C?H Oxygenation Reactions Enabled by Dual Catalysis with Electrogenerated Hypervalent Iodine Species and Ruthenium Complexes

Article abstract of DOI:10.1002/anie.201914226

The catalytic generation of hypervalent iodine(III) reagents by anodic electrooxidation was orchestrated towards an unprecedented electrocatalytic C?H oxygenation of weakly coordinating aromatic amides and ketones. Thus, catalytic quantities of iodoarenes in concert with catalytic amounts of ruthenium(II) complexes set the stage for versatile C?H activations with ample scope and high functional group tolerance. Detailed mechanistic studies by experiment and computation substantiate the role of the iodoarene as the electrochemically relevant species towards C?H oxygenations with electricity as a sustainable oxidant and molecular hydrogen as the sole by-product. para-Selective C?H oxygenations likewise proved viable in the absence of directing groups.

Full text of DOI:10.1002/anie.201914226

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Electro-Catalyzed Hypervalent Iodine Orchestrated for
Ruthenaelectro-Catalyzed C–H Oxygenation-
Authors: Leonardo Massignan, Xuefeng Tan, Tjark H. Meyer, Rositha
Kuniyil, Antonis M. Messinis, and Lutz Ackermann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914226
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10.1002/anie.201914226
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Electro-Catalyzed Hypervalent Iodine Orchestrated for
Ruthenaelectro-Catalyzed C–H Oxygenation
Leonardo Massignan,+ Xuefeng Tan,+ Tjark H. Meyer, Rositha Kuniyil, Antonis M. Messinis, and Lutz
Ackermann*
Abstract: The catalytic generation of hypervalent iodine(III) reagents
by anodic electro-oxidation was orchestrated towards an
unprecedented electro-catalytic C–H oxygenation of weakly-
coordinating aromatic amides and ketones. Thus, catalytic quantities
of iodoarenes in concert with catalytic amounts of ruthenium(II)
complexes set the stage for versatile C–H activations with ample
scope and high functional group tolerance. Detailed mechanistic
studies by experiment and computation substantiated iodoarenes as
the electrochemically relevant species towards C–H oxygenations
with electricity as sustainable oxidant and molecular hydrogen as the
sole by-product. para-Selective C–H oxygenations proved likewise
viable in the absence of directing groups.
molecular challenge that orchestrates the catalytic electro-
regeneration[10] of hypervalent iodine(III) reagents with
ruthenium(II)-catalyzed[11],[12] C–H functionalizations. Salient
features of our findings include (a) first electro-catalyzed C–H
oxygenations by weak coordination, (b) user-friendly
electrochemical generation of hypervalent iodine reagents, (c)
ioda/ruthenaelectro-catalyzed C–H functionalizations that
combine the advantages of ruthenium-catalyzed C–H activation
with the electro-catalytic hypervalent iodine chemistry, and (d)
mechanistic studies by experiment, computation, cyclic
voltammetry and in-operando NMR spectroscopy.
Organic electrochemistry has emerged as an increasingly viable
tool for molecular synthesis.[1] In addition to the unique potential
of electrosynthesis, it is attractive due to the storable, and
sustainable properties of electricity.[2] Thus, the effective
conversion of renewable electricity into value-added chemical
Figure 1. Orchestrating ioda(III)/ruthena(II)-electro-catalytic C–H activation.
products holds major prospect for
a sustainable energy
economy.[1h] In this scenario, the merger of electrosynthesis and
metal-catalyzed C–H activation[3] has recently been identified as
a particularly powerful approach for the resource-economical
transformation of ubiquitous, but otherwise inert C–H bonds.[4]
Despite of indisputable advances by Mei, Sanford, and
Ackermann,[5] electrochemical C–H oxygenations[6] of challenging
arenes by weak-coordination[7] have thus far proven elusive.
Hence, the reported metalla-catalyzed C–H oxygenations largely
required cost-intensive palladium complexes and were inherently
limited to strongly-coordinating N-directing groups, such as
oximes and pyridines.[5] In sharp contrast, C–H oxygenations by
synthetically-useful weak O-coordination have not been realized
in terms of sustainable electro-catalysis. Instead, highly-reactive
We initiated our studies by exploring various reaction conditions
for the envisioned electrochemical orchestrated C–H oxygenation
of substrate 1a in a user-friendly undivided cell setup (Table 1,
and Table S1 in the Supporting Information).[13] Preliminary
experimentation indicated that the reaction could indeed be
accomplished in the presence of catalytic amounts of
iodobenzene and ruthenium(II) carboxylate (entry 1). The ideal
current density was found at 2.67 mA·cm–2 (entries 2-3), and the
C–H activation proceeded equally well under constant potential
conditions at a 2.0 V working potential (entry 4). Interestingly, the
platinum-plate as anode was found to be beneficial in comparison
to the reticulated vitreous carbon (RVC) anode (entries 5-6). Here,
detailed infrared spectroscopic analysis of the RVC anode
indicated its electrocatalytic modification.[13] Control experiments
confirmed the essential role of the electricity, the ruthenium
catalyst, and the iodoarene (entries 7-9). Furthermore,
iodobenzene was found to be the only co-catalyst that enabled
the desired C–H oxygenation, while benzoquinone (entry 10) as
well as chlorine, bromine or chalcogenide redox catalysis[14] fell
short in converting substrate 1a (entries 11 and 12).[12] Notably,
the replacement of electricity by the typical chemical oxidants
mCPBA or Oxone resulted in considerably inferior efficacy
(entries 13-14).
hypervalent
iodine(III)
reagents,[8],[9]
such
as
(diacetoxyiodo)benzene and [bis(trifluoroacetoxy)iodo]benzene,
are required in overstoichiometric quantities, which calls for
strong chemical oxidants for their syntheses and leads to
equimolar amounts of undesired halogenated waste products
during the C–H functionalization process. Contrarily, we herein
present
a mechanistically-distinct strategy to address this
[*]
L. Massignan, Dr. X. Tan, T. H. Meyer, Dr. R. Kuniyil, Dr. A. M.
Messinis and Prof. Dr. Lutz Ackermann
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/
Table 1. Optimization of ioda/ruthenaelectro-catalyzed C–H oxygenation.[a]
[+]
These authors contributed equally to this paper.
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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10.1002/anie.201914226
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Entry
Deviation from standard conditions
Yield[b]
1
2
none
80%
51%
37%
86%[c]
24%
28%
---
j = 4.00 mA·cm–2
3
j = 1.33 mA·cm–2
4
CPE at 2.0 V
5
RVC anode instead of Pt
RVC anode instead of Pt, without PhI
no current
6
7
8
without [Ru]
---
9
without PhI
---
10
11
12
13
14
1,4-Benzoquinone instead of PhI
PhBr or PhCl instead of PhI
PhS-SPh or PhSe-SePh instead of PhI
mCPBA instead of electricity
Oxone instead of electricity
---
---
---
15%
32%
Scheme 1. Electro-catalyzed C–H activation of Weinreb amides 1. [a] Without
nBu4NPF6. [b] Regio-isomer 2j’ was isolated in 2% yield.
[a] Undivided cell, 1a (0.50 mmol), iodobenzene (20 mol %), 3 (5.0 mol %)
electrolyte (1.0 equiv), solvent (3.0 mL), 50 °C, 16 h, Pt-plate electrodes (10 mm
x 15 mm x 0.125 mm), constant current electrolysis (CCE) at 4 mA. [b] Yield of
isolated product. [c] CPE = constant potential electrolysis at 2.0 V vs Ag/Ag+.
TFA = trifluoroacetic acid. TFAA = trifluoroacetic anhydride.
It is noteworthy that the reaction was not limited to Weinreb
amides 1. Indeed, differently substituted amides 1q-w were
efficiently converted to the corresponding oxygenated arenes 2
with excellent efficiency likewise (Scheme 2).
With the optimized reaction conditions in hand, we probed the
versatility of the co-catalytic[15] electrochemical C–H oxygenation
system with a representative set of weakly-O-coordinating amides
1 (Scheme 1). Differently decorated amides bearing para- and
meta-substituents were thus efficiently transformed towards
products 2a-k. Useful electrophilic functional groups, such as
chloro, bromo and even iodo substituents as well as sensitive
benzyl chlorides, were fully tolerated, an invaluable asset in terms
of future late-stage modifications (2l-p).
Scheme 2. Electrooxidative C–H activation of various amides 1. [a] Without
nBu4NPF6.
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To our delight, the outstanding robustness of the
iodine(III)/ruthenium(II)-catalyzed C–H oxygenation co-catalytic
system was further highlighted by its ability to also transform
weakly coordinating ketones 4 (Scheme 3).[7] The versatility of the
electro-catalysis was hence reflected by the successful use of
differently decorated ketones 4. Thereby, various substitution
patterns were well tolerated to deliver products 5e-j. The inherent
selectivity features were probed by intramolecular competition
experiments with diaryl-ketones 4k and 4l, both being
functionalized with an excellent mono- and chemo-selectivity. The
regio-selectivity in the C–H transformation of the unsymmetrically-
substituted substrate 4l further illustrated the inherent preference
for electron-rich arenes (vide infra).
Scheme 4. Ruthenaelectro-catalyzed C–H activation of pyrazolyl substrates 6.
It is noteworthy that the ruthenaelectro-catalyzed C–H
functionalization was not limited to chelation-assisted ortho-
oxygenation. Indeed, the directing group-free[6f] functionalization
in the challenging remote position was likewise sequentially
accomplished with excellent levels of site-selectivity, while the
ruthenium catalyst was found to be essential (Scheme 5).
Scheme 5. Directing group-free remote C–H oxygenation.
The scalability of the orchestrated electrochemical C–H
oxygenation was demonstrated by the gram-scale synthesis of
product 2a without loss in efficiency (Scheme 6).
Scheme 3. Ruthenaelectro-catalyzed C–H activation of ketones 4. [a] 3 mA.
Moreover, the ruthenaelectro-catalyzed C–H oxygenation
enabled the modification of synthetically useful pyrazole
derivatives 6 (Scheme 4).
Scheme 6. Gram-scale iodine/ruthena-electro-catalyzed C–H oxygenation.
Given the efficiency of the unprecedented C–H oxygenation
electrochemical system, we became interested in delineating its
mode of action. To this end, first, the assessment of the
deuterated Weinreb amide in the catalytic reaction unravelled a
reversible C–H activation step (Scheme 7a). This finding
contrasts with C–H oxygenations enabled by the chemical oxidant
PIFA, for which H/D scrambling was not observed.[6g] Second,
kinetic studies provided strong support for a fast and reversible
C–H metalation with a minor kinetic isotope effect (KIE) of only
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kH/kD ≈ 1.6.[13] These observations overall suggest that the C–H
activation is not the rate-determining step, but rather the oxidation
of the cyclometalated species. These experimental data are again
in contrast with the use of chemical oxidants for which the C–H
activation was proposed to be the rate-limiting step with a KIE of
kH/kD ≈ 3.0.[6f] Thirdly, competition experiments, using either the
Weinreb amides 1b and 1d or the difunctionalized ketone 4m,
highlighted electron-rich substrates being preferentially
functionalized (vide supra, Scheme 7b), which can be rationalized
in terms of a base-assisted internal electrophilic-type substitution
(BIES) to be operative for the C–H metalation.[16] Forth, an
intramolecular competition experiment with substrate 1x revealed
the Weinreb amide as more powerful coordination for the iodine-
ruthenium-co-catalyzed C–H transformation (Scheme 7c). Fifth,
we probed the possibility of p-cymene dissociation.[17] Thus,
detailed gas-chromatographic analysis did not provide evidence
for free p-cymene at any point in the reaction mixture.[12]
cyclic voltammetry (Figure 2b).[12] To this end, the oxidation of
different aryl halides was recorded.[13] In trifluoroacetic acid, only
iodobenzene showed irreversible anodic oxidation with an onset
potential of E = 1.25 V vs. ferrocene. By means of computation
we also confirmed that the oxidation potential of the iodobenzene
is 200 mV lower than the ruthenium(II/IV) manifold,[12]
substantiating the iodine-co-catalysis. Notably, other organic
halides are known to undergo oxidation at considerably higher
12]
potentials,[18,
reflecting the unique catalytic competence of
iodine reagents (vide supra, Table 1). The amide 1a and electron-
deficient iodoarenes showed significally higher potentials for
anodic oxidation as compared with unsubstituted and electron-
rich iodoarenes. A mixture of iodobenzene and amide 1a did not
lead to significant changes in the voltammogram, which is in
agreement with the control experiments summarized in Table 1.
Cyclic voltammetry of the independently prepared ruthenacycle
10 in DCE provided support for its facile oxidation.[13]
Figure 2. a) in-operando NMR-studies using constant current electrolysis at
10 mA in trifluoroethanol (TFE) or trifluoroacetic acid (TFA) respectively. NMR
conversion was determined using CH2Br2 as the internal standard. i) Reaction
profile of anodic formation of CH3C6H4I(OCH2CF3)2 (11a). ii) Reaction profile of
anodic synthesis/formation of CH3C6H4I(OCOCF3)2 (11b). b) Cyclic voltammetry
(TFA, 0.1 m nBu4NPF6, 100 mV/s) using glassy carbon as the working electrode.
Cyclic voltammograms of different reaction components and their mixtures as
well as haloarenes.
Scheme 7. Summary of the mechanistic findings.
Next, we studied the reaction profile for the direct anodic
generation of hypervalent iodine reagents by in-operando NMR
spectroscopy (Figure 2a).[12] This combination of electrochemistry
and in-situ spectroscopy offered the possibility to study the
generation of otherwise unstable electrochemically generated
iodine(III) reagents. Initially, the anodic oxidation of iodobenzene
in trifluoroethanol (TFE) was monitored and showed almost full
conversion of the aryl halide after 2.5 h at 10 mA (Figure 2a,i).[10a]
Subsequently, the anodic generation of hypervalent iodine 11b
from TFA and iodobenzene was completed with only slightly
prolonged reaction times within 2.5 h (Figure 2a,ii). Thereafter, we
examined the electrochemical C–H oxygenation by means of
Based on our detailed mechanistic studies, we propose a
plausible catalytic cycle for the ioda/ruthena-electrocatalyzed C–
H oxygenation as depicted in Scheme 8. The mechanism
rationale commences with the C–H activation on amide 1 by a
ruthenium(II) carboxylate. Meanwhile, iodobenzene undergoes a
two electron transfer anodic oxidation generating the hypervalent
iodine(III). The iodine(III) reagent then mediates the oxidation of
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10.1002/anie.201914226
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Scheme 8. Plausible catalytic cycle.
In conclusion, we have devised a novel electrochemical co-
catalytic system for the C–H oxygenation of synthetically useful
amides and ketones by challenging weak-O-coordination. The
versatile
iodine(III)/ruthenium(II)-electro-catalyzed
C–H
functionalization occurred by orchestrating the catalytic
generation of hypervalent iodine(III) reagents with sustainable
electricity as cost-effective terminal oxidant, with the formation of
molecular hydrogen as the sole by-product. Detailed mechanistic
studies by experiment, computation and flow-NMR spectroscopy
provided - in contrast to chemical oxidation - support for a fast and
reversible C–H ruthenation. The ruthenium catalysis also allowed
for the electrochemical remote C–H oxygenations in the absence
of directing groups.
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Acknowledgements
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Conflict of interest
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The authors declare no conflict of interest.
Keywords: oxygenation • C–H activation • electrocatalysis •
electrochemistry • ruthenium • hypervalent iodine
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For detailed information, see the Supporting Information.
This article is protected by copyright. All rights reserved.

10.1002/anie.201914226
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Leonardo Massignan+, Xuefeng Tan+,
Tjark H. Meyer, Rositha Kuniyil, Antonis
M. Messinis, and Lutz Ackermann*
Page No. – Page No.
Electro-Catalyzed Hypervalent Iodine
Orchestrated for Ruthenaelectro-
Catalyzed C–H Oxygenation
Dual catalysis: The joined action of catalytic amounts of iodoarenes and ruthenium(II)
carboxylates enabled the expedient C–H oxygenation by weak coordination with amides and
ketones.
This article is protected by copyright. All rights reserved.