Letter
Controlling the Lewis Acidity and Polymerizing Effectively Prevent
Frustrated Lewis Pairs from Deactivation in the Hydrogenation of
Terminal Alkynes
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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),
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−CCH+Py systems, respectively
Received: March 29, 2021
Published: April 20, 2021
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3685−3690
3685