Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes

Article abstract of DOI:10.1002/anie.202009625

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre-catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β-(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re-enter the main catalytic cycle.

Full text of DOI:10.1002/anie.202009625

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Accepted Article
Title: Reaction Pathways and Redox States in α-selective Cobalt-
Catalyzed Hydroborations of Alkynes
Authors: Lutz Gade, Clemens Blasius, Vladislav Vasilenko, Regina
Matveeva, and Hubert Wadepohl
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10.1002/anie.202009625
Angewandte Chemie International Edition
COMMUNICATION
Reaction Pathways and Redox States in -selective Cobalt-
Catalyzed Hydroborations of Alkynes
Clemens K. Blasius, Vladislav Vasilenko, Regina Matveeva, Hubert Wadepohl, Lutz H. Gade*
Abstract: Cobalt(II) alkyl complexes supported by a monoanionic
NNN pincer ligand are pre-catalysts for the regioselective
hydroboration of terminal alkynes, yielding the Markovnikov products
with :ß-(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II)
vinyl complex appear to determine the main reaction pathway. In a
background reaction the highly reactive hydrido species specifically
converts to a coordinatively unsaturated cobalt(I) complex which was
found to re-enter the main catalytic cycle.
well as a Pd-catalyzed hydroborylation approach,[16] requiring
elevated reaction temperatures of 80 °C. In view of the structural
relevance of -borylated, terminal alkenes, the need for further
synthetic strategies under mild conditions seems apparent.
Based on our previous work on boxmi[17] complexes in
catalytic transformations,[18] we chose the synthesis of alkyl
complexes R’,RboxmiCoCH2SiMe3 (1) as starting point for our
investigation (Scheme 1).
R,R'boxmi-H
(tmeda)Co(CH2SiMe3)2
toluene, r.t., 6 h
The burgeoning field of base metal catalysis has been
stimulated by novel reactivity patterns and their potential in
synthesis.[1] Molecularly defined cobalt complexes have been
O
R'
N
R
SiMe3
R'
N
Co
N
established as versatile tools for
a variety of catalytic
R
transformations.[2] Compared to noble metals, the propensity of
cobalt compounds to engage in one-electron redox processes
has been found as a distinctive difference, such as the tendency
of cobalt(II) alkyl or hydride complexes to be readily convertible
into the corresponding cobalt(I) complexes.[3] Cobalt(II)
hydrides[4,5] – often postulated as transient species – are thought
to be key intermediates in the functionalization of C-C multiple
bonds as well as hydride transfer reactions.[6] On the other hand,
coordinatively unsaturated cobalt(I) complexes have been
efficiently employed in small molecule or inert bond
activations[3b,7] as well as catalytic transformations.[8]
O
1a (R = H, R' = Ph)
1b (R = H, R' = iPr)
Scheme 1. Left: Synthesis of cobalt(II) alkyl complexes 1. Right: Molecular
structure of H,iPrboxmiCoCH2SiMe3 (1b) in the solid-state. Displacement
ellipsoids set at 30 % probability level, hydrogen atoms omitted for clarity, only
one of two independent molecules shown. Selected bond lengths (Å) and
angles (°), values for the second independent molecule are given in square
brackets: Co-C23 1.963(7) [1.981(7)], Co-N1 1.905(6) [1.891(6)], Co-N2
1.963(6) [1.947(5)], Co-N3 1.929(6) [1.917(6)], N2-Co-C23 134.5(3) [130.7(3)],
N1-Co-N3 153.5(2) [160.1(2)].f
Treating dialkyl precursor (tmeda)Co(CH2SiMe3)2 (tmeda
=
Cobalt-mediated hydrometallation reactions provide
a
convenient access to synthetically useful reagents,[1a,9] and in
particular the catalytic hydroboration of alkynes allows for the
synthesis of alkenyl boronates and related species.[10] In case of
terminal alkynes, the potential formation of three different
isomers with either -, (Z)- or -(E)-configuration necessitates
control of regio- and stereoselectivity. Chirik and co-workers
established bis(imino)pyridine cobalt complexes as effective
catalysts for the stereodivergent hydroboration of terminal
alkynes to (Z)-configured alkenyl boronates, which stem from a
syn-hydrometallation step of a alkynyl boronate intermediate.[11]
For the cobalt-mediated synthesis of (E)-alkenyl boronates, both
an isolated -diimine cobalt(II) hydride complex[5] and a bench-
stable cobalt(II) coordination polymer[12] have been recently
reported.
N,N,N’,N’-tetramethylethylen-diamine) with equimolar amounts
of protio-ligand R’,Rboxmi-H provided facile access to cobalt(II)
high-spin complexes 1 in quantitative yields. The molecular
structure of the four-coordinate distorted tetrahedral precatalyst
1b was established by X-Ray diffraction, with all CoN and
CoC23 bond lengths being in good agreement with similar 3d
metal boxmi high-spin complexes.[18e,19] The cobalt complexes 1
were found to be suitable precatalysts for the selective
hydroboration of alkynes. In a first assay, we reacted 1-ethynyl-
4-fluorobenzene (2a) with two equivalents pinacolborane
(HBPin) in the presence of 2.5 mol% H,PhboxmiCoCH2SiMe3 (1a),
finding the selective formation of mono-borylated products 3a in
a remarkable ratio of -3a:-(E)-3a = 8:1 (Figure 1, top).
The -selective hydroboration of terminal alkynes to produce
-borylated alkenes provides access to valuable reagents for the
synthesis of 1,1-disubstituted, terminal alkenes, a common
structural motif in a range of bioactive compounds.[13] Current
synthetic methods include a Ni-catalyzed hydroalumination
procedure,[14] NHC-Cu-mediated hydroboration reactions with
diboron reagents in the presence of tert-butoxide salts,[13,15] as
[a]
C. K. Blasius, Dr. V. Vasilenko, Regina Matveeva, Prof. Hubert
Wadepohl, Prof. L. H. Gade
Anorganisch-Chemisches Institut
Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: lutz.gade@uni-heidelberg.de
Figure 1. Conversion profile for a cobalt-catalyzed hydroboration of 1-ethinyl-
4-fluorobenzene ([Alkyne]0/M = 0.22, [HBPin]0/M = 0.33, [Co]0/M 0.005, room
temperature, toluene) monitored by in situ 19F NMR spectroscopy. Relative
amounts in reaction mixture given.
Supporting information for this article is given via a link at the end
of the document.
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10.1002/anie.202009625
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COMMUNICATION
Notably, the resulting synthetically useful, branched regioisomer
-3a as main product is in stark contrast to previously reported
procedures for cobalt-catalyzed hydroborations, all of which
favor the linear anti-Markovnikov products with (E)-[5,12] or (Z)-
selectivity.[11] As monitored by in situ 19F NMR spectroscopy
(Figure 1, bottom), the c-t profile for the hydroboration by alkyl
complex 1a features a distinct sigmoidal course, indicating a
significant induction period prior to the reduction process.
selective but irreversible transformation, formally resulting from
H2 elimination, to a coordinatively unsaturated cobalt(I) complex
5 (Scheme 2, bottom).[3c,3e]
Upon treatment of the alkyl complexes 1 with an excess of
pinacolborane, slow consumption of HBPin and the emergence
of the -bond metathesis product Me3SiCH2BPin were detected.
From a mass balance point of view, the rise of the latter implies
the presence of a cobalt hydride complex (Scheme 2, middle),
the instability of which precluded its pure isolation. However, the
reaction of H,iPrboxmiCoCH2SiMe3 with pinacolborane in the
presence of an internal alkyne, namely 1-phenyl-1-propyne,
resulted in the generation of vinyl complex 4 (Scheme 2, top)
which confirms the in situ generation of a cobalt hydride complex.
More importantly, the reaction outcome also renders this
insertion reaction a probable elementary step in the catalytic
cycle for the cobalt-catalyzed hydroboration. Although alkyne
insertions into a cobalt hydride bond are frequently postulated
for various transformations,[20] the isolation of catalytically
relevant cobalt(II) vinyl complexes remains scarce. This is in
stark contrast to the fact that the fundamental feasibility of this
transformation has long been known, in fact also for terminal
alkynes.[21] Notably, a Markovnikov-selective hydrometallation
step was very recently found to facilitate the cobalt-catalyzed
Figure 2. Molecular structures of H,iPrboxmiCoC(Me)C(H)Ph
H,iPrboxmiCo]2 5 (right). Displacement ellipsoids set at 30% (4) and 50% (5)
4 (left) and
[
probability level, hydrogen atoms omitted for clarity, only one of four
independent molecules shown for 4. Selected bond lengths (Å) and angles (°),
given as range for all independent molecules in case of 4; 4: Co-C23
1.922(11)…1.956(11), Co-N1 1.921(7)…1.942(7), Co-N2 1.966(8)…1.994(8),
Co-N3 1.916(7)…1.934(7), N2-Co-C23 162.6(4)…167.3(4), N1-Co-N3
171.5(4)…174.8(4). 5: Co1-center(C26-C27) 2.029(2), Co2-center(C12-C13)
2.047(2), Co1-N1 2.002(3), Co1-N2 1.976(3), Co1-N3 2.029(3), Co2-N4
2.062(3), Co2-N5 1.987(3), Co2-N6 2.009(3), C12-C13 1.412(4), C26-C27
1.420(4), N2-Co1-center(C26-C27) 104.6(1), N1-Co1-N3 133.84(12),
N5-Co2-center(C12-C13) 106.0(1), N4-Co2-N6 139.60(12).
The molecular structure of cobalt(I) complex
5 was
determined by X-ray diffraction to be dimeric in the solid-state
(Figure 2, right). With the cobalt centers positioned slightly
above the boxmi ligand plane, additional weak interactions of the
metal to a C=C double bond of the respectively adjacent boxmi
ligand are observed (d = 2.029 Å and 2.047 Å) and give rise to a
distorted tetrahedral coordination sphere around the cobalt
centers. With T-shaped geometries being scarce,[3e,25] this form
of dimerization constitutes a hitherto not observed stabilization
mode for this type of complex, as the majority of coordinatively
unsaturated cobalt(I) compounds is further stabilized by
additional arene,[7b,26] carbon monoxide,[3d] or nitrogen ligands[3a]
or undergo ligand rearrangements upon dimerization.[27]
sequential
hydrogenation/hydrohydrazidation
of
terminal
alkynes.[22] There was no evidence for the formation of a cobalt
boryl species or the occurrence of an oxidative addition step as
discussed for similar catalysts.[11,23]
The magnetic moment of complex 5 was found to be µeff
=
5.07 µB at room temperature in solution, indicating a moderate
antiferromagnetic coupling between the two cobalt centers,
Accordingly, an antiferromagnetic coupling constant of
J
AFC = 54 cm1 was derived from solid-state (SQUID)
experiments (see SI). A strong temperature-dependence of the
paramagnetic 1H NMR spectra was observed, resulting in a non-
Curie and coalescence behavior. As a consequence, the signal
number is diminished from 32 in the low temperature limit, which
is consistent with a nonsymmetrical dimer, to eight at elevated
temperatures which can be attributed to a reversible dynamic
dissociation giving the monomeric form dominating at elevated
temperatures (see SI).
Apart from the absence or presence of an internal alkyne,
both vinyl species 4 and the cobalt(I) species 5 were prepared
under similar conditions, indicating that in the presence of the
alkyne the insertion reaction towards 4 is more favorable than H2
elimination towards 5 and, thus, highlighting the reactivity of the
cobalt(II) hydride species. Moreover, complex 5 was found to
operate as a hydroboration catalyst in its own right, albeit with a
distinctively different kinetic trace (see SI). Its reactivity towards
pinacolborane was found to be sluggish but reaction with
stoichiometric amounts of pinacolborane and 1-phenyl-1-
propyne regenerated vinyl complex 4 as main product (see SI).
Scheme 2. Reaction of precatalyst H,RboxmiCoCH2SiMe3 (1) with
stoichiometric amounts of pinacolborane as well as the internal alkyne 1-
phenyl-1-propyne yielding vinyl complex 4 (top), pinacolborane to generate the
non-isolable hydrido key intermediate (middle) as well as an excess of
pinacolborane to give the dinuclear CoI complex 5 (bottom).
The d7 low-spin vinyl complex 4 adopts a distorted square-planar
coordination sphere with all bond metrics being in the expected
range[19] (Figure 2, left). In solution, well-resolved paramagnetic
1H and 13C NMR spectra with comparatively small shift
dispersions are consistent with the C2 symmetry of the pincer
complex in agreement with other square-planar cobalt(II)
compounds (SI).[24]
All attempts to isolate the hydrido species failed, due to its
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10.1002/anie.202009625
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COMMUNICATION
Scheme 4. Proposed mechanistic cycle for the cobalt-catalyzed hydroboration
of alkynes.
To probe the synthetic potential of the cobalt-catalyzed
hydroboration of terminal alkynes a screening of differently
substituted boxmi ligands and reaction conditions established
the phenyl-substituted complex 1a as the most efficient pre-
catalyst, allowing catalyst loadings as low as 0.5 mol% for full
conversion within 6 h at room temperature (Table 1, for details
see SI).
Table 1. Screening of reaction conditions for the hydroboration
of terminal alkynes.
Scheme 3. Mechanistic control experiments. In all schematic illustrations, [Co]
denotes the complex fragment H,RboxmiCo; a) product distribution given; b)
product distribution with respect to -3b given; c) NMR yield given.
Several catalytic and stoichiometric control experiments
were conducted to obtain insight into the main catalytic pathway
(Scheme 3). The successful hydroboration of an internal alkyne,
such as 1-phenyl-1-propyne, indicated that alkynyl species were
not key intermediates. In this reaction primarily the -(Z) and -
(Z) product in a 1:2 ratio and only trace amounts of the formally
(E)-borylated products were observed (Scheme 3a). Deuterium
labeling studies were performed for the cobalt-catalyzed reaction
of phenyl acetylene with pinacolborane to probe potential
hydride transfer pathways (Scheme 3b). Both the treatment of
PhCCD with 1.1 equivalents of HBPin as well as the same
reaction with reversely deuterated reagents led to complete
proton/deuterium scrambling with all possible H/D isomers
equally represented. This indicates that the selectivity-
determining step is reversible, revealing a fast equilibrium
between the cobalt hydride complex and the insertion product. In
addition, these results imply that there is a rapid equilibration
between the (E)- and (Z)-vinyl species. Such behavior has been
reported previously by Chirik[11] and Ojima[28] for related cobalt
and rhodium complexes and was proposed to occur via a
zwitterionic carbene intermediate. Finally, reacting isolated vinyl
complex 4 with stoichiometric amounts of pinacolborane yielded
the anticipated -bond metathesis product as sole species after
protolytic workup, demonstrating the feasibility of this elementary
reaction within the given context and its importance for the
catalytic process (Scheme 3c).
catalyst
[mol%]
conv.
[%][a]
:[a]
#
R
R’
solvent
t [h]
1
2
3
4
5
6
7
H
Ph
Ph
Ph
iPr
tBu
Bn
Ph
toluene
toluene
toluene
toluene
toluene
toluene
tol/n-hex[b]
2.5
2.5
2.5
2.5
2.5
2.5
0.5
5
5
>99
>99
>99
>99
31
92:8
Me
Ph
H
91:9
5
92:8
3
86:14
74:26
77:23
91:9
H
14
3
Me
H
>99
>99
6
[a] Determined by in situ 19F NMR spectroscopy. [b] Solvent mixture of toluene
and n-hexane (v:v = 1:5). [c] Identical results were obtained for isolated and in
situ generated precatalysts; reactions were performed at 0.1 mmol scale.
With these optimized conditions in hand, we explored the
applicability of the cobalt-catalyzed method and subjected a
variety of alkynes to the Markovnikov-selective hydroboration
with the phenyl-substituted alkyl complex 1a (Scheme 5, top).
Both electron-withdrawing substituents (3a,c,d) and the
presence of ortho-substituents (3h,l,m) appeared to be
beneficial for high α: ratios (up to 97:3), thereby matching the
selectivities obtained from previous approaches.[15] Regarding
terminal alkyl alkynes, cyclohexyl derivative 2n turned out to be
Based on these findings, a catalytic cycle consisting of
detrimental to
a satisfactory enrichment of the -isomer,
reversible alkyne insertion into
a cobalt(II) hydride and
subsequent product release via -bond metathesis is proposed
for the cobalt-catalyzed hydroboration of alkynes (Scheme 4).
whereas only sluggish hydroboration was observed for the
trimethylsilyl substituted alkyne 2o. In the case of ester, cyano,
or nitro groups being attached to an aryl alkyne, slow
conversions of the substrate were accompanied by the formation
of inseparable side products (see SI), preventing the pure
isolation of the target compounds. As mentioned above, internal
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10.1002/anie.202009625
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COMMUNICATION
alkynes (2p) were also tolerated in this cobalt-catalyzed
hydroboration protocol, albeit with inverted α: selectivity. In
addition, compound -3p was obtained as the major product
from the catalytic conversion of 3-phenyl-1-propyne with
pinacolborane, demonstrating the tendency of propargylic
substrates to form the respective internal alkene derivatives (see
SI for details).
Acknowledgements
We acknowledge the award of a predoctoral fellowship to V. V.
from the Landesgraduiertenförderung (LGF Funding Program of
the state of Baden-Württemberg) and generous funding by the
University of Heidelberg as well as the Deutsche
Forschungsgemeinschaft (DFG-Ga488/9-2).
Keywords: cobalt • T-shaped complex • alkynes • pincer ligand
• alkenyl boronates
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compound demonstrates the marked one-electron redox
chemistry 3d metal complexes, an off-cycle reactivity in the case
at hand but opening up new perspectives for future applications
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10.1002/anie.202009625
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Redox Dualism: A cobalt(II) hydride
complex with an ancillary NNN pincer
ligand displays a twofold reactivity. It
acts as catalyst and key intermediate in
the -selective hydroboration of
terminal alkynes. In the absence of
alkynes, however, it provides facile
access to a coordinatively unsaturated
cobalt(I) species which undergoes
reversible dimerization.
C. K. Blasius, V. Vasilenko, R.
Matveeva, H. Wadepohl, L. H. Gade*
Page No. – Page No.
xx
This article is protected by copyright. All rights reserved.