Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C?H Activation

Article abstract of DOI:10.1002/anie.202002258

The merger of cobalt-catalyzed C?H activation and electrosynthesis provides new avenues for resource-economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characteri

Full text of DOI:10.1002/anie.202002258

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Accepted Article
Title: Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced
Reductive Elimination for Twofold C–H Activation
Authors: Tjark H. Meyer, João Carlos Agostinho de Oliveira, Debasish
Ghorai, and Lutz Ackermann
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Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced
Reductive Elimination for Twofold C–H Activation
Tjark H. Meyer, João C. A. Oliveira, Debasish Ghorai and Lutz Ackermann*[a]
Abstract: The merger of cobalt-catalyzed C–H activation and
electrosynthesis provides new avenues for resource-economical
molecular syntheses, but their reaction mechanisms remain
unfortunately poorly understood. Herein, we report the identification
and full characterization of electrochemically generated high-valent
cobalt(III/IV) complexes as crucial intermediates in electrochemical
cobalt-catalyzed C–H oxygenations. Detailed mechanistic studies
provided support for an oxidatively-induced reductive elimination via
highly-reactive cobalt(IV) intermediates. These key insights set the
stage for unprecedented cobaltaelectro two-fold C–H/C–H activation.
twofold C–H/C–H activations by cobaltaelectro-catalyzed C–H
arylations.
C–H activation has emerged as a transformative tool in molecular
sciences[1] with notable applications to inter alia medicinal
chemistry,[2] material sciences[3] and late-stage modifications.[4]
Significant recent momentum was gained by the merger of
electrosynthesis[5] with metal-catalyzed C–H activation with
sustainable electricity as the terminal oxidant,[6] with major
advances in Earth-abundant[7] cobalt catalysis.[8] Despite of
indisputable progress, the mechanistic understanding[9] of their
elementary steps continues to be underdeveloped,[10] strongly
contrasting with their precious 4d and 5d metal homologs.[11]
Particularly, oxidation-induced reductive elimination has recently
been identified as key for rhodium- and iridium-mediated C–H
activation, as described among others by Chang, Jones, and
Tilset. In this context, we and Xu have very recently proposed the
formation of high-valent rhodium(IV) complexes for rhodaelectro-
catalyzed C–H activations.[12,13] Thus, mechanistic studies
revealed the key anodic oxidation of rhodium(III) complexes to the
Figure 1. Oxidation-induced reductive elimination for cobaltaelectro-catalysis.
We set out to rationalize the stoichiometric electrochemical
synthesis of the cyclometalated cobalt(III) complex [Co(III)-I] from
amide 1a (Scheme 1a). Acetonitrile was found as the solvent of
choice. After considerable experimentation, we were able to
isolate and fully characterize the envisioned 18-electron
cobalta(III)-cycle [Co(III)-I]. The potentiostatic electrolysis at low
potential and the thus mild reaction conditions were key to prevent
overoxidation and byproduct formation (vide infra). The overall
electrolysis was stopped upon consumption of 1.0 F/mol. In
addition to electrospray ionization mass spectrometry (ESI-MS),
the structure of [Co(III)-I] was unambiguously confirmed by full
NMR spectroscopic and single-crystal X-ray characterization
(Scheme 1b).
high-valent
rhoda(IV)species.[12]
Cobalt-catalyzed
C–H
activations were thus far largely[14] suggested to occur by a
cobalt(II/III/I) catalytic cycle (Figure 1).[15] In this context, detailed
mechanistic studies on Cp*-free[16]
cobalt-catalyzed C–H
activations continue to be scarce, despite of notable contributions
by Daugulis,[15l] Maiti,[17] Ribas[18] among others,[14g, 19] while single
electron transfer (SET) reactions are predominantly proposed for
base-metal-induced C–Het bond formations.[20] In sharp contrast,
we have now unraveled oxidation-induced reductive elimination
for cobalt-catalyzed electrochemical C–H activation. Salient
features of our findings include a) the isolation and full
characterization of electrochemically generated high-valent
cyclometalated cobalt(III) complexes, b) electroanalytical
characterization of
oxidatively-induced
cobalt(IV) intermediates, and c) an
reductive elimination pathway for
cobalta(III/IV/II) electrocatalysis, which d) mechanistically guided
[a]
T. H. Meyer, Dr. J. C. A. Oliveira, Dr. D. Ghorai, Prof. Dr. L.
Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstraße 2, 37077 Gottingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Scheme 1. a) Electrochemical synthesis of cobaltacycle [Co(III)-I] via C–H
activation. b) X-ray crystal structure analysis of [Co(III)-I] (50% probability
ellipsoids). Hydrogen atoms were omitted for clarity. CCDC 1983755.
Bz = -C(O)Ph.
Motivated by our findings, we became intrigued to investigate
the redox potential of [Co(III)-I] by means of cyclic voltammetry
(Figure 2). Hence, at potentials of Ep,ox = 0.95 V vs. SCE an
irreversible oxidation wave arises, which was assigned to
complex [Co(III)-I]. The experimental oxidation potentials are in
good agreement with the ones that we calculated for [Co(III)-I],
(exp: Ep,2 = 0.89 V vs. SCE, 100 mV/s, calc: Ep,2 = 0.89 V vs.
SCE).[21] The small shoulder at Ep,ox = 0.76 V vs. SCE, however,
could be assigned to intermediate [Co(III)-II], in which the mono-
coordinated substrate 1a was substituted by the solvent MeOH.
The dissociation of the oxygen-coordinated (O5) substrate 1a is
in accordance with ESI-MS, NMR and computational studies on
the calculated redox potentials.[21] The irreversible oxidation
waves were assigned to the anodic generation of Co(IV)
complexes.[22] As can be concluded from the voltammograms, the
electrochemical reaction is chemically irreversible, even at higher
scan rates of up to 1.0 V/s, indicating that subsequent chemical
reactions quickly consume the oxidized cobalt(IV)-complexes
[Co(IV]·+ on the CV time scale. When the CV-experiments were
performed at a lower temperature of 273 K (Figure 2b), we
observes a reversible Co(III)/Co(IV) redox-event with a scan-rate
of 1.6 V/s. At lower scan-rates, the oxidation was still irreversible.
However, with a further decrease in temperature to T = 195 K, we
were able to characterize a reversible redox event with even lower
scan-rates, highlighting the stability of the proposed high-valent
cobalt(IV) complex.[21]
Figure 2. CVs of the electrochemically generated cobaltacycles [Co(III)-I] and
[Co(III)-II] in MeOH (3.5 mM) at different scan rates. The voltammograms were
recorded in 0.1 M [n-Bu4N][PF6]. a) At 298 K. b) At 273 K. C) At 195 K.
To explore the influence of the substitution pattern of the
benzamide 1a on the redox behaviour, we prepared a series of
differently decorated cyclometalated cobalt complexes [Co(III)-I–
VIII] (Table 1).
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Table 1. Electro-C–H activation for cobalt(III) complexes [Co(III)] [a]
The relevance of the thus electro-chemically prepared
cobalt(III) complexes towards cobaltaelectro-catalyzed C–O
formation was emphasized by detailed mass-spectrometric
analysis of stoichiometric experiments with simple Co(OAc)2
versus [Co(III)-I] in the presence of MeOH as the solvent
(Scheme 2). Thus, upon formation of cobalt(II) bisamide [Co(II)-I]
in MeCN, anodic oxidation allowed for the generation of
cyclometalated [Co(III)-I]. It is noteworthy that the
electrosynthesis only occurred in the presence of NaOPiv as the
additive.[21] These findings and the Hammett plot analysis (Figure
3) provide support for a base-assisted internal electrophilic-type
substitution (BIES) C–H activation.[24]
Entry
R
H
[Co(III)]
[Co(III)-I]
Yield / %
46
1
2
3
4
5
6
Noteworthily, the formation of the C–H alkoxylated product 2aa
was solely achieved when electricity was applied, providing
support for an oxidation-induced reductive elimination within a
cobalt(III/IV/II) regime (Scheme 3). Interestingly, analogous
transformations of the cyclometalated cobalt complex [Co(III)-I]
with alkynes or allenes quantitatively delivered the corresponding
annulated products in the absence of additional oxidants.
Collectively, these findings are indicative of different mechanisms
being operative for the C–O versus C–C formations. Here,
decoordination of substrate 1a likely induced coordination of the
alkyne or allene substrate, along with insertion and reductive
elimination to deliver products 3 and 4, respectively.
Me
iPr
[Co(III)-III]
[Co(III)-IV]
[Co(III)-V]
[Co(III)-VI]
[Co(III)-VII]
51
28
OMe
CN
CF3
13[b]
19
13
[a] Reaction conditions: Undivided cell, 1 (1.0 mmol), Co(OAc)2 (0.5 equiv),
NaOPiv (1.0 equiv), MeCN (13 mL), 25 °C, constant potential electrolysis
(CVE) at 1.4 V vs. Ag/Ag+, 1 F/mol, graphite felt anode, Pt-plate cathode.
Yields of isolated complexes are given. [b] C–H acyloxylation was detected
in 3% conversion.
A Hammett plot analysis of voltammetric peak potential
versus the σpara values for a series of different substituents was
performed (Figure 3).[23] The positive slope clearly indicates that
electron-donating substituents facilitate the electro-oxidation,
while electron-withdrawing substituents remove electron-density
on the ligated cobalt and thereby increase the required oxidation
potential of the complex.
Figure 3. CVs of the substituted cobalt(III)cycles [Co(III)] in MeOH (3.5 mM) at
100 mV/s in 0.1 M [n-Bu4N][PF6].
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To rationalize the nature of the C–O forming reductive
elimination step, DFT calculations at the PBE0-D3(BJ)/def2-
TZVP+SMD(MeOH)//PBE0-D3(BJ)/def2-SVP level of theory
were performed. These computational studies were thus in good
agreement with our experimental findings (Figure 5).[21] Hence,
oxidatively-induced
reductive
elimination
through
a
Co(III)/Co(IV)/Co(II) manifold proved to be highly plausible with an
activation barrier of 9.9 kcal mol–1. Noncovalent interactions
between the π-systems of the two coordinated substrates, could
be identified to stabilize the relevant transition state structure.
Scheme 2. ESI-MS monitoring of possible cobaltacycle formation. a) Formation
of [Co(III)-I] upon anodic oxidation of [Co(II)-I] in MeCN. b) Oxidation-induced
C–H alkoxylation of [Co(III)-I] in MeOH.
Figure 5. a) Computed Gibbs free energies (ΔG298.15) in kcal mol−1 for
oxidatively-induced reductive elimination elementary step from and b)
visualization of noncovalent interactions determined through a NCI plot. In the
latter, strong and weak attractive interactions are depicted in blue and green,
respectively, while red corresponds to strong repulsive interactions. All values
include dispersion corrections.
During the synthesis of the cyclometalted cobalt(III)-
complexes [Co(III)] we noticed the formation of a significant
amount of by-product, especially with the electron-rich substrates
1 (Table 1, entries 2 and 3). Based on our CV-studies, we
hypothesized a possible oxidation of [Co(III)-I] to a high-valent
cobalt(IV) complex, which would induce oxidation-induced
reductive elimination for homo-couplings of the coordinated
substrates, while leading to a paramagnetic cobalt(II) complex. To
reduce our hypothesis into practice, we probed various solvents
and adjusted the reaction temperature. The use of solvents other
than polar-aprotic MeCN proved to be unsuitable (Table 2, entries
2–4). Interestingly, at a reaction temperature of 60 °C (Table 2,
Scheme 3. [Co(III)-I] for C–O versus C–C formation.
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entry 5) the desired complex [Co(II)-II] was isolated in high yield,
after cobaltaelectro-double C–H activation. The structure of
[Co(II)-II] was unambiguously verified by single-crystal X-ray
characterization (Figure 4). Overall, the isolation of cobalt(II)
[Co(II)-II] provides further strong support for an oxidation-induced
reductive elimination from a high-valent cobalt(IV) intermediate,
likely featuring two Co–C bonds.
characterized by X-ray diffraction analysis. Our experimental and
computational mechanistic insights are of direct relevance to
cobaltaelectro-catalyzed C–O formations, and enabled the
development of unprecedented cobaltaelectro-mediated double
C–H/C–H arylations.
Table 2. Cobaltaelectro-induced two-fold C–H activation.[a]
Acknowledgements
Generous support by the DFG (Gottfried-Wilhelm-Leibniz award)
is gratefully acknowledged. We thank Dr. Christopher Golz
(Göttingen University) for assistance with the X-ray diffraction and
Dr. Holm Frauendorf (Göttingen University) for support with the
ESI-MS studies.
Keywords: C–H activation • cobalt • electrosynthesis •
mechanism • oxidative catalysis
Entry
Solvent
MeCN
CH2Cl2
HFIP
T [°C]
25
[Co(II)-II] / [%]
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1
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20
---
40
40
---
EtOH
40
---[b]
73
MeCN
60
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Figure 4. X-ray crystal structure analysis of [Co(II)-II] (50% probability
ellipsoids). Hydrogen atoms were omitted for clarity. CCDC 1983756.
In summary, we have identified, isolated and fully
characterized key C–H activated intermediates of cobalta-
electrocatalyzed
cyclometalated
characterized
C–H
cobalt(III)
activation.
complexes
diffraction
Thus,
were
well-defined
structurally
by
X-ray
analysis.
Their
cyclovoltammetric features, kinetic analysis, and mass
spectrometric studies were supportive of an oxidation-induced
reductive elimination within a cobalt(III/IV/II) manifold. The
resulting cobalt(II) complex was also isolated and fully
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10.1002/anie.202002258
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Co(III/IV): Cyclic voltammetric, kinetic
and computational studies identified a
novel mechanism for cobaltaelectro-
catalysis, which enabled
T. H. Meyer, J. C. A. Oliveira, D. Ghorai,
L. Ackermann*
Page No. – Page No.
electrooxidative double C–H
arylations.
Insights into Cobalta(III/IV/II)-
Electrocatalysis: Oxidation-Induced
Reductive Elimination for Twofold C–
H Activation
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