tris(pentafluorophenyl)borane[11] (1) as the
LA component.[12] Because of the hard-type Lewis acidity of
boron in 1 and its inactivation by common oxygen- and/or
nitrogen-containing molecules, careful substrate design was
In an effort to realize this concept, we selected mesityl
borane B(C6F5)2(Mes) (2)[13] as a possible bulky Lewis acid for
an improved FLP catalyst for hydrogenation (Figure 1; Mes =
mesityl = 2,4,6-trimethylphenyl).[14] The methyl groups render
the boron center not only less accessible but also less
electrophilic, which results in a lower intrinsic Lewis acidity
than that of perfluorinated borane 1. Furthermore, the steric
factors are also expected to lower the Lewis acidity, because
the ortho-methyl groups engender further increase of the
front and back strain[15] during the complexation of Lewis
bases with the boron center. Nevertheless, owing to the steric
dependence of the front strain, one would assume to exploit
sufficiently high overall Lewis acidity to affect heterolytic
hydrogen splitting.
˝
[*] G. Eros, Dr. H. Mehdi, Dr. T. Soꢀs
Institute of Biomolecular Chemistry, Chemical Research Center of
Hungarian Academy of Sciences
P.O. Box 17, 1525 Budapest (Hungary)
Fax: (+36)1438-1145
E-mail: tibor.soos@chemres.hu
Dr. I. Pꢁpai, Dr. T. A. Rokob, P. Kirꢁly, Dr. G. Tꢁrkꢁnyi
Institute of Structural Chemistry, Chemical Research Center of
Hungarian Academy of Sciences
P.O. Box 17, 1525 Budapest (Hungary)
[**] We are grateful for the financial support from the OTKA (grant
nos. K-69086, K-60549, and NK-77784) and for the grant GVOP-
3.2.1-2004-04-0210/3.0.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001518.
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Besides the optimization of the LA component, we
toward non complexed LA/LB pairs (Table 1, entries 2–5)
according to the NMR studies.[17] However, these systems
proved to be poor catalysts in the model reaction.
wished to apply commercially available amines as possible
LB components. By combining these sterically and electroni-
cally different amines with borane 2, we began to explore
their applicability in metal-free FLP hydrogenation (Table 1).
The objectives of these investigations were twofold: 1) to test
the capacity of these novel FLPs in the hydrogen-splitting
reaction and 2) to document that the formed ammonium
Next, we evaluated the sterically more accessible, planar
quinoline-type bases 10–12. Despite the dative complex
formation, quinoline (10) was able to promote the hydro-
genation (Table 1, entry 6). When applying pairs of borane 2
and bulkier, slightly more basic quinoline derivatives 11, 12,
however, no improvement in the catalytic performance was
observed (Table 1, entries 7 and 8). Finally, the utilization of
small, but relatively basic amines, such as quinuclidine (13)
and DABCO (14), gave encouraging results (Table 1,
entries 9 and 10).[18] In particular, using the less basic
DABCO gave full conversion of imine 3a into amine 4a at
ambient temperature (Table 1, entry 10).
Table 1: Investigation of Lewis pairs in catalytic metal-free hydrogenation
of imine 3a.[a]
In situ NMR investigations of hydrogen-splitting reac-
tions were carried out for selected 2/LB pairs.[19] The exposure
of a solution of the 2/13 pair in [D5]bromobenzene to an
atmosphere of H2 ( ꢀ 4 atm) resulted in the formation of the
presumed ammonium hydridoborate [Eq. (1)]. The obtained
chemical shift of d = ꢁ22.1 ppm in the 10B NMR spectrum is
comparable to that of the related [HB(C6F5)3][4a] and [HB-
(C6F4H)3][4f] systems (d = ꢁ25.5 and ꢁ23.7 ppm, respectively).
Moreover, the resonances from the 19F NMR spectrum are
consistent with the formation of a tetracoordinate anionic
Entry
1
Lewis base (LB)
d10B [ppm][b]
ꢁ4.3[d]
Yield [%][c]
0
5
6
2
3
70.5
5
0
7
70.1[e]
4
5
8
9
70.1
68.3
2
2
6
7
10
11
0.5
2
3
1
borate.[17] Finally, the H NMR spectra featured a diagnostic
1
11
1
ꢁ
resonance of H B at d = 3.89 ppm with a J( B- H) coupling
of 107 Hz.[17] These data are consistent with formulation of 15
as [CH(CH2CH2)3NH]+ [HB(C6F5)2(Mes)]ꢁ.
68.1
8
12
69.3
1
In parallel to their catalytic performance, the 2/8 and 2/13
pairs showed significant differences in the hydrogen-splitting
capacity during in situ NMR experiments. Although 2/8 could
achieve only 10% conversion after 1 hour, the 2/13 pair could
reach 60% conversion in the same time.[17]
9
13
14
67.8
66.1
48
10
100
The above results highlight the importance of the dual
structural and electronic optimization of both LA and LB
components to attain efficient FLP catalysts, and also support
the validity of the original design framework. However, the
principle illustrated in Figure 1 embodies a dilemma con-
cerning the reaction mechanism of hydrogen splitting. The
question is whether the diminished contact area between the
bulky Lewis acid 2 and “tiny” bases (e.g. 13 or 14) is still
enough to form the presumed encounter pair (the frustrated
complex). In other words, whether the attractive intermolec-
ular interactions can ensure the requisite spatiotemporality,[20]
definable time and distance parameters, for the proposed
cooperative cleavage of hydrogen to be operative.[2]
To address this quandary, and rationalize the base-
dependence of reactivity, we performed quantum chemical
calculations for the 2/13 + H2 and 2/8 + H2 systems.[21] It was
reassuring to find that weakly bound complexes with preor-
ganized active centers can be identified for both FLPs, and the
interaction energies of the most stable forms (ꢁ8.4 and
[a] All reactions were performed with 3a (1 mmol), 10 mol% 2 and
added amine 5–14 in 0.75 mL of [D6]benzene under an atmosphere of H2
(ꢀ4 atm) at ambient temperature. [b] 10B NMR measurements were
carried out in [D8]toluene at ꢁ258C, the predicted amount of the major
component is over 95%. [c] Yields were determined by GC analysis.
[d] The Lewis adduct crystallized out slowly. [e] Color was observed as a
result of the presumed a-hydride abstraction as side reaction.
hydridoborate can reduce the imine functionality of substra-
te 3a.[16] Aliphatic amines 5–9 (Table 1, entries 1–5) with
varying steric demand were first evaluated for a possible
catalytic process. The primary amine 5 afforded a donor–
acceptor complex according to the chemical shift observed in
its 10B NMR spectrum (Table 1, entry 1), and this complex
was not able to catalyze the hydrogenation of 3a at ambient
temperature. In contrast, the combination of bulkier amines
6–9 with borane 2 established an equilibrium, which is shifted
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ꢁ7.6 kcalmolꢁ1 for the 2/13 and 2/8 pairs, respectively) are
similar to previously studied systems.[2,3f,i,6a] Nevertheless, the
size difference between the bases has remarkable effects, as
apparent from the potential energy curves with respect to the
B···N distance (Figure 2).
Table 2: Hydrogenation of selected substrates to evaluate functional-group
tolerance and selectivity of FLP catalysts 2/13 or 2/14.[a]
Entry
Substrate
Yield Yield with
Products
with
14 [%]
13 [%]
1
2
3a
3b
81
75
100[b]
4a
4b
98
3
4
3c
3d
73
49
92
16
4c
4d
Figure 2. Potential energy curves computed at the M05-2X/6-31G*
level for the interaction of 2 with bases 13 and 8. Interaction energies
at the minima calculated at the M05-2X/6-311++G** level are shown
in parentheses.
5
3e
72
97
100
4e
6
7
3 f
24 (33)[c] 4 f
4g
The small quinuclidine base (13) affords favorable
interaction with 2 in a broad d(B···N) range, and even the
datively bound 2···13 complex can be identified on the
potential energy surface with binding energy comparable to
that of 2···13. In contrast, the intermolecular interaction
already becomes repulsive at a rather large B···N distance in
2···8, which may hamper efficient cooperative base!s*(H2)
and s(H2)!borane donations, which is identified as a key
feature for the unique reactivity of FLPs. We indeed found, in
accordance with experiments, that the transition state located
for the heterolytic hydrogen splitting with 2/8 lies higher in
energy than that of the 2/13 + H2 reaction (at 0.0 and
ꢁ2.2 kcalmolꢁ1 with respect to the 2 + LB + H2 level).[22,23]
Having identified the most efficient LA/LB combinations
for metal-free hydrogenation, we investigated their scope,
functional-group tolerance, and possible selectivity (Table 2).
First, we chose substrates which were already studied in
metal-free hydrogenations using intramolecular FLP systems
(Table 2, entries 1–4).[5d,7b] The results obtained demonstrate
that the novel intermolecular FLP catalysts (2/13 and 2/14)
show the expected functional-group tolerance. Not only the
methoxy functionality was tolerated (Table 2, entry 2), but
also the sterically less demanding enamine 3c and benzyl
imine 3d underwent hydrogenation, although the efficiency
of these processes was LB dependent (Table 2, entries 3 and
4).
3g
n.d.[d,e] 87[d,f]
1
[a] Yield was determined by H NMR analysis (average of two experiments).
[b] Reaction time was 24 h instead of 42 h. [c] The yield of butyraldimine as a
partially saturated intermediate is given in parentheses. [d] Used 20 mol% of
catalyst the reaction time was 6 days. The yield was determined by GC analysis.
[e] The polymerization of the product occurred. [f] Diastereomeric ratio is 4.3:1
(trans/cis). n.d.=not determined.
crotyl imine 3 f was examined, but both imine and activated
double bond functionalities were saturated in the case of FLP
2/13 catalyst (Table 2, entry 6). However, the efficiency of the
processes again depends on the basicity of the LB constituent.
Using the less efficient 2/14 pair, not only the reaction rate
decreased but also the presence of an intermediate with a
saturated olefinic bond could be detected.[20] Finally, the
reduction of carvone (3g) was probed because it is a
frequently studied test reaction in transition-metal based
catalytic hydrogenation methods (Table 2, entry 7).[24] Con-
trary to the conventional palladium and platinum-catalyzed
hydrogenation, the FLP catalyst 2/14 could selectively reduce
the activated olefinic bonds and afforded dihydrocarvone
(4 f), thus illustrating the power and potential of FLP-
catalyzed hydrogenation. It is also remarkable that neither
olefin migration nor terminal olefin saturation was observed.
=
Next, the hydrogenation of the more challenging allyloxy
substrate 3e was studied (Table 2, entry 5). Similarly to the
imines 3a,b, a smooth reduction to the secondary amine 4e
took place. Most importantly, neither cleavage of the allyl
group nor the FLP addition to the double bond occurred.
Additionally, the reduction of the non-activated double bond
was not observed during the reaction. In an attempt to
achieve chemoselective hydrogenation, the reduction of
It seems plausible that the steric demands around the C O
bonds in the substrates dictates the chemoselectivity of this
metal-free hydrogenation.
In summary, based on a conceptual framework, we have
developed unique frustrated Lewis acid–base catalytic sys-
tems with unprecedented orthogonal reactivity. Moreover, we
identified intermolecular FLP catalysts in which the efficiency
of the hydrogen activation was dictated by the steric effects.
Angew. Chem. Int. Ed. 2010, 49, 6559 –6563
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Communications
Bergander, R. Frꢅhlich, S. Grimme, D. W. Stephan, Chem.
Commun. 2007, 5072 – 5074; c) S. J. Geier, T. M. Gilbert, D. W.
Stephan, J. Am. Chem. Soc. 2008, 130, 12632 – 12633; d) D. P.
Huber, G. Kehr, K. Bergander, R. Frꢅhlich, G. Erker, S. Tanino,
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Chem. Soc. 2009, 131, 52 – 53; g) A. Ramos, A. J. Lough, D. W.
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Kehr, K. Bergander, B. Wibbeling, R. Frꢅhlich, G. Erker, Dalton
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Organometallics 2009, 28, 5233 – 5239.
Aside from orthogonal reactivity, we have also demonstrated
that chemoselectivity can be achieved in catalytic metal-free
hydrogenations. Efforts to broaden the scope of the above
concept and FLP systems are underway and will be reported
in due course.
Experimental Section
General procedure for the catalytic metal-free hydrogenation
employing FLP Systems (2 with 13 or 14): In a glovebox, substrate
(3a–g, 1 mmol), B(C6F5)2(Mes) (2, 50 mg, 0.10 mmol, 10 mol%),
quinuclidine (13, 12 mg, 0.10 mmol, 10 mol%) or DABCO (14, 12 mg,
0.10 mmol, 10 mol%), and dry [D6]benzene (0.75 mL) were placed
into a 55 mL Schlenk bomb equipped with a small magnetic stirrer
bar. The Schlenk bomb was then attached to a double manifold H2/
vacuum line and degassed (freeze-pump-thaw cycle ꢀ 3). The reaction
mixture was cooled in liquid N2 and 1 atm of H2 was introduced. The
flask was sealed and warmed up to RT. The reaction mixture was then
stirred at 500 rpm, at 208C, where the initial H2 pressure is ꢀ 4 atm.
After 42 h the yield was determined by 1H NMR or GC analysis.
[5] Boron–nitrogen Lewis acid–base pairs for cleavage of hydrogen:
a) P. A. Chase, T. Jurca, D. W. Stephan, Chem. Commun. 2008,
1701 – 1703; b) D. Chen, J. Klankermayer, Chem. Commun.
2008, 2130 – 2131; c) V. Sumerin, F. Schulz, M. Nieger, M.
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J. Am. Chem. Soc. 2009, 131, 3454 – 3455; f) S. J. Geier, D. W.
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P. Pyykkꢅ, T. Repo, B. Rieger, J. Organomet. Chem. 2009, 694,
2654 – 2660; h) C. Jiang, O. Blacque, H. Berke, Chem. Commun.
2009, 5518 – 5520.
[6] Boron–carbene Lewis acid–base pairs for cleavage of hydrogen:
a) D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones, M.
Tamm, Angew. Chem. 2008, 120, 7538 – 7542; Angew. Chem. Int.
Ed. 2008, 47, 7428 – 7432; b) P. A. Chase, D. W. Stephan, Angew.
Chem. 2008, 120, 7543 – 7547; Angew. Chem. Int. Ed. 2008, 47,
7433 – 7437; c) P. A. Chase, A. L. Gille, T. M. Gilbert, D. W.
Stephan, Dalton Trans. 2009, 7179 – 7188.
[7] First FLP-type catalytic hydrogenations: a) P. A. Chase, G. C.
Welch, T. Jurca, D. W. Stephan, Angew. Chem. 2007, 119, 8196 –
8199; Angew. Chem. Int. Ed. 2007, 46, 8050 – 8053; additional
examples: b) P. Spies, S. Schwendemann, S. Lange, G. Kehr, R.
Frꢅhlich, G. Erker, Angew. Chem. 2008, 120, 7654 – 7657; Angew.
Chem. Int. Ed. 2008, 47, 7543 – 7546; c) A. E. Ashley, A. L.
Thompson, D. OꢇHare, Angew. Chem. 2009, 121, 10023 – 10027;
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J. A. Labinger, J. E. Bercaw, J. Am. Chem. Soc. 2010, 132, 3301 –
3303.
Received: March 13, 2010
Revised: April 26, 2010
Published online: June 14, 2010
Keywords: amines · boranes · catalytic hydrogenation ·
.
frustrated Lewis pairs · hydrogen activation
[1] Pioneering work: a) G. C. Welch, R. R. San Juan, J. D. Masuda,
D. W. Stephan, Science 2006, 314, 1124 – 1126; most recent
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[2] T. A. Rokob, A. Hamza, A. Stirling, T. Soꢂs, I. Pꢃpai, Angew.
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[8] a) J. S. J. McCahill, G. C. Welch, D. W. Stephan, Angew. Chem.
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[9] M. Ullrich, K. S.-H. Seto, A. J. Lough, D. W. Stephan, Chem.
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[10] a) M. A. Dureen, D. W. Stephan, J. Am. Chem. Soc. 2009, 131,
8396 – 8397; b) C. Jiang, O. Blacque, H. Berke, Organometallics
2010, 29, 125 – 133.
[11] For reviews on B(C6F5)3 and related fluorinated aryl boranes,
see: a) W. E. Piers, Adv. Organomet. Chem. 2004, 52, 1 – 74;
b) G. Erker, Dalton Trans. 2005, 1883 – 1890; for the fundamen-
tal aspects of the steric and electronic factors in perfluoroaryl
borane complexation and catalytic activity, see: c) P. A. Chase,
L. D. Henderson, W. E. Piers, M. Parvez, W. Clegg, M. R. J.
Elsegood, Organometallics 2006, 25, 349 – 357; d) D. M. Morri-
son, W. E. Piers, Org. Lett. 2003, 5, 2857 – 2860.
[12] Related fluorinated boranes were employed by Berkeꢇs group to
probe the Lewis acid tuning in intermolecular FLPs, see
references [4i,5h]. Additionally, intramolecular FLPs were
developed by Erkerꢇs group (Ref. [4b]) and Repo and Riegerꢇs
groups (Ref. [5d]). Both catalyst systems showed excellent
functional-group tolerance, however, their orthogonal reactivity
[3] Additional theoretical works on the reactivity of FLPs: a) Y.
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A. Hamza, T. A. Rokob, I. Pꢃpai, Chem. Commun. 2008, 3148 –
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Privalov, Chem. Eur. J. 2009, 15, 1825 – 1829; e) T. Privalov,
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Stirling, I. Pꢃpai, J. Am. Chem. Soc. 2009, 131, 2029 – 2036; g) T.
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Stirling, T. A. Rokob, I. Pꢃpai, Int. J. Quantum Chem. 2009, 109,
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[4] Boron–phosphorous Lewis acid–base pairs for cleavage of
hydrogen: a) G. C. Welch, D. W. Stephan, J. Am. Chem. Soc.
2007, 129, 1880 – 1881; b) P. Spies, G. Erker, G. Kehr, K.
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is low[8b] or not yet investigated.[5d] For the only selective
hydrogenation of a bulky exotic system with an intramolecular
FLP, see: S. Schwendemann, T. A. Tumay, K. V. Axenov, I.
Peuser, G. Kehr, R. Frꢅhlich, G. Erker, Organometallics 2010,
29, 1067 – 1069.
from the a position. For related works, see: a) F. Focante, P.
Mercandelli, A. Sironi, L. Resconi, Coord. Chem. Rev. 2006, 250,
170 – 188; b) N. Millot, C. C. Santini, B. Fenet, J. M. Basset, Eur.
J. Inorg. Chem. 2002, 3328 – 3335.
[19] Our efforts to crystallize the zwitterionic product of hydrogen
splitting were unsuccessful.
[13] S. A. Cummings, M. Iimura, C. J. Harlan, R. J. Kwaan, I. V.
Trieu, J. R. Norton, B. M. Bridgewater, F. Jꢆkle, A. Sundarara-
man, M. Tilset, Organometallics 2006, 25, 1565 – 1568.
[14] A similar approach to our concept was probed by Repo, Rieger
and co-workers in reference [5g]. Their modified intramolecular
ansa-aminoborane system, however, showed poor hydrogen-
cleavage activity (the time required for hydrogen splitting
changed to one week instead of minutes or hours).
[20] F. M. Menger, Pure Appl. Chem. 2005, 77, 1873 – 1886.
[21] The systems were investigated at three different levels of
computation (M05-2X/6-31G*, M05-2X/6-311 + + G** and
SCS-MP2/cc-pVTZ). Values given in this communication refer
to M05-2X/6-311 + + G** electronic energies, but our conclu-
sions are independent of the applied method (further details are
provided in the Supporting Information).
[15] H. C. Brown, J. Chem. Soc. 1956, 1248 – 1268.
[22] The free-energy barriers computed with respect to the isolated
reactants reflect the same trend (see the Supporting Informa-
tion).
[23] The relative basicities of 13 and 8 cannot account for the
difference in reactivity towards H2: calculated proton-attach-
ment energies are 243.6 and 243.5 kcalmolꢁ1, respectively.
[24] A. F. Barrero, E. J. Alvarez-Manzaneda, R. Chahboun, R.
Meneses, Synlett 1999, 1663 – 1666.
[16] In a preliminary experiment, we verified that substrate 3a itself
could not serve as a Lewis base partner in the hydrogen splitting,
thus we can rule out its catalytic contribution in the studied
hydrogenation processes. Additionally, the reduced product 4a
proved to be a rather ineffective Lewis base, its autocatalytic
contribution is negligible (< 2%).
[17] See the Supporting Information.
[18] Besides having less steric demands, amines 13 and 14 are
presumably more resistant to a possible hydride abstraction
Angew. Chem. Int. Ed. 2010, 49, 6559 –6563
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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