Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron–Ligand Cooperation in Concert

Article abstract of DOI:10.1002/chem.202001276

The metal-free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H₂ with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal-free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridine activates H₂ is reminiscent of the reactivity of an intramolecular frustrated Lewis pair. However, it is the change in the coordination mode of the boroxypyridine upon H₂ activation that allows the dissociation of the formed pyridone borane complex and subsequent hydroboration of an alkyne. This change in the coordination mode upon bond activation is described by the term boron-ligand cooperation.

Full text of DOI:10.1002/chem.202001276

Accepted Article
Accepted Article
Title: Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane
Complex: Frustrated Lewis Pair reactivity and Boron-Ligand
Cooperation in Concert
Authors: Urs Gellrich, Felix Wech, and Max Hasenbeck
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01/2020

10.1002/chem.202001276
Chemistry - A European Journal
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Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane
Complex: Frustrated Lewis Pair reactivity and Boron-Ligand
Cooperation in Concert
Felix Wech,[a] Max Hasenbeck,[a] and Urs Gellrich*[a]
[a]
B. Sc. Felix Wech, M. Sc. Max Hasenbeck, Dr. U. Gellrich
Institut für Organische Chemie
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 17, 35392 Gießen
E-mail: urs.gellrich@org.chemie.uni-giessen.de
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: The metal-free cis selective hydrogenation of alkynes
catalyzed by a boroxypyridine is reported. A variety of internal alkynes
are hydrogenated at 80 °C under 5 bar H2 with good yields and
stereoselectivity. Furthermore, the catalyst described herein enables
the first metal-free semihydrogenation of terminal alkynes.
Mechanistic investigations, substantiated by DFT computations,
reveal that the mode of action by which the boroxypyridine activates
H2 is reminiscent of the reactivity of an intramolecular frustrated Lewis
pair. However, it is the change in the coordination mode of the
boroxypyridine upon H2 activation that allows the dissociation of the
formed pyridone borane complex and subsequent hydroboration of an
alkyne. This change in the coordination mode upon bond activation is
described by the term boron-ligand cooperation.
termed boron-ligand cooperation. The change in the coordination
mode of the pyridone substituent might enable the dissociation of
the pyridone borane complex 4 in the ligand 6-tert-butylpyridone
5 and Piers borane 6. The Piers borane has been shown to
display the typical reactivity of a trivalent borane, e.g. it effects the
hydroboration of alkenes and alkynes. Such dissociation is not
possible for classic FLPs that, as aforementioned, therefore
rather display borohydride reactivity upon H2 activation
(Scheme 1).
Introduction
The seminal finding that specific combinations of sterically
encumbered Lewis bases and Lewis acids, named “frustrated
Lewis pairs” (FLPs), can activate hydrogen, stimulated the
development of catalytic metal-free hydrogenations.[1] Early
examples included the hydrogenation of (di)imines, nitriles,
aziridines, silyl enol ethers, and enamines, but the scope of FLP
catalyzed hydrogenations was extended to heterocycles, alkenes,
allenes, and aromatic hydrocarbons.[2,3] The heterolytic hydrogen
cleavage by the FLP yields a tetravalent borohydride species.
Therefore, hydrogenations by FLPs consist of a hydride and a
subsequent proton transfer step (or vice versa) and require
activated
alkenes.[3]
A
notable
exception
is
the
semihydrogenation of alkynes catalyzed by an intramolecular FLP
that was reported by Repo et al.[4] In that case, mechanistic
investigations showed that the protolysis of the FLP under the
reaction conditions yields an amine-hydroborane that initiates the
Scheme 1. A classic intramolecular FLP that displays borohydride reactivity and
reversible H2 activation by the boroxypyridine 3 that might display borane
reactivity upon H2 activation and dissociation.
catalytic cycle by hydroboration of the alkyne.[5]
A
Results and Discussion
protodeborylation of the alkenylborane yields then, in a highly
stereoselective reaction, the cis-alkene.[6] We recently reported
reversible H2 activation by the boroxypyridine 3.[7] A distinguishing
feature of this system is that the H2 activation is associated with a
transition of the covalently bound oxypyridine substituent to a
neutral pyridone donor ligand (Scheme 1). This mode of action
was, in analogy to the concept of metal-ligand cooperation,
We envisioned the hydroboration of an alkene to be a valid test
reaction to elucidate whether 3 displays borane reactivity upon
hydrogen activation, since hydroboration requires the presence of
a
trivalent borane. Indeed, when
3
was reacted with
one equivalent of styrene under moderate H2-pressure at r.t., the
1
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formation of the alkyl borane 7 was observed (Scheme 2). The
alkylborane 7 is also formed when styrene is reacted with the
pyridone borane 4, which supports the assumption that 4 is an
intermediate in the formation of 7 starting from 3.
was generated in situ by coordination of 5 to Piers borane 6. An
initial screening of reaction conditions showed that a slight excess
of Piers Borane 6 (1.3 equivalents with respect to 5) is beneficial
to obtain reproducible good yields. Under the same conditions,
cis-2-octene is obtained in very good yields from the
hydrogenation of 2-octyne. Likewise, cis-3-hexene is formed upon
hydrogenation of 3-hexyne in excellent yield after only 8 h reaction
time. The hydrogenation of 4-methyl-2-pentyne leads to the
corresponding cis alkene in a very good yield after 16 h reaction
time. Upon hydrogenation of the respective alkyne, 1-Phenyl-1-
propene is obtained in an excellent yield of 93%. Ethers are
suitable substrates, as proven by the successful hydrogenation of
1-(para-methoxyphenyl)-propyne.
Scheme 2. Hydroboration of styrene upon H2 activation by 3.
The alkylborane 7 does not undergo a protodeborylation.
However, we envisioned that an analogous alkenylborane,
originating from a reaction sequence consisting of H2 activation
and hydroboration of an alkyne might succumb to protonolysis.
This reaction would regenerate the boroxypyridine 3 and close a
catalytic cycle for the hydrogenation of alkynes that consists of H2
activation by 3, hydroboration of an alkyne and protonolysis of the
alkenylborane (Scheme 3).
Scheme 4. Substrate scope of the semihydrogenation of internal alkynes.
Yields were determined by 1H NMR with 1,3,5-trimethoxybenzene as internal
standard and are given as the average of two runs (a) 8 h reaction time; b) 16 h
reaction time).
While 3-hexyne is obtained after 8 h exclusively as cis isomer a
prolonged reaction time of 16 h led to a 1:1 mixture of the cis and
the trans isomer (Scheme 5). After 20 h, the trans isomer is the
major product. Liu et al. reported that Piers borane can isomerize
cis-alkenes via reversible hydroboration.[5] We, therefore, assume
that the catalytic reaction yields first cis-3-hexene that is then
subsequently isomerized by the Piers borane 6 that is present in
the reaction mixture. Thus, both stereoisomers are accessible
with the catalytic protocol described herein.
Scheme 3. Envisioned mechanism of the hydrogenation of alkynes catalyzed
by 3: H2 activation yields the pyridone borane complex 4 that undergoes a
dissociation. Piers borane 6 hydroborates an alkyne, formation of the pyridone
alkenylborane complex 9 and its protolysis are closing the catalytic cycle.
Scheme 5. Stereoselectivity of the hydrogenation of 3-hexyne in dependence
of the reaction time.
Indeed, 2-hexyne was stereoselectively converted to
cis-2-hexene in 87% yield in the presence of catalytic amounts of
4 at 80 °C under 5 bar H2 pressure (Scheme 4). The catalyst 4
The known metal-free protocols for the hydrogenation of alkynes
are limited to internal alkynes. We were pleased to find that the
catalyst described herein is capable to hydrogenate 1-octyne in
2
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good yield with a catalyst loading of 10 mol% (Scheme 6). The
catalytic protocol can also be used for the hydrogenation of other
aliphatic alkynes such as cyclohexyl- and adamantly acetylene.
While aromatic rings are tolerated, the hydrogenation of
The bispyridone complex 8 that was previously described and
1
characterized in detail was observed by H NMR as the resting
1
state of the catalytic reaction (Figure 1).[8] Furthermore, H and
11B NMR proved formation of boroxypyridine 3 with progressing
reaction and hydrogen consumption. This finding strongly
supports the assumption that 3 is part of the catalytic cycle.[9]
phenylacetylene
and
para-(trifluoromethyl)phenylacetylene
yielded the corresponding alkenes in lower yields. Again, ethers
are suitable substrates, as demonstrated by the hydrogenation of
6-methoxy-1-hexylacetylene.
Figure 1. 11B NMR spectra (193 MHz, benzene-d6) obtained by monitoring of
the catalytic reaction (Scheme 6) before heating (blue) and after 15 h at 60 °C
(red).
Scheme 6. Substrate scope of the semihydrogenation of terminal alkynes.
Yields were determined by 1H NMR with 1,3,5-trimethoxybenzene as internal
standard and are given as the average of two runs.
To elucidate whether the envisioned protonolysis of the
alkenylborane can be assumed to be part of the catalytic reaction,
5 was added to the borane 9, derived from the reaction of Piers
borane 6 and 3-hexyne. The reaction progress at r.t. was
monitored by NMR spectroscopy (Scheme 9). Within 30 minutes,
the formation of the expected pyridone alkenylborane complex 10
was observed. Furthermore, signals that were assigned to
cis-3-hexene, the product of the protonolysis, were detected. The
presence of cis-3-hexene implies that boroxypyridine 3,
originating from the protonolysis must be present. Indeed, the
formation of the bispyridone complex 8 that contains one
equivalent 3 was observed.
With these results in hand, we aimed for a mechanistic
understanding of the catalytic reaction. To verify that the pyridone
5 is indeed vital for the reaction, we attempted the hydrogenation
of 2-hexyne only with Piers borane 6 as catalyst (Scheme 7). Less
than 1% product was formed under reaction conditions that are
identical to those reported in Scheme 4, clearly indicating that the
presence of the pyridone 5 is essential for the reaction outcome.
Scheme 7. Attempted hydrogenation with Piers borane 6 as the catalyst.
We then focused on the identification of the resting state of the
catalytic reaction. For this purpose, the catalytic hydrogenation of
3-hexyne was monitored by NMR (Scheme 8). Under 4 bar
H2-pressure, rapid formation of cis-3-hexene was observed at
70 °C in benzene-d6, which implies that the observations made by
this experiment are meaningful regarding the catalytic
transformation.
Scheme 9. Stoichiometric reaction of the alkenylborane
tert-butylpyridone 5.
9 with the
EXSY NMR shows an exchange of the pyridone 5 between 10
and 8 at r.t. That further supports that 8 is not an unreactive,
irreversibly formed species but rather a resting state. The
mechanism of the catalytic reaction was further investigated
computationally at revDSD-PBEP86-D4/def2-QZVPP//PBEh-3c
(Figure 2).[10,11] The SMD model for n-hexane was used to
implicitly account for solvent effects.[12] The hydrogen activation
Scheme 8. NMR monitoring of the catalytic hydrogenation of 3-hexyne.
3
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by 3 requires a free activation energy of 19.6 kcal mol−1. This
elementary step is according to our computations thermoneutral,
which agrees with the previously observed facile reversibility of
the hydrogen activation.[7] The free energy change that is
associated with the dissociation of 4 in Piers borane 6 and the
pyridone 5 is 16.8 kcal mol−1. Relaxed potential energy surface
scans indicate that the dissociation is barrierless. As the
experimental results indicate that the pyridone complex 8 is the
resting state of the transformation, we considered the
coordination of the free pyridone 5 to the boroxypyridine 3. Indeed,
the formation of 8 is according to the computations exergonic. The
hydroboration of the model substrate 2-butyne requires a
moderate activation energy of 4.9 kcal mol−1 and yields the
alkenylborane 11. The bispyridone complex 8 together with 11 is
the resting state of the catalytic transformation.[13] The pyridone 5,
that is bound in complex 8, coordinates than to 11 forming the
pyridone alkenylborane complex 12.
Figure 2. Gibbs free energy profile for the hydrogen activation by 3 computed at revDSD-PBEP86-D4/def2-QZVPP//PBEh-3c. Bulk solvation was considered
implicitly with the SMD model for hexane.
Note that pyridone exchange between 8 and the pyridone
alkenylborane complex 10 was observed experimentally by EXSY
NMR. The activation barrier for the protodeborylation is 22.2 kcal
mol−1, which corresponds to a half-life time of 12 of 35.8 minutes
at 25 °C.[14] This agrees with the experimental observation that the
protodeborylation takes place at r.t. (Scheme 9). The “Energetic
Span”, i. e. the kinetic barrier of the catalytic transformation, is
between the resting state (8 and 11) and the transition state of the
protodeborylation.[15] Classic FLP type catalysts are not suitable
for the hydrogenation of terminal alkynes, presumably because
they are deactivated by an irreversible Csp–H cleavage.[3] To
understand why the catalyst system described herein tolerates
terminal alkynes 3 was reacted with cyclohexylacetylene at r.t. As
previously reported, this reaction led to the formation of the
alkynylborane complex 13 (Scheme 10).[16] Upon addition of
phenylacetylene and heating to 80 °C, 13 was partially converted
to the phenylalkenylborane complex 14.
Scheme 10. Csp–H cleavage of Cyclohexylacetylene by 3 and exchange with
phenylacetylene upon heating to 80 °C.
4
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After 1 h at 80 °C, the ratio of 14 to 13 was 4:1. This experiment
indicates that the Csp–H cleavage is reversible under the reaction
conditions. The assumption that the formation of the
alkynylborane is reversible is further supported by DFT
computations (Figure 3). According to the computations, the
liberation of cyclohexyacetylene from 13 requires a free Gibbs
activation energy of 24.1 kcal mol−1, which corresponds to a half-
life time of 79 seconds at 80 °C. The formation of the phenyl
alkynyl borane complex 14 is kinetically and thermodynamically
favored.
Experimental Section
General Procedure for hydrogenation of alkynes: Piers borane 6
(13.5 mg, 0.039 mmol) and 6-tert-butyl-2-pyridone 5 (4.5 mg, 0.030 mmol)
were dissolved in n-hexane (5 ml) in a Fisher-Porter type 150 ml reaction
vessel equipped with a stirring bar. The respective alkyne (0.60 mmol or
0.30 mmol) was added. The reaction vessel was closed and connected to
an H2 bomb with a gas hose. The hose was rinsed with H2 several times
and the reaction vessel pressurized with H2 (5 bar). The reaction vessel
was placed inside an 80 °C preheated oil bath and stirred at 1000 rpm.
After 20 h, the reaction mixture was cooled to room temperature and the
excess H2 gas was released. An aliquot was taken, and the yield
determined by 1H NMR using 1,3,5-trimethoxybenzene as internal
standard.
Acknowledgements ((optional))
This work was financially supported by the FCI and the DFG. The
authors thank Dr. H. Hausmann for assistance with NMR
experiments. Continuous and generous support by Profes. Dres.
P. R. Schreiner, R. Göttlich, and H. A. Wegner is acknowledged.
Keywords: hydrogenation • boron ligand cooperation •
frustrated Lewis pair • DFT computation • alkyne
[1]
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Figure 3. Gibbs free energy profile for the Csp–H activation of
cyclohxylacetylene
and
phenylacetylene
by
3
computed
at
revDSD-PBEP86-D4/def2-QZVPP//PBEh-3c. Bulk solvation was considered
implicitly with the SMD model for hexane.
The computed Gibbs free energy difference of 0.4 kcal/mol
corresponds to a ratio of 2:1, which is in reasonable agreement
with the experimentally observed proportion of the two alkenyl
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cleavage that allows H2 activation in the presence of a terminal
alkyne and thus the first metal-free hydrogenation of terminal
alkynes.
[3]
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Conclusion
In
summary,
we
have
documented
the
efficient
semihydrogenation of internal and terminal alkynes by
a
boroxypyridine that displays frustrated Lewis pair reactivity and is,
therefore, able to activate hydrogen. However, the change in the
coordination mode of the pyridonate substituent enables
hydroboration as the initial step of the hydrogenation and is thus
vital for the catalytic reaction. We expect this finding to pave the
way for novel metal-free catalytic reactions that rely on this mode
of action.
[4]
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882-889.
[6]
For examples of the semihydrogenation of alkynes by gold nanoparticle
based FLPs see: a) J. L. Fiorio, N. López, L. M. Rossi, ACS Catal. 2017,
5
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[9]
As a slight excess of borane was used (1.3 equivalents), the
alkenylborane 9 was also observed. For details, see the Supporting
Information.
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[13] The excess of Piers borane used in the catalytic experiments (0.3 equiv.)
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6
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Entry for the Table of Contents
The semihydrogenation of alkynes catalyzed by a pyridonate borane complex is reported. While the catalyst can be described as an
intramolecular frustrated Lewis pair, it is the change in the coordination mode of the pyridonate substituent upon H2 bond activation
that enables dissociation of the formed pyridone borane complex and hydroboration as the first C−H bond-forming step. This mode of
action is referred to as boron-ligand cooperation.
7
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