Insights into Ruthenium(II/IV)-Catalyzed Distal C?H Oxygenation by Weak Coordination

Article abstract of DOI:10.1002/chem.202003062

C?H hydroxylation of aryl acetamides and alkyl phenylacetyl esters was accomplished via challenging distal weak O-coordination by versatile ruthenium(II/IV) catalysis. The ruthenium(II)-catalyzed C?H oxygenation of aryl acetamides proceeded through C?H ac

Full text of DOI:10.1002/chem.202003062

Accepted Article
Accepted Article
Title: Insights into Ruthenium(II/IV)-Catalyzed Distal C–H Oxygenation
by Weak Coordination
Authors: Qingqing Bu, Rositha Kuniyil, Shen Zhigao, Elżbieta Gońka,
and Lutz Ackermann
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To be cited as: Chem. Eur. J. 10.1002/chem.202003062
Link to VoR: https://doi.org/10.1002/chem.202003062
01/2020

10.1002/chem.202003062
Chemistry - A European Journal
RESEARCH ARTICLE
Insights into Ruthenium(II/IV)-Catalyzed Distal C–H Oxygenation by
Weak Coordination
Qingqing Bu+, Rositha Kuniyil+, Shen Zhigao, Elżbieta Gońka, and Lutz Ackermann*
Abstract: C–H hydroxylation of aryl acetamides and alkyl phenylacetyl
esters was accomplished via challenging distal weak O-coordination by
versatile ruthenium(II/IV) catalysis. The ruthenium(II)-catalyzed C–H
oxygenation of aryl acetamides proceeded through C–H activation,
ruthenium(II/IV) oxidation and reductive elimination, thus providing step-
economical access to valuable phenols. The p-cymene-ruthenium(II/IV)
manifold was established by detailed experimental and DFT-
Figure 1. Ruthenium-catalyzed C–H oxygenation by distal weak coordination.
computational studies.
Results and Discussion
Introduction
We commenced our studies by probing the envisioned C–H
Phenols are key structural motifs in various natural products and
oxygenation of weakly O-coordinating amide 1a with [RuCl2(p
biologically relevant molecules.[1] In recent years, a number of
cymene)]2 as the catalyst and PhI(TFA)2 as the oxygenation agent
methods for oxidative C−O bond formation on arenes has been
(Table 1). DCE was found to be the solvent of choice, whereas
developed.[2] Especially the catalytic hydroxylation of otherwise inert
toluene, DMF, m-xylene and 1,4-dioxane gave inferior results
C–H bonds under transition metal catalysis represents an
(entries 1-6). Interestingly, TFA/TFAA turned out to be unsuitable for
environmentally-benign as well as economically-attractive method
this reaction (entry 5). Control experiments confirmed the essential
towards an expedient access of substituted phenols.[3,4] The past few
role of the ruthenium catalyst (entry 7). Notably, frequently employed
years have witnessed a considerable growth in the use of less
palladium, nickel, cobalt, and rhodium catalysts fell short in providing
expensive,[5] readily-accessible and versatile ruthenium(II) catalysts
the desired product 2a (entries 8–11). The use of widely employed
for C–H functionalization[6] as an alternative to commonly employed
oxidants such as K2S2O8 and (NH4)2S2O4 fell short in delivering the
cost-intensive palladium and rhodium complexes.[7] To this end,
desired product 2a under otherwise identical conditions (Table S1 in
considerable progress has been made in proximity-induced ortho-C–
the Supporting Information).
H
transformations by employing ruthenium(II) complexes. In
particular, carboxylate assistance has been recognized as a
powerful tool for C–H activations through metal–ligand cooperation
via a six‐membered transition state.[8] Despite these undisputable
advances, ruthenium-catalyzed C–H functionalization with distal,
weakly coordinating directing groups, such as aryl acetamides,
continues to be scarce,[9] mainly due to the formation of unfavourable
six-membered metallacycle intermediates.[10]
Table 1. Optimization of distal C–H oxygenation of acetamide 1a.[a]
Entry
[TM]
Solvent
1,4-dioxane
DMF
Yield (%)[b]
NR
Within our program on ruthenium(II)-catalyzed atom- and step-
economical C–H functionalization,[11] we developed the ruthenium-
catalyzed C–H oxygenation of weakly O-coordinating aryl
acetamides, on which we report herein (Figure 1). Significant
features of our findings include a) an efficient strategy for the
ruthenium(II/IV)-catalyzed C–H hydroxylations via distal weak O-
coordination, b) ample substrate scope with synthetically useful
amides and esters, c) use of mild hypervalent iodine reagents as the
oxidant, and d) unprecedented experimental and computational
mechanistic insights.
1
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
–
2
NR
3
PhMe
15
4
m-xylene
TFA/TFAA[c]
DCE
15
5
NR
6
62
7
DCE
NR
8
Pd(OAc)2
DCE
42
[*]
Dr. Q. Bu, Dr. R. Kuniyil, S. Zhigao, Dr. E. Gońka,
Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstraße 2, 37077 Göttingen
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de/
9
Ni(cod)2
DCE
NR
10
11
[Cp*Co(CO)I2]
[RhCp*Cl2]2
DCE
NR
DCE
NR
[a] Reaction conditions: 1a (0.5 mmol), PhI(TFA)2 (1.0 mmol), [TM] (5.0 mol %),
solvent (2.0 mL), 16 h. [b] Yield of isolated products. [c] Ratio of TFA:TFAA: 1:1.
Key: pentamethylcyclopentadiene (Cp*), 1,2-dichloroethane (DCE), N, N-dimethyl-
formamide (DMF), 1,4-cyclooctadiene (cod), TFA = trifluoroacetate.
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
1
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10.1002/chem.202003062
Chemistry - A European Journal
RESEARCH ARTICLE
With the optimized reaction conditions in hand, we probed the
versatility of the ruthenium(II)-catalyzed C–H oxygenation with
differently decorated phenyl acetamides 1 (Scheme 1). Initially, we
studied the effect exerted by the amide substitution pattern on the
C–H oxygenation. Interestingly, sterically hindered amides such as
1a and 1c served as viable substrates and yielded the corresponding
products 2. The robustness of the ruthenium(II) catalysis was
reflected by the excellent tolerance of valuable functional groups
such as nitro, fluoro and bromo, thereby setting the stage for further
late-stage diversifications. Electron-rich as well as electron-deficient
amides were smoothly transformed into the corresponding
monohydroxylated products 2 in moderate to good yield independent
of the substitution pattern. meta-Bromo-substituted substrate 1h
gave the corresponding product 2h as the sole product with excellent
levels of regioselectivity and yield. Remarkably, no racemization of
stereogenic centre was observed in case of 2m. Furthermore, the
reaction proceeded well with tertiary amide, providing 2o.
Scheme 3. Key mechanistic findings.
Given the excellent efficiency of the ruthenium(II)-catalyzed
C–H oxygenation, we became interested in delineating its mode of
action. To this end, an intermolecular competition experiment
between differently substituted substrates 1 indicated electron-
deficient arene 1g to be inherently more reactive than electron-rich
arene 1e, which can be rationalized by a CMD-type mechanism
(Scheme 3a).[12,13] Furthermore, kinetic isotope effect (KIE) studies
by independent reactions suggested a kinetically relevant C–H
metalation with a KIE of kH/kD ≈ 2.2 (Scheme 3b). Thereafter, we
investigated the possibility of p-cymene dissociation during the
course of the reaction. A careful analysis of the final reaction mixture
did not provide any evidence for the presence of significant amounts
of free p-cymene (5) (Scheme 3c).[14]
Based on our experimental studies, we propose a plausible
catalytic cycle for ruthenium(II)-catalyzed C–H oxygenations to
commence with a kinetically relevant C–H activation on acetamide
1a by ruthenium(II) complex im2 (Scheme 4). Thus generated
ruthena(II)cycle im3 will then undergo oxidation by the hypervalent
iodine reagent PhI(TFA)2, delivering ruthenium(IV) intermediate im7.
Reductive elimination from im7 leads to the formation of a new C–O
bond, generating complex im8. Coordination of a trifluorocarboxylate
anion to the metal centre will liberate product 2a' and regenerate the
active catalyst im1. Alternatively, 2a' will undergo hydrolysis to
generate the final product 2a.
Scheme 1. Ruthenium-catalyzed C–H oxygenation of acetamides 1.
It is noteworthy that the versatile ruthenium(II) catalyst was not
limited to aryl acetamides. Indeed, we were pleased to identify more
challenging, weakly-coordinating phenylacetyl esters as viable
substrates (Scheme 2). To this end, excellent levels of site-selectivity
was accomplished for the hydroxylation of electron-poor as well as
electron-rich phenylacetyl esters providing the corresponding
products 4c and 4d.
Scheme 2. Ruthenium-catalyzed C–H oxygenation of phenylacetyl esters 3.
2
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10.1002/chem.202003062
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RESEARCH ARTICLE
Scheme 4. Plausible catalytic cycle for ruthenium-catalyzed C–H oxygenation
In ordert to probe the catalyst’s mode of action, we became
interested in better understanding the mechanism of C–H
oxygenation by density functional theory (DFT) studies (Figure 2).
Geometry optimizations and frequency calculations were performed
at the B3LYP-D3(BJ)/6-31G*,def2-SVP(Ru,I) level of theory, while
single point energies were calculated at the PBE0-D3(BJ)/
6-311++G**,def2-TZVP(Ru,I)+SMD(DCE) level of theory.[15] Our
findings unravel here that the initial C–H activation occurs from the
intermediate im2 facilitated by the acetate ligand via TS2. The thus
formed ruthenacycle im3 undergoes ligand exchange with PhI(TFA)2
to generate im4. The oxidation process occurs from im4 via two
steps. The first step involves the cleavage of the I–O bond, along
with the generation of Ru–I bond in a concerted fashion via TS3. The
subsequent cleavage of the second I–O bond from im5 via TS4
leads to the formation of the intermediate im6. Upon release of
iodobenzene, the trifluorocarboxylate anion coordinates to the metal
center to form im7. Subsequently, facile reductive elimination occurs
from im6 via TS5 involving a ruthenium(IV/II) process to generate
the final product im8. The calculated energy barriers are overall in
good agreement with the experimental data.
3
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RESEARCH ARTICLE
Figure 2. Computed relative Gibbs free energy in kcal mol−1 for the ruthenium-catalyzed C–H oxygenation reaction of 1a at the PBE0-D3(BJ)/6-311++G**,
def2-TZVP(Ru,I)+SMD(DCE)//B3LYP-D3(BJ)/6-31G*,def2-SVP(Ru,I) level of theory.
Conclusion
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Generous support by the DFG (SPP 1807 and Gottfried Wilhelm
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[3]
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Conflict of interest
The authors declare no conflict of interest.
Keywords: C–H activation • oxygenation • amides • Reaction
mechanisms
4
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[14] For detailed information, see the Supporting Information.
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