Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity

Article abstract of DOI:10.1002/anie.202011790

Here we report the in situ generation of nucleophilic allylboranes from H₂ and allenes mediated by a pyridonate borane that displays frustrated-Lewis-pair reactivity. Experimental and computational mechanistic investigations reveal that upon H₂ activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene. The allylboranes generated in this way are reactive towards nitriles. A catalytic protocol for the formation of allylboranes from H₂ and allenes and the allylation of nitriles has been devised. This catalytic reaction is a conceptually new way to use molecular H₂ in organic synthesis.

Full text of DOI:10.1002/anie.202011790

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Accepted Article
Title: Formation of Nucleophilic Allylboranes from Molecular Hydrogen
and Allenes Catalyzed by a Pyridonate Borane that Displays
Frustrated Lewis Pair Reactivity
Authors: Urs Gellrich, Max Hasenbeck, Sebastian Ahles, Arthur
Averdunk, and Jonathan Becker
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10.1002/anie.202011790
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RESEARCH ARTICLE
Formation of Nucleophilic Allylboranes from Molecular Hydrogen
and Allenes Catalyzed by a Pyridonate Borane that Displays
Frustrated Lewis Pair Reactivity
Max Hasenbeck,[a] Sebastian Ahles,[a] Arthur Averdunk,[a] Jonathan Becker,[b] and Urs Gellrich*[a]
Dedication ((optional))
[a]
[b]
M. Hasenbeck, M. Sc.; Dr. S. Ahles, A. Averdunk, B. Sc.; Dr. U. Gellrich
Institut für Organische Chemie
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 17, 35392 Gießen (Germany)
E-mail: urs.gellrich@org.chemie.uni-giessen.de
Dr. J. Becker
Institut für Anorganische und Analytische Chemie
Justus-Liebig-Universität Gießen
Heinrich-Buff-Ring 17, 35392 Gießen (Germany)
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Here we report the in situ generation of nucleophilic
allylboranes from H2 and allenes mediated by a pyridonate borane
that displays frustrated Lewis Pair reactivity. Experimental and
computational mechanistic investigations reveal that upon H2
activation, the covalently bound pyridonate substituent becomes a
datively bound pyridone ligand. Dissociation of the formed pyridone
borane complex liberates Piers borane and enables a hydroboration
of the allene. The allylboranes generated in this way are reactive
towards nitriles. A catalytic protocol for the formation of allylboranes
from H2 and allenes and the allylation of nitriles has been devised.
This catalytic reaction is a conceptually new way to use molecular H2
in organic synthesis.
Introduction
Scheme 1. a) Formation of B-3,3-dimethallyl-9-BBN 2 by hydroboration of
dimethylallene reported by Brown. b) Structure of Brevianamide A with the
fragment originating from an allylborane highlighted.
Allylboranes are a prevalent class of C-nucleophiles.[1-3] Classic
ways for their preparation include the addition of nucleophilic allyl
Grignard or allyl lithium compounds to electrophilic boron
methoxides.[4] However, already in 1977 Kramer and Brown
reported the formation of nucleophilic allylboranes upon
hydroboration of allenes by 9-borabicyclo(3.3.1)nonane (9-BBN,
1) (Scheme 1).[5] The allylborane 2 prepared in this way was used
in the first total synthesis of Brevianamide A, a highly challenging
target for organic synthesis, reported earlier this year. This
demonstrates the ongoing importance of this class of
nucleophiles for organic synthesis.[6]
We recently reported reversible H2 activation by the pyridonate
borane 3 that can be described as an intramolecular frustrated
Lewis pair (FLP).[7-9] A distinguishing feature of this system is that
the H2 activation is associated with a transition of the covalently
bound pyridonate substituent to a datively bound pyridone ligand
(Scheme 2).
1
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Scheme 2. Envisioned in situ formation of a nucleophilic allylborane upon
hydrogen activation by 3 and hydroboration of an allene.
Scheme 3. Formation of the pyridone allylborane complex 7 upon hydrogen
activation by 3 and its stepwise formation upon hydroboration of phenylallene
by Piers borane 6 and addition of the pyridone 5. Yields were determined by 1H
NMR with 1,3,5-trimethoxybenzene as internal standard.
Piers borane 6 has been shown to display the typical reactivity of
a trivalent borane, e.g. the hydroboration of unsaturated double
bonds.[12] We, therefore, envisioned that Piers borane 6, formed
in situ upon H2 activation by 3 and dissociation of 4, is able to
hydroborate an allene, yielding a nucleophilic allylborane.[13] The
formation of a nucleophilic allylborane from H2 and an allene
would be an atom economic and conceptually new way to use
molecular dihydrogen for the formation of a reactive organic
intermediate. Furthermore, a reaction sequence consisting of
allylation of an electrophile by the allylborane formed in this way,
followed by a protodeborylation mediated by the pyridone 5,
would regenerate the pyridonate borane 3. That would enable us
to realize an allylation that requires only catalytic amounts of the
pyridonate borane 3.
Thus, we were able to demonstrate that it is indeed possible to
form an allylborane from hydrogen and phenylallene. In the next
stage of this research project, we aimed to examine if the
allylboranes generated in this way could be used as nucleophiles
for an allylation reaction. A prerequisite for the use of 3 for an
allylation is that the pyridonate borane 3 is able to activate
hydrogen in the presence of a Lewis basic electrophile. To
elucidate if this condition is met and if 7 serves as a source for
nucleophilic allylboranes, 3 was reacted with phenylallene and
one equivalent acetonitrile under moderated H2 pressure (1.1 bar).
However, this reaction did not yield the expected allylimine, but
the β-diketiminate borane complex 9 together with the bispyridone
complex 10 (Scheme 4).
Results and Discussion
To prove the working hypothesis, phenylallene was added to a
solution of 3 in benzene and the reaction mixture was exposed to
H2 (1.1 bar). This led to the formation of a new borane complex
that was assigned to be the pyridone allylborane complex 7 in
66% yield (Scheme 3). To substantiate the assumed reaction
sequence involving a hydroboration and re-coordination of the
pyridone 5 to the formed allylborane, Piers borane 6 was reacted
with phenylallene (Scheme 3). At –35 °C in diethylether,
phenyallene was quantitatively hydroborated and the allylborane
8 was obtained with a regioselectivity of 81:19. In the next step,
the pyridone 5 was added at r.t. This yielded the pyridone
allylborane complex 7 in quantitative yield.
Scheme 4. Formation of the β-diketiminate borane complex 9 upon the reaction
of 3 with acetonitrile and phenylallene under hydrogen pressure.
The β-diketiminate borane complex 9 that proved to be air stable
was isolated by silica gel column chromatography in 56% yield
and fully characterized by NMR and ESI-MS. The structural
assignment is further supported by single-crystal X-ray diffraction
(SCXRD, Figure 1). The β-diketiminate borane complex 9 is
reminiscent to the core structure of BODIPY dies.[15] Indeed, 9 is
a fluorophore that absorbs at λmax = 395 nm and emits at
λmax(fluorescence) = 409 nm with a molar attenuation coefficient
of ε359 = 6928 cm-1 M-1 (see SI for full spectra).
2
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10.1002/anie.202011790
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experiments revealed that this nitrogen couples to the methyl
group and to the CHPh group of the allylimine 12 (Figure 2, for full
spectra see SI).
Figure 1. Molecular structure of 9 derived from SCXRD (50% probability
ellipsoids, all hydrogens attached to carbons are omitted and C6F5 rings are
shown in stick representation for clarity). Selected bond lengths and angles: N1-
B: 1.528(17) Å, N1-C2 1.336(17) Å, C2-C3 1.403(19) Å, C3-C4 1.390(19) Å, N2-
C4: 1.315(17) Å, N2-B: 1.539(17) Å, C2-N1-B: 125.5(2) °, C4-N2-B: 126.7(2) °.
Figure 2. 15N-1H HMBC NMR spectrum of the allylimine complex 12 (600 MHz,
toluene-d8). The blue arrows indicate the observed 15N-1H correlations. The
signal marked with a star can be assigned to a solvent residue peak from
toluene-d8.
The β-diketiminate borane complex 9 presumably originates from
a nucleophilic attack of the enamine tautomer of an intermediately
formed allylimine to acetonitrile. To verify this hypothesis and to
check if an allylimine was formed as intermediate en route to 9,
the allylborane 8, in situ generated by hydroboration of
phenylallene, was reacted with one equivalent acetonitrile. This
reaction furnished the ketiminoborane 11, which is the allylation
product of acetonitrile, in 83% yield. The formation of 11
Thus, the NMR experiment confirms the proton transfer from the
pyridon 5 to the nitrogen of the ketiminoborane 11 and formation
of an allylimine. The addition of one further equivalent acetonitrile
and pyridonate borane 3 lead to the clean formation of the
β-diketiminate borane complex 9 in 86% yield (Scheme 5). This
experiment strongly supports the assumption that upon the
reaction of 3 with phenylallene and acetonitrile under H2 pressure,
which lead to the formation of 9, the desired allylimine was indeed
formed intermediately (Scheme 4). Therefore, the formation of 9
from phenylallene and acetonitrile proves that 3 is not only able
to activate dihydrogen in the presence of acetonitrile, but also
mediates the subsequent allylation, leading to an allylimine.
However, the B(C6F5)2 fragment is irreversibly bound in 9 which
inhibits any further catalytic reactivity. We envisioned that the
addition of a strong Lewis acid would enable us to capture the
intermediately formed allylimine prior to the β-diketiminate borane
complex formation (Scheme 6).
demonstrates that allylborane
(Scheme 5).
8
is indeed nucleophilic
Scheme 5. Stepwise formation of the β-diketiminate borane complex 9 from
Piers borane 6. Yields were determined by 1H NMR with
1,3,5-trimethoxybenzene as internal standard.
The 11B NMR shift of 21.4 ppm and the C=N stretching vibration
of 1864 cm-1 indicate that 11 is present as monomeric
ketiminoborane with linearity at nitrogen.[14] Next, pyridone 5 was
added to the ketiminoborane 11. This sequence yielded the
allylimine complex 12 (Scheme 5). The NH signal of 12 is found
at 10.90 ppm, indicating an N–H…N hydrogen bond. A 15N–1H
HSQC NMR experiment shows that the proton is bound to a
nitrogen with a shift of 227.3 ppm. Additional 15N–1H HMBC NMR
Scheme 6. Envisioned catalytic cycle for the allylation of nitriles using an
additional Lewis acid that captures the allylimine (grey box) (LA = Lewis acid).
3
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As the liberation of the allylimine from 12 would furthermore
regenerate the pyridonate borane 3, a catalytic allylation can be
envisioned. Indeed, upon addition of B(C6F5)3 the formation of the
allylimine borane complex 13 from phenylallene, acetonitrile, and
dihydrogen in presence of catalytic amounts of the pyridone
borane complex 4 (10 mol%) was observed (Scheme 7).
required to obtain good yields. In the case of tert-butyl phenyl
substituted allene the product 16 is only obtained in moderate
yields. However, an NMR analysis of the crude reaction mixture
showed that the allene is consumed after 16 h reaction time,
indicating that side reactions lower the yield in this case. Aliphatic
allenes are suitable substrates, but again longer reaction times
are required. The formation of the adamantly substituted
allylimine 19 in excellent yield further shows that sterically
demanding substituents are tolerated. The products 17, 18, and
19 did not isomerize upon purification by silica gel
chromatography. However, we demonstrated at the example of
18 that the allylimines with aliphatic substituents can be
isomerized to the respective vinylimines by the addition of a base.
Treatment of 18 with NEt3 at r.t. for 30 minutes allowed us to
isolate the corresponding vinylimine in 95% yield (see SI).
Scheme 7. Formation of the allylimine borane complex 13 catalyzed by 4, and
its isomerization to the vinylimine 13’. One equivalent acetonitrile and B(C6F5)3
were used.
The allylimine borane complex 13 proved to be air-stable and was
isolated in 65% yield. However, on silica gel 13 isomerizes to the
vinylimine 13’ as proven by two-dimensional TLC (see SI). Thus,
the two isomers, fully characterized by NMR and ESI-MS, were
obtained in a 93:7 ratio after column chromatography. The
molecular structure of 13 in the solid-state was further analyzed
by SCXRD (Figure 3). The allylimine borane complex 13 can be
quantitatively isomerized to the vinylamine 13’ by addition of
triethylamine at r.t. within minutes, indicating that 13’ is the
thermodynamically more stable isomer. The molecular structure
of 13’ derived from SCXRD confirms the E-configuration of the
double bond.
Scheme 8. Allylation of acetonitrile catalysed by 4 with different allenes. One
equivalent acetonitrile and B(C6F5)3 were used. All yields are isolated yields. The
number in parenthesis indicate the ratio of allylimine to vinylimine. [a] Reaction
time 5 days. [b] Reaction time 16 hours. [c] Reaction time 7 days.
The mechanism of this novel catalytic transformation was further
addressed by dispersion-corrected Density Functional Theory
(DFT)
computations
at
the
revDSD-PBEP86-D4/def2-
QZVPP//PBEh-3c level (Figure 4).[16-18] The revDSD-PBEP86-D4
functional is one of the best DFT methods for ground-state
thermochemistry and kinetics as shown by benchmark
computations using the GMTKN55 database.[16a,19] The SMD
model for benzene was used to implicitly account for solvent
effects.[20] We used 1,2-butadiene as model substrate and further
assumed, that under the reaction conditions complex 20 forms
upon coordination of acetonitrile to B(C6F5)3. In agreement with
previous investigations, the computations reveal that the catalytic
cycle commences with hydrogen activation by the pyridonate
borane 3. The pyridone borane complex 4 that is formed in this
way dissociates to liberate Piers borane 6. As a stoichiometric
amount of B(C6F5)3 is present, we considered that the pyridone 5
coordinates to the B(C6F5)3, which overall renders the liberation
of Piers borane 6 thermodynamically more favorable. The
hydroboration of the terminal allene requires passing a moderate
barrier of 8.4 kcal mol−1 and is according to the computations
exergonic by 27.6 kcal mol−1.
Figure 3. Molecular structure of 13 and 13’ derived from SCXRD (50%
probability ellipsoids, all hydrogens attached to carbons are omitted and C6F5
rings are shown in stick representation for clarity). Selected bond lengths and
angles: 13: C2-C3 1.528(8) Å, C3-C4 1.514(9) Å, C4-C5 1.310(5) Å, N1-C2-C3:
121.7(8) °, C3-C4-C5: 126.2(4) °; 13’: C1-C3 1.4744(17) Å, C3-C4 1.3408(18)
Å, C4-C5 1.497(2), N1-C1-C3: 118.07(11) °, C3-C4-C5: 124.67(14) °.
To prove that the catalytic activity of 4 is more general, we
explored next the scope and limitations of this novel allylation at
the example of six different allenes (Scheme 8). Electron
withdrawing groups are tolerated, but longer reaction times are
4
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Figure 4. Potential energy surface for the formation of the β-diketiminate borane complex 32 and allylimine borane complex 28 computed at revDSD-PBEP86-
D4/def2-QZVPP//PBEh-3c. The SMD model for benzene was used to implicitly account for solvent effects.
5
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Coordination of acetonitrile to the allylborane 22 precedes the Conclusion
intramolecular allylation via a cyclic six-membered transition state
with a barrier of 16.2 kcal mol−1. The computed structure of the
In summary, we have documented the formation of allylboranes
from molecular hydrogen and allenes mediated by a pyridonate
borane. The stochiometric reaction of these in situ generated
allylboranes with acetonitrile leads to a β-diketiminate borane
complex. By using an additional Lewis acid, we were able to
develop a method for the allylation of nitriles that requires only
catalytic amounts of the pyridonate borane. Mechanistic
investigations reveal that the change in the binding mode of the
pyridonate substituent in course of the hydrogen activation is vital
for the formation of the allylborane. The results presented herein
might stimulate the development of metal-free, atom-economic
catalytic allylations.
ketiminoborane 24 shows a C=N=B angle of 178.7 °, which
agrees with the linearity at nitrogen deduced from the
experimental C=N stretching vibration of 11. Upon dissociation of
pyridone B(C6F5)3 complex 21 and recoordination of 5, a virtually
barrier-less intramolecular proton transfer yields the allylimine
complex 26. Based on our experimental findings, we propose that
the allylimine complex 26 is the common intermediate for the
formation of the β-diketiminate borane complex 32 and the
allylimine B(C6F5)3 complex 28. In the absence of B(C6F5)3, an
intramolecular proton transfer from the methyl group of the
allylimine to the pyridine nitrogen that requires a Gibbs free
energy of 21.3 kcal mol−1 yields the pyridone enamine borane
complex
29.
Dissociation
of
this
complex,
again
thermodynamically favored by formation of the bispyridone
complex 10, enables the nucleophilic addition of the enamine to
acetonitrile via the six-membered transition state TS30/31. While
the C–C bond formation is already exergonic, the tautomerization
to the diketiminate borane complex 32 provides a further decisive
driving force to the reaction. In the presence of B(C6F5)3, the
kinetically favored pathway commences with the dissociation of
the allylimine complex 26 that is endergonic by 14.4 kcal mol−1.
Coordination of the B(C6F5)3 to the free allylimine 27 yields the
allylimine B(C6F5)3 complex 28. The coordination of B(C6F5)3 to
the allyimine 27 imparts a barrier of 25.3 kcal mol−1 for the
tautomerization to the pyridone enamine borane complex 29 and
therefore suppresses the formation of the β-diketiminate borane
complex 32. However, according to the computations, the
allylimine B(C6F5)3 complex 28 must be regarded as the kinetic
product while the β-diketiminate borane complex 32 is the
thermodynamic product of the initial allylation. To verify this
computational result experimentally, the isolated allylimine
B(C6F5)3 complex 13 was reacted with one additional equivalent
MeCN and the pyridonate borane 3 at an elevated temperature of
80 °C (Scheme 9). Within 24 h, the formation of the β-diketiminate
borane complex 9 in 81% yield was observed.
Acknowledgements ((optional))
This work was supported by the FCI (Liebig Fellowship to U.G.)
and the DFG (Emmy-Noether program, GE 3117/1-1). The
authors thank Dr. H. Hausmann and Dr. D. Gerbig for assistance
with NMR and IR experiments, respectively. Continuous support
by Profes. Dres. P. R. Schreiner, R. Göttlich, and H. A. Wegner is
acknowledged.
Keywords: Hydrogen activation • Allylborane • Frustrated Lewis
Pair • DFT computations • Boron-Ligand cooperation
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diketiminate borane complex 9, which is the thermodynamic product of the
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1,3,5-trimethoxybenzene as internal standard.
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RESEARCH ARTICLE
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The in situ formation of nucleophilic allylboranes from molecular hydrogen and allenes is reported. The reaction is catalyzed by
pyridonate borane that displays frustrated Lewis pair reactivity. The allylboranes that are generated in this way are reactive towards
nitriles. Mechanistic investigations show that a change in the binding mode of the pyridonate substituent upon hydrogen activation is
essential for the formation of the allylboranes.
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