Metal-free catalytic olefin hydrogenation: Low-temperature H2 activation by frustrated lewis pairs

Article abstract of DOI:10.1002/anie.201204007

Weak nucleophiles for strong activation: The reversible activation of dihydrogen by an electron-deficient phosphine, (C₆F ₅)PPh₂, in combination with the Lewis acid B(C ₆F₅)₃ at -80 °C was accomplished. The catalytic hydrogenation of olefins proceeds through protonation and subsequent hydride attack. Electron-deficient phosphines and diarlyamines were demonstrated to be viable Lewis bases for the reaction, thus allowing catalyst loadings of 10 to 5 mol %. Copyright

Full text of DOI:10.1002/anie.201204007

Angewandte
Chemie
DOI: 10.1002/anie.201204007
Olefin Reduction
Metal-free Catalytic Olefin Hydrogenation: Low-Temperature
H2 Activation by Frustrated Lewis Pairs**
Lutz Greb, Pascual OÇa-Burgos, Birgitta Schirmer, Stefan Grimme,* Douglas W. Stephan, and
Jan Paradies*
The hydrogenation of double bonds is one of the most
fundamental transformations[1] in organic chemistry, and has
numerous applications in the commodity chemical, agro-
chemical, pharmaceutical, polymer, and food industries.[2]
Despite significant advances in the last 100 years, efforts to
improve metal-based technologies for hydrogenation are still
the focus of current research.[3] In parallel to these continuing
efforts, metal-free strategies for effecting reductions have also
been pursued. While organic reagents such as Hantschꢀs
esters[4] and silanes[5] have been used as stoichiometric
reducing agents, it was not until 2006[6] that the first metal-
free systems, the so-called frustrated Lewis pairs (FLPs),[7]
were shown to reversibly activate dihydrogen. This discovery
allowed the development of FLP-based catalysts for the
reduction of polar unsaturated bonds such as imines,[8]
nitriles,[8a,c] aziridines,[8a,c] enamines,[8b] silylenolethers,[9] and
aromatic reductions of anilines.[10] Herein, we report the
discovery of FLP systems which, while appearing unreactive
at room temperature, in fact are capable of dihydrogen
activation at temperatures as low as ꢀ808C. This finding was
then exploited for the catalytic hydrogenation of olefins at
temperatures between 25 and 708C. Experimental and
computational data support a plausible mechanism involving
protonation of the olefin with subsequent hydride transfer.
These FLPs represent the first metal-free hydrogenation
catalysts for the reduction of olefins bearing carbocation-
stabilizing moieties.
It is well known that the reactions of olefins with Brønsted
acids in the presence of a nucleophilic halide, leads to addition
products according to a protonation/addition mechanism. In
considering the potential of such a mechanism for FLP
=
hydrogenation of C C double bonds, it was recognized that
while the generated borohydride would act as the nucleo-
phile, this pathway would require the generation of a counter-
cation which was sufficiently acidic to effect protonation of
the olefin. While the majority of FLP activations of dihy-
drogen have been demonstrated for phosphine/borane com-
binations,[7b] a variety of other donors including amines,[8a,11]
pyridines,[12] carbenes,[13] and phosphinimines[14] have been
shown to be effective when paired with boron or aluminum
Lewis acids. However, in all of these cases, the generated
cations are only weak Brønsted acids and thus are incapable
of protonation of olefinic double bonds.
Seeking to enhance the Brønsted acidity of the cation
generated by the FLP activation of dihydrogen, we initiated
investigations employing (C6F5)3B (1) in combination with the
weakly basic phosphine (C6F5)Ph2P (2). An NMR spectro-
scopic examination of a 1:1 mixture of 1 and 2 at 258C
resulted in spectra that did not differ from those of the
individual components. Exposure of this FLP to hydrogen
(5 bar) did not lead to significant changes in the NMR spectra
at room temperature. However, the situation altered when
the temperature was gradually lowered to ꢀ808C. The
31P{1H} NMR signal shifts to lower field upon cooling of the
solution. At ꢀ668C a new resonance appeared at d =
ꢀ12.5 ppm, whereas at ꢀ808C the resonance for free
phosphine completely disappeared, and only one sharp (d =
ꢀ12.5 ppm) and one broad signal (d = ꢀ9 to ꢀ15 ppm)
remained (see the Supporting Information). The broad
signal was attributed to the dynamic formation of the Lewis
pair adduct. More importantly, the resonance at d =
ꢀ12.5 ppm was assigned to the phosphonium species
[(C6F5)Ph2PH]+ as a phosphorus to hydrogen coupling of
J = 531 Hz is observed in the 31P NMR spectrum. The
corresponding 11B and 19F NMR spectra are consistent with
the formation of the phosphonium borate salt [(C6F5)Ph2PH]
[(C6F5)3BH] [Eq. (1)]. Upon heating the sample to ꢀ508C the
system readily released dihydrogen and the initial FLP system
was regenerated. These data clearly demonstrate the rever-
sible activation of hydrogen in the temperature range
between ꢀ608C and ꢀ808C.[15] While previous examples
have been reported to effect reversible dihydrogen uptake
and release at room temperature,[16] the present system
[*] L. Greb, Dr. J. Paradies
Karlsruhe Institute of Technology (KIT), Institute of Organic
Chemistry, Fritz-Haber Weg 6, 76131 Karlsruhe (Germany)
E-mail: jan.paradies@kit.edu
Dr. P. OÇa-Burgos
Karlsruhe Institute of Technology (KIT), Institute of Inorganic
Chemistry, Karlsruhe (Germany)
B. Schirmer
Organisch-Chemisches Institut, Universitꢀt Mꢁnster
Mꢁnster (Germany)
Prof. Dr. S. Grimme
Mulliken Center for Theoretical Chemistry, Institut fꢁr Physikalische
und Theoretische Chemie, Universitꢀt Bonn
Beringstr. 4, 53115 Bonn (Germany)
E-mail: grimme@thch.uni-bonn.de
L. Greb, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
Ontario (Canada)
[**] The LGF, FCI, and AvH are gratefully acknowledged by L.G., J.P.,
and P.O.B. for financial support. J.P. thanks S. Brꢀse and F. Breher
for kind support and fruitful discussions. B.S., S.G., and D.W.S.
gratefully acknowledge the financial support from the DFG-SNF
1175 and NSERC of Canada for the award of a Canada Research
Chair.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204007.
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Table 1: Catalytic olefin hydrogenation by FLPs.[a]
provides, to the best of our knowledge, the lowest barrier for
reversible metal-free hydrogen activation reported to date.
Entry
Olefin
Lewis Base
t [h]
Product
Yield
[%]
1
2
3a
3a
3a
3a
2
5
6
7
24
100
24
4a
4a
4a
4a
99
0
99
95
3
4[b]
12
The extremely low temperature for loss of H2 from
[(C6F5)Ph2PH][HB(C6F5)3] is consistent with the increased
Brønsted acidity of the cation. Thus, this prompted efforts to
exploit this feature for the hydrogenation of olefins. Con-
sequently, a 20 mol% mixture of 1 and 2 were exposed to 1,1-
diphenylethene (3a) under 5 bar of dihydrogen. Hydrogena-
tion of the olefin proceeded smoothly at room temperature
and the saturated product 1,1-diphenylethane (4a) was
provided in quantitative yield (Table 1, entry 1).[17] Other
electron-deficient yet sterically demanding phosphines were
also evaluated in the hydrogenation of 1,1-diphenylethene.
For example, when 20 mol% of the FLP consisting of
(C6F5)2PhP (5) and 1 was applied, the catalysis was completely
ineffective (Table 1, entry 2). This was attributed to the low
nucleophilicity of 5 induced by the two C6F5 substiutents.[18,19]
In marked contrast, when the substituted tris(aryl)phosphines
(2,6-C6H3Cl2)3P (6) and tris(1-naphthyl)phosphine (C10H7)3P
(7) were used in combination with 1 to generate catalysts
(20 mol%), the hydrogenation of 3a proceeded in quantita-
tive yield at either room temperature or 508C (Table 1,
entries 3 and 4). The substrate scope of this unprecedented
metal-free olefin hydrogenation was explored utilizing sub-
strates capable of generating stabilized carbenium ions
(entries 5–14). It was found that not all Lewis bases are
compatible with the substrates. The Brønsted acid catalyzed
dimerization was observed as a competing side reaction,
presumably because of the inefficient hydride transfer.
Hence, the balance between the protonation and efficient
nucleophilic attack is critical. This key requirement was met
by the phosphine 7, which displayed the highest substrate
compatibility. For example, 2-phenylpropene (3b) and 2-
tolylpropene (3c) were hydrogenated in 99 and 85% yields,
respectively (Table 1, entries 5 and 6). The more electron-rich
4-methoxy-substituted styrene derivative 3d was hydrogen-
ated in 30% yield and was accompanied by significant
amounts of the dimerization products (entry 7). However,
the less acidic bisphosphine 8 (GemPhos)[9a,20] proved to be an
efficient Lewis base for the hydrogenation of 3d and provided
the saturated product 4d in quantitative yield within 48 hours.
The reaction of the electron-deficient substrate 3e did not go
to completion even after 100 hours at an elevated temper-
ature (708C) when 7 was employed as Lewis base (entry 8).
These results lead to the conclusion that the reactivity of the
unsaturated compounds is dominated by their propensity for
protonation, thus generating a stabilized carbenium ion.
Consequently, it is conceivable to target specifically selected
substrates based on their nucleophilicity parameter N.
Accordingly, when examining the table of nucleophilicity
parameters from Mayr and co-workers[21] we identified 3 f–i as
potential substrates (N = 1.10 to 4.41). Accordingly, 2-neo-
5[b]
3b
3c
7
7
240
96
4b
4c
96
85
6
7[c]
3d
3d
7
8
24
48
4d
4d
30
99
8[d]
3e
7
100
4e
10
9
10
3 f
3 f
2
7
12
12
4 f
4 f
95
95
11[b]
3g
3h
7 (40 mol%)
24
4g
4h
99
99
12[b]
7
240
13[b]
14
15
16
17
18
3i
3a
3a
3a
3e
3 f
7
9
240
40
4ia
4ib 82 + 8
4a
4a
4a
4e
4 f
80
95
87
95
99
12 (10 mol%) 40
12 (5 mol%) 40
12 (20 mol%) 48
12 (5 mol%) 12
[a] Reactions were performed on a 0.1 mmol scale in CD2Cl2 (0.5 mL,
0.2m) using 20 mol% of the Lewis base and (C6F5)3B (1). Yields were
determined by 1H NMR spectroscopy with the residual solvent signal as
an internal standard. [b] Reactions were performed at 508C. [c] Signifi-
cant amounts of dimerization product were observed (60%). [d] Reac-
tion was performed at 708C.
silylpropene (3 f; N = 4.41)[22] was quantitatively reduced at
room temperature within 12 hours by all catalyst systems
(entries 9 and 10). Cyclopentadiene (3g; N = 2.30)[23] was
quantitatively reduced with a catalyst loading of 40 mol%
within 24 hours (entry 11), and the reduction of 2,3-dimethyl-
2
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butadiene (3h; N = 1.17)[23] required heating to 508C to
achieve 99% yield of 2,3-dimethyl-butene (entry 12). Inter-
estingly, reduction of 2-methyl-butadiene (3i; N = 1.10)[24]
was accomplished in 90% yield, and a mixture of the
products, 3-methyl-butene (4ia) and 2-methyl-butene (4ib)
in a ratio of 10:1 was observed, thus showing that the more
easily accessed double bond is preferably hydrogenated
(entry 13). Substrates featuring an exocyclic double bond,
such as methylenecyclopentane (N = 2.82)[24] or methylene-
cyclohexane (N = 1.66)[23] are efficiently isomerized to the
thermodynamically more stable endocyclic double bond,
which was unreactive towards hydrogenation. Thus, these
data clearly demonstrate that efficient hydrogenation is
dependent on two factors: the nucleophilicity of the double
bond that forms the carbocations and the efficiency of hydride
transfer from the hydridoborate [HB(C6F5)3].
The mechanism of the FLP-catalyzed reduction of 3a was
probed. The similar activities of the catalysts derived from 2,
6, and 7 are consistent with similar degrees of steric
congestion about the P atom and the analogous pKa values
of the corresponding phosphonium cations. These findings
infer a mechanism involving the protonation of the double
bond to generate an transient aryl-stabilized carbocation
prior to hydride attack. Although the interaction of double
bonds with Lewis acids[25] has been described, our control
experiments do not support such an activation in the present
process.[26] To gather further insight into the mechanism we
considered a competition experiment to trap the proposed
transient carbocationic species with a p nucleophile, for
example, an arylamine. Erker, Stephan, and co-workers
have recently demonstrated that aniline derivatives can
effect H2 activation, thus generating [PhNMe2H][HB-
(C6F5)3].[27] Following a similar strategy Ph2NMe (9) was
subjected as a Lewis base to the hydrogenation reaction with
the intention to produce the more acidic ammonium cation
[Ph2NMeH][HB(C6F5)3] (10; Scheme 1). Generation of this
strong Brønsted acid in the presence of olefin 3a results in
a transient aryl-stabilized carbocation, which can then react in
two ways: 1) by addition of the hydride to give the hydro-
genation product 4a, or 2) by electrophilic aromatic substi-
tution, thus furnishing Ph2MeC(C6H4)NMePh (11). Indeed,
combination of 1,1-diphenylethene and 1.2 equivalents of
Ph2NMe with 20 mol% 1 resulted in a 69% yield of the
saturated product, and 31% yield of 11. The interception of
the carbocation strongly supports that the FLP-catalyzed
hydrogenation of the olefin proceeds by protonation followed
by hydride attack. Interestingly, use of 20 mol% Ph2NMe led
to an increase of the hydrogenation product, thus affording 4a
in 80% yield together with 20% of 11 (Table 1, entry 14).
However, by blocking of the para position to inhibit the
undesired electrophilic substitution, the even more active
catalyst pTol2NMe (12) was obtained for the hydrogenation of
3a (entries 15 and 16). The catalyst loading could even be
reduced to 5 mol% with only a marginal loss in yield. As
expected from the low pKa value, 12 was not compatible with
substrates prone to Brønsted acid catalyzed dimerization.
Nevertheless, the electron-deficient styrene derivative 3e was
reduced in quantitative yield even at room temperature
(Table 1, entry 17) and the allylsilane 3d was hydrogenated in
quantitative yield within 12 hours (entry 18).
With these mechanistic indications, quantum chemical
studies were initiated which fully support the experimental
findings. Single-point calculations with the highly accurate
double-hybrid density functional B2PLYP[28] and the D3[29]
dispersion correction were carried out for the first four entries
from Table 1. Thermal and entropic corrections at 298.15 K
and 213.15 K and solvent effects (by COSMO-RS for CH2Cl2)
were taken into account, which led to the free reaction
enthalpies given in Table 2[30] (see the Supporting Information
for details).
Table 2: Calculated (B2PLYP-D3) free energy reaction enthalpies (DG) in
CH2Cl2 (kcalmolꢀ1).[a]
Reaction
2
5
6
7
1+PR3![1][PR3]
5.8
(ꢀ0.9)
1.4
(ꢀ1.6)
5.2
(1.8)
3.3
(0.1)
1+PR3 +H2!
[1H][HPR3]
2.7
(ꢀ0.5)
7.4
(4.9)
3.1
(ꢀ0.1)
ꢀ7.7
(ꢀ10.6)
[1H][HPR3]+3a!
1+PR3 +4a
ꢀ26.9
ꢀ31.6
ꢀ27.2
ꢀ16.9
(ꢀ23.6)
(ꢀ29.3)
(ꢀ24.3)
(ꢀ13.9)
[a] Values in parentheses are DG values in CH2Cl2 at ꢀ608C.
According to these data, both the formation of the Lewis
pair adduct [1][PR3] and of the zwitterionic hydrogen
activation product [1H][HPR3] are endergonic for phosphines
2, 5, and 6 at room temperature. This qualitatively agrees with
the experimental observation that these products are not
detected but the DG values are small enough that the species
can be present in significant amounts under equilibrium
conditions. Decreasing the temperature in the calculations to
ꢀ608C leads to even smaller values, for example, for 2, DG is
computed to be 2.7 kcalmolꢀ1 at 298 K and ꢀ0.5 kcalmolꢀ1 at
213 K. This data supports the findings that H2-activation
products and starting materials are detectable by NMR
spectroscopy only at low temperatures. Furthermore, the
reduction of the olefinic double bond as observed exper-
imentally is computed to be highly exergonic as required for
an efficient catalytic cycle. The reaction of phosphine 7 with
1 and H2 yields a negative DG value even at 298 K and should
therefore be observable. Indeed a 1:1 mixture of 7 and
1 reacted cleanly with H2 to give the activation product
[(C10H7)3PH][HB(C6F5)3]. The hydrogen uptake by 7/1 was
experimentally found to be reversible at 408C in solution.
Scheme 1. Reaction of Ph2NMe (9; 1.2 equiv), B(C6F5)3 (1; 20 mol%),
=
Ph2C CH2 (3a; 1 equiv) and H2 (5 bar) for 12 h.
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[7] a) D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J.
Hastie, S. J. Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden,
G. C. Welch, M. Ullrich, Inorg. Chem. 2011, 50, 12338 – 12348;
b) D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50 – 81;
Angew. Chem. Int. Ed. 2010, 49, 46 – 76; c) D. W. Stephan,
Dalton Trans. 2009, 3129 – 3136; d) D. W. Stephan, Org. Biomol.
Chem. 2008, 6, 1535 – 1539.
[8] a) P. A. Chase, T. Jurca, D. W. Stephan, Chem. Commun. 2008,
1701 – 1703; b) P. Spies, S. Schwendemann, S. Lange, G. Kehr, R.
Frçhlich, G. Erker, Angew. Chem. 2008, 120, 7654 – 7657; Angew.
Chem. Int. Ed. 2008, 47, 7543 – 7546; c) 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.
[9] a) L. Greb, P. OÇa-Burgos, A. Kubas, F. C. Falk, F. Breher, K.
Fink, J. Paradies, Dalton Trans. 2012, 41, 9056 – 9060; b) H. D.
Wang, R. Frçhlich, G. Kehr, G. Erker, Chem. Commun. 2008,
5966 – 5968.
[10] T. Mahdi, Z. M. Heiden, S. Grimme, D. W. Stephan, J. Am.
Chem. Soc. 2012, 134, 4088 – 4091.
[11] a) V. Sumerin, K. Chernichenko, M. Nieger, M. Leskela, B.
Rieger, T. Repo, Adv. Synth. Catal. 2011, 353, 2093 – 2110; b) V.
Sumerin, F. Schulz, M. Nieger, M. Leskela, T. Repo, B. Rieger,
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[12] a) C. B. Caputo, S. J. Geier, D. Winkelhaus, N. W. Mitzel, V. N.
Vukotic, S. J. Loeb, D. W. Stephan, Dalton Trans. 2012, 41, 2131 –
2139; b) S. J. Geier, P. A. Chase, D. W. Stephan, Chem.
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Collectively, these experimental and theoretical data
support a mechanism which proceeds by protonation of the
olefin, followed by hydride delivery. These fundamental steps
are analogous to those originally envisioned for the hydro-
silylation of imines[5b,31] and for the hydrogenation by
FLPs.[8a,c,32] However, the present result demonstrates that
even poorly basic donors are suitable to activate H2 in
combination with a Lewis acid, thus, providing an onium salt
which is sufficiently acidic to protonate the olefin.
In summary, a metal-free catalytic route to the hydro-
genation of olefins has been demonstrated employing an FLP
strategy. This development evolved from the recognition that
the inability to observe the activation of H2 by an FLP at room
temperature does not necessarily imply the absence of
reactivity. This rather misleading situation results from
a remarkably facile hydrogen evolution or back reaction.
Such fast equilibriums also explain that selected FLPs have
not been experimentally observed to split dihydrogen
although the activation was computed to be energetically
favored.[33] Nonetheless, in the presence of substrate, the
transient dihydrogen-activation product is intercepted by an
olefin, thus effecting hydrogenation. The optimization of the
present catalysts as well as the potential and utility of other
FLP systems in olefin hydrogenation and catalysis continues
to be the focus of our efforts.
Received: May 23, 2012
Revised: June 27, 2012
Published online: && &&, &&&&
[13] a) P. A. Chase, A. L. Gille, T. M. Gilbert, D. W. Stephan, Dalton
Trans. 2009, 7179 – 7188; b) D. Holschumacher, C. Taouss, T.
Bannenberg, C. G. Hrib, C. G. Daniliuc, P. G. Jones, M. Tamm,
Dalton Trans. 2009, 6927 – 6929; c) P. A. Chase, D. W. Stephan,
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2008, 47, 7433 – 7437; d) D. Holschumacher, T. Bannenberg,
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[14] C. Jiang, D. W. Stephan, Dalton Trans. 2012, DOI: 10.1039/
C1032DT30720K.
[15] The formation of the phosphonium borate salt is additionally
facilitated by the increased dielectric constant of the solvent at
this temperature. See: I. Fernꢁndez, E. Martꢂnez-Viviente, F.
Breher, P. S. Pregosin, Chem. Eur. J. 2005, 11, 1495 – 1506.
[16] M. Ullrich, A. J. Lough, D. W. Stephan, J. Am. Chem. Soc. 2009,
131, 52 – 53.
Keywords: alkenes · frustrated Lewis pairs · hydrogenation ·
.
olefin · phosphanes
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4
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Angewandte
Chemie
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.
Angewandte
Communications
Communications
Olefin Reduction
L. Greb, P. OÇa-Burgos, B. Schirmer,
S. Grimme,* D. W. Stephan,
J. Paradies*
&&&& — &&&&
Weak nucleophiles for strong activation:
The reversible activation of dihydrogen by
an electron-deficient phosphine,
(C6F5)PPh2, in combination with the Lewis
acid B(C6F5)3 at ꢀ808C was accom-
plished. The catalytic hydrogenation of
olefins proceeds through protonation and
subsequent hydride attack. Electron-defi-
cient phosphines and diarlyamines were
demonstrated to be viable Lewis bases
for the reaction, thus allowing catalyst
loadings of 10 to 5 mol%.
Metal-free Catalytic Olefin
Hydrogenation: Low-Temperature
H2 Activation by Frustrated Lewis Pairs
6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!