Transition-Metal-Free Cleavage of CO

Article abstract of DOI:10.1002/chem.201703798

Tertiary silane 1^H, 2-[(diphenylsilyl)methyl]-6-methylpyridine, reacts with tris(pentafluorophenyl)borane (BCF) to form the intramolecular pyridine-stabilized silylium 1⁺-HBCF. The corresponding 2-[(diphenylsilyl)methyl]pyridine, lacking the methyl-group on the pyridine ring, forms classic N(py)→B adduct 2^H-BCF featuring an intact silane Si?H fragment. Complex 1⁺-HBCF promotes cleavage of the C≡O triple bond in carbon monoxide with double C?C_sp2 bond formation, leading to complex 3 featuring a B-(diarylmethyl)-B-aryl-boryloxysilane fragment. Reaction with pinacol generates bis(pentafluorophenyl)methane 4 as isolable product, proving the transition-metal-free deoxygenation of carbon monoxide by this main-group system. Experimental data and DFT calculations support the existence of an equilibrium between the silylium–hydroborate ion pair and the silane–borane mixture that is responsible for the observed reactivity.

Full text of DOI:10.1002/chem.201703798

A Journal of
Accepted Article
Title: Transition Metal-Free Cleavage of CO
Authors: Marc Devillard, Bas de Bruin, Maxime Siegler, and Jarl Ivar
van der Vlugt
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.201703798
Link to VoR: http://dx.doi.org/10.1002/chem.201703798
Supported by

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Transition Metal-Free Cleavage of CO
Marc Devillard,[a] Bas de Bruin,[a] Maxime A. Siegler,[b] and J. I. van der Vlugt*[a]
Abstract:
Tertiary
silane
1H,
2-((diphenylsilyl)methyl)-6-
Mo platform was shown to generate C2O1 species by
stoichiometric CO triple bond scission combined with C-C bond
formation, aided by the use of silyl chlorides as co-reagents,
which inadvertedly generates siloxanes as by-product.[11]
methylpyridine, reacts with tris(pentafluorophenyl)borane (BCF) to
form the intramolecular pyridine-stabilized silylium 1+-HBCF. The
corresponding 2-((diphenylsilyl)methyl)-pyridine, lacking the methyl-
group on the pyridine ring, forms classic N(py)→B adduct 2H-BCF
featuring an intact silane Si-H fragment. Complex 1+-HBCF
promotes cleavage of the C≡O triple bond in carbon monoxide with
double C-Csp2 bond formation, leading to complex 3 featuring a B-
(diarylmethyl)-B-aryl-boryloxysilane fragment. Reaction with pinacol
generates bis(pentafluorophenyl)methane 4 as isolable product,
proving the transition metal-free deoxygenation of carbon monoxide
by this main-group system. Experimental data and DFT calculations
support the existence of an equilibrium between the silylium-
hydroborate ion pair and the silane-borane mixture that is
responsible for the observed reactivity.
Erker 2013
Mes
Mes
C6F5
C6F5
P
C
B
O
CO
Mes2P
B(C6F5)2
H
B
C6F5 C6F5
Stephan 2013
HP(tBu)3
HP(tBu)3
B(C6F5)3
B(C6F5)3
B
O
A
C
O
H2 / CO
O
P(tBu)3
2B(C6F5)3
(C6F5)B
C6F5
(C6F5)2B
C C6F5
C
The coordination chemistry and subsequent reactivity of carbon
monoxide with transition metal complexes is well-developed and
forms the platform for current-day applications of CO as C1
building block, including large-scale industrial processes (e.g.
acetic acid production, hydroformylation).[1] However, the C-O
linkage is typically preserved in these applications. This is in
stark contrast to heterogeneous Fischer-Tropsch catalysis,
where CO (with H2) is utilized as a true C1 building block to make
new C-C bonds concomitant with full deoxygenation of CO.[2]
Valorization of carbon dioxide is currently attracting much
attention.[3] Partial deoxygenation strategies to convert carbon
dioxide with main group elements (e.g. hydrosilylation,
hydroboration) are investigated.[4] Since the pioneering work of
Brown on the carbonylation of alkylboranes,[5] the transition
metal-free chemistry of carbon monoxide with main group
compounds is also rapidly developing. To generate stable or
observable adducts of CO, mainly boron-based compounds[6]
and diaminocarbenes are utilized,[7] but only a handful of
compounds have proven capable of activating CO to the extent
that new C-X bonds (X = O, N, C, H) can be formed. Insertion of
CO in the B-B single bond of azaboriridines,[8a,b] or the M-C bond
of alanes[8c,d] and gallanes[8e] is known, and reductive coupling of
CO has recently been achieved.[9] However, transition metal-free
full deoxygenation of CO is elusive.
H
H
H
3
/
2
This work
H
H+
N
CO
N
C(C6F5)2
Si
B
C6F5
C6F5
C6F5
Ph
Ph
H
O
Si
C
H-B(C6F5)3
Ph
Ph
H
Scheme 1. Top: Rare example of CO activation using main-group (FLP)
systems, including C-H and single C-Csp2 bond formation, respectively.
Bottom: Unique CO triple bond activation of intramolecularly stabilized silylium
hydroborate system with double C-Csp2 bond formation.
Combinations of an electrophilic borane and a phosphine-based
donor, examples of FLPs (Frustrated Lewis Pairs),[12] have been
used to activate CO. The group of Erker described the trapping
of the CO adduct of Piers borane[13] (OC→B(H)(C6F5)2) with
intramolecular phosphine-borane FLP systems, generating a
new C-H bond (Scheme 1, top).[14] Stephan and co-workers
reported that a 2:1 mixture of tris(pentafluorophenyl)borane
(B(C6F5)3 = BCF) and P(tBu)3 leads to the complete splitting of
CO (from syngas), via a boron formyl intermediate, to afford A
featuring one new C-Csp2 bond, together with cyclic product B
(Scheme 1,middle).[15] This report represents the only previous
example of main-group element promoted C-Csp2 bond formation
from CO, to the best of our knowledge. No mechanistic
information is available to shed light on the formation to A. Also,
more extensive CO functionalization (i.e. additional C-C bond
formation) would be very interesting.
Inspired by these recent advances and realizing (i) the ability of
triphenylboron hydrides to activate CO2 and carbonyl groups
owing to the reactive B-H bond,[16] and (ii) the well-known ability
of mixtures of silanes and B(C6F5)3 to promote hydrosilylation
reactions of unsaturated substrates via formation of silyl
cations,[17],[18] we sought to develop a reactive Si---B system that
would be susceptible to CO activation (Scheme 1, bottom).
Well-defined homogeneous transition metal complexes that are
capable of CO triple bond scission normally require highly
reactive co-reagents and/or strongly reducing conditions in order
to enforce (stepwise) C-O cleavage.[10] Recently, a mononuclear
[a]
[b]
Dr. M. Devillard, Prof.Dr. B. de Bruin, Dr.Ir. J.I. van der Vlugt
van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam, the Netherlands
E-mail: j.i.vandervlugt@uva.nl
Dr. M. A. Siegler
Small Molecule X-ray Crystallography, John Hopkins University
3400 N Charles St, Baltimore MD, 21218 USA
Supporting information for this article is given via a link at the end of
the document.
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While the chemistry and catalysis with electrophilic silylium
species is rapidly gaining attention,[19] examples of frustrated
Lewis pair-type chemistry involving silicon are relatively rare.[20]
To address both challenges, we herein report a strategy to
which suggests an intramolecular N→Si donor-acceptor
interaction in the product.[22]
stabilize
a silylium species with a weak donor such as
pyridine.[21] whilst retaining the inherent reactivity of the Si center.
This strategy has enabled transition metal-free rupture of the
triple bond in carbon monoxide and subsequent double C-Csp2
bond formation by C6F5 transfer using a simple and novel
pyridine-stabilized silylium hydroborate system. Through
a
combination of experimental data and supported by DFT
calculations, the role of each component in this hitherto
unknown reaction sequence is detailed.
Synthesis of 2-((diphenylsilyl)methyl)-6-methylpyridine 1H from
lutidine is straightforward (Scheme 2). The Si-H hydrogen
1
resonates at δ 5.01 in the H NMR spectrum (1JSiH = 199.9 Hz).
The hydrogen atoms of the methylene bridgehead (–CH2–)
group appear as a doublet at δ 2.91 through coupling with the
Si-H hydrogen (3JHH = 3.7 Hz). The corresponding 29Si{1H} NMR
signal appears as a singlet at δ -14.6. The analogous species 2H
(2-((diphenylsilyl)methyl)pyridine) was also prepared in this way.
Figure 1. Displacement ellipsoid plot (50% probability level) of 2H-BCF.
Hydrogen atoms are omitted for clarity, except those on C1 and Si1. Selected
bond lengths (Å): N1-B1 1.6547(35); C1-Si1 1.8983(28); Si1-H1 1.366(26).
Abstraction of the hydride to generate cationic pyridine-stabilized
silylium fragment 1+-HBCF (Scheme 2) was further ascertained
by high resolution mass spectrometry (HR-MS; see ESI). To the
best of our knowledge, only one other example of a pyridine-
stabilized silylium derivative has been described to date
(featuring a 5-membered ring), whereas 1+-HBCF features an
N→Si stabilization within a 4-membered ring.[23] Apparently,
introduction of a methyl group ortho to the pyridine nitrogen fully
suppresses the py→B(C6F5)3 adduct in favor of formation of the
[N→Si]+ hydroborate Lewis pair. This species can be considered
isoelectronic with previously described pyridine-stabilized
organoboranes.[24] Silylium salt 1+-HBCF is isolated as an oil,
preventing structure elucidation by single crystal X-ray
crystallography.
N
Si
HB(C6F5)3
1) n-BuLi
Et2O, 0 °C, 1h
BCF
BCF
Ph
Si
Ph
Ph
H
1+-HBCF, 83 %
2) ClSiPh2H
Et2O, -78 °C to rt
Ph
N
R
R
N
1H, R = CH3, 21 %
2H, R = H, 46 %
Ph
Si
N
H
Ph
B
C6F5
C6F5
C6F5
2H-BCF, 72 %
Scheme 2. Synthesis of 1H and 2H and reaction with B(C6F5)3 to pyridine-
stabilized silylium 1+-HBCF and pyridine-borane adduct 2H-BCF, respectively.
H
H
N
N
B
HO
OH
CO (5 bar)
C(C6F5)2
Si
C6F5
C6F5
C6F5
Ph
Ph
Si
HB(C6F5)3
DCM, -78 °C to RT
83 %
DCM, 50 °C, 2h
56 %
O
C
Ph
Upon
addition
of
an
equimolar
amount
of
4
Ph
H
tris(pentafluorophenyl)borane (BCF) to a solution of 2H, selective
formation of species 2H-BCF, featuring a direct N→B adduct, is
observed with an intact hydrosilane unit, according to 1H and
29Si NMR spectroscopy (Scheme 2). The 19F NMR spectrum
shows 15 different resonance signals due to hindered rotation
around the N-B and B-C linkages, resulting in low overall
symmetry. Single crystal X-ray diffraction analysis confirms the
molecular structure of species 2 (Figure 1). The analogous
reaction between BCF and 1H in CD2Cl2 at room temperature
immediately generates a new singlet at δ 23.2 in the HSQC 29Si-
1H NMR spectrum, correlating with a broad singlet at δ -25.5 in
1+-HBCF
3
Scheme 3. Reaction of 1+-HBCF with CO to generate 3 and follow-up
hydrolysis with pinacol to release bis(pentafluorophenyl)methane.
Upon pressurizing a solution of 1+-HBCF in CD2Cl2 with 5 bar of
CO for two hours at 50 °C, a single new resonance at δ -8.6 is
observed in the 29Si DEPT NMR spectrum, revealing a reaction
to a new species 3 featuring a saturated tetracoordinated silicon
(Scheme 3). High resolution mass spectrometry reveals the
incorporation of one molecule of CO or 13CO, respectively. A
new signal is observed at δ 2.3 in the 11B NMR spectrum, while
a new singlet integrating for one hydrogen atom is observed at δ
4.75 in the 1H NMR spectrum. This signal becomes a doublet
(1JCH = 116.8 Hz) when the same reaction is performed with 13C-
labeled CO, supporting the formation of a direct C-H bond, with
the hydrogen atom most likely stemming from the silane. In
agreement with this observation, the proton-coupled 13C NMR
the 11B NMR spectrum. In the corresponding H NMR spectrum,
1
the –CH2– hydrogens appear as a singlet at δ 3.43, indicating
complete Si-H cleavage. In the 1H{11B} NMR spectrum, the
-
formation of the hydroborate H-B(C6F5)3 is evidenced by a
broad singlet at δ 3.57 that nearly disappears in the baseline in
the 11B-coupled spectrum. Furthermore, the para-pyridine H-
atom is strongly shifted downfield upon hydride abstraction,
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spectrum of the labeled compound displays a doublet (1JCH
=
a)
116.8 Hz) at δ 25.9. In the 13C{1H} NMR spectrum, this signal is
very broad (full-width at half-maximum = 20.3 Hz) and nearly
disappears in the baseline in the non-labeled product, which
suggests a direct scalar coupling with the 11B nucleus. The 19F
NMR spectrum displays two sets of signals with an integration
ratio of 2:1, suggestive of (double) C6F5 transfer from boron to
the boron-bound carbon atom.
N
N
N
5 bar CO
5 bar CO
Li
N
No reaction
Si
N
HB(C6F5)3
B(C6F5)4
CD2Cl2, 2h, 50 o
C
CD2Cl2, 2h, 50 o
C
Ph
Ph
1+-BArF
4
b)
1+-HBCF
c)
O
1H
+
N
R
N C
Si
H
CD2Cl2, RT
instantaneous
B(C6F5)3
R
N C
C
Based on multinuclear NMR spectroscopy and literature data on
related organoboron compounds,[25] we conclude that 3 contains
a (C6F5)B-C(H)(C6F5)2 fragment as well as a siloxyborate linkage.
DFT calculated and benchmarked NMR chemical shifts support
the presence of a four-coordinated B-nucleus with an additional
pyridine→boron interaction, as in 3 (Scheme 3 and ESI). It can
be noted that the hydrogens of the methylene bridge in 3 do not
5
B
R = 2,6-dimethylphenyl
Scheme 4. a) Control experiments to highlight the need for silicon, pyridine
and hydroborate in Lewis pair 1+-HBCF to cleave CO. b) Reaction with
isocyanide to give 5, demonstrating equilibrium between 1+-HBCF and
1H+BCF. c) proposed hydride transfer from 1H to OC→B(C6F5)3.
1
display a diastereotopic effect (low temperature H NMR shows
only broadening of the CH2 resonance at -70 °C). This suggests
that 3 is probably in equilibrium with its open form 3’ featuring a
three-coordinated B-nucleus, as supported by DFT calculations
(see ESI for details).[26]
Next, we attempted to translate the observed reactivity with CO
to the electronically related isocyanide moiety. Surprisingly,
reaction of 1+-HBCF with tert-butylisocyanide at r.t. led to
complete regeneration of the silane Si-H bond, as supported by
1H NMR spectroscopy, concomitant with formation of the
tBuNC→B(C6F5)3 adduct 5 (Scheme 4, b). This observation
As a result, the C≡O triple bond in carbon monoxide is
completely cleaved, with the formation of one new C-H and two
new C-C bonds. Although complex 3 proved high air-sensitive
and the oily nature of 3 prohibited direct elucidation of the
structure by single crystal X-ray crystallography, a crystallization
attempt at -20 °C furnished crystals of moderate quality of the
oxidized derivative 3O, which confirms the proposed connectivity
with a six-membered ~C-Si-O-B-N-C~ linkage (see ESI).[27]
Liberation of the organic C(H)(C6F5)2 fragment from 3 was
achieved via alcoholysis with an equimolar amount of pinacol in
dichloromethane (Scheme 3). Gratifyingly, this results in clean
formation and isolation of bis(pentafluorophenyl)methane 4 (as
well as the 13C-labeled derivative, available from 13CO) in 83%
yield, confirming the generation of two new C-C(sp2) bonds from
CO in 3. Compound 4 is fully characterized by multinuclear NMR
spectroscopy, high resolution mass spectrometry, micro-analysis
and single crystal X-ray diffraction (see ESI for details). The
skeletal composition and protocol for formation and reactivity of
3 is unique, to the best of our knowledge, as it represents the
first example of double C-Csp2 bond formation from C-O
cleavage by CO activation promoted by a main-group derivative.
To get additional insights in the reaction mechanism and the
requirement for silylium species 1+-HBCF as reagent, several
suggests that 1+-HBCF is also in equilibrium with the silane and
[28]
the carbonyl adduct OC→B(C6F5)3
under CO pressure and
that intact 1H rather than the silylium derivative is responsible for
the observed reactivity. At this stage we hypothesized that the
reaction could be initiated by a hydride transfer from the silane
to the activated CO carbon of the boron-adduct (Scheme 4, c).
The hydridicity of the silane would be enhanced through
intramolecular stabilization of the corresponding silylium
fragment by the nearby pyridine group. In agreement with this
proposed equilibrium between 1+-HBCF and 1H, a mixture of
methyldiphenylsilane (HSiMePh2), lacking the internal stabilizing
pyridine ring, and B(C6F5)3 do not react with CO.
To provide a qualitative picture of the plausible mechanism of
CO triple bond rupture and formation of 3, featuring the highly
unsual six-membered B-(diarylmethyl)-B-aryl-boryloxysilane
system, we performed DFT calculations (BP86, def2-TZVP,
disp3, m4 grid). The complete pathway from 1H to 3 is displayed
in Figure 2. The separated Lewis acid-base pair (1H + BCF; set
at 0 kcal mol-1 with free CO; starting point) and 1+-HBCF (+2.6
kcal mol-1) are likely in equilibrium under the experimental
reaction conditions. Activation of CO to give A most likely
proceeds via the separated Lewis acid-base pair. The entropy
penalty associated with formation of a van der Waals adduct of
OC-B(C6F5)3 and 1H is largely compensated by relatively strong
π-π stacking interactions. This pre-organization facilitates
hydride transfer, via a low TS1 barrier of +4.5 kcal mol-1 relative
to 1H + BCF, to form zwitterionic silylium-formylborate species B
(the ion-separated conformer B’ is 2.4 kcal⋅mol-1 higher than B).
Intermediate C (-11.8 kcal⋅mol-1), is formed upon interaction of
the silyl cation with the oxygen atom of the formyl group.
Subsequent facile transfer of a C6F5-group from boron to carbon
via transition state TS2 (+5.6 kcal mol-1 relative to 1H + BCF)
generates the open form of the neutral boranylmethoxysilane
derivative D (-19.6 kcal⋅mol-1).[29] The boraepoxide conformer D’
(+0.8 kcal⋅mol-1 relative to D) undergoes a 1,2-oxo-shift via TS3
(+11.9 kcal⋅mol-1 compared to D’) to form the open zwitterionic
control experiments were carried out. Firstly, 1+-BArF , the
4
tetrakis(pentafluorophenyl)borate analogue of 1+-HBCF, lacking
a hydridic B-H fragment, was prepared by the reaction of 1H and
(CPh3)B(C6F5)4 in 76% yield and characterized by multinuclear
NMR spectroscopy and HR-MS analysis (see ESI for details).
This compound is completely unreactive toward CO, which
suggests that a borohydride moiety is necessary for carbon
monoxide activation to occur (Scheme 4, a). Furthermore, the
lithium hydroborate salt LiBH(C6F5)3 also gives no reaction under
the same reaction conditions, thus showing that the pyridine-
silylium fragment is also crucial for the reaction to proceed.
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λ4-boranyloxysilane E with a formal cationic carbon fragment (-
9.6 kcal⋅mol-1). Interestingly, boraepoxides have been proposed
as intermediates in alkyl transfer from boron to carbon.[30] This
reactive species E can undergo direct transfer of a second C6F5-
group from boron to form G over transition state TS4 (+1.9 kcal
mol-1 relative to E). Alternatively, formation of the closed
zwitterionic pyridinium intermediate F (-26.3 kcal mol-1 relative to
E) followed by transfer of a second C6F5-group from boron would
form the open form 3’ over transition state TS5. Given the strong
driving force for formation of intermediate F, this path is deemed
most probable, with the barrier to TS5 (+20.8 kcal mol-1)
representing the rate-limiting step of the reaction. The closed
from 3, featuring an additional NPyàB interaction, is only 5.4 kcal
mol-1 uphill relative to 3’.
transition metal-free deoxygenation of CO. This study shows
that pyridine-appended silanes in combination with B-based
Lewis acids may provide suitable platforms to deoxygenate CO
and to form new C-C bonds. Follow-up research on the scope of
Lewis acids available for this TM-free transformation is ongoing.
Acknowledgements
This research is funded by the European Research Council
through ERC Starting Grant EuReCat 279097 to JIvdV. We
thank Ed Zuidinga for MS analysis and Dr. Andreas Ehlers and
Jan-Meine Ernsting for assistance with NMR spectroscopy.
Ph
H
O
Si
N
B(C6F5)3
Ph
Ph
C
N
N
Ph
O
B'
+5.5
TS1
+4.5
H
Si
OC B(C6F5)3
A
Si
C6F5
C6F5
Ph
Ph
E
B
B'
H
C
B
A
+1.8
+3.1
C6F5
TS2
-5.6
1H, BCF, CO
= 0
TS3
-6.9
O
TS4
-7.7
B(C6F5)3
C
Ph
Si
H
C
-8.7
Ph
N
E
-9.6
Ph
Si
‡
Ph
O
Ph
N
B
Si
C6F5
TS5
O
C
C6F5
Ph
B
C6F5
B
-15.1
N
C
H
C
C6F5
C6F5
H
D'
-18.8
C6F5
D
-19.6
TS5
Ph
Ph
N
N
Ph
Ph
F
-35.9
C6F5
Si
Si
C6F5
O
O
B
C6F5
B
C
Ph
D'
D
C6F5
H
Ph
C
C6F5
Si
H
O
C6F5
C6F5
C6F5
B
N
C
3
C6F5
-50.5
H
3'
-55.9
F
Figure 2. DFT calculated (BP86, def2-TZVP, disp3, m4 grid) pathway for the
conversion of 1H+BCF to 3. For details, see also the Supporting Information.
Keywords: CO • silicon • small molecule activation • boron • C-
C bond formation
In summary, we have prepared silylium species 1+-HBCF using
the pyridine-appended tertiary silane 1H and Lewis acid
tris(pentafluorophenyl)borane. This FLP-like compound is able
to induce full cleavage of the C≡O triple bond to generate 3. This
sequence also results in the formation of one new C-H and two
new C-Csp2 bonds, as supported by multinuclear NMR
spectroscopy and solid state characterization of the oxidized
derivative. Alcoholysis of complex 3 with pinacol provides
bis(pentafluorophenyl)methane as the organic C1 product from
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10.1002/chem.201703798
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Marc Devillard, Bas de Bruin, Maxime A.
Siegler, and J. I. van der Vlugt *
Page No. – Page No.
Transition Metal-Free Cleavage of CO
The transition metal-free complete deoxygenation of CO is achieved using a
combination of tertiary silane 1H, 2-((diphenylsilyl)methyl)-6-methylpyridine and
B(C6F5)3 (BCF), which generates the intramolecular pyridine-stabilized silylium 1+-
HBCF. In the process of CO activation, one new C-H and two new C-Csp2 bonds
are formed. Hydrolysis with pinacol gives H2C(C6F5)2 as isolable product. DFT has
provided insight in the plausible pathway for this remarkable transformation.
N
N
N
B
CO
Si
Si
C6F5
C6F5
C6F5
Ph
Ph
Ph
Ph
Si
HB(C6F5)3
B(C6F5)3
H
O
C
Ph
Ph
H
one new C-H and two new C-C(sp2) bonds
after CO triple bond cleavage!!!
[a]
[b]
Dr. M. Devillard, Prof.Dr. B. de Bruin, Dr.Ir. J.I. van der Vlugt
van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098 XH Amsterdam, the Netherlands
E-mail: j.i.vandervlugt@uva.nl
Dr. M. A. Siegler
Small Molecule X-ray Crystallography, John Hopkins University
3400 N Charles St, Baltimore MD, 21218 USA
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.