Nickel-Catalyzed Electrooxidative C?H Amination: Support for Nickel(IV)

Article abstract of DOI:10.1002/chem.201805441

Nickel-catalyzed electrochemical C?H aminations were accomplished by chemo- and position-selective C?H activation with ample scope. Detailed mechanistic studies highlighted a facile C?H cleavage with unique chemo-selectivity, while cyclovoltammetric analy

Full text of DOI:10.1002/chem.201805441

A Journal of
Accepted Article
Title: Nickel-Catalyzed Electrooxidative C–H Amination: Support for
Nickel(IV)
Authors: Shou-Kun Zhang, Ramesh C. Samanta, Nicolas Sauermann,
and Lutz Ackermann
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To be cited as: Chem. Eur. J. 10.1002/chem.201805441
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Supported by

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COMMUNICATION
Nickel-Catalyzed Electrooxidative C–H Amination: Support for
Nickel(IV)
[
a]
[a]
[a]
[a]
Shou-Kun Zhang, Ramesh C. Samanta, Nicolas Sauermann, and Lutz Ackermann*
Abstract: Nickel-catalyzed electrochemical C–H aminations were
accomplished by chemo- and position-selective C–H activation with
ample scope. Detailed mechanistic studies highlighted a facile C–H
cleavage with unique chemo-selectivity, while cyclovoltammetric
analysis provided support for a nickel(II/III/IV) manifold.
We commenced our studies by probing various reaction
conditions for the envisioned electrooxidative C–H amination by
nickel catalysis (Table 1; Tables S–1-S–5 in the Supporting
[
16]
Information). After considerable orienting experimentation, we
observed that the desired C–H activation could be realized with
.
Ni(OAc)
2
4H
2
O as the catalyst (entry 1). Among various additives
and solvents, NaOPiv and DMA emerged as being optimal,
[
1]
Combining
metal-catalyzed
C–H
activation
with
respectively (entries 2-7). It is noteworthy that, in contrast to
[2]
electrocatalysis holds transformative potential for molecular
syntheses, because it circumvents the use of toxic and
[10c]
cobalt catalysis,
biomass-derived solvent GVL failed to
[
3]
provide the desired product 3aa (entry 5). Subsequent
adjustment of the reaction conditions, including additive
stoichiometry and concentration, further improved the
performance of the nickel/electro-catalysis (entries 8-11). The
key role of the electricity (entry 12) and the nickel complex (entry
[
4]
expensive metal oxidants. The use of electricity as formal
oxidant for C–H activation is likewise attractive since it allows
the sustainable use of renewable energy. While this approach
considerably improved the atom-economy of C–H activation,
advances were until very recently largely limited to the precious
ruthenium and iridium. In
sharp contrast, only very recently we have unraveled the
potential of cobalt catalysts for electrochemical C–H
functionalizations, which has proven instrumental for cobalt-
13) was confirmed by control experiments. It is noteworthy that
[
5]
[6]
[7]
[8]
4d metals palladium, rhodium,
an additional redox-mediator was not required for the nickel-
catalyzed electroamination, contrasting with very recent copper
[14]
catalysis.
[
9]
[
10]
Table 1. Optimization of the Electrochemical Nickel-Catalyzed C–H
mediated transformations.
Despite of this indisputable
progress, electrochemical C–H nitrogenation with the less toxic
[
a]
Amination.
[
11,12]
base metal nickel
has thus far proven elusive. As a direct
[
13]
[14]
consequence, within our program
on nickel-catalyzed C–H
[
15]
activation, we became attracted to probing the opportunities of
joining nickel catalysis with electrochemical C–H activation, the
results of which we report herein. Key features of our findings
include a) first nickel-catalyzed electrochemical C–H aminations,
b) chemical oxidant-free C–H nitrogenations with cyclic and
more challenging acyclic amines, and c) key mechanistic
insights into nickel-electrocatalyzed C–H activation. It is thus
noteworthy that our studies highlight a distinct working mode by
fast C–H scission, involving a scarce nickel(IV) intermediate.
Entry
Solvent
Additive
[Ni]
3aa [%]
.
.
1
2
3
4
5
6
7
8
9
DMA
DMA
DMA
DMA
GVL
NMP
DMF
DMA
DMA
DMA
DMA
DMA
DMA
---
Ni(OAc)
2
4H
2
O
10
35
55
24
---
PivOH
NaOPiv
Ni(OAc) 4H O
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
.
.
.
.
.
.
.
.
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
Ni(OAc)
4H
4H
4H
4H
4H
4H
4H
4H
O
O
O
O
O
O
O
O
2 3
Na CO
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
NaOPiv
44
22
Figure 1. Nickel-catalyzed electrochemical C–H nitrogenation by nickel(IV).
[b]
20
[c]
61
[c, d]
10
11
12
13
72
[
c, d, e]
Ni(DME)Cl
2
77
[
c,d,e,f]
Ni(DME)Cl
---
2
---
[
a]
S.-K. Zhang, Dr. R. C. Samanta, Dr. N. Sauermann, Prof. Dr. L.
Ackermann
[d,e]
---
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstrasse 2, 37077 Göttingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de
[
a] Reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), [Ni] (10 mol %),
Additive (0.50 mmol), nBu NBF (0.50 mmol), Solvent (2.5 mL), CCE = 8 mA,
0 h, N , RVC(+)/Ni foam(-). [b] under air. [c] [Ni] (20 mol %). [d] NaOPiv
0.25 mmol). [e] DMA (3.0 mL). [f] no current. DMA = N,N-Dimethylacetamide,
GVL γ-Valerolactone, NMP N-Methyl-2-pyrrolidinone, DMF N,N-
Dimethylformamide.
4
4
1
(
2
=
=
=
Supporting information for this article is given via a link at the end of
the document.
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With the catalytic conditions being optimized, we next
probed the effect exerted by the N-substituent of benzamides 1
(
Scheme 1). Hence, various N,N-coordination modes (4–7), and
an N,O-bidentate chelation (8) met with limited success, clearly
[
10c]
contrasting with a cobalt catalysis manifold.
Scheme 1. Influence of N-substituents on benzamides 1.
After having established an efficient nickel-catalyzed
electroamination, we tested its robustness with different
benzamides 1 (Scheme 2). Thus, diversely decorated arenes 1
were smoothly converted, thereby fully tolerating valuable
functional groups, such as halo, sulfide or ester substituents.
Intramolecular competition experiments with meta-substituted
substrates 1j–m were fully governed by steric interactions,
exclusively delivering the products 3ja–3ma. The versatile nickel
catalyst was not limited to arene diversification. Indeed, the
heteroarenes 1q and 1r were identified as amenable substrates
likewise.
Scheme 2. Nickel-catalyzed electrochemical C–H amination of arenes 1.
Moreover, the nickel catalyst enabled electrochemical C–H
aminations with differently substituted secondary amines
[
17]
(
Scheme 3).
Thereby, useful functional groups were chemo-
selectively tolerated, featuring among others ester and cyano
substituents. It is noteworthy that thiazine derivatives and even
more challenging acyclic secondary amine 2i could be employed
as well.
Scheme 3. Nickel-catalyzed electrochemical C–H amination with amines 2.
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Given the versatility of the nickel-electrocatalyzed C–H
amination under redox-mediator-free conditions, we felt intrigued
to interrogating its modus operandi. To this end, we first
conducted intermolecular competition experiments (Scheme 4a).
In contrast to all previously reported electrooxidative C–H
[
5]
[6]
[7]
[8]
activations with palladium, rhodium, ruthenium, iridium, or
cobalt catalysts,
[
9,10]
the more electron-deficient arenes reacted
faster here. These findings are indicative of a concerted-
[
18]
metalation-deprotonation mechanism
C–H metalation. In good agreement with this observation, we,
second, did not find H/D scrambling when using D COD as the
D
cosolvents (Scheme 4b). Third, a kinetic isotope effect of k /k =
being operative in the
3
H
[19]
1
.1 by independent experiments was suggestive of a fast C–H
scission (Scheme 4c).
Scheme 4. Mechanistic studies on nickel-electrocatalytic C–H amination.
Finally, we conducted detailed studies by means of cyclic
voltammetry (Figure 2a-c, and Figures S–7-S–12 in the
[
16]
Supporting Information). Hence, the electrode potential for the
nickel(II/III) event was found to be strongly influenced by the
addition of both the substrate and NaOPiv to be thus observed
Figure 2. CV data (DMA, 0.05 M [nBu N][BF ], 100 mV/s) for a) the individual
4
4
components of the catalytic manifold. b) Ni(DME)Cl
2
with the individual
components of the catalysis. c) Ni(DME)Cl
at 120 °C for 30 min.
2
, NaOPiv and 1a after preheating
+
/0
at E
the elevated temperature at which catalysis occurred the
reversible oxidation wave was significantly shifted to E = -0.18
p
= +0.26 V vs. Fc (orange) (Figure 2a and 2b). Notably, at
p
+/0
Based on our mechanistic studies, we propose a plausible
catalytic cycle to initiate by a rate-determining carboxylate-
assisted C–H nickelation to deliver intermediate Ni(II)-B
V vs. Fc (purple), which can be rationalized by the formation of
cyclometalated nickel(III) catalyst. Interestingly, we furthermore
observed an additional reversible oxidation wave at E
vs. Fc , which is furthermore indicative of an oxidation to a
p
= +0.49 V
(
Scheme 5). Thereafter, deprotonation of the coordinated amine
+
/0
[20]
and anodic oxidation generates the nickel(III)amide complex
Ni(III)-C. In consideration of our cyclic voltammetry studies,
electrochemical single electron transfer (SET) finally sets the
stage for an oxidation-induced reductive elimination from
intermediate Ni(IV)-D, thereby yielding the desired product 3,
while regenerating the catalytically competent complex Ni(II)-A.
nickel(IV) species.
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Scheme 5. Plausible catalytic cycle.
[3]
In conclusion, we have reported on the first nickel-catalyzed
electrooxidative C−H amination. The carboxylate-enabled nickel-
electrocatalysis occurred with broad substrate scope and high
levels of chemo- and position-selectivity. In contrast to all
previous metal-catalyzed electrochemical C−H activations, the
nickel electro-regime proved particularly potent in the C−H
nitrogenation of electron-deficient arenes. Detailed mechanistic
studies provided strong support for a fast C−H nickelation and
an oxidation-induced reductive elimination at nickel(IV).
Acknowledgements
Generous support by the CSC (fellowship to SKZ), the DFG
(
Gottfried-Wilhelm-Leibniz award to LA), and the Alexander-von-
Humboldt foundation (fellowship to RCS) is gratefully
acknowledged.
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Conflict of interest
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The authors declare no conflict of interest.
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Keywords: Electrochemistry • C−H activation • Mechanism •
Nickel catalysis • Selectivity • Nickel(IV)
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Q.-L. Yang, X.-Y. Wang, J.-Y. Lu, L.-P. Zhang, P. Fang,
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[
[
15]
16]
For selected examples, see: a) S. Nakanowatari, T. Müller,
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For detailed information, see the Supporting Information.
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Chemistry - A European Journal
10.1002/chem.201805441
COMMUNICATION
COMMUNICATION
Shou-Kun Zhang, Ramesh C. Samanta,
Nicolas Sauermann and Lutz
Ackermann*
Page No. – Page No.
Nickel-Catalyzed Electrooxidative C–
H Amination: Support for Nickel(IV)
II…III…IV: Nickel-catalyzed C–H aminations proved viable with electricity as the
sole oxidant by fast C–H cleavage involving nickel(IV) species.
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