Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes

Article abstract of DOI:10.1021/acs.orglett.1c01073

Two strategies were reported to prevent the deactivation of Frustrated Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh3) with high stability was designed and synthesized. Excellent conversion (up to 99%) and selectivity can be achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh3. This catalytic system works quite well for different substrates. In addition, the P-BPh3 can be easily recycled.

Full text of DOI:10.1021/acs.orglett.1c01073

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Controlling the Lewis Acidity and Polymerizing Effectively Prevent
Frustrated Lewis Pairs from Deactivation in the Hydrogenation of
Terminal Alkynes
Qiang Liu, Liu Yang, Chenfei Yao, Jiao Geng,* Youting Wu,* and Xingbang Hu*
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ABSTRACT: Two strategies were reported to prevent the deactivation of Frustrated
Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity
and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh3) with high stability was
designed and synthesized. Excellent conversion (up to 99%) and selectivity can be
achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh3. This catalytic
system works quite well for different substrates. In addition, the P-BPh3 can be easily
recycled.
or a long time, it has been generally believed that the
activation of hydrogen must involve the participation of
formation of alkynyl borates and retain the ability to split H2 in
the presence of Lewis bases, thus avoiding the deactivation of
FLPs in the hydrogenation.
F
metals,1 especially precious metals.2 In 2006, a metal-free
hydrogen activation method, called frustrated Lewis pairs
(FLPs) catalytic hydrogenation, was first proposed by Stephan
et al.3 FLPs consist of a Lewis base (an electron donor) and a
Lewis acid (an electron acceptor) with steric hindrance.
Subsequently, such a metal-free catalytic system has been
widely studied.1,4
On the basis of a theoretical analysis on the H2 activation
ability and binding energies between different Lewis acids and
terminal alkynes using the B3LYP-D3(BJ)/6-311+G* method
including solvent influence (Supporting Information (SI),
Figures S1 and S2), triphenylboron (BPh3) was chosen as a
potential compound to investigate the possibility of terminal
alkynes hydrogenation catalyzed by FLPs. Although the Lewis
acidity of BPh3 is weaker than that of B(C6F5)3, theoretical
calculations on the H2 activation by BPh3/Py (Py: pyridine)
and B(C6F5)3/Py Lewis pairs gives close energy barriers for H2
splitting (26.2 and 27.2 kcal/mol, respectively). The H−H
distances are 0.949 and 0.807 Å in the transition states TSBPh3
and TSB(C6F5)3, respectively (Figure 1a). The later one is quite
close to those using classical FLPs, such as 0.79 Å for
B(C6F5)3/(tBu)3P,16 0.787 Å for B(C6F5)3/KHCO3,14d and
0.85 Å for Mes2P-C2H4-B(C6F5)2.17
Since the concept of FLPs was proposed, FLPs chemistry
has been widely used in the hydrogen activation5 and the
hydrogenation of many different unsaturated compounds, such
as imines,6 enamines,4f,7 olefins,4c,8 poly aromatics,9 internal
alkynes,10 ketones,11 aldehydes,9a,12 silyl-enol ethers,13 and
CO2.5i,14 Although many successes have been achieved for the
catalytic hydrogenation promoted by FLPs, the terminal alkyne
is an exception. Ansa-aminohydroborane, which showed
excellent reactivity for the hydrogenation of internal alkynes
(up to 100% conversion), has no any catalytic ability for the
hydrogenation of terminal alkynes.10a The pyridone borane
complex showed some catalytic reactivity for terminal alkynes
with low conversion.10d Much experimental and theoretical
researches revealed that the Lewis acids used for the classical
FLPs, such as tris(pentafluorophenyl)borane (B(C6F5)3),
could react with the terminal alkynes via deprotonative
borylation pathway, producing a stable alkynyl borates in
which the terminal carbon of alkynes tightly binds with the
boron of these Lewis acids.10a,15 As a result, the FLPs are
deactivated for the hydrogenation. We rationalize that a borane
compound with reasonable Lewis acidity should prevent the
More importantly, the Gibbs free energy changes to form
the B−C bond are 13.9 and −6.1 kcal/mol for BPh3+Ph−C
CH+Py and B(C6F5)3+Ph−CCH+Py systems, respectively
Received: March 29, 2021
Published: April 20, 2021
https://doi.org/10.1021/acs.orglett.1c01073
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3685−3690
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and Ph-CCH also shows two peaks at −13.9 and −16.4
ppm, which correspond to the products of 1,2-addition and
C−H deprotonation, respectively.5e,15b When using BPh3 as
Lewis acid, the situation is totally different. After adding Ph-
CCH, the 11B signal of BPh3 does not have any shift (Figure
2.2). It suggests that BPh3 does not bind with Ph-CCH. No
matter whether there is Ph-CCH or not, adding Py results in
the shift of 11B signal from 68.7 to 4.2 ppm due to the
formation of BPh3/Py adduct. Even the samples were treated
at 120 °C for 12 h, and the same results can be obtained (SI,
Figure S3). However, when Py was replaced by a stronger base,
such as N(CH2CH2NPri)3P, it could induce deprotonation of
Ph-CCH and [N(CH2CH2NPri)3PH][PhCC-BPh3] was
formed, which exhibited the 11B chemical shift at −13 ppm.15j
Above theoretical and experimental results reveal that, being
different from classical FLPs, BPh3/Py may serve as a catalyst
for the hydrogenation of terminal alkynes. Activation barrier of
H2 splitting for TSBPh3 is a little high, indicating that a higher
temperature is necessary for the cleavage of H2. It is interesting
to find that 11.6% conversion from phenylacetylene to styrene
and ethylbenzene can be obtained in the presence of BPh3/Py
at 120 °C in 12 h (Scheme 1). As expected, B(C6F5)3/Py
Figure 1. Optimized structures of (a) transition states of H2 activation
and (b) BPh3/B(C6F5)3+Ph−CCH+Py complexes. The italic
values are (a) activation energies and (b) Gibbs free energy changes
in kcal/mol. The other values are distances in Å. See SI for calculation
details.
Scheme 1. Terminal Alkynes Hydrogenation Catalyzed by
B(C6F5)3/Py and BPh3/Py
(Figure 1b), indicating that the formation of alkynylborates
between BPh3 and terminal alkyne is highly unfavorable,
whereas that between B(C6F5)3 and terminal alkyne is
obviously exothermic. These theoretical results were further
confirmed by NMR experiments. The 11B signal in the NMR
spectra of B(C6F5)3 and BPh3 appear at 57.5 and 68.7 ppm,
respectively, which are consistent with the literature data.18
When equivalent molar of phenylacetylene (Ph-CCH) was
mixed with B(C6F5)3, the 11B signal shifted to −12.0 ppm
(Figure 2.1), due to the formation of the B−C bond between
cannot promote the reaction at all under the same conditions.
The BPh3/Py system is not stable during the reaction (SI,
Figure S9), which may be responsible for the low conversion of
hydrogenation.
To enhance the stability of BPh3 and provide an example for
recyclable FLPs, a polymeric-BPh3 (P-BPh3) was designed and
synthesized. Alkane was introduced to polymerize with BPh3
by the Friedel−Crafts reaction at room temperature (Scheme
2). The P-BPh3 was characterized by solid-stated NMR, TGA,
XPS, SEM, XRD, and FT-IR (see SI for details). Both the
signals of aromatic carbons, methylene carbons, and boron can
Scheme 2. Synthesis of P-BPh3
Figure 2. 11B spectra of (1) B(C6F5)3 and (2) BPh3 in the presence of
equivalent Py, Ph-CCH, and Py/Ph-CCH, respectively, in
CDCl3.
B(C6F5)3 and the Ph-CCH. In addition, because of the
isomerization of 1,1-carboboration reactions between B(C6F5)3
and the Ph-CCH, a corresponding doublet was observed
(Figure 2.1).15i Even though the Lewis acid−base interaction
between B(C6F5)3 and Py is strong18c (11B signal for
B(C6F5)3/Py adduct: −4.2 ppm), adding Py into the
B(C6F5)3/Ph-CCH system cannot break the B−C bond of
the alkynylborates. The 11B NMR of the mixture of BCF, Py,
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be clearly observed in the solid-state NMR (Figure 3). All of
them show close chemical shifts to those in the liquid NMR,
Table 1. Substrate Scope of Alkynes Hydrogenation
Catalyzed by P-BPh3/Py
a
Figure 3. (1) Solid-state 13C NMR spectrum of P-BPh3 and (2) solid-
state 11B NMR spectrum of P-BPh3/Py. Insets: liquid 13C NMR
spectrum of (a) BPh3 and (b) dichloroethane in CDCl3, and (c)
liquid 11B NMR spectrum of BPh3/Py in CDCl3.
indicating that the polymerization happened and the three-
coordinate structure of B is maintained. Besides NMR, the C−
C bond of the linker in P-BPh3 can also be evidenced by XPS
(SI, Figure S4) and FT-IR (SI, Figure S5). The content of B in
P-BPh3 is 4.4 wt %, determined by EDX of SEM (SI, Table
S1). It is worth noticing that the polymerization with
dichloroethane extremely enhances the stability of P-BPh3.
The decomposition temperature of P-BPh3 is higher than 450
°C, determined by TGA (SI, Figure S8). On the contrary, slow
degradation is observed below 150 °C on the TGA of BPh3.
It is exciting to find that 99% conversion of 4-ethyl-
phenylacetylene is achieved when P-BPh3 was used as the
Lewis acid of FLPs at 120 °C (entry 1, SI, Table S2). Lewis
bases, as the important part of FLPs for H2 splitting, have
crucial influence on the hydrogenation of terminal alkynes.
Reactions with both pyridine and methyl substituted pyridine
give excellent conversions (entries 1−3). However, electro-
philic substitution, which weakens the electron-donor ability of
pyridine, results in poor bases for the hydrogenation (entries 4
and 5). Although bases with steric hindrance are favorable for
H2 splitting according to the concept of FLPs,2−4 this is not
the case for the hydrogenation of terminal alkynes (entries 6−
8), suggesting that steric hindrance may prevent the access of
alkynes.
By using P-BPh3/Py as the catalytic system, we further
explored the substrate scope of terminal alkynes hydrogenation
promoted by this unusual FLPs (Table 1). The investigated
substrates include both aromatic and nonaromatic alkynes. For
all the aromatic alkynes (entries 1−11), whether it is an
electron donor or acceptor modification on the aromatic ring,
more than 99% conversions of alkynes were obtained with 99%
selectivity of alkene and alkane. Comparing with the aromatic
alkynes, the aliphatic alkynes are less reactive (entries 12−18).
Especially, when propargyl amine was used as substrate, no
hydrogenation happened, which may be the result of strong
interaction between the −NH2 group and Lewis acid center.
The recyclability and long-term stability of the catalysts are
vital for practical application in industry. Although FLPs have
achieved great successes for H2 activation, recyclable FLPs
a
Reaction conditions: alkynes (1 mmol), P-BPh3 (10 wt % of the
substrate); base (0.5 mmol); solvent toluene (1 mL); pressure of H2,
b
c
5 MPa, 120 °C, 12 h. Determined by GC and GC-MS. The isolate
yield of alkenes (molar ratio).
were rarely reported.19 P-BPh3 provides a simple example of
recyclable and highly stable Lewis acid for FLPs. After the
hydrogenation of alkynes, the P-BPh3 can be recovered by easy
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Authors
solid−liquid separation. Without further treatment, the P-BPh3
was reused 12 times and an obvious loss of catalytic activity
was observed (Figure 4)due to the high stability of P-BPh3.
Qiang Liu − School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
Liu Yang − School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
Chenfei Yao − School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01073
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by National Natural Science
Foundation of China (no. 21878141) and Jiangsu Province
NSF (BE2019095).
REFERENCES
■
Figure 4. Reusability of P-BPh3 in the hydrogenation of phenyl-
acetylene.
(1) Zhang, L.; Zhou, M.; Wang, A.; Zhang, T. Selective
hydrogenation over supported metal catalysts: from nanoparticles to
single atoms. Chem. Rev. 2020, 120, 683−733.
(2) Wesselbaum, S.; Hintermair, U.; Leitner, W. Continuous-flow
hydrogenation of carbon dioxide to pure formic acid using an
integrated scCO2 process with immobilized catalyst and base. Angew.
Chem., Int. Ed. 2012, 51, 8585−8588.
In summary, we have successfully developed a highly stable
polymeric Lewis acid (P-BPh3) for effective metal-free
hydrogenation of terminal alkynes. In the past, such a
hydrogenation reaction is impossible for classical FLPs due
to the tight bond between the Lewis acid and terminal alkynes,
making the FLPs deactivated.10a,15 In the presence of P-BPh3/
Py, excellent conversion and selectivity can be achieved for the
hydrogenation of terminal alkynes with different functional
groups. In addition, because P-BPh3 is extremely stable with
decomposition temperature being higher than 450 °C, its
catalytic activity remains quite good after recycling 12 times.
This work demonstrates the importance of a rational Lewis
acidity and polymerization for avoiding the deactivation of
FLPs. The simple synthesis route, high stability, and reactivity
of P-BPh3 may provide a new way for FLPs application.
(3) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Reversible, metal-free hydrogen activation. Science 2006, 314, 1124−
1126.
(4) (a) Paradies, J. Metal-free hydrogenation of unsaturated
hydrocarbons employing molecular hydrogen. Angew. Chem., Int. Ed.
2014, 53, 3552−3557. (b) Stephan, D. W. A Perspective on
Frustrated Lewis Pairs. J. Am. Chem. Soc. 2015, 137, 10018−10032.
(c) Stephan, D. W. Frustrated Lewis Pairs. J. Am. Chem. Soc. 2015,
137, 10018−10032. (d) Stephan, D. W.; Erker, G. Frustrated Lewis
Pair Chemistry: Development and Perspectives. Angew. Chem., Int. Ed.
2015, 54, 6400−6441. (e) Stephan, D. W. Frustrated Lewis pairs:
from concept to catalysis. Acc. Chem. Res. 2015, 48, 306−316. (f) Liu,
L.; Cao, L. L.; Shao, Y.; Ménard, G.; Stephan, D. W. A Radical
Mechanism for Frustrated Lewis Pair Reactivity. Chem. 2017, 3, 259−
267. (g) Scott, D. J.; Fuchter, M. J.; Ashley, A. E. Designing effective
’frustrated Lewis pair’ hydrogenation catalysts. Chem. Soc. Rev. 2017,
46, 5689−5700. (h) Lam, J.; Szkop, K. M.; Mosaferi, E.; Stephan, D.
W. FLP catalysis: main group hydrogenations of organic unsaturated
substrates. Chem. Soc. Rev. 2019, 48, 3592−3612. (i) Carden, J. L.;
Dasgupta, A.; Melen, R. L. Halogenated triarylboranes: synthesis,
properties and applications in catalysis. Chem. Soc. Rev. 2020, 49,
1706−1725.
(5) (a) Chase, P. A.; Stephan, D. W. Hydrogen and amine activation
by a frustrated Lewis pair of a bulky N-heterocyclic carbene and
B(C6F5)3. Angew. Chem., Int. Ed. 2008, 47, 7433−7437. (b) Holschu-
macher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M.
Heterolytic dihydrogen activation by a frustrated carbene-borane
Lewis pair. Angew. Chem., Int. Ed. 2008, 47, 7428−7432. (c) Geier, S.
J.; Stephan, D. W. Lutidine/B(C6F5)3: At the Boundary of Classical
and Frustrated Lewis Pair Reactivity. J. Am. Chem. Soc. 2009, 131,
3476−3477. (d) Rokob, T. A.; Hamza, A.; Papai, I. Rationalizing the
Reactivity of Frustrated Lewis Pairs: Thermodynamics of H2
Activation and the Role of Acid-Base Properties. J. Am. Chem. Soc.
2009, 131, 10701−10710. (e) Momming, C. M.; Fromel, S.; Kehr, G.;
Frohlich, R.; Grimme, S.; Erker, G. Reactions of an Intramolecular
Frustrated Lewis Pair with Unsaturated Substrates: Evidence for a
Concerted Olefin Addition Reaction. J. Am. Chem. Soc. 2009, 131,
12280−12289. (f) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. The
mechanism of dihydrogen activation by frustrated Lewis pairs
revisited. Angew. Chem., Int. Ed. 2010, 49, 1402−1405. (g) Lu, Z.;
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01073.
Experimental and theoretical calculation details, original
spectra, and Cartesian coordinates of the optimized
structures (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Jiao Geng − School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China;
Email: gengjiao@nju.edu.cn
Youting Wu − School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China; Email: ytwu@
nju.edu.cn
Xingbang Hu − School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, China;
orcid.org/0000-0002-2995-9704; Email: huxb@
nju.edu.cn
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Cheng, Z.; Chen, Z.; Weng, L.; Li, Z. H.; Wang, H. Heterolytic
cleavage of dihydrogen by ″frustrated Lewis pairs″ comprising
bis(2,4,6-tris(trifluoromethyl)phenyl)borane and amines: stepwise
versus concerted mechanism. Angew. Chem., Int. Ed. 2011, 50,
12227−12231. (h) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J.; de
Kanter, F. J.; Lutz, M.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K.
Preorganized frustrated Lewis pairs. J. Am. Chem. Soc. 2012, 134,
201−204. (i) Sajid, M.; Kehr, G.; Wiegand, T.; Eckert, H.;
Schwickert, C.; Pottgen, R.; Cardenas, A. J.; Warren, T. H.;
Frohlich, R.; Daniliuc, C. G.; Erker, G. Noninteracting, vicinal
frustrated P/B-Lewis pair at the norbornane framework: synthesis,
characterization, and reactions. J. Am. Chem. Soc. 2013, 135, 8882−
8895. (j) Wilson, D. W. N.; Feld, J.; Goicoechea, J. M. A Phosphanyl-
Phosphagallene that Functions as a Frustrated Lewis Pair. Angew.
Chem., Int. Ed. 2020, 59, 20914−20918.
(6) (a) Stephan, D. W.; Erker, G. Frustrated Lewis Pairs: Metal-free
Hydrogen Activation and More. Angew. Chem., Int. Ed. 2010, 49, 46−
76. (b) Liu, Y.; Du, H. Chiral dienes as ″ligands″ for borane-catalyzed
metal-free asymmetric hydrogenation of imines. J. Am. Chem. Soc.
2013, 135, 6810−6813. (c) Sitte, N. A.; Bursch, M.; Grimme, S.;
Paradies, J. Frustrated Lewis Pair Catalyzed Hydrogenation of
Amides: Halides as Active Lewis Base in the Metal-Free Hydrogen
Activation. J. Am. Chem. Soc. 2019, 141, 159−162.
(7) (a) Lindqvist, M.; Borre, K.; Axenov, K.; Kotai, B.; Nieger, M.;
Leskela, M.; Papai, I.; Repo, T. Chiral molecular tweezers: synthesis
and reactivity in asymmetric hydrogenation. J. Am. Chem. Soc. 2015,
137, 4038−4041.
(8) (a) Ines, B.; Palomas, D.; Holle, S.; Steinberg, S.; Nicasio, J. A.;
Alcarazo, M. Metal-free hydrogenation of electron-poor allenes and
alkenes. Angew. Chem., Int. Ed. 2012, 51, 12367−12369. (b) Hasen-
beck, M.; Ahles, S.; Averdunk, A.; Becker, J.; Gellrich, U. Formation of
Nucleophilic Allylboranes from Molecular Hydrogen and Allenes
Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair
Reactivity. Angew. Chem., Int. Ed. 2020, 59, 23885.
Chem., Int. Ed. 2009, 48, 9839−9843. (b) Courtemanche, M. A.;
Legare, M. A.; Maron, L.; Fontaine, F. G. Reducing CO2 to Methanol
Using Frustrated Lewis Pairs: On the Mechanism of Phosphine-
Borane-Mediated Hydroboration of CO2. J. Am. Chem. Soc. 2014,
136, 10708−10717. (c) Romero, E. A.; Zhao, T.; Nakano, R.; Hu, X.;
Wu, Y.; Jazzar, R.; Bertrand, G. Tandem copper hydride−Lewis pair
catalysed reduction of carbon dioxide into formate with dihydrogen.
Nat. Catal. 2018, 1, 743−747. (d) Zhao, T.; Hu, X.; Wu, Y.; Zhang, Z.
Hydrogenation of CO2 to Formate with H2: Transition Metal Free
Catalyst Based on a Lewis Pair. Angew. Chem., Int. Ed. 2019, 58, 722−
726.
(15) (a) Anaya de Parrodi, C.; Walsh, P. J. Terminal Alkyne
Activation by Frustrated and Classical Lewis Acid/Phosphine Pairs.
Angew. Chem., Int. Ed. 2009, 48, 4679−4682. (b) Dureen, M. A.;
Stephan, D. W. Terminal Alkyne Activation by Frustrated and
Classical Lewis Acid/Phosphine Pairs. J. Am. Chem. Soc. 2009, 131,
8396−8397. (c) Momming, C. M.; Kehr, G.; Wibbeling, B.; Frohlich,
R.; Schirmer, B.; Grimme, S.; Erker, G. Formation of cyclic allenes
and cumulenes by cooperative addition of frustrated Lewis pairs to
conjugated enynes and diynes. Angew. Chem., Int. Ed. 2010, 49, 2414−
2417. (d) Freitag, S.; Henning, J.; Schubert, H.; Wesemann, L.
Phosphastannirane: a phosphorus/tin(II) Lewis pair that undergoes
alkyne and alkene addition. Angew. Chem., Int. Ed. 2013, 52, 5640−
5643. (e) Lawson, J. R.; Clark, E. R.; Cade, I. A.; Solomon, S. A.;
Ingleson, M. J. Haloboration of internal alkynes with boronium and
borenium cations as a route to tetrasubstituted alkenes. Angew. Chem.,
Int. Ed. 2013, 52, 7518−7522. (f) Wang, X.; Kehr, G.; Daniliuc, C. G.;
Erker, G. Internal adduct formation of active intramolecular C4-
bridged frustrated phosphane/borane Lewis pairs. J. Am. Chem. Soc.
2014, 136, 3293−3303. (g) Hansmann, M. M.; Melen, R. L.;
Rudolph, M.; Rominger, F.; Wadepohl, H.; Stephan, D. W.; Hashmi,
A. S. Cyclopropanation/Carboboration Reactions of Enynes with
B(C6F5)3. J. Am. Chem. Soc. 2015, 137, 15469−15477. (h) Ma, Y.;
Lou, S.; Luo, G.; Luo, Y.; Zhan, G.; Nishiura, M.; Luo, Y.; Hou, Z. M.
B(C6F5)3/Amine-Catalyzed C-H Silylation of Terminal Alkynes with
Hydrosilanes: Experimental and Theoretical Studies. Angew. Chem.,
Int. Ed. 2018, 57, 15222−15226. (i) Bismuto, A.; Nichol, G. S.;
Duarte, F.; Cowley, M. J.; Thomas, S. P. Characterization of the
Zwitterionic Intermediate in 1,1-Carboboration of Alkynes. Angew.
Chem., Int. Ed. 2020, 59, 12731−12735. (j) Brar, A.; Mummadi, S.;
Unruh, D. K.; Krempner, C. Verkade Base in FLP Chemistry-From
Stoichiometric C-H Bond Cleavage to the Catalytic Dimerization of
Alkynes. Organometallics 2020, 39, 4307−4311.
(16) Rokob, T. A.; Hamza, A.; Stirling, A.; Soos, T.; Papai, I.
Turning frustration into bond activation: a theoretical mechanistic
study on heterolytic hydrogen splitting by frustrated Lewis pairs.
Angew. Chem., Int. Ed. 2008, 47, 2435−2438.
(17) Rokob, T. A.; Bako, I.; Stirling, A.; Hamza, A.; Papai, I.
Reactivity models of hydrogen activation by frustrated Lewis pairs:
synergistic electron transfers or polarization by electric field. J. Am.
Chem. Soc. 2013, 135, 4425−4437.
(18) (a) Blasing, K.; Bresien, J.; Labbow, R.; Michalik, D.; Schulz,
A.; Thomas, M.; Villinger, A. Borane Adducts of Hydrazoic Acid and
Organic Azides: Intermediates for the Formation of Aminoboranes.
Angew. Chem., Int. Ed. 2019, 58, 6540−6544. (b) Santi, M.; Ould, D.
M. C.; Wenz, J.; Soltani, Y.; Melen, R. L.; Wirth, T. Metal-Free
Tandem Rearrangement/Lactonization: Access to 3,3-Disubstituted
Benzofuran-2-(3H)-ones. Angew. Chem., Int. Ed. 2019, 58, 7861−
7865. (c) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar,
A. K. Lewis Acidic Borane Adducts of Pyridines and Stilbazoles for
Nonlinear Optics. Chem. Mater. 1998, 10, 1355−1365.
(19) (a) Niu, Z.; Bhagya Gunatilleke, W. D. C.; Sun, Q.; Lan, P. C.;
Perman, J.; Ma, J.-G.; Cheng, Y.; Aguila, B.; Ma, S. Metal-Organic
Framework Anchored with a Lewis Pair as a New Paradigm for
Catalysis. Chem. 2018, 4, 2587−2599. (b) Shyshkanov, S.; Nguyen, T.
N.; Ebrahim, F. M.; Stylianou, K. C.; Dyson, P. J. In Situ Formation of
Frustrated Lewis Pairs in a Water-Tolerant Metal-Organic Framework
for the Transformation of CO2. Angew. Chem., Int. Ed. 2019, 58,
(9) (a) Mahdi, T.; Heiden, Z. M.; Grimme, S.; Stephan, D. W.
Metal-Free Aromatic Hydrogenation: Aniline to Cyclohexyl-amine
Derivatives. J. Am. Chem. Soc. 2012, 134, 4088−4091. (b) Li, G.; Liu,
Y.; Du, H. B(C6F5)3-catalyzed metal-free hydrogenation of naphthyl-
amines. Org. Biomol. Chem. 2015, 13, 2875−2878.
(10) (a) Chernichenko, K.; Madarasz, A.; Papai, I.; Nieger, M.;
Leskela, M.; Repo, T. A frustrated-Lewis-pair approach to catalytic
reduction of alkynes to cis-alkenes. Nat. Chem. 2013, 5, 718−723.
(b) Liu, Y.; Hu, L.; Chen, H.; Du, H. An alkene-promoted borane-
catalyzed highly stereoselective hydrogenation of alkynes to give Z-
and E-alkenes. Chem. - Eur. J. 2015, 21, 3495−3501. (c) Chernichen-
ko, K.; Kotai, B.; Nieger, M.; Heikkinen, S.; Papai, I.; Repo, T.
Replacing C6F5 groups with Cl and H atoms in frustrated Lewis pairs:
H2 additions and catalytic hydrogenations. Dalton Trans 2017, 46,
2263−2269. (d) Wech, F.; Hasenbeck, M.; Gellrich, U. Semi-
hydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex:
Frustrated Lewis Pair Reactivity and Boron−Ligand Cooperation in
Concert. Chem. - Eur. J. 2020, 26, 13445−13450.
(11) (a) Scott, D. J.; Fuchter, M. J.; Ashley, A. E. Nonmetal
Catalyzed Hydrogenation of Carbonyl Compounds. J. Am. Chem. Soc.
2014, 136, 15813−15816. (b) Gao, B.; Feng, X.; Meng, W.; Du, H.
Asymmetric Hydrogenation of Ketones and Enones with Chiral Lewis
Base Derived Frustrated Lewis Pairs. Angew. Chem., Int. Ed. 2020, 59,
4498−4504.
(12) Liu, Y. B.; Du, H. F. Metal-Free Borane-Catalyzed Highly
Stereoselective Hydrogenation of Pyridines. J. Am. Chem. Soc. 2013,
135, 12968−12971.
(13) (a) Wei, S.; Du, H. A highly enantioselective hydrogenation of
silyl enol ethers catalyzed by chiral frustrated Lewis pairs. J. Am. Chem.
Soc. 2014, 136, 12261−12264. (b) Ren, X.; Li, G.; Wei, S.; Du, H.
Facile development of chiral alkenylboranes from chiral diynes for
asymmetric hydrogenation of silyl enol ethers. Org. Lett. 2015, 17,
990−993.
(14) (a) Ashley, A. E.; Thompson, A. L.; O’Hare, D. Non-Metal-
Mediated Homogeneous Hydrogenation of CO2 to CH3OH. Angew.
3689
https://doi.org/10.1021/acs.orglett.1c01073
Org. Lett. 2021, 23, 3685−3690

Organic Letters
pubs.acs.org/OrgLett
Letter
5371−5375. (d) Meng, Q.; Huang, Y.; Deng, D.; Yang, Y.; Sha, H.;
Zou, X.; Faller, R.; Yuan, Y.; Zhu, G. Porous Aromatic Framework
Nanosheets Anchored with Lewis Pairs for Efficient and Recyclable
Heterogeneous Catalysis. Adv. Sci. 2020, 7, 2000067.
3690
https://doi.org/10.1021/acs.orglett.1c01073
Org. Lett. 2021, 23, 3685−3690